Characterization of Pd− Cu Alloy Nanoparticles on γ-Al2O3-Supported

9214

Langmuir 2006, 22, 9214-9219

Characterization of Pd-Cu Alloy Nanoparticles on γ-Al2O3-Supported Catalysts Vicente Sanchez-Escribano,†,‡ Laura Arrighi,§,| Paola Riani,§,| Rinaldo Marazza,§,| and Guido Busca*,†,| Laboratorio di Chimica delle Superfici e Catalisi Industriale, Dipartimento di Ingegneria Chimica e di Processo, UniVersita` di GenoVa, P.le Kennedy, I-16129 GenoVa, Italy, Departamento de Quimica Inorganica, UniVersidad de Salamanca, Plaza de la Merced, Salamanca, Spain, Dipartimento di Chimica e Chimica Industriale, UniVersita` di GenoVa, Via Dodecaneso 31, 16146 GenoVa, Italy, and Consorzio INSTM, Via Giusti, 9, I-50121 Firenze, Italy ReceiVed June 5, 2006. In Final Form: August 9, 2006

Cu-Pd/Al2O3 bimetallic catalysts have been characterized by XRD, TEM, and EDX techniques. The surface structure has been investigated by FT-IR spectroscopy of low-temperature adsorbed CO in the reduced and in the oxidized state. Evidence has been provided of the formation of Cu-Pd alloy nanoparticles, both of the R-phase (disordered fcc) and of the β-phase (ordered CsCl-type). IR spectra suggest that Cu likely decorates the edges while Pd mostly stays at the main faces. Part of copper disperses as Cu+ on the support even after reduction. The presence of copper seems to modify strongly the sate of oxidized Pd centers in oxidized high-Pd content materials. The redox chemistry of the system, where Pd is reduced more easily than Cu, appears to be very complex.

Introduction Bimetallic catalysts find application in a number of industrial processes such as, for example, catalytic re-forming (Pt-Re on chlorided alumina1), three-way catalysts for car engines gas purification (Pt-Rh on alumina2), and aromatic reduction in diesel fractions (Pd-Pt on silicalumina3). Surface science studies showed that frequently two metals interacting on a surface can form compounds with structures not seen in bulk alloys.4 On the other hand, the actual nature of the nanosized bimetallic particles or of monometallic segregated phases may also depend on the nature of the support5 and is very sensitive to a number variables.6 Palladium-copper bimetallic catalysts have excellent properties both for oxidation reactions and for catalysis in reducing atmospheres. They in fact are of interest as catalysts for the oxidation of CO7 and VOCs,8 for the oxygen-assisted water gas shift reaction,9 and the selective hydrogenation of dienes to olefins.10 They find today much interest for the hydrogenation of nitrates in water.11-13 We recently found interesting properties * Corresponding author. Phone/fax: +39-010-353-6024. E-mail: [email protected]. † Dipartimento di Ingegneria Chimica e di Processo, Universita ` di Genova. ‡ Universidad de Salamanca. § Dipartimento di Chimica e Chimica Industriale, Universita ` di Genova. | Consorzio INSTM. (1) Rønning, M.; Gjervan, T.; Prestvik, R.; Nicholson, D.; Holmen, G. A. J. Catal. 2001, 204, 292. (2) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419. (3) Jacquin, M.; Jones, D. J.; Rozie`re, J.; Jime´nez Lo´pez, A.; Rodrı´guezCastello´n, E.; Trejo Menayo, J. M.; Lenarda, M.; Storaro, L.; Vaccari, A.; Albertazzi, S. J. Catal. 2004, 228, 447. (4) Rodriguez, J. A. Surf. Sci. Rep. 1996, 24, 223. (5) Albertazzi, S.; Busca, G.; Finocchio, E.; Glo¨ckler, R.; Vaccari, A. J. Catal. 2004, 223, 372. (6) Guczi, L. Catal. Today 2005, 101, 53. (7) Choi, K. I.; Vannice, M. A. J. Catal. 1991, 131, 36. (8) Brayner, R.; dos Santos Cunha, D.; Bozon-Verduraz, F. Catal. Today 2003, 78, 419. (9) Bickford, E. S.; Velu, S.; Song, C. Catal. Today 2005, 99, 347. (10) Furlong, B. K.; Hightower, J. W.; Chan, T. Y. L.; Sarkany, A.; Guczi, L. Appl. Catal., A 1994, 117, 41.

