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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.

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.

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.

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.

An alumina-supported rhodium catalyst has been studied with EXAFS. After reduction and evacuation, oxygen was admitted at 100 and 300 K. EXAFS spectra of the catalyst after oxygen admission at 100 K indicated the beginning of oxidation. At 300 K only a small part of the rhodium particles remained metallic and this metallic 'kernel' was partly covered with rhodium oxide. In the rhodium metal to rhodium oxide interface the same 2.7-A Rho-0' distances are present as in the metal-support interface. A model is presented that explains the observed formation of the rhodium metal-rhodium oxide interface.

Reduction of a highly dispersed 2.85 wt% Rh/TiO/sub 2/ catalyst at 473 K after previous calcination at 623 K resulted in EXAFS whose primary contributions are due to nearest rhodium (average coordination number of 3.1 and distance of 2.67 A) and oxygen neighbors (coordination 2.5 and distance 2.71 A). These oxygen neighbors originated at the metal-support interface. The average rhodium-rhodium coordination number did not change in the SMSI state produced by reducing the catalyst at 673 K. However, the average coordination distance contracted by 0.04 A with an accompanying decrease of the Debye-Waller factor of the Rh-Rh bond of 0.0012 A/sup 2/. This is due to the fact that in the SMSI state the surface of the metal particles is not covered with chemisorbed hydrogen. The SMSI state leads to a structural reorganization of the support in the vicinity of the rhodium metal particles. This can be concluded from the appearance of a Rh-Ti bond at 3.42 A in the SMSI state coupled with the fact that the average coordination number of the rhodium-support oxygen bonds does not increase. Other types of rhodium-oxygen bonds could not be detected with EXAFS in this state. Thus, these results provide no evidence formore » coverage of the metal particle by a suboxide of TiO/sub 2/ in the SMSI state.« less

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.

Extended X-ray absorption fine structure (EXAFS) and high-resolution transmission electron microscopy (HRTEM) have been used to study the structure of a Rh/Ti02 catalyst. After reduction in H, at 473 K (when the catalyst is in the normal state) the metal particles contain on the average five rhodium atoms and are situated preferably on edges of the Ti0, crystallites but also on [loll and to a lesser extent on [OOl] anatase crystal faces. Reduction in H2 at 723 K leads to the SMSI state. Besides oxygen neighbors from the support, the rhodium metal atoms in the metal-support interface have Ti“’ neighbors at 3.4 and 4.3 A. These distances and their coordination numbers fit well with a model in which the metal particles rest on a TiO, suboxide. This indicates that the supporting oxide near the metal particle has been reduced to a suboxide of TiO,. In the SMSI state no indication for coverage has been found with either EXAFS or HRTEM. On the contrary, exposing the catalyst in the SMSI state to oxygen at 100 K resulted in changes in the EXAFS spectrum due to physisorption of oxygen. Consequently, in the SMSI state the particles are either not covered or are incompletely covered with TiO,. Since a Rh/AI2O3 catalyst under the same conditions became partly oxidized, it is evident that for the Rh/Ti02 catalyst oxidation has been suppressed. This is most probably the result of an electronic influence from the reduced supporting oxide. Even after oxygen admission at room temperature, the rhodium particles on the TiO, support remain in the metallic state. The TiO, suboxide in the vicinity of the metal particles starts to reoxidize and the metal-support interaction becomes weaker.