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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 influence of the support oxide on the molecular structure of a VO(4) cluster and its interfacial geometry has been determined for SiO(2), Nb(2)O(5), and ZrO(2) as supports. Raman, IR, UV-vis-NIR diffuse reflectance, electron spin resonance, and extended X-ray absorption fine structure (EXAFS) spectroscopies were used to characterize the supported vanadium oxide clusters after dehydration. It has been found that for all supports under investigation the vanadium ion is tetrahedral coordinated and consists of one V=O and three V-O bonds. For a VO(4)/SiO(2) catalyst it has been established that only one O neighbor is shared with the SiO(2) support via a V-O(b)-Si(support) bond with an angle of approximately 101 degrees (+/-0.5 degrees ) and a V...Si distance of 2.61 A. The absence of a second vanadium atom in the vicinity of the vanadium oxide cluster further subverts the classical assignment of the 920 cm(-1) Raman band to a V-O-V related vibration. The EXAFS results combined with structural modeling using Cerius(2) software lead to structural constraints, which imply a similar V-O(b)-M(support) interaction for Nb(2)O(5) and ZrO(2) as well. The V-O(b) and the V...M(support) distances depend on the geometry of each support surface. The results show that the classical model with three V-O(b)-M(support) bonds could not be experimentally observed with EXAFS under the applied measuring conditions. Additional experiments with IR and Raman spectroscopy under experimental conditions mimicking those of the EXAFS measurements reveal the presence of V-OH groups, giving further support for the presence of a O=V(OH)(2)-O(b)-M moiety at the support surface.

Zeolite Y-supported Pt particles were studied by high-resolution transmission electron microscopy (HRTEM), extended X-ray absorption fine structure (EXAFS), and H2 chemisorption experiments. The ionicity (i.e., electron richness of the zeolite oxygen atoms) was altered by using different cations (NaY, MgY, and LaY) and by steaming and introduction of protons (H-USY). Highly dispersed Pt particles were introduced inside the zeolite pores using a very careful synthesis procedure. Computer analysis of HRTEM images exhibited similar Pt particle size distributions with calculated average Pt particle sizes ranging from 0.98 to 1.26 nm for the various zeolite Y-supported Pt particles. The EXAFS results revealed that the first-shell Pt–Pt coordination number was around 6.5, in good agreement with the observed HRTEM results. H2 chemisorption experiments found a much higher H/Pt value for Pt/NaY (1.63) than for Pt/H-USY(0.88), implying no relationship with the Pt dispersion as determined by HRTEM. Using the values for the dispersion (Pts/Pt) as calculated from the HRTEM results, one can obtain a value for the number of chemisorbed hydrogen atoms per surface platinum atom (H/Pts ratio). This value demonstrates a clearly decreasing trend with decreasing electron richness of the support oxygen atom: 1.96 (Pt/NaY) > 1.74 (Pt/MgY) > 1.51 (Pt/LaY) > 1.11 (Pt/H-USY). These results indicate that the hydrogen coverage on supported Pt particles depends strongly on the support ionicity. This has significant consequences for the catalytic behavior of hydrogenolysis and hydrogenation reactions catalyzed by Pt. These results also indicate that one should be cautious in interpreting H2 chemisorption results only in terms of Pt dispersion. This approach is valid only when comparing Pt particles dispersed on the same support. Serious deviations in the real Pt dispersions occur if Pt is supported on oxides with different ionicities.

Carbon nanofiber-supported platinum catalysts were studied to establish the influence of the support surface composition on their performance in the liquid-phase hydrogenation of cinnamaldehyde. By acid–base titration, XPS, and TGA-MS, the number of oxygen-containing surface groups was determined in oxidized carbon nanofiber samples and in platinum catalysts pretreated at 473–973 K. TEM, EXAFS, and H2-chemisorption studies showed the presence of a narrow and stable platinum particle-size distribution (1–2 nm). The overall catalytic activity increased by a factor of 25 after treatment at 973 K, which is tentatively related to the decreasing amount of oxygen of the carbon fibers. With XPS and H 2-chemisorption experiments carried out at 308 K no clear evidence was found for a change in the electronic structure of the platinum particles induced by the oxygen-containing groups present on/in the surface of the carbon nanofibers. A linear decrease of the hydrogenation activity with an increase of the number of acidic groups on the carbon nanofiber support was found. Therefore, it is suggested that hydrogenation is assisted by adsorption of cinnamaldehyde on the carbon support after removal of the oxygen-containing surface groups.

