The Synergy Effect of Ni-M (M = Mo, Fe, Co, Mn or Cr ... - MDPI

catalysts Article

The Synergy Effect of Ni-M (M = Mo, Fe, Co, Mn or Cr) Bicomponent Catalysts on Partial Methanation Coupling with Water Gas Shift under Low H2/CO Conditions Xinxin Dong, Min Song, Baosheng Jin *, Zheng Zhou and Xu Yang Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy & Environment, Southeast University, Nanjing 210096, Jiangsu, China; [email protected] (X.D.); [email protected] (M.S.); [email protected] (Z.Z.); [email protected] (X.Y.) * Correspondence: [email protected]; Tel.: +86-25-8379-4744 Academic Editors: Benoît Louis, Qiang Wang and Marcelo Maciel Pereira Received: 6 January 2017; Accepted: 3 February 2017; Published: 8 February 2017

Abstract: Ni-M (M = Mo, Fe, Co, Mn or Cr) bicomponent catalysts were prepared through the co-impregnation method for upgrading low H2 /CO ratio biomass gas into urban gas through partial methanation coupling with water gas shift (WGS). The catalysts were characterized by N2 isothermal adsorption, X-ray diffraction (XRD), H2 temperature programmed reduction (H2 -TPR), H2 temperature programmed desorption (H2 -TPD), scanning electron microscopy (SEM) and thermogravimetry (TG). The catalytic performances demonstrated that Mn and Cr were superior to the other three elements due to the increased fraction of reducible NiO particles, promoted dispersion of Ni nanoparticles and enhanced H2 chemisorption ability. The comparative study on Mn and Cr showed that Mn was more suitable due to its smaller carbon deposition rate and wider adaptability to various H2 /CO and H2 O/CO conditions, indicating its better synergy effect with Ni. A nearly 100 h, the lifetime test and start/stop cycle test further implied that 15Ni-3Mn was stable for industrial application. Keywords: bicomponent catalyst; partial methanation; water gas shift; low H2 /CO ratio; biomass gas; urban gas

1. Introduction With the rapid development of China’s urbanization, centralized utilization of energy becomes increasingly essential. Biomass gas generated by means of gasification from organic materials, such as agricultural and forestry residues, which exist abundantly in rural places, might be one potential solution to the problem of central gas supply [1]. However, due to assorted biomass resources, as well as gasification conditions and agents [2–6], the H2 /CO ratio of acquired biomass gas varies greatly, and particularly, low H2 /CO ratio biomass gas (H2 /CO < 1) cannot be applied directly by urban residents because of its dreadful toxicity and relatively low heating value. Hence, from the standpoint of safety and efficiency, a series of requisite processing technologies has to be employed in order to eliminate the above two drawbacks before it is transported through a pipeline to downstream users. As is well known, water gas shift (WGS) (Equation (1)) is highly efficient to adjust the H2 /CO ratio by converting CO with H2 O to CO2 and H2 [7]. In addition, methanation (Equation (2)) as one type of CO hydrogenation reaction is capable of reducing CO content and in the meantime generating CH4 that is of high heating value and innocuity. Therefore, a proper connection of these two methods could be theoretically feasible to upgrade low H2 /CO ratio biomass gas into urban gas.

Catalysts 2017, 7, 51; doi:10.3390/catal7020051

www.mdpi.com/journal/catalysts

Catalysts 2017, 7, 51


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CO + H2 O → CO2 + H2 ∆H 0 = −40.6 kJ/mol 0 CO +CO 3H2 H →OCH −206.1 kJ/mol 2 O∆∆H = 40.6 4 + HH → CO kJ/mol

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(1)

(1)

(2)

(2) and In reality, from a great deal exposed the206.1 collocation CO of3H → CH literature, H O∆ kJ/mol of a separate WGS unit methanation unit is frequently mentioned or even has been put in practice. Earlier, this kind of In reality, from a great deal of exposed literature, the collocation of a separate WGS unit and combination was applied to produce synthetic natural (SNG) coalEarlier, [1]. Recently, it of is also methanation unit is frequently mentioned or even has gas been put in from practice. this kind reported that GoBiGas (Gothenburg Biomass Gasification) biomass to the SNG process contains combination was applied to produce synthetic natural gas (SNG) from coal [1]. Recently, it is also the same cooperation, adjusting the H2 /CO ratio in a WGS reactor and yielding CH4 in another reported that GoBiGas (Gothenburg Biomass Gasification) biomass to the SNG process contains the same cooperation, adjusting the H /CO ratio in a WGS reactor and yielding CH 4 in another reactor reactor [8]. Nevertheless, the entire 2production process from biomass gas into urban gas would be [8]. Nevertheless, production process iffrom biomass into merely urban sequentially gas would be connected very very complicated and the thusentire difficult to manipulate the two unitsgas were complicated and thus difficult to manipulate if the two units were merely sequentially connected and and run independently. Moreover, two individual units without integration would be considered to be run independently. Moreover, two individual units without integration would be considered to be not optimal economically. In other words, appropriate compression of WGS and methanation into one not optimal economically. In other words, appropriate compression of WGS and methanation into unit would be very advantageous on account of its convenient operability, desirable stability and low one unit would be very advantageous on account of its convenient operability, desirable stability and maintenance cost for the entire system. As a result, a reasonable concept is proposed to compress the low maintenance cost for the entire system. As a result, a reasonable concept is proposed to compress two units into one unit, and a general scheme of the process chain from low H2 /CO ratio biomass gas the two units into one unit, and a general scheme of the process chain from low H 2/CO ratio biomass to urban gas is depicted in Figure 1. Generally, purified biomass gas passes through an integrated unit gas to urban gas is depicted in Figure 1. Generally, purified biomass gas passes through an integrated containing WGS and methanation to decrease CO content together with synthesized CH 4 , which is unit containing WGS and methanation to decrease CO content together with synthesized CH 4, which moreis more compact than the conventional process. Subsequently, the gas upgrading unit removes part compact than the conventional process. Subsequently, the gas upgrading unit removes part of the CO the heating value. of the CO 2 to increase the heating value. 2 to increase

Figure 1. General scheme of the process chain from biomass gas to urban gas. WGS, water gas shift. Figure 1. General scheme of the process chain from biomass gas to urban gas. WGS, water gas shift.

For the integrated unit, the core part is the catalyst. Single methanation and WGS catalyst have

For the integrated unit, the core part is the catalyst. Single methanation and WGS catalyst been extensively investigated during the last few decades. It is reported that a variety of transition have metals has exhibited their distinct methanation activity, such as Ni, Ru, Rh, Pd and Pt [9]. Meanwhile, been extensively investigated during the last few decades. It is reported that a variety of transition metals has distinct methanation activity, suchMo as Ni, Pt [9]. another group of exhibited transition their metals performs well in WGS, including [10], Ru, Fe Rh, [11], Pd Co and [12], Meanwhile, another group of transition metals performs wellmethanation in WGS, including [10], Fe Mn [13,14] and Cr [15]. However, for the specific WGS plus unit, it is Mo necessary to [11], develop a novel and catalyst, which can simultaneously catalyze the methanation Co [12], Mn [13,14] Cr [15]. However, for the specific WGSWGS plusand methanation unit, itreaction. is necessary Scilicet, this novel catalyst is supposed to have both catalytic characteristics and inherently display a to develop a novel catalyst, which can simultaneously catalyze WGS and the methanation reaction. desirable synergy effect. In theory, if this desired bifunctional catalyst were developed, the catalyzed Scilicet, this novel catalyst is supposed to have both catalytic characteristics and inherently display a WGS could provide additional H 2 for hydrogenation of CO and, further, increase the CO conversion desirable synergy effect. In theory, if this desired bifunctional catalyst were developed, the catalyzed degree. Several creative researchers provided meaningful inspirations. Ma reported that Co3O4 WGS could provide additional H2 for hydrogenation of CO and, further, increase the CO conversion modified nanorod‐NiO/γ‐Al2O3 catalyst exhibited good activity for WGS coupling with methanation degree. Several creative researchers provided meaningful inspirations. Ma reported that Co3 O4 at conditions of CO/H2/H2O = 3:1:1, 593 K and atmospheric pressure [16]. Mei also found that modified nanorod-NiO/γ-Al O catalyst exhibited good activity for WGS coupling with methanation Co/RuO 2/Al2O3 (Ru/Co = 21) 3catalyst calcined at 350 °C owns a unique bimetallic nanoparticle at conditions of CO/H /H O = 3:1:1, 593 K and atmospheric pressure [16]. also found structure, which led 2to a 2high CO conversion in fuel cell techniques due to the Mei co‐occurrence of that ◦ Co/RuO (Ru/Co 1) catalyst we calcined at to 350combine C owns unique bimetallic 2 /Al2 O3[17]. methanation As a = consequence, propose two a different components nanoparticle on one structure, which led to a high CO conversion in fuel cell techniques due to the co-occurrence of support in order to catalyze WGS and methanation simultaneously. However, it should be noted that this work is different from the research above, which specifically focuses on upgrading low H methanation [17]. As a consequence, we propose to combine two different components on one2/CO support ratio biomass gas into urban gas. The feed gas is an authentic, but more complex simulation of this in order to catalyze WGS and methanation simultaneously. However, it should be noted that work is different from the research above, which specifically focuses on upgrading low H2 /CO ratio


