Ind. Eng. Chem. Res. 1998, 37, 887-893
887
Characterization and CO Oxidation Activity of Cu/Cr/Al2O3 Catalysts Paul Worn Park† and Jeffrey S. Ledford*,‡ Department of Chemistry, The Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824-1322
X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) have been used to characterize a series of Cu/Cr/Al2O3 catalysts prepared by stepwise incipient wetness impregnation of first chromium followed by copper (designated “CuCry”). The copper loading was held constant at 8 wt % CuO, and chromium loadings were varied from 0 to 20 wt % Cr2O3. The information obtained from surface and bulk characterization has been correlated with the CO oxidation activity of the catalysts. XPS and XRD results of analogous Cry indicated that the Cr dispersion decreased and the concentration of Cr3+ species increased with increasing Cr content. The decrease in Cu dispersion of CuCry with increasing Cr content has been attributed to the formation of large crystalline CuO and CuCr2O4. Copper addition decreased the Cr dispersion by reacting selectively with a dispersed Cr3+ species to form CuCr2O4 species. However, the Cu addition did not affect the Cr oxidation state distribution compared to that of Cry. For low Cr loading CuCry catalysts (Cr/Al e 0.027), the CO oxidation activity increased with increasing Cr content due to the formation of crystalline CuO on the Cr-modified alumina. This has been attributed to the inhibition of Cu ion diffusion into alumina lattice vacancies by highly dispersed chromium species. The CuCry catalyst of Cr/Al ) 0.054 showed the highest CO oxidation activity due to the formation of CuCr2O4 which was more active than the CuO phase. For Cr-rich catalysts (Cr/Al g 0.080), the decrease in CO oxidation activity has been ascribed to the encapsulation of the active site with Cr2O3 species. Introduction The combination of copper and chromium oxide was recognized as a promising active component for emission control reactions in the early years of pollution control catalysis research (Frazer, 1936; Roth and Doerr, 1961; Hofer et al., 1964; Shelef et al., 1968; Dwyer, 1972). The Cu-Cr catalyst system has been studied extensively for reactions such as the oxidation of CO (Hertl and Farrauto, 1973; Barnes, 1975; Farrauto et al., 1975; Kummer, 1975; Yu Yao, 1975; Yu Yao and Kummer, 1977; Severino and Laine, 1983; Severino et al., 1986; Laine et al., 1987; Laine and Severino, 1990; Bijsterbosch et al., 1992; Dekker et al., 1992; Lo´pez Agudo et al., 1992; Kapteijn et al., 1993; Stegenga et al., 1993; Dekker et al., 1994), hydrocarbons (Hertl and Farrauto, 1973; Barnes, 1975; Farrauto et al., 1975; Yu Yao, 1975; Yu Yao and Kummer, 1977; Rastogi et al., 1980; Chien et al., 1995), alcohols and aldehydes (McCabe and Mitchell, 1983; Rajesh and Ozkan, 1993), sulfurated hydrocarbons (Heyes et al., 1982), and chlorinated hydrocarbons (Subbanna et al., 1988) as well as NO reduction (Tarasov et al., 1990; Kapteijn et al., 1993; Stegenga et al., 1993). Shelef et al. (1968) and Kapteijn et al. (1993) have reported that Cu-Cr catalysts showed higher CO oxidation and NO reduction activity than single oxide catalysts based on Cu, Ni, Co, Fe, Mn, Cr, or V. Barnes (1975) reported that an alumina-supported 8 wt % Cu-7 wt % Cr catalyst showed activity * Author to whom correspondence is addressed. Phone: (315) 986-5365. Fax: (315) 986-5033. † Current address: Center for Catalysis and Surface Science, Northwestern University, Evanston, IL 60208. ‡ Current address: Mobil Chemical Company, Films DivisionsTechnical Center, 729 Pittsford-Palmyra Road, Macedon, NY 14502.
