Combinatorial computational chemistry approach of tight-binding quantum chemical molecular dynamics method to the design of the automotive catalysts

Recently, we have developed a new tight-binding quantum chemical molecular dynamics program “Colors” for combinatorial computational chemistry approach. This methodology is based on our original tight-binding approximation and realized over 5000 times acceleration compared to the conventional first-principles molecular dynamics method. In the present study, we applied our new program to the simulations on various realistic large-scale models of the automotive three-way catalysts, ultrafine Pt particle/CeO₂(111) support. Significant electron transfer from the Pt particle to the CeO₂(111) surface was observed and it was found to strongly depend on the size of the Pt particle. Furthermore, our simulation results suggest that the reduction of the Ce atom due to the electron transfer from the Pt particle to the CeO₂ surface is a main reason for the strong interaction of the Pt particle and CeO₂(111) support.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.     

Reaction of molecular hydrogen with the 100 face of MoO3: II. Kinetics initiated by atomic hydrogen and characterization of the surface electronic state

In order to provide a physical background to the model proposed in Part I, the kinetics of the reaction between the 100 face of a MoO₃ single crystal and, first a mixture of 4% atomic hydrogen in H₂, and later molecular hydrogen only, has been studied. The rate processes of the activation step and of the stationary step are entirely comparable with those observed for one single crystal surface loaded with platinum particles. Thus atomic hydrogen in the gas phase as well as atomic hydrogen produced by the dissociation of molecular hydrogen on a Pt particle may prepare the favourable surface state able to dissociatively chemisorb molecular hydrogen, ruling out — once again — the classical model of the hydrogen spillover process. This “favourable” state has a Fermi level which is 0.25 eV lower than that of initial MoO₃, as shown by measuring the work function with a Kelvin probe. This lowering is in good agreement with the variation of the free energy between MoO₃ and H_1.6MoO₃, measured electrochemically by others. This suggests that the protons inserted into the surface layers transform the initial MoO₃ layers into layers with composition H_1.6MoO₃. The starting material is thus transformed into a biphasic system, the diffusion of the reaction boundary between the two types of layers being the overall rate limiting process. The Fermi energy of H_1.6MoO₃ being known, it is possible to show that in transformed surface layers the conduction and the valence bands overlap, in agreement with the approximate profile of this band observed by XPS for a reacted single crystal surface. The d character of this band would explain why molecular hydrogen can be dissociatively chemisorbed when this favourable surface state is obtained. The fast electron delocalization within the Mo-O-Mo bonds yields fast oscillations in the oxidation states of the molybdenum atom in the surface layer, accounting for the presence of “oxidized” and “reduced” sites. The formal equation observed in Part I for the rate of stationary step is therefore explained, the impinging H₂ molecules reducing temporarily the oxidized fraction of the surface.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

Effects of surface corrugation on the molecular rotational dependence of H2 dissociative adsorption dynamics on Cu(100)

We investigate and discuss how surface corrugation affects the molecular rotational dependence of H₂ dissociative adsorption dynamics on Cu(100) by performing six-dimensional (6D) quantum dynamics calculations. We calculate the dissociative adsorption probability as a function of the initial rotational state J and the normal energy E_norm of incident molecules, and compare with the dissociative adsorption results obtained by four-dimensional (4D) quantum dynamics calculations where the surface is treated as flat. In our calculation, for the case of normal incidence, the increase in dissociative adsorption probability with increasing E_norm and the non-monotonic behavior of dissociative adsorption probability with respect to J are suppressed on a corrugated surface as compared to that on a flat surface.     

Multi-scale theoretical study of support effect on sintering dynamics of Pt

The capability of theoretical durability studies to offer an efficient alternative methodology for predicting the potential performance of catalysts has improved in recent years. In this regard, multi-scale theoretical methods for predicting sintering behavior of Pt on various catalyst supports are being developed. Various types of Pt diffusions depending on support were confirmed by the micro-scale ultra accelerated quantum chemical molecular dynamics (UA-QCMD) method. Moreover, macro-scale sintering behavior of Pt/γ-Al₂O₃, Pt/ZrO₂ and Pt/CeO₂ catalysts were studied using a developed 3D sintering simulator. Experimental results were well reproduced. While Pt on γ-Al₂O₃ sintered significantly, Pt on ZrO₂ sintered slightly and Pt on CeO₂ demonstrated the highest stability against sintering.     

