Search

The structure and dispersion of the active phase present in sulfided Mo/C and Co-Mo/C catalysts are studied by means of Mo K-edge EXAFS. EXAFS measurements were carried out at liquid N2 temperature on freshly sulfided catalysts in situ. Due to a high signal-to-noise ratio of the experimental data, detailed information about the structural parameters of the active phase can be obtained. The Mo/C and Co-Mo/C catalysts consist of very small MoS2-like particles, which contain on average 5 to 6 Mo ions (Mo/C), and 7 to 8 Mo ions (Co-Mo/C). There are indications for the presence of a Mo-C interaction in Mo/C and Co-Mo/C. The Mo-C coordination distance is estimated to be 1.9 ± 0.1 Å. There are also indications for the presence of a Mo-Co interaction. The Mo-Co coordination distance was estimated to be 2.8 ± 0.1 Å.

The three-site adsorption model, previously developed to describe H adsorption on small Pt particles, was used to gain insight into dependence of hydrogen coverage on temperature, pressure, and support ionicity. The three sites, in order of decreasing PtH bond strength, involve H in an atop, a threefold, and an ontop Pt site. The ontop site designates H bonded in an atop site surrounded by occupied threefold sites (hence referred to as ontop site). The model includes an emptying of the H atop sites into H threefold sites with increasing H pressure to reduce lateral interactions. Hydrogen chemisorption on an acidic Pt/H-USY and a basic Pt/NaY and TPD results on a acidic Pt/H-LTL and basic Pt/K-LTL are modeled using a Langmuir isotherm for each H site. The H/M data can be directly compared with Pt L2,3 XANES results on the same samples. A new analysis method (Delta XANES technique) using the difference in the absorption coefficient, Δμ=μ(H/Pt)−μ(Pt), allows an in situ spectroscopic determination of the type of H adsorption site and H coverage. The adsorption enthalpies (ΔH's) for the atop, threefold, and ontop sites are found to be highly dependent on the support ionicity, increasing for ionic (basic) supports consistent with previous results. The calculations using the three-site model confirm that the support-induced changes in the PtH bond strength produce dramatically different H coverages and dominant adsorption sites at catalytic reaction temperatures and pressures. Depending on uptake versus desorption of H, a hysteresis is found in the atop to threefold site rearrangement, believed to result from a requirement for collective rearrangement involving an entire domain or island of H on the surface.

The effect of hydration on the molecular structure of silica-supported vanadium oxide catalysts with loadings of 1–16 wt.% V has been systematically investigated by infrared, Raman, UV–vis and EXAFS spectroscopy. IR and Raman spectra recorded during hydration revealed the formation of V–OH groups, characterized by a band at 3660 cm � 1 . Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H2O. Further hydration resulted in the appearance of a V–O– V vibration band located around 700 cm � 1 pointing to the formation of di- or polymeric species. EXAFS analysis at 77 K indicated structural changes as the oxygen coordination changed from four to five. Moreover, a V ��� V contribution was detected for the hydrated species. The IR, Raman and UV–vis data suggested a pyramidal anchoring of the dehydrated VOx species, whereas, the EXAFS data pointed to the presence of single V–O–Si bonded VOx species. This difference is attributed to water condensation effects at 77 K during EXAFS acquisition, resulting in a partial re-hydroxylation of the dehydrated samples, as confirmed by complementary IR and Raman analysis. Combining the results of this study with data from our previous studies [D.E. Keller, F.M.F. de Groot, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 109 (2005) 10223; D.E. Keller, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 110 (2006) 14313] as well as literature led to a reaction scheme in which a monomeric VOx species anchored by three Si–O–V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VOx species anchored by one Si–O–V bond (umbrella-type structure) by partial hydration of the catalyst material. This results in the formation of both V–O–H and Si–O–H bonds. At higher water pressures, larger vanadium oxide clusters are formed due to full hydration of the catalyst surface and a de-attachment of the vanadium oxide from the support surface. The results of this study provide evidence, that an umbrella-type structure (i.e., Si– O–V O(OH)2) could be present under catalytic conditions where H2O is a reaction product (e.g., partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of alkanes). In other words, both the pyramidal ((Si–O)3–V O) and the umbrella (Si–O–V O(OH)2) model can exist at a support surface, their relative ratio depending on the hydration degree of the catalyst material. This study also illustrates that a corroborative characterization requires the use of multiple spectroscopic techniques applied at the same samples under almost identical measuring conditions.

