Fe phase complexes and their thermal stability in iron phosphate catalysts supported on silica

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1 Hyperfine Interact DOI /s Fe phase complexes and their thermal stability in iron phosphate catalysts supported on silica Venkata D. B. C. Dasireddy K. Bharuth-Ram A. Harilal S. Singh H. B. Friedrich Received: 3 October 2014 / Accepted: 22 December 2014 Springer International Publishing Switzerland 2015 Abstract Comparative XRD and Mössbauer spectroscopy studies have been conducted on the effect of temperature on the phase transformations of an iron phosphate catalyst synthesized using the ammonia gel method (CAT1) and a commercial grade FePO 4 catalyst supported on silica using wet impregnation method (CAT2). The XRD patterns of both catalysts showed the presence of iron phosphate and the tridymite phase of aluminum phosphate. Mössbauer spectra of the catalysts show that the phases present in CAT1 are thermally stable up to 500 C, but CAT2 shows significant changes with the tridymite phase of iron phosphate increasing from 6 % to 29 % of the spectral area at a temperature of 500 C. Keywords Iron phosphate catalyst X-ray diffraction Mössbauer spectroscopy Quartz and trydimite phase 1 Introduction The redox function of a metal oxide is essential for oxygen insertion and oxidative dehydrogenation reactions over the metal oxide catalysts. This implies that oxidation activity is dependent on the reducibility and re-oxidizability of the metal oxide [1, 2]. It has been proposed that the oxidative reaction can proceed selectively when the reducibility of the metal oxide is moderate. Iron phosphate catalysts, which are widely used for the oxidative dehydrogenation reactions [3], exhibit both redox and acidic characteristics. The characteristics are very similar to those of heteropoly-acids, molybdenum and vanadium phosphates Proceedings of the 5th Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2014), Canberra, Australia, September V. D. B. C. Dasireddy ( ) K. Bharuth-Ram A. Harilal S. Singh H. B. Friedrich School of Chemistry and Physics, University of KwaZulu-Natal, Durban, South Africa dasireddy@gmail.com K. Bharuth-Ram Durban University of Technology, Durban 4000, South Africa

2 Venkata D. B. C. Dasireddy et al. which were widely used in the production of Methyl methacrylate (MMA) commercially. However, the iron phosphates show low selectivity compared to the heteropolyacids in the production of acrylic acid, maleic anhydride, and methacryaldehyde [4]. This is attributed to the lack of Fe=O species in the iron phosphates, which then inhibits iron phosphates inducing oxygen insertion reactions. Iron phosphates, on the other hand, showed high selectivity in the oxidative dehydrogenation of lactic to pyruvic acid and glycolic acid to glyoxylic acid which involves an abstraction of hydrogen from the reactant [5]. Information regarding the oxidative dehydrogenation (ODH) over iron phosphate catalysts has been collected in the literature [6, 7]. Typically, the reaction is carried out under fairly mild conditions (< 450 C) with a large quantity of steam added to the gas phase over the catalysts with P:Fe ratios ranging from 1:1 to 2:1. Commonly, the catalysts are formulated with a P:Fe ratio of 1.2:1, a ratio that does not produce a single, pure crystalline iron phosphate phase. Consistent with this fact are reports that samples of FePO 4, Fe 2 P 2 O 7, α-fe 3 (P 2 O 7 ) 2 and β-fe 3 (P 2 O 7 ) 2 all showed catalytic activity and selectivity for the ODH reaction with little variation [7, 8]. Millet and coworkers [7] have published a series of articles concluding that the specific crystalline phases α-fe 3 (P 2 O 7 ) 2 and Fe 2 (PO 3 OH)P 2 O 7 are the most active and selective catalysts for the reaction, with very similar catalytic properties. It is proposed that these two phases develop similar surface species under reaction conditions, namely, a mixture of Fe 2+ and Fe 3+ cations and (PO 3 OH) 2 groups resulting from hydration of the surface [3]. The catalyst considered for this work was a phase specific iron phosphate based catalyst and was chosen because reports have shown that the phosphates of iron are superior to the phosphates of vanadium and molybdenum for the oxidative dehydrogenation of isobutyric acid to methacrylic acid [2, 3]. The α-fe 3 (P 2 O 7 ) 2 phase of the iron phosphate based catalyst has been described as a mixed ferric and ferrous pyrophosphate consisting of Fe 4 (P 2 O 7 ) 3 and Fe 2 P 2 O 7 with P/Fe = 1.33, and this α-phase is believed to be active and selective under catalytic conditions [4]. Although comparison experiments under reduction and reaction atmospheres on the bulk phase catalyst have been reported previously [9, 12, 13], the real transformation processes of the FePO 4 catalyst under oxidation atmosphere were largely unknown due to the mixture of quartz and trydimite phases. Hence, the phase transformations of the FePO 4 precursor of the FePO 4 catalyst were tracked as a function of temperature under in situ conditions using a combination of Mössbauer spectroscopy (MS) and X-ray diffraction (XRD). 2 Experimental Comparative thermal stability studies were conducted on two sets of catalysts: an iron phosphate catalyst synthesized using the ammonia gel method outlined by Muneyama et al. [8] (denoted as CAT1) and a commercial grade iron (III) phosphate dihydrate (Batch No JA, Sigma Aldrich Chemcial Company) and impregnated on colloidal silica (denoted as CAT2). The amounts of iron and phosphate present in the catalysts were determined using a Perkin Elmer Optical Emission spectrometer (Optima 5300 DV) and the data was collected and processed using the Winlab 32 software. The standards used in the analysis were purchased (1000 ppm Fe and P) from Fluka. 57 Fe-Mössbauer measurements were made in conventional transmission geometry with a 57 CoRh source, on samples heated up to 500 C under air with a ramping rate of 10 C/min with a 50 Cinterval.In situ X-ray diffraction patterns were acquired using a Philips X Pert PRO diffractometer system equipped with an ANTON PAAR XRK 600 high temperature

