The Nature of the Chromium Species Formed during the Thermal

Chromium-Promoted Tin(IV) Oxide Catalysts: An EPR and XPS Study ... the impregnation of SnO2 using aqueous CrO3 or chromium(III) nitrate solutions and...
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10672

J. Phys. Chem. B 1998, 102, 10672-10679

The Nature of the Chromium Species Formed during the Thermal Activation of Chromium-Promoted Tin(IV) Oxide Catalysts: An EPR and XPS Study Philip G. Harrison,* Nicholas C. Lloyd, and Wayne Daniell Department of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K. ReceiVed: May 12, 1998; In Final Form: September 25, 1998

Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the nature of chromium-promoted tin(IV) oxide catalyst materials prepared via three routes, by the impregnation of SnO2 using aqueous CrO3 or chromium(III) nitrate solutions and by coprecipitation from aqueous solutions containing both Sn(IV) and Cr(III) ions. The catalyst materials exhibit three different chromium signals (δ, γ, and β) in the EPR, depending on the treatment history of the catalysts. The δ-signal results from dispersed Cr3+ ions and appears in the spectra of the Cr(III)-promoted SnO2 catalyst at all temperatures and also in the spectra for the Cr(VI)-promoted catalyst after calcination. The β-signal detected for the uncalcined Cr(III)-promoted catalyst obtained by coprecipitation is most probably due to the hydrated γ-CrOOH phase but to Cr2O3 at higher calcination temperatures. Both Cr(III)- and Cr(VI)-promoted catalyst materials exhibit the γ-signal after calcination at temperatures of g573K attributed to mixed-valence trimers of the type “Cr(VI)-O-Cr(III)-O-Cr(VI)”, possibly indicating formation of the mixed-valence chromium oxide Cr5O12. Photoreduction of Cr6+ to Cr5+ by the X-ray flux occurs during collection of Cr 2p XPS data for Cr6+ compounds. XPS for Cr(VI)-promoted tin(IV) oxide catalysts dried at 333 K are consistent with surface-adsorbed Cr6+ species (monochromate, dichromate). Cr6+ is still present after calcination at 573 K together with Cr3+, corroborating the formation of the mixed-valence chromium phase Cr5O12. For the material calcined at 673 K, the spectra indicate the presence of some Cr5O12 together with amorphous Cr2O3. Cr 2p XPS spectra for both Cr(III)-promoted catalyst materials dried at 333 K exhibit peaks corresponding to Cr3+ most probably as γ-CrOOH. However, after calcination at 573 K, both types of material exhibit spectra consistent with the formation of some Cr6+, consistent with the presence of the mixed-valence oxide Cr5O12. For all three types of catalyst, Cr2O3 is the sole chromium oxide species present after calcination at calcination temperatures.

Introduction The control of noxious emissions resulting from either the combustion of fossil fuels or from other industrial activities is one of the most immediate and compelling problems faced by nearly every country in the world. The levels of pollutants from automobiles, carbon monoxide (CO), hydrocarbons (HC’s), and nitrogen oxides (NOx), are the subject of ever increasingly stringent legislation controlling the maximum permitted levels of emissions of each substance. Platinum group catalysts currently represent the state-of-the-art in internal combustion engine emission technology. Data from our laboratory1 have demonstrated that Cr/SnO2 and Cu/Cr/SnO2 catalysts exhibit three-way activity which is comparable to conventional noble metal catalysts. These data show that the performance of these catalysts is similar to the Pt/Rh/Al2O3 catalyst for CO and HC oxidation. Despite this very promising observed activity, however, the constitution of these catalyst materials is essentially unknown. Even simple questions such as the oxidation state of chromium in the active catalyst remains unanswered. We have reported the preparation of Cr(VI)/SnO2 catalyst materials by impregnation of tin(IV) oxide using aqueous CrO3 solutions,1 while Solymosi2 has described a SnO2/Cr2O3 catalyst prepared by adding Cr2O3 to a suspension of SnO2 and stirring until the material was homogeneous. The resultant catalyst material was described as containing higher valence chromium ions stabilized

in the surface layer of SnO2 although no characterization data were reported. Thermal pretreatment is a crucial stage in the activation of the this type of catalyst to give optimum performance, and the actual nature of the catalyst material after thermal activation is obviously quite different from that of the freshly prepared material. Additional thermal treatment at higher temperatures usually results in some deactivation, hence indicating that further changes have taken place in the catalyst. Central to an understanding of the behavior of these catalysts, therefore, is a knowledge of the transformations involving the promoting chromium. To overcome the lack of information in this area, here we report an electron paramagnetic resonance and X-ray photoelectron spectroscopy study of Cr/SnO2 catalyst materials prepared via three routes: the impregnation of SnO2 using aqueous CrO3 solutions, the impregnation of SnO2 using aqueous chromium(III) nitrate solutions, and coprecipitation from aqueous solutions containing both tin(IV) and chromium(III) ions. Experimental Section Preparation of Catalyst Materials. (a) Tin(IV) Oxide Gel. Tin(IV) oxide gel was precipitated by the dropwise addition of concentrated 33% (w/w) AnalaR aqueous ammonia solution to a cold, vigorously stirred solution of 0.1 M (13 g, 33.69 mL by volume) tin(IV) chloride (Aldrich) in triply distilled water (ca. 500 mL) to a final pH of 4, to give an average particle size of

10.1021/jp9822135 CCC: $15.00 © 1998 American Chemical Society Published on Web 12/05/1998

