Al2O3−ZrO2. Effect of Zirconia

Characterization of palladium catalysts, supported on alumina and alumina modified by zirconia added by impregnation, was performed by CO adsorption f...
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Langmuir 2004, 20, 10490-10497

Infrared Study of CO Adsorbed on Pd/Al2O3-ZrO2. Effect of Zirconia Added by Impregnation Hugo Tiznado,*,† Sergio Fuentes,‡ and Francisco Zaera§ Programa de Posgrado en Fı´sica de Materiales, CICESE, Ensenada, B.C. 22860, Me´ xico, Departamento de Cata´ lisis, Centro de Ciencias de la Materia Condensada-UNAM, Apartado Postal 2681, Ensenada, B.C. 22830, Me´ xico, and Department of Chemistry, University of California, Riverside, California 92521 Received February 13, 2004. In Final Form: June 30, 2004 Characterization of palladium catalysts, supported on alumina and alumina modified by zirconia added by impregnation, was performed by CO adsorption from 143 to 298 K and monitored by infrared spectroscopy. It was found that the population of the Al3+ octahedral sites in the alumina decreased by the addition of zirconia. In contrast to the case of the pure alumina support, where stabilization of Pd+ was observed, Pd2+ was formed preferentially on samples where zirconia was added, and higher crystallinity in the metallic palladium was observed. Studies of CO adsorption at low temperatures (143 K) gave a better description of the surface species, since at higher temperatures (298 K) the reaction of the CO with some of the palladium oxide particles led to the partial reduction of the latter.

Introduction Because of their low cost, their ability to catalyze the oxidation of hydrocarbons, and their durability under hightemperature conditions, palladium-only three-way catalysts (Pd-only TWC) are commonly used in automotive converters with closed-loop control systems.1-6 In fact, fourth-generation Pd-only TWC automotive catalysts have been available in the market since 1995.7,8 Pd-only catalysts can be quite effective as TWC for the simultaneous removal of NO, CO, and hydrocarbons from automobile exhaust gases,3,9 but they typically display poor NO conversion efficiencies compared to those seen with Rh-based TWC.10,11 Nevertheless, the catalytic activity of Pd-only TWC has been improved by contacting palladium with rare earth oxides. For instance, Muraki * To whom correspondence should be addressed at CCMCUNAM, P.O. Box 439036, San Ysidro, CA, 92143. E-mail: hugot@ ucr.edu. † CICESE. ‡ UNAM. § University of California, Riverside. (1) Hepburn, J. S.; Patel, K. S.; Meneghel, M. G.; Gandhi, H. S. Soc. Automot. Eng. Techn. Pap. Ser. 1994, paper no. 941058. (2) Lemaire, A.; Massardier, J.; Praliaud, H.; Mabilon, G.; Prigent, M. In Catalysis and Automotive Pollution Control III; Frennet, A., Bastin, J.-M., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1995; Vol. 96, pp 97-108. (3) Summers, J. C.; Williamson, W. B. ACS Symp. Ser. 1994, 552, 94. (4) Ciuparau, D.; Bensalem, A.; Pfefferle, L. Appl. Catal. B 2000, 26, 241. (5) Beck, D. D.; Sommers, J. W. Appl. Catal. B 1995, 6, 185. (6) Engler, B. H.; Lindner, D.; Lox, E. S.; Sha¨fer-Singlinger, A.; Ostgathe, K. In Catalysis and Automotive Pollution Control III; Frennet, A., Bastin, J.-M., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1995; Vol. 96, pp 441-460. (7) Yang, Z.; Chen, X.; Niu, G.; Liu, Y.; Bian, M.; He, A. Appl. Catal. B 2001, 29, 185. (8) Heck, R. M.; Farrauto, R. J. Catalytic Air Pollution Control: Commercial Technology; Van Nostrand Reinhold: New York, 1995. (9) Shelef, M.; Graham, G. W. Catal. Rev.-Sci. Eng. 1994, 36, 433. (10) Lindner, D.; Lox, E. S.; Van Yperen, R.; Ostgathe, K.; Kreuzer, T. Soc. Automot. Eng. Tech. Pap. Ser. 1996, paper no. 960802. (11) Monroe, D. R.; Krueger, M. H.; Beck, D. D.; D’Aniello, M. J. In Catalysis and Automotive Pollution Control II; Crucq, A., Ed.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1991; Vol. 71, pp 593-616.

et al.12,14 has reported that lanthana increases the reduction of NO in palladium catalysts supported on R-Al2O3. The catalytic activity of Pd catalysts depends both on their method of preparation15,16 and on the composition of the support.17,18 For Pd supported on Al2O3 catalysts modified with CeO2 and/or ZrO2, the extent of the effect of this promotion on both the reduction of NO and the oxidation of CO depends on the interaction between the support and the palladium, which determine the nature of the Pd species and the size of their particles. Thus, the physical chemical characteristics of the modifier play an active role in these reactions,19,20 and a close control of the composition of the support is therefore important to obtain the best thermal stability and catalytic activity.21,22 One attractive route for the preparation of catalysts is by solgel methods. The sol-gel approach provides the ability to mix several components in a single step and to control the structure and composition of the final solid mix at a molecular level.23 Sol-gel methods also provide more homogeneous Al2O3-MOx binary oxides with enhanced performance for the reduction of NO. In the past, we have reported that palladium catalysts deposited on supports prepared by sol-gel display en(12) Muraki, H. Soc. Automot. Eng. Tech. Pap. Ser. 1991, paper no. 910842. (13) Muraki, H.; Yokota K.; Fujitani, Y. Appl. Catal. 1989, 48, 93. (14) Muraki, H.; Shinjoh, H.; Sobukawa, H.; Yokota, K.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 202. (15) Briot, P.; Primet, M. Appl. Catal. 1991, 70, 87. (16) Park, P. W.; Ledford, J. S. Appl. Catal. B 1998, 15, 221. (17) Cordatos, H.; Bunluesin, T.; Gorte, R. J. Surf. Sci. 1995, 323, 219. (18) Ogata, A.; Obuchi, A.; Mizuno, K.; Ohi, A.; Ohuchi, H. J. Catal. 1993, 144, 452. (19) Ferna´ndez-Garcı´a, M.; Martı´nez-Arias, A.; Iglesias-Juez, A.; Hungrı´a, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. Appl. Catal. B 2001, 31 (1), 39. (20) Martı´nez-Arias, A.; Ferna´ndez-Garcı´a, M.; Iglesias-Juez, A.; Hungrı´a, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. Appl. Catal. B 2001, 31 (1), 51. (21) Bozo, C.; Guilhaume, N.; Garbowski, E.; Primet, M. Catal. Today 2000, 59, 33. (22) Bozo, C.; Guilhaume, N.; Herrmann, J.-M. J. Catal. 2001, 203, 393. (23) Ward, D. A.; Ko, E. I. Ind. Eng. Chem. Res. 1995, 34, 421.

