Characterization of Pd-Cu Alloy Nanoparticles on γ-Al 2O3-SupportedCatalysts

Vicente Sanchez-Escribano,†,‡ Laura Arrighi,§,| Paola Riani,§,| Rinaldo Marazza,§,| andGuido Busca*,†,|

Laboratorio di Chimica delle Superfici e Catalisi Industriale, Dipartimento di Ingegneria Chimica e diProcesso, 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 eChimica 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 surfacestructure has been investigated by FT-IR spectroscopy of low-temperature adsorbed CO in the reduced and in theoxidized state. Evidence has been provided of the formation of Cu-Pd alloy nanoparticles, both of theR-phase(disordered fcc) and of theâ-phase (ordered CsCl-type). IR spectra suggest that Cu likely decorates the edges whilePd mostly stays at the main faces. Part of copper disperses as Cu+ on the support even after reduction. The presenceof copper seems to modify strongly the sate of oxidized Pd centers in oxidized high-Pd content materials. The redoxchemistry of the system, where Pd is reduced more easily than Cu, appears to be very complex.

IntroductionBimetallic catalysts find application in a number of industrial

processes such as, for example, catalytic re-forming (Pt-Re onchlorided alumina1), three-way catalysts for car engines gaspurification (Pt-Rh on alumina2), and aromatic reduction indiesel fractions (Pd-Pt on silicalumina3). Surface science studiesshowed that frequently two metals interacting on a surface canform compounds with structures not seen in bulk alloys.4 On theother hand, the actual nature of the nanosized bimetallic particlesor of monometallic segregated phases may also depend on thenature of the support5 and is very sensitive to a number variables.6

Palladium-copper bimetallic catalysts have excellent proper-ties both for oxidation reactions and for catalysis in reducingatmospheres. They in fact are of interest as catalysts for theoxidation of CO7 and VOCs,8 for the oxygen-assisted water gasshift reaction,9 and the selective hydrogenation of dienes toolefins.10 They find today much interest for the hydrogenationof nitrates in water.11-13We recently found interesting properties

of an alumina-supported Pd-Cu catalyst for the steam re-formingof propane and propene.14,15

Several previous studies have been devoted to the charac-terization of similar materials. The formation of Pd-Cu alloyswith Pd surface segregation has been observed on reduced Pd-Cu/γ-Al2O3 materials by Sun et al.16 using EELS, EDS, andSTEM techniques, and by Batista et al.11 using EXAFS, TEM/EDX, and hydrogen chemisorption, while surface decoration ofPd by Cu was suggested by Furlong et al.10 on the basis ofhydrogen adsorption experiments. The two metals were alsofound to stay in close contact by Sa´ and Vinek.12 However, itis still not clear what alloy structures are formed17in the supportedparticles, either the disordered face centered cubic phase (R) orthe ordered cubic CsClâ structure, as found for some Pd-Cu/MgO samples.18 Additionally, Molenbroek et al.19 found overtheir Pd-Cu/γ-Al2O3catalysts Cu-rich solid solutions, accordingto TEM and EXAFS, while Sun et al.16found, on similar catalysts,Pd surface enrichment on the alloy particles, based on EELS,EDX, and STEM. Copper enrichment has been reported in thecase of Pd-Cu/SiO2 catalysts used for dehydrochlorination of1,2-dichloroethane.20 Pd-Cu/SiO2 alloy materials are also ofinterest for their application as nonlinear optical materials.21

* Corresponding author. Phone/fax:+39-010-353-6024. E-mail:guido.busca@unige.it.

† 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. Today2003, 77, 419.(3) Jacquin, M.; Jones, D. J.; Rozie`re, J.; Jime´nez Lopez, A.; Rodrı´guez-

Castellon, E.; Trejo Menayo, J. M.; Lenarda, M.; Storaro, L.; Vaccari, A.;Albertazzi, S.J. Catal.2004, 228, 447.

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2004, 223, 372.(6) Guczi, L.Catal. Today2005, 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. Today2003,

78, 419.(9) Bickford, E. S.; Velu, S.; Song, C.Catal. Today2005, 99, 347.(10) Furlong, B. K.; Hightower, J. W.; Chan, T. Y. L.; Sarkany, A.; Guczi,

L. Appl. Catal., A1994, 117, 41.

(11) Batista, J.; Pintar, A.; Padez_nik Gomils_ek, J.; Kodre, A.; Bonette, F.Appl.Catal., A2001, 217, 55.

