Search for: All records

Urea is a common waste in agriculture runoff and has also been proposed as a promising oxidizable molecule in urea electrolysis for hydrogen production from wastewater. However, the overpotential of the electrochemical urea oxidation reaction (UOR) is high due to the complicated six-electron transfer process on most metal catalysts. The competition with oxygen evolution reaction (OER) further limits catalyst options for UOR. The most promising and studied catalysts for UOR are Ni-based catalysts. Here we study the reactivity of the basal β-NiOOH(001) surface for UOR and study the effects of metal doping (Mn, Fe, Co, and Cu) on the phase transformation from β-Ni(OH)2 to β-NiOOH, UOR, and OER pathways using density functional theory (DFT) calculations. The introduction of Mn and Fe dopants facilitates the formation of catalytically active β-NiOOH phase, and also favors the adsorption of urea compared to the undoped β-NiOOH surface, thereby significantly benefitting the overall UOR. Moreover, comparison of the effect of dopants on UOR and OER provides fundamental understanding of the competition between UOR and OER and how the dopants influence the reaction selectivity and competition. This work sheds light on the structure-property relationship of Ni-catalysts in urea oxidation and provides design principles for functional Ni-based materials, which will help accelerate the development of efficient UOR catalysts. 

Abstract Ni-based superalloys offer a unique combination of mechanical properties, corrosion resistance and high temperature performance. Near ambient pressure X-ray photoelectron spectroscopy was used to study in operando the initial steps of oxidation for Ni-5Cr, Ni-15Cr, Ni-30Cr and Ni-15Cr-6W at 500 °C, p(O 2 )=10 −6 mbar. The comparison of oxide evolution for these alloys quantifies the outsized impact of W in promoting chromia formation. For the binary alloys an increase in chromia due to Cr-surface enrichment is followed by NiO nucleation and growth thus seeding a dual-layer structure. The addition of W (Ni-15Cr-6W) shifts the reaction pathways towards chromia thus enhancing oxide quality. Density functional theory calculations confirm that W atoms adjacent to Cr create highly favorable oxygen adsorption sites. The addition of W supercharges the reactivity of Cr with oxygen essentially funneling oxygen atoms into Cr sites. The experimental results are discussed in the context of surface composition, chemistry, reactant fluxes, and microstructure.