Dissertation > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalytic reaction

Catalyzed by Gold:Accurate Quantum Chemical Calculations

Author FangZuoCheng
Tutor LiZhenHua
School Fudan University
Course Applied Chemistry
Keywords Accurate quantum chemical calculation Catalyzed by gold CO oxidation Formic acid decomposition Ammonia Borane-ammonia complex
CLC O643.32
Type Master's thesis
Year 2012
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The quantumchemicalcalculation can be applied to the gas phase, solution (solvationmodel), surface catalysis, solid crystals, thermodynamics and kinetics, etc., covering most of the chemical systems and issues which the chemists concerned. As having developed rapidly for many years, the calculation ability of quantum chemistry has been significantly improved comparing to the previous. For the calculation of small molecules, the accuracy, reliability, have reached or even transcended the experimental level. Comparing with the real systems, the average error of energy is less than1kcal/moland average error of optimized bond length is less than0.01A.However, there are some problems in quantum chemical calculations, of which two are particularly important.First, the larger the size of system is, the greater the computational cost is needed. In this situation, sacrificing accuracy or makingvarious approximation is forced to employ to reduce the computational cost.Second, the applicability of different calculation methods in different systems are also hard to define.Sometimes unthinkingly using a method will lead to large error or even qualitative errors. Moreover, precisely because of so many methods available, it always lacks strong basis when choosing one. Particularly for thenonbonding interaction which is very concerned by chemists, the performances of the majority of the DFT methods, including popular PW91, PBE and B3LYP are relatively poor.Now there are a lot of new methods, most of whichare short of the generalratificationof the users.Gold clusters and nanoparticles supported or unsupportedhave been found to have significant catalytic activity eventhough bulk gold is inert as we know. Since1983, Haruta evoke interest in gold clusters andnanoparticles as a catalyst for CO oxidation and many otherimportant catalytic reactions to date.Many models havebeen proposed to explain the high activity of gold clustersand nanoparticles. However, only two aspects are generallyaccepted by researchers. One is the so called size dependent ofthe activity of gold clusters and nanoparticles and the other is that the support also has great effect on the activity bymodifying the electronic properties of the supported goldcatalysts.Recently, two reaction mechanisms for CO oxidationon oxide supported Au is under debating:a dissociativemechanism mostly on inactive supporting materials (e.g. MgO)on which O2breaks O-O bond forming an O-Aux-O intermediatebefore reacting with CO, and an associativemechanism mostly on active supporting materials (e.g. TiO2)on which both CO and O2adsorb on gold forming anAux(OCOO) intermediate and subsequently O2transfersone oxygen atom to CO to form CO2and leave one oxygenatom to the gold. Recently, dehydrogenation of HCOOH is regarded as apotentialhydrogen resource infuel cells.The selectivity todehydrogenation (HCOOH→H2+CO2) and dehydration(HCOOH→H2O+CO) routes in catalytic or non-catalytic conditions is greatly concerned by theoretical studies.Furthermore, the ability to form CO-freeH2in various catalysts is studied by Density Functional Theory (DFT) or direct ab initio methods.In Chapter3, to investigate the performance of DFT methods,42DFTfunctionals have been evaluated and compared with high-level wavefunction based methods.It was found that in order to obtain accurate results the functionals used must treat long rangeinteraction well. The double-hybrid mPW2PLYP and B2PLYP functionals are the two functionalswith best overall performance. CAM-B3LYP, a long range corrected hybrid GGA functional,also performs well. On the other hand, the popular B3LYP, PW91, and PBE functionals do notshow good performance and the performance of the latter two are even at the bottom of the42functionals. Our accurate results calculated at the CCSD(T)/aug-cc-pVTZ//mPW2PLYP/aug-cc-pVTZ level of theory indicate that Au atom is a good catalysis for CO oxidation. Thereaction follows the following mechanism where CO and O2adsorb on Au atom forming anAu(OCOO) intermediate and subsequently O2transfer one oxygen atom to CO to form CO2andAuO. Then AuO reacts with CO to form another CO2to complete the catalytic cycle. The overallenergy barrier at0K is just4.8kcal/mol and free energy barrierat298.15K is20.9kcal/mol.Chapter4is a continuation of Chapter3. For Au2and Au3catalyze CO oxidation, mPW2PLYP/MBS method is employed, and compared with the CCSD(T)/MBS//mPW2PLYP/MBS method. For Au1-3clusters catalyze CO oxidation in CCSD(T)/MBS level, the activity of Au3cluster is the best. The overall reaction energy barrier is only-6.5kcal/mol. Au2has the worst catalytic activity, the total reaction energy barrier of which is36.5kcal/mol. As the parity nature of gold cluster, we can predict that the odd-numbered gold clusters can catalyze CO oxidation, even the CO oxidation on the Au3cluster can occur spontaneously while theactivity of Au2cluster is not as good as the odd one.In Chapter5, mPW2PLYP functional and CCSD(T)/CBS method are employed on small gold clusters catalyzing formic acid decomposition reaction. The mechanismscontain complex competition of path. Even in a same path, there are three key transition states with similar energies which compete with each other. Thus the discussion of the mechanism in the gold clusters requires such a precise method as CCSD(T)/CBS. The free energy barriers of formic acid dehydrogenation catalyzed by Au1-3clusters are37.7,26.2and13.2kcal/mol, respectively. Changes in the free energy-barrier in formic acid solution is small.The differences between them are5kcal/mol or less. Solvent effect is very sensitive to the nature of the structure and polarity. Catalytic activity of Au4-6clusters in similar mechanism is less than that of Au3clusters.The key transition state energiesare in high and low zigzag arrangement which is in line with the discussion of parity nature of the cluster electronic structure. Generally, small gold clusters, especially the Au3cluster have very good catalytic activity for formic acid dehydrogenation.Chapter6reports for the first time that under ammonia atmosphere,ammonia borane (AB) reversibly absorbs up to at least6equiv. ofNH3, forming liquidAB(NH3),,(n=1-6) complexes at0℃. Reasonable structures for AB(NH3),, wereidentified via density functional theory calculations, which indicate that the strongclassical hydrogen bond formed between the lone pair of NH3and the-NH3of AB isthe driving force for the absorption of ammonia by AB. By usingmPW2PLYP/MBS calculations,the enthalpy change (AH) for AB to absorb one NH3was determined to be2.5kcal/mol, which is in good agreement with experiments. Finally, Raman spectra of AB(NH3)n were collected and calculated.

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