Monomolecular cracking of alkanes was investigated over zeolites of different structure types and after different treatments. Large deviations in apparent rates of reaction and apparent kinetic parameters were detected for zeolites of different structure types; smaller changes were observed within a structure type. The Si/Al ratio and different crystal sizes had no effect on turnover frequency; however, steaming of mordenite and ZSM-5 enhances the rate of reaction by a factor of 2–5 due to the formation of Lewis acid sites with enhanced heat of adsorption, although the number of Bronsted sites decreased. The apparent activation energies and preexponential factors showed a linear compensation relation in a Constable plot. Using the Langmuir–Hinshelwood kinetics formalism, the apparent kinetic parameters include enthalpy and entropy of adsorption and surface reaction terms. If the enthalpy and entropy of adsorption are linearly related, which is experimentally observed for unsteamed and steamed mordenite, then the apparent compensation effect implies that the observed differences in rate are dominated by differences in adsorption of the reactant in the pores of the zeolites, rather than by differences in acid strength. Zeolites with smaller pores show enhanced adsorption, and consequently the rate of reaction increases in the order H-Y

The reduction behaviour of Ce x Zr1−x O2 and Pt or Pd supported on Ce x Zr1−x O2 was investigated by means of XANES measured at the Ce LIII edge and magnetic susceptibility measurements. Both techniques agree quite closely for the materials studied. The presence of a noble metal greatly improves the reduction of the Ce x Zr1−x O2 support at low temperatures. The reoxidation that occurred after evacuation of the reduced solids and heating to 500°C suggests that it is linked to a reversible surface reduction process of the sample.

The structure of tin promoted graphite supported platinum catalysts has been studied with extended X-ray absorption fine structure spectroscopy (EXAFS). A newly developed EXAFS cell allows on-line characterization avoiding contact to ambient or drying. Hereto catalyst samples are transferred from a slurry reactor to the EXAFS cell forming a ‘‘bed’’ of catalyst particles in the EXAFS cell. The cell design was based on considerations concerning possible mass transport limitations while performing reactions in the liquid phase. The structures of the tin promoted platinum catalysts were investigated directly after preparation, drying, treatments with hydrogen (363 K) and oxygen (RT) in aqueous phase and a hydrogen gas treatment at 573 K at both the Pt LIII and the Sn K-edge. After preparation, under aqueous hydrogen, reduced platinum can be detected with three coordinations: Pt‐Pt, Pt‐C and Pt‐Sn. Tin appears to be partly oxidic showing a Sn‐O and a Sn‐Pt coordination. A treatment with aqueous oxygen or exposure to ambient leads to oxidized platinum and tin. At the Pt LIII-edge only a Pt‐Pt and Pt‐O coordination for platinum are detected. At the Sn K-edge tin has only a Sn‐O coordination. An aqueous treatment with hydrogen at 363 K reduces platinum showing, however, different coordination numbers for the Pt‐Pt and Pt‐Sn coordination. Tin only shows a Sn‐O coordination. A treatment with hydrogen at 573 K reduces both the platinum and the tin. Platinum shows a Pt‐Pt, Pt‐C and Pt‐Sn coordination. Tin shows a Sn‐Pt and Sn‐O coordination indicating tin deposition on the platinum, tin being bonded via oxygen to the graphite support. Reductive treatments in the aqueous phase appear to reduce platinum and only the tin deposited on the platinum. The effects of drying and consecutive reductive treatments could only be studied since the developed EXAFS cell allowed catalyst preparation and treatments avoiding contact to ambient. # 1998 Elsevier Science B.V.

With a new set‐up for in situ EXAFS spectroscopy the state of a carbon‐supported platinum catalyst during aqueous alcohol oxidation has been observed. The catalyst deactivation during platinum‐catalysed cyclohexanol oxidation is caused by platinum surface oxide formation. The detected Pt–O co‐ordination at 2.10 A during exposure to nitrogen‐saturated cyclohexanol solution is different from what is observed for the pure oxidised platinum surface (2.06 A).