(12) Sa, J.; Vinek, H.Appl. Catal., B2005, 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. HydrogenEnergy2006, 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. B2002,106, 12239.

(17) Subramanian, P. R.; Laughlin, D. E. J. Phase Equilib.1991, 12, 231.(18) Giorgio, S.; Chapon, C.; Henry, C. R.Langmuir1997, 13, 2279.(19) Molenbroek, A. M.; Haukka, S.; Clausen, B. S.J. Phys. Chem. B1998,

102, 10680.(20) Lambert, S.; Heinrichs, B.; Brasseur, A.; Rulmont, A.; Pirard, J.-P.Appl.

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9214 Langmuir2006,22, 9214-9219

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

To characterize the surface properties of metal particles onPd-Cu/γ-Al2O3, several IR studies of adsorbed CO22,23 havebeen performed. Skoda et al.24concluded that Cu-free Pd particlesand Cu on the surface of Pd particles coexist on their catalysts.Similar conclusions were also obtained more recently by Sa´ etal.25 Choi and Vannice7 found terminal carbonyls on coppercenters together with bridging carbonyls over Pd centers. Muchless data are available on oxidized Pd-Cu/γ-Al2O3 catalystswhere copper oxide species were found by IR spectroscopy ofadsorbed CO by Choi and Vannice.7

The production of hydrogen from hydrocarbons may beobtained either in pure steam re-forming conditions, in theconditions of partial oxidation, or finally in the conditions ofoxidative steam re-forming, which is a combination of steamre-forming and partial oxidation. Depending on the reactionconditions, and also very likely on the position in the reactor,the catalyst particles may be in a more or less oxidized or reducedstate. To learn more on the complex chemistry of such system,we tried to characterize alumina-supported Pd-Cu metal particleswith XRD and TEM techniques, and by IR spectroscopy usingCO as a probe in both the oxidized and the reduced states.

Experimental Section

The five catalysts investigated have been prepared by coimpreg-nation of the two metal hydroxides overγ-Al 2O3 (from Degussa)and subsequently dried at 80°C for 48 h. The dried samples werereduced 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 thePd/Cu atomic ratio is∞ (pure Pd on Al2O3 hereinafter denoted assample 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

is in the range 90( 5 m2/g. The samples have been reduced inhydrogen flow at 673 K for 1 h before XRD and TEM measurements.

X-ray powder diffraction patterns of reduced samples have beencarried out with a X-Pert Philips diffractometer using a Cu KRradiation with 0.03 steps in a range from 15° to 100° 2θ with countingtime equal to 4 s per step. The observed diffraction intensities havebeen compared to those calculated using the Pulverix program, andthe values of the lattice parameters were refined using a least-squaresroutine.

The average composition of each sample after reduction has beenevaluated with electron probe microanalysis (EPMA) based onenergy-dispersive X-ray spectroscopy (Oxford Instruments INCA4.04); all of the samples have been analyzed employing anacceleration voltage of 20 kV for a counting time of 50 s, using acobalt standard for calibration to monitor beam current, gain, andresolution of the spectrometer. Finally, the X-ray intensities werecorrected for ZAF effects, using pure elements as standards.

TEM measurements were performed with a transmission electronmicroscope (JEOL 2010). The powders were suspended in 2-pro-panol, and a drop of the resultant mixture was deposited on a goldgrid previously covered with a thin carbon layer. EDX analysis ofelements has been carried out by using a Pentafet Si(Li) detectorwith a probe diameter of 5 nm.

Pressed disks of the pure catalysts powders were activated “insitu” by using an infrared cell connected to a conventional gasmanipulation/outgassing ramp. All catalysts were first submitted toa treatment in air at 673 K, for 30 min, followed by evacuation atthe same temperature before the adsorption experiments. Moreover,to obtain the reduced catalysts, after the mentioned evacuation, thosewere put in contact with a hydrogen pressure equal to 300 Torr at673 K, for 30 min, and successively outgassed at the sametemperature. CO adsorption was performed at 130 K by theintroduction of a well-known dose of the gas (10 Torr) inside thelow-temperature infrared cell containing the previously activatedwafers. IR spectra were collected evacuating at increasing temper-atures between 130 and 270 K.

Results and Discussion

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

(21) Mattei, G.; Maurizio, C.; Sada, C.; Mazzoldi, P.; de Julian Fernandez, C.;Cattaruzza, E.; Battaglin, G.J. Non-Cryst. Solids2004, 345-346, 667.

