CATALYST SYNTHESIS AND ACTIVITY EVALUATION FOR CO2 CONVERSION USING REVERSE WATER GAS SHIFT REACTION

Abstract

Forward and reverse water-gas shift (WGS/RWGS) reactions play a significant role in the production of hydrogen and the reduction of carbon dioxide, respectively. The efficiency of these reactions can be further enhanced by the use of a suitable catalyst. The use of transition metals is an effective alternative to noble metals, since they possess excellent catalytic properties for many of the reforming reactions, especially when incorporated into appropriate supports. Considering the vital role of catalysts in the transformation of hydrocarbon-based energy sources, the majority of research attention has been devoted to the development of catalysts that are highly active, selective, stable, economically feasible, and readily accessible. The primary objective of this research is to study appropriate transition metal-based catalysts, such as Cu and Ni, supported on reducible supports, such as CeO2, for their performance in converting CO2 into CO. To determine the most suitable conditions for synthesizing Cu/CeO2 catalysts through solution combustion synthesis (SCS), two synthesis parameters, e.g., the ratio of fuel to oxidizer (φ) and the metal loading, were adjusted to obtain the highest combustion temperature and gaseous products during the synthesis of catalysts. Bimetallic catalysts were also evaluated for their effect on activity, selectivity, and reducibility on La2O3-supported catalysts in order to determine if there is any synergistic effect caused by the presence of two metals together, e.g., Cu and Ni, on the catalytic performance in RWGS reaction compared to using Cu or Ni alone as active metals supported on La2O3. To evaluate how a suitable additive may eliminate or reduce carbon deposition and boost hydrogen production in the WGS reaction, ammonia, hydrazine, and urea were used in the feed and the distribution of products in the WGS reaction was estimated via thermodynamic calculations. This study also aimed to evaluate the effect of oxygen content and particle size on catalyst performance by manipulating the calcination temperature and metal loading, respectively, prior to the CO2 conversion experiments. Therefore, the active metal composition and calcination temperatures were adjusted to form different crystal structures to investigate metal-support interaction, surface defects, and oxygen vacancies in Cu/ZrO2 catalysts. The activation energies were estimated, and reaction kinetics models were studied in order to understand the governing mechanism of Cu/ZrO2 in RWGS. Overall, under the same experimental conditions, CeO2 as support showed better catalytic activity than La2O3, with 70% compared to 57% activity at 600 oC. However, the addition of Cu to La2O3-supported Ni catalysts improves CO selectivity and activity through the formation of stable alloys and improves the performance in the RWGS reaction by adjusting the reducibility of the active metals. Besides, the calcination temperature and amount of copper present in Cu/ZrO2 catalysts have a significant effect on the CO2 conversion performance of Cu/ZrO2, with the highest conversion of 37% observed at 600 °C for the catalyst with 2wt.% copper and calcined at 800 °C; however, increasing the metal loading to 5 wt.% and calcination temperature to 1000 oC resulted in losing the catalytic activity. Other supports, such as cobalt oxide, may also be investigated in the future since it has some characteristics that make it effective for WGS reaction and is expected to perform well in RWGS reaction as well.