The structure and electronic properties of platinum in WH-LTL after reduction at 300 'C and heating in helium to 500 or 690 'C were determined using X-ray absorption and infrared spectroscopy. After reduction at 300 'C, the platinum particles were metallic, consisted of 4 or 5 atoms, and were located at 2.64 A from the oxygen atoms in the zeolite framework. The particles remained metallic but increased in size to e13 atoms during hydrogen desorption by heating in a helium flow up to 690 'C. Simultaneously, the distance between metal particle and oxygen atoms of the zeolite framework was shortened to 2.05 A. After reduction at 300 'C and in the presence of chemisorbed hydrogen, the platinum atoms in the PT/H-LTL catalyst had more holes in the d-band than bulk platinum. Hydrogen desorption decreased the number of holes of the platinum atoms in the WH-LTL catalyst to levels lower than bulk metal values. The linear CO band shifted from 2071 to 2084 cm-' upon hydrogen desorption, due to the increased particle size andor the change in the structure of the metal-support interface. The apparent contradiction between the shift to higher wavenumbers of the linear CO band and the decreased number of holes in the d-band was attributed to the interaction of CO with filled d-orbitals and the effect of chemisorbed hydrogen on the distribution of the local density of states.

The structure of the metal particles and the metal-support interface of a Pt@c-Al{2}O{3} catalyst was determined by EXAFS after low temperature reduction (LTR: 300}o{ C) and high temperature reduction (HTR: 450}o{ C). The clusters have excellent thermal stability as the particle size remains 12 atoms per cluster upon increasing the reduction temperature from 300}o{ C to 450}o{ C. However, the structure of the metal-support interface is a strong function of the reduction temperature. After LTR and in the presence of chemisorbed hydrogen the metal particles are at a distance of 2.68 a from the support oxygen ions. This long Pt-O distance is due to the presence of hydrogen between the platinum atoms and the support. Increasing the reduction temperature results in the removal of hydrogen from the metal-support interface, simultaneously placing the metal particles in direct contact with the support oxygen atoms at a distance of 2.28 a. The release of interfacial hydrogen during high temperature reduction is changing the metal-support interaction, which in turn changes the electron density distribution in the metal particles. The electronic and thereby the catalytic properties of the platinum metal particles are therefore a function of the reduction temperature. The results show that the influence of the reduction temperature on the structure of the metal-support interface of platinum particles on a amorphous support is similar to platinum particles which reside in cavities of zeolites.

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.

In this study the local structure of the Mo and Co promoter atoms in CoMoS type I and II on alumina, and CoMoS type II on various supports is compared using in situ XAFS spectroscopy as the main technique with XPS and MES in a supporting role. In the CoMoS phase the Co atoms are positioned at the edges of the MoS2 particles, in the same plane as the Mo atoms. The CoMoS phase in all catalysts shows a well defined Co-Mo and a Mo-Co coordination. On alumina the CoMoS type II phase is present as a multilayer structure, whereas type I exists as a single-slab, i.e., monolayer, structure. The silica-supported CoMoS type II closely resembles its alumina-supported counterpart. However, the Mo-S coordination number, the structural ordering and degree of stacking of the carbon-supported type II CoMoS phase is much more similar to the type I CoMoS phase supported on alumina. This contradicts the common opinion that the CoMoS type II phase is fully sulfided and not chemically bonded to the support. Two different types of Co-sites can be distinguished in the suite of catalysts studied in this paper. A Co-site with an approximately fivefold Co-S coordination and possibly a single Co-Mo coordination is predominant in the least active alumina-supported CoMoS type I and CoMoS type II (with Co/Mo = 0.39 at/at) samples. The other Co-site has a sixfold Co-S coordination with possibly a twofold Co-Mo coordination and has the highest activity for HDS. The latter site constitutes a large part of the Co-sites in silica-supported CoMoS phase II and alumina-supported CoMoS type II (with Co/Mo = 0.32) and is exclusively present in the carbon-supported CoMoS phase II.

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.

Hydrogen temperature-programmed desorption (TPD) of supported platinum catalysts, Pt/KLTL, Pt/H-LTL, Pt/K-MAZ, Pt/H-MAZ, Pt/γ-Al 2 O 3 , and Pt/SiO 2 , was performed after hydrogen reduction at 300, 450, or 650°C. For all catalysts, reversible desorption of chemisorbed hydrogen occurred at approximately 175°C. In addition to chemisarbed H 2 , at least three peaks corresponding to higher temperature, irreversibly desorbed H 2 , were observed and assigned to spillover hydrogen. The quantity of spillover hydrogen is related to the number of support hydroxyl groups and the maximum reduction temperature of the catalyst. It is suggested that the irreversibility of the spillover hydrogen results from dehydroxylation of the support at high temperature. The structure of Pt/K-LTL and Pt/H-LTL catalysts has been examined by EXAFS following reduction at 300, 450, and 600°C. After low-temperature reduction, it is proposed that a layer of spillover hydrogen is present between the platinum particle and the support. The distance between the platinum and the support (oxide) surface is ca. 2.6-2.7 A. During high-temperature reduction, or during TPD, the interfacial hydrogen is irreversibly lost, and the platinum-oxygen distance decreases to 2.2 A. The catalysts′ turnover frequency (TOF) for propane hydrogenolysis was dependent on both the reduction temperature and the type of support. Platinum on acidic supports and reduced at the lowest temperature displayed the highest TOF. Qualitatively, the catalysts′ hydrogenolysis activity increased with increasing amounts of spillover hydrogen.