of an alumina-supported Pd-Cu catalyst for the steam re-forming of propane and propene.14,15 Several previous studies have been devoted to the characterization of similar materials. The formation of Pd-Cu alloys with Pd surface segregation has been observed on reduced PdCu/γ-Al2O3 materials by Sun et al.16 using EELS, EDS, and STEM techniques, and by Batista et al.11 using EXAFS, TEM/ EDX, and hydrogen chemisorption, while surface decoration of Pd by Cu was suggested by Furlong et al.10 on the basis of hydrogen adsorption experiments. The two metals were also found to stay in close contact by Sa´ and Vinek.12 However, it is still not clear what alloy structures are formed17 in the supported particles, either the disordered face centered cubic phase (R) or the ordered cubic CsCl β structure, as found for some Pd-Cu/ MgO samples.18 Additionally, Molenbroek et al.19 found over their Pd-Cu/γ-Al2O3 catalysts Cu-rich solid solutions, according to TEM and EXAFS, while Sun et al.16 found, on similar catalysts, Pd surface enrichment on the alloy particles, based on EELS, EDX, and STEM. Copper enrichment has been reported in the case of Pd-Cu/SiO2 catalysts used for dehydrochlorination of 1,2-dichloroethane.20 Pd-Cu/SiO2 alloy materials are also of interest for their application as nonlinear optical materials.21 (11) Batista, J.; Pintar, A.; Padez _nik Gomils _ek, J.; Kodre, A.; Bonette, F. Appl. Catal., A 2001, 217, 55. (12) Sa´, J.; Vinek, H. Appl. Catal., B 2005, 57, 247. (13) Chaplin, B. P.; Roundy, E.; Guy, K. A.; Sharley, J. R.; Werth, C. J. EnViron. Sci. Technol. 2006, 40, 3075. (14) Resini, C.; Arrighi, L.; Herrera Delgado, M. C.; Larrubia Vargas, M. A.; Alemany, L. J.; Riani, P.; Berardinelli, S.; Marazza, R.; Busca, G. Int. J. Hydrogen Energy 2006, 31, 13. (15) Resini, C.; Herrera Delgado, M. C.; Arrighi, L.; Alemany, L. J.; Marazza, R.; Busca, G. Catal. Commun. 2005, 6, 441. (16) Sun, K.; Liu, J.; Nag, N. K.; Browning, N. D. J. Phys. Chem. B 2002, 106, 12239. (17) Subramanian, P. R.; Laughlin, D. E. J. Phase Equilib. 1991, 12, 231. (18) Giorgio, S.; Chapon, C.; Henry, C. R. Langmuir 1997, 13, 2279. (19) Molenbroek, A. M.; Haukka, S.; Clausen, B. S. J. Phys. Chem. B 1998, 102, 10680. (20) Lambert, S.; Heinrichs, B.; Brasseur, A.; Rulmont, A.; Pirard, J.-P. Appl. Catal., A 2004, 270, 201.

10.1021/la0616101 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006

Pd-Cu Alloy Nanoparticles on Alumina

Langmuir, Vol. 22, No. 22, 2006 9215

Figure 1. XRD patterns of the reduced catalysts and of the support: (a) the support, (b) CA, (c) PC5A, (d) PCA, (e) P5CA, and (f) PA. 9, main peaks of the R-phase; b, main peaks of the β-phase.

