Investigation of surface modification of perovskite with different catalysts

Abstract

The increasing energy requirements of mankind have stimulated the need to search for renewable clean energy in order to protect the environment. Proton Exchange Membrane Fuel Cells (PEMFC) are one of the most promising types of fuel cell among the entire range of power generation devices. However, the high quantity of noble metal catalysts used in PEMFCs hinders their commercialization due to the associated high cost. Decreasing the amount of noble metal catalysts without sacrificing the performance of the fuel cell is therefore desirable.

This thesis explores the possibility of modifying the perovskites with small quantities of the most commonly used catalysts (Ni, Ru and Pd) on its surface, with the intention to create a potentially cost-effective electrode material for PEMFC. The perovskites employed in this thesis include two of the most commonly studied perovskite materials, A-site deficient titanate based perovskites and LaCrO₃ based perovskites. The concept of the modification method is to combine two of the most promising state of the art methods, impregnation and exsolution, to improve the properties of the perovskite. Instead of incorporating the catalyst in the whole lattice of the perovskite, the impregnation method was used to dope the catalyst into the surface of the perovskite only. The exsolution of the doped catalysts from the perovskite was then attempted. This would theoretically produce nanoparticles with certain parts of its body anchored inside the perovskite, thus avoiding the catalyst agglomeration problem which has been reported for the normal impregnation method.

The A-site deficient titanate perovskite used to investigate surface doping modifications is La₀.₄Sr₀.₄GaₓTi₁₋ₓO[sub](3-x/2) (LSGT). At first, a Ni catalyst was used to explore the possibility and optimal experimental conditions for catalyst doping of the perovskite from the surface and subsequent exsolution. The microstructure of the LSGT scaffold was optimized for later experiments. The results from the Ni doping study were then applied to the surface doping of Pd and Ru catalysts. This work demonstrates the possibility of incorporating Ni, Ru and Pd catalysts into the surface of A-site deficient titanate LSGT perovskite, using a pre-reduction treatment. The doped Ni and Pd catalysts managed to exsolve to the surface of the perovskite as nanoparticles after reduction treatment. However, the Ru catalyst did not exsolve under the same experimental conditions. It has been found that different catalysts require different conditions to be able to dissolve into the perovskite for example, the heating atmosphere. In addition, the mechanism of incorporating Pd into the A-site deficient titanate has been studied, showing that the Pd is doped into the A-site deficient titanate in the form of Pd²⁺ with square planar 4-fold coordination on the B-site.

The LaCrO₃ based perovskite employed for studying the incorporation of a catalyst into the surface of the perovskite was La₀.₇₅Sr₀.₂₅Cr₀.₅Mn₀.₅O₃ (LSCM). The possibility of doping ruthenium into the surface of LSCM perovskite with the aid of a ball milling process has been explored by VT-XRD. It has been demonstrated that the Ru catalysts are able to dissolve into the lattice of LSCM perovskite on oxidation and exsolve as nanoparticles upon reduction. In addition, the addition of a Ru catalyst into the LSCM has been shown to improve the reducibility of the perovskite. Then the possibility of incorporating a Pd catalyst into the surface of LSCM perovskite was investigated. It was found that the Pd was unable to be doped into the LSCM perovskite due to the fact that the structure of LSCM was not rigid enough to accommodate the deficiencies introduced by Pd.

Catalytic tests of RWGS reactions were carried out with a few selected samples to preliminarily investigate the influence of the catalyst coating on the performance of the perovskite. It has been demonstrated that the catalyst coating technique was helpful in improving the catalytic activity of the perovskites.

The possibility of incorporating the catalyst into the perovskite from the surface and exsolving it afterwards depends on the properties of the host lattice and the catalyst itself, such as the sizes of the host lattice and the catalyst cations, the stability of the host lattice, etc. Through careful matching of the catalyst and the host lattice of the perovskite, the incorporation of the catalyst into the perovskite surface can be a promising method for decreasing the amount of catalyst required and therefore developing cost-efficient electrode materials for PEMFC.