The ring opening of methylcyclopentane (MCP) over well-characterized Pt/SiO{2}(EUROPT-1), Pt/@c-Al{2}O{3}, and Pt/K-LTL catalysts was studied as a function of hydrogen partial pressure and reduction temperature. The MCP ring opening selectivity did not change in the range of H{2}:MCP ratios studied (8-200). The turnover frequency (TOF) went through a maximum as the H{2}:MCP ratio increased. The maximum TOF of the Pt/@c-Al{2}O{3} after reduction at 450}o{C is about three times higher than the maximum specific activity of ttle Pt/K-LTL and Pt/SiO{2} catalysts. The H{2}:MCP partial pressure ratio at which maximum activity is obtained increases in the. series Pt/K-LTL < Pt/@c-Al{2}O{3} < Pt/SiO{2}. This sequence is rationalized using reported adsorption energies of H{2} and assuming a decreased adsorption energy of MCP on Pt/K-LTL. The data can be described with a reaction mechanism that includes the cleavage of a C-C bond as the rate-determining step. Kinetic analysis of the changes in specific reaction rate as a function of the H{2}:MCP ratio showed that the reaction proceeds through multiple adsorbed MCP species. The surface reaction rate is more than an order of magnitude higher for the Pt/@c-Al{2}O{3} catalyst than for the Pt/K-LTL catalyst, but decreases with increasing reduction temperature for both catalysts.

Platinum supported on H-LTL, K-LTL, and H-MAZ was reduced at temperatures from 573 to 873 K and the structure of the metal-support interface was determined by EXAFS. In all samples, the platinum was highly dispersed, with metal particle sizes from 5-1l atoms. After low temperature reduction (LTR, 573 K), the distance between the platinum atoms and the oxide atoms of the support (Pt-O distance) is 2.7 Å, which is significantly longer than the Pt-O distance of 2.2 Å observed after high temperature reduction (HTR, 873 K). At intermediate reduction temperatures both the long and the short Pt-O distances are observed. The shortening of the Pt-O distance, combined with the decrease in both the Debye-Waller factor and the inner potential shift, implies a stronger interaction between the platinum atoms and the oxide support. A structural model is proposed wherein the longer Pt-O distance results from the presence of hydrogen in the interfacial layer between the metal particle and the support. During high temperature reduction, the hydrogen is released from the interface, leaving platinum in direct contact with the support. The shortening of the Pt-O interfacial distance is accompanied by a decrease in the hydrogen chemisorption capacity, and may additionally be related to changes in the catalytic properties.

Catalysts supported on γ-Al2O3 were prepared from [Re2Pt(CO)12], and from Pt (NH3)4(NO3)2 and NH4ReO4. The former samples were characterized by infrared and X-ray photoelectron spectroscopies (XPS) and by temperature-programmed reduction (TPR); the latter were characterized by TPR. [Re2Pt(CO)12] was initially chemisorbed on the γ-Al2O3 surface. Upon treatment in H2 at about 150°C, the cluster fragmented and formed rhenium subcarbonyls, and at about 400°C the sample was decarbonylated. Adsorption of CO and of NO as probe molecules gave evidence of metallic Pt, but there was no evidence of adsorption on Re. The XPS data indicating the Re binding energies give evidence of the presence of low-valent cationic Re in the sample after the treatment at 400°C in H2. In contrast, when a mixture of samples of Re on γ-Al2O3 and Pt on γ-Al2O3 prepared from [H3Re3(CO)12] and [(CH3)2Pt(COD)], respectively, was treated under equivalent conditions, the Re was present in a high-valent cationic form (Re7+), and Pt was metallic. It is concluded that Pt facilitated the reduction of Re and that Pt was likely near the rhenium in the sample prepared from [Re2Pt(CO)12]. The TPR data are consistent with the foregoing results. The TPR data characterizing the samples prepared from the metal salts show that the degree of hydroxylation the γ-Al2O3 support significantly influenced the reduction of the Re and the Pt, but these data are not sufficient to determine the interactions between the two metals.

