In situ Raman spectroscopy of supported chromium oxide catalysts: {sup 18}O{sub 2}-{sup 16}O{sub 2} isotopic labeling studies

The isothermal isotopic exchange reaction of {sup 18}O{sub 2} with {sup 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 {sup 18}O{sub 2} is difficult to achieve and requires several successive butane reduction-{sup 18}O{sub 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. 25 refs., 5 figs.