Ethanol Synthesis from Carbon Dioxide on [Rh10Se]/TiO2Catalyst Characterized by X-Ray Absorption Fine Structure Spectroscopy

Abstract

The active site structures and electronic states were studied for the [Rh10Se]/TiO2catalyst by means of edge spectra and EXAFS. The rate and selectivity of ethanol synthesis from CO2on [Rh10Se]/TiO2had strong dependence on the heating temperature in vacuum (Tevac) (1). Corresponding to the maximum of rate and selectivity when the Tevacwas 623 K, the distance rSe–Rhreached a minimum (2.41 Å) by Se K- and Rh K-edge EXAFS analyses. The contracted [Rh10Se] cluster was found to have an electronic state similar to that of Rh3Se8or RhSe2rather than of metallic Rh. A new reaction path control is proposed by regulating the distance between the interstitial Se atom and the metal framework [Rh10]. A strong peak due to 1s→np transition was observed around the Se K absorption edge. The peak intensity did not exhibit significant change when the Tevacwas varied for [Rh10Se]/TiO2. On the other hand, the area of peak observed around 23,230 eV in Rh K-edge spectra gradually decreased when the Tevacwas elevated for [Rh10Se]/TiO2. Hence, the Se atom surrounded by [Rh10] framework was always kept in anionic state, while the electronic state of Rh atoms gradually changed by the interaction with TiO2surface. The change for Rh atoms was also supported by the gradual increase of rRh–Rhobtained by EXAFS with the temperature increase. In the case of Rh sites with lower total coordination number around Rh ([Rh10Se]/SiO2, [Rh10Se]/Al2O3, and [Rh10Se]/MgO), the reaction CO2→CO(a)+O(a) occurred predominantly and formed CO(a) poisoned the catalysis.