To characterize the surface properties of metal particles on Pd-Cu/γ-Al2O3, several IR studies of adsorbed CO22,23 have been performed. Skoda et al.24 concluded that Cu-free Pd particles and Cu on the surface of Pd particles coexist on their catalysts. Similar conclusions were also obtained more recently by Sa´ et al.25 Choi and Vannice7 found terminal carbonyls on copper centers together with bridging carbonyls over Pd centers. Much less data are available on oxidized Pd-Cu/γ-Al2O3 catalysts where copper oxide species were found by IR spectroscopy of adsorbed CO by Choi and Vannice.7 The production of hydrogen from hydrocarbons may be obtained either in pure steam re-forming conditions, in the conditions of partial oxidation, or finally in the conditions of oxidative steam re-forming, which is a combination of steam re-forming and partial oxidation. Depending on the reaction conditions, and also very likely on the position in the reactor, the catalyst particles may be in a more or less oxidized or reduced state. To learn more on the complex chemistry of such system, we tried to characterize alumina-supported Pd-Cu metal particles with XRD and TEM techniques, and by IR spectroscopy using CO as a probe in both the oxidized and the reduced states. Experimental Section The five catalysts investigated have been prepared by coimpregnation of the two metal hydroxides over γ-Al2O3 (from Degussa) and subsequently dried at 80 °C for 48 h. The dried samples were reduced with hydrogen at 673 K for 1 h. Pd(NO3)2‚2H2O and Cu(NO3)3‚3H2O were used as metal precursors. The metals/Al2O3 ratio is 10/90 (w/w) for all samples, and the Pd/Cu atomic ratio is ∞ (pure Pd on Al2O3 hereinafter denoted as sample PA), 5 (sample P5CA), 1 (sample PCA), 0.2 (sample PC5A), and 0 (pure Cu on Al2O3 sample CA). The surface area of all catalysts (21) Mattei, G.; Maurizio, C.; Sada, C.; Mazzoldi, P.; de Julian Fernandez, C.; Cattaruzza, E.; Battaglin, G. J. Non-Cryst. Solids 2004, 345-346, 667. (22) Vannice, M. A. In Catalysis, Science and Technology; Andeson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1982; Vol. 3, p 139. (23) Hadjiivanov, K. I.; Vayssilov, G. N. AdV. Catal. 2002, 47, 307. (24) Skoda, F.; Asier, M. P.; Pajonk, G. M.; Primet, M. Catal. Lett. 1994, 29, 159. (25) Sa´, J.; Gross, S.; Vinek, H. Appl. Catal., A 2005, 294, 226.

is in the range 90 ( 5 m2/g. The samples have been reduced in hydrogen flow at 673 K for 1 h before XRD and TEM measurements. X-ray powder diffraction patterns of reduced samples have been carried out with a X-Pert Philips diffractometer using a Cu KR radiation with 0.03 steps in a range from 15° to 100° 2θ with counting time equal to 4 s per step. The observed diffraction intensities have been compared to those calculated using the Pulverix program, and the values of the lattice parameters were refined using a least-squares routine. The average composition of each sample after reduction has been evaluated with electron probe microanalysis (EPMA) based on energy-dispersive X-ray spectroscopy (Oxford Instruments INCA 4.04); all of the samples have been analyzed employing an acceleration voltage of 20 kV for a counting time of 50 s, using a cobalt standard for calibration to monitor beam current, gain, and resolution of the spectrometer. Finally, the X-ray intensities were corrected for ZAF effects, using pure elements as standards. TEM measurements were performed with a transmission electron microscope (JEOL 2010). The powders were suspended in 2-propanol, and a drop of the resultant mixture was deposited on a gold grid previously covered with a thin carbon layer. EDX analysis of elements has been carried out by using a Pentafet Si(Li) detector with a probe diameter of 5 nm. Pressed disks of the pure catalysts powders were activated “in situ” by using an infrared cell connected to a conventional gas manipulation/outgassing ramp. All catalysts were first submitted to a treatment in air at 673 K, for 30 min, followed by evacuation at the same temperature before the adsorption experiments. Moreover, to obtain the reduced catalysts, after the mentioned evacuation, those were put in contact with a hydrogen pressure equal to 300 Torr at 673 K, for 30 min, and successively outgassed at the same temperature. CO adsorption was performed at 130 K by the introduction of a well-known dose of the gas (10 Torr) inside the low-temperature infrared cell containing the previously activated wafers. IR spectra were collected evacuating at increasing temperatures between 130 and 270 K.