The structure of the cobalt present in carbon-supported Co and Co-Mo sulfide catalysts was studied by means of X-ray absorption spectroscopy at the Co K-edge and by X-ray photoelectron spectroscopy (XPS). Thiophene hydrodesulfurization activities were used to measure the catalytic properties of these catalysts. By comparison of the EXAFS and XANES spectra of the catalysts with those of c09sS and Cos2 model compounds, it was concluded that all Co atoms in a catalyst prepared with nitrilotriacetic acid as complexing agent were in the "Co-Mo-S" state, while the Co atoms in a conventionally prepared catalyst were partly present in a CO$8-like structure and partly in a "Co-Mo-S" structure. The Co atoms in the To-Mc-S" state have a distorted 5- to 6-fold sulfur coordination, and on the average, every Co atom is in contact with two Mo atoms at a distance of 2.80 A. On the basis of these data, the most likely position for the Co atoms is in front of the square sulfur faces of the MoS6 trigonal prisms along the edges of the MoS, crystallites with two additional sulfur atoms or H2S molecules attached. The Co atoms in the sulfided Co/C catalyst have Co-S and Co-Co coordinations as in c09sg, although the sulfur coordination number is higher.

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 sulfur poisoning of a Pt/BaK-LTL catalyst has been studied with X-ray absorption spectroscopy and hydrogen chemisorption. The fresh catalyst contained highly dispersed platinum inside the zeolite pores. EXAFS analysis determined a Pt-Pt coordination number of 3.7, suggesting an average platinum cluster size of 5 or 6 atoms, consistent with the TEM and chemisorption data (H/Pt = 1.4). The catalyst was poisoned with H2S until the dehydrocyclization activity of n-hexane decreased to 30% of fresh activity. The first-shell PtPt coordination number increased to 5.5, indicating a growth of the average platinum cluster size to 13 atoms. Hydrogen chemisorption measurements of the poisoned catalyst show a decrease in the H/Pt value to 1.0. The EXAFS data also provide evidence for the presence of sulfur adsorbed on the surface of the platinum particles with a PtS bond distance of 2.27 A. The high sensitivity of the Pt/LTL catalyst to poisoning by very low levels of sulfur is attributed to the loss of active platinum surface by adsorption of sulfur and the growth of the platinum clusters. Much of the available platinum surface was found to be capable of chemisorbing hydrogen, but with no activity for dehydrocyclization. Growth of the platinum particle was sufficient to block the pore. In the sulfur-poisoned catalyst, only the sulfur-free platinum atoms exposed through the pore windows remain active. The evidence suggests the location of the sulfur was at or near the metal-zeolite interface. Since both high activity and selectivity require extremely small platinum particles, regeneration of sulfur-poisoned catalysts will require removal of the adsorpted sulfur and restoration of the original particle size.

The room temperature Cu K-edge EXAFS (extended X-ray absorption fine structure) spectrum of reduced and oxidized amicyanin, the blue copper protein from Thiobacillus versutus, was measured at low and high pH. The data interpretation was partly based on independent NMR evidence for the occurrence of a ligand histidine protonation at low pH (pKa = 6.9) in the reduced protein. In the oxidized protein two nitrogen-donors (from two histidines; Cu-N distances 1.95-2.01 A and 1.86-1.89 A) and a sulfur-donor (from a cysteine; Cu-S distance 2.11-2.13 A) were identified and the coordination appears independent of pH. Upon reduction at high pH the Cu-S bond and one of the Cu-N bonds lengthen slightly (from 2.11 to 2.19 A and from 2.01 to 2.18 A, respectively). Upon lowering of the pH one of the N-donors of the Cu in reduced amicyanin disappears from the Cu EXAFS and a second S-donor (from a methionine) becomes visible at 2.41 A from the Cu. The Debye-Waller factors are compatible with a Cu-N vibrational stretch frequency in the range of 150-250 cm-1 and one greater than 285 cm-1, and a Cu-S vibrational stretch frequency of about 150 cm-1 (Cu-Smet; reduced amicyanin at low pH) and one in the range of 230-800 cm-1 (Cu-Scys).