similar to that of a 0.3 wt % Pt catalyst for automotive exhaust gas treatment. In addition, Stegenga et al. (1991) have found that a monolith-supported 10 wt % Cu-Cr/Al2O3 catalyst showed a three-way catalytic activity comparable to that of noble metal catalysts operated under the same conditions. In the copper-chromite catalyst system, copper oxide has been proposed as the active component for CO oxidation, while chromium oxide has been considered a promoter (Severino and Laine, 1983; Stegenga et al., 1993; Chien et al., 1995). The Cr promoter is believed to limit catalyst reduction (Severino and Laine, 1983; Fattakhova, et al., 1986; Severino et al., 1986; Laine et al., 1987; Tarasov et al., 1990), prevent catalyst poisoning (Yu Yao, 1975; Laine and Severino, 1990), inhibit bulk copper aluminate formation (Kapteijn et al., 1993), improve thermal stability (Yu Yao, 1975), and increase catalyst dispersion (Bijsterbosch et al., 1992). Mixed oxide catalysts also show greater activity and stability than catalysts based on single oxides (Dadyburjor et al., 1979; Prasad et al., 1980). In the case of Cu-Cr catalysts, such a synergistic effect has been attributed to electronic interactions between copper and chromium species (Severino and Laine, 1983; Bijsterbosch et al., 1992; Chien et al., 1995). Despite a considerable amount of effort devoted to the study of Cu-Cr catalyst, the effect of catalyst composition and structure on CO oxidation activity is still unclear. Stegenga et al. (1991) reported that Cu/Cr ) 2 was the optimum metal composition for a Cu-Cr/Al2O3 CO oxidation catalyst. However, Severino et al. (1986) suggested that Cu/Cr ) 1 was the optimal metal composition of an alumina-supported catalyst for CO oxidation. Much of the previous research on Cu-Cr catalysts has focused on understanding the activity or stability of the
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888 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
catalysts. Little effort has been devoted to investigating systematically the relationship between the surface structure of Cu/Cr/Al2O3 and its catalytic activity. The present work is a part of a broad study to investigate the effect of base metal oxide promoters on the structure and reactivity of copper oxide based emission control catalysts. In this paper, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to determine the effect of Cr loading on the chemical state and dispersion of chromium and copper oxide phases supported on γ-alumina. The information derived from these techniques has been correlated with CO oxidation activity to develop a more complete understanding of Cu/Cr/Al2O3 catalysts. Experimental Section Catalyst Preparation. Catalysts were prepared by pore volume impregnation of γ-alumina (Cyanamid; γ-Al2O3, surface area ) 203 m2/g, pore volume ) 0.6 mL/ g). The alumina was finely ground ( 99.99%) in the temperature range of 140-230 °C. All activity measurements were obtained under steady-state conditions at conversions of less than 15%. Turnover numbers (TON) were calculated using the CO oxidation rate at 190 °C normalized by surface copper atoms determined from the Cu content and dispersion. Results and Discussion Structure of Cr/Al2O3 Catalysts. Our previous study (Park and Ledford, 1997a) indicated that the dispersion and oxidation state of Cry catalysts strongly depended on the Cr content. XPS results indicated that most of the Cr in Cr1.3 catalyst was present as a highly dispersed Cr6+ species. For Cr2.7, Cr5.4, and Cr8.0 catalysts, XRD results showed no Cr2O3 peaks; however, XPS data indicated that the ratio of Cr3+/Cr6+ species increased with increasing Cr content. For catalysts with high Cr loadings (Cr/Al atomic ratio g0.11), large Cr2O3 crystallites were detected by XRD. Figure 1 shows the variation of the Cr 2p/Al 2p intensity ratio measured for the Cry catalysts as a function of the Cr/ Al atomic ratio. The theoretical line calculated for monolayer dispersion is shown for comparison (Kerkhof and Moulijn, 1979). The Cr/Al intensity ratio measured for the Cr1.3 catalyst is identical, within experimental error, to the value predicted for monolayer dispersion. For higher Cr loadings, the Cr/Al intensity ratios
Figure 1. XPS Cr 2p/Al 2p intensity ratios of Cry (b) and CuCry (O) catalysts plotted versus Cr/Al atomic ratio. Cr 2p/Al 2p intensity ratios calculated for monolayer dispersion (s).
