Sodium percarbonate (sodium carbonate peroxyhy- drate, Na2CO3 ... Sodium percarbonate is stabilized by sodium silicate and polyphosphate. DOI: 10.1134/ ...
ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 9, pp. 1455–1458. © Pleiades Publishing, Inc., 2009. Original Russian Text © A.V. Zhubrikov, E.A. Legurova, V. Gutkin, V. Uvarov, N.V. Khitrov, O. Lev, T.A. Tripol’skaya, P.V. Prikhodchenko, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 9, pp. 1526–1529.
PHYSICAL METHODS OF INVESTIGATION
XPS Characterization of Sodium Percarbonate Granulated with Sodium Silicate A. V. Zhubrikova, E. A. Legurovab, V. Gutkinc, V. Uvarovc, N. V. Khitrova, O. Levc, T. A. Tripol’skayab, and P. V. Prikhodchenkob a
OAO Khimprom, Promyshlennaya ul. 101, Novocheboksarsk, 429952 Chuvash Republic, Russia b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia c Institute of Chemistry, Hebrew University of Jerusalem, Jerusalem, Israel Received October 30, 2008
Abstract—Granular sodium percarbonate has been characterized by X-ray powder diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The O1s binding energy for the solvating hydrogen peroxide molecules is 535.8 eV. Sodium percarbonate is stabilized by sodium silicate and polyphosphate. DOI: 10.1134/S0036023609090198
Sodium percarbonate (sodium carbonate peroxyhydrate, Na2CO3 · 1.5H2O2, PCS), both alone and in combination with other chemicals, is employed as a solid source of active oxygen in the textile, chemical, and other industries and as a bleaching agent in synthetic detergents [1]. PCS, known for over a century [2], was characterized by various physicochemical methods. The PCS structure was determined by X-ray crystallography. It was found that the hydrogen peroxide molecules in PCS are solvating and are bound to the carbonate ion by hydrogen bonds [3, 4]. In PCS production, granulating and stabilizing agents are introduced into the product. PCS granulation is most often ensured by adding sodium metasilicate. PCS granules can be stabilized by encapsulation into a substance unaffected by humid air. Here, we report XPS data for PCS obtained at the research center of OAO Khimprom (Novocheboksarsk). The purpose of this study was to elucidate the roles of the components of the product in PCS stabilization. In addition to clarifying this practical issue, we address the fundamental problem of obtaining XPS data for the oxygen atoms in the hydrogen peroxide molecule. This problem is of vital importance because XPS is widely used in the study of semiconducting nanomaterials, whose properties depend on the oxidation state of their oxygen atoms, which varies upon the adsorption of atmospheric oxygen on the surface [5, 6]. No XPS data have been reported for peroxysolvates to date, and this makes XPS data for other oxygen-containing compounds difficult to interpret. EXPERIMENTAL PCS samples were obtained by crystallization from a water–hydrogen peroxide solution of sodium carbon-
ate in a spray drier at OAO Khimprom. The material examined in this work was 1-mm spherical granules made up of fine PCS crystals (Fig. 1). As determined by permanganatometry [7], the active oxygen content of PCS is 12.8 wt % (the calculated active oxygen content of Na2CO3 · 1.5H2O2 is 15.3 wt %). The phase composition of PCS was determined by X-ray powder diffraction on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) using CuKα radiation. It was found that the PCS granules contain orthorhombic Na2ëé3 · 1.5ç2é2 (Fig. 2) [3, 4]. No other crystalline phases were detected in PCS. X-ray photoelectron spectra were recorded on a Kratos Axis photoelectron spectrometer equipped with an X-ray monochromator and an AlKα source (1486.7 eV). In the surface analysis of PCS granules, the samples were the granules themselves. In order to determine the bulk composition of the granules, they were ground and the surface of the resulting powder was examined. The charging effect arising from photoemission was taken into account using the internal standard technique. The internal standard was the C1s- spectrum of sodium percarbonate, in which the 1s binding energy was taken to be equal to that of sodium carbonate [8]. An analysis of photoemission spectra and calculation of relative element contents were carried out using the Vision Processing program (Kratos Analytical Ltd.) and the CasaXPS program (Casa Software Ltd.). Peak deconvolution into components and determination of their positions and integrated intensities were carried out by fitting to a combination of symmetric Gaussian and Lorentzian functions (30 and 70%, respectively).
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2 mm Fig. 1. SEM images of sodium percarbonate granules.
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Fig. 2. X-ray powder diffraction pattern from sodium percarbonate (—reflections from orthorhombic Na2CO3 · 1.5H2O2).