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biomass gas into urban gas. The feed gas is an authentic, but more complex simulation of biomass gas comprised of CO, H2 , N2 , CO2 and H2 O. Furthermore, unlike the fuel cell system, the contained CO in the biomass gas is not strictly required to be removed as thoroughly as possible. This is where the concept of partial methanation emerges. Therefore, it is essential to develop a novel, but also potentially industrial catalyst that is in good agreement with the biomass gas system. Among the above-mentioned metals, Ni is considered as the most practical choice due to its relatively high methanation activity and comparatively low cost [18]. With respect to the second component M (M = Mo, Fe, Co, Mn and Cr), it is essential to research the discrepancy when synergized with Ni and choose the best fitted one. At last, γ-Al2 O3 material is adopted as the catalyst support due to its abundance and various advantages, such as high specific surface area, favorable mechanical property and satisfactory thermal stability. Particularly, from plentiful exposed literature [19–21], the majority of the researchers utilized powder catalysts (micrometer scale), and the employed reaction tube was comparatively small. It is well known that an inevitable drawback of massive powder catalyst is its induced problems of pressure drop. Arbeláez suggested that pelletized catalysts could solve the problem, as well as reduce the total catalyst volume and eliminate the formation of hotspots [22]. Inspired from that and considering approaching industrial application, the granular support and reaction tube with a larger size were employed in this work. For all that is stated above, in order to investigate the feasibility of integrated methanation coupling with the WGS unit under the catalysis of Ni-M catalysts, the commercial granular-shaped γ-Al2 O3 was adopted as the support and impregnated with Ni and a second metal M, resulting in the expected bicomponent catalysts. The obtained catalysts were characterized using N2 physisorption, XRD, H2 -temperature programmed reduction (TPR), H2 -temperature programmed desorption (TPD) and SEM. The effects of reaction temperature, H2 /CO ratio and H2 O/CO ratio on its activity were investigated aiming to select the best fitted catalyst, which is expected to provide a meaningful strategy for upgrading low H2 /CO ratio biomass gas into urban gas and to show potential for industrial application. 2. Results and Discussion 2.1. Characterization Analysis 2.1.1. Physico-Chemical Analysis Table 1 compiles the surface areas and pore volumes of the support and as-prepared catalysts derived from N2 adsorption analysis (Figure S2). It can be seen expressly that the surface area shrinks after the addition of Ni and/or second metal species, possibly due to the blockage of a few micropores (smaller than 2 nm) in the support framework. Even so, the catalyst samples still possess considerable surface areas (higher than 150 m2 ·g−1 ) and pore volumes, which provide adequate conditions to facilitate the catalytic reactions. Besides, with the addition of Ni and/or the second metal, the average pore size shifts to higher values, indicating increased mesoporous size (2–50 nm) due to surface reconstruction after metal addition. Table 1. Physical and chemical properties of the support and catalysts. Samples

Ni wt % a

M wt % a

SBET (m2 ·g−1 ) b

Vp (cm3 ·g−1 ) c

(nm) d

Average Pore Size (nm) e

γ-Al2 O3 15Ni 15Ni-3Mo 15Ni-3Fe 15Ni-3Co 15Ni-3Mn 15Ni-3Cr

14.6 13.7 14.4 13.8 14.7 15.5

2.7 3.2 2.5 3.1 3.1

241.46 179.15 167.07 162.18 160.13 159.01 179.96

0.34 0.31 0.27 0.29 0.28 0.28 0.27

3.82 3.72 3.79 3.68 3.68 3.68 3.79

6.62 7.02 6.35 7.26 7.06 7.03 7.10

a

dp,peak

Ni Size (nm) f Reduced 17.0 16.5 22.6 22.8 13.2 12.1

Spent 18.3 17.1 23.3 23.8 13.4 13.7

H2 Uptake (µmol·g−1 ) 385.0 294.5 235.1 342.3 416.8 395.5

D (%) g

TOFCO (s−1 ) h

36.1 31.2 21.2 27.1 41.0 42.8

0.0025 0.0024 0.0020 0.0023 0.0040 0.0031

Metal loading of as-prepared catalysts determined by inductively-coupled plasma (ICP). b Surface area of the support and as-prepared catalysts calculated using the BET (Brunauer-Emmett-Teller) method. c Pore volume of the support and as-prepared catalysts obtained from the volume of nitrogen adsorbed at relative pressure of 0.99. d Peak value of the pore size distribution (PSD) curves in Figure S2B. e Average pore size of the support and as-prepared catalysts derived from the PSD curves in Figure S2B. f Crystal size of Ni (111) derived from the Debye–Scherrer equation. g Ni dispersion calculated based on the H2 -temperature programmed reduction (TPR) and H2 -temperature programmed desorption (TPD) results. h Calculated based on the reaction condition of T = 350 ◦ C, H2 /CO = 0.8 and H2 O/CO = 1.

S2B. e Average pore size of the support and as‐prepared catalysts derived from the PSD curves in Figure S2B. f Crystal size of Ni (111) derived from the Debye–Scherrer equation. g Ni dispersion calculated based on the H2‐temperature programmed reduction (TPR) and H2‐temperature programmed desorption (TPD) results. h Calculated based on the reaction condition of T = 350 °C, /CO = 0.8 and H 2O/CO = 1. H22017, Catalysts 7, 51 4 of 18

2.1.2. XRD Analysis 2.1.2. XRD Analysis Figure 2A shows the X‐ray diffraction patterns of the as‐prepared 15Ni catalyst and promoted Figure 2A shows the X-ray diffraction patterns the 15Ni catalystadditionally and promoted 15Ni‐3M bicomponent catalysts (XRD patterns of of γ‐Al 2Oas-prepared 3 support are discussed in 15Ni-3M bicomponent catalysts (XRD patterns of γ-Al O support are discussed additionally in 2 3 Figure S3). The results of Figure 2A reveal that all of the catalysts show two major crystallite phases Figure S3). The results of Figure 2A reveal that all of the catalysts show two major crystallite phases belonging to γ‐Al 2O3 and NiO, which can be indicated with diffraction peaks at 2θ = 37.4, 45.8 and belonging to γ-Al2 O3 and which can be indicated at 2θ =(PDF#47‐1049), 37.4, 45.8 and 67.3° (PDF#04‐0880) and NiO, diffraction peaks at 2θ = with 43.2, diffraction 62.8, 75.4 peaks and 79.4° ◦ (PDF#04-0880) and diffraction peaks at 2θ = 43.2, 62.8, 75.4 and 79.4◦ (PDF#47-1049), respectively. 67.3 respectively. However, it can be carefully noticed that the XRD patterns vary greatly with the However, it can be carefully noticed that the XRD patterns vary greatly with the addition of the addition of the second metal, and they can be briefly classified into three types: (1) spinel structure second they can be briefly classified three types: (1) spinel (b and c); oxide (b and metal, c); (2) and oxide structure (d and e); and into (3) invisible structure (f). structure To be specific, in (2) terms of structure (d and e); and (3) invisible structure (f). To be specific, in terms of 15Ni-3Mo and 15Ni-3Fe, 15Ni‐3Mo and 15Ni‐3Fe, the observed peaks at 2θ = 26.5, 28.5 and 33.0° are related to NiMoO4 the observed peaks at 2θ = 26.5, 28.5 and 33.0◦ are related to NiMoO4 (PDF#12-0348), and the peaks at (PDF#12‐0348), and the peaks at 2θ = 35.6, 62.9 and 75.5° belong to NiFe 2O4 (PDF#10‐0325). In those ◦ 2θ = 35.6, 62.9 and 75.5 belong to NiFe2 O4 (PDF#10-0325). In those two XRD patterns, no separate two XRD patterns, no separate crystalline phase corresponding to FeO x and MoOx can be observed, crystalline phase corresponding to FeO and MoO can be observed, further confirming that the iron x x further confirming that the iron and molybdenum in these two catalysts mainly existed in the form and molybdenum in these two catalysts mainly existed in the form of spinel [23]. As for3O15Ni-3Co and of spinel [23]. As for 15Ni‐3Co and 15Ni‐3Mn, oxide phases are clearly labeled as Co 4 at 2θ = 31.2, ◦ and Mn O /Mn O 15Ni-3Mn, oxide phases are clearly labeled as Co O at 2θ = 31.2, 36.8, 44.8, 59.3 3 4 3 4 2 3 36.8, 44.8, 59.3° and Mn3O4/Mn2O3 at 2θ = 65.2° (PDF#43‐1003) plus 34.0°/55.1° (PDF#04‐0732/41‐1442), ◦ (PDF#43-1003) plus 34.0◦ /55.1◦ (PDF#04-0732/41-1442), respectively. Such an appearance at 2θ = 65.2 respectively. Such an appearance of CoOx and MnOx in the XRD patterns may be due to the of CoOx and MnOx in the XRD patterns may be due to the aggregation of oxide particles, which aggregation of oxide particles, which interacted weakly with the support. However, for 15Ni‐3Cr, no interacted weakly with the support. However, for 15Ni-3Cr, no signs of Cr species are displayed in its signs of Cr species are displayed in its XRD pattern. Liu [24] suggested that the disappearance of the XRD pattern. Liu [24] suggested thatmay be the disappearance of the diffractioninto peaks of the Cr species may diffraction peaks of the Cr species due to their incorporation the support skeleton or be due to their incorporation into the support skeleton or inferior crystallinity. High dispersion may inferior crystallinity. High dispersion may also account for that according to some also account for that according to some other authors [25,26]. other authors [25,26]. γ-Al2O3NiO NiMoO4  NiFe2O4 Co3O4

Mn3O4



(f) (e)



 



(d) (c) (b)

(A)























(a) 20

 γ-Al2O3  Ni



 





Mn2O3



(f) Intensity (a.u.)

Intensity (a.u.)



(B)

(111) 

(200)



(220)





(e) (d) (c) (b) (a)

30

40 50 60 2 Theta (degree)

70

80

20

30


70

80

Figure 2. XRD patterns of the as‐prepared catalysts (A) and reduced catalysts (B): (a) 15Ni, (b) 15Ni‐ Figure 2. XRD patterns of the as-prepared catalysts (A) and reduced catalysts (B): (a) 15Ni, 3Mo, (c) 15Ni‐3Fe, (d) 15Ni‐3Co, (e) 15Ni‐3Mn and (f) 15Ni‐3Cr. (b) 15Ni-3Mo, (c) 15Ni-3Fe, (d) 15Ni-3Co, (e) 15Ni-3Mn and (f) 15Ni-3Cr.