measured for the catalysts are lower than the monolayer values. The increasing deviation of the Cr/Al intensity ratio from the monolayer line indicates that the Cr dispersion decreases continuously with increasing Cr content. We ascribed the decrease in Cr dispersion to the formation of chromium oxide cluster or crystalline Cr2O3 on the alumina support. Three levels of Cr loading will be discussed for Cu/ Cr/Al2O3 catalysts: Cr/Al atomic ratios e0.027 (low loading), 0.054 e Cr/Al e 0.080 (intermediate loading), and Cr/Al g 0.11 (high loading). Structure of Cu/Cr/Al2O3 Catalysts. (a) Chemical State of Cu/Al2O3 Catalyst. XRD patterns obtained for CuCr0 catalysts showed only lines characteristic of the alumina carrier (Figure 2b). The absence of XRD peaks characteristic of crystalline CuO and the small particle size calculated from XPS Cu/Al intensity ratios (Table 2) suggest that copper exists as a well-dispersed phase. The XPS Cu 2p3/2 binding energy (Table 1) measured for the CuCr0 catalyst (935.4 eV) is closer to the Cu 2p3/2 binding energy measured for CuAl2O4 (935.0 eV) than the value for CuO (933.9 eV). Binding energies measured for the standard compounds are consistent with previously reported values (Friedman et al., 1978; Capece et al., 1982; Strohmeier et al., 1985). The XPS and XRD results indicate that a dispersed copper surface phase is predominantly present on the alumina support. These results are consistent with previous work (Friedman et al., 1978; Strohmeier et al., 1985) which showed that the support capacity for the copper surface phase is approximately 8-10 wt % Cu over 200 m2/g of γ-alumina. However, the satellite/main peak ratio (Table 1) measured for the CuCr0 catalysts (0.42) is similar to that measured for CuO (0.45) and lower than the value for CuAl2O4 (0.71). This result conflicts with the XRD pattern which does not show peaks due to crystalline CuO for the CuCr0 catalyst. Strohmeier et al. (1985) suggested that this does not necessarily mean that well-dispersed or amorphous copper oxide is
890 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 2. XRD patterns obtained for (a) alumina, (b) CuCr0, (c) CuCr1.3, (d) CuCr2.7, (e) CuCr5.4, (f) CuCr8.0, (g) CuCr11, and (h) CuCr13 catalysts (b, alumina; 9, CuO; ], cubic CuCr2O4; 1, tetragonal CuCr2O4; +, Cr2O3).
present on the catalysts but that the copper surface species is chemically different from bulk CuO and CuAl2O4. (b) Dispersion of Cu/Al2O3 Catalyst. Figure 3 shows the variation of the XPS Cu 2p3/2/Al 2p intensity ratios measured for the CuCry catalysts as a function of Cr/ Al atomic ratio. The decrease in the XPS intensity ratio with increasing Cr content indicates that the Cu dispersion decreased as the Cr content of the catalysts increased. Copper oxide particle sizes determined from XPS Cu/Al intensity ratios and XRD line broadening are shown as a function of Cr content in Tables 2 and 3, respectively. The particle size of Cu species in CuCr0 catalyst (1.0 nm) calculated from XPS data indicates that Cu is well-dispersed over the alumina carrier. This is consistent with XRD results which do not show any crystalline phase. The Cu particle sizes calculated from XPS data increase from 1.0 to 2.1 nm with increasing Cr content. This result agrees with XRD data for the CuCry catalysts which show the formation of large CuO and CuCr2O4 crystallites with increasing Cr content. CuO and CuCr2O4 particle sizes determined using XRD line-broadening calculations are significantly larger than the particles sizes determined from XPS data. The difference in particle size determined from XPS and XRD results may be understood in terms of the limitations of XRD. Since XRD does not detect highly dispersed species that contribute to the XPS signal, smaller average particle sizes are expected from XPS calculations. (c) Chemical State of Cu/Cr/Al2O3 Catalysts. The average XPS Cu 2p3/2 binding energy measured for CuCry catalysts (935.5 ( 0.1 eV) is identical, within experimental error, to the value obtained for CuCr0 catalyst (Table 1). Figure 4 shows the Cr 2p XPS spectra of CuCry catalysts. The Cr 2p3/2 binding energies of the main peak (580.0 ( 0.1 eV) and shoulder (577.4 ( 0.1 eV) measured for the catalysts have been assigned to the Cr(VI) and Cr(III) species, respectively (Park and Ledford, 1997a). The variation of the XPS
