Effect of metal dispersion and support structure of Ni/silicalite-1 catalysts on non-thermal plasma (NTP) activated CO2 hydrogenation

Non-thermal plasma (NTP) activated heterogeneous catalysis is a promising alternative to thermal catalysis for enabling many challenging reactions (e.g. catalytic CO2 hydrogenation) under mild conditions. However, the mechanistic insight into the interaction between highly energetic electrons and vibrationally-exited reactive species with metal catalyst is still lacking. Here, catalytically active Ni nanoparticles supported on silicalite-1 zeolites with different configurations regarding the location of Ni active sites and support pore structures were comparably investigated using catalytic CO2 hydrogenation under the thermal and NTP conditions. Experimental results revealed that the performance of the NTP-catalysis depends on the configuration of the catalysts significantly. Specifically, catalysts with Ni active sites sit on the outer surface of zeolite crystals (i.e. microporous Ni/S1 and Ni/M-S1@Shell with steam-assisted recrystallised micro-meso-porous structure) showed relatively good catalytic performance at a low applied voltage of 6.0 kV. Conversely, the encapsulated catalyst with hierarchical meso-micro-porous structure (i.e. Ni/D-S1) which has relatively small (i.e. average Ni particle sizes of 2.8 +/- 0.7 nm) and dispersed Ni nanoparticles (i.e. Ni dispersion of ca. 2.5 %) demonstrated comparatively the best catalytic performance (i.e. CO2 conversion of ca. 75 %) at 7.5 kV. Additionally, under the NTP conditions studied, Ni on carbon-templated mesoporous silicalite-1 (Ni/M-S1) showed the worst selectivity to CH4, which was attributed to the poor accessibility of Ni active sites encapsulated in the enclosed mesopores. This study demonstrated the crucial role of catalyst design in NTP activated catalysis.