Plasma driven exsolution from perovskite oxide for catalytic application

Abstract

High-performance nanoparticle platforms can drive catalysis progress to new horizons, delivering environmental and energy targets. Nanoparticle exsolution offers unprecedented opportunities that are limited by current demanding process conditions. Unravelling new exsolution pathways, particularly at low temperatures, represents an important milestone that will enable improved sustainable synthetic route, more control of catalysis microstructure as well as new application opportunities. It is demonstrated that plasma direct exsolution at room temperature represents just such a step change in the synthesis. Moreover, factors were identified that most affect the exsolution process. It is shown that the surface defects produced initiate exsolution under a brief ion bombardment of both low and atmospheric pressure and low-temperature plasma. This resulted in controlled nanoparticles within 100 nm range with very high number densities thus creating a highly active catalytic material which rivals traditionally created exsolved samples.

A non-stoichiometric (A-site deficient) perovskite oxide (POx) having a composition of La0.43Ca0.37Ti0.94Ni0.06O2.955(LCTN) treated in low pressure plasma in argon atmosphere results in exsolution of nanoparticles. Particle size and density of the exsolved nanoparticles were optimized by tuning the treatment time, plasma gas and plasma power, which also helps in understanding the mechanism involved in plasma enhanced exsolution. The average particle size lies within the 19-100 nm range for different plasma parameters. Population density of the exsolved nanoparticles is a parameter directly responsible for achieving higher catalytic activity in such catalyst. Higher population density of the exsolved nanoparticles can be translated into higher catalytic site availability. The population density in thermal plasma generally in the order of 10 ranging from 7-90 nanoparticles/μm-2.In this study, the exsolved nanoparticles density was optimized to ~500 particles/μm2 when process parameters were tweaked. The effect of exsolved nanoparticles was analysed by employing CO catalytic testing to access CO2 production rate. The catalytic reactivity of the plasma exsolved LCTN sample improves significantly when compared with pristine LCTN. In term of catalytic performance plasma exsolved LCTN sample compete well (even gets superior in many case) with other conventionally prepared sample with a Ni loading as low as 0.06 mole fraction on B-site.

Increasing with the understanding of plasma driven exsolution, we have demonstrated the A-site exsolution of silver (Ag) NPs from perovskite oxide scaffold through low and atmospheric pressure plasma. Perovskite oxide (POx) having a composition of La(1-x)AgxFeO3 (LAFO) with varying Ag doping was synthesized and further treated in low pressure and atmospheric pressure plasma. The highlighting feature of this part of research was to demonstrate the capability of plasma to show exsolution not only in transition metals but also in noble metals and also to demonstrate the plasma exsolution of metal from A-site of perovskite oxide matrix. It is pertinent to mention that exsolution of noble mobile metal with low level of doping is very challenging due thermodynamic and kinetic reason. These Ag exsolved nanoparticles has shown promising electrocatalytic properties compared to the pristine sample. In addition, an initial attempt has also been demonstrated using atmospheric pressure micro plasma jet to exsolve nanoparticles at specific location on the sample, which is difficult to achieve during thermal reduction process.

Date of Award	Jan 2024
Original language	English
Supervisor	Davide Mariotti (Supervisor) & Paul Maguire (Supervisor)