State of Supported Rhodium Nanoparticles for Methane Catalytic

Aug 24, 2007 - ... Chimica, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy ... also increases the amount and the dimensions of...
2 downloads 0 Views 185KB Size
Download PDF
Langmuir 2007, 23, 10419-10428

10419

State of Supported Rhodium Nanoparticles for Methane Catalytic Partial Oxidation (CPO): FT-IR Studies Elisabetta Finocchio,*,† Guido Busca,† Pio Forzatti,‡ Gianpiero Groppi,‡ and Alessandra Beretta‡ Dipartimento di Ingegneria Chimica e di Processo, UniVersita` di GenoVa, P.le Kennedy 1, 16129 GenoVa, Italy, and Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy ReceiVed May 18, 2007. In Final Form: June 28, 2007 The effect of pretreatments as well as of rhodium precursor and of the support over the morphology of Rh nanoparticles were investigated by Fourier transform infrared (FT-IR) spectroscopy of adsorbed CO. Over a Rh/alumina catalyst, both metallic Rh particles, characterized by IR bands in the range 2070-2060 cm-1 and 1820-1850 cm-1, and highly dispersed rhodium species, characterized by symmetric and asymmetric stretching bands of RhI(CO)2 gem-dicarbonyl species, are present. Their relative amount changes following pretreatments with gaseous mixtures, representative of the catalytic partial oxidation (CPO) reaction process. The Rh metal particle fraction decreases with respect to the Rh highly dispersed fraction in the order CO ≈ CO/H2 > CH4/H2O, CH4/O2 > CH4 > H2. The metal particle dimensions decrease in the order CH4/O2 > H2 > CH4/H2O > CO > CO/H2. Grafting from a carbonyl rhodium complex also increases the amount and the dimensions of Rh0 particles at the catalyst surface. Increasing the ratio (extended rhodium metal particles/highly dispersed Rh species) allows a shorter conditioning process. The surface reconstruction phenomena going on during catalytic activity are related to this effect.

1. Introduction Catalytic partial oxidation (CPO) of methane is an exothermic process allowing the efficient production of hydrogen for application, for example, in fuel cell technologies.1 The performances of Rh/alumina and Rh/zirconia catalysts for methane CPO have been previously studied by Beretta et al. in a structured wall reactor.2 Results clearly pointed out that the final performance of the supported Rh catalysts are strongly influenced by the conditioning procedure. In a dedicated investigation on the conditioning of highly dispersed Rh/Al2O3 catalysts, it was shown that outstanding performances were achieved after repeated CH4 partial oxidation runs, wherein the reaction temperature was stepwise increased from 573 to 1123 K. Run after run, the conversion of methane and the selectivity of synthesis gas increased until a close approach to equilibrium value was reached even at millisecond contact times. Since the process of methane partial oxidation involves a complex network of structure sensitive reaction steps (e.g., CH4 and CO dissociative adsorptions), it was proposed that the observed evolution of the yield was chemical evidence of a reconstruction of the Rh particles, driven by the high reaction temperatures and the interaction of the surface with the reacting mixture.3 On the other hand, other preparation methods such as the preparation of embedded rhodium nanoparticles in an alumina matrix may produce catalysts which have a better initial performance in methane CPO, not needing repeated activation steps, although the activity reached at a stationary state can be comparable.4 * To whom correspondence should be addressed. Telephone: +39-0103536027. Fax: +39-010-3536028. E-mail: [email protected]. † Universita ` di Genova. ‡ Politecnico di Milano. (1) Rostrup-Nielsen, J. R.; Sehested, J.; Nørskov, J. K. AdV. Catal. 2002, 47, 65. (2) Bruno, T.; Beretta, A.; Groppi, G.; Roderi, M.; Forzatti, P. Catal. Today 2005, 99, 89. (3) Beretta, A.; Bruno, T.; Groppi, G.; Tavazzi, I.; Forzatti, P. Appl. Catal., B 2007, 70, 515.

To understand the chemistry of the phenomena involved in catalyst stabilization, an in-depth study of the surface species formed in different conditions is needed. Fourier transform infrared (FT-IR) spectroscopy of adsorbed carbon monoxide5 is a well-known technique for the characterization of metal surfaces in the cases of both bulk and supported metal catalysts. The spectroscopy of the surface carbonyl species formed upon CO adsorption allows us to have information on the state and the nature of the adsorbing metal species. This technique has been widely applied to supported Rh catalysts.6 CO adsorption on reduced rhodium frequently gives rise to the detection of a typical couple of strong bands due to asymmetric and symmetric stretching modes of RhI(CO)2 gem-dicarbonyl complexes (D bands). These species are quite stable, as a result of a strong covalent σ-bond and π-back bond. A well-known mechanism7-9 involving the reaction of CO with the support OH groups causes the disruption of the very small Rh particles only, or the oxidation of isolated Rh atoms, with the formation of such gem-dicarbonyl complexes:

(1/x)Rhx0 + 2CO + -OH f O2- + RhI(CO)2 + 1/2H2 Thus, the detection of gem-dicarbonyl complexes is considered significant for the existence of highly dispersed Rh metal particles on the catalyst surface. On the other hand, CO adsorption on extended Rh metal particles gives rise, as usual, to linear (L) and bridging (B) Rh0CO species. These Rh0CO complexes are less stable, possibly (4) Montini, T.; Condo`, A. M.; Hickey, N.; Lovey, F. C.; De Rogatis, L.; Fornasiero, P.; Graziani, M. Appl. Catal., B 2007, 73, 84. (5) Lear, T.; Marshall, R.; Lopez-Sanchez, J. A.; Jackson, D. D.; Klapotke, T. M.; Baumer, M.; Rupprechter, G.; Freund, H.-J.; Lennon, D. J. Chem. Phys. 2005, 123, 174706. (6) Hadjiivanov, K. I.; Vayssilov, G. N.; AdV. Catal. 2002, 47, 308. (7) Paul, D. K.; Marten, C. D.; Yates, J. T., Jr. Langmuir 1999, 15, 4508. (8) Yates, J. T., Jr.; Duncan, T. M.; Vaughan, R. W. J. Chem. Phys. 1979, 71, 3908. (9) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Am. Chem. Soc. 1988, 110, 2074.

