Atomic Scale Characterisation of Pt-Re/CeO2 for the Water Gas Shift Reaction

Martin, Richard B

Atomic Scale Characterisation of Pt-Re/CeO2 for the Water Gas Shift Reaction

Master Thesis

2022

Abstract

Inexpensive hydrogen gas is needed to transition to a hydrogen based economy. For this to happen, economically feasible, large scale production of hydrogen gas is required. Currently, electrolysis is unable to meet this demand economically, so steam reforming of hydrocarbons (e.g. methane) is required to produce large volumes of hydrogen. To be used for mobile applications, such as in hydrogen fuel cells, the hydrogen fuel must be free of CO which is poisonous to the fuel cell catalyst. The water gas shift (WGS) reaction is used to convert CO and H2O to CO2 and H2, cleaning the fuel stream and producing additional hydrogen. A medium temperature shift catalyst, operating in a temperature range of 280 to 350 °C can be used to perform the WGS in one step, simplifying the complex two step process used industrially and enabling small scale hydrogen production. The catalytic material used for the medium temperature shift is platinum on a metal oxide support, such as CeO2, however this is prone to deactivation by sintering. It has been shown in literature that the addition of Re enhances the activity and stability of the catalyst, however it requires close contact with the platinum for promotion to take place. Traditional impregnation methods are unable to guarantee this close contact due to the non-uniform distribution of both metals on the catalyst support. Colloidal synthesis methods provide a way to ensure the platinum and rhenium are in close contact with each other. Literature has reported a method to produce homogeneously alloyed Pt3Re nanoparticles. It was hypothesised that using the colloidal method for synthesising Pt-Re/CeO2 catalysts would improve the activity and stability over Pt-Re/CeO2 catalysts synthesised by the traditional impregnation method. In this study the method reported in literature was optimised for small metallic nanoparticles with a spherical shape, which could then be supported on CeO. The optimised method was used to produce Pt/CeO2 and Pt-Re/CeO2 catalysts, while incipient wetness impregnation was used to produce a Pt-Re/CeO2 catalyst for comparison. These catalysts were physically characterised used X-Ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and transmission electron microscopy (TEM). Operando TEM was used to observe the Pt and Pt-Re nanoparticles under water gas shift conditions. The nanoparticles exhibited oscillations between facetted and spherical shapes, which was not present under normal TEM imaging conditions. The cause of these oscillations was attributed to the presence of reactive gases and elevated temperature. The catalytic performance of the catalysts was evaluated at 320 °C and 350 °C, and the Pt-Re/CeO2 synthesised by the colloidal method had the largest activity of all the catalysts, however, it showed more deactivation at 320 °C than the Pt-Re/CeO2 catalyst prepared by impregnation. At 350 °C, all of the catalysts showed an increase in stability. In conclusion, Pt-Re/CeO2 prepared by the colloidal method had a higher activity (at all temperatures) than Pt-Re/CeO2 prepared by the impregnation method, due to the closer contact between Pt and Re, with increasing stability at higher temperatures, the cause of which is currently unknown.

Keywords

Chemical Engineering

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