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A new time-dependent Monte Carlo approach, tdMC, is presented. This allows one to manage the quantum-chemical information relating to surface catalytic processes and rationalize, with atomistic dynamical perspectives, the corresponding reaction mechanism by providing descriptors that can be compared with experimentally obtained data. The approach, which falls into the more general microkinetic paradigm, is strictly self-consistent as it exploits information framed in just one computational method based on the density functional theory. The results simulated by the tdMC algorithm concern the isomerization of but-1-ene to cis- and trans-but-2-ene on Pd surfaces. This reaction was chosen mainly to focus on the development and implementation of the model as well as to point out the characteristics of the code and the soundness of the approach. In order to reach these goals, the simulated findings were compared to related experimental and computational literature data. From the study, it clearly emerges that the tdMC approach, although conceptually very straightforward and simple, is flexible enough to pinpoint the main characteristics of the reaction, which is just seemingly elementary and conversely governed by a complex mechanism involving, besides isomerization, even hydrogenation and dehydrogenation processes. Noticeably, new insights into the title reaction were also provided by the proposed approach.

The preferred location and the corresponding energetics of zeolite-embedded single metal atoms and small metal particles are hot topics within active site optimization and catalyst tuning, even as part of bifunctional materials design. In this context, periodic density functional theory was used to provide insights on the interactions of a platinum atom with the microporous cages of a purely silicious β-zeolite (BEA) framework. Cluster growth was subsequently addressed, up to Pt3@BEA systems, following a one-by-one platinum atom addition; platinum migration between cages was taken into account as well. An unbiased approach was employed, which allowed a wide panorama of structures being considered in addition to a thorough analysis in terms of energetics, cluster geometries, and cavity distortions. Calculations revealed that the optimal interaction geometry for a single platinum atom is realized where two strong Pt–O bonds in almost linear arrangement can form, regardless of the cavity involved. This can cause distortions or even breaking of the zeolite structure, a factor which however is not decisive in determining the energetics of systems with two and three platinum atoms. Platinum migration is associated with energy barriers ranging from 100 to 200 kJ mol–1, depending on the cages. Up to the dimensions considered here, preference for clustering is observed, being the embedded Pt3systems in almost all cases energetically favored with respect to isolated atoms within the BEA framework.

Carbon dots are an emerging family of zero-dimensional nanocarbons behaving as tunable light harvesters and photoactivated charge donors. Coupling them to carbon nanotubes, which are well-known electron acceptors with excellent charge transport capabilities, is very promising for several applications. Here, we first devised a route to achieve the stable electrostatic binding of carbon dots to multi- or single-walled carbon nanotubes, as confirmed by several experimental observations. The photoluminescence of carbon dots is strongly quenched when they contact either semiconductive or conductive nanotubes, indicating a strong electronic coupling to both. Theoretical simulations predict a favorable energy level alignment within these complexes, suggesting a photoinduced electron transfer from dots to nanotubes, which is a process of high functional interest. Femtosecond transient absorption confirms indeed an ultrafast (<100 fs) electron transfer independent of nanotubes being conductive or semiconductive in nature, followed by a much slower back electron transfer (≈60 ps) from the nanotube to the carbon dots. The high degree of charge separation and delocalization achieved in these nanohybrids entails significant photocatalytic properties, as we demonstrate by the reduction of silver ions in solution. The results are very promising in view of using these “all-carbon” nanohybrids as efficient light harvesters for applications in artificial photocatalysis and photosynthesis.

The ethylene glycol, CH2OH–CH2OH, decomposition mechanism, occurring on a subnanometric palladium cluster shaped by 12 atoms, was investigated by means of density functional theory. Different reaction routes were identified leading to H2and CO. The whole reaction network was analyzed, framing the results within the graph theory. The possible decomposition pathways were discussed and compared, allowing one to draw a whole picture of all the parallel, possibly competitive, routes that starting from CH2OH–CH2OH originate H2and CO.