Cr 2p3/2/Al 2p intensity ratio of Cr(VI) and Cr(III) species for the CuCry catalysts determined using nonlinear least-squares curve fitting is shown in Table 4. The Cr(VI)/Al intensity ratio increases with increasing Cr/Al atomic ratio up to 0.054. For higher Cr loadings, the Cr(VI)/Al intensity ratios measured for the catalysts are independent of Cr content. Cr(III) species are observed for all catalysts, and the XPS Cr(III)/Al intensity ratio increases with increasing Cr content. This indicates that the alumina support becomes saturated with Cr(VI) species at the catalyst with Cr/Al atomic ratio ) 0.054 (8 wt % Cr2O3) and further addition of chromium only leads to the formation of Cr(III) species. The XPS binding energies and relative oxidation state distribution of Cr species for the CuCry catalysts are consistent with the results obtained for Cry catalysts within experimental error (Park and Ledford, 1997a). This indicates that the postaddition of Cu does not affect the Cr oxidation state distribution in CuCry catalysts. For CuCry catalysts with low Cr loading (Cr/Al e 0.027), the XRD patterns show peaks characteristic of CuO (Figure 2). Table 5 shows the amount of crystalline CuO, tetragonal CuCr2O4, and Cr2O3 present in the CuCry catalysts as a function of Cr/Al atomic ratio. The amount of crystalline CuO increases from 0.4 to 1.9 wt % as the Cr/Al atomic ratio increases from 0.013 to 0.027. The formation of the CuO phase can be explained in terms of a decrease in the interaction between copper species and alumina support. A significant number of the lattice vacancies normally accessible to the Cu2+ ions during catalyst preparation steps are blocked by preloaded chromium oxide species (well-dispersed Cr6+ species). Thus, many Cu2+ ions are not able to enter the alumina lattice to form the copper surface phase. Upon calcination, these Cu2+ ions agglomerate to form large particles of CuO. The formation of the CuO phase on the Cr-modified alumina indicates that the copper species does not readily interact with the well-dispersed Cr6+ species to form the CuCr2O4 phase. For CuCry catalysts with intermediate Cr loading (0.054 e Cr/Al e 0.080), the XRD patterns show peaks characteristic of tetragonal CuCr2O4 (Figure 2). In addition, a peak observed at 35.9° in the diffraction pattern of the CuCr5.4 catalyst can be assigned to the cubic CuCr2O4 〈311〉 line. The crystalline tetragonal CuCr2O4 content of the catalysts increases from 0.3 to 3.4 wt % as the Cr/Al atomic ratio increases from 0.054 to 0.080 (Table 5). This is consistent with the increase of the XPS Cu 2p3/2 satellite/main peak ratio up to 0.63 (Table 1). The increase in the CuCr2O4 content with increasing Cr(III)/Al XPS intensity ratio (Table 4) in this range of Cr loading indicates that Cu species readily interact with Cr3+ species. For CuCry catalysts with high Cr loading (Cr/Al g 0.11), peaks characteristic of crystalline Cr2O3 are observed in the XRD patterns (Figure 2). The crystalline Cr2O3 content of the catalysts increases from 1.3 to 8.1 wt %; however, the crystalline CuCr2O4 content is independent of the Cr loading as the Cr/Al atomic ratio increases from 0.11 to 0.13 (Table 5). The XPS Cu 2p3/2 satellite/main peak intensity ratios for the Crrich catalysts (Table 1) are close to the value obtained for CuCr2O4. This result indicates that most of the Cu reacts with Cr species to form CuCr2O4 instead of interacting with the alumina support. The comparable Cr2O3 content obtained for the CuCry and the analogous Cry catalysts (Park and Ledford, 1997a) suggests that
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 891 Table 1. XPS Cu 2p3/2 Binding Energies and Shakeup/Main Peak Intensity Ratios Measured for CuCry Catalysts and Standard Compounds catalyst
Cu 2p3/2 B.E. (eV)
shakeup/main peak ratio
catalyst
Cu 2p3/2 B.E. (eV)
shakeup/main peak ratio
CuCr0 CuCr1.3 CuCr2.7 CuCr5.4
935.4 935.3 935.5 935.5
0.42 0.48 0.56 0.62
CuCr8.0 CuCr11 CuCr13
935.5 935.5 935.5
0.63 0.69 0.72
CuO CuAl2O4
933.9 935.0
0.45 0.71
CuCr2O4
934.5
0.70
Table 3. Particle Size of CuO, Cr2O3, and CuCr2O4 Phases Determined from XRD Line-Broadening Calculations particle size (nm) catalyst
CuO
Cr2O3
CuCr2O4
CuCr0 CuCr1.3 CuCr2.7 CuCr5.4 CuCr8.0 CuCr11 CuC r13
a 17 16 a a a a
b a a a a 44 33
b a a 31 29 30 28
a No diffraction lines detected. b Catalyst does not contain chromium.
Figure 3. XPS Cu 2p3/2/Al 2p intensity ratios measured for CuCry catalysts plotted versus Cr/Al atomic ratio. Monolayer dispersion value ) 2.3. Table 2. Particle Size of Chromium and Copper Species Determined from XPS Intensity Ratios particle size (nm)
particle size (nm)
catalyst
chromium
copper
catalyst
chromium
copper
CuCr0 CuCr1.3 CuCr2.7 CuCr5.4
a b 0.7 1.2
1.0 1.3 1.5 1.4
CuCr8.0 CuCr11 CuCr13
2.0 2.0 2.5
1.8 2.1 2.1
a
Catalyst does not contain chromium. b Catalyst dispersion is close to monolayer.
