Insight into carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming by using density functional theory

Abstract

A density-functional theory method has been conducted to investigate carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming. In the process of carbon deposition formation, CH4 dissociation to produce C, further C accumulates to form carbon deposition. In addition, C and its precursor can be eliminated by oxygen species from CO2 dissociation. Based on above analysis about carbon deposition, three possible ways are considered to contribute to resist carbon deposition (1) reducing CH4 dissociation to decrease C produced; if not, (2) accelerating CHx(x=0∼3) oxidation to eliminate C, (3) blocking C accumulation, further suppressing carbon deposition, that is, the rate of CHx oxidation is more than that of C accumulation. The calculation results show that the dissociation of CH4 has almost equal activation energy on perfect and defective NiCo/MgO, and they are lower than those on Ni/MgO and NiCo(111), indicating that it is impossible to reduce C produce through reducing CH4 dissociation when addition a second metal Co to Ni/MgO or using different support MgO; CH oxidation is favorable to produce CHO compared to CH dissociation into C and H, further it is favorable for CHO dissociation into CO and H on perfect and defective NiCo/MgO catalyst. In addition, C oxidation is easier on defective NiCo/MgO than that on perfect NiCo/MgO, meanwhile it is easier on NiCo/MgO than that on NiCo(111), indicating that using defective MgO support accelerates C oxidation, further elimination carbon deposition. Carbon accumulation is easier on perfect NiCo/MgO than that on defective NiCo/MgO. The results indicate that the defective MgO surface support NiCo bimetal catalyst is more resistant to carbon deposition than perfect one. One can modify the Ni-based catalyst behavior via addition a second metal Co or using MgO support with oxygen-vacancy. This might be given a clue for experimental catalyst to develop Ni-based catalyst with the property of resisting carbon deposition.