structure-sensitivity in CO2 methanation over CeO2 supported metal catalysts
Tushar Sakpal is a PhD student in the Catalytic Processes and Materials group. His supervisor is prof.dr.ir. L. Lefferts from the Faculty of Science and Technology.
CO2 methanation reaction often attracts attention in the energy sector, since combined water electrolysis and methanation can store the surplus renewable electrical energy into chemical energy. This reaction was first introduced in 1902 and has been studied extensively since then. A catalyst is required to obtain a better efficiency of CO2 methanation reaction. It has been established that Ni and Ru are the best performing metals in terms of activity, selectivity, and stability. Highly dispersed nanoparticles of these metals on support (usually, thermally stable metal oxide) are generally used during the reaction. There are two types of supports, namely reducible supports, and non-reducible supports. Reducible supports (e.g. CeO2, TiO2) are more active than non-reducible supports (e.g. Al2O3, SiO2) since they provide additional sites for CO2 activation.
CeO2 can easily switch between 4+ and 3+ oxidation without phase change, which results in the formation of abundant oxygen vacancies. As a result of this unique property, CeO2 supported catalysts show excellent activity for CO2 methanation reaction compared to other supported catalysts. In the last decade, significant research was done in studying the CeO2 nano-shapes, with well-controlled crystal planes, such as rods, cubes, and octahedra. Variation in the shape of CeO2 results in variation in properties and activities of these materials. Previous publications reporting on the effect of CeO2 morphology on the activity for CO2 methanation, as well as other reactions, often neglected the effect of metal particle size.
Therefore, this study reports the effect of metal (Ni and Ru) particle size on the activity of catalysts. Moreover, we also studied the morphology effect of CeO2 nano-shapes by keeping identical metal particle size on all three supports. The thesis is mainly divided into two parts, studying the morphology and particle size effects using Ru/CeO2 (chapter 2 and 3) and Ni/CeO2 (chapter 4) catalysts.
In chapter 1, the reader is provided with the motivation for renewable-energy storage, possible ways to store energy, fundamentals of CO2 methanation reaction and properties of materials tested. Last part of chapter summarizes the goals of the thesis.
Chapter 2 compares the performance of rod, octahedra, and cube-shaped CeO2 supported Ru catalysts, with constant Ru particle size, for CO2 methanation. Rod-shaped Ru/CeO2 catalysts exhibit the highest activity of 11.0×10-8 . H2-TPR, Raman and XPS results reveal that the addition of Ru increases the reducibility of CeO2, lowering reduction temperature and generating more oxygen vacancies. Diffusion of these oxygen vacancies into bulk is concluded based on H2-TPR data. Rod-shaped Ru/CeO2 possess higher oxygen vacancy concentration than cubes and octahedra, after oxidative as well as reductive conditions. The catalyst with the highest activity also possesses maximum oxygen vacancies, implying that the oxidation of CeO2 via CO2 adsorption is a rate-determining step of the redox cycle.
In chapter 3 we studied the effect of Ru particle size on the activity for CO2 methanation using rod-shaped catalysts. The activity of the catalysts shows a significant effect of Ru particle size, where 4.8nm Ru/CeO2 catalyst exhibits the highest activity of 0.0045
at 215oC. The primary reason behind the structure-sensitivity in Ru/CeO2 catalysts is the particle size of Ru itself. There is also an effect of particle size on the reducibility of CeO2, contributing to the structure-sensitivity of Ru/CeO2 catalysts. Dissolution of Ru4+ increases with metal loading, while it decreases with increasing reduction temperature. The trend in Ru dissolution agrees well with the trend in activity per Ru surface area, suggesting that the presence of Ru opens a fast pathway to activate CO2 via formation of a HCOO* intermediate. Therefore, based on chapter 2 and 3, we can conclude that the activity of the catalyst for CO2 methanation depends on the Ru particle size. Hence, it is required to keep the Ru particle size identical while studying the effect of CeO2 morphology. Moreover, there are two rate-determining steps influencing the overall reaction rate, one on Ru and one on CeO2 surface respectively.
Chapter 4 of this book reports the effect of CeO2 morphology as well as Ni particle size on CO2 methanation activity using a series of Ni/CeO2 catalysts. Catalysts with different Ni particle size (2.5-4.7 nm) shows different activity, with 2.9nm Ni catalysts showing the maximum activity of 7.54×10-3
at 270oC. The highest activity of 2.9nm Ni particles is attributed to the intermediate strength of Ni-CO interaction. The CO is one of the intermediate species formed on the active metal surface during the reaction. With the help of literature, it is established that weak Ni-CO interaction on small Ni particle cause insufficient activation of the CO bond, while CO poising is caused on large Ni particles due to the stronger interaction between Ni-CO.
Furthermore, the effect of CeO2 morphology was studied by keeping identical Ni particle size (3nm). The maximum activity was observed for rods-shaped catalysts. Characterization techniques reveal the presence of two types of oxygen vacancies: ones created by Ni2+ dissolution (redox inactive), and ones formed during the reduction process via H-spillover (redox-active). The concentration of redox-active oxygen vacancies increases with increasing NiO loading and Ni/CeO2 rods showed the highest concentration of oxygen vacancy.
Rod-shaped Ni/CeO2 exhibits the highest activity as well as possess maximum oxygen vacancies, implying that activation of CO2 on oxygen vacancies is a rate-determining step. Although, the impact of the Ni particle size of activity also indicates that a hydrogenation step of a carbon-containing species on the Ni surface also influences the overall activity. The presence of two rate-determining steps on Ni/CeO2 catalysts is consistent with the conclusions for Ru/CeO2 catalysts, reported in chapter 2 and 3.
Based on this work, we conclude that the CO2 methanation activity of catalysts influenced significantly by variation in metal (Ni and Ru) particle size. Therefore it is very important to maintain identical metal particle size while comparing the nano-shapes of CeO2 for CO2 methanation as well as other reactions.
