Theoretical Study Of The Oxidative Dehydrogenation And Deep Oxidation Of Light Alkanes Over The V₂O₅(001) Surface

The catalytic technology is the base of modern chemical industry.As the foundation stone of modem chemical industry,catalyst plays a very important role in many industrial process,so the manufacture of new type catalysts has become a very important part of modern chemical industry.In recent years,a new method of catalyst devised,"molecular design" has attracted great interests.In order to achieve it,we must first understand the electronic structure and character of catalyst,and the microcosmic mechanism of reaction taking place over the catalyst surface.With the help of experiments,we have acquired lots of microcosmic information about catalytic reaction,but unfortunately,the difficulty in determination of many microcosmic character still exists due to the limit of experimental condition.As an important tool of study on modern chemical research,quantum chemistry calculation can provide a lot of detailed information at the molecular level.In the last decade, with the double-quick development of computer software and hardware,combined with the evolution of quantum chemistry method,the quantum chemistry calculation has been performed in the field of catalytic reaction rese arch broadly,and have achieved great success.To date,the Density Functional Theory(DFT) has been rapidly developed, which contribute to the revolution of quantum chemistry calculation greatly.In addition,the DFT is a practical method to study the condensed phase physics and surface chemistry,and has achieved great success in these fields.So we used the DFT method in all of my thesis.Up to date,cluster models are often used to simulate the metallic oxide surfaces,but this model can not reflect the character of catalyst integrallty since in this model,the structures of the substrate are mostly fixed at the ideal state as that of bulk without considering the relaxtion.So in this thesis,the supercell model is employed to simulate the surface of catalyst,which can consider the relaxtion effect adequately.The catalytic oxidation of light alkanes has been the subject of intensive studies because of its importance for the production of basic chemicals such as alkenes, among which,ethene and propene are two most important blocks of the petrochemical industry.Currently,the direct dehydrogenation of alkanes is still used by industry for the production of light alkenes.But all these reactions are reversible and suffer from several limitations:Thermodynamic restriction on conversion and selectivity;Side reactions;Strong endothermic main reaction and necessity to supply heat at high temperature;Coke formation and resulting catalyst deactivation.Oxidative dehydrogenation of light alkanes has been a research topic of consistent interest from 1970s.Introduction of an oxidant into the reaction mixture allows the oxidation of alkane into alkene and water.The reaction becomes exothermic and is able to proceed at much lower temperature.This in turn reduces the side reactions,such as cracking of alkanes and coke formation,as well as overcomes the thermodynamic limitations.At the present time,Vanadium catalyst is a well-established catalyst for the dehydrogenation of alkanes to alkenes.The V₂O₅ supported on other oxides such as Titania and Alumina gives better activities,such kind of catalyst has gained comprehensive investigation due to its fastness in structure, stabilization in thermodynamics and so on.However,the deep oxidation of alkanes to carbon oxides will reduce the selectivity of alkene,The ODH of light alkanes with high selectivity of alkene is still sought.In order to comprehend ODH of light alkane better,we must first investigate the mechanism of deep oxidation over the surface of catalyst,all these work may help us design excellent catalyst.As for the activation of C-H in propane,we have made detailed research previously.Based on this work,we investigate the mechanism of deep oxidation during the process of propane ODH.Start from the propoxide adsorbed on the surface of V₂O₅,there are two different pathways which lead to products acetone and propene through different type of dehydrogenation.We show that the reactions lead to acetone and propene are two competitive pathways,this new mechanism of acetone formation is different from that of Mamoru Ai et al.Our results show the acetone can be oxidated to formation HCHO and CH₃COOH on the fresh surface of V₂O₅.The energetics and pathways for the conversion from propene to acrolein are determined. We show that(a) the C-H bond of propene can be activated by the bridging lattice O atoms easier than that of terminal O atoms,but the propylene radical can transfer to the O(1) site from the O(2) more easily.