Unraveling hydrogen spillover pathways of reducible metal oxides

抄録

Introduction. Hydrogen spillover is a dynamic phenomenon which is initiated by the dissociation of hydrogen molecules followed by the migration onto the reducible metal oxide supports.[1] Spilled hydrogen atom shows specific behavior compared with gaseous hydrogen, thus it dramatically promotes the performance of various functional materials, such as heterogeneous catalysts, hydrogen storage materials and hydrogen fuel cells. Because of the present movement toward a hydrogen-based society, hydrogen spillover has received increasing interest both in academia and industry.[2] However, understanding of its dynamic behavior, such as at what temperature it can take place, and which pathway it follows, is still lacking because the observation method is not well established.[3] Our group has reported Ru and Ni, an immiscible metal combination, specifically form RuNi solid solution alloy by an assist of hydrogen spillover on the surface of TiO₂.[4] In this work, we evaluated hydrogen spillover pathways in typical reducible metal oxide such as TiO₂, CeO₂ and WO₃ by combining Ru-Ni alloy nanoparticle formation, in-situ techniques, kinetic analysis, and density functional theory calculation.

Experimental/methodology. RuCl₃·3H₂O and NiCl₂·6H₂O were supported on TiO₂ by conventional impregnation method, after which the sample were reduced under H₂ dosage at a heating rate of 5 °C/min up to 300 °C to obtain RuNi/TiO₂. Catalysts with CeO₂ and WO₃ as their supports were also prepared by the same method. In order to evaluate the alloy effect of Ru and Ni in the catalytic reaction, the prepared catalyst was applied to the hydrogen production reaction from ammonia borane (AB ; NH₃BH₃).

Results and discussion. RuNi/TiO₂ and RuNi/CeO₂ showed enhanced activity over those of corresponding monometallic Ru catalysts in the hydrogen production reaction from AB indicating the formation of RuNi solid solution alloy nanoparticles. On the other hand, RuNi/WO₃ didn’t show any activity improvement compared with that of Ru/WO₃ indicating the segregation of Ru and Ni. In-situ XAFS measurement showed that Ru³⁺ and Ni²⁺ were simultaneously reduced over TiO₂ and CeO₂ although the redox potential of Ni²⁺ is much lower than that of Ru³⁺. In contrast, Ru³⁺ and Ni²⁺ were sequentially reduced according to their distinct redox potentials. These results suggest that hydrogen spillover occurred on the surface of TiO₂ and CeO₂, while hydrogen spillover is absent on the surface of WO₃. Systematic characterization combining in-situ DRIFT and MS measurements under the H/D exchange reaction were performed in order to study the dynamic behavior of hydrogen spillover over each support. In-situ DRIFT measurements under the H₂/D₂ switched atmosphere revealed that hydrogen spillover takes place at 50 °C, 150 °C, and 250 °C on the surface of TiO₂, CeO₂, and WO₃, respectively. Moreover, MS measurements demonstrated that hydrogen spillover on TiO₂ and CeO₂ preferentially proceeded on their surface rather than within their bulk phases. Conversely, hydrogen spillover favorably occurred within the bulk prior to the surface over WO₃. (Figure 1).[5]

References

[1] R. Prins, Chem. Rev., 112, 2714 (2012).

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