Dc Diek Koningsberger

Determination of metal particle size of highly dispersed Rh, Ir, and Pt catalysts by hydrogen chemisorption and EXAFS

Abstract 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.

An EXAFS study of the structure of the metal-support interface in highly dispersed Rh/Al2O3 catalysts

Four highly dispersed and fully reduced rhodium on alumina catalysts with different particle sizes in the range 6–12 A were investigated with the EXAFS technique in order to derive information about the structure of the metal–support interface. This information can only be obtained when the signal‐to‐noise ratio of the experimental EXAFS data is high enough and accurate reference compounds and a modified way of data analysis are used. With the aid of phase and amplitude corrected Fourier transforms it was possible to detect a small additional signal which could be ascribed to a Rh–O bond. Since the catalysts were fully reduced and since the intensity of the small signal increased with decreasing particle size, the oxygen neighbor was assigned to be originated from the metal–support interface. From the intensity of the Rh–O bond it was estimated that, on the average, each interfacial rhodium atom is surrounded by 2–3 oxygen ions of the support. The detected Rh–O bond has a coordination distance of 2.7 A which is about 0.6 A larger than the first coordination distance in Rh2O3 (2.05 A). The coordination distance of 2.7 A can be explained by assuming an interaction between metallic rhodium (atomic radius 1.34 A) and ionic oxygen belonging to the support (ionic radius 1.4 A). This would possibly imply an ion‐induced dipole bonding between the metal particle and the support.

An in situ cell for transmission EXAFS measurements on catalytic samples

An in situ cell suitable for transmission EXAFS measurements on catalytic samples is described. The cell can be used for catalyst pretreatments in various atmospheres (including H2, H2S, O2 and CO) in a temperature range upto 700 K. The sample is heated by conducting heat from an external heater to the sample. During measurement the samples can be cooled down to 77 K by conducting heat from the sample to an external liquid nitrogen container. During the pretreatment and the measurement a waterflow through the body of the cell keeps certain crucial parts from overheating or icing up. To avoid radiation leaks in powdery samples these samples are pressed in a selfsupporting wafer and held in a disk‐shaped sampleholder. Tests by various catalytic groups have proven the suitability of the design.

Characterization of supported cobalt and cobalt-rhodium catalysts : III. Temperature-Programmed Reduction (TPR), Oxidation (TPO), and EXAFS of Co---Rh/SiO2

Temperature-Programmed Reduction, Oxidation, and Extended X-Ray Absorption Fine Structure (TPR, TPO, and EXAFS) experiments supply clear evidence for the formation of bimetallic particles in Coue5f8RhSiO2 catalysts. After coimpregnation and drying, as well as after oxidation, the reduction of Coue5f8RhSiO2 catalysts proceeds at lower temperatures than the reduction of comparable CoSiO2 catalysts, indicating that rhodium catalyzes the reduction of the cobalt metal salt and cobalt oxide. EXAFS of the Rh K-edge of the Coue5f8RhSiO2 catalyst shows that after reduction the rhodium atoms in the catalyst have less cobalt neighbors than those in the Coue5f8Rh alloy. The Rhue5f8Co and Rhue5f8Rh peak intensities in the Fourier transform of the Rh EXAFS were only slightly influenced by adsorption of oxygen at room temperature, whereas the EXAFS spectrum of the cobalt K-edge changed completely to that of cobalt oxide. From these results it is concluded that the reduced catalyst contains bimetallic Coue5f8Rh particles, the interiors of which are enriched in rhodium, while the outer layers contain more cobalt.

Roman spectroscopic study of CoMoγ-Al2O3 catalysts

Laser Raman spectroscopy is used to study the structure of molybdenum and cobalt species present in Coue5f8Moγ-Al2O3 catalyst systems. From comparison with Raman spectra of Mo and Co in known structures it is derived that these catalyst systems contain Mo and Co in different modifications depending on the degree of surface coverage. In the absence of Co, four different Mo species are found. At low coverages isolated molybdate tetrahedra are observed. Increasing the surface coverage results in formation of a polymolybdate phase in which Mo is octahedrally surrounded. At higher coverages “bulk” aluminum molybdate is formed. At very high coverages formation of “free” MoO3 occurs. In Coγ-Al2O3 samples the color indicates the presence of Co3O4- and CoAl2O4-like species. When Co is introduced in Moγ-Al2O3 (CoMo atomic ratio, 0.64) various effects occur. “Free” MoO3, as well as Al2(MoO4)3, is converted into “CoMoO4.” Cobalt addition results in a decrease of the isolated Mo tetrahedra concentration in favor of the polymeric molybdate form, which apparently is not qualitatively affected by the presence of Co. In Coue5f8Moγ-Al2O3 most of the Co is present in a structure comparable to CoAl2O4. The influences of the nature of the support, heat treatment, reduction in hydrogen and the effect of sulfiding are discussed briefly.

EXAFS study of the local structure of Ni in Ni-MoS2/C hydrodesulfurization catalysts

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.

ESR Studies on Hydrodesulfurization Catalysts: Supported and Unsupported Sulfided Molybdenum and Tungsten Catalysts

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.

Platinum deactivation: in situ EXAFS during aqueous alcohol oxidation reaction

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 Å during exposure to nitrogen‐saturated cyclohexanol solution is different from what is observed for the pure oxidised platinum surface (2.06 Å).

ESR Studies on Hydrodesulfurization Catalysts: Nickel- or Cobalt-Promoted Sulfided Tungsten- or Molybdenum-Containing Catalysts

ESR spectra of nickel- or cobalt-promoted sulfided tungsten- or molybdenum-containing catalysts are measured together with the influence of equilibration at different H2SH2 ratios and CO adsorption hereon. The ESR measurements are compared with thiophene hydrodesulfurization (HDS) activities. The influence of nickel and cobalt on the ESR spectra is essentially the same. This similarity also holds for the thiophene HDS activities. A new signal VI (g = 2.06, ΔH = 220 G for Niue5f8Mo; g = 2.08, ΔH = 220 G for Niue5f8W) not occurring in unpromoted catalysts is detected. The intensities of signals II, III, IV, and VI are dependent on the composition of the interacting atmosphere. The results are discussed in terms of WS2(MoS2) crystallite edge decoration with promotor ions. Possible surface configurations corresponding with the different paramagnetic sites are described.

Pt clusters in BaKL zeolite : characterization by transmission electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy

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 Å, and a Pt-Ba contribution at 3.8 Å. The Pt/BaKL zeolite is one of the most highly dispersed supported platinum samples and one of the most structurally uniform supported metal catalysts.