(b) Compared to that at the bridging O site, the acrolein production from the propexide at the terminal O is much easier with activation energy only 23.8 kcal/mol,and the desorption energy is 15.8 kcal/mol. While at the bridging O site,the corresponding energies are 37.9 kcal/mol and 40.9 kcal/mol respectively.Similar with that of acetone,the acrolein preferes desorbing from the surface to avoid suffering deep oxidation,and it can be oxidated to form acrylic acid over fresh surface of catalyst rapidly with activation energy only 5.9 kcal/mol.In addition,our calculations determine several pathways leading to CO_x,but these reactions can't compete with that pathway of acrylic acid formation mentioned above,acrylic acid can desorb from surface due to the relative low desorption energy, 3.2 kcal/mol,and it will suffer deep oxidation to CO₂ and CH₂CHO^* on the fresh surface and CH₂CHO^* can further be oxidated to HCHO,and even HCO in the next steps,while the HCO can react with the lattice O atom tempestuously to form CO. The calculation results also show that CO can combine with O(1) to form CO2 easily.In the second part of this thesis,the ODH of ethane on single crystal V₂O₅(001) is studied by periodic density functional theory calculations.We show the ethane ODH over the V₂O₅(001) surface follows a mechanism similar to that of propane.The first C-H bond activation is the rate-limiting step of ethane ODH,leading to the ethoxide intermediate.C-H bond activation over the O(1) site through a radical mechanism is the most feasible route in this step,with an energy barrier of 35.1 kcal/mol.The energy barrier of C-H bond activation over the O(2) site through an oxo-insertion mechanism is 37.6kcal/mol,which is 2.5kcal/mol higher than that of O(1).Ethene can be formed more easily at O(2) than at O(1) through the second C-H bond breaking from the ethoxide intermediate,with energy barriers of 31.6kcal/mol and 34.1kcal/mol,respectively.As the byproduct of ethane ODH,acetaldehyde can be formed at O(1) site via the dehydrogenation of ethoxide species,with a lower energy barrier than that of ethene formation(30.8 vs 34.1kcal/mol).Acetaldehyde formation is an exothermic process and its high stability on surface may lead to deeper oxidation to CO or CO₂.The vacant oxygen sites may be created after the desorption of water or acetaldehyde from V₂O₅(001) surface,and the O₂ in the gas phase may re-oxidate the surface with the O-O bond breaking energy barrier of 35.1 kcal/mol,and the process is calculated to be exothermic by 86.9kcal/mol.The calculated energy barrier for the rate-limiting step of ethane ODH(35.1kcal/mol) is higher than that of propane ODH(29.3kcal/mol),and the ethoxide intermediate does not show higher stabilization energy than iso-propoxide as proposed in former experimental studies.The much lower ODH rate for ethane than for propane can not be accounted for by the entropy effect that determines the preexponential factor.By considering the byproduct selectivity of various lattice oxygens,we propose that the much lower ODH rate of ethane relative to propane may be partly accounted for by the reduction of the number of active sites for ethane ODH due to the poor efficiency of O(1) site for ethene formation.Then we studied the deep oxidation of ethane ODH over V₂O₅(001) surface,and made a comparison with that of propane.The results indicate that the deep oxidation of propane comes from the oxidation of propene,which is first oxidated to acrolein, and then acrylic acid,contributing to the formation of CO_x mostly.While for the ODH of ethane,acetaldehyde formation is the main side-reaction,and the desorbed acetaldehyde can be oxidated to acetic acid easily,which shares the similar mechanism with that of acrolein.But before desorbed from the suface,most of the acetaldehyde has been oxidated to form CO_x.It is obviously,that during the process of ethane ODH,the last oxide COx mainly comes from the oxidation of acetaldehyde. From calculations of the ODH and deep oxidation for light alkanes over V₂O₅ (001) surface,we gained many important results,to explain the experimental phenomenon at the molecular level.So the theoretical method used in this work is reliable to describe the features of title reactions,and may play a key role in the understanding of catalytic reaction mechanism,which will direct us to design more excellent catalysts.Gas-phase studies on "isolated" reactants provide an ideal arena for detailed interpret the energetics and kinetics of any bond-making and bond-breaking process at the strictly molecular level.In last section of this thesis,Density functional calculations have been performed to investigate the single atom catalytic reaction of V⁺ with SCO in gas phase.The quintet and triplet PESs of the title reaction have been explored.The results indicate:Both the reaction of V⁺(⁵D) and V⁺(³F) toward SCO proceed according to the insertion-elimination mechanism.The minimum energy reaction path is found to be neither on the quintet PES nor on the triplet PES.Instead, the minimum energy reaction path requires the crossing of two adiabatic surfaces with different spin states.Specifically,it can be described as: ⁵V⁺SCO→⁵IM1→CP1→³IM2→³VS⁺+CO,which is in line with the previous experiment.