Cr Species Formed by Activation of SnO2 Catalysts 2.0 nm (transmission electron microscopy (TEM)). The gelatinous precipitate formed was washed free of chloride ion by repeated centrifuging and redispersing in triply distilled water. This was confirmed by a simple negative chloride ion test using silver nitrate solution. The solid, white gel obtained was then allowed to air-dry at 333 K for 2-3 days. At this stage the gel was of a granular appearance and approximately one-tenth of its initial volume. These large granules were then broken down by pouring a small amount of triply distilled water over them and then allowed to air-dry further at 333 K for 24 h. The dry, colorless gel was then ground into a fine white powder (BET surface area 180 m2 g-1). (b) Chromium(VI)-Promoted Tin(IV) Oxide by Impregnation. To a suspension of tin(IV) oxide (10 g) was added chromic acid (160 mL), a solution of chromium(VI) oxide (CrO3, Aldrich) dissolved in triply distilled water. The mixture was stirred for 18 h under gentle reflux conditions after which the resultant yellowish suspension was filtered to yield a yellow powder which was dried in air at 333 K for 24 h. Therefore, it can be concluded that targeting a specific loading for chromium(VI) depends on various criteria including the initial chromium(VI) solution concentration, the exposure time, the reflux temperature, and the washing conditions. Samples prepared by this route are referred to as Cr(VI)/SnO2-imp with the Cr:Sn atom ratio denoted in parentheses. (c) Chromium(III)-Promoted Tin(IV) Oxide by Impregnation. To a suspension of tin(IV) oxide gel (10 g) was added an aqueous solution of Cr(NO3)3‚9H2O (Aldrich) dissolved in triply distilled water. These mixtures were kept stirred for 18 h, and the resultant suspension was filtered to yield a pale green gel, which was then air-dried at 333 K for 24 h. The Cr:Sn atomic ratio in the catalyst could be controlled by modifying the initial solution concentration of the chromium(III) nitrate solution, and as with chromium(VI) targeting a specific loading was reproducible under the same preparative conditions. This material is referred to as Cr(III)/SnO2-imp with the Cr:Sn atom ratio denoted in parentheses. (d) Chromium(III)-Promoted Tin(IV) Oxide by Coprecipitation. This was achieved by a modification of the precipitation method above. Cr(NO3)3‚9H2O was allowed to dissolve and homogenize in the vigorously stirred solution of tin(IV) chloride prior to addition of base (concentrated 33% (w/w) AnalaR aqueous ammonia solution). The subsequent workup was as above, affording a dark green gel. The Cr:Sn atom ratio was varied by varying the Cr:Sn ratio in the initial aqueous solution prior to addition of base. Materials prepared by this route are referred to as Cr(III)/SnO2-cop with the Cr:Sn atom ratio denoted in parentheses. The Cr:Sn ratios in the materials studied are representative of the loadings in active catalyst materials. Physical and Spectroscopic Measurements. Catalyst stoichiometries were determined by atomic absorption (for Cr(III)/ SnO2-imp and Cr(VI)/SnO2-imp materials) and X-ray fluorescence (Philips PW1480 wavelength dispersive instrument) (for Cr(III)/SnO2-cop materials). Before examination catalyst materials (approximately 2 g aliquots) were calcined in an alumina boat for 24 h using a Vecstar 91e tube furnace at temperatures of 573, 673, 873, 1073, and 1273 K. Electron paramagnetic resonance (EPR) measurements were recorded at room temperature using a Bruker EMX EPR spectrometer equipped with a Bruker ER 041 XG Microwave bridge in X-band at ca. 9.2 GHz. Experimental g values were determined with reference to diphenyl picryl hydrazyl (DPPH) (g ) 2.0036).

J. Phys. Chem. B, Vol. 102, No. 52, 1998 10673 X-ray photoelectron spectroscopy (XPS) measurements were performed at the Department of Materials Engineering and Materials Design, University of Nottingham, using a VG Ionex spectrometer interfaced with the necessary data handling software. For both material types spectra were recorded under ultrahigh vacuum conditions (10-8 Torr), using Al KR primary radiation (13 kV, 20 mA). Data were collected in the sequence of a survey scan (to determine the C 1s reference), followed by scans of the Cr 2p, O 1s, and Sn 3d regions to minimize the time of exposure to X-rays. As the Cr(VI)/SnO2 samples were dilute, multiple scans (40) of the Cr 2p region were necessary to facilitate spectra of sufficient intensity to enable sensible deconvolution with the data handling software. Each sample was subjected to an X-ray exposure time of ca. 45 min, which resulted in an approximate photoreduction of 52% of the Cr6+ species. After acquisition of spectra, various data handling procedures were carried out on the raw data. To establish peak areas, the spectrum was smoothed and then subjected to a Shirley background subtraction before a normalized peak area could be generated. The peaks during manifold deconvolution were assumed to be Gaussian, and the full width at halfmaximum (fwhm) values assigned to them were constrained to be close to the values reported in the literature.3 This was performed manually before an iterative routine was employed, thus reducing errors by variation of peak attributes (height, width, and position). To compensate for charging effects, which were observed as much as up to +18 eV for the Cr(VI)- and Cr(III)/SnO2 systems, binding energies were normalized with respect to the position of the C 1s signal determined from of a survey spectrum and held constant at 285.0 eV.4 Acquisition parameters for the coprecipitated materials were as for the Cr(VI)/SnO2 system, although 25 scans were sufficient for the Cr 2p doublet region to obtain data to facilitate analysis due to the high concentration of chromium present. Results and Discussion EPR Spectra. EPR spectra of Cr(VI)/SnO2-imp (Cr:Sn, 0.12) and Cr(III)/SnO2-cop (Cr:Sn, 0.32), prior to calcination and after calcination at 573, 873, 1073, and 1273 K, are shown in Figures 1 and 2, respectively. EPR parameters for the various chromium species observed are given in Table 1. Studies by Weckhuysen et al.5,6 on Cr(VI)/SiO2 and Cr(VI)/ Al2O3 catalysts have shown that three different chromium signals are observable in the EPR, depending on the treatment history of the catalysts. These are referred to as the δ-, γ-, and β-signals. The δ- and β-signals are broad and occur in the geff ranges ca. 5 and 1.96-2.45, respectively. The γ-signal is much sharper and occurs at g ) 1.97. The δ-signal results from dispersed Cr3+ ions and appears in the spectra of the Cr(III)/ SnO2-cop material (Figure 2) for the material dried at 333 K and at each calcination temperature. This signal is also observable in the spectra for the Cr(VI)/SnO2-imp material (Figure 1) after calcination, but not of course prior to calcination as Cr(VI) is EPR silent. The intensity of this signal generally increases with chromium loading and calcination temperature. The β-signal is only visible after reduction of chromium catalysts and is assigned to clusters of Cr3+ (such as Cr2O3) in the range of geff ) 1.96-2.45. In the present case, the β-signal detected for uncalcined Cr(III)/SnO2-cop (Table 1) material is most probably due to the hydrated γ-CrOOH phase similar to that observed on Cr(III)/SiO2 catalysts7 rather than Cr2O3, which is present at higher calcination tempereatures. The β-signal is strongly dependent on the size and shape of these clusters, as determined by the broad range of g values.