Decomposition pathways of 1,2-propanediol (1,2-PDO) on platinum were investigated by means of experiments and quantum-mechanical calculations. Different reaction paths on a Pt(111) model surface were computationally screened. Gas and liquid phase products distribution for aqueous phase reforming of 1,2-PDO solutions was experimentally analyzed. A mechanistic approach was used to trace the preferred paths according to calculated activation barriers of the elementary steps; in this way, the presence or absence of some hypothesized intermediates in the experiments was computationally rationalized. Hydroxyacetone was demonstrated to be among the most favored decomposition products. The competition between C–H, O–H, and C–C bond cleavages was investigated, revealing that shortening of the carbon chain occurs most likely via decarbonylation steps.

Nucleation of homo- (Ni, Pd, Re, Pt) and heterometallic (Ni–Pd, Re–Pt) clusters on monovacancy sites of a graphene sheet has been investigated by means of periodic density functional theory calculations. It is shown that a vacant site in graphene is an effective nucleation center for both the monometallic and bimetallic clusters, whose characteristics are described in terms of structural distortions, nucleation energetics, affinities between different metal atoms, metal–carbon interactions, and ease of diffusion of metal atoms on graphene.

Biomass-based feedstocks are often oxygenated compounds characterized by large amounts of hydroxyls. As an example, polyols and sugar alcohols are largely employed as reactants for different chemical catalytic transformations such as oxidation, dehydration, and hydrodeoxigenation, which usually occur in aqueous medium. With the goal of gaining new insights into processes that could be industrially relevant, the adsorption of the OH groups on metal surfaces and in the presence of water has to be correctly reproduced and described, within a chosen theoretical framework. Toward this goal, several tests were performed on the catalytically relevant metal Ru, Pd, and Re, benchmarking their bulk and surfaces properties in the frame of the DFT paradigm and employing numerical atomic orbitals, joined with different exchange-correlation vdW–DF functionals. In particular, methanol, ethylene glycol, glycerol, and erythritol adsorption processes on Ru (0001), Pd (111), and Re (0001) surfaces were investigated, as well as their coadsorption with water.

This interdisciplinary work combines the use of shape- and size-defined Pd nanocrystals (cubes of 10 and 18 nm, and octahedra of 37 nm) with in situ techniques and DFT calculations to unravel the dynamic phenomena with respect to Pd reconstruction taking place during acetylene hydrogenation. Notably, it was found that the reacting Pd surface evolved at a different pace depending on the shape of the Pd nanocrystals, due to their specific propensity to form carbides under reaction conditions. Indeed, Pd cubes (Pd(100)) reacted with acetylene to form a PdC0.13phase at a rate roughly 6-fold higher than that of octahedra (Pd(111)), resulting in nanocrystals with different degrees of carburization. DFT calculations revealed changes in the electronic and geometric properties of the Pd nanocrystals imposed by the progressive addition of carbon in its lattice.

Density functional theory calculations were employed to investigate the nucleation and growth of small palladium clusters, up to Pd9, into a microcavity of the porous hyper-cross-linked polystyrene (HPS). The geometries and the electronic structures of the palladium clusters inside the HPS cavity, following the one-by-one atom addition, are affected by a counterbalance between the Pd–phenyl (Pd−Φ) and Pd–Pd interactions. The analysis performed on energetics, cavity distortions, and cluster geometries indeed suggest that the cluster growth is dominated by the Pd−Φ interactions up to the formation of Pd4aggregates, whereas the metal–metal interactions actually rule the growth of the larger clusters. The elasticity of the hyper-cross-linked polystyrene matrix also plays an important role in the cluster development processes.

In the frame of DFT paradigms, the adsorption of 2-methyl-3-butyn-2-ol (MBY) and 2-methyl-3-buten-2-ol (MBE) on a Pd30cluster, including both {100} and {111} faces, was studied along with the pathways involved in the hydrogenation, taking place on plane and low coordination (corner/edge) sites of given MBY/Pd30and MBE/Pd30surface configurations. The calculated energetics, further validated by gas-phase and water-assisted gas-phase MBY and MBE hydrogenation, performed on well-defined size and shape-controlled Pd nanoparticles supported on SiO2, were able to explain the origin of the structure sensitivity and the high selectivity characterizing the title reaction when occurring in aqueous solution. The CC moiety of the MBY surface species indeed seemed to be mostly activated by plane sites instead of corner/edge atoms, whereas the MBE species appeared to have a different behavior, with their CC moieties typically being activated by low coordination sites. DFT studies excluded that the overhydrogenation paths could be affected by the site topologies; hence, the role of plane, edge, or corner atoms should not be influential in setting the surface reaction mechanism, which as a consequence could be controlled by the adsorption energy, actually distinguished by different values on sites of different topology. The role of water in the selectivity to MBE, which characterizes the catalytic overhydrogenation of MBY on Pd nanoparticles, was also inferred.

