Mechanism of grain reconstruction of Pd and Pd/Ni wires during Pt–catchment

Abstract

We have investigated industrial Pd and Pd/Ni (91.3/8.7 at.%) wires and gauzes used for Pt-catchment during the ammonia oxidation process, focusing on the origin of the grain reconstruction phenomena. In situ X-ray absorption tomography experiments reveal severe reconstruction of polycrystalline Pd and Pd/Ni wires over the course of 10 days, caused solely by the presence of gaseous PtO2 and dry air. A high partial pressure of PtO2 results in higher Pt-catchment rates, which in turn causes more severe reconstruction of the Pd-based wire. Additionally, the Pt-catchment efficiency is reduced when the Pt-content on the wire surface increases. Scanning electron microscopy accompanied by energy dispersive X-ray analysis reveal that Pt diffuses rapidly into the wire core via grain boundaries already after 1 day of Pt catchment. However, quasi monocrystalline (pre-annealed) Pd and Pd/Ni samples show significantly less Pt-catchment and only a little reconstruction compared to the polycrystalline samples. With support from experiments and simulations, we conclude that bulk diffusion is one of the main limitations for Pt-catchment on both poly- and quasi monocrystalline Pd and Pd/Ni wires. The diffusion limitation causes a high surface concentration of Pt, which in turn limits further Pt-catchment due to a gas-surface equilibrium. The polycrystalline wire can overcome the diffusion limitation by utilizing a rapid grain boundary diffusion, transporting both Pt to the wire core and Pd to the wire surface. However, this transport process is also the cause of grain reconstruction.