10674 J. Phys. Chem. B, Vol. 102, No. 52, 1998

Figure 1. EPR spectra for Cr(VI)/SnO2-imp (Cr:Sn, 0.12) after drying at 333 K (a) and after calcination at 573 (b), 873 (c), 1073 (d), and 1273 K (e).

Figure 2. EPR spectra for Cr(III)/SnO2-cop (Cr:Sn 0.32) after drying at 333 K (a) and after calcination at 573 (b), 873 (c), 1073 (d), and 1273 K (e).

The third resonance, centered at g⊥ ) 1.974, is referred to as the γ-signal. Both Cr(III)/SnO2-cop and Cr(VI)/SnO2-imp materials, calcined at temperatures of g573 K, exhibit this resonance at g ) 1.974, which decreases in intensity with increasing calcination temperature. In most cases, this narrow axially symmetric line is assigned to single Cr(V) species (3d1, S ) 1/2) of different coordination sphere (square pyramidal or distorted tetrahedral) on the surface of the corresponding oxide.8-12 Because of different relaxation times expected for square-pyramidal and tetrahedral coordination, the γ-signal observed at room temperature is generally assigned to Cr(V) in

Harrison et al. square-pyramidal coordination. A detailed spectroscopic investigation was reported for the system CrOx/SiO2 by Van Reijen and Cossee.8 However, the γ-signal has also been attributed in CrOx/Al2O3 10,13,14 catalyst materials to mixedvalence trimers of the type “Cr(VI)-O-Cr(III)-O-Cr(VI)” containing both chromium(III) and chromium(VI) in which the average oxidation state of the chromium is +5. A similar model has been proposed for CrOx/TiO2 (rutile) materials.15,16 In the present study, since the preparative methodology employed would not support the formation of an inherently unstable Cr(V) surface species, we propose that the mixed-valence trinuclear cluster pseudo Cr5+ species is responsible for the γ-signal at g ) 1.97. For the Cr(III)/SnO2-cop catalyst material the g resonance at 1.974 is lost after calcination at 873 K. However, for the Cr(VI)/SnO2-imp material this resonance is still present after calcination at 1273 K, suggesting the persistance of trinuclear species even at elevated temperatures. XPS Studies. Cr 2p, Sn 3d, and O 1s photoelectron spectra have been obtained on Cr(VI)/SnO2-imp and both impregnated and coprecipitated Cr(III)/SnO2 materials, dried at 333 K as well as after calcination at temperatures of 575, 673, 873, 1073, and 1273 K. Typical binding energies of the Cr 2p level for chromium compounds3 are listed in Table 2. The XPS spectra of the Cr 2p level for Cr3+ compounds have weak satellites a few electronvolts greater than the principal peak position, and the satellite splitting (∆) for the Cr 2p level for Cr2O3 is shown in Table 3. In contrast, the Cr6+ compounds give no such satellite peak. The spin-orbit (∆Eso) splittings of Cr3+ compounds are larger than those for Cr6+ compounds and are explained in terms of the exchange interaction between 2p electrons and unpaired 3d electrons.17,18 The spin-orbit intensity of the Cr 2p doublet (Cr 2p1/2/2p3/2) has a theoretical value of 0.5. For the measurements made in this study the intensity ratio (Cr 2p1/2/2p3/2) varied dramatically with each calcined sample, and values were quite often lower than the predicted value of 0.5. These discrepancies may be attributed to solid effects. In the case of Cr3+ compounds the satellite peak of Cr 2p3/2 overlaps with the peak of the Cr 2p1/2 manifold. From these observations, chromium in the 3+ and 6+ oxidation states may be distinguished in terms of binding energy, the satellite structure, the spin-orbit intensity ratio, and the spin-orbit splitting. However, in the studies of supported catalysts, the spectra obtained are usually weak, especially when the concentration of the catalytic component is low. Moreover, the information about the satellite is sometimes ambiguous for supported transition metals, especially for supported chromium compounds. Further, the measurements of spin-orbit intensity ratios are considered to be difficult due to uncertainties in the estimation of the base line. Thus information about the binding energy and the spin-orbit splitting is used in order to distinguish the valence states of chromium supported on the carrier. Qualitative analysis of the Cr 2p level for Cr6+ compounds is inherently difficult, as Cr6+ species undergo photoreduction by the X-ray flux during data collection, causing the intensity of the peak due to Cr 2p of Cr6+ to decrease with exposure time. Concurrently, a new Cr 2p peak, having a lower binding energy value, appears with increasing intensity. Figure 3 shows the effect on the Cr 2p doublet for Cr(VI)/SnO2-imp (Cr:Sn, 0.12) dried at 333 K after exposure for various periods in the X-ray beam. The deconvoluted peak at higher binding energy (ca. 579.7 eV) in the Cr 2p3/2 manifold due to Cr6+ decreases in intensity with respect to increased exposure. The reduction of high-valence metal oxides has been documented in the literature from both experimental and theoretical

Cr Species Formed by Activation of SnO2 Catalysts

J. Phys. Chem. B, Vol. 102, No. 52, 1998 10675

TABLE 1: g Values for Cr(VI)- and Cr(III)-Promoted Tin(IV) Oxide at Various Calcination Temperatures catalyst material

calcination temp/K

chromium species

gx

Cr(III)/SnO2-cop (Cr:Sn, 0.32)

333 573

Cr3+ Cr3+ Cr5+ Cr3+ Cr5+ Cr3+ Cr3+ Cr3+ Cr6+ Cr3+ Cr5+ Cr3+ Cr5+ Cr3+ Cr5+ Cr3+ Cr5+

