Konstantin M. Neyman

Support nanostructure boosts oxygen transfer to catalytically active platinum nanoparticles

Interactions of metal particles with oxide supports can radically enhance the performance of supported catalysts. At the microscopic level, the details of such metal-oxide interactions usually remain obscure. This study identifies two types of oxidative metal-oxide interaction on well-defined models of technologically important Pt-ceria catalysts: (1) electron transfer from the Pt nanoparticle to the support, and (2) oxygen transfer from ceria to Pt. The electron transfer is favourable on ceria supports, irrespective of their morphology. Remarkably, the oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt and, thus, is inherently a nanoscale effect. Our findings enable us to detail the formation mechanism of the catalytically indispensable Pt-O species on ceria and to elucidate the extraordinary structure-activity dependence of ceria-based catalysts in general.

Metal nanoparticles as models of single crystal surfaces and supported catalysts: Density functional study of size effects for CO/Pd(111)

Large octahedral and cuboctahedral palladium clusters, ranging from Pd55 to Pd146, have been investigated by means of all-electron relativistic density functional calculations. Adsorption of CO molecules on the (111) facets of these clusters was also studied. In particular, we focused on the interaction of CO (a single molecule per facet) with threefold hollow sites to inspect the variation of the calculated adsorption parameters with cluster size. We considered how observables calculated for that adsorption position on cluster facets relate to adsorption properties of the corresponding site at the single crystal surface Pd(111). We demonstrated for the first time that, with three-dimensional cluster models proposed here, one can reach cluster size convergence even for such a sensitive observable as the adsorption energy on a metal surface. We also addressed size effects on interatomic distances and the cohesive energy of bare Pd nanoclusters whose structure was fully optimized under the imposed Oh symmet...

Cluster embedding in an elastic polarizable environment: Density functional study of Pd atoms adsorbed at oxygen vacancies of MgO(001)

Adsorption complexes of palladium atoms on Fs, Fs+, Fs2+, and O2− centers of MgO(001) surface have been investigated with a gradient-corrected (Becke–Perdew) density functional method applied to embedded cluster models. This study presents the first application of a self-consistent hybrid quantum mechanical/molecular mechanical embedding approach where the defect-induced distortions are treated variationally and the environment is allowed to react on perturbations of a reference configuration describing the regular surface. The cluster models are embedded in an elastic polarizable environment which is described at the atomistic level using a shell model treatment of ionic polarizabilities. The frontier region that separates the quantum mechanical cluster and the classical environment is represented by pseudopotential centers without basis functions. Accounting in this way for the relaxation of the electronic structure of the adsorption complex results in energy corrections of 1.9 and 5.3 eV for electron a...

Dramatic reduction of the oxygen vacancy formation energy in ceria particles: a possible key to their remarkable reactivity at the nanoscale

We address the formation of the energetically most favourable single oxygen vacancies in ceria nanoparticles (CeO2)n focusing on their size dependence. We study a series of structures with increasing number of CeO2 units (n = 21, 30, 40 and 80) that, according to well tested interatomic-potential calculations, approach the global minima for these particle sizes. The structures thus obtained are refined by means of density functional (DF) methods, modified by the on-site Coulomb correction. Subsequent DF calculations are performed to quantify and analyse the depletion of atomic O from the nanoparticles that results in the formation of a vacancy Ovac. We show that (i) removal of a low- (two-)coordinate O atom from ceria species requires the lowest energy, in line with evidence from other metal oxides; (ii) the depletion of such O atoms from the nanoparticles is strongly facilitated compared to extended (even irregular) surfaces; (iii) increase of the particle size is accompanied by a dramatic decrease of the Ovac formation energy, implying that at certain sizes this energy should reach a minimum; (iv) the size dependence of the Ovac formation energy is driven by the electrostatics, thus enabling the prediction of the most easily removable O atoms by analysing the distribution of the electrostatic potential in the pristine stoichiometric (vacancy-free) ceria systems. Our findings provide a key to rationalize the observed spectacularly enhanced reactivity of ceria nanostructures.

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

Complete dehydrogenation of methane is studied on model Pt catalysts by means of state-of-the-art DFT methods and by a combination of supersonic molecular beams with high-resolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH(3) and CH(2) are specially stabilized at sites located at particles edges and corners by an amount of 50-80 kJ mol(-1). This stabilization is caused by an enhanced activity of low-coordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH(3) and CH(2) formation steps are endothermic on Pt(111) but markedly exothermic on Pt(79). An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJ mol(-1) for first C-H bond scission and 40 kJ mol(-1) for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO(2) film on Cu(111). It is shown that CH(3) generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single-crystal surfaces in which CH(3) was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation.

