In this study, we successfully developed nickel (Ni)-modified
tungstate
zirconia (Ni/WO3/ZrO2) as an effective material
for chemical looping dry reforming of methane (CL-DRM) under isothermal
conditions (750 °C). The performance of Ni/WO3/ZrO2 strongly depended on the WO3 loading amount. The
optimal amount of 10.0 wt% WO3, corresponding to low surface
W density (<4 W atoms/nm2), in Ni/WO3(10)/ZrO2 resulted in the exclusive formation of dispersed tungstate
species, such as isolated monotungstate and/or oligomeric polytungstate
species. Increasing the loading amount to 30.0 wt% (higher surface
W density (>4 W atoms/nm2)) resulted in the formation
of
crystalline WO3 species, diminishing the CL-DRM performance
over Ni/WO3/ZrO2. Comprehensive characterization,
including in situ/operando W L3-/L1-, Ni K-, and Zr K-edge X-ray absorption spectroscopy
studies, revealed that surface dispersed tungstate species were reduced
by CH4 to yield CO and H2, and low-valent W
species assignable to W0 to W3+. The low-valent
W species were reoxidized by CO2 to produce CO and regenerate
the original W6+ species. These active tungstate species
cooperatively work with Ni over ZrO2 surface to promote
partial CH4 oxidation, leading to efficient CL-DRM.