5.269 δ 5.269 δ

4.743 δ

5.269 δ

4.743 δ

5.124 δ 5.309 δ

4.844 δ 5.067 δ

4.724 δ 4.811 δ

5.272 δ

4.958 δ

-

2.367 β -

5.246 δ

4.939 δ

4.814 δ

2.301 β

5.251 δ

4.946 δ

4.746 δ

2.301 β

5.251 δ

4.946 δ

4.746 δ

2.301 β

873 1073 1273 Cr(VI)/SnO2-imp (Cr:Sn, 0.12)

333 573 873 1073 1273

gy

gz

g|

g⊥

2.375 β

1.986 β 1.974 γ 1.980 β 1.974 γ 1.962 β 1.980 β 1.981 γ 1.974 γ 2.202 β 1.974 γ 2.202 β 1.974 γ

TABLE 2: Typical Binding Energies of Cr 2p3/2 and Cr 2p1/2, Spin-Orbit Splitting of Cr 2p Level (∆Eso), Satellite Splitting of the Cr 2p Level (∆E), and Full Width at Half-Maximum (fwhm) of Cr 2p3/2 Manifold binding energy/eV compound Cr(VI) CrO3 Cr(VI)/Al2O3a Cr(V) Cr(III) Cr2O3 a

∆Eso/eV ∆/eV fwhm/eV

Cr 2p1/2

Cr 2p3/2

580.3 579.7 577.5

589.4 588.7 585

9.1 9.0 9.0

576.6

586.4

9.8

3.8-4.2 3.8 10.9

3.6

From ref 2.

TABLE 3: Parameters Obtained by Deconvolution of the Cr 2p Doublet in the XPS Spectra for the Cr(VI)/SnO2-imp (Cr:Sn, 0.12) Catalyst Material pretreatment temp/K 333 573 673 873 1073 1273 a

Binding energy/eVa 2p3/2 2p1/2 577.9 (3.5) 579.7 (4.1) 577.0 (3.5) 579.8 (4.1) 576.4 (3.4) 579.3 (3.8) 576.6 (3.6) 579.3 (4.1) 576.9 (3.6) 579.9 (3.8) 576.9 (3.6) 579.9 (3.8)

586.9 (3.6) 588.7 (4.1) 586.4 (3.5) 588.9 (4.1) 586.0 (3.5) 589.7 (3.8) 586.4 (3.6) 590.0 (4.1) 586.7 (3.6) 590.7 (3.8) 586.7 (3.6) 590.6 (3.8)

∆Eso/ 2p3/2 eV area/counts 9.0 9.0 9.4 9.1 9.6 10.4 9.8 10.7 9.8 10.8 9.8 10.7

1741 2052 3180 3375 4906 7085

assigm Cr5+ Cr6+ Cr5+/Cr3+ Cr6+ Cr5+/Cr3+ Cr6+/Cr3+ Cr3+ Cr3+ satd Cr3+ Cr3+ satd Cr3+ Cr3+ satd

Fwhm in parentheses.

points of view.19,20 Various researchers have shown maximal valence oxides such as WO3, TiO2, and V2O5 to be highly susceptible to reduction during XPS analysis, and Knotek and Friedman21 have suggested that this susceptibility is related to bond ionicity. These oxides, whose anion-cation components differ in Pauling electronegativity by more than 1.7 eV (and hence have a strong ionic character in their bonding), were cited as being particularly vulnerable to reduction. CrO3 for example, has a difference of 1.9 eV and is therefore very prone to photoreduction. The cross-section for damage by X-radiation outlined by Cazaux19 is given by

Q ) aσ(C) + bσ(VB) + cσ(SE) + dσ(brems) where a-d are weighting factors. The first term on the right

Figure 3. Smoothed and deconvoluted XPS spectra of the Cr 2p doublet for Cr(VI)/SnO2-imp (Cr:Sn, 0.12) after drying at 333 K, illustrating the effect of exposure to the X-ray flux after (a) 45, (b) 80, (c) 115, and (d) 160 min.

refers to damage resulting from the mechanism of Coulombic explosion. The second refers to damage resulting from the direct ejection of photoelectrons from the valence band, and the third refers to damage resulting from the production of secondary electrons in the materials. The final term refers to damage caused by bremmstrahlung radiation, which according to Wagner20 is responsible for less than 3% of photoreduction by X-rays. It is believed that the dominant mechanism leading to reduction is Coulombic explosion. Here the ejection of a core

10676 J. Phys. Chem. B, Vol. 102, No. 52, 1998

Figure 4. Smoothed and deconvoluted XPS spectra of (a) the Cr 2p doublet, (b) the Sn 3d doublet, and (c) the O 1s peak for Cr(VI)/SnO2imp (Cr:Sn, 0.12) after calcination at 573 K. The lower traces represent the difference between observed and calculated spectra.

photoelectron results in an Auger process in the valence band of O2- in which a temporary positive charge could be imparted to the oxygen ion. Such an ion may be desorbed from the surface or move into a different molecular configuration. The ionization of even deeper core levels could result in Auger or vacancy “cascades”, which may then result in the subsequent removal of many bonding electrons. Such an unstable situation may lead to some degree of atomic rearrangement and changes in the chemical nature of the compound under study. In maximal valence compounds, such as Cr6+ which has no 3d electrons to fill deeper core holes, this effect may lead to changes in the oxidation state. Owing to this photoreduction effect, binding energy and spin-orbit splitting are used in order to characterize the Cr(III)/SnO2 and Cr(VI)/SnO2 materials. Chromium(VI)-Promoted Tin(IV) Oxide. Although measurements were also performed for all three Cr:Sn ratios of Cr(VI)/SnO2-imp materials (Cr:Sn, 0.12, 0.23, and 0.38), no significant difference in the data was observed with different chromium loadings, suggesting that the solid-state behavior of these materials remains uniform irrespective of composition. Data described here refer solely to the Cr(VI)/SnO2-imp (Cr: Sn, 0.12) catalyst, and binding energy and spin-orbit splitting data of the Cr 2p region for this material are shown in Table 3, with a typical spectrum illustrated in Figure 4a. The Cr 2p doublet for the catalyst material dried at 333 K can be resolved into four individual peaks, both with ∆Eso values of 9.0 eV. One pair (Cr 2p3/2/2p1/2) is assigned to the surfaceadsorbed Cr6+ species (monochromate, dichromate) unaffected by photoreduction in the X-ray flux. The other pair possesses a spin-orbit splitting of 9.0 eV with the peak in the Cr 2p3/2 manifold centered at a binding energy of 577.9 eV. These