(22) Vannice, M. A. InCatalysis, 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., A2005, 294, 226.

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 theR-phase;b, main peaks of theâ-phase.

Pd-Cu Alloy Nanoparticles on Alumina Langmuir, Vol. 22, No. 22, 20069215

γ-Al2O3. This pattern is evident in all cases, as expected.Additionally, the patterns of PA and CA show the peaks of theface centered cubic metal phase (cF4, space groupFm3hm), whichis the thermodynamically stable one for both metals. Thecalculated unit cell parameters (0.3887 nm for Pd, 0.3611 nmfor Cu) are in good agreement with the literature data for thebulk metals.17

Also, the samples P5CA and PC5A show the peaks of a cF4fcc phase, denoted as theR-phase in the Pd-Cu state diagram.Only the pattern of the sample PCA shows together the peaksof the alumina support and of the fcc phase, and the peaks ofa third phase, cP2, CsCl type, which is denoted as theâ phasein the Pd-Cu state diagram, where this phase is reported to bethermodynamically stable for Pd atomic percent∼38-45% attemperatures below 598°C.17

The calculation of the lattice parameter for the fcc phase inthe bimetallic samples (0.3857 nm for P5CA, 0.3789 nm forsample PCA, 0.3685 for sample PC5A) shows that theR-phasesare constituted by solid solutions of the two metals. Accordingto literature data,17 bulk R-phase Pd-Cu solid solutions behavein agreement with the Vegard’s law; that is, the dependence ofthe unit cell parameter with respect to the Pd to Cu atomic fractiongives rise to a straight line (Figure 2). The calculated unit cellparameters of our samples are out of this straight line. This islikely due to the fact that the real Cu-Pd composition of themetal particles is different from the virtual Cu-Pd compositionfor the overall catalyst.

On the other hand, we can calculate a theoretical compositionof the alloy particles just by putting the points on the straightline. According to this evaluation, the metal particles are enrichedin 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 ofthe copper is still in the oxidized state even after reduction (seebelow) so that part of it does not participate in the metal alloyparticle formation.

To obtain a rough evaluation of the metal crystal size, we haveapplied the Scherrer formula to XRD peaks. The average crystalsize was calculated to be in the range 7-17 nm for all metallicphase particles.

CharacterizationofReducedCatalysts: TEM.Transmissionelectron microscopy investigations of reduced Pd/γ-Al2O3 andPd-Cu/γ-Al2O3 catalysts showed that samples are composed ofsupportingγ-Al 2O3 seen as elongated and partly transparent

particles that form large agglomerates in which dark particlesare observed uniformly distributed on the supported material.By EDX analysis, the dark particles were revealed to be Pd orPd-Cu alloys with different Pd/Cu ratios depending on theanalyzed sample. Some of these particles are faceted, indicatingthat they are single crystallites, whereas others seem to beagglomerates of smaller particles. In the single crystallites, thecrystallographic planes are visible, and the metallic particle siteswere determined to be in the range of 4-40 nm, most beingbelow 15 nm. As an example of a TEM micrograph, in Figure3a is shown the TEM image of the sample PCA. The presenceof a black spot is underlined, containing palladium and copper,whose radius is about 6.5 nm (the beam dimension for qualitativemicroanalysis is 5 nm) and where lattice planes are well defined.

Surface Characterization of the Materials through IRSpectra of Low-Temperature Adsorbed Carbon Monoxide.In Figure 4, the IR spectra of CO adsorbed at low temperatureover PA catalyst after oxidizing (upper spectra) and reducingpretreatment (lower spectra) are reported. In the oxidized catalysts,a strong band composed of a main maximum at 2149 cm-1 anda pronounced shoulder at 2160 cm-1 is evident. This band is notobserved in the case of the reduced catalyst. This band is certainlydue to carbonyl species adsorbed over oxidized Pd centers, whichare reduced by the treatment in hydrogen. The position of thisband allows its assignment to carbonyls over Pd2+ sites.23

According to the literature, carbonyl species on Pd+ are expectedto be observed between 2140 and 2115 cm-1: we have no bandsin this region, and thus we do not find monovalent Pd sites inthese conditions.

At even higher frequencies, a “continuous” weak absorptionis found only in the presence of CO gas in the region 2200-2170cm-1 for the oxidized catalyst. In the reduced one, a quite definite,

Figure 2. Variation of the lattice parameters of the disordered fccR-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 composi-tion).

