Raman spectroscopic studies on the sulfation of cerium oxide

B ENVlRCNMENlAL Applied Catalysis B: Environmental 12 (19971 309-324

Raman spectroscopic studies on the sulfation of cerium oxide Jen Twu av*, Chung Jen Chuang a, Kuang I Chang a, Ching Hsiang Yang a, Kuei Hsien Chen b a Department ofChemistry, Chinese Culture University, Yang Ming Shari,, Taipei, Taiwan b Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan

Received 7 May 1996; revised 28 August 1996; accepted 29 August 1996

Abstract In the present study, we have examined sulfation of cerium oxide via impregnation of (NH,),SO,, followed by heating in the temperature range of 220-720°C using Raman Spectroscopy. Based on the S-O and S=O stretching frequencies in the range of 900-1400 cm-‘. a wide range of surface oxysulfur species and bulk cerium-oxy-sulfur species are identified. At 220°C a mixture of (NH,l,SO, crystals, SO&q, and HSO&,, is found to have formed on ceria’s surface, whereas complete conversion of (NH&SO, to SO&, and HSO&&, occurs at 280°C. At 35O”C, formation of a mixture of surface pyrosulfate S,O$&a, consisting of two S =O oscillators and a bulk type compound identified as Ce(IV)(SO,),(SO,),_, (0 < x < 2) have been observed. Upon introduction of moisture, the former transforms to HSO&,,,, whereas the latter remains unchanged. At 4OO”C, only the bulk type compound can be observed. At 450°C only Ce,(SO,), is generated and remains stable until 650°C. Further increase in the temperature to 720°C results in the formation of CeOSO,. The present study not only provides a more thorough understanding of the sulfation of cerium oxide at a molecular level, but also demonstrates that Raman spectroscopy is a highly effective technique to characterize sulfation of metal oxides. 0 1997 Elsevier Science B.V.

1. Introduction Cerium oxide is being widely used in Three-Way Catalyst [l] and Fluid Catalytic Cracking Catalyst [2] systems to provide oxygen storage capacity [3]

* Corresponding author. 0926.3373/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PZZ SO926-3373(96)00085-9

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and thermal stabilization [4] for the former, and for promotion of SO, oxidation [5] of the latter. In the presence of catalyst, cerium oxide is known to react with sulfur oxides over a wide range of temperature and plays a crucial role in the storage of sulfur oxides and release of H,S [6,7]. Interactions between sulfur oxides and various metal oxides have recently attracted a considerable attention due to their vital roles in emission control [8] and superacid [9]. Hence, extensive efforts have been devoted to acquire a more thorough understanding of the structures and configurations of various surface intermediates which are critical in the performance of these catalysts [lo-241. An XPS study involving the adsorption of SO, on cerium oxide, has revealed modification of the oxidation states of both cerium and sulfur and formation of Ce,(SO,), [19,20]. Other studies involving SO, uptake by supported cerium oxide have indicated the presence of intermediates involving sulfate; however, specific product compounds could not be identified by X-ray diffraction or IR spectroscopy [21]. Transformation of sulfite into sulfate on ceria surface has been proposed to be responsible for its thermal stability at temperatures higher than 500°C [22]. Other compounds such as CeOSO, [23] and Ce,O(SO,), [24] have been reported to be present in sulfated catalysts. Although the function of cerium oxide on TWC and FCC has been well understood, there is a lack of proper understanding of the structural and mechanistic chemistry between sulfur oxides and cerium oxide as compared to that of the other metal oxides such as A&O, [lo], ZrO, [11,12], SiO, [13], MgO [14], MgAl,O, [15], Fe,O, [16], TiO, [17] and MgFe,O, [18], which have been investigated extensively at a molecular level by IR spectroscopy. Raman spectroscopy, a highly effective vibrational spectroscopic technique, has only been used recently for characterization of the supported oxide catalysts [25], and to examine the interactions between sulfur oxides and metal oxides. For instance, Raman spectroscopic studies involving sulfated ZrO, [26] have assigned the bands at 1017, 1382 and 1399 cm-’ to S-O and S=O stretching modes of SO, surface species, having a ( - O),S=O structure. A similar structure has also been identified on the surface of Al,O, [27] by Raman spectroscopy by analyzing the bands at 103 1 and 138 1 cm-‘. On the other hand, Raman bands appearing at 1150 cm-’ and 1340 cm- ’ have been attributed to physisorbed SO,, whereas a very weak band at 1060 cm-’ has been attributed to surface sulfite species on Al,O, [28]. In the present study, sulfation of cerium oxide by impregnating (NH,),SO,, followed by heating in the temperature range 220-720°C has been studied by Raman spectroscopy. Raman bands of oxysulfur species, consisting of characteristic S-O and S=O stretching features, allow us to discriminate between alternate molecular structures of surface and bulk species generated by different preparations. This study provides a more thorough understanding of the sulfation of cerium oxide.