Harrison et al. parameters cannot be attributed to a Cr3+ species, which has a larger spin-orbit splitting value (9.8 eV), as shown in Table 2. Studies by Okamoto et al.3 on chromia-alumina catalysts have assigned this species to chromium possessing a valence state of +5 having one unpaired 3d electron. Chemical shift data are generally not useful for determining the valence state of chromium, especially for materials that experience high charging effects. Therefore the ∆Eso values are considered to be more powerful and reliable in discriminating the valence state of chromium. The valence state of chromium obtained by photoreduction under ultrahigh vacuum conditions of Cr6+ species is concluded to be Cr5+ with binding energies which are in good agreement with literature values (Table 2) and on chromia-titania systems22,23 (577.7 and 577.4 eV, Cr 2p3/2). Further studies by Okamoto et al.3 have shown that, for catalysts with a high chromium concentration, the photoreduced species are a combination of Cr3+ and Cr5+ species. These spectra containing both Cr3+ and Cr5+ are deduced to have spin-orbit splittings of an intermediate value between 9.0 and 9.8 eV, where the spectra of Cr3+ and Cr5+, whose binding energies are almost the same, overlap each other. However, for the Cr(VI)/SnO2 catalysts with higher ratios (Cr:Sn, 0.23 and 0.38), no Cr3+ species were observed to form by photoreduction. It should be stressed that the formation of Cr5+ species is merely an undesirable feature in the XPS spectra of chromium(VI) compounds as a direct result of photoreduction in the X-ray flux and do not exist as such in the chromium-promoted tin(IV) oxide catalyst materials themselves. After calcination at 573 K, Cr6+ is still observed in the Cr(VI)/SnO2-imp (Cr:Sn, 0.12) material, unaffected by photoreduction. However, the spin-orbit splitting between the peaks at 577.0 and 586.4 eV has a value of 9.4 eV, intermediate between 9.0 and 9.8 eV. For the reasons described above, these deconvoluted peaks are a combination of Cr5+ (photoreduced from Cr6+) and Cr3+, consistent with either separate chromium(III) and chromium(VI) components or a single mixed-valence chromium phase such as Cr5O12.24 For the material calcined at 673 K, four peaks are again resolvable within the Cr 2p doublet. The peaks assigned to Cr5+/Cr3+ (Table 3) after calcination at this temperature are due to the presence of amorphous Cr2O3, some Cr5O12 (Cr3+) character, and Cr5+ photoreduced from Cr6+. This can be rationalized by the spin-orbit splitting of 9.6 eV, arising from the fact that Cr5+ and Cr3+ species lie so close in energy that they cannot be resolved separately. Further evidence for the existence of Cr2O3 at this temperature can be determined from the increased spin-orbit split of 10.4 eV, between binding energy values of 579.3 and 589.7 eV. This increased spinorbit splitting is attributed to a Cr3+ satellite peak at higher binding energy from the principal peak (576.4 eV), although the expected value of 10.9 eV (Table 2) is not observed due to the unresolvable overlap between this satellite and the very weak unreduced Cr6+ peak. At calcination temperatures of g873K, the fit parameters (Table 2) are all in accordance with Cr2O3 being the sole chromium oxide species present in the Cr(VI)/ SnO2-imp system. Peak intensities in XPS are directly related to surface concentration with the observed Cr 2p intensities related to the concentration of chromium on the surface. It has been shown for chromia-silica systems4,25 that the crystallite size of the thermally produced chromia phase increases with temperature in the range 773-973 K. Large platelets of Cr2O3 form around 773-873 K and cover the support surface. Hence the surface concentration of chromium is greater than that of the initially prepared sample. This is reflected by the increase in area of

Cr Species Formed by Activation of SnO2 Catalysts TABLE 4: Parameters Obtained by Deconvolution of the Sn 3d Doublet in the XPS Spectra for the Cr(VI)/SnO2-imp (Cr:Sn, 0.12) Catalyst Materiala pretreatment temp/K 333 573 673 873 1073 1273 a

binding energya/eV 3d5/2 3d3/2 486.3 (2.1) 486.0 (2.1) 486.0 (2.2) 486.1 (2.3) 486.1 (2.4) 486.4 (2.3)

J. Phys. Chem. B, Vol. 102, No. 52, 1998 10677 TABLE 6: Parameters Obtained by Deconvolution of the Cr 2p Doublet in the XPS Spectra for the Cr(III)/SnO2-imp (Cr:Sn, 0.38) Catalyst Material

Sn 3d5/2 area/counts

pretreatment temp/K

7743 7711 15928 12468 6452 6256

333

494.7 (1.9) 494.5 (2.0) 494.3 (2.3) 494.3 (2.2) 494.5 (2.1) 494.2 (2.1)

573 673 873

Fwhm in parentheses. 1073

TABLE 5: Parameters Obtained by Deconvolution of the O 1s Peak in the XPS Spectra for the Cr(VI)/SnO2-imp (Cr:Sn, 0.12) Catalyst Material pretreatment temp/K 333 573 673 873 1073 1273 a

O 1s binding energya/eV 528.2 (2.2) 528.7 (2.0) 529.0 (2.2) 529.2 (2.2) 529.3 (2.2) 529.3 (2.0)

530.4 (1.9) 530.3 (2.0) 530.4 (2.0) 530.3 (2.0) 530.4 (1.9) 530.4 (1.9)

532.0 (2.2) 531.9 (2.0) 532.0 (2.2) 531.9 (2.2) 532.0 (2.2) 532.0 (2.0)

1273 a

576.7 (3.4) 579.3 (3.8) 577.1 (3.5) 579.5 (3.9) 577.0 (3.4) 579.6 (4.0) 576.8 (3.5) 579.8 (3.8) 576.6 (3.4) 579.9 (4.1) 576.6 (3.6) 579.9 (4.0)