10.1021/la7014622 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007

10420 Langmuir, Vol. 23, No. 20, 2007

Finocchio et al.

because of a weak σ-bond. Low Rh concentration and high dispersions allow the detection of prevailing gem-dicarbonyl species following CO adsorption at room temperature. On the other hand, the linear-to-bridging (L/B) band intensity ratio may also be taken as an indication of the mean size of the Rh metal particles. There are several factors affecting the spectra characteristic of the rhodium/carbonyl species, and among them are the pretreatment of the samples, the rhodium concentration, the nature of the support, and the method of deposition (i.e., Rh precursors). In this work, the effect of pretreatments in several gaseous mixtures (representative of the CPO reacting mixtures) on the rhodium species at the catalyst surface has been examined by FT-IR. These treatments, performed in the IR cell, aimed at simulating and analyzing the interactions which arise between the surface and the reacting mixture; this is characterized by the co-presence of various reducing agents (CH4, CO, or H2) together with CO2 and H2O. The effect of the rhodium precursors (carbonyls or nitrate salts) as well as the effect of the support (alumina versus zirconia) have also been considered. CO adsorption at room temperature following the described procedures allowed us to obtain a comparative evaluation of the surface species detected. 2. Experimental Section R-Alumina, obtained by calcination in air at 1373 K for 10 h of a commercial γ-Al2O3 (Sasol), was used as the support. The measured Brunauer-Emmett-Teller (BET) surface area was 12 m2/g. Rh (0.5% w) was deposited both by grafting of Rh4(CO)12 (Rh/Al2O3cb sample)2 and by impregnation of Rh(NO3)3 via the incipient wetness technique (Rh/Al2O3nt sample).3 Following Rh deposition, the resulting catalysts show a comparable specific surface area. X-ray diffraction (XRD) analyses confirm the presence of the R phase, but a careful examination of the pattern allows the detection of minimal θ phase traces in the support used for the Rh/Al2O3nt preparation. The impregnated materials were dried at 383 K and then submitted to a slurry treatment in an acidic solution (HNO3/powder; H2O/ powder) before deposition on ceramic tubular supports by dipping them into the slurry for reaction tests in the structured reactor.2 Powder samples of the dried slurries, representative of the deposited layers, have been used in the IR experiments. Rh dispersion was evaluated by H2/O2 titration to be ∼35%, corresponding to a particle diameter of 3.2 nm for the Rh/Al2O3nt sample, and 35% by hydrogen chemisorption for the Rh/Al2O3cb sample. For the sake of comparison, we reported also data on CO adsorption over a Rh/zirconia catalyst prepared by the grafting of Rh4(CO)12 (Rh/ZrO2cb sample) with a metal phase dispersion of 48%. In this case, a commercial ZrO2 (Mel Chemicals) has been calcined at 1223 K for 10 h, obtaining a monoclinic phase, as confirmed by XRD and spectroscopic analysis. The surface area was comparable to the one of the alumina support (15 m2/g). Temperature-programmed reduction (TPR) and temperatureprogrammed oxidation (TPO) experiments were performed by Beretta et al.3 to verify the support inertness to Rh inclusion. Detailed preparation procedures and activity data on methane CPO have been reported by Bruno et al.2 and by Beretta et al.3 The skeletal spectra of the powder samples in the region above 400 cm-1 have been recorded with KBr pressed disks. For the surface IR studies, the pure powders were pressed into self-supported disks of ∼20 mg and activated in situ in a conventional IR cell with NaCl windows connected to an evacuation/gas manipulation apparatus. All the experiments have been performed in static conditions. The pretreatments of the powders were as follows: 1 h reduction step at 773 K in (i) pure hydrogen (∼30 kPa), (ii) pure methane (∼10 kPa), (iii) a mixture of methane/oxygen (∼10 kPa total pressure, 2/1 “partial oxidation conditions”), (iv) methane/water (∼10 kPa total pressure, large excess of methane, approximately 10/1), (v) pure CO (∼20 kPa), and (vi) a mixture of CO/hydrogen (∼10 kPa total

Figure 1. (A) FT-IR spectra of the skeletal region of the Rh/Al2O3nt (a) and Rh/Al2O3cb (b) samples in KBr disks. (B) FT-IR spectra (OH stretching region) of Rh/Al2O3nt outgassed at 773 K in vacuum (c) and heated at 773 K in hydrogen (d) and Rh/Al2O3cb outgassed at 773 K in vacuum (e). pressure, 1/2 “syngas conditions”), followed by 1 h of outgassing (0.013 Pa) at the same temperature. For each IR test, a fresh powder disk (i.e., not already pretreated with catalyst) was used. By this way, the effect of several gas mixtures on the possible reconstruction of the metal phase at the catalyst surface was simulated in the IR cell. The IR spectra of the powders have been recorded following CO adsorption at room temperature and after outgassing at 373 and 473 K. FT-IR spectra have been recorded with Thermo Nicolet Nexus and Prote´ge´ 460 FT instruments.

3. Results and Discussion 3.1. FT-IR Characterization of the Fresh Rh/Alumina Catalysts. The IR skeletal spectra of the Rh/Al2O3nt and Rh/ Al2O3cb samples (Figure 1A) show features typical of R-Al2O3 (corundum) powders.10,11 The position of the fundamental modes, all found below 700 cm-1, is related to the octahedral coordination of the Al3+ ions in the hcp lattice. The broad absorption tailing toward 800 cm-1 has been assigned to split longitudinal optical (LO) modes of the alumina fundamental modes. In the spectrum of the Rh/Al2O3nt sample, the detection of a number of weak components superimposed to the main bands in the range 800600 cm-1 (whose maxima can be barely seen at 766 and 722 cm-1) points out the presence of residual transitional aluminas (θ-Al2O3 phase).10,11 The sharp band at 1380 cm-1 is due to the residual nitrate arising from the slurry step in the acidic solution (HNO3) of the preparation method. The IR spectra in the νOH region of the same samples are reported in Figure 1B, recorded following outgassing at 773 K. The spectrum of the Rh/Al2O3nt sample (Figure 1Bc) shows sharp (10) Busca, G.; Resini, C. Vibrational spectroscopy for the analysis of geological and inorganic materials. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; Wiley: Chichester, 2000; p 10984. (11) Busca, G.; Lorenzelli, V.; Ramis, G.; Willey, R. J. Langmuir 1993, 9, 1492.

FT-IR Studies of Rh Nanoparticles for Methane CPO

Langmuir, Vol. 23, No. 20, 2007 10421

Figure 2. FT-IR spectra of Rh/Al2O3nt reduced at 773 K in hydrogen (a), reduced at 773 K in CO/H2 (b), reduced at 773 K in CO (c), reduced at 773 K in CH4/H2O (d), reduced at 773 K in CH4/O2 (e), and reduced at 773 K in CH4 (f). Inset: detail of the band at 1590 cm-1. Table 1. FT-IR Spectral Data Following Pretreatment Conditions (Rh/Al2O3nt Catalyst) treatment conditions

Tmax%

vacuum H2 CO/H2 CO CH4/O2 CH4/H2O CH4

66 61 55 43 43 2

a

cm-1.

b

A1590 cm-1 (a.u.)