10.1021/la049606h CCC: $27.50 © 2004 American Chemical Society Published on Web 10/30/2004

Effect of Impregnated ZrO2 in Pd/Al2O3-ZrO2

hanced reducibility properties, and that those changes influence the activity of the catalyst toward NO reduction by hydrogen.24 It was suggested in that work that the higher selectivity toward N2 production found in Pd/ Al2O3-La2O3 compared to Pd/Al2O3 at temperatures below 523 K is due to the extent of interaction between the Pd particles and reduced lanthana species. Also, palladiumonly TWC containing additives such as Zr, Ce, V, and La, prepared by sol-gel techniques, all presented high activity, good thermal stability and resistance to SO2.25 These results are consistent with previous work suggesting that addition of zirconia improves the thermal stability of ceria and palladium in such catalysts.26 The effect of zirconia on the catalytic properties of palladium has been demonstrated in other reactions too. For instance, Shen et al.27 have reported that the nature of the support influences the catalytic activity and selectivity of palladium in CO hydrogenation. It was shown there that while palladium supported on zirconium oxide leads to CO conversions of 4.7% and selectivities toward methanol above 80%, palladium supported on alumina results in a CO conversion of only 2.3% and selectivity to methanol of 19%. This difference was attributed to metalsupport interactions: it was proposed that the cationic Pd species detected by XPS, present in Pd/zirconiasupported interfaces but not in Pd/alumina, may be responsible for the improvement in catalytic performance seen in that work. The reports summarized above point to the need for a systematic understanding of the role of zirconia in the Pd/Al2O3-ZrO2 system. It also gives incentive for the search of new solids with improved catalytic properties using sol-gel preparation methods. To establish a reference point toward those goals, the present work addresses the effect of zirconia over the oxidation state of palladium when incorporated via impregnation methods. A systematic study is presented below on the characteristics of Pd/Al2O3-ZrO2 solids as a function of oxidation and reduction treatments based on infrared spectroscopy characterization of adsorbed carbon monoxide from low (LT: ∼143 K) to room (RT: 298 K) temperatures. Experimental Section Catalyst Preparation. Two supports were used in the studies reported below. The first was a commercial pure γ-alumina (AlfaAesar, 99.8%), which has a surface area of 86 m2/g as received and 65 m2/g after calcination at 923 K. The infrared experiments were performed with the as-received powder, designated as Alref. The second support was a zirconia-modified solid, prepared by adding zirconium 2,4-pentanedionate dissolved in ethanol to the as received commercial γ-alumina mixed with water in a reflux system to yield a nominal 12 wt % of ZrO2. The latter mix was heated from room temperature to 393 K, kept at that temperature for 4 h, and distilled at a reduced pressure. The temperature in the latter step was kept at 393 K for 12 h, and the resulting powder was transferred to a furnace and calcined at 673 K in a nitrogen atmosphere for 3 h, and finally oxidized in air at 923 K for another 3 h. This second support, designated as Al-Zr12, displayed a surface area of 83 m2/g even after calcination. Clearly, zirconia prevents the loss of surface area in the alumina. For the preparation of the palladium catalysts, each support was mixed in a beaker with water at 323 K for 3 h, and a palladium (24) Barrera, A.; Viniegra, M.; Bosch, P.; Lara, V. H.; Fuentes, S. Appl. Catal. B 2001, 34(2), 97. (25) Noh, J.; Yang, O.-B.; Kim, D. H.; Woo, S. I. Catal. Today 1999, 53, 575. (26) So, H. S.; Yang, O. B.; Kim, D. H.; Woo, S. I. In Proceedings of the 12th International Congress in Catalysis; Corma, A., Melo, F. V., Mendioroz, S., Fierro, J. L. G., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 2000; Vol. 130B, pp 1379-1384. (27) Shen, W. J.; Okumura, M.; Matsumura, Y.; Haruta, M. Appl. Catal. A: Gen. 2001, 213, 225.

