Elucidation of Active Sites of Gold Nanoparticles on Acidic Ta2O5 Supports for CO Oxidation

Praseodymia–titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides

Modulating the active sites for controllable tuning of the catalytic activity has been the goal of much research, however, this remains challenging. The O vacancy is well known as an active site in reducible oxides. To modify the activity of O vacancies in praseodymia, we synthesized a series of praseodymia–titania mixed oxides. Varying the Pr : Ti mole ratio (2 : 1, 1 : 2, 1 : 1, 1 : 4) allows us to control the electronic interactions between Au, Pr and Ti cations and the local chemical environment of the O vacancies. These effects have been studied study by X-ray photoelectron spectroscopy (XPS), CO diffuse reflectance Fourier transform infrared spectroscopy (CO-DRIFTS) and temperature-programmed reduction (CO-TPR, H₂-TPR). The water gas shift reaction (WGSR) was used as a benchmark reaction to test the catalytic performance of different praseodymia–titania supported Au. Among them, Au/Pr₁Ti₂O_x was identified to exhibit the highest activity, with a CO conversion of 75% at 300 °C, which is about 3.7 times that of Au/TiO₂ and Au/PrO_x. The Au/Pr₁Ti₂O_x also exhibited excellent stability, with the conversion after 40 h time-on-stream at 300 °C still being 67%. An optimal ratio of Pr content (Pr : Ti 1 : 2) is necessary for improving the surface oxygen mobility and oxygen exchange capability, a higher Pr content leads to more O vacancies, however with lower activity. This study presents a new route for modulating the active defect sites in mixed oxides which could also be extended to other heterogeneous catalysis systems.

Schematic illustration of H₂O activation on the Pr-TiO_x support and the following reaction with CO in the Au–oxide interface.

Au3+ Species Boost the Catalytic Performance of Au/ZnO for the Semi-hydrogenation of Acetylene

Au nanoparticles have garnered remarkable attention in the chemoselective hydrogenation due to their extraordinary selectivity. However, the activity is far from satisfactory. Knowledge of the structure-performance relationship is a key prerequisite for rational designing of highly efficient Au-based hydrogenation catalysts. Herein, diverse Au sites were created through engineering their interactions with supports, specifically via adjusting the support morphology, that is, flower-like ZnO (ZnO-F) and disc-like ZnO (ZnO-D), and the catalyst pretreatment atmosphere, that is, 10 vol % O₂/Ar and 10 vol % H₂/Ar (denoted as -O and -H, respectively). The four samples of Au/ZnO were characterized by various techniques and evaluated in the semi-hydrogenation of acetylene. The transmission electron microscopy results indicated that the Au particle sizes are almost similar for our Au/ZnO catalysts. The charge states of Au species demonstrated by X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule, and simulation based on density functional theory, however, are greatly dependent on the ZnO shape and pretreatment atmosphere, that is, the percentage of Au³⁺ reduces following the order of Au/ZnO-F-O > Au/ZnO-F-H > Au/ZnO-D-O > Au/ZnO-D-H. The testing results showed that the Au/ZnO-F-O catalyst containing maximum of Au³⁺ possesses the optimal activity with 1.8 × 10^-2 s^-1 of specific activity at 200 °C, around 16.5-fold of that for Au/ZnO-D-H. More interestingly, the specific rate at 200 °C and the average conversion/selectivity in the entire operating temperature range are well correlated with the redox states of the Au species, indicating that Au³⁺ sites are more active for acetylene hydrogenation. A plausible explanation is that the Au³⁺ species not only facilitate acetylene adsorption via electrostatic interactions but also favor the heterolysis of H₂ via constructing frustrated Lewis pairs with O.