3 Fe phase complexes and their thermal stability in iron phosphate stage setup and an X Celerator detector. The source of radiation was Co K radiation using a wavelength of Å. 3 Results and discussion The XRD spectra of the synthesized iron phosphate catalyst which was heated in air showed peaks at 2θ values at 23,24,31,40 and 43, and resembled the quartz type iron phosphate phase. There is a minor contribution from the tridymite-like phase of FePO 4 was also observed [9]. Although there is no crystallographic information available for the tridymite phase of FePO 4, this phase was identified based on the isotypic AlPO 4 analogue of this phase. The synthesized catalyst which was calcined at 500 C in air showed a mixture of the quartz and tridymite type phases [10], in agreement with the phase changes suggested in the literature. The XRD patterns of the two catalysts were matched with the quartz type phase of iron phosphate (JCPDS no ) and the tridymite phase of aluminum phosphate (JCPDS no ). With increase in temperature the in situ XRD spectra reflected an increase in the crystallinity for both the catalysts (Fig. 1). The ICP-OES results give a calculated P/Fe ratio of 1.2, which supports the formation of the tridymite phase at lower temperatures and validates the observation and assignments of the XRD patterns, respectively. Sample CAT1 showed a surface area of 18 m 2 /g. The ICP results of CAT2 showed a P/Fe ratio of 1.0. This supported the absence of the tridymite phase in the XRD patterns for this catalyst, since the formation of the tridymite phase would require a P/Fe ratio of greater than 1. CAT2 showed a BET surface area of 15 m 2 /g, which was marginally lower than that of CAT1 [10]. Fig. 1 In situ XRD of CAT 1 under air

4 Venkata D. B. C. Dasireddy et al. Fig. 2 Mössbauer spectra of catalysts (a) CAT 1 and (b) CAT 2 under air, at the temperatures indicated The Mössbauer spectra of catalysts CAT1 and CAT2, together with their spectral components, are presented in Fig. 2a and b. The spectra were fitted with four paramagnetic Table 1 Isomer shift (IS), electric quadrupole splitting (QS), Areal fractions (A), phase assignments and the attributed phases, determined from the Mössbauer spectra of CAT1 and CAT2 at T 300 C Spectral IS QS <A> Fe Phase component (mm/s) (mm/s) (%) CAT1 D1 0.34(3) 0.36(3) 8(2) Fe 3+ FePO 4 -tdm D2 0.34(2) 0.69(3) 29(3) Fe 3+ FePO 4 -quartz D3 0.34(2) 1.08(4) 43(3) Fe 3+ FePO 4 -quartz D4 0.35(3) 1.58(5) 21(4) Fe 3+? CAT2 D1 0.34(3) ) 6(2) Fe 3+ FePO 4 -tdm D2 O.39(3) 0.67(4) 28(3) Fe 3+ FePO 4 -quartz D3 0.36((3) 1.03(4) 41(3) Fe 3+ FePO 4 -quartz D4 0.41(4) 1.54(5) 24(4) Fe 3+? The isomer shifts are expressed relative to α-fe

