High reduction temperature generally induces the agglomeration of supported noble metals. Howerve, we found that high temperature reduction did not induce Pd particles sintering but improved Pd dispersion. Multiple methods were further carried out to illuminate the abnormal phenomenon. The results indicated more surface oxygen defects and diffusion of Pd particles were simultaneously induced by high temperature reduction. During diffusion process of Pd particles, they were trapped by the oxygen defects because of the strong metal-support interaction, which led to improvement of Pd dispersion on the Pd/TiO₂-450R catalyst. In addition, more surface oxygen vacancies on the Pd/TiO₂-450R catalyst resulted in more active sites of H₂O activation to form abundant surface OH groups which further enhanced adsorbed O₂ activation and mobility, and then opening a more effective pathway for HCHO oxidation, which result in its high activity of Pd/TiO₂-450R for ambient HCHO oxidation.