Papers by Author: Yoshinobu Saito

Abstract: Photocatalytic proton reduction and water oxidation have been studied in a tris(2,2’-bipyridyl)ruthenium complex-catalyst system. Pyrochlore-type oxides have been used as proton reduction catalysts with a sacrificial electron donor (Na2EDTA) at pH 7 and as water oxidation catalysts with a sacrificial electron acceptor (K2S2O8) at pH 3. Rate constants for the proton reduction were estimated on the basis of photochemical processes. Yb2Ru2O7-δ was found to be the most active catalyst for proton reduction and water oxidation catalyst in this system.

Abstract: The activities of the pyrochlores Pb2Ru2-xMxO7-δ (M=Mn,Fe,Co,Ni)[PRM(0x)] toward electrochemical oxygen reduction (EOR) were examined in 0.1 M KOH and 0.05 M H2SO4 aqueous solutions. The onset voltage (Vo) and the efficiency (E4) of 4-electron reduction of oxygen for PRM(0x) were evaluated by semi-steady state voltammetry with rotating ring-disk electrodes. In PRM01, the order of the EOR activities was PRMn01 > PRCo01 > PRNi01 > PRFe01 in both solutions. This was consistent with the variation of the lattice parameters, i.e. PRM01 with a smaller lattice parameter showed higher EOR activity. In addition, PRMn05 showed higher EOR activity than PRMn01. The onset voltages Vo were 0.95 and 0.50 V vs. reversible hydrogen electrode in 0.1 M KOH and 0.05 M H2SO4, respectively, while E4 was almost 100 % in both solutions. These results indicate that the EOR activity of PRM(0x) depends on the incorporated metal ions on the B-sites and their contents.

Abstract: The activity of electrochemical oxygen reduction (EOR) for the pyrochlores Ln2Ru2-XO7-δ (Ln=Pr,Nd,Sm) [LnR] were examined in 0.1 M KOH aqueous solution at 70oC. The onset voltage (Vo) of the oxygen reduction current and the efficiency (E4) of 4-electron reduction of oxygen were evaluated by semi-steady state voltammetry with rotating ring-disk electrodes. PrR with the highest EOR activity showed Vo = ~ 0.85 V vs. reversible hydrogen electrode and E4 values above 80 %. Their Vo and E4 values show that LnR containing Ln with a smaller atomic number has a higher EOR activity, i.e. the order of the activity is PrR > NdR > SmR. This was in good agreement with that of the lattice parameters of LnR. These results indicate that the EOR activity of LnR depends on the kind and/or the size of the lanthanide metal ion on the A-site. PrR and NdR exhibited higher E4 values than known excellent Pb2Ru2O7-δ electrocatalyst containing toxic Pb.