Effect of oxidation–reduction treatments on the infrared spectra of carbon monoxide chemisorbed on a Ru/TiO2 catalyst

Abstract

The measurement of IR spectra of CO adsorbed on H₂-reduced Ru/TiO₂(1% Ru, Degussa P-25 TiO₂) is made difficult by the very low transparency, caused by partial reduction of the support; reduction at 758 K leads to complete opacity, but spectra can be obtained by multiple scanning on a slightly oxidised sample (transparency 0.07%). Reduction at either 433 or 623 K, however, gives products with which spectra are readily observed. On samples well reduced at 623 K the adsorption of CO gives as prevailing species Ru⁰—CO with ν(CO) at 2045 cm^–1 at full coverage, shifting to lower frequency up to 1990 cm^–1 as the coverage is decreased. At high pressures at room temperature, and especially on samples not well reduced and containing Cl^–, bands are observed at 2151–2132 and 2090–2072 cm^–1 which are attributed to Ru³⁺(CO)₂ or Ru²⁺(CO)₃ species. These species are removed by evacuation at 473 K, whereas Ru²⁺(CO)₂ species (bands at 2101–2080 and 2038–2023 cm^–1) are stable up to 573 K. At 473 K CO is an effective reductant for incompletely reduced samples. The Ru²⁺(CO)₂ species are intermediates in the reduction by CO of the Ru/TiO₂ system: at 573–623 K all the oxidised Ru species are reduced to Ru⁰. Spectra obtained by CO adsorption on samples reduced at 433 K are similar, save for a shoulder on the low-frequency side of the 2045 cm^–1 linear Ru⁰—CO band. The presence of a low-frequency component at 1990 cm^–1 is the fingerprint of isolated Ru⁰—CO species formed on very small and less perfect Ru crystals. This suggests that Ru is well dispersed. Evacuation at 758 K of unreduced RuCl₃/TiO₂ also results in the formation of some Ru⁰.