The reaction path leading to the partial and total reduction of alkynols and alkyndiols with general formula R–CH2–CC–CH(OH)–R′ and R–CH(OH)–CC–CH(OH)–R′ (R, R′ = H, CH3) on a D3hsymmetry Pd9cluster where atomic hydrogen is available have been analyzed by means of calculations based on density functional theory. The results, analyzed and discussed in atomistic details, suggest that small palladium clusters could be selective on the partial hydrogenation of triple bonds. The first energy barrier for the hydrogenation of the alkene derivatives is in fact much higher than the one corresponding to the hydrogenation of the triple bond. Further, the products of the partial hydrogenation, also when adsorbed on the Pd9cluster, are largely more stable than the corresponding alkynol or alkyndiol parents.

The catalytic dehydration of propan-2-ol over H-Y and H-ZMS-5 aluminated zeolite models, mimicking both internal cavities and external surfaces, was studied by DFT calculations to investigate the reaction mechanism. After the adsorption of propan-2-ol on the zeolite, the dehydration mechanism starts with alcohol protonation, occurring by one acidic –OH group of the zeolite fragment, followed by a concerted β-elimination to give propene. The catalytic activity is affected by the size of the zeolite cavity, which is larger in the H-Y than in the H-ZMS-5 zeolite. The adsorption energy of the reagent, as an example, decreases in the order: H-Y cavity ≃ H-ZMS-5 surface > H-ZMS-5 cavity, pointing that the adsorption process should preferentially occur either on open surface or inside larger cavity. More interestingly, confinement effects play a twofold role in driving the reaction pathway, resulting in two different effects on the reaction outcomes. The thermodynamic stability, evaluated by the standard free energy difference of the products (water and propene) with respect to the reactant (propan-2-ol), would indeed suggest that the reaction more smoothly could occur for the systems: H-ZMS-5 surface > non-catalyzed> H-Y cavity > H-ZMS-5 cavity. The activation standard free energy of the process conversely decreases in the order: non-catalyzed> H-ZMS-5 surface > H-ZMS-5 cavity > H-Y cavity, suggesting that the reaction is faster inside zeolite cavities. Experimental and computational results are in agreement, giving confidence on the atomistic-level insights provided.

Adsorption of five l-arabinose tautomers, one acyclic and four cyclic (α, β, pyranose, and furanose) species, on a ruthenium surface was studied as a precursor process of the, nowadays more and more, industrially important sugar catalytic hydrogenation on metal surfaces in water medium. The study was mostly referred to a 37 atom metal catalyst fragment even though border effects on the adsorption processes were also checked employing a 61 atom metal fragment. To figure out conformational effects on the title process, the tautomer flexibility was, at first, investigated by the genetic algorithm based code Balloon considering the conformational spaces of the different aquo tautomers. On the whole, 30 l-arabinose conformers, representing the complete conformational set (of a realistic water solution), were isolated by the genetic algorithm based code Balloon. These were further refined at density functional theory (DFT) level and then were analyzed when interacting with a ruthenium surface, always at DFT level, by SIESTA. It was found that (1) cyclic l-arabinose tautomers give rise to less strong adsorption than the acyclic tautomeric form; (2) l-arabinose molecules preferentially adsorb perpendicularly to the metallic surface; (3) one among the α-pyranose and one among the β-furanose derivatives are largely the most abundant adsorbed species; (4) the dominant l-arabinopyranose and l-arabinofuranose surface configurations are clearly related to corresponding not-adsorbed species that preserve both conformations and intramolecular hydrogen bonds during their adsorption. The consideration of the points above allowed us to pick out significant properties characterizing l-arabinose adsorption on ruthenium.