RAIRS studies of CO adsorption on Pd/CeO2−x(1 1 1)/Pt(1 1 1)

Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on CeO_2−x-supported Pd nanoparticles at room temperature. The results show that when CeO_2−x is initially grown on Pt(1 1 1), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto CeO_2−x/Pt(1 1 1), most of the Pd grows directly on top of the CeO_2−x(1 1 1). RAIR spectra of CO adsorption on 1 ML Pd/CeO_2−x/Pt(1 1 1) show a broad CO-Pd band, which is inconsistent with a single crystal Pd surface. However, the 5 ML and 10 ML Pd/CeO_2−x/Pt(1 1 1) spectra show vibrational bands consistent with the presence of Pd(1 1 1) and (1 0 0) faces, suggesting the growth of Pd nanostructures with well defined facets.     

Detailed balancing and quasi-equilibrium in the adsorption of hydrogen on copper

The apparent equilibrium relationship between adsorption and desorption distributions measured in recent molecular beam experiments is discussed. Through an equilibrium synthesis and the application of detailed balancing, the energy and angle dependence of the dissociative adsorption probability of hydrogen on copper is shown to predict the non-cosine angular distributions of desorption. In a similar construction, the velocity distribution of H₂ desorbing from nickel predicts the peaked desorption angle distributions observed. The implications of the apparent local equilibrium and detailed balancing at low pressure on the mechanism of surface catalyzed isotopic exchange and on the dynamics of chemical processes on surfaces are discussed.     

Theoretical investigation of the adsorption and diffusion of hydrogen on the surface and in the bulk of the intermetallic compound Mg2Ni

The intermetallic compound Mg₂Ni as a potential material for hydrogen storage has been investigated theoretically. The sorption and diffusion of a hydrogen atom in the bulk and on the surface of this material, as well as the step-by-step process of dissociative chemisorption of a H₂ molecule on the surface, have been considered. The dependence of the sorption energy of atomic hydrogen on the structural characteristics of the intermetallic compound Mg₂Ni has been analyzed.     

The adsorption of hydrogen on iron; A surface orbital modified occupancy — bond energy bond order calculation

A Surface Orbital Modified Occupancy — Bond Energy Bond Order (SOMO-BEBO) model calculation of hydrogen adsorption on iron is presented. This calculation represents a novel approach to the CFSO-BEBO method in that the calculation is correlated in a consistent way with the thermal desorption spectra of the hydrogen-iron system. Heats of molecular adsorption calculated are ?32.88, ?35.68 and ?49.57 kJ/mol for the iron (110), (100), and (111) surfaces, respectively. Heats of dissociative adsorption calculated are ?54.40, ?75.30 and ?87.90 kJ/mol for the three states on the iron (111) surface; ?51.21 and ? 73.62 kJ/mol for the two states on the iron (100) surface; and ?63.78 kJ/mol for the one state on the iron (110) surface. Activation energies for dissociative adsorption were found to be small or zero for the iron (111) surface while non-zero activation energies of 49.27 and 45.05 kJ/mol were calculated for the iron (100) and (110) surfaces, respectively. The FeH single-order bond energy has been calculated to be 298.2 kJ/mol. The radius of the hydrogen surface atom has been estimated to be 1.52 × 10^?10 m consistent with the expected size of an H^? ion. The elimination of certain surface sites for molecular adsorption as a result of the ferromagnetism of iron is suggested by the calculation. The reason for the absence of well defined LEED patterns for hydrogen adsorption on the iron (111) and (100) surfaces [Bozso et al., Appl. Surface Sci. 1 (1977) 103] is explained on the basis of the size of the H^? surface ion. The adsorption of hydrogen on the iron (110) surface is consistent with a relatively stable, small-sized H⁺₂ surface ion giving, therefore, a regular LEED pattern and a positive surface potential upon adsorption of hydrogen on this surface.     

Influence of nano-AL2O3-catalyst sizes on hydrogen formation at the water radiolysis under ionizing radiation

The aim of this study was to investigate the regularities of molecular hydrogen formation from water dispersing Al₂O₃ nanoparticles irradiated with gamma ray. It was established that formed molecular hydrogen’s yield changed depending on the size of the catalyst, so that yield of molecular hydrogen formed on the surface with small size is 1.4–1.6 times greater than the one with big size. Equal distribution of nanocatalyst in water medium and much more adsorption of water molecule on the catalyst surface result in more efficient radiolysis process.     

