Cold plasma for preparation of Pd/C catalysts toward formic acid dehydrogenation: Insight into plasma working gas

Cold plasma has been proved to be an efficient method for synthesizing high performance Pd/C catalysts. However, the influence mechanism is still in obscure. In this work, the influence of cold plasma working gas, including air, Ar, O2 and H2, on the structure and performance of the Pd/C catalysts were investigated. Formic acid (HCOOH) dehydrogenation was selected as a probe reaction to investigate the performance of the Pd/C catalysts, and the catalytic activity of the catalysts follows the order: Pd/C-H2P > Pd/C-ArP > Pd/C-AirP ≈ Pd/C-O2P. The Pd/C-H2P catalyst possessed the highest activity, and the TOFinitial was as high as 552.4 h−1. The discharge parameters indicated that few micro discharge channels were generated in H2 plasma, and the power of a single-channel discharge is high. It facilitates the migration of the Pd active species from the pores to the outer surface of the support, and the mean size of the Pd nanoparticles were 2.6 ± 1.0 nm. In addition, the atomic ratio of Pd/C and the content of metallic Pd in Pd/C-H2P, determined by XPS, were as high as 0.0229 and 50.8%, respectively. These are beneficial to the reaction, and ensured the highest catalytic activity of the Pd/C-H2P catalyst. In contrast, the discharge in Ar plasma was mild and the regulation effect was not as strong as that in H2 plasma. Therefore, the catalytic activity of Pd/C-ArP was inferior to Pd/C-H2P. In addition, since there was a severe ablation of the carbon support and agglomeration of Pd nanoparticles in O2 plasma and air plasma, both of Pd/C-O2P and Pd/C-AirP exhibited poor catalytic activity toward HCOOH dehydrogenation.