586.9 (3.5) 590.1 (3.8) 586.6 (3.5) 589.9 (4.0) 586.5 (3.4) 590.2 (4.0) 586.6 (3.6) 590.6 (4.1) 586.4 (3.6) 590.7 (4.0) 586.4 (3.6) 590.8 (4.1)

∆Eso/ 2p3/2 eV area/counts 9.8 10.8 9.5 10.4 9.5 10.6 9.8 10.8 9.8 10.8 9.8 10.9

3519 670 1217 3281 4263 5803

assigm Cr3+ Cr3+ satd Cr5+/Cr3+ Cr3+ satd/Cr6+ Cr5+/Cr3+ Cr3+ satd/Cr6+ Cr3+ Cr3+ satd Cr3+ Cr3+ satd Cr3+ Cr3+ satd

Fwhm in parentheses.

TABLE 7: Parameters Obtained by Deconvolution of the Cr 2p Doublet in the XPS Spectra for the Cr(III)/SnO2-cop (Cr:Sn, 0.32) Catalyst Material pretreatment temp/K 333

Fwhm in parentheses. 573

the Cr 2p3/2 manifold with increasing calcination temperature. This effect is accompanied by a decrease in the peak area (Sn 3d5/2) for the Sn 3d doublet above 873 K, as illustrated in Table 4. The Sn 3d binding energy positions are in good agreement with those of Ansell et al. (486.3 eV)26 for tin(IV) oxide, and a typical spectrum is illustrated in Figure 4b. The O 1s region, centered at ca. 530 eV, provides useful information that supports the observations made in the Cr 2p region. Binding energy data for the O 1s peak are represented in Table 5 with a typical spectrum shown in Figure 4c. The parameters derived for the O 1s peak for the Cr(VI)/SnO2-imp material are in good agreement with the literature values for lattice oxygen for SnO2 (530.4 eV) and surface hydroxyl groups (532.0 eV),27 and these positions remain uniform for all calcination treatments. The binding energy position at lower energy with respect to the lattice oxygen peak becomes marginally higher with increased calcination temperature (528.8 f 529.3 eV). These peaks are due to oxygen ions in the oxochromium species with oxygen atoms due to adsorbed chromates observable in the material dried at 333 K and Cr5O12, after calcination at 573 K. At calcination temperatures g673 K the spectra are due to oxygens of Cr2O3. All of these binding energy values are in good agreement with those assigned by Allen et al.28 (Cr6+ in CrO3, 528.5 eV; Cr3+ in Cr2O3, 529.3 eV). Chromium(III)-Promoted Tin(IV) Oxide. Binding energy data and spin-orbit splitting data of the Cr 2p doublet region for Cr(III)/SnO2-imp (Cr:Sn, 0.38) and Cr(III)/SnO2-cop (Cr: Sn, 0.32) catalyst materials are listed in Tables 6 and 7, respectively. As for the Cr(VI)/SnO2-imp materials, the Cr 2p doublet for both Cr(III)/SnO2 materials dried at 333 K can be resolved into four individual peaks. Both manifolds contain satellite peaks corresponding to Cr3+ with a spin-orbit splitting of 10.7/8 eV in accordance with the reference described in Table 2. The maximal peaks both correspond to a Cr3+ species with appropriate spin-orbit splittings. For the Cr(III)/SnO2-cop (Cr: Sn, 0.32) catalyst material, the maximal peak in the Cr 2p3/2 manifold lies at a binding energy of 577.0 eV, which is slightly higher than expected. This can be assigned to Cr3+ in γ-CrOOH, with the binding energy position being in agreement

binding energya/eV 2p3/2 2p1/2

873 1273 a

binding energya/eV 2p3/2 2p1/2 577.0 (3.2) 579.4 (3.9) 577.0 (3.1) 579.4 (3.7) 576.6 (3.6) 579.4 (3.7) 576.5 (3.6) 579.1 (3.8)

∆Eso/ 2p3/2 eV area/counts

586.7 (3.3) 9.7 590.1 (4.0) 10.7 586.3 (3.3) 9.3 588.8 (4.1) 9.4 586.4 (3.7) 9.8 590.1 (3.9) 10.7 586.3 (3.6) 9.8 590.0 (3.9) 10.9

8145 7715 8153 11311

assigm Cr3+ Cr3+ satd Cr5+/Cr3+ Cr3+ satd/Cr6+ Cr3+ Cr3+ satd Cr3+ Cr3+ satd

Fwhm in parentheses.

with studies by Brooks et al.29 (577.0 eV). Analysis of the O 1s peak (see later) further supports the existence of a γ-CrOOH phase for the Cr(III)/SnO2-cop (Cr:Sn, 0.32) dried at 333 K. After calcination at 573 K, both types of material exhibit an unresolved maximal peak in the Cr 2p3/2 manifold at 577.0/1 eV assigned to both Cr3+ and a Cr5+ species, the latter having been photoreduced from Cr6+. This is rationalized by the spinorbit splitting values between the maximal peaks of the Cr 2p3/2 and Cr 2p1/2 manifolds of 9.5 and 9.3 eV for impregnated and coprecipitated materials, respectively. For the coprecipitated material, a spin-orbit splitting of 9.4 eV between the deconvoluted peaks at 579.1/588.8 eV suggests a degree of Cr6+ character present within the material, unaffected by the X-ray flux. However, for the impregnated material this spin-orbit splitting is greater with a value of 10.4 eV, suggesting a higher proportion of photoreduction of the Cr6+ sites. However, the fit parameters strongly indicate the existence of a Cr6+ species in both material types after calcination at 573 K consistent with the presence of the mixed-valence oxide Cr5O12. Typical examples of deconvoluted spectra of the Cr 2p doublet for Cr(III)/SnO2-imp (Cr:Sn, 0.38) and Cr(III)/SnO2-cop (Cr:Sn, 0.38) after calcination at 573 K are shown in Figure 5a,b, respectively. At calcination temperatures of g873 K, the fit parameters for both impregnated and coprecipitated Cr(III)-promoted SnO2 are all in accordance with Cr2O3 being the sole chromium oxide species present in the sample. Binding energy positions for the Sn 3d region together with peak areas (Sn 3d5/2) are shown in Table 8 for both impregnated and coprecipitated Cr(III)/SnO2 catalyst materials. As previously demonstrated for the Cr(VI)/SnO2 materials, these peak areas decrease at elevated calcination temperatures for both Cr(III)/SnO2 catalyst materials due to increased surface coverage by chromia. The decrease in the Sn 3d5/2 peak area for the coprecipitated material at 573 K (Table 8) is a reflection of the

10678 J. Phys. Chem. B, Vol. 102, No. 52, 1998

Harrison et al.

