First principles investigation of Ti2C MXene as catalyst and support in CO2 activation and CO oxidation reactions

Abstract

Two-dimensional materials are a sought-after class in the domain of material science, owing to its applications in a plethora of fields. A recent addition to this family is called MXene, a class of transition metal carbides (TMCs)/nitrides/carbonitrides, This discovery in 2011 turned out to be a game-changer in this realm as it offers a lot of flexibility in their chemical nature (unlike its predecessors) mainly because of the diverse parent MAX phases available and also different surface terminations that arise from synthesis conditions. This in addition to having a higher surface-to-volume ratio has made MXenes a popular choice (both as support and catalyst) in the field of heterogeneous catalysis, especially for environmental remediation. For example, Ti2C is found to be efficient for the CO2 abatement process and Cu3 cluster doped Mo2CO2 has good stability and CO oxidation activity. This constitutes the backdrop of my talk, where we have investigated the performance of these novel catalysts for such environmentally relevant processes. To design effective catalysts, it is pertinent to have an understanding of the reaction mechanisms associated with the process. Computational techniques are commonly used as a complementary tool to gain better insight into the thermodynamic and kinetic aspects in heterogeneous catalysis. In this work, we have employed a technique called density functional theory (DFT), which provides a fine balance between computational cost and accuracy. In the first part of the thesis, we have examined the structural, electronic, and magnetic properties of the most stable 1T phase of Ti2C MXene along with a newly predicted metastable 1H phase. We also put forward the phase transition mechanism involving a barrier of 1.48 eV as the Ti-C-Ti stacking changes from ABC (1T) to ABA (1H). 1T (1H) structure is an antiferromagnetic semiconductor (ferromagnetic half-metal) and Monte Carlo simulations confirm the magnetism maintained well above room temperature. The second part focuses on Ti2C MXene based catalysts for CO2 dissociation and CO oxidation reactions. CO2 activation and dissociation are the preliminary steps involved in CO2 utilization processes. Bulk transition metal carbides (TMCs) have displayed good activity towards these reactions with the stoichiometry, composition, etc having a profound effect. Our studies indicate barrierless chemisorption for CO2 and very low barrier (0.13 eV) for CO2 dissociation when bare Ti2C surface is used as the catalyst. CO oxidation is the most simple and effective way to convert CO to CO2 and finds applications in automobile exhausts, proton-exchange membrane fuel cells, etc. Gold nanoparticles deposited on oxide support is a prominent catalyst for the reaction but poses an issue due to sintering. Doping with copper is a popular strategy deployed to improve stability, increase oxyphilicity and decrease cost. The catalyst we designed is an ultrasmall gold-copper bimetallic cluster on O-terminated Ti2C MXene. This catalyst maintained stability through the course of the reaction and the highest barrier required was lower than the precious metal surfaces used for the same.