DENSITY FUNCTIONAL STUDIES OF THE MITIGATION OF SOME ENVIRONMENTALLY HAZARDOUS GASES

Abstract

The greenhouse gases, such as carbon dioxide (CO2), sulfur dioxide (SO2) and nitrous oxide (N2O), are emitted into the atmosphere due to the burning of fossil fuels, industrial processes, and agriculture. Along with these, several other gases such as methane (CH4), carbon monoxide (CO), and hydrogen cyanide (HCN) are also released and is a major environmental issue with far-reaching effects on the climate and ecosystem of the globe. This causes global warming, rising sea levels, depletion of the ozone layer, extinction of species, and drought. The storage and conversion of harmful gases into various products is an important strategy in the fight against climate change. In such methods, the captured gases from diverse sources are stored permanently or transformed into value-added products. In the past decades, different research groups have carried out numerous investigations in this direction. Different types of materials, such as zeolites, metal-organic framework, covalent organic framework, metal oxides, transition metal clusters, etc., have been investigated to capture these gas molecules and their conversion. Clusters of nickel, platinum, and palladium are used as catalysts for different reactions, including the oxidation of carbon monoxide, oxygen reduction reaction, hydrocyanation of acetylene, etc. The reactivity of metal clusters is known to get altered over carbon support such as graphene. Considering the above, a computational study on the small-sized transition metal clusters in their free and carbon-supported forms for the catalytic conversion of carbon dioxide and hydrogen cyanide to value-added chemicals is carried out in the present thesis. Considering the fact that confinement affects the energy barrier of reactions, the conversion of carbon dioxide to carbonate is examined inside two types of confinement, viz. cucurbituril and carbon nanotubes of different diameters. Carbon dioxide adsorption over the metal oxide cluster is also investigated to know the nature of interactions between the adsorbate and adsorbent. In Chapter 1 of the thesis, the harmful effects of different gases, which are emitted into the environment with the burning of different fuels are discussed. Different approaches adopted in the mitigation of hazardous gas molecules are reviewed and potential materials for storing the harmful gases are briefed. Various methods used to convert greenhouse gases to products, such as formic acid, methanol, and propylene carbonate, are also briefly explained. Chapter 2 of the thesis reviews the computational methods used in the present study. Computational methods such as Hartree Fock, post-Hartree Fock, and density functional theoretical methods are briefly reviewed. A brief discussion on different types of functional and basis sets is provided. The theoretical background on the transition states using the intrinsic research coordinates (IRC) methods, is also elucidated. The catalytic conversion of HCN and acetylene to vinyl isocyanide is investigated and discussed in Chapter 3. Pt3 cluster and its carbon-supported form are modeled for the transformation of hazardous HCN to vinyl isocyanide, using the density functional BPW91 in conjunction with 6-311G** and LANL2DZ basis set. The analysis of frontier molecular orbitals inferred that the reaction commences with the adsorption of acetylene. The investigation revealed that acetylene and hydrogen cyanide are preferably adsorbed over the Pt catalysts. The activation energy barrier for the reaction is reduced by ~10 kcal/mol using a carbon-supported platinum cluster (coronene-Pt3). The reaction barrier is decreased appreciably for coronene-3Pt, where platinum atoms are dispersed over the carbon layer. It is also inferred that the catalytic conversion of HCN over the free and carbon-supported cluster is free from poisoning. In Chapter 4 of the thesis, the conversion of two greenhouse gases, CH4 and CO2, to CH3COOH is investigated using small-sized group 10 transition metal clusters employing the density functional BPW91 with 6-311G** and LANL2DZ basis set. The adsorption energy for CH4 and CO2 is negative for Ni and Pt clusters. The energy barrier for the conversion is significantly reduced over such clusters compared to that in the free state. The notable finding of the analysis is that only a single metal site is used for the conversion when the reaction commences from the adsorption of CH4 over the cluster. On the other hand, two metal sites are required, and a relatively higher energy barrier is observed if the reaction is initiated with the adsorption of CO2. The conversion of CO2 and CH4 over the free and carbon-supported clusters is also found to be free from catalytic poisoning. Chapter 5 investigates the addition of propylene oxide and CO2 to form propylene carbonate inside the cages of different diameters using the density functional B3LYP and 6- 311G** basis set. The study unveils the interaction between the guest species and the host cages as feasible forming stable host-guest complexes. It is also inferred that the reaction is attainable inside the cages, forming propylene carbonate. Among the cages, the lowest energy barrier is observed for the reaction inside cucurbituril [6]. The energy barrier for the reaction is slightly increased (~3 kcal/mol) when encapsulated in a carbon nanotube of nearly the same diameter. The effect of electron-donating and withdrawing groups is also examined, and it is found that the energy barrier is decreased inside the cavity upon the substitution of electron donating groups, despite an appreciable change in the energy barrier was not observed in the free state. The clustering of carbon dioxide around Zn12O12 cage employing density functional B3LYP and 6-311G** and LANL2DZ basis set is discussed in Chapter 6 of the thesis. CO2 molecules are physisorbed over the cluster via van der Waals forces. A remarkable finding of the study is the sudden drop in the sequential adsorption energy with the adsorption of the twenty-sixth CO2 molecule along with a large distance between the cage and CO2, suggesting that twenty-five CO2 molecules belong to the first coordination shell of the cage. During the adsorption, transfer of electron density occurs from CO2 molecules to the cluster, ascertained by natural bond orbital analysis and the charges on various atoms. The bond critical point analysis revealed that the adsorption occurs via interaction between Zn atom of the cage with O atom of CO2 and that between O atom of the cage with C atom of CO2. An inverse relationship between stabilization energy and the molecular electrostatic potential minimum suggests that the less electron-rich nature of the oxygen atoms of the cage promotes the adsorption of CO2 molecules. The conclusion and future scope of the thesis are highlighted in Chapter 7 of the thesis.