Theoertical Studies On The Adsorption And Catalysis Of Pd_n Clusters And Their Suppotred Catalyst

The adsorption and catalytic properties of Pd_nclusters and theirsupported catalytic materials are researched by using density functionaltheory (DFT) at B3LYP and B3PW91methods. The GEN, effective corepseudopotential (ECP) and ONIOM methods are also applied in this study.The significant contents include the hydrogenation catalyzed by Pd_ncluster, the adsorption and catalytic properties of the Pd_nclusterssupported catalytic materials, and the interaction between the Pd_nclusterand amino acid molecules.The DFT method has been employed to study the adsorbility of Hand C2H₂, the mechanisms of acetylene hydrogenation and their turnoverfrequency (TOF) ratios on Pd_n(n=2-8) clusters. The calculations presentthat the stable reactant is formed by the adsorption of dissociated H atomson hollow sites (the fold of Pd-Pd-Pd) and C2H₂with di-σ type bond onPd_nclusters. There are two feasible pathways of the acetylenehydrogenation to ethane via ethylene on Pd_ncluster. One pathway willproceed via the vinyl (Pd_n(H)···CH=CH₂) and ethene (Pd_n···CH₂-CH₂)intermediates to form the ethane, and the other go along the vinylidene(Pd_n(2H)···C=CH₂), ethylidyne (Pd_n(2H)···C=CH₃) and ethylidene(Pd_n(2H)···CH=CH₃) to ethane. The intermediates in the two pathways,such as the complexes of acetylene and vinylidene, ethylidyne and vinyl,ethylene and ethyl, can transform into each other, giving a complicated reaction profile of selective hydrogenation. On the other hand, thenumber n of Pd_ncluster affects the mechanisms of acetylenehydrogenation on palladium clusters directly. When the n numbers of Pd_nis equal or lesser than4(n≤4), the favorable pathway isPd_n(2H)···CH≡CH→Pd_n(H)···CH=CH₂→Pd_n···CH₂-CH₂→Pd_n(2H)···CH₂-CH₂→Pd_n(H)···CH₂-CH₃→Pd_n···CH₃-CH₃, and when n>4, it isPd_n(2H)···CH≡CH→Pd_n(H)···CH=CH₂→Pd_n···CH-CH₃→Pd_n(2H)···CH-CH₃→Pd_n(H)···CH₂-CH₃→Pd_n···CH₃-CH₃. For the calculation of TOF ratio,it is found that the formation of ethylene contributes most to the rate ofthe cycle, and also indicates that the selective hydrogenation of acetyleneon Pd_nclusters is more efficient than the formation of ethane, which is inaccordance with the experimental result.The adsorption and catalytic properties Pd_nclusters supported ZSM-5functional materials are researched by ONIOM2methods, its contentsinclude: the interaction between Pd_nclusters of ZSM-5and the adsorptionand cracking reaction mechanism of NO on Pd_n/ZSM-5. Pd_nclustershave good catalytic activity and selectivity, but in the actual reaction, it isvulnerable to impact by the environment, leading to sinter and poison,even lose its catalytic activity. ZSM-5zeolite can not only be wellprotected the catalytic activity of Pd_nclusters, but also increase theirsurfaces to improve the thermal stability. In order to understand theinteraction between the Pd_nclusters and ZSM-5zeolite and its effect,88Tcluster are taken from crystallographic data of ZSM-5zeolite to researchby using the two-layer ONIOM2methodology. For the high level (7Tcluster), the calculations are used the B3LYP functional. The LANL2DZbasis set and effective core pseudo-potentials (ECP) method are used forthe palladium atoms of clusters, and6-311+G (d, p) basis set for the otheratoms. The universal force field approach (UFF) is employed for the lowlevel calculations with no charge assigned to atoms while for the highlevel. The calculated results show that the coordination sites of the Pd_nsupported ZSM-5forming complexes are focus on the O atoms bondedwith the Al atom of the zeolite framework, and the interaction sites of Pdclusters are only one Pd-Pd bond whatever the size of clusters.Meanwhile, the Pd_nmove out the straight channel of ZSM-5with the increasing of metal atoms because of the long-range and electrostaticinteraction between the adsorbates and the zeolite framework. Basing onthe previous results, the adsorption and dissication of NO molecule inPd_n/ZSM-5catalyst are researched. The results indicate that the Pd clustercatalyst activity has been enhanced after supported, and there are twopathways for the pyrolysis reaction. For one pathway, N and O atomsadsorbed on Pd cluster, and then pyrolysis atomic final adsorption ontransition metal clusters; in another way, the NO bond dissociated after Natoms adsorbed on the cluster, and the final completion of the pyrolysisprocess.Zeolite as carrier provide a good acidic environment for the Pd_ncluster, continue to study Pd_nclusters supported on hydrotalcite alkalinecarrier such as active components of the interaction between the carrier tofurther clear the carrier itself supported catalytic materials. The Mg/Al=2hydrotalcite laminates accumulation of the crystal model are simulatedPXRD spectra to find a reasonable hydrotalcite laminate surface model.Basing on the above research, the surface model of different Pd_nclustersupported hydrotalcite laminate are obtained, and calculated by thegeneralized gradient functional (GGA) PW91of Dmol3in MaterialStudio (MS) program. The analysis of the structure and energies of Pd_nclusters supported hydrotalcite laminate surface show that the Pd-Pd bondtends to parallel adsorption. Comparing the results, it is found that the Pd_nclusters can not be in accordance with its symmetry, must periodicallyarranged in the surface of the hydrotalcite laminates. It is based on ashort-range disorder, long-range order load of layered materials with aspecial crystal surface structure formed in the laminate surface.For further in-depth study of the adsorption and catalytic propertiesof Pd_nclusters supported catalytic material, the interaction betweenamino acid molecules and Pd_nclusters are studied. The results show thatthe electrons transfer from the transition metal to the amino acidmolecules during the interaction process, which also present in the macrochange of the structure and energy. According to the results, thephoto-isomerization mechanism of no-interact and interact L-tyrosinehave been researched. Two independent channels of no-interact L-tyrosine photoisomerization are found, which are symmetrical andunsymmetrical, respectively. In symmetrical channel, the proton ofL-tyrosine is transferred from carbon atom to the carboxyl oxygenforming the intermediate after excited to S₁state. Then, the intermediateis converted from S₁to T₁to continue the racemization until theD-tyrosine forms. Meanwhile, the D-tyrosine is transited from T₁to S₀state which accompanies the release of phosphorescence. There are twodifferent enol intermediates in symmetrical channel. When L-tyrosinetransited from S₁to T₁state, that proton migration is taken place inunsymmetrical channel through different intermediates and transitionstates. Subsequently, it follows the remainder steps as the symmetricalchannel to complete the racemization reaction. In contrast, theisomerization can not occur in interact tyrosine for the high barrier higher.