νRh0-CO (L band)a

trace 0.05 0.06 0.19

2066 2072 2067 2063 2067 2060

L/Bb

linear band/ dicarbonyl bandb

4.0 10.0 5.2 3.5 4.3

0.6 1.6 1.6 1.2 1.5 1.0

A graphical method of spectra resolution has been used.20

bands at 3770, 3785 (shoulder), 3720, and 3680 cm-1. A weak and broad band is also found at 3565 cm-1. This pattern of bands is typical for transitional aluminas, including θ-Al2O3, while it contrasts the typical OH group spectrum of R-Al2O3.12 This shows the presence of a very small amount of transitional alumina (θphase) at the surface of the Rh/Al2O3nt sample. The IR spectrum of the Rh/Al2O3cb sample (Figure 1Be) shows some parallelism with the one described for Rh/Al2O3nt, as the complexity of the IR absorption is concerned. However, the intensity of the OH stretching bands is by far lower, pointing out that a number of OH groups is reduced, as is typical of R-Al2O3 surfaces. Reduction treatment in hydrogen does not significantly affect the position or the intensity of the OH group IR absorption (Figure 1Bd); the effect of other treatments will be discussed in detail in the following paragraphs. 3.2. Effect of Pretreatments: FT-IR Spectra of the Catalyst Surface (Rh/Al2O3nt Catalyst). The spectrum of the pure powder catalyst activated in hydrogen is reported in Figure 2a in the percent transmittance scale. The maximum transmission attained is 68% at wavenumbers at ∼1800 cm-1. Treatments in pure CO and in a CO/H2 mixture slightly lower this transmittance value (55% and 63%, respectively). Thermal treatments in mixtures such as CH4/O2 and CH4/H2O result in spectra showing a diffuse absorption in the all mid-IR range, significantly lowering the transmittance values down to 42-43%, as reported in Table 1. In the case of pretreatment with pure methane, a dramatic decrease in the transmitted IR light is evident, which is now below 1% (Figure 2f). The general decrease in the baseline transmission (12) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497.

can be taken as a measure of absorption by the growing coke particles. A peak at ∼1590 cm-1, growing during the different pretreatments, has been previously assigned to a vibrational mode of a highly carbonaceous coke species, consisting partly of pseudographitic structures and partly of disorganized materials, as discussed by Blackmond et al.13 According to Eischens, this band could also be associated to the asymmetric stretching mode of the carboxylate species fraction in the coke.14 This is a reasonable alternative assignment, which has been proven following olefin adsorption at a high temperature over alumina. The suppression of the species producing this band has been related to the effect of noble metal deposition. In our spectra, however, there is very little evidence of the corresponding carboxylate symmetric stretching mode, typically found at 1460 cm-1, and thus, the former assignment proposed is likely to be more accurate. The intensity of this band increases as the transmission value decreases for the samples pretreated with methane and mixtures where methane is present, in agreement with its assignment to carbonaceous species (Table 1). On the other hand, this peak is barely detectable or absent in the spectra of samples pretreated with pure CO and CO/H2, although they also present the phenomenon of decreased transmittance. OH groups are strongly influenced by coke formation: the growth of coke deposition is accompanied by a decrease in the concentration of OH groups. It seems all types of OH groups are similarly involved in the coke growth. Reaction of pure methane with the surface causes the complete disappearance of the free OH bands, leading to the detection of a broad and not resolved infrared absorption centered at ∼3600 cm-1 and tailing toward lower frequencies, assigned to perturbed OHs. From these data, it is thus clear that the surface is heavily covered in carbon following methane decomposition, with coke deposition being partially limited by the co-treatment with water and O2. On the other hand, high-temperature treatment of the catalyst in pure CO and in the CO/H2 mixture could lead to a lower formation of surface carbon species, and, possibly, to the formation of another kind of carbon which cannot be related to the band at 1590 cm-1. Alternatively, the decrease in transmittance can be related to a variation in the size of the metal particles, although the metal particle dimensions following treatments in CO and CO/H2 appear to be actually smaller with respect, for example, to the ones detected following pretreatment in hydrogen (see following section). On the other hand, the existence of two types of carbonaceous materials has also been detected by TPO experiments following pretreatment in a reaction mixture.3 We may assume that carbon deposits on the surface mainly as a result of a disproportionation Boudouard type reaction over Rh particles, as also reported by several authors to occur over Rh containing catalysts15,16

2CO f CO2 + C as well as methane decomposition

CH4 f 2H2 + C (13) Blackmond, D. G.; Goodwin, J. G., Jr.; Lester, J. E. J. Catal. 1982, 78, 34. (14) Najbar, J.; Eischens, R. P. In Proceedings of the 9th International Congress on Catalysis; Phillips M. J., Ternan, M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. 3, p 1434. (15) Erdohely, A.; Solymosi, F. J. Catal. 1984, 84, 446. (16) Iordan, A.; Zaki, M. I.; Kappenstein, C.; Geron, C. Phys. Chem. Chem. Phys. 2003, 5, 1708.

10422 Langmuir, Vol. 23, No. 20, 2007

Finocchio et al.

Figure 3. FT-IR spectra of the surface species arising from CO (133 Pa) adsorption at room temperature over the Rh/Al2O3nt sample submitted to the following pretreatments: outgassed at 773 K (a), reduced at 773 K in hydrogen (b), reduced at 773 K in CO/H2 (c), reduced at 773 K in CO (d), reduced at 773 K in CH4/O2 (e), reduced at 773 K in CH4/H2O (f), and reduced at 773 K in CH4 (g).

