Herewith, detailed information about present research is given. A full list of publications is given below.

Main research areas

Material Science This subs-section is devoted to the understanding of the chemistry of metal oxides at the molecular level (especially alumina and magnetite). Different studies were/are performed in order to obtain more details about: - alpha-Al2O3(0001) and Fe2O3(0001) surface relaxation - metals (Pd, Ir, Rh, Co) adsorbed on alumina - oxygen vacancies on bulk and surface alpha-Al2O3 - metal-substrate bonding mechanism More details are given in a separate section. If you are interested please click on the picture on the right.
Thermochemistry (Organic and Inorganic compounds) This subs-section is devoted to computational studies concerning the thermodynamic properties of different families of compounds. These may divided in two classes, organic or inorganic. The former class is composed by amines, phenol derivatives, piperidines, quinoxalines, etc. Inorganic structures are mainly transition metal complexes. The gas-phase thermodynamic properties calculated are enthalpies of formation, (O-H, N-H, N-O, M-S, M-O, etc.) enthalpies of dissociation, proton affinities, electron affinities, ionization potentials, etc.
Organic Chemistry Density functional theory has been applied successfully to organic chemistry problems. The calculations yield important information that was used to explain/support experimental results. For example, this was useful on the understanding of different experimental yields coming from the reaction of aminoacids with similar sulfonamide-derived compounds. For more details click on the picture.
Gemini Surfactants These kind of surfactants offer new major advantages over conventional amphiphiles but there is a regrettable lack of systematic studies on their thermodynamic and strutural properties. This is the main topic that incited us to perform a combined experimental and computational study on this kind of compounds. From the experimental side, this involves the synthesis and caractherization of these molecules. In parallel, both ab-initio and classical methods will be employed to obtain further information about the structure of single surfactant molecules. Moreover, detailed studies will be performed for larger systems and in some cases we hope to study systems that resemble stable vesicles or cylindrical micelles. These studies will be carried out in the gas-phase and in water solution.

Publications

Book Chapters:

Elementary steps of catalytic processes on metallic and bimetallic surfaces

Coauthored with Francesc Illas, Carme Sousa, Anna Clotet and Josep Manel Ricart.

Appeared in Theoretical Aspects of Heterogeneous Catalysis, Progress in Theoretical Chemistry and Physics: Vol. 8, Edited by M. A. Chaer-Nascimento, Kluwer Academic Publishers, Dordrecht, 2001.

Papers:

33- Thermochemistry of 2 - amino - 3 - quinoxalinecarbonitrile - 1,4 - dioxide. Evaluation of the mean dissociation enthalpy of the (N-O) bond

Submitted for publication.

Coauthored with Maria D. M. C. Ribeiro da Silva, Jorge M. Gonçalves, Emanuel A. Sousa, Siddharth Pandey and William E. Acree, Jr..

Submitted for publication.

32- Onde param os electrões? Análise topológica da natureza das ligações intra e intermoleculares

Submitted for publication.

Coauthored with Bernard Silvi and Francesc Illas.

Submitted for publication.

31- A DFT study of Co, Rh and Ir atoms deposited on the alpha-Al₂O₃(0001) surface

Submitted for publication.

Coauthored with Norge Cruz-Hernández, Javier F. Sanz, António Márquez and Francesc Illas.

Submitted for publication.

30- A theoretical study of the decomposition of azomethane on Pt(111)

In preparation.

Coauthored with Francesc Illas and Josep Maria Bofill.

In preparation.

29- Combined experimental and computational study of the energetics of methylpiperidines

In preparation.

Coauthored with Joana I. T. A. Cabral and Manuel A. V. Ribeiro da Silva.

In preparation.

28- Kinetics and mechanism of hydrolysis of N-acyloxymethyl derivatives of azeditin-2-one

The pH-independent, acid-catalysed and base-catalysed hydrolyses of Nacyloxymethylazetidin-2-ones all occur at the ester function. The pH-independent hydrolysis involves rate-limiting alkyl C-O fission and formation of an exocyclic Beta-lactam iminum ion. This iminium ion is then trapped by water at the exocyclic iminium carbon atom, rather than at the Beta-lactam carbonyl carbon atom, to form the corresponding N-hydroxymethylazetidin-2-ones. Calculations carried out at the B3LYP/6-31+G(d) level of theory also support that nucleophilic attack by water takes place at the exocyclic carbon rather than at the Beta-lactam carbonyl carbon of the iminium ion. The mechanism for the acid-catalysed pathway involves a pre-equilibrium protonation, probably at the Beta-lactam nitrogen, followed by rate-limiting alkyl C-O fission with formation of an exocyclic iminum ion. The base-catalysed hydrolysis involves rate-limiting hydroxide attack at the ester carbonyl carbon. These results imply formation of a Beta-lactam system containing a positively charged amide nitrogen atom that hydrolyses via a pathway that preserves the Beta-lactam structure in the product and provide further evidence that cleavage of the Beta-lactam C-N bond is not as facile as is commonly imagined.

