Catalytic activity and stability of nanometic Rh overlayers prepared by pulsed arc-plasma deposition and r.f. magnetron-sputtering

Highlights: • Nanometric Rh overlayer catalysts prepared by arc-plasma deposition and magnetron-sputtering are compared. • Both catalysts demonstrated 50-fold high turnover frequencies for NO−CO reaction compared to powder catalysts. • The overlayer catalyst prepared by arc-plasma deposition exhibits higher stability under TWC reaction condition. 50 μm-thick Fe−Cr−Al metal foils covered by 7 nm-thick Rh overlayers were prepared by pulsed arc-plasma (AP) and r.f. magnetron sputtering technique to compare their catalytic activities. As-prepared metal foil catalysts were wrapped into a honeycomb structure with a density of 900 cells per square inches and the stoichiometric NO−CO−C₃H₆−O₂ reaction was performed at space velocity of 1.2 × 10⁵ h⁻¹. During temperature ramp at 10 °C min⁻¹, honeycomb catalysts showed steep light-off of NO, CO, and C₃H₆ at above 200 °C and their conversions soon reached to almost 100%. Both catalysts exhibited high turnover frequencies close to or more than 50-fold greater compared with those for a reference Rh/ZrO₂ powder-coated cordierite honeycomb prepared using a conventional slurry coating. When the temperature ramping was repeated, however, the catalytic activity was decreased to the different extent depending on the preparation procedure. Significant deactivation occurred only when prepared by sputtering, whereas the sample prepared by AP showed no signs of deactivation. The deactivation is associated with the formation of passivation layers consisting of Fe, Cr, and Al oxides, which covered the surface and decreased the surface concentration of Rh. The Rh overlayer formed by AP was found to be thermally stable because of the strong adhesion to the metal foil surface, compared to the sample prepared by sputtering.