Genesis of Surface and Bulk Phases in Rhodium−Copper Catalysts

X-Ray Absorption Spectroscopy and Anomalous Wide Angle X-Ray Scattering: Two Basic Tools in the Analysis of Heterogeneous Catalysts. D. Bazin , J. Lyn...
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Langmuir 1999, 15, 5295-5302

5295

Genesis of Surface and Bulk Phases in Rhodium-Copper Catalysts M. Ferna´ndez-Garcı´a,*,† A. Martı´nez-Arias,† I. Rodrı´guez-Ramos,† P. Ferreira-Aparicio,† and A. Guerrero-Ruiz‡ Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus Cantoblanco, 28049-Madrid, Spain, and Departamento de Quı´mica Inorga´ nica, UNED, Senda del Rey, 28040-Madrid, Spain Received June 12, 1998. In Final Form: April 28, 1999 The temperature-programmed reduction of four copper-rich, Rh-Cu/Al2O3 bimetallic catalysts was studied using X-ray absorption near-edge spectroscopy (XANES), infrared spectroscopy using CO as a probe molecule, and electron paramagnetic resonance (EPR). In the initial, fully oxidized state, copper and rhodium do not form mixed oxide phases; that is, only copper aluminates and rhodium oxide are detected. The nature of the copper aluminate is controlled by the Rh loading and pH of the preparation solution. Below 1.0 wt % of rhodium charge, there is extensive formation of a superficial copper aluminate phase, which is coreduced with rhodium oxide to yield an alloyed phase. The bimetallic particles formed have a heterogeneous composition with a Rh-rich core and a surface enriched in copper. Rh is electronically perturbed in these binary particles in relation to a metallic reference and shows a net positive charge. The analysis of the genesis mechanism of the zerovalent phases provides evidence of the critical role of the support and superficial interactions between oxidized phases in alloy formation.

Introduction The catalytic interest of Rh-Cu bimetallic systems has been emphasized since the late 1970s by several works in connectionwithhydrocarbonactivationandtransformation.1-6 Depending on the reaction in which the binary Rh-Cu system is probed, the base metal can act as a simple inert diluent disrupting the ensembles of rhodium atoms or can vary the electronic properties of the active metal (Rh), affecting in both cases the catalytic properties. Rhodium and copper are partially miscible and their binary phase diagram shows the formation of alloy phases.7 However, alloy formation in a bimetallic system finely dispersed on an oxidic carrier depends on both cluster size and metal-support interaction, yielding a more complex situation that is now under debate. For an inert support, such as SiO2, early characterization studies by Sinfelt et al.8 using EXAFS successfully identified alloy formation but, apparently, surface enrichment in copper and little “bulk” miscibility of both metals were also observed. This Rh-Cu/SiO2 catalyst has been reported as an efficient system for the enhancement of ethane production during methane dehydrogenation.2 Furthermore, Rh-Cu/SiO2 catalysts have been also applied for hydrocarbon transformation reactions.3 From this last † ‡

CSIC. UNED.

(1) Clarke, J. K. A.; Peter, A. J. J. Chem. Soc., Faraday Trans 1 1975, 72, 1201. (2) Solymosi, F.; Cserenyi, J. Catal. Lett. 1995, 78, 882. (3) Coq, B.; Dutartre, R.; Figueras, F.; Rouco, A. J. Phys. Chem. 1989, 93, 4904. (4) Mendes, F. M. T.; Schmal, M. Appl. Catal. A: Gen. 1997, 151, 393. (5) Guerrero-Ruiz, A.; Bachiller-Baeza, B.; Ferreira-Aparicio, P.; Rodrı´guez-Ramos, I. J. Catal. 1997, 171, 374. (6) Kacimi, S.; Barbier, J.; Taba, R.; Duprez, D. Catal. Lett. 1993, 22, 343. (7) (a) Massalsiki, T. B.; Okamoto, H.; Subramanian, P. R.; Kacprazak, L. Binary Alloy Phase Diagrams, 2nd ed.; ASM: Materials Parks, Ohio, 1992. (b) Irons, L.; Mini, S.; Brower, B. E. Mater. Sci. Eng. 1988, 98, 309. (8) Meitner, G.; Via, G.; Lytle, F.; Sinfelt, J. J. Chem. Phys. 1983, 78, 882.