little CuCr2O4 phase is formed by the interaction of Cu with large Cr2O3 crystallites. This is readily understood when one considers that large Cr2O3 crystallites have relatively less surface area available to react with Cu species than small Cr2O3 crystallites. (d) Dispersion of Cu/Cr/Al2O3 Catalysts. Figure 1 shows the variation of the XPS Cr 2p/Al 2p intensity ratios measured for CuCry catalysts (open circles) as a function of Cr/Al atomic ratio. The Cr/Al intensity ratios measured for the CuCry catalysts with Cr/Al atomic ratio e 0.027 are similar to the values measured for Cry catalysts. For the CuCry catalysts with Cr/Al atomic ratio g0.054, the Cr/Al intensity ratios are lower than the values obtained for the analogous Cry catalysts. This indicates that Cu addition decreased the dispersion of the Cr species. This is consistent with XRD results which showed a large crystalline CuCr2O4 phase for the catalysts with Cr/Al atomic ratios g 0.054. Cr particle
Figure 4. XPS Cr 2p3/2 spectra obtained for (a) CuCr1.3, (b) CuCr2.7, (c) CuCr5.4, (d) CuCr8.0, (e) CuCr11, and (f) CuCr13 catalysts. Table 4. XPS Cr 2p3/2/Al 2p Intensity Ratios of Cr(VI) and Cr(III) Species in CuCry Catalysts Determined Using Nonlinear Least-Squares Curve Fitting (NLLSCF) intensity ratio catalyst CuCr1.3 CuCr2.7 CuCr5.4
Cr(VI)/Al Cr(III)/Al 0.16 0.30 0.42
0.06 0.09 0.16
intensity ratio catalyst CuCr8.0 CuCr11 CuCr13
Cr(VI)/Al Cr(III)/Al 0.36 0.42 0.45
0.27 0.42 0.49
sizes determined from XPS Cr/Al intensity ratios and XRD line-broadening calculations are shown as a function of Cr loading in Tables 2 and 3. Cr particle sizes calculated from XPS data generally increase with increasing Cr loading. As for the copper species, the Cr2O3 particle sizes determined for CuCry catalysts using XRD line-broadening calculations are significantly larger than the values determined using XPS.
892 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Table 5. Concentration of Crystallinea Phases in CuCry Catalysts Calculated from Quantitative XRD Data crystalline phase (wt %)
b
catalyst
Cr2O3
CuO
CuCr2O4
CuCr0 CuCr1.3 CuCr2.7 CuCr5.4 CuCr8.0 CuCr11 CuCr13
b 0.0 0.0 0.0 0.0 1.3 8.1
0.0 0.4 1.9 0.0 0.0 0.0 0.0
b 0.0 0.0 0.3 3.4 3.4 3.1
a Valid for crystalline phases with particle sizes > 3.0 nm. Catalyst does not contain chromium.
Table 6. Specific Turnover Numbers and Activation Energies of CO Oxidation for CuCry Catalysts
catalyst CuCr0 CuCr1.3 CuCr2.7 CuCr5.4
activation activation TONa energyb TONa energyb 2 -1 2 × 10 ) (kcal/mol) catalyst (s × 10 ) (kcal/mol)
(s-1
0.6 2.7 5.3 5.9
17.9 16.5 14.4 15.8
CuCr8.0 CuCr11 CuCr13
2.9 3.1 2.7
16.2 17.5 16.3
a
Determined from reactive rate at 190 °C normalized by Cu dispersion. b Estimated from Arrhenius plots within 15% conversion.