Figure 4. Second derivative (bold line) of the surface species FT-IR spectra arising from CO (133 Pa) adsorption over the Rh/Al2O3nt sample outgassed at 773 K (A) and reduced at 773 K in CH4/H2O (B). Original spectra are indicated by the dotted line.

possibly producing two different coke species. The presence of excess hydrogen only slightly limited the formation of the carbonaceous deposits. A spillover effect from the metal particles to the alumina support can also occur. 3.3. Effect of Pretreatments: FT-IR Characterization of Rhodium Nanoparticles by CO Adsorption (Rh/Al2O3nt Catalyst). In a second set of experiments, the different pretreatments at 773 K have been followed by CO adsorption at room temperature and outgassing at increasing temperatures. The resulting FT-IR spectra recorded in the presence of 133 Pa equilibrium pressure of CO are reported in Figure 3. The spectrum of the activated sample has been subtracted. CO adsorption over the sample previously heated in vacuum (10-2 Pa) at 773 K (Figure 3a) results in the detection of two main IR bands at 2093 and 2021 cm-1, due to the symmetric and asymmetric CO stretching modes of gem-dicarbonyl species RhI(CO)2 (D bands), in agreement with data extensively reported in the literature,6-8,17,18 and due to the reaction of CO with atomically dispersed Rh or Rh0 nanoparticles and OH groups of alumina, with the evolution of hydrogen. Another weak and broad absorption band centered at 1830 cm-1 is assigned to multibonded carbonyl, likely bridging Rh02(CO) species (B band), thus indicating the presence of metallic Rh particles. The corresponding band due to CO being linearly adsorbed over Rh metal particles (L band), expected in the range (17) Lavalley, J. C.; Saussey, J.; Lamotte, J.; Breault, R.; Hindermann, J. P.; Kiennemann, A. J. Phys. Chem. 1990, 94, 5941. (18) Kraus, L.; Zaki, M. I.; Kno¨zinger, H.; Tesche, B. J. Mol. Catal. 1989, 55, 55.

2070-2060 cm-1, can be masked by the intense dicarbonyl absorptions. This hypothesis is also confirmed by the second derivative of spectrum of Figure 3a reported in Figure 4A showing clearly three components in the 2200-2100 cm-1 region. A very weak and broad band at ∼2250 cm-1 can be assigned to the CO coordinated over the exposed Al ions of the support, although this component is usually reported to be below 2245 cm-1.6 Activation of the catalyst at 773 K in pure hydrogen leads to the increase of the component at 2066 cm-1 in the spectrum of adsorbed CO (Figure 3b), which is related to the presence of quite extended Rh metal particles. Bands due to gem-dicarbonyl are still evident at 2088 and 2016 cm-1 and thus at slightly lower frequencies than in the previous case, and they are partially superimposed to the band of CO coordinated over metallic Rh. The band due to bridging CO is also well evident (centered at 1860 cm-1). Following pretreatment in a CO/H2 mixture (Figure 3c), the dominant spectral feature of adsorbed CO consists of a strong band of linear Rh0-CO at 2072 cm-1; other sharp bands are detectable at 2092 and 2019 cm-1, assigned to gem-dicarbonyl species, and at 1889 cm-1 (broad and weak), assigned to bridging CO species over Rh metal particles. Pretreatment in pure CO (Figure 3d) allows the detection of the strong band due to CO linearly coordinated over Rh metal particles at 2067 cm-1, overlapped with the two bands of dicarbonyl species (shoulders around 2095 and 2035 cm-1). This spectrum shows little evidence of any bridging CO species, pointing out the formation of a very well dispersed Rh metallic phase. This result agrees with data

FT-IR Studies of Rh Nanoparticles for Methane CPO

Langmuir, Vol. 23, No. 20, 2007 10423

from Zaki et al. who reported that CO at high temperatures acts as a reducing agent, inducing a reconstruction of the surface metal particles.18,19 Pretreatment of the sample with a methane “partial oxidation” mixture (O2/CH4 around 0.5) also leads to the detection of a relevant amount of Rh0 particles characterized by the adsorbed CO stretching band at 2063 cm-1 overlapped with the twin bands due to gem-dicarbonyls (Figure 3e). Correspondingly, the band due to CO bridging species is broad and complex, with a component centered at higher frequencies (∼1900 cm-1) and a component at 1854 cm-1. The two components correspond to those observed by Lavalley et al.,17 who assigned two similar bands detected over 5% Rh-supported catalysts to bridging CO on two high coordination planes, such as Rh(100) and Rh(111). Rhodium metal particles thus seem more “structured” than those in the other cases. Alternatively, a shoulder at ∼1920 cm-1 has been assigned to a structure such as Rh2(CO)3, that is, a bridging species with a stoichiometry of 3:2.20 Activation of the sample in a CH4/H2O mixture (large CH4 excess) results in a further increase of the relative intensity of the band due to linear CO over metal particles (2067 cm-1, Figure 3f), which becomes the main band of the spectrum, strongly overlapping with the twin dicarbonyl bands at 2091 and 2033 cm-1. The second derivative of the spectrum in this region has also been reported to confirm the detection of three separate components (Figure 4B). The band at 1850 cm-1 represents the adsorption of CO bridging over metallic rhodium, whose relative intensity is slightly lower here than in the previous case. Heating the sample in pure methane at 773 K results in a dramatic weakening of the characteristic νCO bands of both the geminal and linear species (Figure 3g); however, three IR components are still distinguishable at 2083, 2060, and 2017 cm-1, showing that poisoning by coking of the surface affects at a similar extent all the rhodium species present at the catalyst surface. As reported by Anderson,21 gem-dicarbonyl species are retained following methane treatment. It is interesting to notice also the presence of the band due to bridging CO, showing the availability of adjacent Rh metal atoms. Thus, methane reduces the rhodium catalyst at 773 K, and the reaction is accompanied by carbon deposition, as already stated in the previous paragraph. The effect of the interaction of methane with Rh/alumina catalysts has also been studied by Tian et al. using diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy experiments in a higher temperature range (823-973 K).22 They found, as we did, that methane reduced the catalyst and that a further interaction of methane with the reduced surface led to carbon deposition covering the rhodium atoms, but not completely. The wavenumber values of the main bands obtained by CO adsorption at room temperature over the Rh/Al2O3nt catalyst following the different pretreatments are summarized in Table 1. Some differences can be noted regarding the position of the νRh0CO band (L band) in the range 2070-2060 cm-1. If we take as a reference the frequency and relative intensity of this band recorded following CO adsorption over the hydrogen-reduced sample, the following considerations can be proposed: (i) Upon reducing treatments under methane and CH4/O2, this band reaches the lowest wavenumber (∼2060 cm-1); this downward shift has to be ascribed in part to a decrease in the dipole coupling between adjacent molecules caused by poisoning

