Monolayer supported CuOx/Co3O4 as an active and selective low temperature NOx decomposition catalyst

Abstract

A NO_x decomposition catalyst, copper supported on cobalt oxide (CuO_x/Co₃O₄), is reported with areal activity and selectivity to N₂ exceeding that of a Cu-ZSM5 catalyst at reaction temperatures as low as 350 °C. A series of the CuO_x/Co₃O₄ catalysts were synthesized with incremental Cu loadings and characterized to determine the monolayer coverage. X-ray diffraction confirmed the presence of bulk-like Cu(II)O microcrystals at high loadings (≥13.2 Cu per nm²); typical for coverages exceeding monolayer. However, the added surface sensitivity of XPS more precisely identified the transition from surface CuO_x species to three-dimensional CuO microcrystallites at 10.2 Cu per nm², thus approximating the monolayer coverage. Catalytic activity testing was performed revealing a maximum areal activity for NO decomposition of 0.0022 [(μmol NO converted to N₂) m⁻² s⁻¹] at 450 °C over the catalyst closest to monolayer coverage: 9.9 Cu per nm² CuO_x/Co₃O₄. The function of the monolayer CuO_x species was examined by performing oxygen adsorption/desorption cycles, which revealed that highly dispersed CuO_x surface species (monolayer and sub-monolayer) better retain oxygen storage capacity compared to bulk CuO microcrystals. Finally, an internally consistent comparison to a Cu-ZSM5 catalyst was made, and the areal activity of the monolayer CuO_x/Co₃O₄ catalyst was an order of magnitude higher at 350 °C, largely due to a 34% increase in selectivity to N₂. This structure–activity relationship for NO decomposition over an oxide catalyst is expected to guide future design of improved NO decomposition catalysts and continue to push the boundary for activity to lower temperatures.