Improvement of Pt-based catalyst supported on CeZrO2 used for the high-temperature water-gas shift reaction

Sammanfattning: The iron-chromium (Fe/Cr) catalyst is used for high-temperature water-gas shift (HT WGS) reaction. A major concern using the Fe/Cr catalyst is that it contains hexavalent chromium (Cr+6) which is a potent carcinogen causing health and environmental issues. In order to reduce the usage of chromium in industrial HT WGS applications, a chromium-free catalyst needs to be developed. The aim of this thesis project was to develop a chromium-free catalyst which shows high activity, selectivity, and stability during HT WGS operating conditions. Six different formulations, catalysts A-F, were prepared for the experimental part of the project. The prepared catalysts were then compared to the activity and selectivity performance of a commercial Fe/Cr catalyst. The tests were performed in a reactor containing 3 ml of the catalyst and 3 ml of inert material (alfa-alumina). The particle size of the catalysts and the inert material was 1-2 mm. Feed containing 10.8 % CO, 29.8 % H2O, 43.2 % H2 and 16.2 % CO2 was mixed and introduced to the reactor (Gas hourly space velocity: 20,000 h -1). The outlet dry gas was analysed by gas chromatography at temperatures 350 °C, 400 °C, 450 °C and 500 °C. The system was operating at atmospheric pressure. The short-term activity and selectivity tests show that the catalysts E exhibits CO conversion up to 72 % at 350 °C (the maximum CO conversion is 79.3 % at 350 °C due to equilibrium). Compared to the commercial Fe/Cr catalyst, which exhibits 37.8 % at 350 °C, the activity of the catalyst E is significantly higher. None or very low concentrations of hydrocarbons were produced, implying that the catalyst is selective towards the desired products, H2 and CO2. The other prepared catalysts, Pt/Re/CeZrO2 and Pt/Re/TiO2, performed poorly in terms of selectivity. Pt/Re/CeZrO2 produced as much as 5.97 % methane at 450 °C. The Pt/Re/TiO2 catalyst produced 0.16 % methane at 450 °C. Because of the methane formation, it was concluded that the Pt/Re/CeZrO2 and Pt/Re/TiO2 catalyst formulations are not suitable for HT WGS applications, and were therefore not further studied in the long-term stability test. The long-term stability tests were performed on the catalyst E. No deactivation signs were noted during the 135 hours on-stream and reactor temperature at 400 °C. The thesis project presents successful results from the experiments performed on the developed chromium-free catalyst for HT WGS applications. In terms of the activity and selectivity, it is concluded that catalyst E exceeds the performance of a commercial Fe/Cr catalyst. Stability results show that the catalyst is stable for at least 135 hours, not indicating any deactivation signs.