study it is deduced that copper atoms are preferentially occupying low coordination sites (edges and corners) while rhodium atoms are placed in dense planes of alloyed particles. Here, the relative loading of Rh and Cu is suggested to play a key role in determining the structure, morphology, and existent phases. For a more reactive support such as alumina and in the framework of characterization studies, it was concluded that the interaction of copper with the support prevents the redispersion and/or oxidation of rhodium crystallites.4 Recently, the formation of alloyed phases has also been detected in Rh-Cu/Al2O3 systems but only for low Rh/Cu atomic ratios.5 In addition, chemisorption and protonNMR results on this system have been explained by assuming the formation of two different alloy phases, whose existence ranges depend on the relative content of Rh and Cu.9 In the case of another reactive support, CeO2, copper is an excellent promoter of the oxygen storage capacity of ceria, conferring also an improved resistance toward sintering of the precious metal.6 In an attempt to shed some light on the genesis mechanism of binary zerovalent phases and characterize their bulk and surface characteristics in bimetallic RhCu/Al2O3 catalysts, we have performed a multitechnique analysis of four specimens differing in the Rh/Cu atomic ratio. The strong interaction of Cu10 and Rh11 with alumina and the existence of binary oxides (aluminates) in the first case complicate the understanding of the Rh-Cu system evolution upon reduction. To unravel this complex picture (vide supra), XANES combined with factor analysis has been used to follow the temperature-programmed reduction (TPR) of the catalysts;12-14 this will yield information concerning the most abundant phases and their reactivity toward hydrogen. Due to the known (9) Chou, S.-C.; Yeh, C.-T.; Chang, T.-H. J. Phys. Chem. B 1997, 101, 5828. (10) Ferna´ndez-Garcı´a, M.; Ferreira-Aparicio, P.; Rodrı´guez-Ramos, I.; Guerrero-Ruiz, A. J. Catal. 1998, 178, 253. (11) Ferreira-Aparicio, P.; Bachiller-Baeza, B.; Rodrı´guez-Ramos, I., Guerrero-Ruiz, A.; Ferna´ndez-Garcı´a, M. Catal. Lett. 1997, 49, 163. (12) Malinowsky, E. R. Factor Analysis in Chemistry; Wiley: New York, 1991.

10.1021/la980689+ CCC: $18.00 © 1999 American Chemical Society Published on Web 06/30/1999

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Table 1. Main Characteristics of Rh-Cu Systems metal loading (%) samples Rh 0.05RhCu 0.14RhCu 0.23RhCu 0.43RhCu Cu

Cu 4.6 4.2 3.8 3.0 4.8

Rh

pH

H/Rh

1.25 0.45 0.95 1.4 2.05

1.3 0.6 0.5 0.3 0.2 1.9

0.38 0.39 0.32 0.36 0.14

limitations of XANES in surface phase detection and in getting information concerning aggregation phenomena, this bulk characterization technique will be complemented by infrared spectroscopy using the CO probe molecule (FTIR CO) to detect surface species and follow their evolution. Finally, minor (oxidized) copper phases have been attempted to be detected and their physicochemical properties analyzed by using electron paramagnetic resonance (EPR). Experimental and Computational Details Catalysts Preparation. Samples were prepared by impregnation of γ-alumina (Puralox, Condea, SBET ) 176 m2‚g-1) with aqueous solutions of the corresponding concentrations of RhCl3‚ xH2O (Aldrich) and Cu(NO3)2‚3H2O (Merck) salts. Bimetallic systems were prepared by coimpregnation maintaining the total loading close to 5 wt % at a pH dictated by the hydrolysis of precursors salts (Table 1). After impregnation, catalysts were dried overnight at room temperature (RT) and calcined in air at 773 K for 3 h. In Table 1, the composition of the systems as measured by atomic absorption and the H/Rh ratio obtained from hydrogen chemisorption are listed. The H2 chemisorption experiments were performed using a Micromeritis 2700 Probe Chemisorb apparatus, after sample reduction at 673 K. The Rh/Cu ratios obtained after the preparation step (0.05, 0.14, 0.23, and 0.43) will be used to denote the catalysts. CuO, bulk and superficial CuAl2O4, CuAlO2, Cu2O, Cu foil, Rh2O3, Rh(CO)4Cl2, and Rh foil reference compounds were obtained as described in refs 11, 13, and 14. XANES. Experiments at the Rh and Cu K-edges were carried out in the ID-24 line at the ESRF Synchrotron, Grenoble, France. All samples (catalysts and pure reference compounds) were measured in an energy-dispersive, transmission mode with simultaneous calibration of the energy scale with the aid of a Rh or Cu foil. A bent perfect Si crystal in a Bragg configuration was used as a dispersive monochromator. Self-supporting wafers of the samples were placed in a controlled-atmosphere (10% H2 in He) cell and submitted to a heat treatment of 3 K‚min-1 from room temperature to 673 K. Typically, a XANES spectrum was obtained every 10-15 K in a few seconds recording process. Notice that although the temporal resolution inherent to a dispersive XANES experiment is not absolutely necessary to follow the reduction process, it certainly minimizes the average over the compositional (phase) change always present in conventional XANES data. This point may be of particular significance in the region of maximum variation of the reduction process. The set of XANES spectra taken during the reduction treatment was analyzed using principal component factor analysis (PCA), details of which can be found in ref 13. The PCA analysis assumes that the absorbance in a set of spectra can be mathematically modeled as a linear sum of individual components, called factors, which correspond to each one of the copper species present in a sample, plus noise.12 Notice that this analysis does not intend to decompose a specific copper species in a linear (13) (a) Ferna´ndez-Garcı´a, M.; Ma´rquez-Alvarez, C.; Haller, G. L. J. Phys. Chem. 1995, 99, 12565. (b) Ma´rquez-Alvarez, C.; Rodrı´guez-Ramos, I.; Guerrero-Ruiz, A.; Haller, G. L.; Ferna´ndez-Garcı´a, M. J. Am. Chem. Soc. 1997, 119, 2905. (14) (a) Ferna´ndez-Garcı´a, M.; Anderson, J. A.; Haller, G. L. J. Phys. Chem. 1996, 100, 16247. (b) Ferna´ndez-Garcı´a, M.; Ma´rquez-Alvarez, C.; Rodrı´guez-Ramos, I.; Guerrero-Ruiz, A.; Haller, G. L. J. Phys. Chem. 1995, 99, 16380.