Results and Discussion Characterization of Reduced Catalysts: XRD. Figure 1 shows the XRD patterns of Pd-Cu/Al2O3 samples. The alumina support shows the typical pattern of transitional alumina, mainly

9216 Langmuir, Vol. 22, No. 22, 2006

Sanchez-Escribano et al.

Figure 2. Variation of the lattice parameters of the disordered fcc R-type solid solution phase: 9, bulk literature data from ref 17; b, experimental data for our catalysts (nominal catalyst composition); 2, experimental data for our catalysts (theoretical particle composition).

γ-Al2O3. This pattern is evident in all cases, as expected. Additionally, the patterns of PA and CA show the peaks of the face centered cubic metal phase (cF4, space group Fm3hm), which is the thermodynamically stable one for both metals. The calculated unit cell parameters (0.3887 nm for Pd, 0.3611 nm for Cu) are in good agreement with the literature data for the bulk metals.17 Also, the samples P5CA and PC5A show the peaks of a cF4 fcc phase, denoted as the R-phase in the Pd-Cu state diagram. Only the pattern of the sample PCA shows together the peaks of the alumina support and of the fcc phase, and the peaks of a third phase, cP2, CsCl type, which is denoted as the β phase in the Pd-Cu state diagram, where this phase is reported to be thermodynamically stable for Pd atomic percent ∼38-45% at temperatures below 598 °C.17 The calculation of the lattice parameter for the fcc phase in the bimetallic samples (0.3857 nm for P5CA, 0.3789 nm for sample PCA, 0.3685 for sample PC5A) shows that the R-phases are constituted by solid solutions of the two metals. According to literature data,17 bulk R-phase Pd-Cu solid solutions behave in agreement with the Vegard’s law; that is, the dependence of the unit cell parameter with respect to the Pd to Cu atomic fraction gives rise to a straight line (Figure 2). The calculated unit cell parameters of our samples are out of this straight line. This is likely due to the fact that the real Cu-Pd composition of the metal particles is different from the virtual Cu-Pd composition for the overall catalyst. On the other hand, we can calculate a theoretical composition of the alloy particles just by putting the points on the straight line. According to this evaluation, the metal particles are enriched in Pd in all cases with respect to the catalyst composition: 28% versus 16.67% Pd/Cu a.r. for PC5A, 65% versus 50% for PCA, and 90% versus 83.33% for P5CA. We can suppose that part of the copper is still in the oxidized state even after reduction (see below) so that part of it does not participate in the metal alloy particle formation. To obtain a rough evaluation of the metal crystal size, we have applied the Scherrer formula to XRD peaks. The average crystal size was calculated to be in the range 7-17 nm for all metallic phase particles. Characterization of Reduced Catalysts: TEM. Transmission electron microscopy investigations of reduced Pd/γ-Al2O3 and Pd-Cu/γ-Al2O3 catalysts showed that samples are composed of supporting γ-Al2O3 seen as elongated and partly transparent

Figure 3. Transmission electron micrograph of the PCA reduced catalyst.