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2. Experimental Reagent grade (NH,),SO,, Ce(SO,>, and Ce,(SO,>, were obtained from * . Aldrich and CeO, (BET surface area: 2.5 m”/g) was obtained from Koch. These chemicals were used without any further purification. Sulfated Cerium Oxide @CO), the precursor, was prepared by wet impregnation of 20% (NH4j2S04 on CeO,, followed by drying at 100°C overnight. Sulfation of cerium oxide was carried out by heating SC0 in the temperature range of 220-720°C. Each sample was subjected to a heating rate of 20”C/min up to the designated temperature and then heated for 60 min under either N,, ambient or saturated moisture atmospheres in a Carbolite tube furnace (Model 4510). The Raman Spectrometer used in this study was a Renishaw system 2000 micro-Raman spectrometer. A 25mW HeNe laser, operating at 632 nm, formed the excitation source. A 5-pm exit slit offered resolution better than + 1 cm-‘. Raman spectra were recorded at ambient temperature after completion of heat treatment. Thermogravimetric analysis (TGA) was performed using a Du Pont Model 2200 at a heating rate of lO”C/min in N, atmosphere with a flow rate of 100 cc/min. Powder X-ray diffraction pattern was recorded on a Rigaku Geigerflex D/Max 2B diffractometer using Cu K (Y radiation. FTIR spectra were recorded on Nicolet 510P spectrometer at room temperature. Finally, evolved gas analysis was conducted using Cahn TG-121 equipped with a Thermolab Quadrupole Mass Spectrometer.

3. Results and discussion The results of TGA of SC0 are presented in Fig. 1. Four major weight losses appear in the temperature ranges of 220-280°C 300-350°C 360-420°C and 610-750°C. Based on the stoichiometric analysis and results published in the literature,[l3] the first two weight losses can be attributed to losses of NH,, H,O and unsupported (NH4)2S04 crystal (dec. 280°C) respectively; whereas the third and fourth losses are due to the loss of sulfur oxides. When the temperature is raised above 750°C desorption of sulfur oxides is completed, leaving only ceria. A large temperature difference between the last two losses suggests the presence of two different types of bonding structures between sulfur oxides and cerium oxide. However, due to the nature of thermal analysis, no molecular structure can be proposed at this stage. Raman studies were then conducted to gain further understanding of the results of the TG study. Fig. 2 shows the Raman spectrum of CeO,, which exhibits a strong band at 465 cm- ’ associated with the F,, mode [29]. A very weak and broad band at 1180 cm-‘, arising due to mixing of A,, + E, + Fzg modes [29], is also listed in Fig. 2. This band will be subtracted as background throughout this study, for the sake of clarity. No other band with significant intensity appears between 950