combination of well-defined references, as erroneously assumed in ref 15. With this statistical tool it is possible (vide supra) to draw a complete picture of the phase behavior of the system, extracting the number of “pure” chemical species that contributes to the XANES set and quantifying their evolution throughout the treatment. To determine the number of individual components, an F-test of the variance associated with factor k and the summed variance associated with the pool of noise factors is performed. A factor is accepted as a “pure” species (factor associated with signal and not noise) when the percentage of significance level of the F-test, % SL, is lower than a test level recommended by previous experience (5%).13,14 The ratio between reduced eigenvalues, R(r), which must approach one for noise factors, will be also used in reaching this decision. Once the number of individual components is set, XANES spectra corresponding to individual copper or rhodium chemical species and their concentration profiles are generated by a varimax rotation followed by iterative transformation factor analysis.13,14 EPR spectra were obtained at 77 K after with a Bruker ER200D spectrometer working in the X-band and calibrated with DPPH (g ) 2.0036). Quantification of paramagnetic species was made by comparing doubly integrated intensities with those of a CuSO4‚5H2O standard; background functions were subtracted for this operation covering the same magnetic field range in all cases. Small quantities of sample (ca. 40 mg) were placed in a quartz probe cell equipped with greaseless stopcocks, where they could be subjected to high vacuum, heating, and reduction or adsorption treatments. FTIR data were collected with a Nicolet 5ZDX Fourier transform spectrometer (4 cm-1 resolution, 128 scans). Powder samples were pressed into self-supported wafers of ca. 10 mg/ cm2 thickness and mounted in a Pyrex IR cell, assembled with greaseless stopcocks and NaCl windows, where catalysts could be treated in the temperature range 323-773 K. Samples reduced in hydrogen were subsequently evacuated in dynamic vacuum at 473 K and contacted with 50 Torr of CO. Spectra were recorded, after removing the gas phase at RT, at beam temperature (ca. 320 K). IR spectra of adsorbed species were extracted through subtraction of background contributions and baseline correction.

Results XANES. The % SL and R(r) values reported in Tables 2a and 3a indicate the existence of three copper and two rhodium chemical species for the 0.14RhCu sample. The 0.05RhCu specimen is completely parallel in its behavior to the 0.14RhCu catalyst and, for the sake of brevity, will not be reported here. For 0.43RhCu, the analysis of the number of components, Tables 2b and 3b, reveals again the existence of three copper and two rhodium chemical species. The Cu and Rh K-edges of 0.14RhCu contain contributions from two and one, respectively, oxidized species. Comparison with reference compounds (Figures 1a and 2a) allows clear assignment to a superficial copper aluminate, called CuAl2O4-o because copper only occupies octahedral positions,10 a bulk-like copper aluminate, called CuAl2O4-t/o because now copper is in both octahedral and tetrahedral sites,10 and a Rh2O3-like phase. The superficial copper aluminate reference, CuAl2O4-s, has only copper in octahedral positions and the first continuum resonance (CR) is located at 8995-6 eV, while the bulk reference, CuAl2O4-b, contains cations on both octahedral and tetrahedral sites and shows splitting of this CR with maxima located at 8997 and 9002 eV. This splitting is not observed for the CuAl2O4-t/o species detected in the 0.14RhCu specimen; however, the copper aluminates contained in this sample can be differentiated (i.e., superficial vs bulk-like phases) by XANES due to the larger line width of the 9000 eV CR presented by bulk-like component. The broadening of the first CR of species (15) Bazin, D. C.; Sayers, D. A.; Rehr, J. J. J. Phys. Chem. B 1997, 101, 11040.

Genesis of Binary Phases in Rhodium-Copper Catalysts

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Figure 1. Factor analysis generated XANES spectra for individual copper species (solid lines) and reference compounds (dashed lines): 1, CuAl2O4-o; 2, CuAl2O4-t/o; 3, Cu0; 4, CuAlO2; CuAl2O4-s, superficial copper aluminate reference; CuAl2O4-b, bulk copper aluminate reference; Cu, copper foil reference. Key: (a, top) 0.14RhCu; (b, bottom) 0.43RhCu.

Figure 2. Factor analysis generated XANES spectra for individual rhodium species (solid lines) and reference compounds (dashed lines); 1, Rh2O3-like; 2, RhxCuy alloy; 3, Rh0; Rh2O3, rhodium oxide reference; Rh, rhodium foil reference. Key: (a, top) 0.14RhCu; (b, bottom) 0.43RhCu.

