A theoretical study on catalytic steps of hydrogen production and purification

Abstract

The aim of this thesis is to obtain information on catalytic activity of transition metals used in different steps of hydrogen production and purification. Density functional theory calculations with periodic slabs have been utilized on energetically stable Pt-Sn and Pt-Ni bimetallic alloys, to analyze the electronic structure and reactivity in specific reactions, namely methane oxidative steam reforming (OSR) in the case of Pt-Ni and CO oxidation in the case of Pt-Sn. The studies performed within the framework of the current thesis can be briefly summarized as follows: (i) Oxygen chemisorption on Pt-Ni surface alloy has been studied aiming to shed light on the decreased oxidation tendency of the Ni surface after Pt doping. The results showed that oxygen chemisorption is weakened on surface alloys, compared with monometallic Pt and Ni surfaces and bulk Ni3Pt alloy. (ii) Methane dehydrogenation is one of the most important steps of OSR mechanism. Methane dehydrogenation reaction was studied on Pt-Ni surface alloy and it was found that methane dehydrogenation proceeds on Pt-sites during OSR. (iii) Experimental studies confirmed that Pt3Sn is the main active phase of Pt-Sn system supported on nitric acid oxidized activated carbon. The strength of CO adsorption on Pt3Sn alloy has pronounced effects on the activity and performance stability characteristics of the catalyst. In the current study, CO adsorption on Pt(111) and on Pt3Sn(111) were analyzed for CO surface coverages ranging between 0-0.5 ML. The effect of CO surface coverage on electronic reconstruction of free sites, extent of adsorbate-adsorbate and adsorbate-surface interactions, and adsorption structure of CO were parametrically studied. It was found that adsorbed CO molecules form clusters on Pt(111) surface, which hinder oxygen chemisorption and prevent CO oxidation reaction. (iv) In the final part of the study, the adsorptive properties of the Pt3Sn alloy were investigated; here the results of the previous part were extended further to coadsorption of CO and O on Pt3Sn(111), aiming to analyze the changes in the binding strength of O and CO on the surface led by coadsorption. Electronic structure analysis is performed throughout the calculations to bring an explanation to the results.