Langmuir, Vol. 20, No. 24, 2004 10491 chloride solution (pH ∼ 1.5) was slowly added to yield a nominal 0.3 wt % palladium. The stirring was kept for 3 more hours, after which the mixture was filtered, dried at 323 K, and calcined in air at 923 K for 3 h. At that point the color of catalysts was slightly yellow. A low palladium loading was chosen here in order to improve its dispersion and favor metal support interactions. The catalysts produced by this method were designated as Pd/ Alref and Pd/Al-Zr12, respectively. Infrared Spectroscopy Characterization. The IR spectra of CO adsorbed on the catalysts were recorded at a resolution of 4 cm-1 by using a Bruker Equinox 55 Fourier transformed infrared (FTIR) spectrometer in transmittance mode.28 In situ experiments were carried out in a quartz cell with NaCl windows capable of working at temperatures from 143 to 823 K and pressures from 10-2 to 760 Torr. The powder catalyst samples were pressed into self-supporting disks 13 mm in diameter and less than 20 mg in weight and placed at the center of the cell. They were then exposed to set pressures of carbon monoxide (Matheson Research grade) after purification of the gas by using a liquid nitrogen-cooled trap. To minimize changes on the catalyst surface due to reactions of the CO probe with adsorbed oxygen, the temperature of adsorption was initially lowered to 143 K (LT) and then gradually increased at a rate of approximately 10 K/min by means of a homemade temperature controller until reaching 298 K (RT). Experiments were carried out both in the presence of the CO gas and after pumping of the cell. The CO IR spectra were obtained by ratioing against background traces obtained before CO adsorption. The CO-titration FTIR experiments were performed after pretreating the samples with combinations of oxidation (oxygen, 100 Torr) and reduction (hydrogen, 100 Torr) cycles, each of 1 h of duration, followed by evacuation for 2 min at the pretreatment temperature. These treatments are labeled in this report by a capital letter to denote the type of treatment (O, oxidation; H, reduction) followed by a number to indicate the temperature used. For instance, oxidation at 823 K followed by reduction at 823 K is indicated as O823/H823. Using that convention, the two sets of experiments performed in these studies can be denoted as follows: (1) two O673, CO IR, O673/H673, CO IR cycles; and (2) two O823, CO IR, O673/H823, CO IR, O823/H823, CO IR cycles. CO IR spectra were registered after each step within these cycles, as indicated. The multiple-cycle experiments were carried out to test the reproducibility of the CO spectra after the oxidation and reduction treatments. As a standard cleaning procedure, all samples were heated under vacuum from RT to 673 K, sequentially oxidized and reduced at 673 K, and evacuated for 2 min at the same temperature, prior to the performance of the very first CO adsorption cycle. The supports were taken directly to the abovedenoted cycle (2) after the cleaning procedure. For the sake of clarity and brevity, this report will focus on the results from the O823 and O823/H823 experiments; those including any 673 K pretreatment will be considered only when required.

Results and Discussion Due to the large number of bands found in this work, it is convenient to summarize them in a quick reference table (Table 1). Alumina Support, Alref. The CO IR spectra obtained for the Alref support after an O823/H823 treatment, recorded at LT, are shown in Figure 1A. Four peaks were detected under 5 Torr of CO, namely, S1 at 2130 cm-1, S2 at 2160 cm-1, S3 at 2189 cm-1, and S4 (shoulder) at 2236 cm-1 (Figure 1a). These bands are characteristic of γ-alumina, as reported by Zecchina et al.,29 and correspond to CO adsorbed on the different sites of the alumina surface. First, the S1 band is in close agreement with that reported at 2135-2140 cm-1 for multilayer CO physisorption. (28) Zaera, F. Int. Rev. Phys. Chem. 2002, 21, 433. (29) Zecchina, A.; Escalona-Platero, E.; Otero-Area´n, C. J. Catal. 1987, 107, 244.

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Figure 1. Low-temperature (LT: ∼143 K) CO infrared spectra for the Alref (A) and Al-Zr12 (B) supports. The samples were O823/H823 pretreated prior to these CO adsorption experiments. Spectra are shown after 20 min under 5 Torr CO loading (a, e) and following evacuation for 0.5 (b, f) and 45 (c, g) min. Spectra d and h correspond to O823 pretreated samples, obtained after evacuation for 45 min. Table 1. Summary of Bands Observed in This Work band

wavenumber/cm-1

assignment and reference

S1 S2 S3 S4 Z1 L0 L0 L1 L1 L2 L2 M1

2130 2160 2184-2210 2225-2240 2171-2178 2091-2110 2071-2078 2116-2123 2135-2140 2153-2158 2178 1968-1981

M2 M3 M4

1942-1955 1920-1930 1880-1885

multilayer CO on alumina (physisorbed)29 CO on octahedral Al3+ 29 CO on tetrahedral Al3+ 29 CO on strong Lewis acid Al3+ 29 CO on Zr4+ located in regular patches31 linear CO on terraces of Pd0 34,38 linear CO on low coordination Pd0 37,41 linear CO on Pd+ 34 linear CO on ionic Pd (previously reported,38 but not assigned) linear CO on Pd2+ 34 (two species: easy and hard to reduce Pd2+) linear CO on highly dispersed Pd2+ supported on zeolites35 compressed 2-fold CO on Pd(100)34,42-44 or 2-fold CO on the edges of palladium aggregates with Pd(111) facets45 2-fold CO on Pd(100)34,37 2-fold CO on Pd(111)37,40 3-fold CO on Pd(111)34,37

Second, the S2 and S3 bands have been assigned to CO weakly σ-bonded to octahedral (Al3+oct) and tetrahedral (Al3+tet) aluminum surface ions, respectively. Finally, the S4 band has been assigned to CO σ-bonded to strong cationic Lewis acid sites. After evacuation for 0.5 min, an initial strong decrease of the S2 band is seen (Figure 1b). However, a stable remanent persists which is almost unaffected even after 45 min of pumping (Figure 1c). A similar, although less pronounced, behavior is observed for the S1 band. On the other hand, the intensity of the S3 band decreases more slowly and blue-shifts gradually up to 2207 cm-1. According to Zecchina et al.,29 this shift is due to an increase in the acceptor ability of the Al3+tet sites as the CO coverage decreases on the surface, since this allows for a depopulation of the CO 5σ antibonding orbital, which in turn strengthens both the C-O bond and its stretching frequency. To investigate the effect of treatment procedure on the properties of the surface, the same Alref sample was characterized directly after oxidation at 823 K, O823. S1, S2, S3, and S4 IR bands similar to those seen for the O823/H823 treatment were observed here with 5 Torr of CO (not shown). However, after as little as 10 min of evacuation, the S1 and S2 bands completely disappear, and after 45 min of pumping, only the S3 and S4 IR bands remain in the spectra (Figure 1d). These results show that oxygen treatments without subsequent hydrogen reduction decrease the CO adsorption on the octahedral Al3+ sites (as well as the extent of the physisorption). A subsequent O823/H823 treatment restores the intensities