particles that form large agglomerates in which dark particles are observed uniformly distributed on the supported material. By EDX analysis, the dark particles were revealed to be Pd or Pd-Cu alloys with different Pd/Cu ratios depending on the analyzed sample. Some of these particles are faceted, indicating that they are single crystallites, whereas others seem to be agglomerates of smaller particles. In the single crystallites, the crystallographic planes are visible, and the metallic particle sites were determined to be in the range of 4-40 nm, most being below 15 nm. As an example of a TEM micrograph, in Figure 3a is shown the TEM image of the sample PCA. The presence of a black spot is underlined, containing palladium and copper, whose radius is about 6.5 nm (the beam dimension for qualitative microanalysis is 5 nm) and where lattice planes are well defined. Surface Characterization of the Materials through IR Spectra of Low-Temperature Adsorbed Carbon Monoxide. In Figure 4, the IR spectra of CO adsorbed at low temperature over PA catalyst after oxidizing (upper spectra) and reducing pretreatment (lower spectra) are reported. In the oxidized catalysts, a strong band composed of a main maximum at 2149 cm-1 and a pronounced shoulder at 2160 cm-1 is evident. This band is not observed in the case of the reduced catalyst. This band is certainly due to carbonyl species adsorbed over oxidized Pd centers, which are reduced by the treatment in hydrogen. The position of this band allows its assignment to carbonyls over Pd2+ sites.23 According to the literature, carbonyl species on Pd+ are expected to be observed between 2140 and 2115 cm-1: we have no bands in this region, and thus we do not find monovalent Pd sites in these conditions. At even higher frequencies, a “continuous” weak absorption is found only in the presence of CO gas in the region 2200-2170 cm-1 for the oxidized catalyst. In the reduced one, a quite definite,

Pd-Cu Alloy Nanoparticles on Alumina

Figure 4. FT-IR spectra of carbon monoxide adsorbed on Pd/γAl2O3 catalyst, after previous calcinations in air at 673 K (upper spectra) and after reduction in hydrogen 1 atm at 673 K (lower spectra). The first spectra in each series have been obtained in contact with CO 10 Torr at 130 K for 5 min. The other spectra were recorded upon outgassing (10-3 Torr) on warming to 270 K.

although also weak, band is found at 2195 cm-1. This feature is likely due to CO interacting weakly with Al3+ ions on the alumina support. The larger absorption in the case of the oxidized sample suggests that in this case some highly oxidized Pd4+ cations may be present too. In both oxidized and reduced surfaces, a strong band is found centered near 2100 cm-1 with an additional component envisaged as a shoulder in the region near 2060 cm-1. The maximum tends to shift in both cases to lower frequencies by outgassing and decreasing CO coverage, down to 2085 cm-1. These features are quite confidently assigned to terminal carbonyls adsorbed on zerovalent Pd atoms. In agreement with this, CO adsorption on Pd monocrystals shows that terminal carbonyls are found near 2095 cm-1 on both 111 and 100 planes.26 Following Lear et al.,27 the Pd-carbonyls absorbing in the range 2075-2090 cm-1 are on particle corners, while Pd-carbonyls absorbing at 2050-2060 cm-1 are on particle edges. At lower frequencies, only a very weak and broad absorption centered at 1930 cm-1 is found in the case of the oxidized catalyst. On the contrary, a very strong band is observed in the case of the reduced catalyst with a main maximum centered at 1981 cm-1, and a pronounced shoulder near 1900 cm-1. By outgassing, the main component decreases fast in intensity and its maximum tends to shift down to 1940 cm-1, while the lower frequency shoulder does not decrease in intensity and becomes predominant at lower CO coverages. The vibrational range of these absorptions allows their assignment to bridging carbonyls over Pd metal particles. According to Binet et al.28 and to Lear et al.,27 the sharper band in the region 1980-1940 cm-1 should be assigned to CO on Pd particles exposing a 100 type face, while the broader and more strongly bonded band in the 1930-1850 cm-1 region should be assigned to species adsorbed on Pd particles exposing a 111 type face. The analysis of the intensity of the bands shows that the reduction treatment causes the complete disappearance of the bands of oxidized Pd centers and the simultaneous strong growth of the bands of bridging carbonyls on zerovalent Pd. However, also the bands due to terminal carbonyls on zerovalent Pd grow (26) Bradshaw, A. M.; Hoffmann, F. M. Surf. Sci. 1978, 72, 513. (27) Lear, T.; Marshall, R.; Lopez-Sanchez, J. A.; Jackson, S. D.; Klapo¨tke, T. M.; Ba¨umer, M.; Rupprechter, G.; Freund, H.-J.; Lennon, D. J. Chem. Phys. 2005, 123, 174706. (28) Binet, C.; Jadi, A.; Lavalley, J. C. J. Chim. Phys. 1989, 86, 451.

