Chemical Looping Dry Reforming of Methane over Ni-Modified WO<sub>3</sub>/ZrO<sub>2</sub>: Cooperative Work of Dispersed Tungstate Species and Ni over the ZrO<sub>2</sub> Surface
Shinta Miyazaki, Zirui Li, Lingcong Li, Takashi Toyao, Yuta Nakasaka, Yasushi Nakajima, Ken‐ichi Shimizu, Zen Maeno
Abstract
In this study, we successfully developed nickel (Ni)-modified tungstate zirconia (Ni/WO 3 /ZrO 2 ) as an effective material for chemical looping dry reforming of methane (CL-DRM) under isothermal conditions (750 °C). The performance of Ni/WO 3 /ZrO 2 strongly depended on the WO 3 loading amount. The optimal amount of 10.0 wt% WO 3, corresponding to low surface W density (<4 W atoms/nm 2 ), in Ni/WO 3 (10)/ZrO 2 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/nm 2 )) resulted in the formation of crystalline WO 3 species, diminishing the CL-DRM performance over Ni/WO 3 /ZrO 2 . Comprehensive characterization, including in situ / operando W L 3 -/L 1 -, Ni K-, and Zr K-edge X-ray absorption spectroscopy studies, revealed that surface dispersed tungstate species were reduced by CH 4 to yield CO and H 2, and low-valent W species assignable to W 0 to W 3+ . The low-valent W species were reoxidized by CO 2 to produce CO and regenerate the original W 6+ species. These active tungstate species cooperatively work with Ni over ZrO 2 surface to promote partial CH 4 oxidation, leading to efficient CL-DRM.