Chemisorption of small molecules on palladium in terms of local density functional calculations (original) (raw)
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Carbon-supported palladium catalysts. Molecular orbital study
Journal of Catalysis, 2003
Geometric and electronic structures of palladium clusters supported on activated carbon (Pd n /AC) are analyzed using semiempirical quantum chemical modeling for n = 1-22. Qualitative reliability of the results is checked by DFT calculations for n = 1-6. Supported Pd atoms and clusters are shown to be strongly bound to unsaturated and defect surface sites. In such positions, interaction of Pd atoms with the support is much stronger than that with each other. That provides the driving force for atomic dispersity of Pd/AC catalysts. Geometry of small clusters is determined by morphology of an adhesion position. Nanosized particles form compact three-dimensional structures with close-packed triangular surfaces. AC support causes notable excitations in the electronic structure of metal atoms directly bound to the support, resulting in the direct nucleation of fcc-like structures. These excitations are quickly extinguishing when moving far from the support surface.
Physical Chemistry Chemical Physics, 2007
We investigated the decomposition and (partial) oxidation of methanol on Pd based catalysts in an integrated attempt, simultaneously bridging both the pressure and the materials gap. Combined studies were performed on well-defined Pd model catalysts based on ordered Al 2 O 3 and Fe 3 O 4 thin films, on well-defined particles supported on powders and on Pd single crystals. The interaction of Pd nanoparticles and Pd(111) with CH 3 OH and CH 3 OH/O 2 mixtures was examined from ultrahigh vacuum conditions up to ambient pressures, utilizing a broad range of surface specific vibrational spectroscopies which included IRAS, TR-IRAS, PM-IRAS, SFG, and DRIFTS. Detailed kinetic studies in the low pressure region were performed by molecular beam methods, providing comprehensive insights into the microkinetics of the reaction system. The underlying microscopic processes were studied theoretically on the basis of specially designed 3-D nanocluster models containing B10 2 metal atoms. The efficiency of this novel modelling approach was demonstrated by rationalizing and complementing pertinent experimental results. In order to connect these results to the behavior under ambient conditions, kinetic and spectroscopic investigations were performed in reaction cells and lab reactors. Specifically, we focused on (1) particle size and structure dependent effects in methanol oxidation and decomposition, (2) support effects and their relation to activity and selectivity, (3) the influence of poisons such as carbon, and (4) the role of oxide and surface oxide formation on Pd nanoparticles.
2019
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721 302, West Bengal, India E-mail: anoop@chem.iitkgp.ac.in Manuscript received online 03 May 2019, revised and accepted 22 May 2019 The formation of active catalyst species in the Pd-catalyzed reactions is studied using Density Functional Theory calculations. Taking palladium acetate and simple phosphine as the models, we estimated the energetics for the addition of phosphines, release of acetates, and associative exchange or ligands which convert the unreactive PdOAc<sub>2</sub> to reactive species that takes part in the catalysis. Two consecutive addition of two phosphines, PdOAc<sub>2</sub> + 2PH<sub>3</sub> → Pd(OAc)<sub>2</sub> (PH<sub>3</sub> )<sub>2</sub> ) are exergonic steps with the free energies of activation 8.16 kcal mol<sup>–1</sup> and 5.33 kcal mol<sup>–1</sup>. The associative exchange of acetat...
Langmuir, 1991
Ab initio SCF studies have been performed to study the molecular properties of several single-bonded palladium compounds, PdH, PdC, PdO, PdF, Pdz, and PdCO, which are important in surface and materials science. Electron correlation effects were evaluated by a second-and third-order Mdler-Plesset (MP) perturbation theory and a size-consistency-corrected configuration interaction with single and double substitutions (CISC). Relativistic effects were investigated for PdH and PdF. The ground state of PdC has been calculated at the CISC level to be a 311 state which is only 0.26 eV below the 32state (previously assigned ground state) and 0.51 eV below the 1Z+ state. PdC is predicted to be stable in the gas phase, and the possibility of preparing this compound is investigated. The bonding in CO chemisorbed on palladium is studied by using the model Pd-CO system. The effect of d,-a* back-bonding, discussed at the Hartree-Fock and CI level, is compared with results from multiple-scattering Xa calculations. The C-0 stretching frequency shift for CO on palladium was analyzed at various levels of theory, and the results indicated that the decrease in the CO force constant associated with chemisorption is not solely the result of d,-a* back-bonding. At the CISC level, the calculated chemisorption shift of Au =-72 cm-l in the C-0 stretching frequency compared well with the infrared experimental value (-95 cm-l) for PdCO monomer in a Kr matrix and with terminal CO on the surface of palladium hydrosols (-78 cm-l), while the calculated MP2 value of Aw =-122 cm-' overestimated the experimental result. The calculations indicated that the CO bond distance decreases only slightly (-0.007 A) as a result of chemisorption on single palladium atoms.