Effect of Catalyst Structure on CO Oxidation Activity. Table 6 shows the specific turnover numbers (TON) and activation energies of CuCry catalysts for CO oxidation as a function of Cr/Al atomic ratio. The specific TON increases by 1 order of magnitude upon increasing the Cr/Al atomic ratio from 0.0 to 0.054. However, further addition of Cr decreases the specific TON for CO oxidation. The activation energies of CuCry catalysts are independent of Cr content (16.4 ( 1.1 kcal/mol). The CO oxidation activity can be attributed predominantly to the Cu species in CuCry catalysts, since Cr species show little CO oxidation activity compared to Cu catalysts (Hertl and Farrauto, 1973; Severino and Laine, 1983; Stegenga et al., 1993; Chien et al., 1995). In this study, Cry catalysts showed negligible CO oxidation activity compared to CuCr0 catalyst at the same reaction conditions. Addition of Cr up to a Cr/Al atomic ratio of 0.027 increases the CO oxidation rate by a factor of 9 compared to the CuCr0 catalyst. We attribute this to the formation of an active CuO phase which was observed in XRD patterns for CuCr1.3 and CuCr2.7 catalysts. In our previous work (Park and Ledford, 1997b), we concluded that crystalline CuO was more active for CO oxidation than the Cu surface phase. This has been attributed to the redox ability of CuO being more favorable than the dispersed Cu surface phase during CO oxidation reaction (Severino et al., 1986). CuCr5.4 catalyst shows the highest specific TON among the CuCry catalysts, although Cu and Cr dispersion decreases continuously as a function of Cr content. This can be correlated to the formation of the CuCr2O4 phase which was detected by XRD. Therefore, we attribute the activity improvement of CuCr5.4 catalyst to the formation of the CuCr2O4 phase. CuCr2O4 is known to be a more active phase than copper oxide for CO oxidation (Shelef et al., 1968; Severino and Laine, 1983; Stegenga et al., 1993). Severino and Laine (1983) have studied alumina-supported and unsupported CuCr catalysts for CO oxidation. They suggested that chromium enhanced the catalytic activity by providing
structural oxygen (O2-) to copper species and electron transfer from copper to chromium. Kapteijn et al. (1993) have also proposed recently that the Cu-Cr oxide catalyst was more active than a single oxide for CO oxidation due to high reactivity of surface oxygen on the mixed oxide catalyst. It is generally accepted that CO oxidation involves redox phenomena on the surface of oxides. Therefore, the high CO oxidation activity of the CuCr2O4 phase can be attributed to more facile generation of surface oxygen during the CO oxidation reaction compared to the CuO or copper surface phase. The further addition of Cr (Cr/Al atomic ratio g 0.080) decreases CO oxidation activity for CuCry catalysts. XPS and XRD data indicate that the dispersions of both Cu and Cr are decreased significantly and a large amount of CuCr2O4 crystalline phase is formed on the catalyst. In addition, excess Cr2O3 phase, which is not active for CO oxidation reaction, is present on the catalyst surface. Excess Cr phase can cover active sites and decrease the CO oxidation activity for Cr-rich catalysts. Conclusions The combined use of several techniques to investigate the effect of Cr addition on the structure and CO oxidation activity of Cu/Cr/Al2O3 catalysts leads to the following conclusions. 1. Cu addition decreased Cr dispersion in CuCry catalysts (Cr/Al g 0.054) compared to the Cr dispersion in analogous Cry catalysts. The dispersion of Cu species in CuCry catalysts also decreased with increasing Cr content. The decrease in dispersion of both Cu and Cr species has been attributed to the formation of large crystalline CuO, CuCr2O4, and Cr2O3 phases on the alumina support. 2. For CuCry catalysts with low Cr contents (Cr/Al e 0.027), the presence of a highly dispersed Cr6+ phase inhibited the formation of the copper surface phase and enhanced the formation of crystalline CuO over the alumina support. The CO oxidation activity increase in these catalysts (CuCr1.3 and CuCr2.7) has been attributed to an increase in the amount of crystalline CuO. 3. The CuCr5.4 catalyst showed the highest CO oxidation activity in this study. This is due to the formation of the active CuCr2O4 phase. Postloaded copper species interacted with dispersed Cr3+ species to form CuCr2O4 rather than with highly dispersed Cr6+ species or large Cr2O3 crystallites. 4. The CuCry catalysts with Cr/Al g 0.08 showed decreased CO oxidation activity. This has been attributed to an encapsulation of the active site with excess Cr2O3. Acknowledgment Financial support from an All-University Research Initiation Grant (AURIG) administered by Michigan State University and the Center for Fundamental Materials Research (CFMR) is gratefully acknowledged. Literature Cited Barnes, G. J. A comparison of platinum and base metal oxidation catalysts. Adv. Chem. Ser. 1975, 143, 72. Bijsterbosch, J. W.; Kapteijn, F.; Moulijn, J. A. In situ FT-IR study of copper-chromium oxide catalysts in CO oxidation. J. Mol. Catal. 1992, 74, 193.