Factors affecting the strength of nitrogen physisorption at monolayer coverage on different catalytic oxide carriers (e.g., ZnO, MgO, Al2O3, ZrO2, TiO2, and SiO2) have been addressed. Isotherm elaboration by the two-parameter BET equation provides C-constant values (80–200) inversely related to the polarizing power (PP) of the oxide adsorbent irrespective of the surface area exposure. The energy of monolayer formation depends on the extent of charge-delocalization characterizing the surface cation-oxygen bond, which determines the acid–base character of the oxide and strength of van der Waals interactions with adsorbate molecules. Density functional theory (DFT) calculations on MgO and TiO2systems support experimental BET findings, showing enhanced electron-density gradients and adsorption energy of the N2molecule on the former system owing to a stronger polarity of the Mg–O bond.

Isomerization of trans-but-2-ene to cis-but-2-ene and double bond migration of trans-but-2-ene to but-1-ene have been investigated by means of density functional theory calculations on a suitable model of H-ZSM-5 surface. The study has been afforded on outer surface sites by considering the hydroxyl group of either a SiO2(OH)2or a AlO2H(OH)2moiety. On these outer surface sites, one alkoxide species occurs as a stable intermediate both along the isomerization and double bond migration pathways. The latter process could also occur via a single-step mechanism, which involves a six-center transition state. The energy barriers of the outer surface processes above do not take any advantage by the occurrence of Al↔Si substitutions on the surface, resulting the considered butene transformations negatively affected, indeed. This occurrence can be explained since the reactions on the external surface do not involve acidic hydrogen atoms of the AlO2H(OH)2moiety.

The isomerization of cis-but-2-ene to trans-but-2-ene within a 22T H-ZSM-5 zeolite model, also in the presence of two adsorbed Pd atoms, has been studied by DFT calculations. The results obtained allow us to state that the cis/transbut-2-ene isomerization can easily proceed inside unsupported zeolite cavities. In this case, differently than in the gas phase reaction, the trans-but-2-ene is less stable than the cis-but-2-ene, when adsorbed on the zeolite inner surface. Excluding the adsorption−desorption steps, the isomerization process involves two intermediates and three transition states, whose energy content is always very low with respect to that of reagents and intermediate species. The reaction is in principle allowed also in the presence of two Pd atoms embedded inside the zeolite cavity. However, strong H-Pd interactions seem to cause higher activation energies along the formation of the involved intermediates and transition states. To evaluate the confining effects of the zeolite room on the cis/transisomerization process, the latter has been also analyzed on protonated (Pd2H+) and unprotonated (Pd2) bare palladium fragments at different multiplicity states. The but-2-ene adsorption on the considered systems and the mutual influence occurring between the metal atoms and the hydrogen acidic sites at different multiplicity states have also been taken into consideration.

A ZSM-5 fragment, containing 52 tetrahedral moieties, each of them formed by one silicon or one aluminum atom surrounded by four oxygen atoms, was employed to model (52Tsystems) by quantum chemical calculations (i) the influence of the positions of the acidic sites on the energetics of 22 aluminum monosubstituted and bisubstituted 52Tacidic zeolite (H−ZSM-5) systems and (ii) the local adsorption properties and acidic strength of the corresponding −OH sites. The energetics and the structural properties of simpler acid H−ZSM-5 systems containing only five tetrahedral moieties (5Tsystems) were also modeled for comparison. B3LYP/6-31G(d,p) partial geometry optimization routines were performed on the 5Tand 52Tsystems. On the latter, ONIOM(B3LYP/6-31G(d,p)AM1) calculations and an alternative approach, i.e., the ONIOM method followed by a single-point at the above B3LYP/6-31G(d,p) level, aimed to decrease the need of computational resources, were also employed to analyze the properties of the different H−ZSM-5 models. The whole results showed that the orientation and the position of the acidic hydrogen atoms within the zeolite channel strongly affect the stability of the model systems, irrespective of the starting local topology characterizing the Al ↔ Si substitution site. Brönsted gas-phase acidity strength and adsorption-ability were evaluated through the analysis of the energy involved in (i) the proton dissociation from the acidic sites and (ii) the cis-but-2-ene and trans-but-2-ene adsorption on the same acidic sites. Both were affected, although to a very different extent, by the location and number of the considered −OH acidic groups. In particular, 2 among the 12 modeled acidic sites resulted in a highly stabilized zeolite structure, pointing out that the Al ↔ Si substitutions in the synthesis of aluminated ZSM-5 zeolites, and hence the corresponding catalytic activity, could preferentially occur on special sites. The choice of the computational method along with the size and the cutoff of the mimicked structures influenced the reliability of the calculations. The suggested alternative approach (that is, the ONIOM followed by the DFT single-point calculation) provided reasonable findings at very low computational cost.