5 Fe phase complexes and their thermal stability in iron phosphate Fig. 3 Temperature dependence of the relative intensities of the spectral components required to fit the data showninfig.2 of catalysts (a) CAT1 and (b) CAT2 doublets with fit parameters listed in Table 1. The parameters (isomer shift and quadrupole splitting) are characteristic of Fe 3+ species corresponding to FePO 4, except for a small unknown component. The presence of these two doublets is not due to the existence of Fe 3+ atoms in two distinct crystal sites but rather to a distribution of the quadrupole doublets, with the large quadrupole splitting an indication that the environment around the Fe 3+ ion is distorted Table 1. Parameters for component D1, with the smaller quadrupole splitting (QS), indicate Fe 3+ in sites of relatively low coordination (tetrahedral). The hyperfine interaction parameters for this environment are very similar to those of bulk FePO 4.The small differences in the isomer shift and quadrupole splitting for bulk FePO 4, and FePO 4 in the supported catalysts, are within the experimental uncertainty. Component D2 has higher quadrupole splitting, indicating Fe 3+ in sites of higher coordination. Hyperfine interaction parameters for this component are similar to those reported for iron in trigonal bi-pyramid coordination and [11] suggestthat interaction with the support increasesthe number of oxygen atoms in the iron coordination sphere for a fraction of the supported phase. The spectral components of CAT1 Fig. 2a show no change with temperature, reflecting the thermal stability of the catalysts. Although in situ calcination in air at 500 C resulted predominantly in the formation of the quartz phase, small contributions from the tridymite phase as well as an unknown component were also evident. The quadrupole splittings are quite similar to that of the bulk phase FePO 4, indicating that the ferric cations should be tetrahedrally coordinated with oxygen. This is supported by the observation made from in situ XRD which is

6 Venkata D. B. C. Dasireddy et al. contrary to reports in the literature that thermal treatment under air at 550 C will lead to the formation of the quartz phase only [11, 14]. The Mössbauer spectra of CAT2 Fig. 2b wereverysimilartothoseofcat1uptoa temperature of 300 C, and were also fitted with four paramagnetic doublets with parameters very similar to CAT1 (Table 1). In our studies, in situ calcination in air at 550 Cdid not lead exclusively to the formation of the quartz phase, nor did it enhance the intensity of quartz phase. Although the calcination in air lead predominantly to the formation of the quartz phase, small contributions from the tridymite phase as well as an unknown component were also evident.in contrast to the thermal stability of CAT1, the commercial catalyst CAT2 as shown in Fig 3b, showed quite significant change at T=500 C, with the tridymite phase increasing from 6(2) % at T 400 C, to 29 % of the spectral area. 4 Conclusion Powder X-ray diffractograms and Mössbauer spectroscopy data were collected for an iron phosphate catalyst synthesized using the ammonia gel method and a commercial grade FePO 4 catalyst supported on silica using wet impregnation method. Our results show that iron is present in the quartz type and the tridymite type iron phosphate phases. These phases showed little change with temperature in the catalyst synthesized using the ammonia gel method. However, the commercial grade catalyst, although thermally stable up to 300 C, showed significant change at 500 C, with the tridymite phase of FePO 4 increasing from 6 % to 29 %. The hyperfine parameters for the catalysts are similar to those reported for iron in trigonal bi-pyramid coordination, and suggest that interaction with the support increases the number of oxygen atoms in the iron coordination sphere for a fraction of the supported phase. References 1. Grasselli, R.: Top. Catal 21, (2002) 2. Campanati, M., Fornasari, G., Vaccari, A.: Catal. Today 77, (2003) 3. Spivey, J.J., Anderson, J.A., Dooley, K.M., Castaldi, M.J.: Catalysis. Royal Society of Chemistry, London (2009) 4. Suib, S.L.: New and Future Developments in Catalysis: Catalysis for Remediation and Environmental Concerns. Elsevier Science, Netherlands (2013) 5. Bullock, R.M.: Catalysis without Precious Metals. Wiley, London (2011) 6. Miller, J.E., Gonzales, M.M., Evans, L., Sault, A.G., Zhang, C., Rao, R., Whitwell, G., Maiti, A., King- Smith, D.: Appl. Catal. A 231, (2002) 7. Bonnet, P., Millet, J.M.M., Leclercq, C., Védrine, J.C.: J. Catal. 158, (1996) 8. Muneyama, E., Kunishige, A., Ohdan, K., Ai, M.: J. Catal. 158, (1996) 9. Khan, F.B., Bharuth-Ram, K., Friedrich, H.B.: Hyperfine Interact 197, (2010) 10. Ai, M., Muneyama, E., Kunishige, A., Ohdan, K.: Appl. Catal. A 109, (1994) 11. Miglierini, M., Petridis, D.: Mössbauer Spectroscopy in Materials Science. Springer, Netherlands (1999) 12. Millet, J.-M.M., Vedrine, J.C.: Appl. Catal. A 76, (1991) 13. Millet J.-M.M: Catalysis Reviews 40, 1 38 (1998) 14. Long, G.J.: Mössbauer Spectroscopy Applied to Inorganic Chemistry. Springer, Netherlands (1984)