γ al2o3

Supported Pt NPs catalysts (3 wt%) were prepared by the impregnation method. γ-Al₂O₃ (97 %, STREM Chemicals Inc.) and SiO₂ (99.9 %, Fujifilm Wako Pure Chemicals Co.) were calcined at 973 K for 5 h in the air. The samples were immersed in aqueous H₂PtCl₆ and dried up at 393 K to obtain the 3 wt% Pt precursors. Pt/Al₂O₃ and Pt/SiO₂ were synthesized by the reduction of the precursors under H₂ (36 mL min⁻¹) flow at 773 K for 2 h. The calcination of the precursor synthesized PtOx/Al₂O₃ at 773 K for 5 h in air. Supported Pt NPs were analyzed by FE-TEM (JEM-2010F, JEOL Ltd.) and XRD (MiniFlex600, Rigaku Co. Ltd.). Dielectric properties of as-prepared and vacuum-dried (383 K, 72 h) catalysts were measured by the perturbation method^{42 ,43 (link)} using a 2.45 GHz cavity resonator (TM₀₁₀ mode) equipped with a vector network analyzer (ZND, Rohde & Schwarz, Supplementary Table 1). The amounts of water adsorbed on Pt/Al₂O₃ and Pt/SiO₂ were determined using a thermo-gravimetric analyzer (TGA-51, Shimadzu Co.) under 20 mL min⁻¹ Ar flow (Supplementary Fig. 1).

Rutile TiO₂ (JRC-TIO-6) and anatase TiO₂ (JRC-TIO-8) were kindly supplied by the Catalysis Society of Japan. MgO was purchased from Wako Pure Chemical Industries, Ltd. Ethanol, RhCl₃, CuCl₂·2H₂O, and ZrO₂ were purchased from Nacalai Tesque. γ-Al₂O₃ was obtained from Strem Chemicals, Inc. Ammonia borane (AB, NH₃BH₃) was obtained from the Aldrich Chemical Co. All commercially available compounds were used as received. Distilled water was employed as the reaction solvent.