Satoshi Hinokuma

Effect of thermal ageing on the structure and catalytic activity of Pd/CeO2 prepared using arc-plasma process

Pd nanoparticles were deposited onto CeO2 powders using an arc-plasma process in order to study their thermal behaviour and catalytic activity for CO oxidation. As-prepared catalyst exhibited a higher catalytic activity than catalysts prepared via conventional wet-impregnation because the metallic state of Pd responsible for CO oxidation activity is abundant. The activity was decreased by thermal ageing at 600 °C in air, which oxidized metallic Pd to the oxide interacting with the surface of CeO2. On the other hand, ageing at 900 °C in 10% H2O/air significantly enhanced the activity, regardless of sintering of CeO2, to show the highest activity, as observed in our previous work for the catalyst prepared by conventional wet-impregnation. The activation by thermal ageing is associated with the thermodynamic dissociation of the oxidic Pd to metallic Pd species at 900 °C.

CO oxidation activity of thermally stable Fe–Cu/CeO2 catalysts prepared by dual-mode arc-plasma process

Fe–Cu bimetal nanoparticles were deposited on CeO2 powders at a low loading level (≤0.2 wt%) by the dual-mode arc-plasma (AP) process. The bimetal particles enabled the formation of Fe–Cu oxide after being thermally aged at 900 °C, which was characterised by a highly dispersed Cu+ and Fe3+ on the surface of CeO2. The CO oxidation activity was enhanced by thermal aging at 900 °C when prepared by the AP process compared to the conventional wet-impregnation method, despite the significant decrease in surface area from 170 m2 g−1 to less than 10 m2 g−1. The higher activity can be rationalised by the more efficient formation of Fe–Cu oxide, the Cu+ species of which plays the role of CO chemisorption site. The reaction rate of CO oxidation exhibited partial orders of ~0.9 and ~0.0 with respect to CO and O2, respectively. Pulsed C16O–18O2 reactions revealed that CO adsorbed on Cu+ reacted mainly with lattice oxygen, which implies that the reaction proceeds via the Mars–van Krevelen mechanism.

Nanoparticle catalyst preparation using pulsed arc plasma deposition

A novel nanoparticle preparation technique using pulsed arc plasma deposition has been developed as a dry catalyst preparation process in complete contrast to conventional wet processes. An overview of the principle, fundamental features of the technique and a discussion of the recent progress are presented using examples from our recent publications. The potential for preparing several types of supported metal and metal oxide catalysts with higher activities at minimum metal loadings is also highlighted.

A nanometric Rh overlayer on a metal foil surface as a highly efficient three-way catalyst.

Pulsed arc-plasma (AP) deposition of an Rh overlayer on an Fe–Cr–Al stainless steel foil produced a composite material that exhibited high activity for automotive three-way catalysis (TWC). The AP pulses deposited metallic Rh nanoparticles 1–3 nm in size, whose density on the surface increased with the number of pulses. This led to coalescence and grain growth on the foil surface and the eventual formation of a uniform two-dimensional Rh overlayer. Full coverage of the 51 μm-thick flat foil by a 3.2 nm-thick Rh overlayer was achieved after 1,000 pulses. A simulated TWC reaction using a miniature honeycomb fabricated using flat and corrugated foils with the Rh overlayers exhibited successful light-off at a practical gaseous hourly space velocity of 1.2 × 105 h−1. The turnover frequency for the NO–CO reaction over the metallic honeycomb catalyst was ca. 80-fold greater than that achieved with a reference Rh/ZrO2-coated cordierite honeycomb prepared using a conventional wet impregnation and slurry coating procedure. Despite the nonporosity and low surface area of the foil-supported Rh overlayer compared with conventional powder catalysts (Rh/ZrO2), it is a promising alternative design for more efficient automotive catalysts that use less Rh loading.

Local structures and TWC activity of Pd supported on Ni-substituted aluminium oxide borates

A series of transition metal-substituted aluminium oxide borates, MxAl20−xB4O36 (M–10A2B), were investigated as supports for Pd catalysts. The Ni-substituted compound (Ni–10A2B, x = 1.0) exhibited the highest metal dispersion and catalytic activity under simulated three-way catalysis conditions. X-ray absorption fine structure analysis and density functional theory calculations suggested that Ni2+ occupies an octahedral Al site in the bulk structure. Although Ni2+ ions exposed on the surface of the support are unlikely to contribute to chemisorption under stoichiometric and/or lean conditions, considerable NO adsorption on the Ni sites occurred when the catalyst was partially reduced under rich conditions. Because the as-formed nitrosyl species on Ni–10A2B reacted with hydrogen, which was generated by C3H6–H2O and CO–H2O reactions, and subsequently activated by Pd, the Pd/Ni–10A2B catalyst significantly promoted NO conversion under rich conditions.