Evaluating bimetallic Ni-Co oxygen carriers for their redox behavior and catalytic activity toward steam methane reforming

This work deals with the synthesis of bimetallic Ni-Co oxygen carriers with ZrO2 as structure stabilizer via a one-pot sol–gel auto-combustion method and the evaluation of their redox properties and catalytic activity of their reduced state for steam methane reforming. Fresh materials were composed of Ni-Co mixed monoxides, which could be reduced to form a Ni-Co alloy. Conducting reforming experiments for 10 h time-on-stream at 650 °C showed that Ni-Co alloy sites of reduced bimetallic materials, in addition to the high reforming activity, can improve the stability compared to Ni sites of a monometallic material, due to a higher tendency of the latter toward sintering and coke formation. Redox behavior was tested via CH4 reduction and air oxidation cycles in a thermogravimetric analyzer. Increasing the Co content led to a slight decrease of reduction degree, which, however, was not altered over cycles. Both reduction and oxidation steps were found to proceed via a nucleation and nuclei growth reaction mechanism, with Co addition promoting the oxidation kinetics. Ultimately, addition of Co with low Co/(Ni + Co) ratios (≤0.20) could balance adequate degree of reduction (≥80 %) and oxygen transfer capacity (≥0.07 g O/g material) in tandem with stable catalytic activity for steam methane reforming in the reduced state of oxygen carrier.