Density Functional Theory Study Of Small Gas Molecules Dissociative Chemisorption On Transition Metal Catalysts

Transition metals have been shown to be the most effective catalysts in heterogeneous catalysis due to their highly catalytic reactivity, stability and selectivity. Among which, platinum (Pt) and nickel (Ni) have been received the most attention and already applied in many industrial chemical processes such as hydrogenation and dehydrogenation in organic and petrochemical productions, toxic gas reduction of automobile exhausts, oxidation and reduction in electrolysis reactions. For a hydrogen involved reaction, the H₂ molecular adsorption with the successive activation and the H atoms desorption were regarded as the most important steps in the catalytic processes. A detailed knowledge of the dissociative Chemisorption of hydrogen molecules on the transition metal clusters or crystalline surfaces at an atomistic/molecular level would be very important to gain useful insight into the dynamic behaviors of surface adsorption and migration, and the nature of the interaction between the adsorbates and substrates, and consequently to understand the underlying catalytic mechanisms. Such an understanding would be benefit on finding appropriate solutions towards specific industrial catalytic problems. In this thesis, we present a systematic density functional theory (DFT) study of hydrogen dissociative Chemisorption on Pt crystalline surfaces and the alumina supported Pt clusters using a more realistic model which is capable of describing the catalytic processes. To address the CO poisoning issue, we further adopted a hydrogen-saturated subnano Pt and Pt/Ru cluster model to explore the CO removal mechanisms via O₂ oxidation and/or hydrogenation. We further present the comparably study of hydrogen sequential dissociative Chemisorption on Ni and Pt clusters. The structural characteristics, energetics and electronic structures of the hydrogen+catalyst complex systems were thoroughly calculated and analyzed by considering the H coverage dependence. The nature of the interaction between the adsorbates and substrate had also been discussed.Hydrogen sequential dissociative Chemisorption on precious metal Pt crystalline surfaces was first studied using DFT-based methods according to the study of hydrogen adsorption behaviors on Pt clusters models. Successive H₂ decomposition and sequential H desorption and the geometric and electronic structures of metal hydrides at full H saturation were identified. The results indicate that the difference is the preferred sites for H atom loading on Pt(111), Pt(100) and Pt(531) surfaces. The energies are very close for H adsorption on the top, bridge and fee hollow sites of Pt(111) surface, while the bridge site is more preferable than on-top and hollow sites of Pt(100) surface. The first bridge and top sites are energetically the most favorable for H dissociation on Pt(531) surface. In contrast to the most stable adsorption energy, the first bridge site of Pt(531) is about 0.06 eV higher than that of Pt(lOO) surface, and about 0.15 eV higher than the on-top and bridge sites of Pt(111) surface. We further found that dissociative Chemisorption energy of H₂ and desorption energy of H atom in general decline with H coverage. For the three Pt crystalline surfaces at the threshold of saturation, the H₂ Chemisorption energies fall within a narrow range of 0.60-0.88 eV, which are slightly lower than the values 0.90-1.1 eV of Pt cluster models at high coverages. The calculated thresholds of H desorption energy vary in a range of 2.08-2.84 eV, which are comparable to the values of 2.02-2.70 eV on Pt cluster models. H₂ dissociative Chemisorption is largely controlled by charge transfer from metal atoms to H atoms. Bader charge population analysis indicates that charge transfer increases with H loading, resulting in sequential change of metallic bonds to covalent bonds in the metal hydrides. Moreover, our calculations suggest that the capacity of Pt crystalline surfaces to adsorb H atoms is essentially much lower than what was found for Pt clusters at full saturation.Based on the studies of sequential H₂ dissociative Chemisorption on small Pt clusters, we attempt to investigate different CO mitigation techniques by employing a small Pt subnano cluster as model to understand the hydrogenation and oxidation of CO poison in the presence of H atoms. At low H coverage, CO oxidation by oxygen to form CO₂ on the selected Pt₆ cluster was found to be endothermic with moderate overall thermochemical energy. However, the subsequent CO₂ desorption from the cluster is highly endothermic. Kinetically, the oxidation process is also unfavorable and needs to overcome a significant activation barrier. The unfavorable energetics makes the CO oxidation at low H-coverage unlikely to occur. Upon saturation of the Pt₆ cluster by H atoms, the activation energy required to form a transition state that leads to the formation of surface CO₂ is reduced substantially and, thermochemically, the oxidation reaction becomes exothermic. Our results suggest that CO oxidation by oxygen on the H-saturated Pt₆ cluster would be difficult at low temperatures due to the moderate activation energy in spite of the favorable thermochemical energy. However, at an elevated temperature, the relatively moderate barrier can be readily overcome and thus the reaction can become facile. This conclusion is consistent with experimental observations. Many experimental results suggest that CO tolerance can be signifieantly improved by doping the Pt catalysts with Ru. To reveal the role of Ru in the CO₂ removal process, we calculated the thermochemical energies and activation barriers for the CO oxidation on a Pt₅Ru cluster fully covered by H atoms. Our results indicate that O atoms are chemisorbed on the Pt-Ru bond more strongly than on the Pt-Pt bond. As a consequence, the oxidation process becomes much more exothermic. Depending on where the attacking O atom resides, the