On the Relation between Particle Morphology, Structure of the Metal-Support Interface, and Catalytic Properties of Pt/γ-Al2O3

Abstract

The relation between the catalytic activity, electronic properties, morphology, and structure of the metal-support interface was studied for Pt/γ -Al 2 O 3 after low (300 • C, LTR) and high temperature reduction (450 • 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 2.7Å 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 2.2Å 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 III and L II X-ray absorption edge intensities after LTR and HTR revealed that the number of holes in the d-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 the d-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

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

“…A long Pt-O coordination distance of 2.65 Å is observed after reduction, which, according to other studies of our group [43][44][45], is due to the presence of chemisorbed hydrogen in the metal-support interface. After evacuation, the Pt-O distance is shortened to 2.24 Å for Pt/H-USY and to 2.28 Å for Pt/NaY due to the removal of the chemisorbed hydrogen from the metal-support interface [44,45]. HRTEM data collected on the EXAFS Table 1) were prepared with all Pt particles dispersed inside the zeolite.…”

Structure and Reactivity of Bimetallic FeIr/SiO2 Catalysts after Reduction and during High-Pressure CO Hydrogenation

“…In order to fit the spectrum at high k-values where oxygen has no contribution can be assigned to an interaction of reduced iridium with oxygen in the support or in iron oxide. The low value of E 0 (−19 eV) has been observed previously for metal-support interactions (45) and is most likely due to differences in oxidation state between the metal in the reference compound and in the sample. In this case, first shell Ir-Ir and Ir-Fe coordination numbers of 6.5 and 2.7 are observed (Table 2), which are in much better agreement with the higher shell coordination numbers than the previous model.…”

The Nature of the Pt−H Bonding for Strongly and Weakly Bonded Hydrogen on Platinum. A XAFS Spectroscopy Study of the Pt−H Antibonding Shaperesonance and Pt−H EXAFS

“…It has been shown earlier that the oxygen backscatterers originate from the metal-support interface. [30][31][32] The variances in imaginary and absolute parts were used to determine the fit quality. The best-fit results are given in Table 1.…”