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and 1400 cm- ‘, thereby making it easy to observe S-O and S =0 stretching vibrations [30]. Fig. 3 shows Raman spectra of Ce(SO,>, crystal at room temperature, 450°C and 700°C under ambient conditions. Strong bands at 1028 and 1042 cm- ‘, a moderate band at 1051 cm- ’ and weak bands at 964, 980, 1005, 1071, 1088 and 1111 cm-’ can be observed for Ce(SO,), (Fig. 3~). At 450°C strong bands appear at 1010 and 1028 cm-‘; and weak bands at 993, 1046, 1080, 1107, 1121, 1146, 1189 and 1213 cm-’ (Fig. 3b). Whereas at 700°C a spectrum similar to that at 450°C is obtained except that the 993 cm- ’ band disappears (Fig. 3a). Thermal decomposition of Ce(SO,), and transformation to CeOSO, have been reported to occur around 200°C [19] and remain stable until 720°C [31]. This leads to the assignment of Fig. 3a to CeOSO,, which is further supported by the X-ray diffraction result [32]. In this study, the Raman spectrum of CeOSO, has been observed to begin to appear at around 450°C accompanied by the 993 cm - ’ band; and complete transformation to CeOSO, is observed to occur at around 700°C. In Fig. 3b, the band at 993 cm-i is due to the formation of a minor and transitional intermediate. This band has also been observed for other samples and is discussed later in this paper. Fig. 4 shows Raman spectra of Ce,(SO,), crystal at room temperature, 600°C

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and 700°C under ambient conditions. Strong bands at 980, 1007, 1015, 1130 and 1200 cm- ’ and weak bands at 1053, 1092 and 1154 cm- ’ can be observed for Ce,(SO,),, as shown in Fig. 4c. Temperature-dependent Raman studies of this crystal indicate that Ce,(SO,), remains stable until 600°C as can be seen from Fig. 4b. When the temperature is raised to 700°C the Raman spectrum shown in Fig. 4a is obtained. Transformation of Ce,(SO,), to CeOSO, can thus be concluded, based on the similarity between the Raman spectra in Fig. 3a and Fig. 4a. Raman spectra of the intermediates, which are produced due to thermal decomposition of Ce(SO,), and Ce,(SO,), between 350°C and 7OO”C, provide no observable evidence for interconversion between them. Normal-coordinate analysis has not been conducted so far for the various coordination geometries and molecular speciation of oxysulfur species. However, by considering characteristic features of Raman spectra as comparison standards, the products and intermediates of sulfation of cerium oxide can be identified and analyzed. Fig. 5 shows Raman spectrum of (NH,),SO, crystal at room temperature and that of SC0 samples at 220°C and 280°C under ambient conditions. Fig. 5c displays a sharp band at 975 cm-’ which arises due to symmetric S-O stretching of (NH,),SO, [33]. The spectrum recorded at 220°C (Fig. 5b) shows

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a sharp band at 975 cm-’ and a broad shoulder between 980 and 1040 cm-‘, whereas at 280°C (Fig. 5a), it displays two bands at 984 cm-’ and 1000 cm- ‘. The corresponding TGA result indicates that the release of NH, occurs between 220°C and 280°C suggesting that the broad shoulder in Fig. 5b can be assigned to an intermediate species, which is responsible for the high degree of similarity in the Ran-ran spectra of Fig. 5a and Fig. 5b in the range of 980-1040 cm- ‘. The fact that desorption of NH, has resulted in the formation of H,SO, leads to the conclusion that Fig. 5a should be assigned to H,SO,-related species. Raman spectroscopic study of (NH,),SO,-impregnated Al,O, at room temperature has been reported in the literature [27]. This study has attributed the band at 982 cm-’ to sulfate species, whereas broadening of the 982 cm- ’ band on high frequency side is attributed to another kind of sulfate species. However, no detailed structure was suggested. The Raman frequencies of H,SO,-related aqueous species such as H,SO, (1155 and 910 cm-‘) [34] SO&) (984 cm-‘) [35], and HSO&q, (1050 cm-‘) [35] are known. Previous investigations involving crystals of KHSO, [36], Cs,SRb,,,HSO, [37] and TlHSO, [38] has revealed that Raman frequencies of HSO&,, are highly sensitive to the intermolecular interactions. For instance, the band at 1010 cm-’ arises from a chain-like

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Fig. 6. Raman spectra of SC0 being heated at 350°C under (a) saturated (c) N, atmosphere.

