Na-promoted Ni/ZrO2 dry reforming catalyst with high efficiency: details of Na2O–ZrO2–Ni interaction controlling activity and coke formation

Abstract

Herein, a 0.6 wt% Na-promoted 3% Ni/ZrO₂ dry reforming catalyst and its unpromoted counterpart are discussed in detail. Structural investigations were carried out using TEM, TPR, XRD, XPS, and CO pulse chemisorption followed by TPD and DRIFTS methods. In the presence of a dry reforming mixture, bidentate carbonates were detected on Na-promoted Ni/ZrO₂, while on Ni/ZrO₂ effective hydrogenation by metallic Ni converted bicarbonates to formate species. In continuous flow atmospheric catalytic tests in a high excess of methane, a reactive-type coke was formed on the promoted sample, which did not cause significant deactivation. Temperature ramped ¹³CO₂ isotope labeled dry reforming experiments in a closed loop sub-atmospheric circulation system revealed ¹³CH₄ formation on Ni/ZrO₂, while in the case of the promoted catalyst methanation was retarded until the complete consumption of oxidants (from ¹³CO₂). In isothermal experiments in the same circulation system carbon monoxide disproportionation was observed on Ni/ZrO₂ leaving carbon on Ni, besides the coke formed from the CH₄ source, while on the promoted catalyst carbonaceous deposit under the same conditions did not form from CH₄. The superb catalytic properties of Na-promoted Ni/ZrO₂ are explained by a proposed catalytic cycle compiling the dynamic participation (formation and decomposition) of the surface Na₂CO₃ or NaHCO₃ species surrounding the NiO_xH_y active sites on a ZrO₂ support that is able to accommodate the labile Na₂O promoter capturing and releasing CO₂ oxidant.