Langmuir, Vol. 22, No. 22, 2006 9217

Figure 5. FT-IR spectra of carbon monoxide adsorbed on Cu/γAl2O3 catalyst, after previous calcinations in air at 673 K (upper spectra) and after reduction in hydrogen 1 atm at 673 K (lower spectra). The first spectra in each series have been obtained in contact with CO 10 Torr at 130 K for 5 min. The other spectra were recorded upon outgassing (10-3 Torr) on warming to 270 K. Inset: subtraction spectrum, (in contact with CO 10 Torr at 130 K) - (after outgassing at 150 K).

by a factor of 2 during reduction. This suggests that zerovalent Pd particles are already present in the oxidized sample but grow in dimension and number upon reduction. In Figure 5, the spectra of CO adsorbed on oxidized (top) and reduced (bottom) CA catalyst are reported. The spectra are slightly different from those reported recently by Topsøe and Topsøe.29 In both cases of oxidized and reduced samples, a main feature is present with two components, one near 2100 cm-1 and the other near 2115 cm-1. However, the intensity of this band is enhanced by a factor of 3 by the reducing treatment. This feature has, for the position and the relevant stability with respect to outgassing, the typical characteristics of carbonyls on cuprous ions, although the possible presence of CO species adsorbed on Cu zerovalent clusters is also possible. In the case of the oxidized catalyst, additional weak features disappearing by outgassing are observed at 2152 cm-1 (well defined though weak maximum) and near 2185 cm-1 (broad and even weaker). The position and the behavior agree with those expected for carbonyls on Cu2+ 23,30,31 (band near 2185 cm-1) and hydrogen bonded to the surface OH’s (band at 2152 cm-1). In the case of the reduced catalyst, a broad absorption tail appears below 2100 cm-1, whose maximum is observed at 2073 cm-1, with perhaps one or more components at lower frequencies. This feature corresponds to very weakly adsorbed species. These characteristics allow the assignment of this feature to terminal carbonyls on zerovalent Cu. It is, in fact, well known that CO adsorbs weakly on Cu monocrystal faces in a terminal way, giving rise to CO stretching bands in the region 2100-2070 cm-1.32-35 The spectra relative to CO adsorption on oxidized bimetallic catalysts are reported in the upper part of Figures 6-8. The spectrum of CO adsorbed on oxidized PC5A catalyst (Figure 6, top) shows, as in the case of oxidized Cu/Al2O3, a main band centered at 2116 cm-1, whose maximum shifts to (29) Topsøe, N. Y.; Topsøe, H. J. Mol. Catal. A: Chem. 1999, 141, 95. (30) Busca, G. J. Mol. Catal. 1987, 43, 225. (31) Gallardo Amores, J. M.; Sanchez Escribano, V.; Busca, G.; Lorenzelli, V. J. Mater. Chem. 1994, 41, 965. (32) Pritchard, J.; Catterick, T.; Gupta, R. K. Surf. Sci. 1975, 53, 1. (33) Horn, K.; Pritchard, J. Surf. Sci. 1976, 55, 706. (34) Horn, K.; Hussain, M.; Pritchard, J. Surf. Sci. 1977, 63, 244. (35) Choi, K. I.; Vannice, M. A. J. Catal. 1991, 131, 22.

9218 Langmuir, Vol. 22, No. 22, 2006

Figure 6. FT-IR spectra of carbon monoxide adsorbed on PdCu5/ γ-Al2O3 catalyst, after previous calcinations in air at 673 K (upper spectra) and after reduction in hydrogen 1 atm at 673 K (lower spectra). The first spectra in each series have been obtained in contact with CO 10 Torr at 130 K for 5 min. The other spectra were recorded upon outgassing (10-3 Torr) on warming to 270 K.