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structure, whereas the band at 1003 cm-’ is associated with a dimeric structure [36,38]. Based on results in Fig. 5 and from literature, the band at 984 cm-’ can be assigned to SO;&, and the band at 1000 cm- ’ can be attributed to HSO&$. The assignment of the 1000 cm-’ band, although somewhat ambiguous, agrees with the stabilities of aqueous H,SO,,,,, HSO&iq, and SO&, which determine their distribution on aqueous CeO, surface. The reason that this frequency is lower than the corresponding frequency observed for free HSO&$ ion is probably due to the intermolecular interaction in dimeric configuration and basicity of cerium oxide [39]. The exact nature of such interaction, however, remains unclear. From these data, we can conclude that heating SC0 at 280°C under ambient condition results in formation of HSO&iq, and SO&,, species on the surface of cerium oxide. Fig. 6 shows the Raman spectra of SC0 at 350°C under different atmospheres. When heated under the N, atmosphere, the Raman spectrum (Fig. 6c) shows strong bands at 975, 992, 1370 and 1400 cm-‘; and weak bands at 1008 and 1111 cm-‘. When SC0 is heated under ambient condition (Fig. 6b), a spectrum similar to that in Fig. 6c, except a dramatic increase in intensity of 1008 cm- ’ band and a slight reduction in the intensities of bands at 1370 and

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Fig. 7. Raman spectrum of SC0 being heated at 400°C

1400 cm-‘, is obtained. On the other hand, when SC0 is heated under a saturated moisture, the Raman spectrum (Fig. 6a) shows a strong band at 1008 cm-‘, a moderate band at 992 cm- ’ and weak bands at 1067 and 1127 cm- ‘. It may be noted that the bands at 1370 and 1400 cm-’ have diminished drastically and only their residues can be observed. Fig. 7 shows Raman spectrum of SC0 at 400°C under ambient condition. A strong band at 993 cm- ‘, two moderate bands at 975 and 1009 cm- ’ and a weak band at 1111 cm- ’ can be observed. Several additional results are worth listing. First, after keeping the samples in vacuum at room temperature for 12 h, no significant change in the intensities of bands at 1370 and 1400 cm-’ ( sh own in Fig. 6c) can be observed. Secondly, although IR studies of sulfated ceria have reported no specific features [21], the present study shows a moderate band at 1423 cm-’ and a strong broad band at 1130 cm- ‘. Thirdly, analysis of the evolved gas indicates that gaseous SO, molecule is the major desorbed product. Fourth, a spectrum similar to that in Fig. 7 is obtained when SC0 is heated at 400°C under nitrogen atmosphere. It may be mentioned that TGA results have indicated dehydration of SC0 to occur between 300°C and 350°C. Hence, sulfation of ceria by SO, is apparently responsible for the bands appearing in Fig. 6c and Fig. 7. In general, sulfation of metal oxides is known to produce two types of products: bulk (or bulk-like) metal sulfate and a variety of surface oxysulfur species such as SO:-, SO:-, and SO,-like species [ 10,181. Among these products, surface HSO;-, S,O;sulfate species have attracted major attention in the previous investigations because of their critical roles in catalysis. IR spectroscopic studies involving

