Effect of morphological characteristics of TiO 2-supported noble metal catalysts on their activity for the water–gas shift reaction

Abstract

The catalytic activity of M/TiO 2 catalysts (M=Pt, Rh, Ru, Pd) for the water–gas shift (WGS) reaction has been investigated in the temperature range of 150–450 °C with respect to the structural and morphological properties of the dispersed metallic phase and the support. It has been found that the turnover frequency (TOF) of CO conversion varies in the order of Pt>Rh>Ru>Pd, with Pt being about 20 times more active than Pd. The activation energy of the reaction is practically the same for all metals examined. Conversion of CO at a given temperature increases significantly with increasing metal loading in the range of 0.1–5.0 wt%. However, activation energy and TOF do not depend on the morphological and structural characteristics of the metallic phase, such as loading, dispersion, and crystallite size. This has been clearly shown in the case of Pt/TiO 2 catalysts, where the TOF of CO conversion was found to be independent of the Pt crystallite size in the range of 1.2 to 16.2 nm. The effect of the morphology of the support on catalytic performance has been investigated over Pt catalysts supported on four commercial titanium dioxide carriers with different structural characteristics (surface area, primary crystallite size of TiO 2). It has been found that conversion of CO at low temperatures (<300 °C) is significantly improved when Pt is dispersed on TiO 2 samples of low crystallite size. The turnover frequency of CO increases exponentially with decreasing the primary crystallite size of TiO 2, accompanied by a substantial decrease of the apparent activation energy of the reaction. In particular, the rate per surface Pt atom increases by more than two orders of magnitude with decreasing crystallite size of TiO 2 from 35 to 16 nm, with a parallel decrease of activation energy from 16.9 to 11.9 kcal/mol. It is concluded that noble metal catalysts highly dispersed on titanium dioxide carriers with small TiO 2 crystallite size are promising candidates for use in low-temperature WGS reactors for fuel cell applications.