Paper HC+SA+SS-WeA7
In-situ Investigation of Methane Activation on MO_x/CeO₂ (111) Surfaces {M=Co, Ni and Cu} using Ambient-Pressure XPS

Wednesday, November 1, 2017, 4:20 pm, Room 24

Natural gas has transformed the energy landscape of this nation and has fast become a cheap and abundant fuel stock. Methane is the primary component of natural gas but is difficult to convert it to upgraded fuels or chemicals due to the strong C-H bond in methane (104 kcal/mol). This challenge constitutes one of the most difficult problems in heterogeneous catalysis. We have discovered a catalyst with small Ni nanoparticles supported on ceria that has shown promising activity for both methane activation and dry reforming of methane. Then we expanded the study to other transition metals (Co and Cu) supported on ceria in order to rationalize the structure-reactivity relationships for methane activation. Due to the chemically inert nature of methane, the experiment needs to be conducted at elevated pressure via the utilization of Ambient Pressure of XPS. Nanoparticles or clusters of Co and Cu were deposited onto the well-defined CeO₂(111) surface. Strong metal-oxide interactions were found upon annealing the deposited surfaces to 700 K, leading to the generation of MO_x. In-situ AP-XPS showed that the CoO_x/CeO₂(111) interacted strongly with 50 mTorr of methane, resulting in the formation of Co/CeO_x(111), while no obvious changes were observed on the CuO_x/CeO₂(111) surface (figure 1). By comparing it with the NiO_x/CeO₂(111) surface, it can be found that the methane activation on these MO_x/CeO₂ (111) surfaces follow the order: Co > Ni > Cu. The methane dry reforming activity was also investigated on the CoO_x/CeO₂(111) surface by sequentially adding another 50 mTorr of CO₂ into the system. The slight reoxidation of the ceria surface indicates the participation of CO₂ in the catalytic cycle by the following steps: CH₄(g) → CH_4-x(a) + H(a) with x=1-4; CO₂(g) → CO(a) + O(a); C(a) + O(a) → CO(g); H(a) + H(a) → H₂(g).