X-ray absorption spectra characterizing the metal-support interface in supported metal complexes and supported metal catalysts are summarized and evaluated. Single-metal-atom transition metal complexes on non-reducible metal oxide supports are bonded with metal-oxygen bonds with metal-oxygen distances of approximately 2.15 A; the bonding distance is only weakly sensitive to the oxidation state of the metal. Nearly this same metal-oxygen distance is characteristic of the metal-support interface in metal-oxide-supported metal clusters following high temperature reduction in H2 (HTR:T > 450 °C). The metals at the interface may be polarized sufficiently that they bond with the oxygen of the support much as the cations in mononuclear complexes bond with it. When the supported metals are treated in H2 at low temperatures (LTR:T < 350 °C) or are prepared under He with partially hydroxylated supports, a longer metal-support oxygen distance is observed, typically 2.5–2.7 A. This distance is suggested to characterize interactions between zero-valent metals and support oxygen. Changes in the performance of supported metal catalysts resulting from differences in the temperature of pretreatment in H2 are attributed to changes in the electronic properties and/or morphology of the metal clusters, which are suggested to be related to the concomitant changes in the structure of the metal-support interface.

The structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied by means of Mo K-edge EXAFS. In situ EXAFS measurements were carried out at liquid N2 temperature on freshly sulfided (at 673 K) catalysts. Due to the high signal-to-noise ratio of the experimental data, detailed information about the structural parameters of the active phase could be obtained. The catalysts probably consist of small MoS2-like particles with an auerage local ordered structure of5-6 Mo atoms (Mo/C) and 7-8 Mo atoms (Co-Mo/C). Comparison with literature data shows that the MoS2 dispersion in these carbon-supported catalysts is probably as good as in their alumina-supported counterparts. A detailed EXAFS data analysis shows the presence of carbon neighbors next to the Mo atoms in both catalysts with a Mc-C coordination distance of 2.1 * 0.1 A. This short distance could imply an intimate interaction between the active phase and the carbon support, which may explain the high active-phase dispersion. From the value of 2.1 8, it can be inferred that the Mo-C coordination is restricted to the exposed Mo atoms and that a support carbon atom takes the place of a sulfur vacancy. The high-quality EXAFS spectra show evidence for the existence of cobalt neighbors next to the Mo atoms in the Co-Mo/C catalyst, with a Mo-Co coordination distance of 2.8 f 0.1 A.

MgO-supported catalysts were prepared from [HRe(CO)5] and [H,Re3(CO),,] and characterized by extraction of surface organometallics, infrared and ultraviolet/visible spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS analysis and other data show that [H,Re,(CO),,] was initially deprotonated on the MgO surface, giving a surface-bound anion with a structure comparable to that of the salt [Ph,As] [H2Re,(C0)12] and having a Re-Mg distance of 2.39 8,. Heating of the supported cluster anion in helium to 225 OC led to oxidation and breakup of the cluster framework, giving a mononuclear complex formulated as [Re(C0)3(0MgJ3(]w here the braces refer to groups terminating the bulk oxide). The distances characterizing the bonding of the Re to the support are Re-O = 2.15 8, and Re-Mg = 2.80 8,. A Re-Re distance of 3.94 8, was observed, consistent with the decomposition of the cluster on the support to form ensembles consisting of three of the Re subcarbonyls, for which a structural model is presented. Treatment of this sample in hydrogen at 350 'C gave a Re species with oxygen neighbors at average distances of 1.94 and 2.45 8,. Heating of this sample to 500 OC in hydrogen led to reduction and conversion of most of the Re into metal crystallites. The several samples were tested as catalysts, as described in a companion paper.

