Effect of oxidation–reduction treatments on the infrared spectra of carbon monoxide chemisorbed on a Ru/TiO2catalyst

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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.