Preferential Oxidation of H₂in CO-Rich Streams over a Ni/γ-Al₂O₃Catalyst: An Experimental and First-Principles Microkinetic Study

The shift toward unconventional CO/CO2 sources for chemicals production is accompanied by several challenges, e.g., those associated with their composition. The use of CO/CO2-rich steel mill gases as an alternative feedstock promotes carbon circularity, contributing to reducing the industry's carbon footprint. The upgrading of steel mill gases as a carbon source in some cases requires efficient and selective H2 removal. This study investigates the preferential oxidation of H2 in CO-rich streams, resembling blast furnace steel mill gas, over a 15 wt % Ni/γ-Al2O3 catalyst combining experimental and modeling techniques. A reaction temperature of 310 °C was selected to prevent carbonyl-induced sintering for CO partial pressures of 26 kPa and higher. Despite the excess CO, an O2-to-H2O selectivity of 65% was achieved at full O2 conversion for an optimum H2/O2 inlet ratio of 3.3. By investigating the effects of the CO and H2 inlet partial pressures, a relation between the CO/H2 inlet ratio and the water selectivity was established. Catalyst stability was confirmed over a 24 h oxidation test. In situ TPO showed negligible amounts of deposited carbon, and subsequent XRD analysis showed only a minor change in diffraction patterns. First-principles microkinetic modeling attributes the high water selectivity over O*-saturated Ni(111) to a 33 kJ mol-1 difference in barrier between H∗ and CO∗ oxidation, which compensates the low H*/CO∗ coverage ratios. The model further highlights the relation between the CO/H2 inlet ratio and the water selectivity with changes in the CO*/H∗ coverage ratio. The microkinetic model predicts a water selectivity of 91% at 310 °C, significantly higher than the experimental data. Combining several experimental tests with characterization techniques, we attribute the somewhat lower experimental selectivity to WGS activity, possible formation of surface NiO species that are highly active for CO-PROX, and undercoordinated Ni sites that are active for CO dissociation.