The effect of the point of zero charge (PZC) of the support oxide (Al(2)O(3), Nb(2)O(5), SiO(2) and ZrO(2)) on the molecular structure of hydrated vanadium oxide species has been investigated with EXAFS spectroscopy for low-loaded vanadium oxide catalysts. It was found that the degree of clustering (i.e., the V[dot dot dot]V coordination number) and the V...V distance increase with decreasing PZC of the support oxide; i.e., Al(2)O(3) (8.7) < ZrO(2) (7) < Nb(2)O(5) (3.3) < SiO(2) (2). Upon hydration the silica-supported vanadium oxide exhibited a clear alteration in the position of the oxygen atoms surrounding the central vanadium atom and the number of oxygen atoms around vanadium increased to five. In contrast, only minor changes in the molecular structure were detected for the alumina-, niobia- and zirconia-supported vanadium oxide catalysts. Based on a detailed analysis of the EXAFS data a semi-quantitative distribution of vanadium oxide species present on the surface of the different support oxides can be obtained, which is in good agreement with earlier characterization studies primarily making use of Raman spectroscopy.

The electronic and geometric effects induced by hydrogen chemisorption on small platinum particles supported on high surface-area saponite clay and zeolite LTL were studied by near edge X-ray absorption fine structure (XAFS) spectroscopy. A new subtraction procedure was developed to separate the electronic from geometric effects. A significant Pt-H extended X-ray absorption fine structure (EXAFS) scattering (structural effect) was found for energy values between 0 and 20 eV. In addition, the Pt-H antibonding state (electronic effect) was found to produce a shape-resonance and was isolated from the near edge of the L3 X-ray absorption spectrum. Moreover, for Pt/LTL the shape and energy of the shape-resonance was found to strongly depend on the acidity/alkalinity of the support material, implying a direct influence of the support on the electronic properties of the platinum particles. The results of the study of the resonance state and the Pt-H EXAFS scattering demonstrate the potential of these techniques for characterization of hydrogen chemisorption, metal-promoter, and metal-support effects in catalysis research.

The relation between the catalytic activity, electronic properties, morphology, and structure of the metal-support interface was studied for Pt/@c-Al{2}O{3}after low (300}o{C, LTR) and high temperature reduction (450}o{C, HTR). EXAFS revealed that after LTR the platinum particles were three-dimensional, contained 11 Pt atoms on average, and were at a distance of 2.7 A from the support oxygen. During HTR, the morphology of the platinum particles changed from three-dimensional to rafts with a structure similar to Pt(100), as indicated by a decrease in the Pt-Pt coordination number and the absence of the third coordination shell in the EXAFS spectrum. After HTR the Pt-O distance was shortened to 2.2 A due to the desorption of hydrogen from the metal-support interface. The shortening of the Pt-O distance upon HTR treatment agrees with previous studies on zeolite supported platinum. However, zeolite supported platinum particles retained their three-dimensional structure upon HTR. The changes in the structure of the catalyst affected the catalytic, chemisorption, and electronic properties. After HTR the selectivity for hydrogenolysis of both neopentane and methylcyclopentane to methane decreased. At the same time the specific activity for neopentane isomerization and methylcyclopentane ring opening increased. The hydrogen chemisorption capacity after HTR was lower than after LTR. HTR shifted the asymmetric linear CO infrared absorption from 2063 to 2066 cm}-{}1{. Comparison of the Pt L{I}{I}{I}and L{I}{I}X-ray absorption edge intensities after LTR and HTR revealed that the number of holes in thed-band of the platinum atoms increased by 9.5% during HTR. It was suggested that the decrease in hydrogen chemisorption capacity, hydrogenolysis selectivity, and number of holes in thed-band are related to the change in the structure of the metal-support interface. The increase in specific activity for isomerization of neopentane and the shift in the CO infrared absorption band with a raise in reduction temperature agreed with reported results for single crystals and was attributed to the higher concentration of atoms with Pt(100) symmetry in the catalyst after HTR.

The structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied by means of Mo K-edge EXAFS. In situ EXAFS measurements were carried out at liquid N2 temperature on freshly sulfided (at 673 K) catalysts. Due to the high signal-to-noise ratio of the experimental data, detailed information about the structural parameters of the active phase could be obtained. The catalysts probably consist of small MoS2-like particles with an auerage local ordered structure of5-6 Mo atoms (Mo/C) and 7-8 Mo atoms (Co-Mo/C). Comparison with literature data shows that the MoS2 dispersion in these carbon-supported catalysts is probably as good as in their alumina-supported counterparts. A detailed EXAFS data analysis shows the presence of carbon neighbors next to the Mo atoms in both catalysts with a Mc-C coordination distance of 2.1 * 0.1 A. This short distance could imply an intimate interaction between the active phase and the carbon support, which may explain the high active-phase dispersion. From the value of 2.1 8, it can be inferred that the Mo-C coordination is restricted to the exposed Mo atoms and that a support carbon atom takes the place of a sulfur vacancy. The high-quality EXAFS spectra show evidence for the existence of cobalt neighbors next to the Mo atoms in the Co-Mo/C catalyst, with a Mo-Co coordination distance of 2.8 f 0.1 A.

Platinum supported on BaKL zeolite was characterized by Transmission Electron Microscopy (TEM), hydrogen chemisorption, and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The results of all three techniques indicate the presence of highly dispersed platinum in the zeolite pores. There is no evidence of platinum outside the zeolite pores. The EXAFS data determine a Pt-Pt coordination number of 3.7, suggesting that the average platinum cluster in the zeolite consists of 5 or 6 atoms, consistent with the TEM and chemisorption data. The EXAFS data also provide evidence of the platinum-zeolite interface, indicated by Pt-O contributions at 2.14 and 2.70 A, and a Pt-Ba contribution at 3.8 A. The Pt/BaKL zeolite is one of the most highly dispersed supported platinum samples and one of the most structurally uniform supported metal catalysts.

The influence of the alkalinity of the support on the chemisorption of oxygen on Pt particles has been studied using Pt L3 and L2 X-ray absorption near edge data from Pt/Al2O3 doped with F or Rb. The O binding site is determined by using a procedure similar to that previously utilised for H chemisorption. The Pt L3 X-ray absorption edge data of the sample in vacuum is subtracted from the data obtained after chemisorption of oxygen. The signature of this difference spectrum is compared with that resulting from a similar procedure applied to theoretical data, the latter obtained with the help of full multiple scattering ab initio calculations on small model Pt4 clusters using the FEFF8 code. After O2 exposure of the F doped alumina, oxygen is found in hcp/sub-surface binding sites, whereas in the Rb doped case oxygen is present in the bridged/fcc sites. These results imply that the properties of the support have a strong effect on the oxygen chemisorption properties of small Pt particles. Since O is known to bond in a bridged position near step edges or corners at low coverage, followed by 3-fold fcc sites on flat faces or terraces at higher coverage, and then finally at hcp sites and/or in the subsurface region, these data indicate that the Pt–O bond strength and hence coverage increases with support acidity. This is in contrast to that found previously for the Pt–H bond, which decreases in strength with support acidity. Consistent with these results, it is concluded from density functional theory (DFT) calculations on model Pt4 clusters that the isolated Pt–O bond strength is indeed significantly altered by the support and in the opposite direction to that reported previously for Pt–H bonding. The calculations also show that the Pt–O bond ionic character is dependent on the properties of the support. The increased Pt–O ionic character in a basic support gives rise to a stronger O–O lateral repulsion. This lateral interaction, along with the decreased Pt–O bond strength, decreases the O coverage for basic supports. A survey of the recent literature reporting DFT and experimental results on H and O adsorption on Pt(111) and Pt clusters is given.

