Energetics and Dynamics of Water Dissociation on Bimetallic Alloy Surfaces

Ghosh, Smita (2020) Energetics and Dynamics of Water Dissociation on Bimetallic Alloy Surfaces. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Smita Ghosh (14RS077)) 14RS077.pdf - Submitted Version Restricted to Repository staff only Download (7MB)

Abstract

The adsorption and dissociation of water molecule on metal surfaces is a topic of fundamental interest due to its vital role in many scientific and technological processes, in particular, its relevance to heterogeneous catalysis and electrochemistry are noteworthy. Therefore, a detailed understanding of the structure and reactivity of water with metal surfaces under ambient conditions is necessary. Water gas shift reaction is an industrially important reaction where the water dissociation is the rate-determining step. Water gas shift reaction plays an essential role in the methane steam reforming process. Therefore, a molecular level understanding of the adsorption and dissociation of water molecule on metal surfaces is needed. For water dissociation in to H and OH fragments, it has to overcome a large barrier by activating the chemisorption of water on metal surfaces. This barrier for water dissociation is lowered in the presence of transition metals as well as nobel metals, where they act as a catalyst to accelerate the rate determining step by lowering the activation energy barrier for the dissociation. Although experimental and theoretical attempts in this direction were carried out to understand the water splitting reaction on metal surfaces, still many other aspects need to be addressed. Here, we are interested in understanding the energetics and dynamics of water dissociation on Cu/Ni(111) and Ag/Ni(111) bimetallic alloys surfaces. The prime objective of this thesis is to obtain a promising a catalyst upon alloying a very low reactive pure metal to its alloy and furthermore, how bimetallic alloy acts in lowering the activation energy barrier to the water dissociation. Bimetallic alloys have unique electronic and catalytic properties as compared to the respective monometals. The effects of alloying on the dissociative chemisorption of water and other molecules like methane, etc., have been explored both experimentally and theoretically through DFT calculations. Dynamics studies on these reactions enable us the opportunity to control the outcome and efficacy of the reaction. Our quantum dynamical investigations on the dissociative chemisorptions of water revealed the mode and bond selective nature of this reaction. Moreover, inclusion of surface temperature effect significantly improves the dissociation probability of water on metal surfaces and hence improving the theoretical results to match with the experimental ones. Owing to the difficulty of studying this sort of reactions in full dimensions, a simple model can often provide the essential physics underlying this problem in a qualitative manner and is thus acceptable. Therefore, we followed the reaction path Hamiltonian approach which is a simple but robust technique where an effective potential along the reaction path is used to compute the dynamics. Electronic structure calculations using density functional theory are performed to construct the reaction path. In this thesis, the dynamical investigations of water dissociation on Cu/Ni(111) and Ag/Ni(111) surfaces reveal how the surfaces alloy compositions influence the mode selectivity of this reaction. The thesis is outlined as follows: Chapter 1 provides a general introduction, the motivation of this research and its present challenges. Chapter 2 presents a detailed overview of the theoretical methods used in this study. Chapter 3 reports the water adsorption and dissociation on Cu/Ni(111) bimetallic surface alloys by including the surface temperature effect on reactivity. In chapter 4, the effects of alloying on the mode-selectivity of water dissociation on Cu/Ni bimetallic surfaces is investigated using the reaction path Hamiltonian approach. In chapter 5, water adsorption and dissociation on Ag/Ni bimetallic surface alloys is reported including the surface temperature effect. In chapter 6, the reaction path Hamiltonian based dynamical study is carried out to study the effect of mode selectivity of water dissociation on Ag/Ni(111) bimetallic surfaces. Chapter 7 summarizes the important findings of this thesis and concludes with future research prospects.

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