DFT calculations have been performed on a palladium cluster adsorbed on two different carbonaceous supports, namely, two stacked polycircumcoronene units mimicking a double layer of graphite and a portion of an armchair (6,6) carbon nanotube. All of the systems have been subjected to geometry optimization and electronic structure investigation. This work, which is part of an extensive computational study on heterogeneous catalytic systems, is devoted to identify electronic and geometrical changes in which metal clusters and supports are involved upon interaction. Such analysis is helpful in designing new heterogeneous metallic catalysts, namely, new metal-supported carbonaceous catalysts. Calculations reveal a major geometrical distortion occurring in the palladium cluster supported on both graphite and nanotubes, which is caused by strong Pd−C interactions. The curvature of the nanotube surface seems to provide the basis for a stronger interaction with respect to the flat surface of graphite. This evidence is also pointed out by the atomic orbital overlap occurring between the cluster and the nanotube, as revealed by the density of states analysis.

The X-ray absorption spectra of CuII and CuIII complexes of N,N′-1,2-phenylenebis(2-mercapto-2-methylpropionamide) were recorded and analysed in solid phase. The EXAFS spectra of the two samples were refined with full multiple scattering path. Geometry optimisations on the CuII and CuIII complexes were performed by the B3LYP density functional method, with the 6-31G(d,p) basis set, considering different spin multiplicities. The singlet state of the CuIII complex was shown to be more stable than the triplet state, and a good agreement between the calculated and the corresponding experimental structure was found. Further single-point calculations on the optimised geometry were carried out at the Hartree−Fock level for obtaining molecular orbital eigenvectors and eigenvalues. The latter were employed as parameters in a new fitting approach to rationalise the shape of the K-edge of the XAS spectra. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

SCSAis an algorithm designed to get information on molecular conformational properties. The most stable conformers are determined by the homemade SCSAcode, performing a multistep systematic conformational search, which involves energy and structure quantum chemical optimizations at low-level and high-level. The SCSAmethod was employed to analyze the conformational space of the in vacuo cyclopeptide renieramide at AM1 and B3LYP/6-31G(d) levels. Calculations at B3LYP level of the GIAO 13C NMR chemical shifts were also performed on the final conformers. In fact, to validate the conformational search results experimental and calculated 13C NMR spectra of renieramide were compared. Slight disagreements observed between experimental and calculated spectra could be attributed to solute−solvent interactions, which were not taken into account in the algorithm proposed here.

Ab initio calculations at the Hartree–Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) ¹³C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an open-chain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental ¹³C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8–13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The ¹³C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental ¹³C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2002/f3743_s.pdf or from the author.

Geometry optimization and GIAO (gauge including atomic orbitals) ¹³C NMR chemical shift calculations at Hartree–Fock level, using the 6-31G(d) basis set, are proposed as a tool to be applied in the structural characterization of new organic compounds, thus providing useful support in the interpretation of experimental NMR data. Parameters related to linear correlation plots of computed versus experimental ¹³C NMR chemical shifts for fourteen low-polar natural products, containing 10–20 carbon atoms, were employed to assess the reliability of the proposed structures. A comparison with the hybrid B3LYP method was carried out to evaluate electron correlation contributions to the calculation of ¹³C NMR chemical shifts and, eventually, to extend the applicability of such computational methods to the interpretation of NMR spectra in apolar solutions. The method was tested by studying three examples of revised structure assignments, analyzing how the theoretical ¹³C chemical shifts of both correct and incorrect structures matched the experimental data. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2002/f3744_s.pdf or from the author.