Coauthored with Emília Valente, Rui Moreira and Jim Iley.

Journal of Organic Chemistry, 69 3359 (2004).

27- Thermodynamic properties of quinoxaline-1,4-dioxide derivatives - A combined experimental and computational study

The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-R-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b) and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids, and their enthalpies of sublimation. In parallel, accurate density functional theory based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported.

Coauthored with Maria D. M. C. Ribeiro da Silva, Jorge M. Gonçalves, Emanuel A. Sousa, Siddharth Pandey and William E. Acree, Jr..

Journal of Organic Chemistry, 69 2785 (2004).

26- On the geometric structure of the (0001) hematite surface

A theoretical study of the surface structural relaxation of the (0001) hematite is presented which explores the influence of the size of a slab model, the number of relaxed surface layers and various computational approaches, all of them based on density functional theory (DFT). Four different slab models including 9, 12, 15, and 18 layers were analyzed. In each model, sequences from 1 to 7 layers were relaxed while keeping the bottom layers fixed to mimic the constraints imposed by the bulk. In the geometry optimization procedure, four different DFT approximation levels were employed, namely, the non self-consistent Harris functional, the local spin-density approximation, LSDA, the PW91 or BP meta GGA functionals and the hybrid B3LYP method. It was found that the relaxation predicted for up to four layers is independent of the slab thickness. Except for the Harris functional, all methods and models predict consistent values for the relaxation of the innermost layers. It is concluded that the Harris functional can be used to explore the adequacy of a given model but not to provide an accurate enough structure of the relaxed hematite (0001) surface.

Coauthored with Fernando Alvarez-Ramírez, J. Manuel Martínez-Magadán and Francesc Illas.

Surface Science, 558 4 (2004).

25- Topological analysis of the metal-support interaction: The case of Pd atoms on oxide surfaces

The metal support interaction was analyzed by means of AIM and ELF approaches taking Pd adsorbed on MgO(001), alpha-Al₂O₃(0001) and SiO₂ surfaces as examples of simple ionic, complex ionic and covalent oxide surfaces, respectively. On the most favorable adsorption sites, the polysynaptic basins found indicate the presence of chemical bonds. The basins populations are low in the case of the two ionic oxides while on a specific site of the SiO₂ surface, a polysynaptic basin with ~2 electrons was found illustrating the variety and richness of the chemical bonding between a metal and an oxide.

Coauthored with Francesc Illas and Bernard Silvi.

Chemical Physics Letters, 388 132 (2004).

24- A DFT study on the thermodynamic properties of aminophenols

Density Functional Theory has been used to investigate several gas-phase thermodynamic parameters of the o-, m- and p-aminophenol isomers. Within the DFT approach, the B3LYP method and the 6-31G(d) and 6-311+G(2d,2p) basis sets were used to compute standard enthalpies of formation. Calculated data is in excellent agreement with the experimental work of Nuñez et al. [J. Chem. Thermodynamics 18, 575-579 (1986)] but differs significantly from the values of Sabbah et al. [Can. J. Chem. 74, 500-507 (1996)]. In this work, other properties such as homolytic O-H and N-H bond dissociation energies, gas-phase acidities and proton or electron affinities were also obtained and confirm the few experimental results available for these properties in this kind of compounds, except for O-H homolytic dissociation energy of 2-aminophenol.

Coauthored with Manuel A. V. Ribeiro da Silva.

International Journal of Quantum Chemistry, accepted for publication.

23- Solvent and structural effects in the N-H bond homolytic dissociation energy

In this work, the gas-phase homolytic N-H bond dissociation enthalpy (BDE) was investigated for a large series of molecules containing at least one N-H bond by means of accurate Density Functional Theory (DFT) calculations. The molecules studied belong to different classes of compounds, namely, amines, amides, and anilines, amino acids, phenoxazines, indolamines and other compounds of general interest, such as anti-inflammatory drugs. To achieve these purposes, the (RO)B3LYP/6-311+G(2d,2p)// B3LYP/6-31G(d) level of theory was used. The calculated gas-phase N-H BDEs, at T = 298.15 K, are in the range 499.6 kJ/mol - 203.9 kJ/mol, for purine and HNO, respectively. Further, the calculated BDEs are in excellent agreement with a significant number of available experimental BDEs. Solvent effects were also taken in account and rather significant differences are found among N-H BDEs computed in the gas-phase and in heptane, dimethylsulfoxide or water.

