Hiroaki Himei

Density functional theory calculations of the reaction pathway for methane activation on a gallium site in metal exchanged ZSM‐5

Density functional theory is used to describe the reaction profile for methane dissociation on Ga‐exchanged ZSM‐5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.Density functional theory is used to describe the reaction profile for methane dissociation on Ga‐exchanged ZSM‐5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.

Computer-aided design of active catalysts for the removal of nitric oxide

Abstract We report here our computational studies on metal-exchanged ZSM-5 catalysts using molecular dynamics and computer graphics techniques. The structure and the dynamic behavior of copper, gallium and cerium cations in cation-exchanged ZSM-5 framework were studied. Various oxidation states of these metals were simulated and their relevance in the catalytic removal of nitric oxide is discussed. The high Si/Al ratio in ZSM-5 explains the existence of an equilibrium between the different oxidation states of exchanged copper cation, which favors NO reduction. The higher mobility of the oxygen atom attached to gallium and cerium, and the low coordination of these cations are also reasons for their catalytic activity.

The distribution of framework aluminum atoms and extraframework exchanged cations in faujasite as studied by molecular dynamics, NMR simulation, neutron diffraction simulation and computer graphics

Abstract The effectiveness and applicability of molecular dynamics (MD), NMR simulation, and computer graphics to the investigation of the sites and distribution of framework aluminum atoms in faujasites, which cannot be derived from experimental techniques is shown. By using MD calculations, we successfully predicted the adequate aluminum distribution in NaY zeolite ( Si Al =2.43 ), as it reproduces the sites and occupancies of Na + cations which have been reported by neutron diffraction techniques. The validity of our aluminum distribution model in the NaY was strongly confirmed by the simulated 29 Si MAS NMR spectra in agreement with the experimental chemical shifts and intensities. Although the sites and occupancies of the exchanged cations in faujasites can be determined by analytical experimental techniques, their atomistic detailed distribution cannot be derived experimentally. A methodology is proposed to predict the detailed distribution of the Na + cations in NaY by using neutron diffraction spectrum simulation.

Quantum chemical investigation of reactants in selective reduction of NOx on ion exchanged ZSM-5

Abstracts A combination of quantum chemistry, molecular dynamics and computer graphics methods was applied in the investigation of the adsorption and activation mechanism of methane and NO x molecules on Ga-exchanged ZSM-5 catalyst. For CH 4 we found that the initial step is a weak physical adsorption on the Ga site at the distance between gallium and carbon atoms of 2.9 A and the adsorption energy was −4.9 kcal/mol. In the next step the dissociative adsorption of methane was studied. The dissociated complex with CH 3 attached to Ga at 2.0 A and H bonded to the extraframework oxygen appeared to be very favorable and led to energetic stabilization of −63.0 kcal/mol. Another dissociated adsorption state was found when a hydrogen atom was attached to Ga and CH 3 bonded to the extraframework oxygen. In this case stabilization energy was estimated to be −31.4 kcal/mol. Moreover, we described the transition state of methane dissociation. The activation barrier on the Ga site was 31.3 kcal/mol, which means kinetically favorable reaction. For the NO x molecules there is no energy barrier before chemisorption. The adsorption energy for NO and for NO 2 were −18.4 and −10.3 kcal/mol, respectively. It is found that NO is slightly, while NO 2 is significantly activated. The spin distribution analysis also shows an activated NO 2 part after adsorption. It is also observed that the Ga atom becomes stable, tetrahedrally coordinated after each of the adsorption processes.

2.16 Structure and Dynamics of Exchanged Cations in Zeolites as Investigated by Molecular Dynamics and Computer Graphics

Abstract The application of molecular dynamics and computer graphics techniques to understand the structure and the dynamics of different exchanged metal cations in zeolite-A, ZSM-5 and mordenite is described. The specific information derived from the above investigations include the distribution of the exchanged cations as well as the aluminum ions in the zeolite frameworks. The relevance of this information to their adsorption and catalytic properties, in many industrially important processes are studied. The illustrative examples include the molecular sieving effect of zeolite-A for the separation of O 2 and N 2 , the role of multivalent cations in ZSM-5 for the deNO x process and the distribution of aluminum in mordenite.

The role of the multi-body interaction in the de-NOx process on solid catalysts investigated by density functional method

Abstract We have performed density functional calculations to investigate the bimolecular CH 4 + NO x ( x = 1, 2) and the trimolecular NO + CH 4 + NO 2 reactions which can be assumed to occur in the vicinity of an active site on a de-NO x catalyst. We have shown that the trimolecular reactions can take place very smoothly on a singlet energy hypersurface, while the bimolecular reactions producing unstable radicals are not favorable. We have explained these results with the high energy of the reaction products in the bimolecular reactions and the formation of HNO x molecules yielding negative stabilization energy in the trimolecular reactions with the extra NO x molecules which can be considered as spin traps in these processes. We present the geometrical changes together with the electronic changes occurring in the reactions. The significance of this study is that we could demonstrate that the increased concentration of the reaction partners due to an active site of the catalyst allows reactions which otherwise would be unlikely.

Theoretical estimation of ordered metal species in zeolite pores

Abstract The combination of computational techniques such as computer graphics (CG), molecular dynamics (MD) and quantum chemistry (QC) methods are demostrated to be effective for investigating the dynamics and formation of ordered metal species in zeolite pores. The behavior of metal atoms in all-silicon mordenite and ion-exchanged mordenite with Si/Al = 5 have been studied. The MD calculations on mordenite containing many metals in its pore confirm that the zeolite framework is rigid enough to hold the metal particles. In addition to the cohesive force, the concentration, the size of metal particles and size matching to the pore structure of zeolite also influence the ordering and the stability of metals. It is suggested from MD calculations and confirmed by QC calculations that the exchanged cations stabilize the ordering of metal particles in the zeolite pore.