Compared to the as‐prepared catalysts, the XRD patterns of the reduced catalysts in Figure 2B Compared to the as-prepared catalysts, the XRD patterns of the reduced catalysts in Figure 2B seem more similar to each other. As can be seen, apart from the distinct diffraction peaks attributed seem more similar to each other. As can be seen, apart from the distinct diffraction peaks attributed to the γ‐Al2O3 support, the orderly diffraction peaks of elemental Ni can be explicitly observed at to the γ-Al2 O3 support, the orderly diffraction peaks of elemental Ni can be explicitly observed 2θ = 44.5, 51.8 and 76.3°, which are correspondingly ascribed to its (111), (200) and (220) crystal plane at 2θ = 44.5, 51.8 and 76.3◦ , which are correspondingly ascribed to its (111), (200) and (220) crystal (PDF#04‐0850). In spite of the similarity, some discrepancy still can be traced concerning Ni crystal plane (PDF#04-0850). In spite of the similarity, some discrepancy still can be traced concerning Ni size (Table 1) after reduction. Among them, we can find that 15Ni‐3Mn and 15Ni‐3Cr have the crystal size (Table 1) after reduction. Among them, we can find that 15Ni-3Mn and 15Ni-3Cr have the smallest Ni crystal size, indicating that the addition of the appropriate amount of Mn or Cr species smallest Ni crystal size, indicating that the addition of the appropriate amount of Mn or Cr species can greatly decrease the size of reduced Ni nanoparticles in 15Ni, whereas other species, like Fe and can greatly decrease the size of reduced Ni nanoparticles in 15Ni, whereas other species, like Fe and Co, function in an opposite way. Besides, it seems that the second metal species shows no prominent Co, function in an opposite way. Besides, it seems that the second metal species shows no prominent diffraction peaks in all reduced catalysts. Considering the comparatively low content of the second diffraction peaks in all reduced catalysts. Considering the comparatively low content of the second metal species, their interaction with coordinated sites of the γ-Al2 O3 support may cause them to be below the detection limit of XRD measurements [27]. Another reason may be that the reduced second metal species is highly dispersed on the surface of catalysts by forming small particles [28].


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metal species, their interaction with coordinated sites of the γ‐Al2O3 support may cause them to be below the detection limit of XRD measurements [27]. Another reason may be that the reduced second Catalysts 2017, 7, 51 5 of 18 metal species is highly dispersed on the surface of catalysts by forming small particles [28]. 2.1.3. H22-TPR and H22‐TPD Analysis -TPD Analysis 2.1.3. H ‐TPR and H

15Ni-3Fe

15Ni-3Mo

α

α

15Ni 200

γ

β

α

γ

β

β

400 600 Temperature (℃)

(B)

(A) H2 TPR signal (a.u.)

H2 TPR signal (a.u.)

The TPR technique not only renders us the ability to differentiate the complicated metal-support The TPR technique not only renders us the ability to differentiate the complicated metal‐support interactions, but also provides us with a unique perspective, which elucidates the role of the second interactions, but also provides us with a unique perspective, which elucidates the role of the second metal as additives in the reduction period and the influence of one or more phases on the reducibility metal as additives in the reduction period and the influence of one or more phases on the reducibility of a specific compound in a multi-component system [28,29]. Figure 3 displays the TPR results of of a specific compound in a multi‐component system [28,29]. Figure 3 displays the TPR results of the the as-prepared catalysts. The overlapped peaks are deconvolved into several elementary peaks as‐prepared catalysts. The overlapped peaks are deconvolved into several elementary peaks using 2 > using Gaussian-type functions on basis the basis R20.990, > 0.990, which are plottedwith witha a dashed dashed line line for Gaussian‐type functions on the of Rof which are plotted for differentiation. As can be seen expressly in both Figure 3A,B, the addition of the second component differentiation. As can be seen expressly in both Figure 3A,B, the addition of the second component can greatly change the reduction characteristics of monometallic Ni-based catalyst. Judging from can greatly change the reduction characteristics of monometallic Ni‐based catalyst. Judging from the the peak temperatures in the TPR profiles and referring to relevant reported literature, the reducible peak temperatures in the TPR profiles and referring to relevant reported literature, the reducible NiO NiO species could be sorted into three categories: α-type,free freenickel nickeloxide oxidespecies species [30] [30] or species could be sorted into three categories: (1) (1) α‐type, or bulk bulk NiO [28], which interact weakly with the support; (2) β-type, Ni-rich phases, which have medium NiO [28], which interact weakly with the support; (2) β‐type, Ni‐rich phases, which have medium interaction with the support; (3) γ-type, Al-rich phases, which interact strongly with the support. interaction with the support; (3) γ‐type, Al‐rich phases, which interact strongly with the support. In In Figure 3A, for 15Ni-3Mo and 15Ni-3Fe, there is no distinct reduction peak ascribed to the NiMoO44 Figure 3A, for 15Ni‐3Mo and 15Ni‐3Fe, there is no distinct reduction peak ascribed to the NiMoO and Given their relatively high reduction temperature (NiMoO than 44 spinel 4 : higher and NiFe NiFe2 O 2O spinel structure. structure. Given their relatively high reduction temperature (NiMoO 4: higher ◦ C [31]; and NiFe O : 402–715 ◦ C [32]), their reduction process may be overlapped by that of NiO. 500 2 4 than 500 °C [31]; and NiFe2O4: 402–715 °C [32]), their reduction process may be overlapped by that of However, in Figure there exists anexists individual peak labelled I. According to the XRDto analysis in NiO. However, in 3B, Figure 3B, there an individual peak aslabelled as І. According the XRD Figure 2A, it can be deduced that the individual peak I for 15Ni-3Co is ascribed to Co3 O4 reduction analysis in Figure 2A, it can be deduced that the individual peak І for 15Ni‐3Co is ascribed to Co3O4 (Co3 O4 → CoO → Co0 ) [33],0 and for 15Ni-3Mn, it corresponds to Mn2 O3 reduction reduction (Co3O4 → CoO → Co ) [33], and for 15Ni‐3Mn, it corresponds to Mn2O3 reduction (Mn2O3 (Mn2 O3 → Mn3 O4 ) [30]. For 15Ni-3Cr, the individual peak I is attributed to the reduction of Cr → Mn3O4) [30]. For 15Ni‐3Cr, the individual peak І is attributed to the reduction of Cr species [24]. High dispersion and the small particle size (Table 1) may account for its low reduction species [24]. High dispersion and the small particle size (Table 1) may account for its low reduction temperature and invisible structure in the XRD patterns in Figure 2A. temperature and invisible structure in the XRD patterns in Figure 2A.

γ 800

α β

Ⅰ

α

Ⅰ Ⅰ

200

α

15Ni-3Cr γ

β

15Ni-3Mn γ

β

15Ni-3Co γ

400 600 Temperature (℃)

800

Figure 3. TPR profiles of the as-prepared catalysts (A) and (B). Figure 3. TPR profiles of the as‐prepared catalysts (A) and (B).

Considering the the much much lower lower loading loading of of second second component component species, species, we we mainly mainly focus focus on on the the Considering reduction on NiO. For comparison, the corresponding peak temperatures and proportions of every reduction on NiO. For comparison, the corresponding peak temperatures and proportions of every type type are Table The quantitative results that addition with the of component, the second are shownshown in Tablein S1. The S1. quantitative results show thatshow with the of addition the second all reducible NiO precursors except 15Ni‐3Mo component, the fraction of γ‐type in the fraction of γ-type in all reducible NiO precursors except 15Ni-3Mo decreased, suggestingdecreased, a general suggesting a general weaker interaction between NiO and alumina support and an increased amount weaker interaction between NiO and alumina support and an increased amount of easily reducible of easily reducible NiO. Moreover, according to the exposed literature [28,29], the reduction of α‐type NiO. Moreover, according to the exposed literature [28,29], the reduction of α-type and β-type would ◦ C). On and β‐type favor the low temperature reaction (lower than 450 the °C). On the favor the lowwould temperature methanation reactionmethanation (lower than 450 the contrary, γ-type is ◦ contrary, the γ‐type is beneficial to high temperature methanation (600–700 °C) due to the formation beneficial to high temperature methanation (600–700 C) due to the formation of small Ni particles. of small Ni particles. Figure 4 presents the H2 -TPD curves of the reduced catalysts, and the calculated hydrogen uptakes Figure 4 presents H2‐TPD curves the desorption reduced catalysts, and the calculated hydrogen are listed in Table 1. Forthe 15Ni, there are two of main peaks, which are located at ca. 288 ◦ C ◦ uptakes are listed in Table 1. For 15Ni, there are two main desorption peaks, which are located at and 553 C, respectively. However, for 15Ni-3M catalysts, the temperature of the second peak differs from that of 15Ni, implying that the Ni dispersion in the bicomponent catalysts is distinct from that in ca. 288 °C and 553 °C, respectively. However, for 15Ni‐3M catalysts, the temperature of the second the single-component 15Ni catalyst. In accordance with others [24,30], the former peak is attributed to the chemisorbed hydrogen on the highly dispersed Ni nanoparticles with a large density of surface