associated activation barrier can be moderately reduced or slightly increased. CO occupation on Ru was found to be energetically more favorable than on Pt, and thus makes the CO removal via oxidation at this site more difficult. We further investigated the feasibility of removing CO from the H-saturated Pt₆ cluster by considering CO reduction by H atoms to form formaldehyde. This was done by allowing two H atoms adsorbed nearby the active site to sequentially attack the CO molecule. It was found that each reaction step is moderately endothermic. However, the calculated activation barriers are relatively high. Even at 500K, no formaldehyde formation was observed in our ab initio MD simulations. The results suggest that CO reduction by H atoms on the Pt₆ cluster is energetically difficult. The present study utilizes an exceedingly small Pt cluster to represent the catalyst particles for exploration of CO removal mechanisms. However the catalyst model is undoubtedly oversimplified, we catch an eye on the dependency of CO poison and removal mechanisms on the size of Pt clusters.Oxide supported precious metals play an important role in many heterogeneous catalytic reactions. We present a systematic study using the DFT method to understand the adhesion of small Pt_n clusters for n up to 6 on theγ-Al₂O₃ (001) surface and the catalytic behaviors of Pt_n/γ-Al₂O₃ system with respect to H coverage. Our calculations indicate that the catalytic performance of supported Pt subnano-catalyst is dependent upon the size and shape of metal particles. Results show that (1). The Pt clusters can be stably anchored on the surface. Energetically the most favorable adsorption sites were identified and substantial structural relaxation upon adsorption was observed. The significantly higher adsorption energy at the O site is largely attributed to the face that the charge transfer from the Pt atoms to the O atoms makes the Pt atom positively charged. The Al atom underneath Pt atom is also positively charged. The repulsion between the two positively charged atoms, Pt and Al, leads to much weaker bonds. The calculated average adhesion energies were found to be size and shape dependent. The adhesion energy of Pt atoms in general decrease with the size of cluster increases. Since the Pt-Pt interaction would become stronger than Pt-O and Pt-Al at large cluster size, the formation of metal cluster would be strongly preferred upon high Pt loading, consequently, the growth of metal films on theγ-Al₂O₃(001) surface is unlikely to be smooth and agglomeration could occur under certain conditions; (2). The support changes the site preference of H₂ adsorption, increasing H₂ activation for bridge sites but decreases it for on-top sites. Our results also indicate that H₂ dissociative Chemisorption on supported small Pt clusters in general decrease with the coverage of H atoms. At H high coverage, the structural distortion and relaxation of metal clusters and substrate occur with the selected cluster size and shape, the H₂ dissociative Chemisorption energy and H desorption energy fluctuate in the range of 1.00-1.10 eV, 2.46-2.80 eV for larger clusters, respectively. The capacity of Pt clusters on support at full saturation decreases to be 1:3(Pt:H ratio) compared with that of bare Pt clusters to absorb H atoms due to the fact that some of the possible orientations of the Pt atoms toward H were occupied by the Pt-substrate bonding. Bader charge analysis indicates that charge transfer from Pt clusters to H atoms increases with H loading, resulting in the interaction between Pt clusters and the substrate decreases.For purpose of comparison, we also studied the hydrogen sequential dissociative Chemisorption on subnano Ni clusters using the same computational scheme, together with our previous results on small Pt clusters to discuss the difference of Chemisorption/desorption behaviors between Pt and Ni. Our results show that at H low coverage, the edge and hollow sites are energetically the most favorable for H₂ dissociation, which are different to the Pt clusters. Dissociative Chemisorption of H₂ on the Ni clusters is facile with exothermic reaction energies and small activation barriers. However, the Chemisorption energy is generally lower than that of on Pt clusters. H₂ dissociative Chemisorption energies and H desorption energies are strongly coverage dependent. These energies in general decline with H coverage and for various sizes and shapes of Ni clusters at the threshold of saturation, the H₂ Chemisorption energies each fall within a narrow range of 0.71-1.00 eV, which are slightly lower than the values of 0.91-1.10 eV of Pt clusters at a high coverage. The calculated threshold values of H desorption vary in a range of 2.08-2.73 eV, which are comparable to the values of 2.02-2.70 eV on Pt clusters. The favorable orbital overlaps between the HOMO of metal clusters and the LUMO of H₂. Hirshfeld population analysis indicates that charge transfer from Pt clusters to H atoms increases with H loading, resulting in sequential change of metallic bonds to covalent bonds in the metal hydrides. However, the difference is that some of Ni metal hydrides still remain the magnetic characters, such as Ni₈ and Ni₁₃ clusers. Our calculations also suggest that the capacity of Ni clusters to adsorb H atoms is nearly constant at full saturation, except when some of the metal atoms residing at the core of the clusters are not accessible to H atoms. However, the H capacity on Ni clusters is essentially half of what was found for Pt clusters. Despite the relatively smaller size of the Ni clusters chosen in the present study compared with the size of catalyst particle size used in practice, some of the properties may not change significantly with the particle size and shape of catalysts. Useful insight into the catalytic activity of transition metal catalyst toward H₂ can be gained at atom and molecule levels. It will be very helpful in understanding real heterogeneous catalytic processes.