Modulating the surface defects of titanium oxides and consequent reactivity of Pt catalysts

Abstract

In heterogeneous catalysis, it is widely believed that the surface states of catalyst supports can strongly influence the catalytic performance, because active components are generally anchored on supports. This paper describes a detailed understanding of the influence of surface defects of TiO₂ supports on the catalytic properties of Pt catalysts. Pt was deposited on reduced (r-), hydroxylated (h-), and oxidized (o-) TiO₂ surfaces, respectively, and the different surface states of TiO₂ not only lead to differences in metal dispersion, but also distinct electronic interactions between the metal and the support. The highest reactivity for catalytic CO oxidation can be achieved over the Pt catalyst supported on reduced TiO₂ with surface oxygen vacancies. The turnover frequency (TOF) of this catalyst is determined to be ∼11 times higher than that of Pt supported on oxidized TiO₂. More importantly, the reactivity is seen to increase in the sequence of Pt/o-TiO₂ < Pt/h-TiO₂ < Pt/r-TiO₂, which is well consistent with the trend of the calculated Bader charge of Pt.

Fig. 1. Bulk and surface properties of TiO₂ supports. (A) XRD patterns of different TiO₂ supports; (B) Raman spectra of different TiO₂ supports; (C) XPS Ti 2p peaks of different TiO₂ supports. The inset in (C) shows the Ti³⁺ species on reduced TiO₂.

Fig. 2. HAADF-STEM and size distribution of Pt/TiO₂ catalysts. (A–C) and (D–F) are HADDF-STEM and size distribution of Pt/r-TiO₂, Pt/h-TiO₂ and Pt/o-TiO₂ before and after reaction, respectively.

Fig. 3. Reactivity, kinetic properties and DRIFTS of CO adsorption on Pt/TiO₂ catalysts. (A) Light-off curves of CO conversion on different catalysts. The CO oxidation was conducted with a gas composition of 1% CO, 20% O₂ and He balanced, keeping GHSV at 18 000 ml g_cat.^–1 h^–1. Each point is tested at a fixed temperature three times and then ramped to higher temperature. (B) Arrhenius plot of CO oxidation on different catalysts. (C) DRIFTS in CO oxidation (1% CO, 20% O₂ and He balanced) at 80 °C. The spectra were acquired after 5 minutes of reaction. (D) Plot of CO adsorption amount as a function of temperature in an Ar atmosphere. CO was pre-adsorbed at room temperature.

Fig. 4. Calculated CO oxidation over Pt/TiO₂ catalysts. (A) Free energy barriers for CO oxidation. (B–D) Models from left to right represent the initial state (IS), transition state (TS), final state (FS) and CO₂ desorption of CO oxidation on Pt/r-TiO₂, Pt/h-TiO₂ and Pt/o-TiO₂, respectively.