Transition Metal Oxides as Accelerants for Direct Interspecies Electron Transfer in Syntrophic-Methanogenic Associations: A Dft Hse Study

Abstract

In biogas production, ineffective charge shuttling in the bio-digestion process leads to the accumulation of fatty acids over methane. Research into the understanding and engineering of the direct interspecies electron transfer (DIET) based syntrophic process has emerged to improve methanogenesis kinetics in anaerobic digestion by reducing fatty acid accumulation. Although oxides help in this regard, they have been identified to play either a catalytic or inhibitory role in biogas production. Previous experimental investigations show that transition metal oxides like tungsten oxide and hafnium oxide accelerate DIET, whiles zinc oxide and copper oxide inhibit DIET and, thus, methanation. The density functional theory (DFT) method at the Heyd-Scuseria-Ernzerhof (HSE) hybrid exchange-correlation functional level of theory has been employed in this work to elucidate the microscopic phenomena and to understand the role of the metal oxide chemistry on the initial mechanisms of the DIET. The electronic properties, the quantity of charge shuttled from the acetogen into the methanogen at transition metal oxide surfaces, and the effect of conductivity of the cost-effective transition metal oxides (i.e., ZnO, CuO, HfO2 and WO2) on the anaerobic digestion process have been studied. The results show that an enhanced DIET process can be ascribed to the superior electron transmission ability of HfO2 and WO2 in anaerobic digestion systems. It is also found that the Meso-diaminopimelic acid, N-acetylglucosamine and N-acetylmuramic acid form more stable complexes in all cases.