Rhenium surface species, derived by treating [H,Re,(CO),,] adsorbed on y-A1203 in hydrogen at 400 'C, formed extremely small surface grouping of rhenium atoms having an average Re-Re coordination number of 2, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Results of X-ray photoelectron spectroscopy and threshold resonance experiments show that the rhenium was cationic, with oxidation states of about +4 to +6. Infrared spectroscopy was used to follow the decomposition of surface species derived from [H3Re3(C0)12a]d sorbed on y-A1203,t he data indicating that Re subcarbonyls such as [Re(CO)3(O-AI)2(HO-Al)] were formed as intermediates. On the basis of the Re-O and Re-Re coordination parameters determined by EXAFS spectroscopy, a structural model of cationic trirhenium rafts on the y-A1203 surface is suggested. This structure is characterized by a Re-Re distance of 2.67 A, which is greater than the distances characterizing the quadruple and triple Re bonds (2.3 A) in Re complexes but shorter than the distance in bulk metallic Re (2.74 A). The Re-Re distance suggests an intermediate bond order, roughly 1.5. The trirhenium rafts are suggested to be tethered to surface oxygens with a Re-0 distance of 2.05 A indicated by the EXAFS data. A longer Re-O distance, 2.58 A, is also indicated by the EXAFS data, and this is suggested to be indicative of interactions of the rhenium with surface hydroxyl groups. Another y-Al2O3-supported Re sample was prepared by a conventional method with aqueous NH4ReOr as the precursor, with treatment in hydrogen at 450 OC. The resulting surface species were highly nonuniform, consisting of surface rhenium oxide species and metallic rhenium particles.

The structure of the cobalt present in carbon-supported Co and Co-Mo sulfide catalysts was studied by means of X-ray absorption spectroscopy at the Co K-edge and by X-ray photoelectron spectroscopy (XPS). Thiophene hydrodesulfurization activities were used to measure the catalytic properties of these catalysts. By comparison of the EXAFS and XANES spectra of the catalysts with those of c09sS and Cos2 model compounds, it was concluded that all Co atoms in a catalyst prepared with nitrilotriacetic acid as complexing agent were in the "Co-Mo-S" state, while the Co atoms in a conventionally prepared catalyst were partly present in a CO$8-like structure and partly in a "Co-Mo-S" structure. The Co atoms in the To-Mc-S" state have a distorted 5- to 6-fold sulfur coordination, and on the average, every Co atom is in contact with two Mo atoms at a distance of 2.80 A. On the basis of these data, the most likely position for the Co atoms is in front of the square sulfur faces of the MoS6 trigonal prisms along the edges of the MoS, crystallites with two additional sulfur atoms or H2S molecules attached. The Co atoms in the sulfided Co/C catalyst have Co-S and Co-Co coordinations as in c09sg, although the sulfur coordination number is higher.

[Ir4(C0)12o] n the surface of MgO reacts to give [HIr4(CO)ll]-,w hich upon treatment with co at 200 OC gives [Ir6(C0)15]2- on the surface in high yield. Treatment of the latter with a solution of [PPN][CI] leads to extraction of [Ir6(C0)15]2b-y cation metathesis. The surface-bound [Ir6(CO)15]2w- as characterized by extended X-ray absorption fine structure (EXAFS) sk :e icnt rcolsocsoep ayg,r weeimthe tnhte w diatht at hined piucabtliinshge adn c rayvsetraalgloeg Irra-plrh icc odoartad ifnoart itohnis naunmiobne rin o tfh 4e. 1so alindd s taant ea.v Terhaeg er eIarc-Itrio ndsi sotfa nthcee iorfi d2iu.7m7 carbonyls on the basic MgO surface closely parallel those of iridium carbonyl anions in basic solutions. Treatment of the MgO-supported [h6(CO)IS]2- in He followed by H2 at 300 OC gives decarbonylated surface species shown by EXAFS spectroscopy to have iridium cluster frames substantially unchanged from those of the carbonyl anion precursor. The decarbonylated iridium clusters are among the most nearly uniform supported metals.

