Single-Atom Cationic Ag and Metallic Ag Nanoparticles Supported on Al<sub>2</sub>O<sub>3</sub> as Catalysts for Water-Resistant and CO<sub>2</sub>-Selective HCHO Oxidation
Xueyan Chen, Shuo Hu, Xiaoxiao Qin, Min Chen, Jianghao Zhang, Xiaolei Bao, Changbin Zhang
Abstract
Ag-based catalysts have been extensively investigated in HCHO oxidation on account of the low price of the Ag precursor and their considerable low-temperature activity. It is known that H 2 pretreatment has a remarkable promotion effect on Pt- and Pd- catalysts for HCHO oxidation, whereas the effect of H 2 reduction on Ag-based catalysts has been rarely reported, probably due to the coexistence of multiple states of Ag species resulting from the high Ag loading. Herein, we prepared a 1% Ag/Al 2 O 3 (Ag/Al-fresh) catalyst with Ag species in a uniform state and then reduced Ag/Al-fresh with H 2 at 600 °C to obtain the Ag/Al-600H 2 catalyst. We observed that H 2 reduction had a significant influence on the performance of Ag/Al 2 O 3 nanocatalysts in HCHO oxidation. In a dry reaction atmosphere, Ag/Al-fresh and Ag/Al-600H 2 exhibited comparable HCHO conversion levels, but Ag/Al-600H 2 showed much higher CO 2 selectivity than Ag/Al-fresh. The presence of water vapor severely suppressed the HCHO conversion of the Ag/Al-fresh catalyst, while Ag/Al-600H 2 showed high water resistance, maintaining high HCHO conversion and CO 2 selectivity at 35% RH. Characterization results showed that the Ag species on Ag/Al-fresh and Ag/Al-600H 2 were single-atom cationic Ag and metallic Ag nanoparticles, respectively. It was revealed that the Ag 0 nanoparticles were more conducive than single-atom Ag + to formate decomposition, as well as O 2 and H 2 O activation, thus accelerating the conversion of HCOO – and CO intermediates to CO 2 and contributing to the higher activity and water resistance of the Ag/Al-600H 2 catalyst.