Figure 7. FT-IR spectra of carbon monoxide adsorbed on PdCu/ γ-Al2O3 catalyst, after previous calcinations in air at 673 K (upper spectra) and after reduction in hydrogen 1 atm at 673 K (lower spectra). The first spectra in each series have been obtained in contact with CO 10 Torr at 130 K for 5 min. The other spectra were recorded upon outgassing (10-3 Torr) on warming to 270 K.

lower frequencies (down to near 2095 cm-1) by decreasing CO coverage. This feature, which resists outgassing, should be due, for the same reasons discussed above, to CO adsorbed on cuprous ions, although terminal carbonyls on zerovalent Pd may also contribute to it. An additional unresolved absorption centered near 2056 cm-1 and a weak peak at 1972 cm-1 with a tail toward lower frequencies, both resisting outgassing, are also observed. The last feature is certainly due to bridging CO adsorbed over two Pd centers on metal particles. On the contrary, the feature near 2056 cm-1 corresponds to terminal CO, and for its position and stability looks also similar to that observed on Pd metal particles. The spectrum of CO adsorbed on oxidized PCA catalyst (Figure 7, top) shows again a main band, but this maximum is now found at a distinctly higher frequency, that is, 2127 cm-1. This band is at an intermediate position between that observed for oxidized

Sanchez-Escribano et al.

Figure 8. FT-IR spectra of carbon monoxide adsorbed on Pd5Cu/ γ-Al2O3 catalyst, after previous calcinations in air at 673 K (upper spectra) and after reduction in hydrogen 1 atm at 673 K (lower spectra). The first spectra in each series have been obtained in contact with CO 10 Torr at 130 K for 5 min. The other spectra were recorded upon outgassing (10-3 Torr) on warming to 270 K.

PA (maximum at 2149 cm-1) and for oxidized CA (maximum at 2113 cm-1), suggesting that a mixed Pd-Cu oxide species may be formed to which CO adsorbs linearly. By outgassing, a lower frequency component at 2103 cm-1 becomes more evident and even grows in intensity. This band could again contain components due to Cu+ and Pd0 terminal carbonyls. The weak feature at 2052 cm-1 may be again assigned to CO on Pd0. The band of bridging CO is almost absent. In the case of oxidized P5CA, the main band is apparently a triplet, with a component shifting from near 2125 to 2140 cm-1, a second one, more labile, shifting from 2120 to near 2115 cm-1, and a third one shifting from near 2100 to 2083 cm-1. The higher frequency component may be assigned, as discussed before, to CO adsorbed on a mixed Pd-Cu oxide species. The other two behave similar to CO on Cu+ and Pd0 species. Again, the weak features near 2057 cm-1 and in the region below 1980 cm-1 suggest that Pd metal particles, where CO may adsorb linearly and bridging, may also exist. The spectra relative to CO adsorption on reduced bimetallic catalysts are reported in the upper part of Figures 6-8. In the region of terminal carbonyls (2200-2000 cm-1), a main component is always observed, at the higher coverages, shifting from 2098 to 2115 cm-1 by increasing Cu content. This band almost always contains actually a component at higher frequency that decreases more rapidly by outgassing and another at lower frequencies that appears to correspond to a more stable species. These two features are assigned to carbonyls on “isolated” Cu+ and on Pd0 metal particles, respectively. The shift of the maximum may be associated with the change of the overlapping of the two bands whose intensities depend on the content of Cu and Pd. Only in the case of the P5CA catalyst is another band at 2129 cm-1 observed, possibly due to CO on unreduced Pd-Cu oxide species, as discussed above. In all cases, a well evident additional band shifting from near 2075 to near 2060 cm-1 by outgassing is present. This band is not observed in the two monometallic samples. This band, whose position is typical of terminal carbonyls of zerovalent copper but whose stability corresponds to carbonyls on zerovalent Pd, is assigned to CO terminally bonded on a Pd-Cu alloy.

Pd-Cu Alloy Nanoparticles on Alumina

In the region of bridging carbonyls (