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peak differs from that of 15Ni, implying that the Ni dispersion in the bicomponent catalysts is distinct from that in the single‐component 15Ni catalyst. In accordance with others [24,30], the former peak Catalysts 2017, 7, 51 6 of 18 is attributed to the chemisorbed hydrogen on the highly dispersed Ni nanoparticles with a large density of surface defects, which commonly serve as capture traps for hydrogen diffusion, leading to defects, which commonly serve as capture traps for hydrogen diffusion, leading to a capability of a capability of reducing the activation energy of hydrogen dissociation. The latter peak is supposed reducing the activation energy of hydrogen dissociation. The latter peak is supposed to be derived to be derived from the H 2 adsorbed in bulk or on poorly‐dispersed Ni particles. Velu [34] proposed from the H2 adsorbed in bulk or on poorly-dispersed Ni particles. Velu [34] proposed that the former that the former peak is ascribed to the H 2 desorbed from the surface Ni particles, which indicates the peak is ascribed to the H desorbed from the surface Ni particles, which indicates the exposed fraction 2 exposed fraction of Ni atoms, and the latter one belongs to the H 2 located in the subsurface layers of Ni atoms, and the latter one belongs to the H located in the subsurface layers and/or to spillover 2 and/or to spillover H2. Despite diverse theoretical explanations, they all disclosed the nature of two H2 . Despite diverse theoretical explanations, they all disclosed the nature of two different forms of Ni different forms of Ni particles. Furthermore, it is seen from Table 1 that among all of the catalysts, particles. it ishave seencomparatively from Table 1 that among all of the catalysts, and−115Ni-3Cr 15Ni‐3Mn Furthermore, and 15Ni‐3Cr high H2 uptake of 416.8 and 15Ni-3Mn 405.5 μmol∙g and Ni −1 and Ni dispersion of 41.0% and have comparatively high H uptake of 416.8 and 405.5 µmol · g 2 dispersion of 41.0% and 42.8%, respectively, indicating that the addition of the proper second 42.8%, respectively, indicating that the addition of the proper second component can highly increase component can highly increase the dispersion of Ni nanoparticles, which are closely related with the the dispersion of Ni nanoparticles, which are closely related with the H2 chemisorption ability in the H2 chemisorption ability in the process of catalytic reaction. process of catalytic reaction. 2.2. Catalytic Performances of Catalysts 2.2. Catalytic Performances of Catalysts 2.2.1. Performance Tests 2.2.1. Performance Tests The results of partial methanation coupling with the WGS of biomass gas catalyzed by 15Ni‐3M The results of partial methanation coupling with the WGS of biomass gas catalyzed by 15Ni-3M bicomponent catalysts are shown in Figure 5. It is clear that the combination of the second component bicomponent catalysts are shown in Figure 5. It is clear that the combination of the second component M with Ni Ni obviously obviously influences influences the the catalytic catalytic performances. performances. As the M with As shown shown in in Figure Figure 5A, 5A, for for all all of of the catalysts, the CO conversion increases gradually with increasing reaction temperature. However, as catalysts, the CO conversion increases gradually with increasing reaction temperature. However, the reaction temperature is over 350 350 °C, ◦ C, the the CO CO conversion starts to decrease or or level off off due to as the reaction temperature is over conversion starts to decrease level due thermodynamic limitations, except for with to thermodynamic limitations, except for15Ni‐3Co, 15Ni-3Co,which whichmaintains maintainsits its upward upward momentum momentum with temperature. This special phenomenon could be due to the prevailing performance of WGS temperature. This special phenomenon could be due to the prevailing performance of WGS over over methanation, which is verified by the ascending CO methanation, which is verified by the ascending CO22 growth rate in Figure 5C and descending CH growth rate in Figure 5C and descending CH44 selectivity in Figure 5B from 375–400 °C. Generally speaking, in contrast with monometallic Ni‐based selectivity in Figure 5B from 375–400 ◦ C. Generally speaking, in contrast with monometallic Ni-based catalyst, 15Ni‐3Mn and 15Ni‐3Cr possess higher CO conversion, while the other three bicomponent catalyst, 15Ni-3Mn and 15Ni-3Cr possess higher CO conversion, while the other three bicomponent catalysts, including 15Ni-3Mo, 15Ni‐3Mo, 15Ni‐3Fe 15Ni‐3Co, present CO conversion activities, catalysts, including 15Ni-3Fe andand 15Ni-3Co, present lowerlower CO conversion activities, which which correspond well to the TOF CO value in Table 1. correspond well to the TOFCO value in Table 1.

288℃ H2 TPD signal (a.u.)

506℃

50

522℃

15Ni-3Cr 15Ni-3Mn

512℃

15Ni-3Co 15Ni-3Fe

560℃ 15Ni-3Mo 553℃ 15Ni 200

350 500 650 Temperature (℃)

800

Figure 4. H Figure 4. H22‐TPD curves of the reduced catalysts. -TPD curves of the reduced catalysts.

Figure 5B,C exhibits the CH4 selectivity and CO2 growth rate as a function of temperature, Figure 5B,C exhibits the CH4 selectivity and CO2 growth rate as a function of temperature, respectively. From Figure 5B, it can be seen that the CH4 selectivity of 15Ni‐3Mn within the measured respectively. From Figure 5B, it can be seen that the CH4 selectivity of 15Ni-3Mn within the measured temperature range is the highest (68.03% on the top) compared to other catalysts. The trend for all of temperature range is the highest (68.03% on the top) compared to other catalysts. The trend for all the catalysts remains accordant. The CH4 selectivity increases gradually with temperature and passes of the catalysts remains accordant. The CH4 selectivity increases gradually with temperature and through a maximum at 350 °C, and beyond this temperature spot, a decrease in methanation catalytic passes through a maximum at 350 ◦ C, and beyond this temperature spot, a decrease in methanation activity is observed. This trending pattern meaningfully suggests us to take 350 °C as the optimum catalytic activity is observed. This trending pattern meaningfully suggests us to take 350 ◦ C as the optimum operation temperature. Different from Figure 5B,C, a less universal trending pattern of CO2


7 of 18

growth rate with temperature is displayed. This behavior may be caused by the addition of the second component, which greatly influences the WGS catalytic activity and its synergy effect [17,35,36] with Ni methanation catalytic activity. However, it can still be apparently perceived that 15Ni-3Mn and 15Ni-3Cr have overall higher CO2 growth rates than the other catalysts. Through comprehensive comparison, 15Ni-3Mo, 15Ni-3Fe and 15Ni-3Co generally present lower CO conversion, CH4 selectivity and CO2 growth rate than the monometallic 15Ni catalyst, possibly owing to their larger Ni particle size and lower Ni dispersion (Table 1) with the addition of the second component Mo, Fe or Co species. Their low activities, including the capability to catalyze methanation and WGS reaction, are undesirable for further industrial application. On the contrary, 15Ni-3Mn and 15Ni-3Cr seemingly are agreeable for our purposes. Their superior performance is not only on account of the relatively small Ni particles and high Ni dispersion, but also due to large proportions of α-type and β-type NiO (Table S1) in their matching as-prepared catalysts. Further comparison of these two catalysts would be continued in the following sections to select the best candidate for our use. 2.2.2. XRD and TG Analysis after Performance Tests Figure 6A shows the XRD patterns of spent catalysts after performance tests. As is shown, the diffraction peaks of γ-Al2 O3 support and active metal Ni still can be noted clearly compared with the XRD patterns of the reduced catalysts in Figure 2B. The crystal size of Ni (111) in the spent catalysts is calculated and listed in Table 1. By referring to the reduced catalysts, the Ni crystal size of all catalysts increases to various extents after performance tests, and particularly, there is almost no change in the size of Ni particles in the 15Ni-3Mn catalyst, indicating the slightest sintering degree of Ni particles and the best thermal stability. In addition to the Ni crystal size changes, some phases that did not exist in the reduced catalysts have newly emerged in the spent catalysts. Specifically, for 15Ni-3Fe, 15Ni-3Co and 15Ni-3Cr, the phases with the spinel structure (NiFe2 O4 , NiCr2 O4 and NiCo2 O4 ) are newly formed after the performance test. This type of spinel structure not only consumes part of the exposed Ni atoms, which would debase the H2 chemisorption ability in methanation, but also could enhance high temperature shift (HTS) [37]. As is proven by Figure 5B,C, the CH4 selectivity decreases while the CO2 growth rate remains at a relatively high level within the range of 350–400 ◦ C Catalysts 2017, 7, 51 8 of 19 for 15Ni-3Fe, 15Ni-3Co and 15Ni-3Cr.

CH4 selectivity SCH4 (%)

70

(A)

80 70

15Ni 15Ni-3Mo 15Ni-3Fe 15Ni-3Co 15Ni-3Mn 15Ni-3Cr

60 50

300

(B)

325 350 375 Temperature (℃) 90 CO2 growth rate RCO2 (%)

CO conversion XCO (%)

90

60 15Ni 15Ni-3Mo 15Ni-3Fe 15Ni-3Co 15Ni-3Mn 15Ni-3Cr

50

40

300


400

400

(C)

80 70 15Ni 15Ni-3Mo 15Ni-3Fe 15Ni-3Co 15Ni-3Mn 15Ni-3Cr

60 50 40

300


400

Figure 5. Catalytic performances of the reduced catalysts for partial methanation coupling with WGS Figure 5. Catalytic performances of the reduced catalysts for partial methanation coupling with WGS at H2/CO = 0.8 and H2O/CO = 1: (A) CO conversion; (B) CH4 selectivity; and (C) CO2 growth rate. at H2 /CO = 0.8 and H2 O/CO = 1: (A) CO conversion; (B) CH4 selectivity; and (C) CO2 growth rate.  γ-Al2O3  Ni

(f)





(A)

NiFe2O4 NiCr2O4 HNiCo O 2 4  (200) (220) 



100 Ⅰ



(e) (d)

95 H

H

H

(%)

(a.u.)

(111) 

Ⅱ

Ⅲ

Ⅳ

(B)

300

C

C

15Ni-3Mn 15Ni-3Cr

40


40

400

15Ni-3Mn 15Ni-3Cr

300


400

Figure 5. Catalytic performances of the reduced catalysts for partial methanation coupling with WGS at H2/CO = 0.8 and H2O/CO = 1: (A) CO conversion; (B) CH4 selectivity; and (C) CO2 growth rate. 8 of 18


 γ-Al2O3  Ni





(f)



(A)

NiFe2O4 NiCr2O4 HNiCo O 2 4  (200) (220) 



100



(c) (b) (a)

20

Ⅲ

Ⅳ

(B)



(e) (d)

Ⅱ

Ⅰ

95 H 

30

H  



H

H






Mass (%)

Intensity (a.u.)

(111)

70

80

15Ni 15Ni-3Cr 15Ni-3Mo 15Ni-3Fe 15Ni-3Mn 15Ni-3Co

90

85

0

200

CO(a) Cα(a)

Cα(a)+O(a) 4H(a) CH4(g) Cβ(s)


Cc(s)

1000

Figure 6. XRD patterns (A) and TG curves (B) of spent catalysts after performance tests: (a) 15Ni, Figure 6. XRD patterns (A) and TG curves (B) of spent catalysts after performance tests: (a) 15Ni, (b) (b) 15Ni-3Mo, (c) 15Ni-3Fe, (d) 15Ni-3Co, (e) 15Ni-3Mn and (f) 15Ni-3Cr. 15Ni‐3Mo, (c) 15Ni‐3Fe, (d) 15Ni‐3Co, (e) 15Ni‐3Mn and (f) 15Ni‐3Cr.