Effects of ligand coverage on properties of palladium clusters. A density functional theory study
Physical Chemistry Chemical Physics, 2004
Ligand stabilized clusters of palladium are treated at density functional theory level to investigate effects of ligand coverage on properties, such as preferred coordination sites, metal-ligand binding energies, and structure. Systems investigated include cores of Pd n , for n ¼ 1,2,13,16,19,23,35,39, stabilized by CO, PH 3 , and P(CH 3) 3 , which are either known experimentally or have been proposed as intermediates in syntheses. Some of these systems, e.g. Pd 13 (CO) i , served as models to study aspects of ligand metal binding. The main results are the following. CO binds preferably as m 3 such that coordination sites share at most one vertex atom. The ligand coverage weakens Pd-Pd bonding and quenches the high spin of pure metal clusters. Ligands tend to stabilize Pd cores showing icosahedral Pd 13 structural units such as biicosahedra in Pd 19 and Pd 23 , whereas the corresponding bare metal clusters are most stable as octahedron and decahedron, respectively. The BP86 density functional yields reliable structure parameters and ligand metal binding energies in cases where comparison with experiment or accurate calculations is possible, only Pd(CO) i , i ¼ 1-4, appears to be an exception.
Density Functional Study of the Interactions between Dihydrogen and Pdn (n = 1−4) Clusters
Journal of Physical Chemistry A, 2000
The dihydrogen interactions with Pd n (n) 1-4) clusters was investigated using hybrid density functional Becke3LYP method and two ECP basis sets. The local minima configurations for a number of H 2 molecule approach modes toward Pd n clusters are presented. Some of these states may be interpreted as a physical adsorption and others as dissociative interaction of the H 2 molecule with the palladium cluster. Both geometric and energetic characteristics of weakly bonded pre-dissociated complexes on Pd 3-4 clusters are very close to those on a bulk Pd (111) surface, while the stable states with dissociated hydrogen molecules show significant differences. In contrast to the bulk surface, 2-fold coordination positions exhibit slightly higher stability of hydrogen bonding in Pd 3 and Pd 4 clusters than 3-fold ones. The binding energy is significantly higher for small clusters than for the bulk surface. The Pd 2 cluster was found to be the most active toward hydrogen capture in accordance with the experimental results.
Applied Catalysis A: General, 2004
Carbon supported palladium catalysts have been used in a wide range of industrial reactions, especially hydrogenation in the fine chemical industry. However, there are a lot of difficulties to characterize this system, mainly due to intrinsic characteristics of the carbon materials. This work aimed to evaluate the effect of dispersion and electronic density of palladium on the catalytic performance of different reactions, besides characterizing Pd/C system by several techniques. Two samples of activated carbon and graphite were used in this work. Some pretreatments of activated carbon samples with HCl, HNO 3 and O 2 were performed before the impregnation with palladium. Supports and catalysts were characterized by atomic absorption spectroscopy, N 2 adsorption isotherms at 77 K, temperature programmed desorption (TPDHe), temperature programmed reduction (TPR), CO chemisorption and X-ray photoelectron spectroscopy (XPS). Four reaction probes were performed with the catalysts: cyclohexane dehydrogenation, 1,3-butadiene hydrogenation, dichlorodifluoromethane (CFC-12) and trichloromethane hydrodechlorination. Depending on the electronic density, which is affected by the chemical environment of metal, carbene intermediate formation can be favored, yielding a particular selectivity pattern. The dispersion seems to influence the activity of the reactions, but not selectivity patterns. #
DFT Studies of Palladium Model Catalysts: Structure and Size Effects
Journal of Cluster Science, 2011
An important task for theory is the multi-scale modeling of catalytic properties of nanocrystallites with sizes ranging from clusters of few metal atoms to particles consisting of 10 3 -10 4 atoms. To explore catalytic properties of nanosized metal catalysts, we developed an approach based on three-dimensional symmetric model clusters of 1-2 nm (*100 metal atoms) with fcc structure, terminated by low-index surfaces. With this modeling technique one is able to describe at an accurate DFT level various catalytic and adsorption properties of metal nanoparticles in quantitative agreement with experimental studies of model catalysts deposited on thin oxide films. Metal nanocrystallites exhibit properties that can significantly vary with their size and shape.