Density Functional Theory Investigation of the NiO@Graphene Composite as a Urea Oxidation Catalyst in the Alkaline Electrolyte

Abstract

Developing efficient and low-cost urea oxidation reaction (UOR) catalysts is a promising but still challenging task for environment and energy conversion technologies such as wastewater remediation and urea electrolysis. In this work, NiO nanoparticles that incorporated graphene as the NiO@Graphene composite were constructed to study the UOR process in terms of density functional theory. The single-atom model, which differed from the previous heterojunction model, was employed for the adsorption/desorption of urea and CO 2 in the alkaline media. As demonstrated from the calculated results, NiO@Graphene prefers to adsorb the hydroxyl group than urea in the initial stage due to the stronger adsorption energy of the hydroxyl group. After NiOOH@Graphene was formed in the alkaline electrolyte, it presents excellent desorption energy of CO 2 in the rate-determining step. Electronic density difference and the d band center diagram further confirmed that the Ni(III) species is the most favorable site for urea oxidation while facilitating charge transfer between urea and NiO@Graphene. Moreover, graphene provides a large surface for the incorporation of NiO nanoparticles, enhancing the electron transfer between NiOOH and graphene and promoting the mass transport in the alkaline electrolyte. Notably, this work provides theoretical guidance for the electrochemical urea oxidation work.

Remarkable COx tolerance of Ni3+ active species in a Ni2O3 catalyst for sustained electrochemical urea oxidation

“…However, as evident from experimental studies, under the inuence of applied electrode potential in alkaline medium the catalyst surface will undergo surface reconstruction resulting in a hydroxylated active NiOOH surface before the UOR and this phenomenon is well explored in the literature. 11,43 Accordingly, we have constructed in situ generated dynamic hydroxylated surfaces and found out that the formation of a hydroxylated surface is more facile for Ni In a typical UOR process, the primary step is the adsorption of CO(NH 2 ) 2 (urea) on the surface of the electrocatalyst. Then the subsequent deprotonation of CO(NH 2 ) 2 occurs in an alkaline medium in the presence of OH À ions resulting in CO 2 as the oxidized product.…”

“…Then the subsequent deprotonation of CO(NH 2 ) 2 occurs in an alkaline medium in the presence of OH À ions resulting in CO 2 as the oxidized product. 11,43,44 The detailed mechanism of urea electro-oxidation is given in the ESI (Table S6 †). However, as proposed earlier, the adsorption of CO(NH 2 ) 2 and desorption of CO 2 are the key processes in the electrochemical UOR.…”

A review of hetero-structured Ni-based active catalysts for urea electrolysis

“…97,98 It can be used to study the intermediates, energies, structures, and electron congurations of atoms or molecules in many kinds of electrocatalytic reactions. [99][100][101] It has the advantage to visualize the atomic and electronic structure of real materials and reveal the underlying catalytic mechanism as well as specic reaction pathways. 102 In urea electrocatalysis, some critical results can be obtained by DFT, including Gibbs free energy, charge density analysis, the density of states, and band structures.…”

DFT Study on Methanol Oxidation Reaction Catalyzed by PtmPdn Alloys

“…The DFT calculation was performed using the DMol 3 package integrated into the Materials Studio project [30][31][32]. The exchange-correlation energy was calculated using the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) [33].…”