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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.

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 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.

A series of Pt/LTL zeolite catalysts with varying Pt loading and support acidity was studied by hydrogen temperature-programmed desorption (TPD) and reaction of neopentane and propane. Specific desorption peaks in the TPD were associated with hydrogen spilled-over onto the support. The ability of the spilled-over hydrogen to migrate on the support surface was limited to approximately 35 nm from the platinum cluster. Saturation capacity for spilled-over hydrogen was related to the support acidity/basicity, with a maximum of about 1018 atoms/m2 for the most acidic support studied. Spilled-over hydrogen was inert to oxygen treatment at 100°C, and did not contribute to catalytic activity for neopentane or propane conversion. Instead, hydrogenolysis TOF and the amount of spilledover hydrogen were mutually dependent on support acidity/basicity. Both declined with increasing K/Al ratio of the support, and continued to decline even for basic supports with K/Al ratios greater than 1. The continued decline in activity for K/Al > 1 indicates than the metal-support interaction is not dependent on acidic hydroxyls.

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.