Ab initio calculations at the Hartree–Fock level with full‐geometry optimization using the 6‐31G(d) basis set, and GIAO (gauge including atomic orbitals) 13C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low‐polar organic compounds having an open‐chain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental 13C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force‐field molecular dynamics, pointed out the existence of 8–13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The 13C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental 13C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers.

Geometry optimization and GIAO (gauge including atomic orbitals) 13C NMR chemical shift calculations at Hartree–Fock level, using the 6‐31G(d) basis set, are proposed as a tool to be applied in the structural characterization of new organic compounds, thus providing useful support in the interpretation of experimental NMR data. Parameters related to linear correlation plots of computed versus experimental 13C NMR chemical shifts for fourteen low‐polar natural products, containing 10–20 carbon atoms, were employed to assess the reliability of the proposed structures. A comparison with the hybrid B3LYP method was carried out to evaluate electron correlation contributions to the calculation of 13C NMR chemical shifts and, eventually, to extend the applicability of such computational methods to the interpretation of NMR spectra in apolar solutions. The method was tested by studying three examples of revised structure assignments, analyzing how the theoretical 13C chemical shifts of both correct and incorrect structures matched the experimental data.

N-diphenylmethyl-2-propenamide (NDP) was synthesised and characterised. Pharmacological in vitro tests pointed out that NDP had a cytotoxic activity on a human ovarian carcinoma comparable to that of doxorubicin. Hypothesising that this in vitro cytotoxic activity could be mainly due to intercalating interactions, between the drug and DNA fragments, ab initio calculations, at the Hartree–Fock (HF) level, were performed on the structure, and on the conformational properties of NDP, whereas its interaction with an (AC)(TG) dinucleotide triphosphate duplex (DD) was studied by the ONIOM method, at HF and PM3 level for NDP and DD, respectively. The supposed intercalation process with the DNA fragment was discussed in terms of the co-planarity of the aromatic rings present in the NDP molecule.

The molecular geometry, energetics and electronic charge distribution of diorgano‐ and triorgano‐tin(IV) complexes of [protoporphyrin‐IX] and [meso‐tetra(4‐carboxyphenyl)porphine] derivatives were determined at semi‐empirical and ab initiolevels. To study the molecular details of the complexes, simpler molecule models were calculated by the ab initiopseudo‐potential method. The molecular properties of these complexes are essentially independent of the presence of the peripheral tin atoms. Agreement was always found among the results of the different computational approaches, as well as between the theoretical and the experimental findings on the molecular geometry of the hypothesized complexes. Interaction modes between water and the organo‐tin systems considered were affected strongly by the presence of peripheral tin atoms. Copyright © 2001 John Wiley & Sons, Ltd.

A time-dependent Monte Carlo algorithm was employed to study the effects of carbonaceous surface species on the hydrogenation mechanism of acetylene–ethylene mixtures on a Pd catalyst. Simulations of tail-end and front-end mixture hydrogenation were performed employing the same set of predetermined event probabilities. The involvement of the steric hindrance of the surface species was essential to simulate the experimental data. The catalyst activity and selectivity were promoted or inhibited by different concentrations of surface polymeric species formed along with the hydrocarbon hydrogenation. Tools to get a new interpretation of the catalytic reaction mechanism and an innovative basis to perform catalyst design were suggested by the approach illustrated.

Abstract.: The hydrogenation mechanism of acetylene–ethylene mixtures on Pd catalysts under different experimental conditions was studied by employing a time-dependent Monte Carlo approach set to use a fixed series of event probabilities. The dependence of the catalyst activity and selectivity on the sizes of the metal particles was simulated at microscopic level and the results, also refined by fitting procedures, suggested proper explanations for the apparent nonuniformity of the related experimental findings. The use of the steric hindrance parameter of the surface species and the available surface energy on the metallic catalyst sites was decisive for reproducing the experimental results.

A new computational approach that combines kinetic and thermodynamic methods to mimic surface reactions is reported. Thermodynamics is mainly used to calculate, from experimental and/or quantum chemistry data, the parameters to be used in kinetic studies performed by Monte Carlo simulations. Monte Carlo algorithms are formulated in terms of the time dependent version. An innovative statistical approach is presented for cutting down the computational time required for the simulations. The reported discontinuity in the Arrhenius plot for the hydrogenation of ethylene on Pt/silica catalysts, and other peculiar properties of the above reaction on platinum catalysts, are extensively discussed as an example of reliability of the method. A new approach to metal catalysts and their role in catalytic reactions is also presented.