Analysis of in situ EXAFS measurements on a 2.4 wt % Rh/A120, catalyst, reduced at 473 K after calcination at 623 K, shows the presence of two different rhodium-oxygen bonds (viz. 2.05 and 2.68 A). The oxygen neighbors of rhodium at a distance of 2.05 A disappear after reduction at 673 K. The coordination distance 2.05 A is the same as found for Rh203 and indicates that this rhodium-oxygen bond is present due to incomplete reduction of the catalyst at 473 K. Rhodium-oxygen bonds with a coordination distance of 2.68 A are present at both reduction temperatures. These bonds arise from a coordination of the interfacial rhodium atoms with the oxygen ions of the support. Assuming a half-spherical shape for the rhodium metal crystallites, numerical values for the average interfacial rhodium-oxygen coordination number (N = 2.5) and the crystallite diameter (d = 11 A) are derived from the experimental EXAFS parameters.

Five different signals have been analyzed in ESR spectra obtained from sulfided molybdenum- or tungsten-containing catalyst samples. Signal I (oxo-Mo5+, g = 1.933 for Mo/γ-Al2O3; and oxo-W5+, g = 1.78 for W/γ-Al2O3), and possibly signal III arise as a result of interactions with the support. Signal II (g = 1.985 for Mo/SiO2, and g = 1.91 for W/γ-Al2O3) and signal IV (g = 1.995 for W/γ-Al2O3, and g = 2.01 for WS2 bulk) have been detected both on supported and on unsupported sulfided samples. These two signals show a complementary behavior upon evacuation and H2S adsorption and are therefore ascribed to paramagnetic surface species in the MoS2 and WS2 phases. Some surface configurations are proposed to describe the origin of these paramagnetic surface species. The origin of signal V which has been detected in supported and unsupported samples is still unknown.

Hydrogen-to-metal ( H M ) ratios exceeding unity for Pt and Rh and exceeding 2 for Ir were measured for highly dispersed Pt, Rh, and Ir catalysts supported on Al 2 O 3 and SiO 2 . Since the coordination of hydrogen to metal atoms is unknown for such highly dispersed catalysts, the metal surface area of these catalysts cannot be calculated from the hydrogen chemisorption values. Therefore EXAFS (extended X-ray absorption fine structure) measurements were performed to determine the metal particle size and thereby to calibrate hydrogen chemisorption results. The H M ratio determined by hydrogen chemisorption is a linear function of the average metal coordination number determined by EXAFS. This linear relationship is independent of support but varies with the metal with the H M ratio increasing in the order Pt H M values are discussed. Spillover and subsurface hydrogen are excluded as explanations and only multiple adsorption of hydrogen on metal surface atoms is shown to be capable of explaining all experimental observations. The H M surface stoichiometry differs among Pt, Rh, and Ir in the order H Pt H Rh H Ir , analogous to the order of stability of corresponding metal polyhydride complexes and of theoretical expectation.

Analysis of in situ EXAFS measurements on a 2.4 wt % Rh/A120, catalyst, reduced at 473 K after calcination at 623 K, shows the presence of two different rhodium-oxygen bonds (viz. 2.05 and 2.68 A). The oxygen neighbors of rhodium at a distance of 2.05 A disappear after reduction at 673 K. The coordination distance 2.05 A is the same as found for Rh203 and indicates that this rhodium-oxygen bond is present due to incomplete reduction of the catalyst at 473 K. Rhodium-oxygen bonds with a coordination distance of 2.68 A are present at both reduction temperatures. These bonds arise from a coordination of the interfacial rhodium atoms with the oxygen ions of the support. Assuming a half-spherical shape for the rhodium metal crystallites, numerical values for the average interfacial rhodium-oxygen coordination number (N = 2.5) and the crystallite diameter (d = 11 A) are derived from the experimental EXAFS parameters.