of some adsorption sites, combined with an increased electron donation from the carbonaceous material to rhodium, increasing its back-donation to adsorbed CO.21 (ii) Other treatments, such as adding water vapor during the pretreatment in methane, reduce the effect of coking as discussed in the previous paragraph (see the T% values reported in Table 1), and the IR band due to adsorbed linear CO can be found at slightly higher wavenumbers. Two effects are likely competing: the formation of larger particles due to the treatment in methane/ water or methane/oxygen mixtures results in an increased coupling effect between adsorbed CO molecules, while the residual presence of coke at the surface and the resulting back-bonding from the carbonaceous material to the metal particles favor a shift of the same band toward lower wavenumbers, as previously reported.23 (iii) Treatment in the CO/H2 mixture leads to the highest frequency value detected for the νRh0-CO band (∼2072 cm-1) and thus to the lowest back-bonding effect, which could be explained by some withdrawing of the electron density from the metal particles or by the increased coupling effect among the CO molecules adsorbed over larger particles, as reported above. The intensity ratios of the bands due to linear (L) and bridging (B) coordinated CO on rhodium metal particles are reported to depend on the metal dispersion,24 at least for “mild” reduction conditions (i.e., reduction temperatures below 873 K). In our conditions, this ratio can be carefully calculated with respect to the different pretreatment conditions (Figure 3), leading to an evaluation of the metal particle fraction exposed at the surface. Values of L/B are reported in Table 1, and they decrease in the following order of reducing agent: CO/H2 > CO > CH4/H2O > H2 > CH4/O2. Taking into account the extinction coefficients reported in the literature for the linear and bridging CO coordinated species (13 and 42 cm µmol-1, respectively),20,25 in our case, the amount of micromoles of linearly coordinated CO ranges between 45 and 21 times the amount of micromoles of bridging CO. This points out that Rh in the Rh/Al2O3nt catalyst is always very well dispersed and it keeps the dispersion following each treatment. Treatments in pure CO and CO/H2 lead to the highest L/B relative intensity. This implies the lowest rhodium metal particle dimensions, rich in low coordination metal atoms. Heating in the CH4/O2 mixture allows the formation of high coordination planes on metal particles, where there is an increased number of adjacent metal atoms. The intensity ratio between the Rh0(CO) bands (L) and gemdicarbonyl stretching bands (D) is also reported in Table 1. Although this measure is difficult due to the overlapping of the bands in our spectra, we chose to carefully relate the intensity of the band at ∼2070 cm-1 assigned to linearly coordinated CO over Rh metal particles with the intensity of the band at ∼2030 cm-1 (asymmetric RhI(CO)2 stretching). This value is decreasing in the order CO ≈ CO/H2 > CH4/H2O, CH4/O2 > CH4 > H2. The contribution of the Rh02(CO) bridging band intensity has also been considered, but it is negligible. This ratio can be taken as an indicator of the Rh particle morphology,26 with the L (and B) bands being related to rhodium metal particles and the D bands being related to highly dispersed rhodium species. Activation in CO and in the CO/H2 mixture allows the formation of Rh metal particles prevailing over highly

(19) Zaki, M. I.; Tesche, B.; Kraus, L.; Kno¨zinger, H. Surf. Interface Anal. 2004, 12, 239. (20) Rasband, P. B.; Hecker, W. C. J. Catal. 1993, 139, 551. (21) Anderson, J. A.; Rochester, C. H.; Wang, Z. J. Mol. Catal. A: Chem. 1999, 139, 285. (22) Tian, Z.; Dewaele, O.; Marin, G. B. Catal. Lett. 1999, 57, 9.

(23) Maroto-Valiente, A.; Rodriguez Ramos, I.; Guerrero-Ruiz, A. Catal. Today 2004, 93-95, 567. (24) Sheu, L. L.; Sachtler, W. M. H. J. Mol. Catal. 1993, 81, 267. (25) Fischer, I. A.; Bell, A. T. J. Catal. 1996, 162, 54. (26) Kaspar, J.; de Leitenburg, C.; Fornasiero, P.; Trovarelli, A.; Graziani, M. J. Catal. 1994, 146, 136.

10424 Langmuir, Vol. 23, No. 20, 2007

Finocchio et al.

Figure 5. FT-IR spectra of the surface species arising from CO adsorption at room temperature and thermal evolution over the Rh/Al2O3nt sample reduced in hydrogen in the presence of CO (133 Pa) at room temperature (a), followed by outgassing at room temperature (b) and prolonged outgassing at room temperature (c), 373 K (d), and 473 K (e).

dispersed (easily disrupted) Rh species. We can propose the existence of two-dimensional Rh metal rafts, characterized by an high number of edge and corner atoms, coordinating linearly CO molecules and strongly interacting with the support (see the high Rh0-CO stretching band wavenumber). However, rhodium atoms in these rafts should still have a metallic character rather than an ionic character (RhI) as proposed instead by Yates et al. in the early 1980s.27 Treatments in methane/oxygen or methane/water slightly favor the appearance of a highly dispersed phase, while the metal particles, still detectable, are quite larger and more structured with respect to the previous case. The treatment in CH4/O2 and CH4/H2O likely provokes the reduction of the roughness of the small particles, which evolve and expose high coordination planes (such as Rh(111) and Rh(110)).23 We have to take into account the effect of coke over these surfaces. Clearly, the number of rhodium sites accessible to CO adsorption is limited as pointed out by the decrease in the intensity of the bands reported in Figure 3. It seems however that there is no direct proportion between coke deposition, evaluated through the decrease in T%, and the trend of the L/B and L/D band intensity ratios. Thus, the effect we notice is to be related more to the different pretreatment procedure than to any selective poisoning of sites by coke deposition. 3.4. Effect of Pretreatments: Thermal Evolution of the Adsorbed Surface Species. Following CO adsorption at room temperature, we also studied the evolution of the surface species upon outgassing from room temperature to 473 K to evaluate the strength of the CO/surface interaction. In Figure 5, the evolution of CO adsorbed over the sample previously reduced in pure hydrogen is reported, which can be taken as representative of this study. As a matter of fact, regardless of the pretreatment conditions, we detected a common trend, which can be described as follows. After outgassing at room temperature and at increasing temperatures, the band due to CO linearly adsorbed over metallic Rh decreases in its intensity and its maximum continuously shifts toward lower frequencies (as reported in Figure 6). This shift in the vibrational frequency is due to the reduction of dipole-dipole coupling as a result of a low coverage of adsorbed CO. Linear carbonyls disappear at temperatures higher (27) Yates, D. C. J.; Murrel, L. L.; Prestridge, E. B. J. Catal. 1979, 57, 41.