The quantity of deposited carbon spent catalysts catalysts was gauged by TG analysis, analysis, and the The quantity of deposited carbon on on the the spent was gauged by TG and the obtained results are are depicted Figure Generally, the TG curves be classified four obtained results depicted in in Figure 6B. 6B. Generally, the TG curves can be can classified into fourinto sections: sections: (1) Section I: the weight loss between room temperature and 280 °C mainly derived from (1) Section I: the weight loss between room temperature and 280 ◦ C mainly derived from absorbed absorbed surface water and volatiles compound; (2) Section II: weight gain between 280 and 450 °C surface water and volatiles compound; (2) Section II: weight gain between 280 and 450 ◦ C due to the due to the oxidation of Ni particles [30]; (3) Section III: weight loss originated from the oxidation of oxidation of Ni particles [30]; (3) Section III: weight loss originated from the oxidation of deposited deposited α‐type carbon (C α-type carbon (Cα ) betweenα) between 450 and 670 °C; and (4) Section IV: weight loss ascribed to the 450 and 670 ◦ C; and (4) Section IV: weight loss ascribed to the oxidation ◦ C [24]. oxidation of β‐type (Cβ) plus graphitic carbon above 670 °C [24]. to the of β-type carbon (Cβ )carbon plus graphitic carbon (Cc) above 670(Cc) According to theAccording literature [24,38], literature [24,38], Cα formed is the active species formed by dissociation, which for is the the formation essential Cα is the active species by CO dissociation, which is CO the essential intermediate of methane, and Cβ is one kind of polymeric carbon species that can be produced by Cα reacting. intermediate for the formation of methane, and C β is one kind of polymeric carbon species that can Cc is normally generated at high temperatures that can be transformed from Cβ , whereas it is be produced by C α reacting. Cc is normally generated at high temperatures that can be transformed responsible for the deactivation of catalysts. Relevant simplified formulas are inserted in Figure 6B. from C β, whereas it is responsible for the deactivation of catalysts. Relevant simplified formulas are The carbon deposition rate is also compiled in Table 2. As is shown, except for 15Ni-3Mn and 15Ni-3Cr, inserted in Figure 6B. The carbon deposition rate is also compiled in Table 2. As is shown, except for the other four catalysts have less carbon deposition and smaller carbon deposition rates. This may be due to their low activity as analyzed above. In terms of 15Ni-3Mn and 15Ni-3Cr catalysts with high activity, the former has less carbon deposition, as well as a lower carbon deposition rate than the latter, indicating better resistance of carbon formation for 15Ni-3Mn. Nevertheless, there is no diffraction peak in Figure 6A attributed to deposited carbon for all of the catalysts, probably owing to the limited amount of deposited carbon, which is below the detection limit or in the amorphous state. Table 2. Carbon deposition data of spent catalysts after performance tests. Samples

Carbon Deposition Rate a (mmol·min−1 )

Carbon Deposition b (wt %)

15Ni 15Ni-3Mo 15Ni-3Fe 15Ni-3Co 15Ni-3Mn 15Ni-3Cr

4.882 6.425 3.870 4.212 5.305 7.450

1.82 1.95 1.54 1.69 2.40 3.06

a

Average value of carbon deposition rates at five temperature spots in performance tests based on the carbon balance between inlet and outlet species. b Obtained from Section III and Section IV in the TG curves.

2.3. Comparative Study on 15Ni-3Mn and 15Ni-3Cr From the above analysis, we preliminarily concluded that both 15Ni-3Mn and 15Ni-3Cr have better activity than the other four catalysts for partial methanation coupling with WGS of biomass gas. In order to filter out the best candidate for industrial application, we carried out a series of comparative investigations on 15Ni-3Mn and 15Ni-3Cr.


9 of 18

2.3.1. Effect of H2 /CO The effect of H2 /CO on partial methanation coupling with WGS of biomass gas is summarized in Figure 7. As can be seen, for both 15Ni-3Mn and 15Ni-3Cr, the CH4 selectivity in Figure 7B is promoted while the CO2 growth rate in Figure 7C is impeded with increasing H2 /CO. This is mainly due to increased H2 concentration that the larger H2 /CO incurs, which is favorable for methanation, whereas detrimental for WGS. However, the changing trend of CO conversion for 15Ni-3Mn and 15Ni-3Cr exhibits an adverse pattern. As Figure 7A shows, it is obvious that CO conversion for 15Ni-3Mn first decreases and then increases with further increasing H2 /CO, of which 91.7% CO conversion is obtained at H2 /CO = 0.8. While for 15Ni-3Cr, CO conversion increases firstly and then decreases with H2 /CO, along with the highest 94.3% CO conversion at H2 /CO = 0.8. Considering that these two catalysts have same upward trend of CH4 selectivity and downward trend of the CO2 growth rate with H2 /CO, we can safely conclude that for 15Ni-3Mn, methanation prevails over WGS, while for 15Ni-3Cr WGS surpasses methanation in the H2 /CO range of 0.8–1.0. This adverse pattern may be caused by the difference of the synergy effect, as well as Ni dispersion and its interaction with the 10 of 19 supportCatalysts 2017, 7, 51 between Ni-Mn and Ni-Cr.

90

80 15Ni-3Mn 15Ni-3Cr 70

CH4 selectivity SCH4 (%)

(A)

0.6

(B)

15Ni-3Mn 15Ni-3Cr

75

0.7

65 60 55 0.7

0.8 H2/CO

0.9

1.0

110

15Ni-3Mn 15Ni-3Cr

100

70

0.6

0.8 H2/CO

CO2 growth rate RCO2 (%)

CO conversion XCO (%)

100

0.9

1.0

(C)

90 80 70 60 50

0.6

0.7

0.8 H2/CO

0.9

1.0

◦CH Figure 7. Effect H2/CO partial methanation coupling WGS at 350 °C and 2O/CO = 1: = 1: Figure 7. Effect of H2of /CO on on partial methanation couplingwith with WGS at 350 and H2 O/CO (A) CO conversion; (B) CH4 selectivity; and (C) CO2 growth rate. (A) CO conversion; (B) CH4 selectivity; and (C) CO2 growth rate.

2.3.2. Effect of H2O/CO

Besides, according to Table 1, the Ni dispersion of 15Ni-3Mn is comparable with that of 15Ni-3Cr, H2O plays an important role in the partial methanation coupling with WGS of biomass gas since but the it is not only the reactant of WGS reaction, but also serves as the product of methanation. Its dual H2 chemisorption ability of 15Ni-3Mn is larger than 15Ni-3Cr, which may account for the constantly higher CH4 selectivity of 15Ni-3Mn than2O/CO on catalytic performance, and the results that of 15Ni-3Cr in the whole range of H2 /CO, identity makes it worthwhile to probe the effect of H further are shown in Figure 8. As displayed in Figure 8B and Figure 8C, both 15Ni‐3Mn and 15Ni‐3Cr show indicating that the former catalyst has better adaptability to various H2 /CO conditions and is a downward trend of CH 4 selectivity, while an upward tendency of the CO2 growth rate as a function more eligible for industrial application. of H2O/CO. Hence, we can conclude that H2O could inhibit partial methanation, but otherwise

promote the WGS reaction. As for its integrated influence on catalytic performance, it can be explicitly 2.3.2. Effect of H2 O/CO

seen in Figure 8A that the CO conversion increases with H2O/CO, indicating that H2O favors WGS

H2reaction over methanation. In addition, despite the comparable CO O plays an important role in the partial methanation coupling with WGS of biomass gas since it 2 growth rate and CO conversion at high water content (1.0–1.5 H 2 O/CO) for both catalysts, 15Ni‐3Mn presents better CH 4 selectivity is not only the reactant of WGS reaction, but also serves as the product of methanation. Its dual identity than 15Ni‐3Cr, implying that the Mn species is more competent than Cr as the second component in makes it worthwhile to probe the effect of H2 O/CO on catalytic performance, and the results are the Ni‐M bicomponent catalyst since the former is more efficient in converting CO to CH4. shown in Figure 8. As displayed in Figure 8B,C, both 15Ni-3Mn and 15Ni-3Cr show a downward trend of CH4 selectivity, while an upward tendency of the CO2 growth rate as a function of H2 O/CO. Hence, 2.3.3. Heating Value Analysis we can conclude that H2 O could inhibit partial methanation, but otherwise promote the WGS reaction. The urban gas has to be of a certain heating value before it is transported in a pipeline to downstream residents. As mentioned previously, biomass gas after partial methanation coupling with WGS needs subsequent removal of CO2 in order to satisfy the heating value standard of urban gas. Apart from traditional upgrading technologies (e.g., water scrubbing, chemical and physical scrubbing and membrane adsorption), there are plentiful reported literature works concerning CO2 removal, such as the low‐temperature CO2 removal process [39], pressure/vacuum swing adsorption


10 of 18

As for itsCatalysts 2017, 7, 51 integrated influence on catalytic performance, it can be explicitly seen in Figure11 of 19 8A that the Catalysts 2017, 7, 51 11 of 19 CO conversion increases with H2 O/CO, indicating that H2 O favors WGS reaction over methanation. −3, which completely reaches the Type II standard heating value for both catalysts is beyond 10 MJ∙Nm In addition, despite the comparable CO2 growth −3rate and CO conversion at high water content heating value for both catalysts is beyond 10 MJ∙Nm , which completely reaches the Type II standard of the Chinese National Standard GB/T 13612‐2006 corresponding to manufactured gas (dotted line (1.0–1.5 of the Chinese National Standard GB/T 13612‐2006 corresponding to manufactured gas (dotted line H2 O/CO) for both catalysts, 15Ni-3Mn presents better CH4 selectivity than 15Ni-3Cr, implying in Figure 9). Moreover, biomass gas catalyzed by 15Ni‐3Mn steadily presents a higher heating value that thein Figure 9). Moreover, biomass gas catalyzed by 15Ni‐3Mn steadily presents a higher heating value Mn species is more competent than Cr as the second component in the Ni-M bicomponent than 15Ni‐3Cr, suggesting that 15Ni‐3Mn is more suited for industrial application in converting low than 15Ni‐3Cr, suggesting that 15Ni‐3Mn is more suited for industrial application in converting low catalyst H since the former is more efficient in converting CO to CH4 . 2/CO ratio biomass gas to urban gas.

CO conversion XCO (%) CO conversion XCO (%)

H2/CO ratio biomass gas to urban gas.