RESULTS AND DISCUSSION The results of elemental analysis of PCS samples by XPS are presented in the table. According to these XPS data, the major elements of PCS are carbon, sodium, and oxygen. The relative amounts of these elements are consistent with the elemental composition of PCS, the main phase of the samples. The surface is poorer in oxygen than the sample bulk. This can be explained by PCS decomposition to sodium carbonate on the surface and by the stabilization of peroxo compounds in the granule bulk. It follows from the elemental analysis data and PCS synthesis conditions that the samples contain sodium silicate, sulfate, chloride, and polyphosphate. The sulfate, chloride, and silicate percentages are higher in the granule bulk, while most of the phosphate is on the granule surface. Therefore, the PCS granules are encapsulated in sodium polyphosphate. No other elements were detected in the samples. It is
interesting that the silicon content of the granule bulk is over 2.5 wt %; hence, the Na2SiO3 content will be at least 10 wt % even if the water of crystallization is ignored. Nevertheless, no crystalline phases other than orthorhombic PCS were detected by X-ray diffraction (Fig. 2). This is indirect evidence that X-ray-amorphous, gellike sodium persilicate forms during the production of the material and it is this compound that binds the sodium percarbonate crystals into granules. Although the sodium persilicate structure and formation conditions have not been determined for certain [9–12], an analysis of preliminary data together with the PCS synthesis conditions suggests that the product contains sodium persilicate. The X-ray photoelectron spectrum of PCS shows Na1s, C1s, Cl2p, S2p, P2p, and Si2p lines (Fig. 3), which 2– 2– are due to the Na+, CO 3 , Cl–, SO 4 , phosphate, and
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silicate ions, respectively, and also a complex é1s peak (Figs. 3, 4). This peak has two maxima of different intensities; however, it cannot be deconvolved satisfactorily into only two components (even when asymmetric functions are used). The é1s peak can be deconvolved into three components at binding energies of 531.1, 532.6, and 535.8 eV with integrated intensity ratios of 4.6 : 1.1 : 1.0. The strongest peak, which occurs at a binding energy of 531.1 eV, is due to the oxygen atoms in the carbonate ions of sodium carbonate [8, 13]. The component occurring at the highest binding energy, 535.8 eV, should be assigned to the peroxide oxygen atoms in PCS. The comparatively low intensity of this component is likely due to PCS decomposition on the sample surface under conditions of a high vacuum and X-ray irradiation. This yields sodium carbonate, gives rise to a strong peak from the oxygen atoms in the carbonate ion, and reduces the é1s peak components due to the initial compound. No XPS data have been reported for solvating hydrogen peroxide, and the data available on peroxo derivatives are scanty. This is likely explained by the instability of most peroxo com-
XPS elemental analysis data for PCS
Peak
PCS granules (surface analysis)
PCS powder (bulk analysis)
Atomic con- Weight con- Atomic con- Weight concentration, % centration, % centration, % centration, %
Na1s O1s C1s* Cl2p S2p P2p Si2p
28.12 54.19 15.70 0.37 0.01 1.03 0.58
36.65 49.15 10.69 0.75 0.02 1.82 0.92
23.60 58.03 15.95 0.37 0.24 0.26 1.56
O1s
Na1s O KLL
Na KLL
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Fig. 3. X-ray photoelectron spectrum of the sodium percarbonate powder. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
31.26 53.51 11.04 0.76 0.44 0.47 2.52
pounds under the severe conditions of XPS measurements. We obtained the first data for compounds containing hydrogen peroxide molecules. The é1s binding energy for PCS (535.8 eV) is much higher than the same binding energy for sodium and strontium peroxides (530.8 and 531.1 eV, respectively) [14]. Our data
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ACKNOWLEDGMENTS This study was supported by the Russian Foundation for Basic Research (project nos. 08-03-00537 and OFI 0703-12199) and by the Russian Federal Agency for Science and Innovation (grant no. MK-3120.2008.3).
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REFERENCES
540
530 Binding energy, eV
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Fig. 4. O 1s line in the X-ray photoelectron spectrum of the sodium percarbonate powder.
are in agreement with the XPS data for tin dioxide pretreated with oxygen [15, 16], for which the comparatively high é1s binding energy of 532.6 eV [15] (532.7 eV [16]) was assigned to peroxo derivatives of tin. Indeed, the X-ray photoelectron spectrum of potassium hexahydroperoxostannate K2Sn(OOH)6, in which the peroxide is in the form of hydroperoxo groups coordinated to the tin atom, exhibits contains an é1s component with a well-defined maximum at 533.6 eV [17]. Thus, the assignment of the 535.8 eV line in the spectrum of PCS to the oxygen atoms of solvating hydrogen peroxide is quite substantiated. It is likely that the peak component at 532.6 eV in the spectrum of PCS is due to the carbonate oxygen atoms hydrogen-bonded with hydrogen peroxide molecules [3, 4]. The similar integrated intensities of the two é1s components assigned to the oxygen atoms of sodium percarbonate are consistent with the ratio of carbonate oxygen to peroxide oxygen in Na2ëé3 · 1.5ç2é2 (1 : 1).
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