Figure 6. Plot of the linear Rh0-CO stretching mode wavenumbers following CO adsorption and desorption at room temperature and at 373 K.

than 373 K. Correspondingly, the broad band at ∼1850 cm-1, due to CO bridging species, slightly shifts toward even lower frequencies. It is however quite difficult to estimate the shift of the bridging species features, due to the very low intensity of the signal in our spectra. Bands due to geminal RhI(CO)2 species are more resistant to outgassing, being detectable up to 473 K. Their position is not significantly affected by outgassing, as can be expected for an atomically dispersed system where dipole-dipole coupling does not exist. The stability of these species has been ascribed to electronic effects because of the achievement of a stable 18-16 electron configuration of those surface complexes.28,29 Evacuation of the sample at high temperature leads to a decrease in the intensity of the two bands, while their apparent relative intensity significantly changes: the 2090 cm-1 band from the symmetric νCO stretching mode decreases faster in intensity (Figure 5). However, this effect is possibly due to the diminished overlapping of the band attributed to linear CO, which decreases in intensity while also shifting toward lower wavenumbers. Up to 473 K in our conditions, we had no evidence of reductive agglomeration of RhI sites.30 In the spectra recorded at 473 K, we also detected a weak, complex component centered at 2230 cm-1, assigned to CO adsorbed over alumina support. At the same temperature, a shoulder become visible at ∼1970 cm-1, which could be assigned to bridging CO in a Rh2(CO)3 configuration as reported by Fischer et al.25 However, these authors pointed out that these species are becoming less evident at increasing temperature. One alternative (28) van’t Blik, H. F. J.; van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koninsberger, D. C.; Prins, R. J. Am. Chem. Soc. 1985, 107, 3139. (29) Wang, H.; Yates, J. T., Jr. J. Catal. 1984, 89, 79. (30) Hadjiivanov, K.; Ivanova, E.; Dimitrov, L.; Knozinger, H. J. Mol. Struct. 2005, 661-662, 459.

FT-IR Studies of Rh Nanoparticles for Methane CPO

Langmuir, Vol. 23, No. 20, 2007 10425

Figure 7. FT-IR spectra of the surface species arising from CO adsorption at room temperature over the Rh/Al2O3cb sample outgassed at 773 K (a) and reduced at 773 K in hydrogen (b).

explanation might be that they represent bridge-bonded CO on RhI , a species proposed by Traumann et al.31 The comparison of the reported thermal evolution for all the pretreatments considered points out that there is no effect of the pretreatments; thus, coking of the catalysts and/or the morphology changes of the rhodium particles have no significant effect on the strength of the CO interaction with rhodium particles. 3.5. Effect of the Rh Precursor: FT-IR Spectra of CO Adsorbed over the Rh/Al2O3cb Catalyst. Figure 7 shows the spectra arising from CO (133 Pa) adsorbed over a Rh/Al2O3cb sample following thermal treatment in vacuum and in hydrogen, and they can be compared with the spectra in Figure 3a and b. Following reduction in hydrogen, the bands detected at 2093 (shoulder) and 2032 cm-1 are due to gem-dicarbonyl species (D bands), while bands due to CO linearly and bridging coordinated over metallic Rh particles are detected at 2072 and 1870 cm-1, respectively. Qualitatively, these features are similar to those reported in the previous paragraph for CO adsorption over Rh/Al2O3nt; however, the intensities of the signals due to Rh0CO linear species are strongly enhanced in the spectra of the Rh/Al2O3cb sample (the ratio L/D is equal to 1.5), revealing an increased tendency to form metal particles with respect to the formation of a highly dispersed Rh phase detected as RhI(CO)2. It is worth noticing that, even following thermal treatment in vacuum, the band due to CO coordinated over metallic rhodium is clearly detectable (Figure 7a). Moreover, rhodium metal particles seem to be more structured, showing a non-negligible signal due to bridging (Rh0)2CO (the intensity ratio L/B is equal to 4.3, following reduction in hydrogen). RhI formation is detectable in any case: other techniques such as in situ DRIFT32 and extended X-ray absorption fine structure (EXAFS)33 also proved the formation of RhI(CO)2 particles over low Rh content catalysts (0.1% rhodium) from the decomposition of a Rh4(CO)12 precursor. In these papers, it was also reported that higher Rh loading led to the formation of large metal particles (31) Trautmann, S.; Baerns, M. J. Catal. 1994, 150, 335. (32) Basini, L.; Marchionna, M.; Aragno, A. J. Phys. Chem. 1992, 96, 9431. (33) Grunwaldt, J.-D.; Basini, L.; Clausen, B. S. J. Catal. 2001, 200, 321.

upon reduction in hydrogen. Aggregative decarbonylation reactions occurred during decomposition of the carbonyl precursor. The different Rh precursor used in catalyst preparation thus significantly changed the state of rhodium at the surface, and this effect is particularly evident in the spectrum in Figure 7b recorded after reduction in pure hydrogen. This is not surprising, because the high-oxidation-state RhIII cation impregnated in the case of Rh/Al2O3nt may disperse better on the ionic alumina surface than the neutral species utilized in the preparation of Rh/Al2O3cb. Additionally, in the grafting procedure, the Rh carbonyl is already a polynuclear species and it should be possible, in principle, to obtain small metal aggregates of similar nuclearity at the catalyst surface, provided the complex decomposition is carried out a low temperature. Thermal desorption of coordinated CO (spectra not reported here) does not evidence any significant variation in the strength of the metal-CO interactions related to the Rh precursor used. 3.6. Effect of the Support: Rh/Al2O3cb and Rh/ZrO2cb Catalysts. IR spectra of the surface species arising form CO adsorption at room temperature over the Rh/ZrO2cb catalyst and their thermal evolution are reported in Figure 8, following pretreatment in pure hydrogen. As in the case of an alumina support, Rh deposition from the carbonyl complex precursor leads to a system where large Rh metallic particles can be formed following reduction treatment. The main band at 2076 cm-1 is assigned to CO linearly coordinated over Rh0 particles (L band, Figure 8a), while the band due to CO bridging species appears quite complex with two maxima centered at 1900 and 1870 cm-1. The twin bands (D) are also detectable as shoulders around 2095 and 2030 cm-1. A weak but sharp band at 2200 cm-1 is assigned to CO adsorbed over exposed Zr4+ ions. The complexity of the bridging band clearly points out the formation of surface metal particles exposing different high coordination planes. More questionable is the analysis of the other features of the spectrum. IR adsorption bands of gem-dicarbonyl species are independent of the coverage, consisting of isolated species on the surface; thus, the gem-dicarbonyl structure seems adequate for the study on the influence of the support on the adsorbed CO stretching modes.29 However, in the open literature, somehow conflicting

10426 Langmuir, Vol. 23, No. 20, 2007

Finocchio et al.