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these surface species have revealed that sulfuryl bands, due to the S=O antisymmetric stretching vibrations, can be used in distinguishing between different type of sulfates present on the surfaces [12,40-421. The bands appearing below 1400 cm- ’ have been ascribed to isolated sulfate species and the bands appearing beyond 1400 cm-’ have been ascribed to poly-sulfate species such as pyrosulfate [12,40-421. In conjunction with the reported studies on isotope exchange, as well as with those concerning sensitivities toward moisture and thermal treatment, the following three structures are identified [ 10-181. First, a ( - O),S = 0 structure consisting of a single S = 0 oscillator, as shown by structure I, is identified on surfaces of Al,O, [lo], TiO, [17], ZrO, [11,12], MgAl,O, [15]. Second, a (-Oh S g 8 structure consisting of two S =0 oscillators, as shown by structure II, is identified on the surface of Fe,O, [16] MgO [14] and SiO, [13]. Third, a O=SO,-0-O,SO=(S,O,) structure, also consisting of two S=O oscillators, as shown by structure III, and usually appearing upon a high sulfur loading,

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has been proposed to exist on the surface of Al,O, [lo] and ZrO, [11,12]. Close examination of the spectra in Fig. 6 reveals that the bands appearing at 1008, 1370 and 1400 cm- ’ get modified by the introduction of moisture. In addition, when heated at 4OO”C, the bands at 1370 and 1400 cm-’ disappear, whereas the bands at 975, 992, 1008 and 1111 cm-‘, as shown in Fig. 7, are unaffected by these two factors. Two features of these behaviors are worth noting. First, the 1008 cm-’ band, appearing in Fig. 6 and Fig. 7, behaves differently towards thermal and moisture treatment. As can be seen from Fig. 6, the intensity of this band increases markedly after the introduction of moisture at 350°C as shown in Fig. 6a and Fig. 6b. However, upon heating at 400°C (Fig. 7), the intensity of this band initially identified increases and then remains unaffected under ambient condition. Hence, this band can be interpreted differently, depending on its hydration behavior. Second, although bands at 975,993 and 1111 cm- ’ appears to be unaffected by the introduction of moisture (shown in Fig. 6b) and heating at 400°C (shown in Fig. 7), their intensities can be seen to have reduced under a

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saturated moisture (shown in Fig. 6a). This may be related to the relative enrichment of aqueous oxy sulfur species, as evidenced by the presence of the corresponding bands in the range of 1000-1200 cm-’ in Fig. 6a which affects its composition upon introduction of saturated moisture. However, its exact nature is unclear. Based on the difference between thermal and hydration behaviors exhibited by these bands, and comparison with spectra of Ce,(SO,),, Ce(SO,), and CeOSO,, formation of a mixture of surface species (1370 and 1400 cm-‘) and new bulk cerium oxysulfur species (975, 993, 1008 and 1111 cm-‘) can be concluded from Fig. 6c. Close examination of the 1370 and 1400 cm - ’ bands in Fig. 6a, Fig. 6b and Fig. 6c reveals that the intensities of these two bands diminish with the reduction in the degree of moisture at a fixed ratio. These features, in conjunction with the IR band at 1423 cm- ‘, strongly suggest that these two bands should be ascribed to antisymmetric S =0 stretching of pyrosulfate surface species, present in structure III, which has been proposed to exist on the surfaces of Al,O, [lo] and ZrO, [ 11,22,42] upon a high degree of sulfation. In contrast, a recent Raman study of the sulfated ZrO, has assigned the bands at 1017 and 1399 cm-’ to symmetric stretching of S-O and antisymmetric stretching of S =0 from ( - O&S =0 and the unresolved shoulder at 1382 cm-’ is attributed to surface heterogeneity [26]. Another Raman study of sulfated Al,O, by the same author has attributed bands at 1031 and 1381 cm-’ to the same surface structure [27]. The fact that S-O symmetric stretching of pyrosulfate is not accounted for in the present study is due to the assignment of bulk type species for bands in the range of 970- 1010 cm- ‘. Other species such as SO,-like species have also been proposed to account for structure III on Al,O, surface [lo]. Since vacuum treatment does not change the intensities of 1370 and 1400 cm-i bands, physisorption appears unlikely. Chemisorption of gaseous SO, molecule would affect S-O stretching frequencies drastically, which has not been observed either [43]. However, the possibility of existence of other species cannot be ruled out since Raman study of sulfation of metal oxides is only beginning to be more thoroughly understood. The spectra in Fig. 6 obviously reveal that intensity of the 1008 cm-’ band increases at the expense of 1370 and 1400 cm- ’ bands, as a function of water content. Since vibrational frequencies of HSOi- species appear between 1002 and 1050 cm-’ (depending on the mutual interaction discussed earlier), the 1008 cm-’ bands appearing in Fig. 6a and Fig. 6b may be assigned to HSO&“,,,species formed by the reaction of surface pyrosulfate with moisture (Fig. 6a and Fig. 6b) Formation of HSO&rf., by intentionally introducing moisture has been observed by IR spectroscopy on various supportssuch as TiO, [44,46] and ZrO, [45]. The fact that the vibrational frequency of these species is lower than those observed for a free ion (1050 cm- ‘) and close to that of the chain - like (1010 cm- ‘) configuration suggests that anchoring to the surface, instead of chainlike configuration, of cerium oxide may be responsible. In contrast, SC0 results in formation of HSO&$ at 280°C and displays a band

