Effect of crystal phases and defect formation on photocatalysis of reduced Pt/ZrO_2−x

抄録

Introduction

Zirconium oxide (ZrO₂) has attracted attention as a catalyst material, which exhibits activity in various reactions because of its excellent chemical properties. On the other hand, it has a large band gap of about 5 eV, making it difficult to utilize sunlight as a photocatalyst. Defect introduction into semiconductor oxide is a useful technique to narrow its inherent band gap which leads to a broadening of the light absorption for photocatalytic applications.^{[1, 2]} Recently, this technique has been used to endow large band gap metal oxides, which have been difficult to be used as photocatalysts, with photocatalytic activity. Reduced ZrO_2−x, in which oxygen vacancies (V_O) or Zr³⁺ are formed, is one of the promising materials which exhibit photocatalytic activity by defect formation.^[3]

Crystal phases of ZrO₂ can be changed by different temperatures and pressures. Among them, Monoclinic and Tetragonal can exist stably under mild conditions, which makes them suitable for catalytic reaction. However, the effect of crystal phases of ZrO₂ on the photocatalytic activity has rarely been studied.

In this study, we performed hydrogen reduction treatments on two types of ZrO₂, Monoclinic and Tetragonal, to investigate the effect of crystal phases and defect formation on the photocatalytic activity.

Experimental

ZrO₂ with monoclinic (m-) or tetragonal (t-) crystal phase was synthesized in a previously reported method.^[4] Then, Pt was deposited on ZrO₂ at 1 wt%. The Pt/ZrO₂ was subsequently reduced at 200, 400, or 600 °C under H₂ flow to obtain Pt/ZrO_2−x. The prepared Pt/ZrO_2−x photocatalysts were evaluated by X-ray diffraction (XRD), electron spin resonance (ESR), and so on. The photocatalytic activity in H₂ production was investigated with aqueous methanol solution under UV light (λ = 365 nm) irradiation.

Results and Discussion

Fig. 1 (a, b) shows the XRD patterns of the Pt/ZrO_2−x after reduction under various conditions. No phase transition was observed for Pt/m-ZrO_2−x. For Pt/t-ZrO_2−x, the diffraction pattern did not change after reduction below 400 °C, but a peak attributed to Monoclinic appeared after 600 °C reduction.

Fig. 1 (c, d) shows the ESR spectra of Pt/ZrO_2−x after reduction under various conditions. In the Pt/m-ZrO_2−x spectrum, the signal attributed to Zr³⁺ was observed before reduction, and a new signal appeared at 200 °C reduction, which was attributed to oxygen vacancies. When the reduction temperature increased above 400 °C, the signal attributed to Zr³⁺ disappeared, while the intensity due to oxygen vacancies increased. On the other hand, in Pt/t-ZrO_2−x, oxygen vacancies were present even before reduction, and their amount increased significantly with increasing reduction temperature. In addition, the amount of oxygen vacancies was much larger in Pt/t-ZrO_2−x than in Pt/m-ZrO_2−x. Therefore, it is clear that, Zr³⁺ disappears and oxygen vacancies are formed by reduction treatment in Pt/m-ZrO_2−x, while the amount of oxygen vacancies increases by reduction treatment in the Pt/t-ZrO_2−x.

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