Hydrogen as a Modifier of the Structure and Electronic Properties of Platinum in Acidic Zeolite LTL: A Combined Infrared and X-ray Absorption Spectroscopy Study

Abstract

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 e 1 3 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 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

“…22 Both the Pt L 2 and L 3 edge XAFS spectra were recorded at liquid nitrogen temperature in transmission mode.…”

Pore curvature and support composition effects on the electronic properties of supported Pt catalysts: An infrared spectroscopy study with CO as probe molecule

“…A pressure of 3 bar was applied during 10 s to prevent destruction of the pore structure of the support [18]. The wafer was placed in an IR transmission cell equipped with CaF 2 windows as previously described [19]. The cell was evacuated to 10 À8 bar followed by drying of the wafer at 323 K overnight prior to reduction by a 10 ml/min flow of H 2 (quality 4.6; Hoekloos).…”

Analysis of in situ EXAFS data of supported metal catalysts using the third and fourth cumulant

“…The supported Pt catalyst was pressed into a self-supporting wafer and placed in a stainlesssteel in-situ cell. 24 The catalysts were reduced in pure hydrogen at the temperature specified in Table 1 …”