Figure 5. Smoothed and deconvoluted Cr 2p XPS spectra of for (a) Cr(III)/SnO2-imp (Cr:Sn, 0.38) and (b) Cr(III)/SnO2-cop (Cr:Sn, 0.32) after calcination at 573 K. The lower traces represent the difference between observed and calculated spectra.

Figure 6. Smoothed and deconvoluted O 1s XPS spectra of for (a) Cr(III)/SnO2-imp (Cr:Sn, 0.38) and (b) Cr(III)/SnO2-cop (Cr:Sn, 0.32) after drying at 333 K. The lower trace represent the difference between observed and calculated spectra.

aggregation of chromium atoms in the formation of Cr5O12 as the extensive polymeric γ-CrOOH surface species becomes destroyed thermally. This is accompanied by a decrease in the Cr 2p3/2 manifold area at 573 K in comparison with the untreated material, as shown in Table 7. The O 1s region centered at ca. 530 eV again imparts useful information which supports the observations made for the Cr 2p doublet region. Binding energy data for the O 1s peak are presented in Table 9 for both impregnated and coprecipitated Cr(III)/SnO2 catalyst materials, and typical spectra are illustrated in Figure 6a,b, respectively. The peak with a binding energy at ca. 530 eV observed for both Cr(III)/SnO2 materials dried at 333 K materials corresponds to tin(IV) oxide lattice oxygen. The second peak at higher binding energy (ca. 532 eV) is characteristic of oxygen atoms present as surface hydroxyl groups present on the surface of the tin(IV) oxide particles as

well as hydroxyl groups of the γ-CrOOH phase (Cr(III)/SnO2cop material) and coordinated water of sorbed hexaaqua {Cr(H2O)6}3+ ions (Cr(III)/SnO2-imp material). Three peaks can be fitted in the O 1s peak for both material types after calcination at temperatures g573K. The peak at lower binding energy to the lattice oxygen maximal peak becomes marginally greater in binding energy (528.8 f 529.3 eV) as the thermal transformation of Cr5O12 to Cr2O3 occurs, as previously observed with the Cr(VI)/SnO2-imp catalyst materials. Summary The nature of chromium-promoted tin(IV) oxide catalyst materials prepared via three routes, the impregnation of SnO2 using aqueous CrO3 solutions, the impregnation of SnO2 using

TABLE 8: Parameters Obtained by Deconvolution of the Sn 3d XPS Spectra for the Cr(III)/SnO2-imp (Cr:Sn, 0.38) and Cr(III)/SnO2-cop (Cr:Sn, 0.32) Catalyst Materials Cr(III)/SnO2-imp pretreatment temp/K 333 573 673 873 1073 1273 a

binding energya/eV 3d5/2 3d3/2 486.1 (2.3) 486.1 (2.2) 486.3 (2.2) 486.1 (2.5) 486.5 (2.5) 486.3 (2.3)

494.4 (2.4) 494.3 (2.4) 494.7 (2.3) 494.6 (2.3) 495.0 (2.3) 494.2 (2.1)

Cr(III)/SnO2-cop Sn 3d5/2 area/counts 3217 3474 4897 9235 10563 7870

binding energya/eV 3d5/2 3d3/2

Sn 3d5/2 area/counts

486.0 (2.3) 486.4 (2.3)

494.4 (2.1) 494.8 (2.1)

8351 6246

486.4 (2.4)

494.8 (2.2)

13448

486.3 (2.5)

494.8 (2.3)

9452

Fwhm in parentheses.

TABLE 9: Parameters Obtained by Deconvolution of the O 1s XPS Spectra for the Cr(III)/SnO2-imp (Cr:Sn, 0.38) and Cr(III)/SnO2-cop (Cr:Sn, 0.32) Catalyst Materials Cr(III)/SnO2-imp O 1s binding energya/eV

pretreatment temp/K 333 573 673 873 1073 1273 a

Fwhm in parentheses.

528.9 (2.2) 529.0 (2.2) 529.2 (2.2) 529.3 (2.2) 529.3 (2.0)

530.3 (2.2) 530.4 (2.3) 530.4 (2.2) 530.3 (2.1) 530.4 (2.0) 530.4 (2.0)

Cr(III)/SnO2-cop O 1s binding energya/eV 531.8 (2.2) 532.0 (2.2) 532.0 (2.1) 532.0 (2.2) 532.0 (2.1) 531.9 (2.0)

528.8 (2.1)

530.2 (2.3) 530.3 (2.2)

531.5 (2.3) 531.9 (2.2)

529.2 (1.9)

530.3 (1.9)

531.9 (2.0)

529.2 (1.9)

530.3 (2.0)

531.8 (2.1)