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at 1000 cm-’ which is assigned to dimeric species. Two weak bands at 1067 and 1127 cm-’ can also be observed in Fig. 6a. Formation of a variety of aqueous species such as SZO&, [48] and S,O&,, [49] (both exhibiting band at 1067 cm- ‘> and their related species probably get influenced by temperature, moisture and the presence of ceria. However, the exact assignments of these two bands and the nature of these oxysulfur species cannot be precisely determined partially due to the weak and broad features of these bands and complexity due to overlapping of symmetric and asymmetric S-O stretching vibrations in this region. Results of heating SC0 at 350°C and 400°C indicate that the bands at 975, 993, 1008 and 1111 cm-’ are unaffected by the introduction of moisture and increase of temperature from 350°C to 400°C as can be seen from Fig. 7. Based on this behavior, they may be attributed to bulk-type cerium-oxy-sulfur species rather than surface oxysulfur species generated at 280°C and 350°C. By comparing with the Raman spectra of Ce(SO,),, Ce,(SO,), and CeOSO,, we can conclude that none of these cerium sulfate compounds are related to the bands in Fig. 7. Formation of sulfite ions on the surfaces of various metal oxides [16,47] including ceria, brought about by adsorption of SO, or SO, molecules and via impregnation of (NH,),SO,, has been confirmed and is already discussed in Section 1 [19,20,22]. More importantly, the reducing power of sulfite species [22] plays a critical role in forming cerium sulfite Ce,(SO,),, observed in the temperature range of 420-610°C. This is discussed later in this paper. A Raman study [28] of SO, adsorption on Al,O, has attributed very weak band at 1060 cm- ’ to surface sulfite species which is not observed in the present study [28]. In addition, characteristic bands of the sulfite-related species appearing at 965 cm-’ (free SO& ,>, 1050 cm-’ (HSO&,> and 1023 cm-’ (HOSOG,,,) are not observed as well 9481. The inability to detect surface sulfite species leads to the conclusion that sulfite species may exist either in bulk or bulk-like form of cerium-sulfite or sulfite-related compositions. The Raman spectra of a number of metal oxy sulfur compounds are available from published literature [33,49,51]. From these results, characteristic features for identification of metal sulfite compounds are known to appear in the range of 950-990 cm- ‘, due to the symmetric S-O stretching vibrations [33,50]. In contrast, Raman spectra of metal sulfate compounds exhibit bands between 965-1035 cm-’ and 1080-l 180 cm-’ due to symmetric and asymmetric S-O stretching vibrations [33,49]; whereas the Raman spectra of metal disulfite [51] and metal disulfate [49] compounds exhibit characteristic S-O stretching frequencies between 1050 and 1100 cm-‘. Ce(III),(S0,),(S03) [30], which was synthesized for comparison, exhibits Raman bands at 940,980, 1003 and 1030 cm- ’ and can be excluded as well. However, a composition consisting of both sulfite as well as sulfate is highly probable for the spectrum in Fig. 7 since all of the compounds such as metal-sulfate, -sulfite, -disulfite and -disulfate exhibit significant disagreement with Fig. 7. This leads us to ascribe the composition of