Figure 3. Transmission electron micrograph of the PCA reducedcatalyst.

9216 Langmuir, Vol. 22, No. 22, 2006 Sanchez-Escribano et al.

although also weak, band is found at 2195 cm-1. This featureis likely due to CO interacting weakly with Al3+ ions on thealumina support. The larger absorption in the case of the oxidizedsample 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-1with an additional component envisagedas a shoulder in the region near 2060 cm-1. The maximum tendsto shift in both cases to lower frequencies by outgassing anddecreasing CO coverage, down to 2085 cm-1. These features arequite confidently assigned to terminal carbonyls adsorbed onzerovalent Pd atoms. In agreement with this, CO adsorption onPd monocrystals shows that terminal carbonyls are found near2095 cm-1 on both 111 and 100 planes.26Following Lear et al.,27

the Pd-carbonyls absorbing in the range 2075-2090 cm-1 areon particle corners, while Pd-carbonyls absorbing at 2050-2060cm-1 are on particle edges.

At lower frequencies, only a very weak and broad absorptioncentered 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 ofthe reduced catalyst with a main maximum centered at 1981cm-1, and a pronounced shoulder near 1900 cm-1. By outgassing,the main component decreases fast in intensity and its maximumtends to shift down to 1940 cm-1, while the lower frequencyshoulder does not decrease in intensity and becomes predominantat lower CO coverages. The vibrational range of these absorptionsallows their assignment to bridging carbonyls over Pd metalparticles. According to Binet et al.28 and to Lear et al.,27 thesharper band in the region 1980-1940 cm-1 should be assignedto CO on Pd particles exposing a 100 type face, while the broaderand more strongly bonded band in the 1930-1850 cm-1 regionshould be assigned to species adsorbed on Pd particles exposinga 111 type face.

The analysis of the intensity of the bands shows that thereduction treatment causes the complete disappearance of thebands of oxidized Pd centers and the simultaneous strong growthof the bands of bridging carbonyls on zerovalent Pd. However,also the bands due to terminal carbonyls on zerovalent Pd grow

by a factor of 2 during reduction. This suggests that zerovalentPd particles are already present in the oxidized sample but growin dimension and number upon reduction.

In Figure 5, the spectra of CO adsorbed on oxidized (top) andreduced (bottom) CA catalyst are reported. The spectra are slightlydifferent from those reported recently by Topsøe and Topsøe.29

In both cases of oxidized and reduced samples, a main featureis present with two components, one near 2100 cm-1 and theother near 2115 cm-1. However, the intensity of this band isenhanced by a factor of 3 by the reducing treatment. This featurehas, for the position and the relevant stability with respect tooutgassing, the typical characteristics of carbonyls on cuprousions, although the possible presence of CO species adsorbed onCu zerovalent clusters is also possible.

In the case of the oxidized catalyst, additional weak featuresdisappearing by outgassing are observed at 2152 cm-1 (welldefined though weak maximum) and near 2185 cm-1 (broad andeven weaker). The position and the behavior agree with thoseexpected 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 appearsbelow 2100 cm-1, whose maximum is observed at 2073 cm-1,with perhaps one or more components at lower frequencies. Thisfeature corresponds to very weakly adsorbed species. Thesecharacteristics allow the assignment of this feature to terminalcarbonyls on zerovalent Cu. It is, in fact, well known that COadsorbs weakly on Cu monocrystal faces in a terminal way,giving rise to CO stretching bands in the region 2100-2070cm-1.32-35

The spectra relative to CO adsorption on oxidized bimetalliccatalysts 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, amain band centered at 2116 cm-1, whose maximum shifts to

(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.; Baumer, 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.

(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.

Figure 4. FT-IR spectra of carbon monoxide adsorbed on Pd/γ-Al 2O3 catalyst, after previous calcinations in air at 673 K (upperspectra) and after reduction in hydrogen 1 atm at 673 K (lowerspectra). The first spectra in each series have been obtained in contactwith CO 10 Torr at 130 K for 5 min. The other spectra were recordedupon outgassing (10-3 Torr) on warming to 270 K.

Figure 5. FT-IR spectra of carbon monoxide adsorbed on Cu/γ-Al 2O3 catalyst, after previous calcinations in air at 673 K (upperspectra) and after reduction in hydrogen 1 atm at 673 K (lowerspectra). The first spectra in each series have been obtained in contactwith CO 10 Torr at 130 K for 5 min. The other spectra were recordedupon outgassing (10-3 Torr) on warming to 270 K. Inset: subtractionspectrum, (in contact with CO 10 Torr at 130 K)- (after outgassingat 150 K).

