Water-Gas Shift Reaction on Pt/Ce_(1-x)Ti_xo_(2-δ): The Effect of Ce/Ti Ratio

摘要

Pt nanoparticles (1.2—2.0 nm size) supported on Ce_(1-x)Ti_xo_(2-δ) (x = 0, 0.2, 0.5, 0.8, and 1.0) carriers synthesized by the citrate sol—gel method were tested toward the water—gas shift (WGS) reaction in the 200—350 °C range. A deep insight into the effect of two structural parameters, the chemical composition of support (Ce/Ti atom ratio), and the Pt particle size on the catalytic performance of Pt-loaded catalysts was realized after employing in situ X-ray dim-action (XRD), high-resolution transmission electron microscopy (HR-TEM) and HAADF/STEM, scanning electron microscopy (SEM), in situ Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies under different gas atmospheres, H2 temperature-programmed reduction (H2-TPR), and temperature-programmed desorption (NH3-TPD and CO2-TPD) techniques. The 0.5 wt % Pt/Ce_(.08)Ti_(0.2)O_(2-δ) solid (d_(Pt) = 1.7 nm) was found to be by far the best catalyst among all the other solids investigated. In particular, at 250 °C the CO conversion over Pt/Ce_(.08)Ti_(0.2)O_(2-δ) was increased by a factor of 2.5 and 1.9 compared to Pt/TiO2 and Pt/CeO2, respectively. The catalytic superiority of the Pt/Ce_(.08)Ti_(0.2)O_(2-δ) solid is the result of the support's (i) robust morphology preserved during the WGS reaction, (ii) moderate acidity and basicity, and (iii) better reducibility at lower temperatures and the significant reduction of "coking" on the Pt surface and of carbonate accumulation on the Ce_(.08)Ti_(0.2)O_(2-δ) support. Several of these properties largely influenced the reactivity of sites (k, s~(-1)) at the Pt—support interface. In particular, the specific WGS reaction rate at 200 °C expressed per length of the Pt~support interface (μmol CO cm~(-1) s~(-1)) was found to be 2.2 and 4.6 times larger on Pt supported on Ce_(.08)Ti_(0.2)O_(2-δ) (Ti~(4+)-doped CeO2) compared to TiO2 and CeO2 alone, respectively.