Monolayer supported CuOx/Co3O4 as an active and selective low temperature NOx decomposition catalyst
A NOx decomposition catalyst, copper supported on cobalt oxide (CuOx/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 CuOx/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 CuOx 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 CuOx/Co3O4. The function of the monolayer CuOx species was examined by performing oxygen adsorption/desorption cycles, which revealed that highly dispersed CuOx 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 CuOx/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.