Synergistic effects of plasma and catalyst in formaldehyde conversion over a Ni/α-Al2O3 catalyst in a dielectric barrier discharge reactor

Abstract

Plasma-catalytic conversion of formaldehyde, as a representative volatile organic compound, in a dielectric barrier discharge reactor over a NiOx/α-Al2O3 catalyst was studied. Effects of plasma treatment on the physicochemical properties of the catalyst were studied and XPS analysis showed that plasma treatment increased the surface contribution of more active sites (i.e. Ni2+ and adsorbed O). However, the physical properties of the catalyst, including BET surface area and porosity, did not significantly change after plasma exposure. Presence of oxygen in the plasma-catalytic process improved conversion efficiency. Individual roles of plasma and catalyst in the selective conversion of formaldehyde to CO2, as the desired product, were decoupled. It was found that, in the oxygen-containing atmosphere, plasma can enhance formaldehyde conversion due to the facilitating role of O2-derived intermediates, such as ozone, while catalyst improves selectivity toward CO2 through catalyzing the complete oxidation path. Catalysts can make a substantial improvement in CO2 selectivity compared to the corresponding plasma-only conditions, even in an oxygen-free environment. Comparison between plasma-only, catalyst-only, and plasma-catalytic conditions with in-plasma catalyst (IPC) and post-plasma catalyst (PPC) configurations showed that CO2 yield decreased in the following order: IPC > PPC > catalyst-only > plasma-only. Superior performance under IPC conditions was due to: i) the improved catalyst properties upon plasma treatment; ii) facilitated CHO formation due to the impacts of both plasma and catalyst; iii) dominance of catalytic reaction steps after CHO formation resulting in the improved CO2 selectivity in the presence of a catalyst. The CO2 yield in the IPC system was greater than the summation of CO2 yield in the plasma-only and catalyst-only conditions, demonstrating the synergistic effects of plasma and catalyst. These results provide new insights into the facilitating role of oxygen in formaldehyde conversion, the beneficial role of catalyst in CO2 selectivity, and synergistic effects of plasma and catalyst in formaldehyde conversion over a NiOx/α-Al2O3 catalyst.