In Situ Raman Spectroscopy of Supported Chromium Oxide Catalysts:  ¹⁸O₂−¹⁶O₂ Isotopic Labeling Studies

Abstract

The isothermal isotopic exchange reaction of 18 O 2 with 16 O of chromium(VI) oxide supported on zirconia, alumina, and titania has been investigated with in situ laser Raman spectroscopy. The isotopic exchange reaction is dependent on the support type, the Cr loading, and the reaction temperature. Complete isotopic exchange of chromium(VI) oxide with 18 O 2 is difficult to achieve and requires several successive butane reduction-18 O 2 oxidation cycles at relatively high temperatures. The efficiency of the isothermal isotopic exchange reaction increases from alumina over titania to zirconia and with increasing Cr loading and reduction temperature. The observed Raman shifts upon isotopic labeling are consistent with a mono-oxo surface chromium oxide(VI) species.

In Situ Raman Spectroscopy of Supported Transition Metal Oxide Catalysts: 18O2−16O2 Isotopic Labeling Studies

“…A monooxo species (structure A) should give rise to two Raman vibrations, Figure 1. Molecular structure of a supported monooxo species (structure A), dioxo species (structure B), and trioxo species (structure C), and their corresponding 18 8,[18][19][20][21]26 The transition metal oxide loadings of these catalysts correspond with high coverages, but no crystalline phases were observed. Thus, well-dispersed supported transition metal oxides were studied in all cases.…”

Diffuse Reflectance Spectroscopy of Supported Chromium Oxide Catalysts: A Self-Modeling Mixture Analysis

“…spectroscopy (EXAFS-XANES) (13,14), Raman spectroscopy (15)(16)(17)(18)(19)(20)(21), and diffuse reflectance spectroscopy (DRS) (22)(23)(24)(25)(26)(27)(28)(29)(30). The last one is the least expensive and, in principle, the most versatile technique, as both d-d and charge transfer transitions can be probed (31).…”

In Situ Diffuse Reflectance Spectroscopy of Supported Chromium Oxide Catalysts: Kinetics of the Reduction Process with Carbon Monoxide

“…In order to study real catalytic conditions as closely as possible, in situ measurements are recommended. Up to now, measurements of the catalytic action under in situ conditions were possible with Fourier transform infrared, [1][2][3][4] diffuse reflectance infrared Fourier transform, [5][6][7] Raman, [8][9][10][11] X-ray powder diffractometry, 12 and extended X-ray absorption fine structure spectroscopies. 13 Although diffuse reflectance spectroscopy (DRS) is ideally suited for probing the oxidation state and coordination environment of TMI and for their quantitative speciation, 14 its application on supported metal oxide catalysts was not yet possible at high temperature in the presence of substrates.…”