Date of Award

Fall 11-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

Abstract

With recent advances in modeling and simulations methods along with state-of-the- art supercomputers, theoretical studies are becoming a cheaper choice for scientific predictions and provide complementary support to the experiments. Intense research in catalysis has been done in the past two decades experimentally as well as theoretically. This works employs first-principle studies to predict the most promising catalysts for Fischer-Tropsch (FT) reaction based on size and composition. The transition metals Fe, Co, Ni, Ru, Pd, and Pt, have been explored in pure and alloyed nanocluster forms for the effectiveness of catalytic properties.

The first and most crucial step of the FT reaction is carbon-mono-oxide adsorption on the surface of the catalyst, followed by its dissociation to form long-chain hydrocarbons. The studies done in this work explores the natural potential of the metals towards the CO adsorption and dissociation and provide a reference for further studies to find the best catalyst for the FT reaction. In this work density, functional theory calculations were carried out using Generalized Gradient Approximation with RPBE functional on two sizes of pure and bimetallic nanoclusters viz. ~0.5 nm and ~1.2 nm consisting of 13 and 55 atoms respectively. Core-shell icosahedron geometry of nanoclusters in the form of A1B12 (0.5 nm) and A13B42 (1.2 nm) is used. Bimetallic nanoclusters are formed using a combination of the above-mentioned metals. 13-atom clusters pure and binary clusters of Ru, Pd, and Pt, are explored with DND and DNP basis sets while 55-atom nanoclusters studies are done using plane-wave basis sets. Based on the CO adsorption and dissociation energies, an initial predictor, percentage difference was proposed to identify potentials catalyst systems. In 13-atom pure systems Ru was found to have the highest value of the % difference. In 55-atom clusters of Ru, Ni, Pd, and Co, Ru was found to have a maximum value of the percentage difference, hence greater catalytic performance.

In bimetallic systems, only systems showing better excess energy were considered for further studies. Surface energy was seen to be the dominant factor in the binding of metal atoms in a core-shell arrangement. In bimetallic 55-atom nanoclusters, Fe13Ru42 was found to be the best catalyst among all the binary combinations explored. Ni and Pt are better than Ru, Co, and Fe (in decreasing order of preference) in the core of cluster when shell metal is Pd. Fe13Co42 nanocluster was found to have greater value of percentage difference than bare Co nanocluster of same size. Ru, Co, and Fe (in decreasing order of preference) preferred to be in the core of the cluster where host(shell) element is Ni than the pure Ni cluster. Fe13Pt42 was found to be better than any other element in the core of cluster, when shell was composed of Pt. The initial predictor proposed in this work predicted the order of preference of potentials catalyst (top 6 candidates) as follows: Fe13Ru42 > Ru55 > > Ru13Ni42 > Pd55 > Co13Ni55 > Fe13Ni42.

Download

Share

COinS