65 65 60 60 55 55 0.50 0.50

(A) (A)

15Ni-3Mn 15Ni-3Mn 15Ni-3Cr 15Ni-3Cr

93 93 90 90 87 87 0.50 0.50

0.75 1.00 1.25 1.50 0.75 H21.00 1.50 O/CO 1.25 H2O/CO 100 (B) 100 15Ni-3Mn (B) 15Ni-3Mn 15Ni-3Cr 95 15Ni-3Mn 15Ni-3Cr 95 15Ni-3Cr 15Ni-3Mn 90 15Ni-3Cr 90 85 85 80 80 75 75 70 0.75 1.00 1.25 1.50 0.75 1.00 1.25 70 0.50 0.75 H21.00 1.50 0.50 0.75 H21.00 O/CO 1.25 O/CO 1.25 H2O/CO H2O/CO

CO growth rate R RCO2 (%) CO (%) 2 2 growth rateCO2

CH selectivity S SCH4 (%) CH (%) 4 4 selectivityCH4

70 70

96 96

(C) (C)

1.50 1.50

-3

-3 Lower Lowerheating heatingvalue value(MJ (MJNm Nm) )

2O/CO on partial methanation coupling with WGS at 350 °C and H 2/CO = 0.8: Figure 8.Figure 8. Effect of H Effect of H2 O/CO on partial methanation coupling with WGS at 350 ◦ C and H2 /CO = 0.8: Figure 8. Effect of H 2O/CO on partial methanation coupling with WGS at 350 °C and H2/CO = 0.8: 4 selectivity; and (C) CO2 growth rate. (A) CO conversion; (B) CH (A) CO conversion; (B) CH selectivity; and (C) CO growth rate. 4 2 (A) CO conversion; (B) CH4 selectivity; and (C) CO2 growth rate.

14 14

15Ni-3Mn 15Ni-3Mn 15Ni-3Cr 15Ni-3Cr

12 12 10 10 8 8 0 0

1 1

2 3 2Condition3 Condition

4 4

Figure 9. Lower heating value of biomass gas under different conditions. Condition 1 refers to the

Figure 9. Lower heating value of biomass gas under different conditions. Condition 1 refers to the Figure 9. Lower value of biomass gas under different Condition 1 refers outlet state heating after partial methanation coupling with WGS at 350 conditions. °C, H2/CO = 0.8 and H2O/CO = 1. to the outlet state after partial methanation coupling with WGS at 350 °C, 2/CO = 0.8 and H2O/CO = 1. is removed, respectively. = 1. Conditions 2–4 refer to the assumed state, for which 30%, 40% and 50% CO outlet state after partial methanation coupling with WGS at 350 ◦ C,HH = 0.8 and H2 O/CO 22/CO Conditions 2–4 refer to the assumed state, for which 30%, 40% and 50% CO2 is removed, respectively. Conditions 2–4 refer to the assumed state, for which 30%, 40% and 50% CO2 is removed, respectively.

2.3.4. SEM Analysis after Comparative Study 2.3.4. SEM Analysis after Comparative Study

2.3.3. Heating Value Analysis The urban gas has to be of a certain heating value before it is transported in a pipeline to downstream residents. As mentioned previously, biomass gas after partial methanation coupling with WGS needs subsequent removal of CO2 in order to satisfy the heating value standard of urban gas. Apart from traditional upgrading technologies (e.g., water scrubbing, chemical and physical scrubbing


11 of 18

and membrane adsorption), there are plentiful reported literature works concerning CO2 removal, such as the low-temperature CO2 removal process [39], pressure/vacuum swing adsorption using improved 13X zeolite [40] and using activated carbon as the adsorption bed layer in the pressure swing adsorption (PSA) unit [41]. Thus, it is practical to construct a CO2 removal unit as the gas upgrading segment with WGS plus the methanation unit as portrayed in Figure 1. Figure 9 displays the lower heating value of biomass gas under different conditions. It can be seen that the lower heating value increased after removing part of the CO2 due to the enhanced concentration of combustible components, such as H2 , CO and CH4 . Besides, after removing 30% CO2 , the lower heating value for both catalysts is beyond 10 MJ·Nm−3 , which completely reaches the Type II standard of the Chinese National Standard GB/T 13612-2006 corresponding to manufactured gas (dotted line in Figure 9). Moreover, biomass gas catalyzed by 15Ni-3Mn steadily presents a higher heating value than 15Ni-3Cr, suggesting that 15Ni-3Mn is more suited for industrial application in converting low H2 /CO ratio biomass gas to urban gas. 2.3.4. SEM Analysis after Comparative Study Catalysts 2017, 7, 51

12 of 19

The structures of the two compared catalysts were further analyzed by SEM and EDS analysis. The structures of the two compared catalysts were further analyzed by SEM and EDS analysis. As Figure 10A,C presents, both reduced catalysts display a porous structure belonging to the Al2 O3 As Figure 10A,C presents, both reduced catalysts display a porous structure belonging to the Al 2O3 support with a large specific surface area, which provides prerequisites for highly dispersed Ni support with a large component. specific surface area, which provides prerequisites for Ni highly dispersed Ni particles and the second Besides, it can be clearly seen that nanoparticles (possibly mixed particles and the second component. Besides, it can be clearly seen that Ni nanoparticles (possibly with the second component) are distributed on the support surface without showing large mixed with the second component) are distributed on the support surface without showing large aggregations, indicating the high dispersion that is consistent with the H2 -TPD results in Table 1. aggregations, indicating the high dispersion that is consistent with the H2‐TPD results in Table 1. Figure 11 displays the total elemental mapping images and corresponding EDS analysis of the two Figure 11 displays the total elemental mapping images and corresponding EDS analysis of the two catalysts. As can be seen in Figure 11E,F, the Ni content of both catalysts is higher than the prepared catalysts. As can be seen in Figure 11E,F, the Ni content of both catalysts is higher than the prepared contentcontent due to the limitation of EDS as a surface analysis technology instead of bulk analysis. The due to the limitation of EDS as a surface analysis technology instead of bulk analysis. The bulk contentbulk is listed inis Table based ICP detection. Moreover, according to the elemental mapping content listed 1in Table on 1 based on ICP detection. Moreover, according to the elemental imagesmapping in Figure S4, for 15Ni-3Mn and 15Ni-3Cr, Al, Ni and Mn/Cr species are all in a homogeneous images in Figure S4, for 15Ni‐3Mn and 15Ni‐3Cr, Al, Ni and Mn/Cr species are all in a distribution, which distribution, is consistent withis both a smaller particles size derivedsize from XRD from analysis and homogeneous which consistent with Ni both a smaller Ni particles derived XRD and the larger H 2 adsorption amount in Table 1, further conforming their high the larger Hanalysis adsorption amount in Table 1, further conforming their high dispersion on the support. 2 dispersion on the support. The relative amount can be judged from their density in mapping images The relative amount can be judged from their density in mapping images with a sequence of Al > Ni > with a sequence of Al > Ni > Mn ≈ Cr, which is in accordance with the prepared content. Mn ≈ Cr, which is in accordance with the prepared content.

(A)

Ni particles

(B)

Deposited carbon Porous structure

(C)

(D) Porous structure

Ni particles Deposited carbon

FigureFigure 10. SEM (scanning electron microscope) images of reduced catalysts: (A) 15Ni‐3Mn; (C) 15Ni‐ 10. SEM (scanning electron microscope) images of reduced catalysts: (A) 15Ni-3Mn; 3Cr; and spent catalysts: (B) 15Ni‐3Mn; (D) 15Ni‐3Cr. (C) 15Ni-3Cr; and spent catalysts: (B) 15Ni-3Mn; (D) 15Ni-3Cr. After the comparative study, carbon filaments and whiskers can be observed on the surface of both catalysts (Figure 10B,D). However, the amount of deposited carbon for 15Ni‐3Cr is apparently larger than that for 15Ni‐3Mn, which could be proven by TG analysis in Table 2. As is well known, deposited carbon is responsible for blocking support pores and covering active components, leading to a decreased catalytic activity. Thus, due to more deposited carbon, 15Ni‐3Cr is less active than 15Ni‐3Mn during the comparative study.

Catalysts 2017, 7, 51 Catalysts 2017, 7, 51

12 of 18 13 of 19

(A)

(B)

(C)

(D)

Al+Ni+Mn

Al+Ni+Cr

Au O Ni 0.0

Mn 2.5

Ni

5.0 7.5 Energy (keV)

Ni

Au 10.0

(F)

Al

(E) cps (eV)

cps (eV)

Al

Au O Ni 0.0

Cr 2.5

Ni

5.0 7.5 Energy (keV)

Ni

Au 10.0

Figure 11. SEM images (A,B), corresponding total elemental mapping images (C,D) and EDS Figure 11. SEM images (A,B), corresponding total elemental mapping images (C,D) and EDS (energy (energy dispersive spectroscopy) analysis (E,F) of reduced catalysts. ((A,C,E) for 15Ni-3Mn and dispersive spectroscopy) analysis (E,F) of reduced catalysts. ((A,C,E) for 15Ni‐3Mn and (B,D,F) for 15Ni‐ (B,D,F) for 15Ni-3Cr). 3Cr).

After the comparative study, carbon filaments and whiskers can be observed on the surface of 2.4. Stability of 15Ni‐3Mn Catalyst both catalysts (Figure 10B,D). However, the amount of deposited carbon for 15Ni-3Cr is apparently Through the comparative study, we found out that 15Ni‐3Mn performs better than 15Ni‐3Cr larger than that for 15Ni-3Mn, which could be proven by TG analysis in Table 2. As is well known, over a wide range of H 2/CO and H2O/CO, impelling us to select 15Ni‐3Mn as the best candidate for deposited carbon is responsible for blocking support pores and covering active components, leading partial methanation coupling with WGS of low H 2/CO ratio biomass gas into urban gas. However, to a decreased catalytic activity. Thus, due to more deposited carbon, 15Ni-3Cr is less active than from the industrial and engineering perspective, the catalyst must withstand start/stop cycles and 15Ni-3Mn during the comparative study. have a desirable lifetime. As a result, in this section, we designed a start/stop cycle test and a lifetime 2.4. Stability of 15Ni-3Mn Catalyst test on the 15Ni‐3Mn catalyst with the purpose of examining its performance in practical occasions. In the start/stop cycle test, the 15Ni‐3Mn catalyst is confronted with continuous start‐up/cessations Through the comparative study, we found out that 15Ni-3Mn performs better than 15Ni-3Cr of the engine. The cycles were realized cooling the as system down to room over a wide range of series H2 /COof and H2 O/CO, impellingby us firstly to select 15Ni-3Mn the best candidate for temperature and then passing the reaction feed gas including steam through the catalyst bed for partial methanation coupling with WGS of low H2 /CO ratio biomass gas into urban gas. However, 30 min. Finally, the system was reheated to the chosen temperature of 350 °C [22]. The condensed from the industrial and engineering perspective, the catalyst must withstand start/stop cycles and water, which is lifetime. located As in the pore inof catalyst, considered one of the harshest have a desirable a result, this section,is we designed aas start/stop cycle test andconditions a lifetime for catalysts. test on the 15Ni-3Mn catalyst with the purpose of examining its performance in practical occasions. In theThe heating value of urban gas is closely associated with the content of combustible components, start/stop cycle test, the 15Ni-3Mn catalyst is confronted with continuous start-up/cessations of such as H 2, CO and CH4; thus, we concentrate on their content in the outlet. The experimental result the engine. The series of cycles were realized by firstly cooling the system down to room temperature in Figure 12A revealed that during the former two stop/cycles (Stop/Cycles 1 and 2), the content of and then passing the reaction feed gas including steam through the catalyst bed for 30 min. Finally, H2, CO and CH4 remained as the state without shutdown (Stop/Cycle 0), indicating that the 15Ni‐3Mn catalyst is robust to some extent. However, in Stop/Cycles 3 and 4, the content of CH4


