Density Functional Studies On The Design And Catalytic Mechanism Of Novel Molybdenum/Graphene Nitrogen-fixation Catalyst

Nitrogen exits in all organisms, primarily in form of amino acids (proteins) andnucleic acids (DNA and RNA). The most common industrial transformation method,the Haber process, transfers N2into NH3under high pressure and temperature whilerequiring tremendous amounts of energy. Thus, efficient catalysts working at mildconditions have been explored by chemists for decades.Two catalysts Mo[HIPTN3N]and Mo(L)(N2)2]2(-N2) are able to fix nitrogen under ambient pressure andtemperature, while the catalytic efficiency still needs improvement. The loss ofligand leads to the decrease of reactivity of catalysts (Mo[HIPTN3N] andMo(L)(N2)2]2(-N2)). Theoretical investigations proved that the efficiency ofnitrogen fixation depends on reduction processes. A nitrogen-fixation catalyst basedon graphene, which is hoped to combine the merits of molybdenum (efficientcatalytic activity) and graphene (electron bridge and reservoir), is computationallyevaluated in the present work.The possible reaction mechanism of a graphene catalyst (Mo/N-dopedgraphene) based N2fixation has been studied within density functional theory. Twoclassic (Schrock and enzymatic) routes for the N2fixation have been explored indetail. The enzymatic route is energetically more feasible for N2fixation onMo/N-doped graphene, as it is more energetically favorable for the simultaneousbonding of two N atoms to Mo than the end-on adsorption on Mo. Most steps in thereaction are exothermic while a few processes are slightly endothermic.[MoNH2NH2] tends to generate NH2NH2in weak acid environment because of itsweak Mo-Nα and Mo-Nβ interaction, while [MoNH2NH3] and [MoNH3] could formin strong acid environment.The variation of atomic charge along the reaction processes reveals that thecatalyst can be divided into three function parts with different catalytic roles. TheMo/N is the active center, and the body graphene serves as electronic bridge forelectron transmission, while the graphene periphery region stores and provideselectrons for protonation and reduction. With high charge carrier mobility, graphenefunctioning as electron transmitter and electron reservoir could broaden itsapplications in catalysis.