The sections in this article are Introduction Physical Principles of X-Ray Adsorption and Electron Scattering Absorption of X-Rays Scattering of the Photoelectron Isolating the Oscillatory Part (χAX + χEX) from the Absorption Data Pre-Edge Subtraction and Determination of Eedge Post-Edge Background Removal and Normalization EXAFS The EXAFS Equation EXAFS Data Analysis Fourier Transformation Fourier Filtering Phase Shifts and Backscattering Amplitudes Derived from EXAFS Data Obtained from Reference Compounds Obtaining R, N and Δσ2, k-Space and R-Space Fitting The Use of Theoretical References Difference File Technique Structural Analysis with EXAFS of a Heterogeneous Single-Site Cr/SiO2 Ethylene Trimerization Catalyst. An Illustrative Example Introduction Raw XAS Data of New Cr/SiO2 Catalysts and CrCl3–TAC Reference Compound Fabrication and Calibration of Ab Initio Phase Shifts and Backscattering Amplitudes EXAFS Data Analysis Conclusions: EXAFS Data Analysis Atomic XAFS (AXAFS) Introduction Physical Principles of AXAFS The Field and Inductive Interactions Separation of AXAFS from EXAFS Data The Interatomic Potential Model for Metal–Support Interaction ΔXANES Technique Introduction Calculations of Pt L3 ΔXANES Using the FEFF8 Code Experimental ΔXANES Data from Pt/H-USY and Pt/NaY Experimental Information ΔXANES Spectra Relevance for Pt-Catalyzed Hydrogenolysis/Hydrogenation Reactions General Applicability of the ΔXANES Technique Acknowledgments Keywords: physical principles of XAS; EXAFS data-analysis; atomic XAFS; delta XANES

The size of Pt particles introduced into the channels of ZSM-5 zeolite by two different preparation methods has been studied with EXAFS. ZSM-5 was loaded with 2 wt 5% Pt by ion exchange and by impregnation. By careful calcination of the catalysts the dispersion was maintained. The EXAFS measurements were performed on the reduced system. Analysis of the data showed that in the impregnated sample the platinum was present in particles which on average were smaller than the platinum particles in the ion-exchanged sample. These results corroborate earlier high-resolution electron microscopy measurements performed ex situ on the calcined catalysts. Moreover, the average particle size found from EXAFS analysis agrees well with the HREM results, indicating that dispersion is maintained during reduction. Both techniques indicate particles that exceed the dimensions of a zeolite cage at the intersection of the channels. The fact that the impregnated system has smaller platinum particles than the ion-exchanged catalyst can be explained by the presence of additional potassium species in the pores of the impregnated zeolite. These species inevitably will hinder the migration of platinum to form larger particles.

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.

To study the local structure of the Ni promoter atom, the Ni and Mo K edge EXAFS spectra of Ni-MoS2/C hydrodesulfurization catalyst were measured in an in-situ EXAFS cell at 77 K. The Ni atom is situated in a square pyramid of five S atoms at a distance of 2.21 Å from the S atoms. In addition an EXAFS contribution due to a Mo atom at 2.82 Å from the Ni atom could be identified. This local structure indicates that the Ni atoms are situated on top of the S4 squares at the MoS2 edges in millerite-type Ni sites. The Ni atoms are situated in the planes of the Mo atoms and not in the intercalation plane midway between successive MoS2 sandwich layers.

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.

Desorption of hydrogen at high temperature leads to a contraction of the Ir–Ir coordination distance of highly dispersed iridium metal particles. The long metal–support oxygen distance (2.55 A) observed if the metal crystallites are covered with chemisorbed hydrogen disappears after treatment in vacuum at 623 K. Instead, a metal–oxygen coordination in the metal–support interface is now detected with a distance of 2.19 A. It is concluded that the long metal-oxygen distances found for oxide–supported metal catalysts after reduction in H2 originate from an M0–(OH)– interaction or from the presence of chemisorbed hydrogen in the metal–support interface. Admission of O2 at 77 K does not lead to a corrosive oxidation of the metal particles. The adsorption of O2 resulted in the same type of Ir—O bonds as present in the metal–support after evacuation.

[Ir4(CO)12] was used as a precursor for the preparation of a MgO-supported catalyst with a uniquely simple metal structure, Ir4. The precursor was adsorbed on MgO from hexanes with the metal framework remaining intact, as determined by extended X-ray absorption fine structure (EXAFS) spectra. The surface-bound species was inferred to be predominantly [HIr4(CO)11]−, which could be extracted from the surface by cation metathesis and identified in solution by infrared spectroscopy. After treatment of the MgO-supported iridium carbonyl in He followed by H2 at 300 °C, the sample was characterized again by EXAFS spectroscopy; the results provide evidence that the predominant surface species are clusters of four Ir atoms with an average Ir-Ir distance of 2.69 A.