A Monte‐Carlo model for the simulation of alkene hydrogenation on metallic catalysts has been developed and implemented in Fortran language. We describe the model employed for ethylene hydrogenation on platinum and show the flow chart of the program. Computational characteristics such as number of necessary calculations to reach steady state, running times on different platforms, and effect of the size of the catalyst matrix, are presented. Good correlation between simulated and experimental data was observed. A subroutine allows for visual observation of the reaction. This approach is very useful for obtaining a personal impression of the important factors governing the reaction. By using this example the advantages of Monte‐Carlo simulation to test the level of understanding of catalytic phenomena are discussed. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 396–403, 1998

The catalytic hydrogenation of ethylene on Pt catalysts was simulated by a Monte Carlo model. Elementary events such as adsorption, diffusion, and desorption of the species involved (hydrogen, ethylene, and ethane) and reaction on the surface were considered. Based on comparison between experimental and calculated data no distinction has to be made between competitive and non-competitive hydrogen adsorption sites nor has adsorbed hydrogen activation to be taken into consideration. However, steric hindrance caused by adsorbed ethylene and ethane has been included in the model. Not only was the basic physical and chemical behavior reproduced by this very simple model, but also important experimental findings such as the trend of the turnover frequency versus hydrogen pressure (pH2) and the magnitudes of reaction orders for hydrogen and ethylene. In addition this model can predict some aspects of hydrocarbon hydrogenation not investigated experimentally so far.

A new route for obtaining 1,4-cyclooctadiene by catalytic isomerization of 1,5-cyclooctadiene at low temperature in presence of H2in a three-phase reactor is presented. Wei, Prater and Silvestri methods are used for the kinetic studies. A comparison of the activity-selectivity patterns of the Pd/pumice catalysts with Pd on silica and Pt and Pd–Pt on pumice catalysts is performed. Although Pt catalysts are not active in the hydro-isomerization of 1,5-cyclooctadiene, the presence of small amount of Pt in Pd/pumice catalysts increases the yield to 1,4-cyclooctadiene. Besides the catalyst characteristics, temperature and H2pressure influence the production of 1,4-cyclooctadiene. A mechanism of the process is proposed.

An easy and flexible interface, Empathes (Extensible Minimum PATH EStimator), that allows to perform Nudged Elastic Band calculation for the determination of transition states is presented. The code is designed to be easily modified, in order to be associated with the user's preferred calculation software, even with those which implement composite approaches. In particular, the interfaces to Gaussian and Siesta programs are discussed in details, being the former only used for testing purpose, while the latter can be productively employed for transition states search with that commonly used density functional theory software for periodic calculations. Program Title: Empathes CPC Library link to program files: https://doi.org/10.17632/v525mwf3cc.1 Developer's repository link: https://github.com/marberti/empathes Code Ocean capsule: https://codeocean.com/capsule/2394233 Licensing provisions: GPLv3 Programming language: Fortran 08 Nature of problem: The search for the structure of transition states through computational methods, essentially based on Density Functional Theory, is of overwhelming importance for the determination of the elementary steps forming a reaction mechanism. Allowing to develop basic knowledge, these investigations can be used to direct experimentalists towards a more efficient realization of chemical compounds synthetic processes. In cases where it is necessary to describe the reactive system through periodic calculations, which is very common in heterogeneous catalysis, this research must be done through the use of non-analytical methods. Solution method: In case of lacking of analytical procedures, the search for the transition states associated with the elementary stages that make up chemical reactions must take place through numerical methods. The Nudged Elastic Band (NEB) approach is, together with its variants, one of the most used for this purpose. In accordance with the NEB algorithm, a chain of geometric structures, generated by interpolating between the reactant and product geometries and joined by fictitious springs, is relaxed on the minimum energy path, allowing the association of the transition state to the maximum along this path. The NEB method involves the determination of molecular energies and forces acting on the nuclei of the system, which is generally carried out through a program for electronic structure calculation. The present code is a useful general interface. [ABSTRACT FROM AUTHOR]