CuAl2O4-t/o with respect to the CuAl2O4-s reference indicates the presence of tetrahedrally coordinated cations in addition to octahedral ones. Certainly, it is not possible to detail the exact active metal site distribution of the CuAl2O4-t/o species, although the percentage of tetrahedral cations is considerably lower than 60% (the one corresponding to the CuAl2O4-b reference). At this point it is interesting to note that XANES probes only the local order around photoexcited (Cu, Rh) centers and the comparison with well-crystallized references only refers to this similarity; of particular importance is to note that this comparison does not reflect additional similarities, particularly the long-range or three-dimensional order. For 0.43RhCu, the existence of the CuAl2O4-t/o and Rh2O3-like phases can be inferred from the XANES analysis (comparison with references compounds can be found in Figures 1b and 2b). Notice that the CuAl2O4-t/o phase presents the ≈8998 CR shifted 0.5 eV to higher

energies with respect to the corresponding CR of the 0.14RhCu CuAl2O4-t/o phase. As the CRs have a 1/R2 dependence of the coordination distance, this shows that copper centers located in this phase have a 1.3% contraction of the first Cu2+-O2- coordination distance with respect to the 0.14RhCu analogue. In the 0.43RhCu case, the superficial aluminate is below our detection limit (about 5% of the total copper). All oxidized phases disappear by the end of the treatment giving one chemical species in each edge (see Figures 1 and 2). The Rh foil XANES shape, Figure 2, has notable differences with respect to the Rh species obtained for the 0.05RhCu (not shown) and 0.14RhCu samples. In contrast, Rh foil exhibits the CRs at about the same energy position than the Rh species detected in the 0.43RhCu specimen although with larger intensity; the nearly constant shift to higher energies detected for the CRs of this sample with respect to the foil is most certainly due

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Figure 4. EPR spectra of samples calcined (a, c) and reduced in H2 at 403 K (b, d): 0.14RhCu (a, b); 0.43RhCu (c, d).

Figure 3. Concentration profiles of the pure components along the reduction coordinate: (a, top) 0.14RhCu; (b, bottom) 0.43RhCu; open symbols, Cu K-edge species; closed symbols, Rh K-edge species. Species are numbered with a superscript referring to their assignment in Figures 1 and 2. Solid lines are only included as a guide for the eyes. See text for details.

to a small decrease of the Rh-Rh coordination distance induced by the small particle size. In the first case (0.05and 0.14RhCu), a RhxCuy disordered fcc alloy must be formed since binary ordered phases are not known in the Rh-Cu system7,9 while, in the second case (0.43RhCu), particles of zerovalent Rh with a small coordination number must be formed after reduction. Note however that the 0.05RhCu and 0.14RhCu Cu K-edges do not include a contribution from any alloy phase. This can be rationalized by considering the limited miscibility between both metals; for Rh-rich compositions at working temperatures, total miscibility (one phase) is observed only in a small region close to the Rh abscissa that contains up to a 6 at. % of Cu; at lower rhodium atomic percentages, there is a region of solid limited miscibility with the presence of two solid phases containing about 6 at. % (the Rh-rich phase) and 80 at. % (the Cu-rich phase) Cu content.7 So, it would mean that only a maximum of 1 wt % of the total copper loading (corresponding to a 6 at. % of copper content of the binary phase) is involved in the alloying process, justifying the absence of this contribution in the Cu K-edge XANES signal. Therefore, the Cu K-edge just shows the existence of well-developed zerovalent Cu particles (coordination number above 7) in all cases. The concentration profiles associated with the Rh,Cu species assigned already are presented in Figure 3. Copper aluminates present characteristic reduction temperatures of 410 (CuAl2O4-t/o of 0.43RhCu), 463 (CuAl2O4-t/o of 0.14RhCu) and 428 K (CuAl2O4-o of 0.14RhCu) while Rh2O3-like phases undergo for both samples a smooth,

progressive reduction process in a broad range of temperatures (from 373 to 623 K), as previously observed in the monometallic Rh/Al2O3 system.11 A pronounced decay is observed in the 0.43RhCu Rh3+ concentration profile (better in the derivative) at around 433 K showing that some minor part of the rhodium (about 30%) is forming big particles with a weak or null interaction with the support while the majority (around a 1.5 wt % of rhodium charge) is in strong interaction with alumina. More interestingly, the presence of the Cu+ intermediate (CuAlO2) of the copper aluminates reduction (species 4 in Figure 2), well described for the monometallic Cu/Al2O3 system,10 is only observed for the 0.43RhCu specimen, being absent in the 0.14RhCu and 0.05RhCu catalysts. EPR. Figure 4 shows the EPR spectra of 0.14RhCu and 0.43RhCu samples in the initial calcined state and after reduction in hydrogen at 403 K. The corresponding spectra of the oxidized materials after prolonged outgassing at RT are quite similar (Figures 4a,c). They are characterized by the overlapping of one dominant, broad, and, as a consequence, structureless signal showing extremes at g ) 2.33 and 2.04 (a) and a narrower axial signal with g| ) 2.311 and g⊥ ) 2.05, in which hyperfine pattern of four lines can be resolved in its parallel component with A| ) 145 G (b). The shape and parameters of trace a are similar to those attributed to Cu2+ ions situated in local environments of a copper aluminate spinel, its large line width being caused by magnetic dipolar interactions between Cu2+ ions.16 Trace b must be assigned to isolated Cu2+ cations, in view of the good resolution of its hyperfine structure; its EPR parameters are consistent with a tetragonally distorted octahedral symmetry.16,17 For the calcined specimens under vacuum, the copper(II) percentage detected decreases with the copper loading (Figure 5A). Spectra obtained under an air atmosphere (which leads to a large broadening of signals due to surface species by magnetic interactions with paramagnetic oxygen and thus allows a semiquantitative estimation of signals corresponding to bulk species)18 suggest that all (16) Martinez-Arias, A.; Catalun˜a, R.; Conesa, J. C.; Soria, J. J. Phys. Chem. 1998, 102, 809. (17) Berger, P. A.; Roth, J. F. J. Phys. Chem. 1967, 71, 4307.