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 893 Capece, F. M.; Di Castro, V.; Furlani, C.; Mattogno, G.; Fragale, C.; Gargano, M.; Rossi, M. “Copper Chromite” catalysts: XPS structure elucidation and correlation with catalytic activity. J. Electron Spectrosc. Relat. Phenom. 1982, 27, 119. Chien, C.-C.; Chuang, W.-P.; Huang, T.-J. Effect of heat-treatment conditions on Cu-Cr/γ-alumina catalyst for carbon monoxide and propene oxidation. Appl. Catal. A. 1995, 131, 73. Dadyburjor, D. B.; Jewur, S. S.; Ruckenstein, A. E. Selective oxidation of hydrocarbons on composite oxides. Catal. Rev.sSci. Eng. 1979, 19, 293. De Angelis, B. A. On the surface reduction of some chromium compounds during X-ray photoelectron spectroscopy. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 81. Deffosse, C.; Canesson, D.; Rouxhet, P. G.; Delmon, B. Surface characterization of silica-aluminas by photoelectron spectroscopy. J. Catal. 1978, 51, 269. Dekker, N. J. J.; Hoorn, J. A. A.; Stegenga, S.; Kapteijn, F.; Moulijn, J. A. Kinetics of the CO oxidation by O2 and N2O over Cu-Cr/Al2O3. AIChE J. 1992, 38, 385. Dekker, F. H. M.; Dekker, M. C.; Bliek, A.; Kapteijn, F.; Moulijn, J. A. A transient kinetic study of carbon monoxide oxidation over copper-based catalysts for automotive pollution control. Catal. Today 1994, 20, 409. Dwyer, F. G. Catalysis for control of automotive emission. Catal. Rev.sSci. Eng. 1972, 6, 261. Farrauto, R. J.; Hoekstra, K. E.; Shoup, R. D. Copper chromitealumina catalysts having high-temperature stability. U.S. Patent 3,870,658, 1975. Fattakhova, Z. T.; Ukharskii, A. A.; Shiryaev, P. A.; Berman, A. D. Reduction and reoxidation of cobalt-chromium oxide catalysts by means of IR spectroscopy and X-ray phase analysis. Kinet. Katal. 1986, 27, 884. Fleisch, T. H.; Mains, G. J. Reduction of copper oxides by UV radiation and atomic hydrogen studied by XPS. Appl. Surf. Sci. 1982, 10, 51. Frazer, J. C. W. Adsorption on chromite catalysts. U.S. Patent 2031475, 1936. Friedman, R. M.; Freeman, J. J.; Lytle, F. W. Characterization of Cu/Al2O3 catalysts. J. Catal. 1978, 55, 10. Hertl, W.; Farrauto, R. J. Mechanism of carbon monoxide and hydrocarbon oxidation on copper chromite. J. Catal. 1973, 29, 352. Heyes, C. J.; Irwin, J. G.; Johnson, H. A.; Moss, R. L. The catalytic oxidation of organic air pollutants. Part 2. Cobalt molybdate and copper chromite catalysts. J. Chem. Technol. Biotechnol. 1982, 32, 1034. Hofer, L. J. E.; Gussey, P.; Anderson, R. B. Specificity of catalysts for the oxidation of carbon monoxide-ethylene mixtures. J. Catal. 1964, 3, 451. JCPDS, Powder Diffraction File, Inorganic Phases, 1983. Kapteijn, F.; Stegenga, S.; Dekker, N. J. J.; Bijsterbosch, J. W.; Moulijn, J. A. Alternatives to noble metal catalysts for automotive exhaust purification. Catal. Today 1993, 16, 273. Kerkhof, F. P. J. M.; Moulijin, J. A. Quantitative analysis of XPS intensities for supported catalysts. J. Phys. Chem. 1979, 83, 1612. Klug, H. P.; Alexander, L. E. X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 1st ed.: Wiley: New York, 1954. Kummer, J. T. Oxidation of CO and C2H4 by base metal catalysts prepared on honeycomb supports. Adv. Chem. Ser. 1975, 143, 178. Laine, J.; Severino, F. Changes in alumina-supported copper and copper-chromite catalysts by the introduction of water during carbon monoxide oxidation. Appl. Catal. 1990, 65, 253. Laine, J.; Albornoz, A.; Brito, J.; Carı´as, O.; Castro, G.; Severino, F.; Valera, D. Development of a copper-chromite catalyst for carbon monoxide automobile emission control. In Catalysis and Automotive Pollution Control; Crucq, A., Frennet, A., Ed.; Elsevier: Amsterdam, The Netherlands, 1987; pp 387-393. Lo´pez Agudo, A.; Palacios, J. M.; Fierro, J. L. G.; Laine, J.; Severino, F. Activity and structural changes of aluminasupported CuO and CuCr2O4 catalysts during carbon monoxide oxidation in the presence of water. Appl. Catal. 1992, 91, 43. McCabe, R. W.; Mitchell, P. J. Oxidation of ethanol and acetaldehyde over alumina-supported catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 212.