Density functional study of Pd nanoparticles with subsurface impurities of light element atoms

Atomic H, C, N and O at the surface and in the subsurface region of Pd nanoparticles were studied theoretically using an all-electron scalar relativistic density functional approach. We modelled nanosize metal clusters by the three-dimensional crystallites Pd79 and Pd116 chosen as octahedral fragments of bulk Pd; these clusters expose (111) and (001) facets. Adsorbed atoms were located at the three-fold hollow sites in the centre of (111) facets. Migration of the atoms from the surface of the cluster Pd79 to the octahedral subsurface (oss) site below was considered. Migration of C from the surface hollow site to the oss position was found to be almost isoenergetic; migration of H is somewhat endothermic (by 0.5 eV). For N and O, a lager endothermicity was calculated. Both H and C species exhibit moderate activation barriers for the diffusion to the oss site. C and O atoms in the tetrahedral subsurface (tss) position of the cluster Pd116 were also studied. For both species, this location is energetically disfavoured, although the endothermic effect of O atom migration to the tss position is ∼0.5 eV smaller than to the oss site. Subsurface C impurities were calculated to reduce the adsorption energy of CO molecules at Pd clusters.

N2 and CO molecules as probes of zeolite acidity: an infrared spectroscopy and density functional investigation

The interaction of N2 with Brønsted acid centers of H-ZSM-5 zeolite has been investigated employing Fourier transform infrared spectroscopy and cluster model calculations based on a gradient corrected density functional method. A comparison is made with CO, which is widely used as a probe for surface acidity. It is shown that the computational approach is capable of almost quantitatively reproducing a number of sensitive parameters of the H-bonded dinitrogen and carbonyl complexes, like adsorption energy, adsorption-induced changes of the vibrational frequencies and of their intensities. According to a constraint space orbital variation analysis, the carbonyl and dinitrogen complexes mainly differ by the somewhat stronger σ donation ability of CO as compared to N2. It is concluded that dinitrogen may serve as a convenient probe for the acidity of zeolites.

ADSORPTION OF PD ATOMS AND PD4 CLUSTERS ON THE MGO(001) SURFACE : A DENSITY FUNCTIONAL STUDY

Abstract We performed gradient-corrected density functional calculations on the adsorption of Pd atoms and Pd 4 clusters on the MgO(001) surface. The surface was represented by clusters embedded in point charge arrays and total ion model potentials. Pd atoms adsorb on top of the O ions with a binding energy of 0.9±0.1 eV and a distance of 2.2 A. A Pd 4 square cluster is almost perfectly accommodated to the MgO substrate, suggesting a preference for pseudomorphic growth of large Pd particles. PdPd distances are substantially elongated compared to the bulk metal, in agreement with observations on small supported Pd clusters.

CO bonding and vibrational modes on a perfect MgO(001) surface: LCGTO-LDF model cluster investigation

Abstract The adsorption of isolated CO molecules on a perfect MgO(001) surface is investigated theoretically by the LCGTO-LDF cluster method. Internuclear distances, adsorption energies as well as frequencies and absolute intensities of the MgOCO and CO vibrational modes are calculated for a number of MgO/CO and MgO/OC model clusters containing 2–18 substrate atoms and a large array of surrounding point charges. The convergence of the computed observables with cluster size is discussed. The LDF results are compared to those of previous HF studies. MgOCO adsorption is found to be mainly due to the electrostatic interaction of the CO molecule with the Madelung field on the surface. A small charge transfer of about 0.1 au from the CO 5σ orbital to the substrate also takes place and, besides Pauli repulsion, contributes to the overall blue shift of the CO vibrational frequency. The cluster models predict an approximate doubling of the CO vibrational intensity upon adsorption. This intensity enhancement derives to a large extent from a change of the π component of the CO dynamical dipole moment due to Pauli repulsion between the adsorbate at the cation atop position and the nearest neighbour surface anions. The calculated change of the CO vibrational intensity is at variance with that obtained in a previous analysis of the observed coverage dependence of the CO frequency. The MgOCO vibrational mode is calculated to have a frequency lower than 200 cm −1 and an absolute intensity of about 0.5 km/mol.

How the C-O bond breaks during methanol decomposition on nanocrystallites of palladium catalysts.

Experimental findings imply that edge sites (and other defects) on Pd nanocrystallites exposing mainly (111) facets in supported model catalysts are crucial for catalyst modification via deposition of CH(x) (x = 0-3) byproducts of methanol decomposition. To explore this problem computationally, we applied our recently developed approach to model realistically metal catalyst particles as moderately large three-dimensional crystallites. We present here the first results of this advanced approach where we comprehensively quantify the reactivity of a metal catalyst in an important chemical process. In particular, to unravel the mechanism of how CH(x) species are formed, we carried out density functional calculations of C-O bond scission in methanol and various dehydrogenated intermediates (CH3O, CH2OH, CH2O, CHO, CO), deposited on the cuboctahedron model particle Pd79. We calculated the lowest activation barriers, approximately 130 kJ mol(-1), of C-O bond breaking and the most favorable thermodynamics for the adsorbed species CH3O and CH2OH which feature a C-O single bond. In contrast, dissociation of adsorbed CO was characterized as negligibly slow. From the computational result that the decomposition products CH3 and CH2 preferentially adsorb at edge sites of nanoparticles, we rationalize experimental data on catalyst poisoning.