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Figure 6. IR spectra of CO adsorbed on the 0.14RhCu sample reduced at increasing reduction temperatures: (a) 373 K; (b) 423 K; (c) 473 K; (d) 523 K; (e) 573 K; (f) 673 K.

Figure 5. (A) EPR-detected Cu(II) percentage, with respect to the total copper amount, of calcined samples treated under vacuum and in an air atmosphere. (B) EPR-detected Cu(II) percentage, with respect to the total copper amount, after different treatments. Inset: EPR-detected Cu(II) percentage after reduction at 403 K, expressed with respect to the initial amount of copper, in function of the Rh,Cu loadings of the catalysts. Key: white, 0.14RhCu; gray, 0.23RhCu; black, 0.43RhCu. See text for details.

samples have similar initial surface-to-bulk ratio (i.e. particle size). A general decrease of the EPR signal intensities is achieved upon reduction up to 403 K (Figure 5B). Samples 0.23RhCu and 0.43RhCu maintain most of the initial intensity after reduction at 373 K. For the 0.14RhCu specimen, the intensity goes through a maximum after reduction at 373 K, suggesting certain breaking of magnetic interactions between prior EPR-silent Cu2+ species. If the reduction temperature is increased to 403 (18) Che, M.; Tench, A. J. Adv. Catal. 1983, 32, 1.

Figure 7. IR spectra of CO adsorbed on the 0.43RhCu sample reduced at increasing reduction temperatures: (a) 373 K; (b) 423 K; (c) 473 K; (d) 523 K; (e) 573 K; (f) 673 K.

K, the EPR-detected copper intensity decreases strongly for the 0.23RhCu and 0.43RhCu specimens while the 0.14RhCu sample suffers only a moderate reduction (inset in Figure 5B shows this decrease with respect to the Rh and Cu loadings of the samples). The air-contacted samples (data not shown) evidence that mainly bulk-like species are responsible for the EPR signal after the last reduction treatment. FTIR. Spectra of CO adsorbed on 0.14RhCu and 0.43RhCu specimens reduced at increasing temperatures are shown in Figures 6 and 7, respectively. The assignment of carbonyls adsorbed on copper and rhodium centers is

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Table 2. Cu K-Edge Principal Components Factor Analysis Results factor

eigenvalue

% SL

R(r)

1 2 3 4 5 6 7 8

258.432 2.796 93 0.087 16 0.008 23 0.004 85 0.001 61 0.000 48 0.000 32

(a) 0.14RhCu 0.00 81.13 0.00 278.27 0.40 9.03 10.93 1.41 13.49 2.44 10.12 2.60 18.74 1.10 40.08 0.79

1 2 3 4 5 6 7 8

463.453 3.816 54 0.019 05 0.001 13 0.000 33 0.000 12 0.000 10 0.000 09

(b) 0.43RhCu 0.00 101.45 0.00 175.32 0.07 14.53 8.23 2.90 22.76 2.09 40.59 0.98 43.32 0.87 44.49 1.05

variancea 98.926 1.070 0.003

99.128 0.765 0.006 0.002

a Variance is given in percentage. Values lower than 10-3 are not reported.

Table 3. Rh K-Edge Principal Components Factor Analysis Results factor

eigenvalue

% SL

R(r)

1 2 3 4 5 6 7 8

452.652 0.892 07 0.031 04 0.001 86 0.001 02 0.000 51 0.000 42 0.000 36

(a) 0.14RhCu 0.00 471.43 0.00 266.32 6.33 1.54 8.89 1.68 15.27 1.83 27.54 1.10 29.22 1.06 30.14 1.07

1 2 3 4 5 6 7 8

460.872 0.857 20 0.014 98 0.001 07 0.000 71 0.000 57 0.000 35 0.000 31

(b) 0.43RhCu 0.00 496.123 0.00 526.342 14.36 1.28 16.14 1.37 20.27 1.13 20.40 1.46 27.75 0.97 25.54 1.12

variancea 99.801 0.197 0.001

99.813 0.186

a Variance is given in percentage. Values lower than 10-3 are not reported.

