Self-regeneration of a Pd-perovskite catalyst for automotive emissions control

Abstract

Catalytic converters are widely used to reduce the amounts of nitrogen oxides, carbon monoxide and unburned hydrocarbons in automotive emissions. The catalysts are finely divided precious-metal particles dispersed on a solid support. During vehicle use, the converter is exposed to heat, which causes the metal particles to agglomerate and grow, and their overall surface area to decrease. As a result, catalyst activity deteriorates. The problem has been exacerbated in recent years by the trend to install catalytic converters closer to the engine, which ensures immediate activation of the catalyst on engine start-up, but also places demanding requirements on the catalyst's heat resistance. Conventional catalyst systems thus incorporate a sufficient excess of precious metal to guarantee continuous catalytic activity for vehicle use over 50,000 miles (80,000 km). Here we use X-ray diffraction and absorption to show that LaFe(0.57)Co(0.38)Pd(0.05)O(3), one of the perovskite-based catalysts investigated for catalytic converter applications since the early 1970s, retains its high metal dispersion owing to structural responses to the fluctuations in exhaust-gas composition that occur in state-of-the-art petrol engines. We find that as the catalyst is cycled between oxidative and reductive atmospheres typically encountered in exhaust gas, palladium (Pd) reversibly moves into and out of the perovskite lattice. This movement appears to suppress the growth of metallic Pd particles, and hence explains the retention of high catalyst activity during long-term use and ageing.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

“…Changing the reaction atmosphere has also been exploited for improving the catalyst lifetime: In so‐called “intelligent catalysts”, the oxidation of Pd nanoparticles was used to incorporate Pd into a perovskite lattice (e.g. Pd and LaFeO 3 to LaFe 0.95 Pd 0.05 O 3 ) 36, 37. Under reducing conditions Pd moves back to the surface inducing a self‐regeneration of the catalyst since the metal particles are effectively redispersed on the surface.…”

“…Sintering (Figure 4 b) describes the agglomeration and gradual growth of nanoparticles. Usually this is connected to a decrease of the catalytic activity of the catalyst which is a common problem in heterogeneous catalysis,13, 36, 37, 44 as well as in electrocatalysis 16. Systems composed of several components often feature strong metal–support interactions.…”

Combustion activities of the Ru catalysts supported on hexagonal YbFeO3

“…1) 5) Orthorhombic REFeO 3 (o-REFeO 3 ) with the perovskite structure has recently received much attention in the catalyst field. For example, Nishihata et al 6) reported that LaFe 0.57 Co 0.38 Pd 0.05 O 3 with the perovskite structure is successfully applied as a catalyst for the vehicle exhaust treatment. Andrews et al reported that LaFe 0.95 Pd 0.05 O 3 is an effective catalyst for Suzuki Miyaura coupling reaction in organic chemistry.…”

Preparation of LaFe0.95Pd0.05O3 perovskites and their catalytic performances in methane combustion

“…Partial substitution of B-site ions with other ions is expected to modify the perovskite structure and thus improve the catalytic performance. A typical example is that doping very small amounts of Pd to LaFeCoO 3 and LaFeO 3 can dramatically promote the catalytic durability in vehicle exhaust abatement [4,6]. LaFe 0.57 Co 0.38 Pd 0.05 O 3 , even after long-term ageing at 1173 K, still displayed superior activity and stability to a Pd/Al 2 O 3 catalyst [6].…”

“…A typical example is that doping very small amounts of Pd to LaFeCoO 3 and LaFeO 3 can dramatically promote the catalytic durability in vehicle exhaust abatement [4,6]. LaFe 0.57 Co 0.38 Pd 0.05 O 3 , even after long-term ageing at 1173 K, still displayed superior activity and stability to a Pd/Al 2 O 3 catalyst [6]. The Pd species, in response to the inherent redox fluctuations in the automobile exhausts, reversibly and rapidly incorporated into and moved out of the perovskite lattice and thereby the sintering of Pd grains could be suppressed [4,6−8].…”