of the S1 and S2 peaks (not shown), a result that indicates the reversibility of these processes. It is interesting to note that Hsiao et al.30 observed no CO adsorption on single-crystal γ-alumina. They suggested that their surface was oxygen-terminated and had no Al3+ sites available for CO adsorption. Similarly, we believe that in our case some of the Al3+ sites may end up coordinated to oxygen atoms after O2 treatments. In that case the sites may act as a Lewis base and may not be able to adsorb CO (a soft base). The oxygen coordinated to the aluminum ions can then be removed during H2 treatment and the Al3+ sites consequently be restored. Alumina-Zirconia Support, Al-Zr12. For the alumina-zirconia support, Al-Zr12, several differences were observed with respect to Alref. For one, after O823/H823 treatments and under a 5 Torr CO atmosphere, the intensity of the S2 band is smaller (relative to the S3 band) than in Alref (Figure 1e). This indicates that the addition of zirconia decreases the amount of octahedral Al3+ sites. There is also a new signal at 2177 cm-1 (Z1) in the spectra for Al-Zr12, which can be assigned to CO adsorbed on Zr4+ sites located in regular patches of lowindex crystal planes.31 It has previously been reported that, because of the larger relative radius of the Zr4+ ion, only a limited amount of zirconia can be incorporated into the structure of alumina to form solid solutions, ∼5 wt % ZrO2 in γ-alumina.32 To check if zirconia can be incorpo(30) Hsiao, G. S.; Erley, W.; Ibach, H. Surf. Sci. 1998, 405, L465. (31) Morterra, C.; Cerrato, G.; Pinna, F. Spectrochim. Acta, Part A 1999, 55, 95. (32) Low, I. M.; McPherson, R. J. Mater. Sci. 1989, 24, 892.

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Figure 2. CO infrared spectra for the Pd/Alref catalyst after an O823 pretreatment. The sample was exposed to 5 Torr of CO for 20 min at LT (∼143 K) and evacuated for 0.5 (a) and 45 (b) min. After that, the temperature was ramped to 223 (c) and 298 (d) K. At 298 K the IR cell was reloaded with 5 Torr of CO, and spectra were taken after 0.5 (e) and 20 (f) min of this exposure and after evacuation for 2.5 (g) and 45 (h) min.

rated into the structure of the alumina, a special sample with 3 wt % zirconia (less than the 12 wt % used here in the other experiments and the 5 wt % in the reference cited above) was prepared. A noticeable decrease of the S2 signal relative to the S3 band was seen, a result that advocates for the selective disappearance of the Al3+oct sites. However, since a small signal for the Z1 band was still detected, it was not possible to conclusively confirm if the zirconia actually replaces the Al3+oct ions or if it just blocks them. On the other hand, in the Al-Zr12 sample, even if a portion of the zirconia replaces Al3+oct ions, the large intensity of the Z1 signal strongly suggests that the rest is present in the form of large pure zirconia crystallites on the surface of alumina. This was confirmed by the transmission electron microscopy (TEM) and powder X-rays diffraction (XRD) experiments, where the presence of crystallites for the tetragonal phase of zirconia was clearly identified. The Z1 band completely disappears after evacuation for 3 min and is also not detected in a pure zirconia sample calcined at either 723 or 1173 K,31 indicating that the adsorption of CO in these Zr sites is weak. It is also noticed that the S4 band for the Al-Zr12 support can be detected only under 20 Torr of CO, and even there only as a very weak peak. This can be interpreted as the result of zirconia inhibiting the formation of strong cationic Lewis acid sites, perhaps via the loss of acidity in the surface caused by the zirconia. According to observations of Damyanova et al.,33 the strong Lewis acid sites in ZrO2/Al2O3 oxides prepared by impregnation of zirconium alkoxide on alumina decrease with zirconia content. After evacuation for 0.5 min, the intensity of all bands in Figure 1e decrease (Figure 1f), and after 45 min the S1 and S2 signals are barely detected (Figure 1g). Finally, after oxygen pretreatment of the Al-Zr12 support (without further reduction), only the S3 band is seen following CO exposure plus evacuation for 45 min (Figure 1h). The absence of the S1 and S2 bands in that case reinforces the observation that CO adsorption is significantly weaker after oxygen pretreatments. Palladium on the Reference Alumina, Pd/Alref. The low-temperature IR spectra of CO adsorbed on Pd supported on commercial γ-alumina, Pd/Alref, recorded after oxidation at 823 K, are shown in Figure 2. After only 0.5 min of evacuation, all the IR bands for the support are clearly observed and, in fact, dominate the overall spectra (33) Damyanova, S.; Grange, P.; Delmon, B. J. Catal. 1997, 168, 421.