Figure 8. FT-IR spectra of the surface species arising from CO adsorption at room temperature and thermal evolution over the Rh/ZrO2cb sample reduced in H2 in the presence of CO (133 Pa) at room temperature (a), followed by outgassing at room temperature (b) and prolonged outgassing at room temperature (c), 373 K (d), and 473 K (e).

data have been reported. Erdohelyi et al.34 found that adsorption of CO on differently supported Rh samples (TiO2, Al2O3, SiO2, and MgO) produced almost identical IR spectra. Although the twin CO IR bands are sensitive to preparation methods and pretreatment, the electronic interaction between Rh and the support does not result in any shift of the positions of these bands. On the other hand, Tanaka et al. investigated the state of Rh supported over Al2O3 and ZrO2, finding a shift of the dicarbonyl bands to lower frequencies on Rh/zirconia, but no further explanations were proposed for this effect. The same authors found that a ZrO2 support favored the formation of twin CO species at increasing temperature, with respect to the alumina support.35 Miessner et al. evaluated by FT-IR spectroscopy CO adsorption over reduced Rh catalysts prepared using Na/zeolites and several oxides as supports. They found out a decreasing back-bonding ability of supported rhodium in the order NaX < Al2O3 < NaY < TiO2.36,37 In our conditions, it is clear from a comparison of Figures 7b and 8a that wavenumbers of the gem-dicarbonyl bands detected as shoulders in the spectrum of the Rh/ZrO2cb sample appear to be almost the same as those in the Rh/Al2O3cb sample spectrum, although their position is quite difficult to determine due to the strong overlapping with the linear band. In both cases, the dominant feature of the spectrum is the L Rh0-CO band, whose slight increase in frequency for the Rh/ZrO2cb sample can be explained by an increased population of CO bound terminally on structured metal particles. The substantial constancy of the νCO wavenumber of the monocarbonyl linear species and gemdicarbonyl species, whatever the sample composition, thus allows the exclusion of all long-range electronic interactions between the support and Rh.34 As for reactive CO adsorption, the enhanced formation of adsorbed carbonate and bicarbonate species over this sample (the bands at 1630, 1420, and 1217 cm-1 are typical of bicarbonate species, and those at 1550, 1315, complex, and 1059 cm-1 are assigned to carbonate species; see Figure 9a) (34) Erdohelyi, A.; Solymosi, F. J. Catal. 1983, 84, 446. (35) Iizuka, T.; Tanaka, Y.; Tanabe, K. J. Mol. Catal. 1982, 17, 381. (36) Miessner, H.; Gutschick, D.; Ewald, H.; Mu¨ller, H. J. Mol. Catal. 1986, 36 (3), 359. (37) Trunschke, A.; Ewald, H.; Miessner, H.; Marengo, S.; Martinengo, S.; Pinna, F.; Zanderigh, L. J. Mol. Catal. 1992, 74, 365.

suggests that CO disproportionation to C and CO2 is more favored on this surface than on an alumina surface (Figure 9b), in agreement with data previously reported38 and likely related to the exposed zirconium surface ions. In Figure 8, spectra corresponding to the thermal evolution of the species arising from CO adsorption over the Rh/ZrO2cb sample reduced in pure hydrogen are also reported. Upon desorption at room temperature and at 373 and 473 K, the strong L band at 2076 cm-1 shifts toward lower wavenumbers, due to the decreased CO coverage, and its intensity is also decreased. Only a minor decrease in the intensity of the broad absorption of bridge-bonded CO on Rh0 is detected. gem-Dicarbonyl species are resistant to outgassing up to 473 K with no shift in the wavenumbers, with the asymmetric stretching band becoming even more complex and the symmetric component appearing to be by far the weakest band. This effect can be explained by taking into account an unusual resistance of the linearly coordinated CO molecule on Rh0, whose characteristic IR band, still strong, shifts toward lower wavenumbers overlapping with the asymmetric component of the twin bands. The detection of a broad, although weaker, band centered at 1850 cm-1 resisting outgassing and due to bridging species supports the hypothesis of an increased resistance of CO adsorbed over metal particles. 3.7. Rhodium FT-IR Characterization: CO Adsorption Following the Preoxidation Step. An oxidation step, before any reducing treatments, seems to further assist the formation of an atomically dispersed phase. In the case of the Rh/Al2O3nt and Rh/Al2O3cb samples pretreated in air at 773 K and then reduced in hydrogen at the same temperature, RhI species are largely formed and the corrsponding gem-dicarbonyl bands are predominant (Figure 10a). A comparison of the spectra in Figures 10a and 3b, recorded in the same conditions, shows that in the spectrum in Figure 10a another component at higher wavenumbers is present (at ∼2120 cm-1, as evidenced by the second derivative; see Figure 10 inset), which can be assigned to the carbonyls of the rhodium ions. It has been reported that linear Rhn+-CO complexes (n > 1) give rise to bands at wavenumbers above 2120 cm-1. In particular, CO adsorbed over Rh3+ and Rh2+ ions (38) Tanaka, Y.; Izuka, T.; Tanabe, K. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2215.

FT-IR Studies of Rh Nanoparticles for Methane CPO

Langmuir, Vol. 23, No. 20, 2007 10427

Figure 9. FT-IR spectra of the surface species arising from CO adsorption at room temperature over the Rh/ZrO2cb sample reduced in H2 (a) and over the Rh/Al2O3cb sample reduced in hydrogen (b). (Low-frequency region.)

Figure 10. FT-IR spectra of the surface species arising from CO adsorption at room temperature over the Rh/Al2O3nt (a) and Rh/Al2O3cb (b) samples with a preoxidation step in air before reduction at 773 K in hydrogen. Inset: second derivative of spectrum (a).

is characterized by stretching bands in the range 2150-2120 cm-1. These species are unstable and disappear after evacuation at room temperature.6 We can thus suggest that in our conditions Rh3+/2+ species are present. Recent reports have also proposed the formation of new Rhn+(CO)2 species formed on highoxidation-state rhodium in cationic positions in zeolite catalysts and characterized by bands in the 2170-2140 cm-1 region.39 However, the two IR components characterizing these particular dicarbonyl species could not be detected in our spectra. Metal rhodium particles can barely be detected (shoulder at ∼2065 cm-1, also evidenced in the derivative spectrum; see Figure 10 inset), and no absorptions due to bridging carbonyl species appear. The spectrum of the surface species arising from CO adsorption over the Rh/Al2O3cb catalyst (Figure 10b) shows the same features discussed above: bands due to gem-dicarbonyl species predominate, but some Rh metal particles are also present, characterized by IR bands due to both linear and bridge-bonded CO. In our reduction conditions, that is, at low temperature, the sintering effect described by Wong et al.40 could not be observed. (39) Ivanova, E.; Mihaylov, M.; Thibault-Starzyk, F.; Daturi, M.; Hadjiivanov, K. J. Catal. 2005, 236, 168.