La technique EXAFS peut etre utilisee pour determiner la taille des particules d'un catalyseur metallique meme si celui-ci n'est pas completement reduit. Mais il est preferable de connaitre le degre de reduction avant d'employer cette technique

Controversies exist in literature about the structure and oxidation state of highly dispersed Rh/Al2O3 catalysts. Some authors conclude on the basis of 00 infrared data that rhodium monatomatically dispersed as RH1+ ions (1–3). However, electron microscopy studies shows the presence of small metallic rhodium particles (4–5).

Extended X-ray absorption spectroscopy (EXAFS) has been applied to study the Rh K-edge of two ultradisperse Rh/Al{2}O{3} catalysts containing 0.47 and 1.04 wt.% rhodium respectively. The structural properties of the Rh crystallites were determined after reduction with H{2}, evacuation at elevated temperatures, CO admission at room temperature, CO desorption, CO admission at 523 K (at which temperature the Boudouard reaction takes place) and after hydrogenation of CO at 523 K.During reduction of the catalyst, metallic rhodium crystallites are formed which are attached to the support via Rh}0{-O}2{}-{ bonds, implying zero-valent rhodium. The Rh-Rh nearest-neighbour distance in the particles is contracted to 2.63 @9 in vacuum and relaxes to 2.68 @9 (similar to the bulk value) upon hydrogen chemisorption. Adsorption of CO at room temperature onto the reduced catalysts results in a partial disruption of the small Rh crystallites, ultimately leading to rhodium geminal dicarbonyl species. A subsequent desorption of CO leads to rearrangement of the metal clusters. The oxidation state of rhodium alters from 0 to 1+ upon CO chemisorption, and becomes 0 again after CO desorption. After CO admission at 523 K, the rhodium particles break up completely, probably because of intercalation of carbon and/or oxygen atoms. The highly dispersed Rh/Al{2}O{3} catalysts change to poorly dispersed systems after reaction between CO and H{2} at 523 K, due to sintering of the metallic crystallites.

The X-ray absorption spectrum of the Rh K edge of a highly dispersed reduced 0.57 wt % Rh/ y-A1203 catalyst shows EXAFS oscillations due to rhodium-rhodium nearest neighbors, which proves the existence of rhodium metal crystallites. Adsorption of CO at room temperature on the reduced catalyst significantly decreases the amplitude of these EXAFS oscillations. This implies that CO adsorption on very small rhodium crystallites leads to a disruption of a significant number of the metal-metal bonds.

The X-ray absorption spectrum of the Rh K-edge of a highly dispersed reduced Rh/Al2O3 catalyst shows EXAFS oscillations due to Rhodium-Rhodium nearest neighbors. Adsorption of CO at room temperature on the reduced catalyst significantly decreases the amplitude of the EXAFS oscillations. This implies that CO adsorption on very small metal crystallites leads to a disruption of a significant number of the Rhodium-Rhodium metallic bonds.

EXAFS measurements have been carried out on an 0.8 wt% Ir/¿-Al2O3 catalyst after the following treatments : 1) reduction at 773 K, 2) evacuation at 623 K, 3) O2 adsorption at 77 K, 4) subsequent heating to 100 K. The tentative results show that chemisorbed hydrogen influences the structure of the metal particles and their interaction with the support. Oxygen adsorption at 77 K (most probably physisorption) seems to produce the same kind of metal-oxygen bonds as detected for the metal-support interface, when the metal crystallites are under vacuum. Subsequent heating to 100 K resulted in the onset of oxygen chemisorption.