The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and alumina-supported vanadium oxide clusters has been investigated. Using Na3VO4 and V2O5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been determined; this led to a 50 % decrease in the AXAFS intensity and to an increase in the centroid position. Furthermore, the influence of the vanadium oxide loading has been assessed. A non-linear relationship between the vanadium oxide loading and the AXAFS intensity has been found, indicating that the AXAFS intensity is sensitive to the formation of V-O-V bridging bonds between the vanadium VO4 clusters. The results show that AXAFS can be used to probe the relative energy level of the vanadium valence orbitals.

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.

Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions. A unique, previously not observed, pre-edge feature is detected in zeolites H-Mordenite and steamed and unsteamed H-Beta at temperatures above 675 K. Spectra simulations using the full multiple scattering code Feff8 identify the unique pre-edge feature as three-coordinate aluminum. The amount of three-fold coordinated aluminum is a function of temperature and pretreatment of a zeolite: a steamed zeolite Beta contains more three-coordinate aluminum than an unsteamed sample. No clear differences between zeolites H-Mordenite and H-Beta were observed. Octahedrally coordinated aluminum forms in zeolites H-Mordenite and H-Beta at room temperature in a stream of wet helium. This octahedrally coordinated aluminum is unstable at temperatures higher than 395 K, where it quantitatively reverts to the tetrahedral coordination.

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.

Two new techniques in X-ray absorption spectroscopy (XAS) have recently been developed which provide previously unobtainable information on supported noble metal catalysts. Atomic X-ray absorption fine structure (AXAFS) provides direct information on the average changes in interatomic potential of the metal particles induced by metal-support effects. Changes in the interatomic potential can now be directly related to changes in catalytic reactivity. Analysis of shape resonances near the X-ray absorption edge provides information on the changes in the nature of the bonding of adsorbates (like hydrogen) to the metal cluster as induced by the properties of the support. The strong decrease in activity of supported platinum clusters for neo-pentane hydrogenolysis with increasing alkalinity of the support can be ascribed to a decrease in ionization potential of the metal particles directly influenced by the alkalinity of the support.

The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by the acidity/alkalinity of the support but are relatively independent of the metal (Pd or Pt) or the type of support (zeolite LTL or SiO{2}). As the alkalinity of the support increases, the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a decrease in the XPS binding energy and a shift from linear to bridge bonded CO in the IR spectra. Analysis of the shape resonance in XANES spectra indicates that in the presence of chemisorbed hydrogen the difference in energy between the Pt-H antibonding orbital and the Fermi level decreases as the alkalinity of the support increases. Based on the results from the IR, XPS, and shape resonance data a new model is proposed in which the interaction between the metal and support leads to a shift in the energy of the metal valence orbitals. The EXAFS structural analysis indicates that the small metal particles are in contact only with the oxide ions of the support. Finally, a new spectroscopic characterisation, Atomic XAFS, is presented which provides new insights into the origin of the electronic changes in the metal. As the alkalinity of the support increases, there is decrease in the metal ionisation potential. The primary interaction is a Coulomb attraction between metal particle and support oxygen ions, which affects the metal interatomic potential. This model for the metal-support interaction explicitly excludes the need for electron transfer, and it can account for all observed changes in the catalytic, electronic, and structural properties of the supported metal particles induced by support acidity ranging from acidic to neutral to alkaline.

To investigate the anchoring of precious metals onto carbon fibrils Pd/C-fibril samples were investigated in both the precursor and the reduced state by X-ray absorption fine structure (XAFS) spectroscopy. Analysis of the XAFS data of the freshly prepared catalyst showed a palladium–tetraamine complex in interaction with carbon. The proposed model involves [Pd(NH3)4]2+ coordinated to the carbon fibril surface, most probably stabilised by the carboxylic groups (through O–H bridging and charge balance) and the π-electron system of the support. After insitu reduction, analysis pointed to small palladium particles of ca. 15 A present on the fibrils with a significant Pd–C interaction.

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.

Atomic XAFS on [PtCl(NCN)-Z] pincer complexes shows it to be a sensitive probe for the determination of the electron density on the metal atom, similar yet complementary to NMR.

UV-Vis, combined with ED-XAFS shows, for the first time, the evolution of inactive Pd dimers and trimers, that are a possible first stage in the deactivation process of important palladium catalysed reactions, leading to larger palladium clusters and eventually palladium black.