Pd-Cu Alloy Nanoparticles on Alumina Langmuir, Vol. 22, No. 22, 20069217

lower frequencies (down to near 2095 cm-1) by decreasing COcoverage. This feature, which resists outgassing, should be due,for the same reasons discussed above, to CO adsorbed on cuprousions, although terminal carbonyls on zerovalent Pd may alsocontribute to it.

An additional unresolved absorption centered near 2056 cm-1

and a weak peak at 1972 cm-1with a tail toward lower frequencies,both resisting outgassing, are also observed. The last feature iscertainly due to bridging CO adsorbed over two Pd centers onmetal particles. On the contrary, the feature near 2056 cm-1

corresponds to terminal CO, and for its position and stabilitylooks also similar to that observed on Pd metal particles.

The spectrum of CO adsorbed on oxidized PCA catalyst (Figure7, top) shows again a main band, but this maximum is now foundat a distinctly higher frequency, that is, 2127 cm-1. This bandis at an intermediate position between that observed for oxidized

PA (maximum at 2149 cm-1) and for oxidized CA (maximumat 2113 cm-1), suggesting that a mixed Pd-Cu oxide speciesmay be formed to which CO adsorbs linearly. By outgassing, alower frequency component at 2103 cm-1 becomes more evidentand even grows in intensity. This band could again containcomponents due to Cu+ and Pd0 terminal carbonyls. The weakfeature at 2052 cm-1 may be again assigned to CO on Pd0. Theband of bridging CO is almost absent.

In the case of oxidized P5CA, the main band is apparently atriplet, 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 higherfrequency component may be assigned, as discussed before, toCO adsorbed on a mixed Pd-Cu oxide species. The other twobehave similar to CO on Cu+ and Pd0 species. Again, the weakfeatures near 2057 cm-1 and in the region below 1980 cm-1

suggest that Pd metal particles, where CO may adsorb linearlyand bridging, may also exist.

The spectra relative to CO adsorption on reduced bimetalliccatalysts are reported in the upper part of Figures 6-8.

In the region of terminal carbonyls (2200-2000 cm-1), a maincomponent is always observed, at the higher coverages, shiftingfrom 2098 to 2115 cm-1 by increasing Cu content. This bandalmost always contains actually a component at higher frequencythat decreases more rapidly by outgassing and another at lowerfrequencies that appears to correspond to a more stable species.These two features are assigned to carbonyls on “isolated” Cu+

and on Pd0metal particles, respectively. The shift of the maximummay be associated with the change of the overlapping of the twobands whose intensities depend on the content of Cu and Pd.

Only in the case of the P5CA catalyst is another band at 2129cm-1 observed, possibly due to CO on unreduced Pd-Cu oxidespecies, as discussed above.

In all cases, a well evident additional band shifting from near2075 to near 2060 cm-1 by outgassing is present. This band isnot observed in the two monometallic samples. This band, whoseposition is typical of terminal carbonyls of zerovalent copper butwhose stability corresponds to carbonyls on zerovalent Pd, isassigned to CO terminally bonded on a Pd-Cu alloy.

Figure 6. FT-IR spectra of carbon monoxide adsorbed on PdCu5/γ-Al 2O3 catalyst, after previous calcinations in air at 673 K (upperspectra) and after reduction in hydrogen 1 atm at 673 K (lowerspectra). The first spectra in each series have been obtained in contactwith CO 10 Torr at 130 K for 5 min. The other spectra were recordedupon outgassing (10-3 Torr) on warming to 270 K.

Figure 7. FT-IR spectra of carbon monoxide adsorbed on PdCu/γ-Al 2O3 catalyst, after previous calcinations in air at 673 K (upperspectra) and after reduction in hydrogen 1 atm at 673 K (lowerspectra). The first spectra in each series have been obtained in contactwith CO 10 Torr at 130 K for 5 min. The other spectra were recordedupon outgassing (10-3 Torr) on warming to 270 K.

Figure 8. FT-IR spectra of carbon monoxide adsorbed on Pd5Cu/γ-Al 2O3 catalyst, after previous calcinations in air at 673 K (upperspectra) and after reduction in hydrogen 1 atm at 673 K (lowerspectra). The first spectra in each series have been obtained in contactwith CO 10 Torr at 130 K for 5 min. The other spectra were recordedupon outgassing (10-3 Torr) on warming to 270 K.

