Differentiating and Understanding the Effects of Bulk and Surface Mo Doping on CO<sub>2</sub> Hydrogenation over Pd/Anatase-TiO<sub>2</sub>
Linxiao Chen, Libor Kovařík, Debora Meira, János Szanyi
Abstract
Doping oxide supports with alter-valent cations is a viable strategy to modulate supported metal catalysts. Fundamental understanding on the dopant effects is lacking, as the roles of various dopant species have not been differentiated, and the physical origins of these effects were not clearly revealed. Here we present a systematic investigation on the effects of bulk and surface Mo dopants in anatase TiO2 on CO2 hydrogenation over supported Pd particles. On the one hand, Mo in the near-surface region enhances the reducibility of support surfaces, thus increasing the electron density on Pd under reducing reaction conditions, as shown by infrared (IR), X-ray photoemission (XPS), and X-ray absorption (XAS) spectroscopy. The adsorption of *CO and *H on electron-rich Pd is weaker, shown by IR and XAS, respectively, thus suppressing CH4 formation (methanation). On the other hand, bulk Mo6+ substitutes lattice Ti4+ and reduces the band gap of TiO2. The changes in the bulk electronic property of TiO2 increase Pd dispersion, as shown by scanning transmission electron microscopy (STEM) and XAS, resulting in a higher fraction of peripheral sites, thus promoting CO formation (reverse water–gas shift, rWGS) over them. This work clearly differentiates the roles that surface and bulk Mo dopants play in regulating supported metal catalysts and reveals the physical origin of the dopant effects. These fundamental insights enhance the understanding of metal–support interaction and offer guidelines for tuning metal catalysts through support engineering.