Cr Species Formed by Activation of SnO2 Catalysts aqueous chromium(III) nitrate solutions, and coprecipitation from aqueous solutions containing both tin(IV) and chromium(III) ions, has been investigated by electron paramagnetic resonance and X-ray photoelectron spectroscopy. The catalyst materials exhibit three different chromium signals in the EPR depending on the treatment history of the catalysts (δ-, γ-, and β-signals). The δ- and β-signals are broad and occur in the geff ranges of ca. 5 and 1.96-2.45, respectively. The γ-signal is much sharper and occurs at g ) 1.97. The δ-signal results from dispersed Cr3+ ions and appears in the spectra of the Cr(III)-promoted SnO2 catalyst at all temperatures and also in the spectra for the Cr(VI)-promoted catalyst after calcination. The intensity of this signal generally increases with chromium loading and calcination temperature. The β-signal detected for uncalcined Cr(III)-promoted catalyst obtained by coprecipitation is most probably due to the hydrated γ-CrOOH phase but to Cr2O3 at higher calcination temperatures. Both Cr(III)- and Cr(VI)-promoted catalyst materials exhibit the γ-signal at g ) 1.97 after calcination at temperatures of g573 K, which decreases in intensity with increasing calcination temperature. This is attributed to mixed-valence trimers of the type “Cr(VI)-O-Cr(III)-O-Cr(VI)” containing both Cr(III) and Cr(VI) in which the average oxidation state of the chromium is +5, possibly indicating formation of the mixed-valence chromium oxide Cr5O12 (average oxidation state +4.8). Caution needs to be exercised in recording Cr 2p XPS data for Cr6+ compounds because of photoreduction by the X-ray flux during data collection. Thus, XPS spectra of materials containing Cr6+ exhibit peaks due to both Cr6+ and Cr5+ formed by the photoreduction process, and this phenomenon can be used additionally to ascertain the presence of Cr6+. XPS for chromium(VI)-promoted tin(IV) oxide catalysts dried at 333 K are consistent with surface-adsorbed Cr6+ species (monochromate, dichromate), and Cr6+ is still present after calcination at 573 K together with Cr3+, lending corroboration to the formation of the mixed-valence chromium phase Cr5O12. For the material calcined at 673 K, the spectra indicate the presence of some Cr5O12 together with amorphous Cr2O3. Cr 2p XPS spectra for both Cr(III)/SnO2 catalyst materials prepared by impregnation and coprecipitation and dried at 333 K exhibit peaks corresponding to Cr3+, most probably as γ-CrOOH. However, after calcination at 573 K, both types of material exhibit spectra consistent with the formation of some Cr6+, consistent with the presence of the mixed-valence oxide Cr5O12.

J. Phys. Chem. B, Vol. 102, No. 52, 1998 10679 For all three types of catalyst, Cr2O3 is the sole chromium oxide species present after calcination at high temperatures. Acknowledgment. We thank the Commission of the European Community (Contract No. AVI* CT92-0012) and the EPSRC (for Research Grant Nos. GR/J76026 and GL/L24397) for support and funds for the purchase of the EPR spectrometer. References and Notes (1) Harrison, P. G.; Harris, P. J. U.S. Patent 4 908 192, 1990; U.S. Patent 5 051 393, 1991. (2) Solymosi, F.; Kiss, J. J. Chem. Soc., Chem. Commun. 1974, 509. (3) Okamoto, Y.; Fuji, M.; Imanaka, T.; Teranishi, S. Bull. Chem. Soc. Jpn. 1976, 49, 859. (4) Best, S. A.; Squires, R. G.; Walton, R. A. J. Catal. 1977, 47, 292. (5) Weckhuysen, B. M.; Schoonheydt, R. A.; Mabbs, F. E.; Collison, D. J. Chem. Soc., Faraday Trans. 1 1996, 92, 2431. (6) Weckhuysen, B. M.; De Ridder, L. M.; Grobert, P. J.; Schoonheydt, R. A. J. Phys. Chem. 1995, 99, 320. (7) Fendorf, S. E.; Lamble, G. M.; Stapleton, M. G.; Kelley, M. J.; Sparks, D. L. EnViron. Sci. Technol. 1994, 28, 284. (8) van Reijen, L. L.; Cossee, P. Discuss. Faraday Soc. 1966, 41, 277. (9) Cimino, A.; Cordischi, D.; de Rossi, S.; Ferraris, G.; Gazzoli, D.; Indovina, V. M.; Occhiuzzi, M.; Valigi, M. J. Catal. 1991, 127, 761. (10) O’Reilly, D. E.; MacIver, D. S. J. Phys. Chem. 1962, 66, 276. (11) Cornet, D.; Burwell, R. L. J. Am. Chem. Soc. 1968, 90, 2489. (12) Katansky, V. B.; Turkevich, J. J. Catal. 1967, 8, 231. (13) Ellison, A.; Sing, K. S. W. Discuss. Faraday Soc. 1966, 41, 315. (14) Ellison, A.; Sing, K. S. W. J. Chem. Soc., Faraday Trans. 1 1978, 73, 2907. (15) Evans, J. C.; Relf, C. P.; Rowlands, C. C.; Egerton, T. A.; Pearman, A. J. J. Mater. Sci. Lett. 1985, 4, 809. (16) Amorelli, A.; Evans, J. C.; Rowlands, C. C.; Egerton, T. A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3541. (17) Okamoto, Y.; Nakaro, H.; Imanaka, T.; Teranishi, S. Bull. Chem. Soc. Jpn. 1975, 48, 1163. (18) Frost, D. C.; MacDowell, C. A.; Woolsey, I. S. Mol. Phys. 1974, 27, 1473. (19) Cazaux, J. Appl. Surf. Sci. 1985, 20, 457. (20) Wagner, C. D. Surf. Inter. Anal. 1984, 6, 90. (21) Knotek, M. L.; Friedman, P. J. Surf. Sci. 1979, 90, 78. (22) Michel, G.; Cahay, R. J. Raman Spectrosc. 1986, 17, 4. (23) Venezia, A. M.; Palmisano, L.; Sciavello, M.; Martin, C.; Martin I.; Rives, V. J. Catal. 1994, 147, 115. (24) Wilmhelmi, K. A. Acta Chem. Scand. 1965, 19, 165. (25) Dyne, S. R.; Butt, J. E.; Haller, G. L. J. Catal. 1972, 25, 378. (26) Ansell, R. O.; Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electrochem. Soc. 1977, 124, 1360. (27) Katayama, A. J. Phys. Chem. 1980, 84, 376. (28) Allen, G. C.; Curtis, M. T.; Hooper, A. J.; Tucker, P. M. J. Chem. Soc., Dalton Trans. 1973, 1675. (29) Brooks, A. R.; Clayton, C. R.; Doss, K.; Lu, Y. C. J. Electrochem. Soc. 1986, 133, 2459.