9218 Langmuir, Vol. 22, No. 22, 2006 Sanchez-Escribano et al.

In the region of bridging carbonyls (<2000 cm-1), noabsorption is found for PC5A as on CA. A weak band is foundat 1981 cm-1, with a large tail toward for the sample PCA, whilethe band is very strong for the sample P5CA. This confirms thatCO may bridge over Pd-based metal particles while it does notbridge over Cu-based metal particles. This also suggests thatmetal particles exposing only Pd do not exist (or are not detectable)on sample PC5A.

Conclusions

XRD and TEM data provide strong evidence for the formationof alloy nanoparticles on our Pd-Cu/γ-Al2O3. XRD data showthat a fcc solid solution (R-phase) is present in all cases, althoughit is depleted in Cu with respect to the virtual catalyst composition.This may be explained on the basis of the IR study, which showsthat part of copper is dispersed on the surface as a monovalentcation even after catalyst reduction. On the contrary, oxidizedPd centers are not detected after reduction even on the purePd-containing sample PA.

The XRD pattern also shows that an additional ordered CsCl-typeâ-phase Pd-Cu solid solution is found for the PCA sample.In this case, the virtual catalyst composition (Pd-Cu 1:1) is nearthe range for which theâ-phase is thermodynamically stable inthe bulk. This phase contains, according to the literature, a slightdefect of copper with respect to the Pd-Cu ratio 1:1, while thecopresentR-phase should have, according to our data, see Figure2, a slight excess of Pd with respect to the 1:1 ratio.

TEM data confirm XRD data showing that most bimetallicparticles have a crystal size of the order of less than 15 nm.

The results of the IR study show that in both oxidized andreduced states bimetallic Pd-Cu materials on alumina showfeatures not present on the corresponding monometallic samples.

In the oxidized state, a band shifting from 2140 to 2115 cm-1

by increasing Cu/Pd ratio is present only in the mixed samples.This band has been assigned to a “mixed” Pd-Cu oxide phase.It is actually very difficult to discriminate whether the “phase”is actually constituted by mixed oxide particles or by a dispersionof ions on the support surface where, however, the two metalions interfere with each other. Actually, according to the

literature,23 the bands above 2140 cm-1 should be assigned toPd2+ carbonyls, while the band in the region 2140-2115 cm-1

should most likely be due to Pd+ carbonyls.In the case of the reduced catalysts, a well-defined band in the

region 2070-2060 cm-1 is only present in bimetallic samples.In agreement with this, a band in the region 2070-2050 cm-1

has been found also upon CO adsorption on Cu-Pd orderedalloy monocrystals.36,37This band seems to be independent fromthe presence of the band of bridging carbonyls (bands below2000 cm-1) and is most evident in the sample P5CA. As wasalready said, the frequency of this band suggests its assignmentto Cu0 carbonyls whose stability should be enhanced by thecopresence of palladium. This behavior suggests that this bandmay be a proof of a surface segregation of copper on Pd-Cualloys. On the other hand, the behavior of this band together withthat of bridging carbonyls may be in agreement with the formationof Pd-metal particles whose main faces may adsorb CO in thebridging form, but whose edge or corners may be “decorated”by copper ions where terminal carbonyls form, in agreementwith the suggestions of Furlong et al.10

The data confirm also the easier reducibility of Pd with respectto copper, which largely stays in the monovalent state also afterreduction by hydrogen, although it is evident that the two metalsinteract strongly with each other and modify their reciprocalelectronic state. The Pd-Cu/support systems certainly presenta very complex behavior from the point of view of the redoxchemistry, in particular for reactions where reductant and oxidantspecies are present together and may behave with parallel paths,like in the case of the oxidative steam re-forming reactions.

Acknowledgment. V.S.-E. acknowledges the University ofSalamanca for a grant. This work has been funded by the MIUR(Rome)-FISR program.

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(36) Lu, S. H.; Yao, J.; Zhu, L.; Liu, G. L.; Fiu, F. Q.; Wu, S. C.Phys. ReV.B 1992, 45, 6142.

(37) Wadayama, T.; Sasaki, Y.; Shiomitsu, K.; Hatta, A.Surf. Sci.2005, 592,72.

Pd-Cu Alloy Nanoparticles on Alumina Langmuir, Vol. 22, No. 22, 20069219