Table 4. IR Assignment of Carbonyl Bands over Cu and Rh Centers freq (cm-1)

species

phase

refs

2111 2133 2090, 2020 2060 1845 2120 2142

Cu+-CO Cu+-CO Rh+(CO)2 Rh0-CO Rh-CO-Rh Rh2+-CO Rh3+-CO

copper aluminate sintered copper aluminate isolated Rh0 atoms crystallites of Rh0 crystallites of Rh0 oxidized Rh oxidized Rh

5, 19, 20 5, 19, 20 21-23 21-23 21-23 24 24

done in Table 4 on the basis of literature reports (refs 5 and 19-24).20-24 CO exposure of 0.14RhCu reduced at 373 K yields IR bands at 2110 and 2093 and 2022 cm-1 attributed to Cu+ species in the aluminate phase and the Rh+ gem-dicarbonyl, respectively. The last (Rh) species (19) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504. (20) Van’t Blik, HF. J.; Van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. Phys. Chem. 1983, 87, 2264. (21) Solymosi, F.; Pa´sztor, M. J. Phys. Chem. 1985, 89, 4789. (22) Rice, R. A.; Warley, S. D.; Curtis, C. W.; Guin, J. A.; Tarrer, A. R. J. Phys. Chem. 1981, 74, 6487. (23) Lokhov, Yu A.; Davydov, A. A. Kinet. Catal. 1979, 20, 1498. (24) Sepulveda, A.; Ma´rquez, C.; Rodrı´guez-Ramos, I.; Guerrero-Ruiz, A.; Fierro, J. L. G. Surf. Interface Anal. 1993, 20, 1067.

IR intensity increased with reduction temperature, reaching a maximum near 473 K. At this temperature, new contributions at the lower and higher frequency sides of the Rh+ gem-dicarbonyl bands are observed. The lower frequency, located around 1999 cm-1, is characteristic of Rh-Cu alloys5 and has a decreasing intensity at higher reduction temperatures. The higher frequency, located around 2140 cm-1, is characteristic of isolated Cu+ ions and has an increasing intensity at higher reduction temperatures. At 673 K, a small contribution from CO adsorption on Rh0 is also observed. The 0.43RhCu sample reduced at low temperature (e433 K) and subsequently exposed to CO yield maxima similar to those already described, providing evidence of the presence of Cu+ and Rh+ ions at the surface, although with different intensities. However, some subtle but important differences appear at higher reduction temperatures. At 473 K, in addition to the growth of the Rh+related bands, a characteristic shoulder around 2060 cm-1, attributable to Rh0, is observed. The intensity of this last contribution clearly increases at higher reduction temperatures (above 473 K) in parallel to an intensity decrease of the Rh+ bands. Small signals of carbonyls adsorbed on isolated Cu+ ions may also be observed to increase mildly with reduction temperature. Discussion The IR, EPR, and particularly the XANES results indicate that copper and rhodium do not form mixed phases in their fully oxidized state. The hypothetical mixed phase could be, in this case, a solid solution of both components or a Rh2O3-like phase with copper occupying some of the octahedral vacancies. However, the obtention of XANES spectra showing local order and anion-cation distances characteristic of the monometallic phases (Figures 1 and 2) as well as the thermal evolution of the rhodium compound, which is typical of a monometallic sample, rule out both possibilities. IR results of samples reduced at low temperatures suggest that the specimens studied contain copper and rhodium surface species that are similar. This is in agreement with the XANES spectra, which contain contributions only from copper aluminates and rhodium oxide. EPR intensities show however that the 0.43RhCu specimen has a moderately larger copper dispersion, although considering the high copper loadings used, overall the samples seems to be highly dispersed. Of course, this point is raised with caution, as EPR does not detect copper ions in well-crystallized, aggregated phases.16 The surface/volume ratio, approximated here by using the intensity ratio between samples under vacuum and samples contacted with air, is almost constant through the series of catalysts analyzed. So, on the basis of EPR results, it can be concluded that the 0.43RhCu sample has a more homogeneous copper aluminate (the dominant phase) particle size distribution. Phases which are in minority (not detected by XANES) are evidenced in the EPR signals. These correspond to isolated Cu2+ cations in tetragonally distorted octahedral symmetries, which seem relatively more stable upon reduction, and are probably induced by a strong interaction with the support.16 As noted in the previous section, the reduction of the oxidized phases starts from 400 K. In the 0.05RhCu and 0.14RhCu catalysts, the CuAl2O4-o and Rh2O3-like components are coreduced while XANES and EPR indicate that the CuAl2O4-t/o phase initiates its reduction at higher temperatures. In contrast, the similar CuAl2O4-t/o phase of the 0.23RhCu (EPR results) and 0.43RhCu (EPR and

Genesis of Binary Phases in Rhodium-Copper Catalysts

Figure 8. IR spectra of CO adsorbed on samples reduced at 573 K: (a) Rh; (b) 0.43RhCu; (c) 0.23RhCu; (d) 0.14RhCu; (e) 0.05RhCu.