Surface chemistry studied by in situ X-ray photoelectron spectroscopy

Time-dependent X-ray photoelectron spectroscopy is used to study the kinetics and dynamics of simple surface reactions. Combining high-resolution core level spectroscopy with a supersonic molecular beam in one experimental setup, processes such as the dissociative adsorption of methane on both Pt(111) and Ni(111), the coadsorption of water and CO on Pt(111), and the oxidation of CO on Pt(111) have been studied. In the case of methane, the observed vibrational fine structure in C 1s spectra is used to identify the adsorbed species (CH₃) and further thermal dehydrogenation steps. While simple dehydrogenation via CH is observed on Pt(111), a C–C coupling reaction to acetylene is found on Ni(111). In the coadsorbate phase, CO is found to be able to replace predosed water from the bilayer into multilayers. Water, in turn, leads to a site change of the CO molecules, which are preferably adsorbed at bridge sites in the presence of water, as opposed to on-top adsorption on clean Pt(111). For the truly bimolecular surface reaction, the CO oxidation on Pt(111), the ability of the molecular beam to create a relatively high CO pressure was found essential to study the kinetics of the basic step (CO+OCO₂) without influence of adsorption or diffusion rate. An activation energy of 0.53 eV and a preexponential factor of 5×10⁶ s^-1 are found. PACS 68.43.Mn; 79.60.Dp; 82.20.Pm     

Effect of lattice strain on the oxygen vacancy formation and hydrogen adsorption at CeO2(111) surface

Using first-principles calculation, the effect of lattice strain on the oxygen vacancy formation at CeO₂(111) surface has been investigated. The tensile strain facilitates the oxygen vacancy formation at the surface and the compressive strain hinders the process. This is in part due to the strengthening or weakening of the surface Ce–O bond under the lattice strain. On the other hand, a more open surface with a larger lattice constant can better accommodate the larger Ce³⁺ and thus facilitate the structural relaxation of the reduced surface. The studies on the strain effect on the atomic hydrogen adsorption at the defect-free CeO₂(111) surface show that the adsorption strength monotonously increases with the increase of the lattice strain, further confirming the tunable surface chemical activity by lattice strain.     

Resistive switching effects in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3/Pt heterostructures prepared by pulse laser deposition method

The heterostructural junctions of CeO₂/La_0.7(Sr_0.1Ca_0.9)_0.3MnO₃/Pt (CeO₂/LSCMO/Pt) were prepared using pulse laser deposition technique. Their resistive switching (RS) behavior was investigated. As compared to the metal/manganite/Pt junction, the CeO₂/LSCMO/Pt device displayed an improved switching characteristic. The RS effects with characteristics of bipolar, threshold, and complementary were realized by adjusting the thicknesses of the CeO₂ layer in the CeO₂/LSCMO/Pt junctions. Under a higher external bias voltage, the threshold and complementary switching modes of the junctions could turn into bipolar switching mode. The switching behavior shows strong dependence on the O₂ partial pressure during the fabrication, indicating that the amount and behavior of the oxygen at the interface play an important role in the determination of the RS behavior. The observed switching behavior is related to the modification of the accumulation/depletion layers as well as the interfacial potential barrier due to the migration of the oxygen vacancies.     

The adsorption of hydrogen and the reaction of hydrogen with oxygen on Pt (100)

The adsorption of hydrogen on Pt (100) was investigated by utilizing LEED, Auger electron spectroscopy and flash desorption mass spectrometry. No new LEED structures were found during the adsorption of hydrogen. One desorption peak was detected by flash desorption with a desorption maximum at 160 °C. Quantitative evaluation of the flash desorption spectra yields a saturation coverage of 4.6 × 10¹⁴ atoms/cm² at room temperature with an initial sticking probability of 0.17. Second order desorption kinetics was observed and a desorption energy of 15–16 kcal/mole has been deduced. The shapes of the flash desorption spectra are discussed in terms of lateral interactions in the adsorbate and of the existence of two substates at the surface. The reaction between hydrogen and oxygen on Pt (100) has been investigated by monitoring the reaction product H₂O in a mass spectrometer. The temperature dependence of the reaction proved to be complex and different reaction mechanisms might be dominant at different temperatures. Oxygen excess in the gas phase inhibits the reaction by blocking reactive surface sites. At least two adsorption states of H₂O have to be considered on Pt (100). Desorption from the prevailing low energy state occurs below room temperature. Flash desorption spectra of strongly bound H₂O coadsorbed with hydrogen and oxygen have been obtained with desorption maxima at 190 °C and 340 °C.     