(Figure 2a). After 45 min, on the other hand, new bands are uncovered at 2121 (L1) and 2158 cm-1 (L2) (Figure 2b). Although the positions of the L2 and S2 bands are close, they display significantly different behavior as a function of both time of pumping and temperature. For the Alref sample after a O823 pretreatment, the S2 band practically disappears after 45 min of evacuation (Figure 1d), but the L2 band remains and is still seen even after heating to 298 K (see below). These observations indicate that the species associated with the L2 band are of a different nature than those due to the S2 band. Thus, we can assign the L1 and L2 peaks to CO adsorbed on Pd+ (CO-Pd+) and Pd2+ (CO-Pd2+) species, respectively.34 The L1/L2 intensity ratio in this sample amounts to less than unity (0.3). Figure 2 also shows the effect of raising the temperature of the catalyst. At 223 K, the S4 band disappears and the S3 signal decreased significantly, the same as in Alref (Figure 2c). Concurrently, a new band appears at 2178 cm-1 (L2′), which we assign to Pd2+ highly dispersed species such as those reported for Pd on zeolites.35 The L2 band decreases 35% in intensity, while the L1 band grows 20% with increasing temperature, suggesting that the Pd2+ species are converted to Pd+. A weak signal for carbonates at 1684 and 1785 cm-1 (not shown) indicates that some adsorbed CO is converted to CO2. Finally, the S3 disappears during ramping of the temperature to 298 K, and the intensities of both IR peaks due to CO adsorbed on palladium decrease, the L2 peak being the most affected (Figure 2d). These changes are mainly due to CO desorption, but some conversion to CO2 is also detected. In addition, the L1′ peak appears at 2138 cm-1, perhaps indicating the formation of an intermediate species between Pd+ and Pd2+. Figure 2e shows the IR spectra obtained 0.5 min after reloading the cell with CO. All L1, L1′, L2, S3, and S4 (trace) bands are observed there. However, after 20 min, the intensity of the S3 signal decreases again (Figure 2f). This effect is observed for both supports, oxidized or reduced, but only when the CO is reloaded immediately after ramping of the temperature. We believe that this effect is due to the stabilization of the actual temperature of the sample by the gas over time (it is possible that the temperature reading from the thermocouple used in these (34) Tessier, D.; Rakai, A.; Bozon-Verduraz, F. J. Chem. Soc., Faraday Trans. 1992, 88(5), 741. (35) Davydov, A. In Molecular Spectroscopy of Oxide Catalyst Surfaces; Sheppard, N. T., Ed.; Wiley: Hoboken, NJ, 2003; Chapter 3.

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Figure 3. CO infrared spectra for the O823/H823-pretreated Pd/Alref catalyst. The sample was exposed to 5 Torr of CO for 20 min at LT (∼143 K), and evacuated for 0.5 (a) and 45 (b) min. Afterward, the temperature was ramped to 223 (c) and 298 (d) K. At 298 K the sample was then reloaded with 5 Torr of CO, and CO IR spectra were taken after 0.5 (e) and 20 (f) min of this exposure and after evacuation for 2.5 (g) and 45 (h) min.

experiments, which is located on the outside of the IR cell, lags behind the actual temperature of the catalyst under vacuum during the heating process). The L1/L2 ratio increases to more than one (1.6), indicating a partial reduction of Pd2+ to Pd+, and some absorption is also observed in the 2050-2100 cm-1 region in the form of a broad and not well-defined band. This is the spectral region associated with linear bonding of CO on top of metallic palladium,34 likely from reduction of the PdO species with the gas-phase CO at room temperature.27,34, Thus, the Pd2+ detected in the oxidized sample is attributed to easy-toreduce PdO. Additional evidence for the formation of Pd0 upon RT CO exposures is given by two new bands at 1957 (M2) and 1882 cm-1 (M4), previously assigned to bridgedbonded CO on Pd (100) and to 3-fold bridged CO on Pd (111) faces.34,37 Outgassing of the previous sample for 2.5 min at RT leads to the disappearance of the S3 band and to the uncovering of the L2′ peak (Figure 2g). After 45 min, a shoulder for L2′ is still visible, and the M2 band intensity increases, indicating further reduction to Pd0 (Figure 2h). Spectra for the Pd/Alref sample after hydrogen reduction at 823 K are shown in Figure 3. After CO exposure followed by 0.5 min of evacuation, the IR bands due to the support mask again those due to CO on palladium (Figure 3a). Then, after 45 min of pumping, the S2 (not present in the data for O823), S3 and S4 bands are downsized, and the S1 is suppressed (Figure 3b). As observed with the pure supports, the IR signals due to the support in these Pd catalysts are more intense after hydrogen pretreatments. The remaining signal in Figure 3b is seen as a broad peak from 2000 to 2160 cm-1, mostly a combination of L1 and L0 (2110 cm-1) peaks, the latter associated with CO linearly bonded to the terraces of metallic palladium. The presence of the L1 and L2 signals indicates that cationic palladium species are still present in the alumina support even after high-temperature reduction. The broad band observed from 1840 to 2000 cm-1 corresponds to CO multicoordinated on metallic palladium. At 223 K, the intensity of the signal for L1 (2120 cm-1) in the CO IR spectra for Pd/Alref appears only as a shoulder and the more stable L0 (2109 cm-1) band is the main contribution, confirming that Pd+ species are present at LT (Figure 3c). According to Juszczyk et al.38 a possible (36) Cubeiro, M. L.; Fierro, J. L. G. Appl. Catal. A: Gen. 1998, 168, 307. (37) Palazov, A.; Kadinov, G.; Bonev, Ch.; Shopov, D. J. Catal. 1982, 74, 44.