XANES results) samples presents a characteristic reduction close to 400-410 K and the Cu+ intermediate of the reduction process is clearly detected in the last case, both events paralleling the behavior of monometallic Cu/Al2O3 catalysts.10 In all samples, the Rh2O3-like component interacts with alumina in a way similar to that observed in the corresponding monometallic reference system.11 These facts allow the reduction behavior of both samples to be interpreted: clearly rhodium has the strongest interaction with alumina and is able to cover most of the available surface area with approximately a 1.0 wt % loading. Specifically, below some point between 1.4 and 0.95 wt % of rhodium charge, some surface area of alumina becomes free and there is extensive formation of the CuAl2O4-o phase, which is coreduced with the Rh2O3-like simply by physical proximity. Above this point, only a CuAl2O4-t/o phase of easier reducibility (see Figure 5B) is majority formed; this phase starts its reduction before the Rh2O3-like component. As can be seen in Figure 8, which present the IR CO of samples reduced at 573 K (from ref 5 and this work), the final result of the treatment is the formation of a Rh-Cu alloy (band at 1999 cm-1)5 only for the 0.05RhCu and 0.14RhCu specimens. Of course, there is a second variable that, in addition to the rhodium loading, may also influence the situation: the decreasing pH observed with increasing rhodium loading of bimetallic systems (Table 1) can also induce a larger dissolution of alumina and, therefore, a predominance of the CuAl2O4t/o phase (which is formed by reaction of dissolved copper and aluminum species) with respect to the CuAl2O4-o one.10 The CuAl2O4-t/o compound observed for 0.14RhCu is reduced at very high temperatures (see XANES and EPR results). Although our study does not give a clear interpretation of this fact, previous analyses of monometallic copper systems suggest that the surface characteristics (i.e. vacancies) of copper phases are the key factor in governing their reduction behavior.10 The existence of a lower amount of vacancies in the 0.14RhCu catalyst (and, consequently, a higher characteristic reduction temperature) would be deduced from the lengthening of the first Cu2+-O2- coordination distance observed with

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respect to the corresponding phase of the 0.43RhCu specimen. The consequent nondetection of the Cu+ intermediate of the reduction process can be assumed to be due to the important hydrogen spillover from rhodium atoms at high temperature. The reduction process resulted in zerovalent phases above 623 K. The 0.14RhCu sample XANES gives evidence of the quasi-complete rhodium consumption in the generation of an alloyed phase. As mentioned above, the 0.05RhCu and 0.14RhCu specimens reduced at 573 K exhibit a shoulder at 2060 cm-1 in the IR CO experiments (Figure 8). This fact shows that the Rh-Cu alloy partially segregates into Rh0 and Cu0 at high temperature. The apparent discrepancy between XANES and IR can be resolved considering their different probing depth: the bulk-averaged XANES result clearly indicates that monometallic phases are minor components while IR suggests a preferential location of these components at the surface. The Rh-Cu binary diagram shows that the main, alloyed phase must correspond to a disordered fcc alloy with a 6 at. % maximum copper content (Rh94Cu06). Note, however, that the 1999 cm-1 band, assigned to a RhxCuy contribution, is progressively lost with increasing of the reduction temperature while XANES shows that the alloy phase is still present. The combination of both facts implies that the surface of the alloy is continuously enriched in copper, as observed previously for Rh-Cu/SiO2 systems.25,26 For copper alloys with noble metals, accumulation of the coin metal on the surface with proclivity to clustering is generally observed in systems with limited miscibility (Rh-Cu, Ru-Cu) but is noticeably restricted for those miscible (Pt-Cu, Pd-Cu).27 In the first case, the surface is enriched in the lower melting component as can be predicted by a simple broken-bond model, while in the second case strain forces partially counteract this phenomenon.27 The picture that has emerged from the study for the Rh-containing phase of the 0.14RhCu specimen after 673 K reduction is thus a heterogeneous phase with a Rh-rich alloy core superficially covered with copper. Although we cannot give the total copper percentage involved in the binary particle formation, the fact that XANES did not detect any Cu alloy gives an upper limit (rough estimation) of a 30% average atomic copper content. It is interesting to point out that similar (although less detailed) model of the binary phases has been previously reported by Chou et al.;9 for global Cu-rich systems (our case), an alloy that has a Rh-rich core and a Cu-rich exterior was proposed. However, the copper percentage above which this situation exists is clearly superior to our case probably by the different preparation procedures used. Additionally, these authors do not study the genesis mechanism of binary phases in Rh-Cu/Al2O3 catalysts. Rh atoms in the alloy compound have Rh K-edge 5sp and 4f CRs (located around 23 235 and 23 265 eV),27 Figure 3, at 0.6 and 0.4 eV below those of Rh particles in aluminasupported systems (see, for instance, sample 0.43RhCu). The CRs positions can include contributions from geometrical (they have a 1/R2 dependence with the coordination distance) as well as electronic parameters. However, the decreasing first coordination distance around rhodium atoms expected by the lower lattice constant of the alloy,7 as well as by the small particle size, should produce a shift to higher energies. The observed shift to lower energies indicates the primacy of electronic factors in (25) Sinfelt, J. H. Acc. Chem. Res. 1987, 20, 134. (26) Toshi, K.; Udayawa, Y.; Harada, M.; Ueno, A. Jpn. J. Appl. Phys. 1986, 11, 1553. (27) Ponec, V.; Bond, G. C. Catalysis by metals and alloys; Elsevier: Amsterdam, 1995; Chapter 2.