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the system was reheated to the chosen temperature of 350 ◦ C [22]. The condensed water, which is located in the pore of catalyst, is considered as one of the harshest conditions for catalysts. The heating value of urban gas is closely associated with the content of combustible components, such as H2 , CO and CH4 ; thus, we concentrate on their content in the outlet. The experimental result in Figure 12A revealed that during the former two stop/cycles (Stop/Cycles 1 and 2), the content of H2 , Catalysts 2017, 7, 51 14 of 19 CO and CH4 remained as the state without shutdown (Stop/Cycle 0), indicating that the 15Ni-3Mn catalyst is robust to some extent. However, in Stop/Cycles 3 and 4, the content of CH4 decreased and decreased and CO increased, suggesting a deactivation in the 15Ni‐3Mn catalyst. This may be due to CO increased, suggesting a deactivation in the 15Ni-3Mn catalyst. This may be due to destruction in destruction in the porous structure of the support and a wastage of active components caused by the porous structure of the support and a wastage of active components caused by excessive condensed excessive condensed water. Nevertheless, this start/stop cycle test demonstrated indirectly that water. Nevertheless, this start/stop cycle test demonstrated indirectly that frequent shutdowns have frequent shutdowns have a negative effect on the performance of catalyst, and we should keep a negative effect on the performance of catalyst, and we should keep systematic continuity as much systematic continuity as much as possible. as possible. 50

H2

CO

CH4

30

(A) Content (%)

Content (%)

30 20 10 0

CH4

25

40

(B)

H2 CO

20 15 10 5

0

1 2 3 Start/stop cycle

4

0

0

20

40 60 Time (h)

80

100

Figure 12. Start/stop cycle test (A) and lifetime test (B) of 15Ni-3Mn catalyst at 350 ◦ C, H2 /CO = 0.8 Figure 12. Start/stop cycle test (A) and lifetime test (B) of 15Ni‐3Mn catalyst at 350 °C, H 2/CO = 0.8 and H O/CO = 1. and H22O/CO = 1.

The lifetime lifetime test test is is considerably considerably critical critical in in the the appraisal appraisal of of catalyst, catalyst, and and aa catalyst catalyst with with good good The stability isis crucial crucial industrial processes. Figure 12B presents the lifetime test of the stability forfor industrial processes. Figure 12B presents the lifetime test results of results the 15Ni-3Mn 15Ni‐3Mn catalyst carried out at 350 °C, H catalyst carried out at 350 ◦ C, H2 /CO = 0.82/CO = 0.8 and H and H2 O/CO =2O/CO = 1. It can be seen that during the 1. It can be seen that during the nearly nearly 100‐h lifetime test, the content of H , CO and CH 4 in the outlet gas remains relatively stable 100-h lifetime test, the content of H2 , CO 2and CH4 in the outlet gas remains relatively stable and and basically fluctuates over the average value (dashed line in Figure 12B), suggesting that the basically fluctuates over the average value (dashed line in Figure 12B), suggesting that the 15Ni-3Mn 15Ni‐3Mn catalyst could work with a satisfying over an extended This be catalyst could work with a satisfying stability over anstability extended period. This may period. be because themay added because the could added Mn species could the surface rapid loss specific area caused by and Ni Mn species inhibit the rapid lossinhibit of specific areaof caused bysurface Ni sintering (Table 1) sintering the (Table 1) carbon and decrease the (Table rate of 2). Similar results are also decrease rate of deposition 2).carbon Similardeposition results are(Table also reported elsewhere [30,42,43]. The mean content of H2 is 22.07% accompanied by 19.35% CH4 and 8.37% CO on average. In addition, reported elsewhere [30,42,43]. The mean content of H 2 is 22.07% accompanied by 19.35% CH 4 and the content of CO is below 10%, which could guarantee the safety requirements for downstream 8.37% CO on average. In addition, the content of CO is below 10%, which could guarantee the safety residents. The coexistence of H2 and CH4 with a high concentration demonstrates that the 15Ni-3Mn requirements for downstream residents. The coexistence of H 2 and CH 4 with a high concentration catalyst could that splendidly catalyzecatalyst partial methanation coupling withpartial WGS of biomass gas, which demonstrates the 15Ni‐3Mn could splendidly catalyze methanation coupling exposes its great industrial potential. with WGS of biomass gas, which exposes its great industrial potential. 2.5. Synergy Effect between Ni and MnOxx Species Species 2.5. Synergy Effect between Ni and MnO On the basis of the above results and discussions, a process of partial methanation coupling On the basis of the above results and discussions, a process of partial methanation coupling with with of biomass gas catalyzed by Ni-Mn bicomponent catalyst is outlined in Figure WGS WGS of biomass gas catalyzed by Ni‐Mn bicomponent catalyst is outlined in Figure 13. As 13. it As it illustrates, highly dispersed Ni particles and MnO species serve as active sites for CO x illustrates, highly dispersed Ni particles and MnOx species serve as active sites for CO hydrogenation, hydrogenation, as well as WGS. Compared to monometallic 15Ni catalyst, thex species on the one addition of MnOx as well as WGS. Compared to monometallic 15Ni catalyst, the addition of MnO species on the one hand weakens the interaction between2ONiO and the Al2 O3 support, which facilitates hand weakens the interaction between NiO and the Al 3 support, which facilitates the reduction of the reduction of NiO particles, while on the other hand increases theof dispersion reduced well NiO particles, while on the other hand increases the dispersion reduced of Ni, as well Ni, as as the H2 as the H uptake, leading to an increased activity for methanation. Moreover, the addition of MnO x uptake, 2leading to an increased activity for methanation. Moreover, the addition of MnOx species species could enhance the WGS reaction, demonstrating an outstanding synergy effect between Ni could enhance the WGS reaction, demonstrating an outstanding synergy effect between Ni and MnOx

species. Compared to CrOx species, MnOx species could also inhibit the rate of carbon deposition, showing its better anti‐coking ability when submitted to various H2/CO and H2O/CO situations. Finally, the synergy effect between Ni and MnOx species presents a nonlinear relationship with the amount of Mn addition, and there exists an optimal value, as shown in Figure S5.


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and MnOx species. Compared to CrOx species, MnOx species could also inhibit the rate of carbon deposition, showing its better anti-coking ability when submitted to various H2 /CO and H2 O/CO situations. Finally, the synergy effect between Ni and MnOx species presents a nonlinear relationship Catalysts 2017, 7, 51 15 of 19 with the amount of Mn addition, and there exists an optimal value, as shown in Figure S5.

methanation CO

H2

CH4

H 2O

H2O

CO2

H2

WGS Ni

MnOx

deposited carbon

Ni

Al2O3 support

Figure 13. Schematic between Ni Ni and and MnOxx species partial methanation methanation Figure 13. Schematic of of the the synergy synergy effect effect between species for for partial coupling with WGS. coupling with WGS.