explanation for the appearance of this Pd+ (the L1 band) is via oxidation of Pd0 by surface hydroxyl groups in the Pd/Al2O3 catalyst. However, it should be mentioned that OH groups have been also proposed as reducing agents in oxidizing environments.34 In any case, no evidence of this OH-induced chemistry was found under the experimental conditions used in our work (no changes were observed in the intensity of the O-H stretching region of the IR spectra, although that would be hard to detect because of the small number of Pd atoms compared to OH groups on the support). At 298 K all bands from the support disappear, and the intensities of the L0 and L2 peaks decrease (Figure 3d). A red shift of the L0 peak is detected, down to 2102 cm-1, likely due to a decrease in CO coverage.39-41 A stronger signal is also seen for multicoordinated CO species at RT (the broad band around 1830-1890 cm-1), maybe because of a conversion of the linear species as the temperature increases.34,39 Figure 3e shows the spectra obtained 0.5 min after reloading with 5 Torr of CO. There, the L0 band increases and shifts up to 2109 cm-1 because of an increase in CO coverage. The L2 band is the only signal observed that can be associated with cationic palladium, and two bands corresponding to the support, the S3 (2204 cm-1) and S4 (∼2237 cm-1) peaks, are also detected. After 20 min of adsorption the intensity of the L2 peak remains the same, denoting that the Pd2+ species detected in the reduced catalyst are more stable than those in the oxidized catalyst (Figure 3f). The intensity of the L0 band increases 18% but is accompanied by a red-shift back to 2104 cm-1, in contradiction with what would be expected from an increase in coverage. Such shift may be explained by the formation of more L0 sites due to the reduction of cationic palladium, perhaps the Pd+ present in trace amounts. Alternatively, CO could induce some Pd redispersion. The broad band from multicoordinated CO species observed at 298 K before the CO is reloaded is now resolved in two peaks at 1885 (M4) and 1940 cm-1 (M2), and a broad shoulder around 1970 cm-1 (M1) appears as well. The latter could possibly be ascribed to compressed bridged species on Pd (100),34,42-44 or alternatively to CO bridge(38) Juszczyk, W.; Karpinski, Z.; Ratajczykowa, I.; Stanasiuk, Z.; Zielinski, J.; Sheu, L.-L.; Sachtler, M. H. J. Catal. 1989, 120, 68. (39) Gelin, P.; Siedle, A. R.; Yates, J. T. J. Phys. Chem. 1984, 88, 2978. (40) Bradshaw, A. M.; Hoffman, F. M. Surf. Sci. 1978, 72, 513. (41) Blyholder, G. J. Phys. Chem. 1964, 68(10), 2772.

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Figure 4. CO infrared spectra for the O823-pretreated Pd/Al-Zr12 catalyst. The sample was exposed to 5 Torr of CO for 20 min at LT (∼143 K) and evacuated for 0.5 (a) and 45 (b) min. Afterward, the temperature was ramped to 223 (c) and 298 (d) K. At 298 K the sample was reloaded with 5 Torr of CO, and CO IR spectra were then taken after 0.5 (e) and 20 (f) min of this treatment and after evacuation for 2.5 (g) and 45 (h) min.

bonded on the edges of palladium aggregates with Pd(111) facets (see results for Pd/Al-Zr12 at RT).45 After 2.5 min of evacuation, the CO IR bands due to the support disappear, and the L0 peak decreases significantly and shifts down in frequency due to a lowering in CO coverage on these sites (Figure 3g). The intensities of the multicoordinated CO remain the same, indicating their higher stability compared with that of the linearly bonded species. Nevertheless, the IR intensity due to the multicoordinated CO decreases and red-shifts again, forming a broad band after 45 min of pumping (Figure 3h). Also, the L0 and L2 peaks are still present in the spectra, although with smaller intensities. Worth noticing from these data is the clear indication of the presence of Pd2+ ions on the surface (the L2 band), which is still detected above 448 K in samples reduced at 673 K. It is difficult to say what the origin and chemical nature of these species may be. One possibility is Clcontaining Pd2+ oxo complexes on γ-alumina, since here the palladium was deposited by impregnation of the support with PdCl2 solutions. However, no chlorine was detected by XPS in any of our solids (data not shown), and the species in question are in any case seen at high temperatures (448 K) well above those needed for the reduction of the Cl-Pd2+ complexes (around 393 K).46 Another possibility is the formation of two-dimensional palladium species with a strong interaction with the surface of the alumina support.47,48 However, those species are reducible around 623 K49 and thus difficult to associate with our Pd2+. All we can conclude for now is that the Pd2+ detected in the reduced samples of this work is more stable than that observed in the oxidized samples and that its concentration appears to be quite low in all cases. Palladium on Alumina-Zirconia, Pd/Al-Zr12. In the case of the palladium catalyst supported on the aluminazirconia support, the CO IR data obtained after CO (42) Ortega, A.; Hoffman, F. M.; Bradshaw, A. M. Surf. Sci. 1982, 119, 79. (43) Szanyi, J.; Kuhn, W. K.; Goodman, D. W. J. Vac. Sci. Technol. A 1993, 11(4), 1969. (44) Rainer, D. R.; Wu, M.-C.; Mahon, D. I.; Goodman, D. W. J. Vac. Sci. Technol. A 1996, 14(3), 1184. (45) Wolter, K.; Seiferth, O.; Kuhlenbeck, H.; Ba¨umer, M.; Freund, H.-J. Surf. Sci. 1998, 399, 190. (46) Frusteri, F.; Arena, P.; Parmaliana, A.; Mondillo, N.; Giordano, N. React. Kinet. Catal. Lett. 1993, 51(2), 331. (47) Chen, J. J.; Ruckenstein, E. J. Phys. Chem. 1981, 85, 1606. (48) Ruckenstein, E.; Chen, J. J. J. Colloid Interface Sci. 1982, 86(1), 1. (49) Lieske, H.; Vo¨lter, J. J. Phys. Chem. 1985, 89(10), 1841.