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describing the Rh-Cu interaction. The electronic basis of this interaction has been analyzed in Cu/Rh(100) model systems, but the work was centered in Cu properties.28 In any case, the intensity of the 23 235 eV CR proves that alloyed rhodium has a larger 5sp density of unoccupied states (larger intensity) than rhodium in small particles supported in alumina.11 Due to the lack of resolution at the Rh K-edge energy, we are not able to complete the electronic information giving details of the Rh 4d subband (which is located just over the edge).29 However, the severe loss of 5sp electronic density may suggest that rhodium is positively charged in the alloy. This fact is in agreement with the usual interpretation given to the negative XPS shift observed for the Cu2p3/2 level,28 which is explained by assuming that copper in contact with rhodium has a negative electronic density. A word of caution should however be raised about this argument, as the closely (structurally) related disordered PdCu fcc alloys exhibit Cu2p3/2 negative shifts, although copper centers are positively charged. This happens because the XPS shift is here dominated by polarization and not charge-transfer effects.30 Nevertheless, the chemical or charge-transfer components in the Rh-Cu bond are much more important than in Pd-Cu, lending further proof that the Rh centers are positively charged. The rhodium electronic modification has notable chemical implications. Let us mention, for instance, that the thermodynamics of hydrogen chemisorption is strongly altered31 and, consequently, H/Rh ratios of Table 1 cannot be considered as indicators of Rh dispersion. Other chemical effects can be deduced from the interaction of CO with the 0.14RhCu sample. First of all, some leaching of rhodium from the alloy phase, responsible for the Rh+ gem-dicarbonyl signal, is observed just above the onset temperature of alloy formation (at 473 K). This may be viewed as a parallel process to the CO disruption of small Rh particles; however, at higher reduction temperatures, the covering of the alloy particles with copper seems to stop this process, simultaneously producing Cu+ species. This last phenomenon can be interpreted by considering that rhodium is able to dissociate CO at low temperatures on rough surfaces and that positively charged species may favor this process.32 For the 0.43RhCu specimen, the copper and rhodium phases are reduced separately and the formation of binary (28) Rodriguez, J. A.; Campbell, R. A.; Goodman, D. W. J. Phys. Chem. 1991, 95, 2477. (29) Sham, T. K. Phys. Rev. B 1985, 31, 1888. (30) Ferna´ndez-Garcı´a, M.; Conesa, J. C.; Clotet, A.; Ricart, J. M.; Lo´pez, N.; Illas, F. J. Phys. Chem. B 1998, 102, 141. (31) Zauwen, M. N.; Crucq, A.; Degols, L.; Lienard, G.; Frennet, A. Catal. Today 1989, 5, 237. (32) Gorodetski, V. V.; Nieuwenhuys, B. E. Surf. Sci. 1981, 105, 299.

Ferna´ ndez-Garcı´a et al.

compounds is absent. In this case, the increase of the reduction temperature induces aggregation of metallic rhodium, shown by the growth of the 2060 cm-1 IR band. The differential behavior of the Rh-Cu catalysts studied gives evidence that the Rh/Cu atomic ratio affects the genesis of zerovalent phases through the presence of the CuAl2O4-o compound, whose existence is, in turn, governed by the Rh3+-Al2O3 interaction and, possibly, by the preparation pH. Consequently, the metal-support interaction in the calcined materials governs the genesis of bimetallic phases. Obviously, this fact makes the support role critical and implies that alumina- and silica-supported systems should present very different alloy genesis mechanisms. Conclusions The evolution of four Rh-Cu bimetallic catalysts in a reducing atmosphere with Rh/Cu atomic ratios of 0.05, 0.14, 0.23, and 0.43 was analyzed. The calcined materials show separated copper and rhodium phases. The existence, for samples with ratio equal or lower than 0.14, of a CuAl2O4-o phase has been related to the formation of a Rh-Cu alloyed phase. The final state of the catalysts also includes a Cu0 phase. The electronic state of rhodium in binary, zerovalent particles is strongly perturbed by the presence of copper, and rhodium centers are believed to be positively charged. The bimetallic phase has a heterogeneous nature, containing a Rh-rich core (close to Rh94Cu06) and a surface enriched in copper; the extension of this last phenomenon grows with reduction temperature. The Cu,Rh-Al2O3 interaction in the oxidized state is proposed to be the key parameter in determining the existence of copper intermediates of the reduction process and the final, zerovalent state of the noble element. In alumina-supported systems, only Rh/Cu atomic ratios that allow both components to interact with the surface of the support will drive alloy formation. Acknowledgment. We thank the scientific staff of line ID-24 at the ESRF Synchrotron, Grenoble, France, for their help during XANES experiments and Dr. J. Soria for the use of the EPR spectrometer. M.F.-G. and A.M.-A. acknowledge the Consejo Superior de Investigaciones Cientı´ficas (CSIC) and the Comunidad de Madrid (CAM), respectively, for Postdoctoral Contracts. P.F.-A. thanks the Comunidad de Madrid for a scholar-ship grant. We acknowledge with pleasure the helpul comments of one of the reviewers. This work was partially supported by CICYT and DGICYT (Spain) under Projects MAT96-0859 and PB94-0077, respectively. LA980689+