3. Materials and Methods 3. Materials and Methods 3.1. Catalyst Preparation 3.1. Catalyst Preparation The incipient wetness wetness The supported supported nickel‐based nickel-based bicomponent bicomponent catalysts catalysts were were prepared prepared by by the the incipient co-impregnation method. The γ-Al22O O33 granular support (particle size: 2–3 mm) (Henan Chuangmei granular support (particle size: 2–3 mm) (Henan Chuangmei co‐impregnation method. The γ‐Al Environmental Protection Co., Ltd., Gongyi, China) was first pretreated by drying in an electric oven Environmental Protection Co., Ltd., Gongyi, China) was first pretreated by drying in an electric oven at 110 ◦ C for 12 h, so as to remove the adsorbed impurities. Then, a precalculated amount of solution at 110 °C for 12 h, so as to remove the adsorbed impurities. Then, a precalculated amount of solution containing Ni(NO ·6H22O O and and the the second asas (NH) ·9H22O, O, containing Ni(NO33)22∙6H second metal metal salt, salt, such such (NH) 4Mo 7O ∙4H22O, O, Fe(NO Fe(NO33))22∙9H 4 Mo 7O 2424·4H Co(NO33))22∙6H ·6H2O, O, Mn(NO ) · 4H O and Cr(NO ) · 9H O, was mixed with the support material. All 2 Mn(NO3)32∙4H 2 2O 2 and Cr(NO3)32∙9H 2 2O, 2 was mixed with the support material. All chemicals mentioned above were commercially provided by Sinopharm Chemical Reagent Co., Ltd. chemicals mentioned above were commercially provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and were of analytical purity. In order to obtain catalysts with uniformly-dispersed (Shanghai, China) and were of analytical purity. In order to obtain catalysts with uniformly‐dispersed bicomponent Ni-M species (M = Mo, Fe, Co, Mn or Cr), the heterogeneous mixture was kept in a bicomponent Ni‐M species (M = Mo, Fe, Co, Mn or Cr), the heterogeneous mixture was kept in a water bath at 60 ◦ C for 12 h. Later, the samples were dried at 110 ◦ C and calcined by a temperature water bath at 60 °C for 12 h. Later, the samples were dried at 110 °C and calcined by a temperature programmed muffle furnace at 450 ◦ C for 4 h. The nominal load of nickel in each of the acquired programmed muffle furnace at 450 °C for 4 h. The nominal load of nickel in each of the acquired catalyst is 15 wt %, and that of the second metal is 3 wt %, unless noted elsewhere. For convenience, catalyst is 15 wt %, and that of the second metal is 3 wt %, unless noted elsewhere. For convenience, the abbreviations the catalysts in this workwork are denoted as 15Ni-3M. 15Ni catalyst was also was prepared the abbreviations ofof the catalysts in this are denoted as 15Ni‐3M. 15Ni catalyst also in the same manner for reference. prepared in the same manner for reference. 3.2. Catalyst Characterizations 3.2. Catalyst Characterizations Inductively-coupled plasma (ICP) atomic emission spectrometry (Optima5300DV, PerkinElmer Inductively‐coupled plasma (ICP) atomic emission spectrometry (Optima5300DV, PerkinElmer Instrument, Waltham, MA, USA) was used to determine the real contents of loaded metal in the Instrument, Waltham, MA, USA) was used to determine the real contents of loaded metal in the as‐ as-prepared catalysts. N2 isothermal adsorption at −196 ◦ C was analyzed using an automated gas prepared catalysts. N2 isothermal adsorption at −196 °C was analyzed using an automated gas adsorption analyzer (BELSORP-Mini, Osaka, Japan) in order to calculate the specific surface area adsorption analyzer (BELSORP‐Mini, Osaka, Japan) in order to calculate the specific surface area according to the BET method and the pore size distribution by the BJH (Barrett-Joyner-Halenda) according to the BET method and the pore size distribution by the BJH (Barrett‐Joyner‐Halenda) method. Powder X-ray diffraction (XRD) patterns were recorded by an X-ray diffractometer method. Powder X‐ray diffraction (XRD) patterns were recorded by an X‐ray diffractometer (Smart (Smart Lab, Tokyo, Japan) with Cu Kα radiation under a 0.02◦ scanning step and 20◦ ·min−1 scanning Lab, Tokyo, Japan) with Cu Kα radiation under a 0.02° scanning step and 20°∙min−1 scanning speed speed operated at 40 kV and 100 mA. The wavelength λ was 0.154 nm, and the scanning 2θ angle operated at 40 kV and 100 mA. The wavelength λ was 0.154 nm, and the scanning 2θ angle range range was from 20◦ –80◦ . was from 20°–80°. H2 -TPR and H 2 -TPD characterizations were carried out on an automated chemisorption analyzer H 2‐TPR and H2‐TPD characterizations were carried out on an automated chemisorption analyzer (chemBET pulsar TPR/TPD, Quantachrome, Boynton Beach, FL, USA). For the H2 -TPR test, typically, (chemBET pulsar TPR/TPD, Quantachrome, Boynton Beach, FL, USA). For the H 2‐TPR test, typically, a 0.10 g sample was first placed at 300 ◦ C for 1 h under Ar flow to remove moisture and impurities a 0.10 g sample was first placed at 300 °C for 1 h under Ar flow to remove moisture and impurities as the pretreatment. Following, the sample was cooled down to room temperature and reheated as the pretreatment. Following, the sample was cooled down to room temperature and reheated to 1000 °C at 10 °C∙min−1 under the flow of a 10 vol % H2/Ar mixture gas (40 mL∙min−1). The outlet signal was recorded continuously by a thermal conductivity detector (TCD) detector (Shimadzu Corporation, Kyoto, Japan). For the H2‐TPD test, a 0.20 g sample was initially reduced in situ by H2 flow and subsequently cooled down to room temperature and saturated with H2 for 1 h. After the


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to 1000 ◦ C at 10 ◦ C·min−1 under the flow of a 10 vol % H2 /Ar mixture gas (40 mL·min−1 ). The outlet signal was recorded continuously by a thermal conductivity detector (TCD) detector (Shimadzu Corporation, Kyoto, Japan). For the H2 -TPD test, a 0.20 g sample was initially reduced in situ by H2 flow and subsequently cooled down to room temperature and saturated with H2 for 1 h. After the physically adsorbed H2 was removed by purging with Ar for 2 h, the sample was subsequently heated to 800 ◦ C at a ramping rate of 10 ◦ C·min−1 in Ar flow (30 mL·min−1 ). Likewise, the released H2 was detected continuously as a function of increasing temperature by a TCD detector. The dispersion of Ni was calculated based on the volume of chemisorbed H2 using the simplified equation referring to the literature [44]: D (%) = 100 ×

2 × Vad × M × SF m × P × Vm × dr

(3)

where Vad (mL) is the volume of chemisorbed H2 in the TPD measurement at standard temperature and pressure (STP) conditions; M is the molecular weight of Ni (58.69 g·mol−1 ); SF is the stoichiometric factor, which originates from the assumption of adsorption stoichiometry H/Ni = 1:1; m is the mass of the sample (g); P is the weight fraction of Ni in the sample determined by ICP; Vm is the molar volume of H2 (22.4 L·mol−1 ) at STP; and dr is the reduction degree of Ni calculated on the basis of H2 -TPR. The properties and the amount of carbon deposits over spent catalysts were investigated through thermogravimetric (TG) analysis (NETZSCH STA 409 PC/PG, NETZSCH, Selb, Germany). The sample was heated in flowing air from room temperature to 1000 ◦ C at a heating rate of 10 K·min−1 . The morphology of catalyst samples was determined by a HITACHI S-4800 scanning electron microscopy (SEM) operated at 5 kV with an energy dispersive X-ray spectroscopy (EDS) detector for elemental analysis. 3.3. Catalyst Testing The catalyst testing was carried out in a continuous fixed-bed reactor equipped with a stainless steel tube (inner diameter: 50 mm, length: 1000 mm) at 0.1 MPa and a temperature range of 300–400 ◦ C. In a typical catalyst test, the reactor with 50 mL of catalysts was initially heated to 450 ◦ C (heating rate: 4 ◦ C·min−1 ) in N2 flow, and then the gas flow was switched to pure H2 (Gas Hour Space Velocity (GHSV): 1000 h−1 ) for catalyst reduction at the same temperature for 4 h. After the reduction period was finished, the reactor was cooled down to 250 ◦ C, and the feed gas was switched to dry reactant gas flow with a composition of 20% CO2 , 8% N2 and 0.6–1.0 H2 /CO (total GHSV: 2000 h−1 ); and the additional required steam is based on the molar flow ratio of H2 O and CO (0.5–1.0 H2 O/CO). The outlet gas was analyzed by a Shimadzu GC2018 gas chromatography (Shimadzu Corporation, Kyoto, Japan) equipped with a 3 m Porapak Q column and a 2 m molecular sieve column, of which helium was used as the carrier gas. The concentrations of H2 , CO2 , N2 , CH4 and CO were analyzed by a TCD, while C2 H4 and C2 H6 were analyzed by a flame ionization detector (FID, Shimadzu Corporation, Kyoto, Japan). This combination of two detectors allowed highly sensitive analysis of all components in the gas mixture. The detailed experimental setup is displayed in Figure S1. The percentage conversion of CO (XCO ) was calculated using Equation (4) to determine the overall catalytic activity. The CH4 selectivity (SCH4 ) and CO2 growth rate (RCO2 ) were determined using Equations (5) and (6) to measure the degree of methanation and WGS, respectively. XCO (%) = ([ FCO ]in − [ FCO ]out )/[ FCO ]in × 100

(4)

SCH4 (%) = [ FCH4 ]out /([ FCO ]in + [ FCO2 ]in − [ FCO ]out − [ FCO2 ]out ) × 100

(5)

RCO2 (%) = ([ FCO2 ]out − [ FCO2 ]in )/[ FCO2 ]in

(6)

where F is the molar flow of the inlet and outlet species.


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4. Conclusions Partial methanation coupling with WGS of low H2 /CO ratio biomass gas into urban gas was investigated over γ-Al2 O3 -supported Ni-M (M = Mo, Fe, Co, Mn or Cr) catalysts. The bicomponent catalysts with fixed 15 wt % Ni and 3 wt % M were prepared by the co-impregnation method and further characterized by N2 isothermal physisorption, XRD, H2 -TPR, H2 -TPD and SEM. The comprehensive results of the characterization analysis, performance tests and comparative study showed that: (1)

(2)

(3) (4)

(5)

15Ni-3Mo, 15Ni-3Fe and 15Ni-3Co catalysts present lower CO conversion, CH4 selectivity and CO2 growth rate than one-component 15Ni catalyst, mainly due to their low Ni dispersion and weak H2 chemisorption ability with the addition of the second component, indicating the deficient synergy effect between Mo/Fe/Co and active Ni particles. Though both 15Ni-3Mn and 15Ni-3Cr have relative high Ni dispersion and small Ni particle size, as well as comparable H2 chemisorption ability, 15Ni-3Mn is more suitable for our use due to its lower carbon deposition rate, smaller Ni sintering degree and wider adaptability to various H2 /CO and H2 O/CO conditions in comparison with 15Ni-3Cr. 15Ni-3Mn is robust to some extent during the start/stop cycle test and has satisfactory stability during a nearly 100-h lifetime test, showing its qualification for industrial and engineering use. The lower heating value of the low H2 /CO ratio biomass gas catalyzed by 15Ni-3Mn through partial methanation coupling with WGS could be over 10 MJ·Nm−3 , which is eligible as urban gas for downstream users. MnOx species as an additive or second component cooperate well with Ni, for increasing methanation activity, as well as enhancing WGS reaction, indicating its favorable synergy effect with active Ni particle.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/7/2/51/s1: Figure S1: Schematic diagram of catalyst testing. Figure S2: N2 adsorption isotherms (A) and PSD curves (B) of the support and as-prepared catalysts: (a) γ-Al2 O3 , (b) 15Ni, (c) 15Ni-3Mo, (d) 15Ni-3Fe, (e) 15Ni-3Co, (f) 15Ni-3Mn and (g) 15Ni-3Cr. Figure S3: XRD patterns of the γ-Al2 O3 support: (a) before calcination and (b) after calcination at 450 ◦ C for 4 h. Table S1: Detailed data of deconvolved TPR profiles. Figure S4: Elemental mapping images of reduced catalysts: (A,C,E) 15Ni-3Mn; (B,D,F) 15Ni-3Cr. Figure S5: Effect of Mn loading in 15Ni catalyst on the partial methanation coupling with WGS at 350 ◦ C, H2 /CO = 0.8 and H2 O/CO = 1. Acknowledgments: The financial support to the reported research by the International S & T Cooperation Program of China (ISTCP, 2014DFE70150) is gratefully acknowledged. Author Contributions: Baosheng Jin, Min Song and Xinxin Dong conceived of and designed the experiments. Xinxin Dong, Zheng Zhou and Xu Yang performed the experiments. Xinxin Dong analyzed the data. Zheng Zhou and Xu Yang contributed reagents/materials/analysis tools. Xinxin Dong wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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