Reactive molecular dynamic simulations of hydrocarbon dissociations on Ni(111) surfaces

Empirical potential parameters for H, C and Ni elements have been developed for the ReaxFF force field in order to study the decomposition of small hydrocarbon molecules on nickel using molecular dynamics simulations. These parameters were optimized using the geometrical and energetic information obtained from density functional (DFT) calculations on a subset of hydrogen and methane reactions with nickel (111) surfaces. The resulting force field was then used to obtain a molecular perspective of the dynamics of the methane dissociative adsorption on Ni(111) as well as two other small alkane molecules, ethane and n-butane. NVT simulations of dissociative adsorption of methane over a range of temperatures enabled the estimation of the sticking coefficient for the adsorption as well as the activation energy of the first C–H bond breaking. The rate constants of each elementary step (both forward and reverse) of CH_x dissociation on Ni(111) were obtained by monitoring the surface species and a microkinetic model was constructed as a result. Qualitative analyses of the simulations of ethane and n-butane decompositions on Ni(111) demonstrate that such reactive MD technique can also be used to obtain useful information on complex reaction networks.     

Reaction of molecular hydrogen with the 100 face of MoO3: I. Kinetics in the low pressure range (10−8–10−6 Torr) in the presence of platinum particles

Three successive processes are observed when the 100 face of a MoO₃ single crystal covered with a specific density of platinum particles of known diameter is exposed to molecular hydrogen in a pressure (P) domain between 10^?8 and 10^?6 Torr. During the first step, called the activation step, the rate process is a function of the surface area of the Pt particles and it is slightly thermally activated. During this activation step, H₂ dissociatively chemisorbed on the platinum surface and the hydrogen atoms are inserted into the MoO₃ surface layers. The inserted atom is partially ionized, the electrons given to the lattice reducing Mo⁶⁺ into Mo⁵⁺ and Mo⁴⁺. The rate of hydrogen insertion (V_a) in the activation step increases about linearly with time. The activation step is followed by a stationary step where V is independent of the extent of the Pt surface, V = 0.7Z(1?X), where Z is the number of collisions between H₂ and the MoO₃ surface and X the electronically reduced fraction of the surface area. $\text{X =}\text{a}\text{P}\text{(1 + a}\text{P}\text{)}$, where a is a function of the temperature only. In the stationary step, the platinum particles do not play any role and molecular hydrogen is dissociatively chemisorbed on the MoO₃ surface. The stationary step is followed by a deceleration step. The slowing down of the reaction rate is due to the formation of fractures within the crystal. During all these three steps, the surface is self-cleaning, since protons diffuse into the bulk. The final product of the insertion process would be H_1.6MoO₃, the properties of which have been studied in detail elsewhere. The model which is summarized above is very different from that which would result from the classical views on the so-called hydrogen spillover process since the Pt surface producing the H atoms spilling over the surface would operate only during the activation step.     

Competition between hydrogen and oxygen dissociation on palladium surfaces at atmospheric pressures

A large change in the surface and interface potential of Pd-films on SiO₂ is observed at a constant ratio between hydrogen and oxygen pressure in the ambient. This observation, which is most probably due to a competition between hydrogen and oxygen for the same dissociative adsorption sites, is described in this communication. Furthermore the potential measurements indicate the presence of another type of hydrogen adsorption sites on the oxygen covered surface.     

Active oxidation of Cu3Au(1 1 0) using hyperthermal O2 molecular beam

Oxidation of Cu₃Au(1 1 0) using a hyperthermal O₂ molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O₂ molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O₂ exposures. Dissociative adsorption of O₂ on Cu₃Au(1 1 0) is as effective at the thermal O₂ exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O₂ implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu₃Au(1 1 0) similarly as on Cu₃Au(1 0 0). The growth of Cu₂O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu₃Au(1 0 0).     

The temperature programmed desorption of hydrogen from platinum and platinum-gold films

The thermal desorption of hydrogen from Pt and PtAu films has been measured in an ultra-high vacuum system by means of a mass spectrometer. On the average, hydrogen is more loosely bound on the alloys than on pure Pt. About 50% of the adsorbate is desorbed by pumping at 78 K from the alloys while only a very small percentage is desorbed from Pt at this temperature. After maximum coverage of Pt films by hydrogen adsorption three desorption peaks have been observed: γ (120 K), β₁ (200 K) and β₂ (330 K). The same peaks have been found for the alloys as well but the relative population of the various adsorption types was different. The relative peak heights vary with the alloy composition.