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Ce(IV>(SO,>,(SO,),_, (0 , has also been proposed. Characterization of SO, uptake on supported cerium oxide by XRD and IR suggests the presence of highly dispersed, amorphous sulfate; however, no specific structure has been identified. From the weight gain observed during the TGA, the formation of Ce,(SO,), is proposed. In contrast, comparison of Raman spectra furnishes an unambiguous confirmation of Ce,(SO,), in the present study. Raman spectra of samples being heated between 450°C and 600°C exhibit the same pattern as that of Ce,(SO,),. This results correlates well with the TGA result, in which no weight loss appears to occur between 420°C and 610°C as can be seen in Fig. 1 and the thermal behavior of Ce,(SO,>, crystal shown in Fig. 4. The presence of unresolved features at 1007

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and 1015 cm-’ bands is probably due to broadening caused by dispersion of Ce,(SO,), on the host cerium oxide. A similar effect has been observed for BaCO, dispersed on TiO, [52]. In the absence of reducing agents, reduction of CeO, occurs around 800°C under 0, atmosphere [19]. Confirmation of the structure supports the composition of of Ce,(SO,), strongly Ce(IV>(SO,>,(SO,>,_, at 400°C since the presence of SO;- species is critical for cerium’s reduction when the temperature is raised from 400°C to 450°C. When heated to 720°C strong bands at 1010 and 1028 cm-’ and weak bands at 993, 1046, 1080, 1107, 1121, 1146, 1189 and 1213 cm-’ can be observed (Fig. 9). Except for the weak band at 993 cm- ‘, this spectrum correlates well with the spectra of CeOSO, which are shown in Fig. 3a and Fig. 4a obtained either by decomposition of Ce(SO,), or Ce,(SO,),. The TGA data in Fig. 1 reveals that a major weight loss takes place in the temperature range of 610-750°C. This leads us to conclude that CeOSO, is an intermediate formed in the transition of Ce,(SO,), to CeO,. In contrast, the formation and decomposition of CeOSO, were proposed to occur in the temperature range of 575-727°C and 727-927°C in the studies concerning splitting of SO, with CeO, [31]. Fig. 3b and Fig. 9 suggest that the band at 993 cm- ’ can be ascribed to a minor transitional species, accompanied with CeOSO,, which is produced from the thermal decomposition of Ce(SO,), at 450°C and ceria-supported Ce,(SO,), at 720°C. However, this band does not appear in Fig. 4a for CeOSO, which is produced from the decomposition of Ce,(SO,),. Since only the 993 cm-’ band can be observed for this intermediate, determination of its structure cannot be accomplished at present and further investigations are currently being carried out.

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4. Conclusion Raman spectroscopy has been applied to study the structures of intermediates and product compounds, generated by thermal treatment of cerium-oxy-sulfur compounds and sulfated cerium oxide between 220°C and 720°C. Based on the S-O and S =0 stretching frequencies, evolution of surface oxysulfur species and bulk cerium-oxy-sulfur species such as HSOGa P SO&) and SzO&f) such as Ce(IV)&O,),(SO,),_, (0
Acknowledgements One of the authors (J.T.) would like to thank the National Science Council of the ROC for financial support under contract No. NSC85-2113-M-034-004.

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