X-ray absorption spectroscopy was used to characterize the metal-support interface in catalysts consisting of very small Ir clusters of nearly uniform nuclearity on the surface of MgO powder. [Ir4(CO)12] on MgO was converted in high yield into [HIr4(CO)11]- and separately into [Ir6(CO)15l2-. Extended X-ray absorption fine structure (EXAFS) data are consistent with the hypothesis that each cluster retained its nuclearity upon decarbonylation. The average Ir-Ir coordination in Ir,/MgO is 2.6 neighbors at a distance of 2.72 A, and the structure is modeled as a mixture of 40-50% tetrahedra and the remainder four-atom rafts on MgO. The average Ir-Ir coordination in Ir6/MgO is 2.7 neighbors at a distance of 2.12 A, and the structure is modeled as a mixture of about 25% Ir6 octahedra and the remainder Ir rafts on MgO. Another sample was treated in H2 at 623 K to give a typical supported metal catalyst having a distribution of Ir clusters averaging 10 8, in diameter. The Ir-support interfaces in Ir4/MgO and Irb/MgO are characterized by single Ir-Mg coordinations at 1.6 A and fourfold Ir-O coordinations at 2.6 A; the interface It atoms are suggested to be atop Mg ions in the (100) faces of MgO. However, this structural model is simplified, and the Ir-Mg interactions are not fully understood. The results demonstrate that the structurally simple supported Ir clusters are among the simplest supported metals; the metal-oxygen distances of approximately 2.6 Aare quite generally characteristic of metal particles and clusters supported on metal oxides and zeolites; the long distance is attributed to metal atoms interacting with oxygen of the support with hydrogen present at the interface.

A platinum, non–acidic K–LTL catalyst, reduced at 300, 450, and 600°C, was characterized by extended X–ray absorption spectroscopy (EXAFS), hydrogen chemisorption, hydrogen temperature programmed desorption (H2–TPD) and methylcyclopentane hydrogenolysis. Reduction at 300°C produces small platinum crystallites with an interfacial layer of hydrogen. Reduction at 450°C increases the particle size, releasing interfacial hydrogen. This irreversible hydrogen desorption is observed in the TPD around 300°C. Reduction at 600°C results in further growth of the platinum cluster with the loss of the remaining interfacial hydrogen. In the TPD, a second high temperature H2 desorption is observed at around 610°C. Because of the confined space within the zeolite pore, as the platinum particle size approaches the pore size, there is a reduction in hydrogen chemisorption capacity and catalytic activity compared to a particle of equivalent size on an amorphous support.

The adsorption of carbon monoxide on a non-acidic Pt/K-LTL catalyst has been studied by diffuse reflectance and transmission IR spectroscopy. The CO spectrum is strongly dependent on the experimental conditions. Adsorption on the small Pt clusters in the presence of water gives linear-CO bands between 2060 and 1990 cm−1 and a bridging-CO band around 1800 cm−1. In the absence of water, the linear bands are red shifted to about 1940 and 1720 cm−1, respectively. The frequency shift is attributed to an ion-dipole interaction between adsorbed CO and support cations. The ion-dipole interaction is screened by the adsorbed water leading to a smaller red shift in the CO stretching frequency.

The selective catalytic oxidation of NH3 to N2 over unsupported and silica-supported MoO3 catalysts has been investigated. The defect structure of the catalysts greatly affects the selectivity towards N2O. The performance of the silica supported catalysts is controlled by the thermal pretreatment and the structure, which is installed by the preparation procedure. Water dramatically decreases the selectivity to N2 of MoO3–on-SiO2 catalysts with low loadings. The selectivity to N2 highly depends on the ability of the catalyst to decompose released N2O, which proceeds on oxygen vacancies.

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.

A Longoni-Chini cluster, [Ptl5(C0),l2-, was prepared on MgO powder by a surfacemediated synthesis from Na2PtCb in the presence of CO. The formation of [Pt15(CO)30]2a-n d its decarbonylation at 120 'C under vacuum were characterized by infrared and X-ray absorption spectroscopies. The decarbonylated cluster had an average Pt-Pt bond distance of 2.76 A and an average Pt-Pt first-shell coordination number of 3.7, indicating that the trigonal prismatic framework structure of the Longoni-Chini cluster was retained after decarbonylation; thus the decarbonylated cluster is represented as PtIS with a trigonal prismatic structure. When this sample was recarbonylated, the Longoni-Chini cluster was not re-formed. The average Pt-Pt bond distance increased to 2.78 A and the Pt-Pt first-shell coordination number increased to 6.0, indicating that a CO-induced morphology change gave approximately hemispherical Pt clusters with adsorbed CO.

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.