exposure followed by outgassing for 0.5 min at LT show bands for the support (S1, Z1, and S3), except for the expected S2 signal (Figure 4a). Adsorption of CO on palladium oxide is also observed, with a band centered at 2146 cm-1. We suggest that this band may be the result of contributions from the L1′ and L2 peaks (supporting evidence given below). After 45 min of pumping, the Z1 band is no longer present in the CO IR spectra; it disappears after 5 min (Figure 4b). A new band at 2078 cm-1 (L0′) develops during the evacuation, which, although associated with CO linearly adsorbed on Pd0, is of a different nature than those found in Pd/Alref (L0). The lower frequency of L0′ with respect to L0 (2078 vs 2110 cm-1) suggests that the CO is linearly bonded on Pd0 particles of lower metal coordination numbers, those associated with steps and other defects.41 This assignment has been demonstrated on Pt crystals.50-52 Alternatively, the L0′ peak may represent a unique site at the Pd-ZrO2 interface. The effect of increasing the temperature of the Pd/AlZr12 catalyst after CO adsorption is also shown in Figure 4. At 223 K the band at 2146 cm-1 decreases 5%, and its maximum shifts down to 2142 cm-1 and broadens on the high-frequency side (Figure 4c), merging with the L1′ and L2 bands at 298 K (Figure 4d). The L0′ peak increases about 4 times in intensity (relative to the peak observed at ∼143 K), indicating the further formation of ZrO2modified metallic palladium, but no bands for Pd+ (L1) are evident. Given that the amount of Al3+oct sites decreases in the presence of zirconia (see above), it can be suggested that those are the sites involved in the stabilization of the Pd+ species found in Pd/Alref.38 Next, it can be seen in Figure 4 that 0.5 min after CO reloading leads to increases in the intensities of the L1′ (30%) and L2 bands (110%), which are clearly resolved into two peaks (Figure 4e). The region for linear CO on Pd0 (2025-2115 cm-1) broadens, denoting that, besides L0′, at least one more band is also present in this spectrum, perhaps the L0 feature. The M2 and M4 bands are visible as well, in contrast with the case of Pd/Alref (where they appeared only after 20 min), strongly suggesting that the reduction of cationic palladium is easier when zirconia is present on the support. After 20 min of CO exposure, the (50) Zaera, F.; Liu, J.; Xu, M. J. Chem. Phys. 1997, 106(10), 4204. (51) Xu, J.; Henriksen, P. N.; Yates, J. T. Langmuir 1994, 10, 3663. (52) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G. Surf. Sci. 1985, 149, 394.

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Figure 5. CO infrared spectra for the O823/H823-pretreated Pd/Al-Zr12 catalyst. The sample was exposed to 5 Torr of CO for 20 min at LT (∼143 K), and evacuated for 0.5 (a) and 45 (b) min. Afterward, the temperature was ramped to 223 (c) and 298 (d) K. At 298 K the sample was reloaded with 5 Torr of CO, and CO IR spectra were taken after 0.5 (e) and 20 (f) min of this exposure and after evacuation for 2.5 (g), 25 (h), and 45 (i) min.

L1′ and L2 band intensities decrease; reduction of cationic palladium by CO continues (Figure 4f). The L0 (2108 cm-1) band clearly emerges, indicating adsorption of CO in terraces, as expected for higher CO pressures. Also, the M1 band grows in, as it was not the case for the Pd/Alref sample. It has been reported that the intensity of this band can be enhanced by large palladium particles, in particular if those lead to a better ordering and an increased crystallinity of the metal.34,45,53 Thus, it can be suggested that the metallic palladium phase formed on the Pd/Al-Zr12 catalyst is more crystalline than on the Pd/Alref. After 2.5 min of evacuation, the L1′ signal decreases, and a weak peak for L1 appears (Figure 4g). The L0 band also decreases 40% and red-shifts to 2097 cm-1, again because of a lowering in CO coverage. Finally, after 45 min (Figure 4h), the proportion of metallic palladium relative to palladium oxide is 25% larger than in the absence of zirconia (Pd/Alref). Following a combined O823/H823 treatment of the Pd/ Al-Zr12 sample, the IR data obtained after CO adsorption at LT followed by 0.5 min of pumping show S1, S3, and traces of the Z1 bands, but no signals for S2 and S4 (Figure 5a). This indicates that the zirconia decreases the amount of octahedral and strong cationic Lewis acid sites in alumina, the same as in the pure supports, without any added palladium. After 45 min of evacuation, the bands associated with CO linearly coordinated to palladium are seen at 2112 (L0), 2147 (L2), and 2124 cm-1 (L1, small trace) (Figure 5b). The presence of the L1 peak may be explained by the oxidizing effect of the OH groups, as previously proposed for Pd/Alref. The IR spectra in the OH region is quite similar for the Pd/Alref and Pd/Al-Zr12 catalysts (Figure 6); thus, the same effect could be expected in both. The intensity of the L2 band relative to L0 is higher in Pd/Al-Zr12 than in Pd/Alref, indicating the formation of more hard-to-reduce Pd2+ when zirconia is present. In connection with the multicoordinated CO, its IR signal is centered on the M1 peak, but the absorption signal extends to 1840 cm-1, denoting the presence of additional bands, likely M2 and M3. Increasing the temperature of the Pd/Al-Zr12 solid to 223 K causes the L1 band to completely disappear (Figure 5c). Then, at 298 K, the intensity of M1 clearly increases, likely because of the conversion of the linear CO to bridge (53) Hicks, R. F.; Qi, H.; Kooh, A. B.; Fischel, L. B. J. Catal. 1990, 124, 488.