Okamoto, Y.; Fujii, M.; Imanaka, T.; Teranishi, S. X-ray photoelectron spectroscopic studies of chromia-alumina catalysts. Bull. Chem. Soc. Jpn. 1976, 49, 859. Park, P. W.; Ledford, J. S. Characterization and CH4 oxidation activity of Cr/Al2O3 catalysts. Langmuir 1997a, 13, 2726. Park, P. W.; Ledford, J. S. The influence of surface structure on the catalytic activity of alumina supported copper oxide catalysts: Oxidation of carbon monoxide and methane. Appl. Catal. B 1997b, in press. Penn, D. R. Quantitative chemical analysis by ESCA. J. Electron. Spectrosc. Relat. Phenom. 1976, 9, 29. Prasad, R.; Kennedy, L. A.; Ruckenstein, A. E. Catalytic combustion of propane using transitional metal oxides. Combust. Sci. Technol. 1980, 22, 271. Proctor, A. Provided Software, University of Pittsburgh, Pittsburgh, PA, 1993. Rajesh, H.; Ozkan, U. S. Complete Oxidation of Ethanol, Acetaldehyde, and Ethanol/Methanol Mixtures over Copper Oxide and Copper-Chromium Oxide Catalysts. Ind. Eng. Chem. Res. 1993, 32, 1622. Rastogi, R. P.; Singh, G.; Dubey, B. L.; Shukla, C. S. Solid State chemistry of copper chromite used as a catalyst for the burning of ammonium perchlorate/polystyrene propellants. J. Catal. 1980, 65, 25. Rosencwaig, A.; Wertheim, G. K. X-ray photoemission satellites; surface or bulk effect? J. Electron. Spectrosc. Relat. Phenom. 1972, 1, 493. Roth, J. F.; Doerr, R. C. Air pollution studies Oxidation-Reduction Catalysis. Ind. Eng. Chem. 1961, 53, 293. Scierka, S. J.; Proctor, A.; Houalla, M.; Fiedor, J. N.; Hercules, D. M. Determination of the Distribution of Chromium Oxidation States in Reduced Cr/Al2O3 Catalysts from XPS by Factor Analysis and Curve Fitting. Surf. Interface Anal. 1993, 20, 901. Scofield, J. H. Hartree-Slater subshell photoionization crosssections at 1254 and 1487 eV. J. Electron. Spectrosc. Relat. Phenom. 1976, 8, 129. Severino, F.; Laine, J. Effect of composition and pretreatments on the activity of copper-chromium-based catalysts for the oxidation of carbon monoxide. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 396. Severino, F.; Brito, J.; Carı´as O.; Laine, J. Comparative study of alumina-supported CuO and CuCr2O4 as catalysts for CO oxidation. J. Catal. 1986, 102, 172. Shelef, M.; Otto, K.; Gandhi, H. The oxidation of CO by O2 and by NO on supported chromium oxide and other metal oxide catalysts. J. Catal. 1968, 12, 361. Stegenga, S.; Dekker, N.; Bijsterbosch, J.; Kapteijn, F.; Moulijn, J.; Belot, G.; Roche, R. Catalytic automotive pollution control without noble metals. In Catalysis and Automotive pollution Control II; Crucq, A., Ed.; Elsevier: Amsterdam, The Netherlands, 1991; pp 353-369. Stegenga, S.; van Soest, R.; Kapteijn, F.; Moulijn, J. A. Nitric oxide reduction and carbon monoxide oxidation over carbon-supported copper-chromium catalysts. Appl. Catal. B 1993, 2, 257. Strohmeier, B. R.; Leyden, D. E.; Field, R. S.; Hercules, D. M. Surface Spectroscopic Characterization of Cu/Al2O3 catalysts. J. Catal. 1985, 94, 514. Subbanna, P.; Greene, H.; Desal, F. Catalytic oxidation of polychlorinated biphenyls in a monolithic reactor system. Environ. Sci. Technol. 1988, 22, 557. Tarasov, A. L.; Osmanov, M. O.; Shvets, V. A.; Kazanskii, V. B. IR spectroscopic study of adsorbed NO and CO, state of CuCr/Al2O3 catalyst surface, and mechanism of reduction of NO by carbon monoxide. Kinet. Catal. 1990, 31, 565. Wallbank, B.; Johnson, C. E.; Main, I. G. Surface reduction of solid 3d transition metal compounds during X-ray photoelectron spectroscopy. J. Electron. Spectrosc. Relat. Phenom. 1974, 4, 263. Yu Yao, Y. F. The oxidation of CO and C2H4 over metal oxides. J. Catal. 1975, 39, 104. Yu Yao, Y. F.; Kummer, J. T. A study of high temperature treated supported metal oxide catalysts. J. Catal. 1977, 46, 388.
Received for review July 15, 1997 Revised manuscript received November 12, 1997 Accepted November 12, 1997 IE970494H
