Effects of Calcination Temperature and Acid-Base Properties on Mixed Potential Ammonia Sensors Modified by Metal Oxides

Mixed potential sensors were fabriated using yttria-stabilized zirconia (YSZ) as a solid electrolyte and a mixture of Au and various metal oxides as a sensing electrode. The effects of calcination temperature ranging from 600 to 1,000 °C and acid-base properties of the metal oxides on the sensing properties were examined. The selective sensing of ammonia was achieved by modification of the sensing electrode using MoO3, Bi2O3 and V2O5, while the use of WO3, Nb2O5 and MgO was not effective. The melting points of the former group were below 820 °C, while those of the latter group were higher than 1,000 °C. Among the former group, the selective sensing of ammonia was strongly dependent on the calcination temperature, which was optimum around melting point of the corresponding metal oxides. The good spreading of the metal oxides on the electrode is suggested to be one of the important factors. In the former group, the relative response of ammonia to propene was in the order of MoO3 > Bi2O3 > V2O5, which agreed well with the acidity of the metal oxides. The importance of the acidic properties of metal oxides for ammonia sensing was clarified

Silver Cluster Catalysts for Green Organic Synthesis

Development of platinum-group-metal (PGM)-free　　catalysts for green chemical synthesis is an important topic in synthetic catalysis, because PGM will soon be in short supply in the near future. Our group has paid attention to the catalytic functions of Ag clusters. Here, we summarize our recent work on size- and support-specific catalysis of Ag clusters for green organic synthesis. Ag clusters supported on Al2O3 act as effective heterogeneous catalysts for (1) oxidant-free dehydrogenation of alcohols to carbonyl compounds, (2) coupling of alcohols with amines to form amides and H2, (3) N-alkylation of anilines with alcohols, (4) C_C cross-coupling reaction of alcohols, (5) selective hydrogenation of nitroaromatics, and (6) direct synthesis of N-substituted anilines from nitroaromatics and alcohols. To establish a catalyst design concept, effects of Ag particle size and acid-base character of support oxides are investigated. The structure-activity relationships for all reactions show similar tendencies; metallic Ag clusters with smaller size and acid-base bifunctional nature of the support oxide are preferable. We propose that cooperation between coordinatively unsaturated Ag sites of Ag clusters and acidbase pair sites at the metal-support interface is a key concept for the design of Ag cluster catalysts for the above reactions

A Theoretical Insight into Enhanced Catalytic Activity of Au by Multiple Twin Nanoparticles

Recently, it has been reported that the morphology of Au nanoparticles (NPs) affects the catalytic activity of CO oxidation; twin crystal NPs show higher activity for CO oxidation than single-crystal NPs. In this study, density functional calculations have been carried out to investigate the morphology effect of Au NPs using CO as a probe molecule. In the case of Au NPs with a size of more than 2 nm, CO adsorption energy on the Au NPs is mainly determined by a coordination number (CN) of adsorption sites. CO binding to a multiple twin NP with a size of about 1 nm is stronger than that on a single-crystal NP with the same size. A simple CN explanation cannot be applied to the enhancement of CO binding to the small multiple twin NP. This enhancement is related to a deformation of the NP structure before and after CO adsorption. It is suggested that the multiple twin NP with a size of less than 1 nm, which shows the deformation upon CO adsorption, contributes to the higher activity for CO oxidation