Figure 6. Infrared spectra in the O-H stretching region for the O823/H823-pretreated Pd/Alref (a) and Pd/Al-Zr12 (b) catalysts, both recorded at LT.

species (Figure 5d).34,39 After 0.5 min of reloading with CO at RT, all band intensities increase (Figure 5e), but L0′ is masked by L0. After 20 min of exposure to CO, the coverage on metallic palladium increases, as indicated by the intensity increase of the M1 peak at 1981 cm-1, which may be associated with well-ordered crystalline facets (Figure 5f). The intensity of the L2 signal remains almost invariable, pointing to the stability of the Pd2+ species against reduction, the same as with Pd/Alref. Perhaps these species are stabilized mainly on the surface of the alumina. The L0 band intensity also increases 10% and shifts down, as in Pd/Alref. Under vacuum, all IR intensities due to linearly adsorbed CO decrease rapidly, an effect already evident after 2.5 min (Figure 5g), while the M1 band grows somewhat. A small new signal at 1927 cm-1 (M3), attributed to CO bridge-bonded to Pd(111), start to develops.37,40 After 25 min, the signal at L0 decreases considerably, and the L0′ band is uncovered (Figure 5h). This indicates that the species associated with the L0′ band are more stable than those for L0, as expected because a lower C-O vibrational frequency typically correlates with a higher Pd-CO frequency, and that with a stronger Pd-CO bond. The linear Pd-CO bond is weaker on flat terraces, where the palladium atoms have a higher coordination number to other Pd atoms than those on steps, kinks, and other defects.41,50-52 At the same time, the M1 band intensity decreases significantly too, but the signal for M3 increases. Although the M2 band appears to gain some intensity after 25 min of evacuation, this is probably due to its overlapping with the M1 band, which

Effect of Impregnated ZrO2 in Pd/Al2O3-ZrO2

blue-shifts; in fact, after 45 min of evacuation, its intensity decreased a little (Figure 5i). A similar process has been described by Gelin et al.39 for Pd/SiO2 (10 wt %, 75 Å mean particle diameter), who provided evidence for a reversible CO coverage-induced bridged-to-linear conversion process with no permanent changes in the Pd crystal structure. That process involved the disappearance of both a linear band at 2103 cm-1 and a bridge band at 1995 cm-1 and the concomitant formation of another adjacent bridge state at 1979 cm-1. However, the IR band seen for bridged CO on Pd(111)54 at 1996 cm-1 does not behave like the feature at 1995 cm-1 reported for the Pd/SiO2 system. This rules out the possible assignment of our M1 band to CO bridgebonded on Pd(111) facets and may instead be associated with Pd(100) planes,42-44 or possibly with CO bridgebonded on the edges of palladium aggregates.45 Regardless of the specific assignments, however, the stronger intensity of the M1, and in general of all multicoordinated bands compared to those seen in the spectra for Pd/Alref, is likely to be associated with a higher degree of crystallinity.45 Since higher crystallinity is often seen in catalysts with big metal particles,45,53 it is tempting to conclude that our catalysts contain large crystalline particles. To investigate if this is the case, the palladium dispersions were measured via hydrogen uptake at 353 K and estimated at 26 and 31% for the Pd/Alref and Pd/Al-Zr12 catalysts, respectively. Clearly, no dramatic differences in dispersion could be identified by this technique to explain the CO infrared spectra. Further characterization by transmission electron microscopy (TEM) was somewhat hampered by the low concentration of palladium (0.3 wt %) in our samples (which made a statistical estimation of the particle sizes not viable), but useful information was obtained by detailed characterization of individual particles. The typical Pd particle of the Pd/Alref catalyst was found to be large (∼30 nm of diameter), of irregular shape, and probably multiple-twinned (given the several gray tones seen within each particle). On the other hand, the typical Pd particle imaged for the Pd/Al-Zr12 catalyst is smaller (∼16 nm of diameter), of better-defined shape (with more angular features), and with a homogeneous gray tone, suggesting that it may be a single crystallite. Thus, although the palladium particles in the Pd/Al-Zr12 catalyst appear to be smaller, they seem to have a higher degree of crystallinity: hence the higher intensity of the M1 band observed. (54) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107.

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After 45 min of pumping of that Pd/Al-Zr12 sample (Figure 5i), the L0 and M1 peaks become quite small, but the more stable M3, M2, and L0′ bands for metallic palladium and L2 for cationic palladium remain, as expected. Conclusions In this study, the nature of the surfaces of palladium catalysts supported on zirconia-modified alumina was characterized by infrared spectroscopy using carbon monoxide as a probe in conjunction with electron microscopy and hydrogen uptake measurements. Reference experiments on the supports, pure γ-alumina and γ-alumina impregnated with 12 wt % of zirconia, indicated that the addition of zirconia diminishes the amount of octahedral and strong Lewis acid aluminum sites in alumina. It was also determined that the presence of zirconia in the support favors the formation of single Pd particles. The reduction of PdO particles is facile and leads directly to the formation of crystalline Pd0 particles. On γ-alumina, on the other hand, the reduction of the Pd2+ species involves isolatable Pd+ intermediates. Larger particles of Pd0 are formed upon reduction, but those appear to have a more amorphous morphology. Additional hard-to-reduce Pd2+ species were detected in both reduced samples, the specific chemical nature of which could not be fully understood. From the experimental point of view, it was determined that since the reductive effect of CO is minimized at low temperatures (143 K), the spectra obtained at those temperatures are more representative of the state of the surfaces than those obtained at room temperature, specially for oxidized samples. Our temperature-dependent studies highlighted the chemical process behind this CO reduction. Acknowledgment. H.T. thanks UC-Mexus Conacyt for financial support during his visit to UC Riverside and CONACYT for his Ph.D. fellowship. This project was financed by Conacyt Project 41331-Y, UC Mexus-CONACYT, and the U.S. Materials Corridor Initiative. The authors also acknowledge E. Flores, P. Casillas, M. Sainz, J. Peralta, A. Tiznado, and F. Ruiz from CCMC-UNAM, and S. Sheldon from UC Riverside, for technical support. LA049606H