Also, diffusion of rhodium in the alumina bulk is unlikely to occur below 773 K. 3.8. Activity Results and Surface Species. The behavior of these catalysts in the methane partial oxidation (CPO) process has been described elsewhere in detail.2,3 Relevant results are herein briefly recalled for clarity. To better understand the nature of the transient behavior of the Rh-supported catalysts during repeated CH4 partial oxidation runs, TPO and CH4 decomposition measurements were performed at an early stage and a final stage of the conditioning process. It was found that the as-prepared catalyst is much more prone to C deposition than a fully conditioned catalyst. Also, chemisorption measurements showed that the high-temperature reaction process caused a contraction of the metal surface. These results were interpreted as an indication that the surface of the fresh catalyst presents small metal clusters (rich with defect sites), while, after conditioning, the surface is characterized by larger clusters with more densely packed crystal faces; C-forming reactions such as CH4 and CO decompositions are in fact more energetically favored on step and kink sites than on flat Rh(111) sites.41 The present IR study provides direct (40) Wong, C.; McCabe, R. W. J. Catal. 1989, 119, 47. (41) Beretta, A.; Tavazzi, I.; Bruno, T.; Groppi, G.; Dal Santo, V.; Sordelli, L.; Mondelli, C. La Chimica e l’Industria 2006, 5, 50.

10428 Langmuir, Vol. 23, No. 20, 2007

evidence of some key elements of this picture. First, IR measurements of adsorbed CO performed in “static” conditions showed evidence of the presence of both rhodium metal particles (Rh0) and RhI-forming species (thus very highly dispersed) at the catalyst surface, whose ratio increases along the following series: “fresh” catalyst (following only thermal treatment) < hydrogen-reduced catalyst < catalyst pretreated in CH4/O2 and CH4 /H2O mixtures < catalyst pretreated in CO and CO/H2. By considering that the highly dispersed Rh species are almost atomically dispersed species or, more generically, atoms that interact strongly with the support (this is the case of rafts, that is, small two-dimensional particles27), the IR measurements are consistent with the existence of defect sites on the surface of the fresh catalyst. Also, the tendency of the rhodium species to reorganize under reaction conditions with the formation of metal particles is confirmed; the process is likely favored by the mobility of the defect sites, promoted by the interactions with CO in the gaseous mixture. Moreover, if we take into account the L/B ratio as an indicator of the Rh particle morphology, the lowest value of this parameter is reached following heating in CH4/O2, pointing out the formation of the “largest” Rh surface metal particles, likely favored by the presence, under reaction conditions, of sintering promoters, such as H2 and H2O. An indication is thus provided that conditioning runs in the reaction mixture should allow the prevailing formation of rhodium metal particles, with respect to highly dispersed Rh species, thus leading to the formation of the “smooth” particles supposed by Beretta et al.3,41 Concerning the effect of the preparation procedure, activity data showed that the conditioning process was much faster for Rh/Al2O3 catalysts obtained via Rh4(CO)12 deposition: few repeated CH4 partial oxidation runs were in fact needed to reach high and stable performances. The present IR measurements confirm that the RhI species are less abundant and the formation of extended metal particles is clearly favored on catalysts prepared from the organometallic precursor. Finally, it is recalled that, after the initial conditioning, the activity of the Rh/ZrO2cb catalyst was found to be lower than that of the Rh/Al2O3cb catalyst. The present IR measurements are not conclusive in this respect, since for both systems CO adsorption at the hydrogen-reduced surface presented similar features. Other factors could be involved in the different catalytic behaviors, such as the increased formation of carbonate and bicarbonate species at the catalyst surface or an increased tendency for coke deposition. Further investigation is currently ongoing on the effects of pretreatments in methane/oxygen over Rh/zirconia catalysts.

4. Conclusions Reduction of the Rh/Al2O3nt catalyst in different conditions representative of the CPO reaction process has been performed and studied by FT-IR. Room-temperature CO adsorption over a hydrogen reduced catalyst leads to the detection of linear- and

Finocchio et al.

bridge-bonded carbonyls coordinated over metallic Rh particles, characterized by IR bands in the ranges 2070-2060 cm-1 (L) and 1820-1850 cm-1 (B), respectively, signifying of the existence of rhodium metal particles that are extended and not subjected to disruption processes. RhI(CO)2 gem-dicarbonyl species are also formed, characterized by symmetric and asymmetric stretching bands around 2090 and 2020 cm-1 (D bands) and related to the presence of highly dispersed rhodium species. The results obtained can be summarized as follows: (i) Effect of the pretreatments: The Rh/Al2O3nt sample contains highly dispersed Rh species as well as more extended Rh metal particles in a different relative amount, depending on the different compositions of the pretreatment gaseous mixtures. The ratio L/D decreases in the order

CO ≈ CO/H2 > CH4 /H2O, CH4/O2 > CH4 > H2 while the rhodium metal particle dimensions (ratio B/L) decreases in the order

CH4/O2 > H2 > CH4/H2O > CO > CO/H2 Thus, the pretreatment with CO and CO/H2 allows the formation of an increased amount of rhodium metal particles, with respect to other treatments, while pretreatment with the CH4/O2 “reaction” mixture allows the growth of such extended and more structured metal particles. Coke formation affects the support surface as well as the exposed rhodium particles (both the Rh metal particles and highly dispersed Rh species). Two kinds of carbonaceous materials seem to be formed from CH4 pretreatment and from CO pretreatment. (ii) Effect of the rhodium precursor: Following the reduction of the Rh/Al2O3cb catalyst in hydrogen, the Rh carbonyl is decomposed to structured Rh metal particles which are the main species detectable at the catalyst surface. The same effect was also found following the hydrogen reduction of a Rh/ZrO2cb catalyst. (iii) Effect of the support: An enhanced formation of reactive adsorption products (carbonate and bicarbonate species) over the Rh/ZrO2 catalyst could be observed. (iv) A preoxidation step improves the formation of an even highly dispersed Rh phase: in the case of the Rh/Al2O3nt and Rh/Al2O3cb samples pretreated in oxygen, gem-dicarbonyl bands are clearly predominant. Moreover, Rhn+ (n > 1) species are also detectable even following a reduction step in hydrogen. (v) A parallel between the activity results and IR characterization results can be proposed: increasing the ratio (extended rhodium metal particles/highly dispersed Rh species) allows a shorter conditioning process. The surface reconstruction phenomena going on during catalytic activity are to be related to this effect. LA7014622