Applications of in-situ wide spectral range infrared absorption spectroscopy for CO oxidation over Pd/SiO2 and Cu/SiO2 catalysts

Abstract

Infrared (IR) absorption spectroscopy has been widely used for dynamic characterization of catalysts and mechanism of catalytic reactions. However, due to the strong infrared absorption of heterogeneous catalysts (mainly oxides, or supported metal and metal oxides, etc.) below 1200 cm−1, and the intensity of regular infrared light source rapidly decays at low-wavenumber range, most in-situ infrared spectroscopy studies are limited to the detection of surface adsorbates in the range of 4000–900 cm−1. The change of catalytically active component itself (M–O, M–M bond, etc., 1200–50 cm−1) during the reaction is hard to be tracked under reaction conditions by in-situ IR. In this work, a home-made in-situ IR reactor was designed and a sample preparing method was developed. With such progresses, the changes of reactants, products, surface adsorbates, and catalysts themselves can be measured under the same reaction conditions with a spectral range of 4000–400 cm−1, providing a new opportunity for in-situ characterization of heterogeneous catalysis. CO oxidation on Pd/SiO2 and Cu/SiO2 catalysts were taken as examples, since both the two catalytic systems were extensively used commercially, and moreover reduction and oxidation of palladium and copper occur during the examined reaction conditions. The characteristic bands of Pd2+–O (670, 608 cm−1), Cu+–O (635 cm−1) and Cu2+–O (595, 535 cm−1) were observed by IR, and the changes during CO oxidation reaction were successfully monitored by IR. The oxidation/reduction of palladium and copper were also confirmed by ex-situ XPS. Moreover, Pd0 in Pd/SiO2 and Cu+ in Cu/SiO2 were found as the thermal dynamically stable phases under the examined conditions for CO oxidation.