Coauthored with Maria D. M. C. Ribeiro da Silva and Manuel A. V. Ribeiro da Silva.

Journal of Physical Chemistry A, 108 2119 (2004).

22- Theoretical study of bulk and surface oxygen and aluminium vacancies in corundum

The formation energy, geometry and electronic structure of isolated oxygen and aluminium vacancies in bulk and on the (0001) surface of corundum (alpha-Al₂O₃) have been investigated by means of periodic calculations in the framework of Density Functional Theory within the Generalized Gradient Approximation and large supercells. The energy cost to form an oxygen vacancy in the bulk is estimated to be of the order of 10 eV whereas that corresponding to the formation of Al vacancies is found to be at least a 30% larger. The relaxation of the material is rather small for both defects. The removal of an oxygen atom in bulk alpha-Al₂O₃ is accompanied by the appearance of an impurity level in the gap which is a strong indication of electron localization. This has been further confirmed by integration of the Density of States in the energy interval corresponding to the impurity level and by several other theoretical analyses. For the alpha- Al₂O₃(0001) surface, the formation of oxygen and aluminum vacancies exhibits many similarities with the bulk; the energy cost to form Al vacancies is much larger than for O vacancies and, in both cases, it is accompanied by rather small atomic displacements. However, there are also significant differences between bulk and surface oxygen and aluminium vacancies. Thus, the formation energy of one of these point defects in the surface is rather smaller as expected and, more importantly, the degree of electronic delocalization is also larger.

Coauthored with Javier Carrasco and Francesc Illas.

Physical Review B, 69 064116 (2004).

21- Amino acids as selective sulfonamide acylating agents

Acylation of antimalarial and bacteriostatic sulfonamides with N-protected amino acids and peptides was carried out using standard peptide coupling methods. These acylation reactions are regioselective for the N⁴ nitrogen atom of diazine-containing sulfonamides. In contrast, only N¹ coupling was found for sulfisoxazole, an isoxazole-based sulfonamide. Computational studies suggest that a combination of geometrical, thermodynamic and electronic factors are responsible for the different reactivities reported.

Coauthored with Paula Gomes, Maniela Rodrigues and Rui Moreira.

Tetrahedron, 59 7473 (2003).

20- Standard molar enthalpies of formaton, vapour pressures and enthalpies of sublimation of 2-chloro-4-nitroaniline and 2-chloro-5-nitroaniline

The standard (p^o = 0.1 MPa) molar enthalpies of formation for crystalline 2-chloro-4-nitroaniline and 2-chloro-5-nitroaniline were derived from the standard molar enthalpies of combustion, in oxygen, at the T = 298.15 K, measured by rotating bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure the vapour pressures of the crystals as function of temperature, and the standard molar enthalpies of sublimation, at the temperature 298.15 K, were derived by the Clausius-Clapeyron equation. Direct measurements of the standard molar enthalpies of sublimation, using Calvet microcalorimetry, for 2-chloro-4-nitroaniline and 2-chloro-5-nitroaniline confirmed the values derived from the Knudsen technique.

Additionally, standard enthalpies of formation were estimated by employing two different methodologies. One is based on the Cox scheme and the other one, much more accurate, is based on first-principles calculations. The theoretical calculations were performed at the AM1, B3LYP/6-31+G** and BP86/6-31+G** levels of theory. Estimated values are in good agreement with the reported experimental numbers.

Coauthored with M. A. V. Ribeiro da Silva, L. M. S. S. Lima, L. M. P. F. Amaral, A. I. M. C. L. Ferreira

Journal of Chemical Thermodynamics, 35 1343 (2003).

19- Thermochemical and structural studies of Cu(II) and Ni(II) complexes with N,N- diethyl-N'- pivaloylthiourea

The standard (p⁰ = 0.1 MPa) molar enthalpies of formation of crystalline bis (N,N-diethyl-N´- pivaloylthioureato) copper(II), Cu(PVET)₂, and bis (N,N- diethyl-N´- pivaloylthioureato) nickel(II), Ni(PVET)₂, were measured, at T = 298.15 K, by solution-reaction isoperibol calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, of both complexes were obtained using a Knudsen effusion technique. These values were used to derive the standard molar enthalpy of formation of Cu(PVET)₂ and Ni(PVET)₂ in gaseous phase, and to evaluate the difference between the mean metal-ligand and the hydrogen-ligand bond dissociation enthalpies, in these compounds.

The N- H homolytic bond dissociation enthalpy in N,N- diethyl-N´- pivaloylthiourea ligand (HPVET) was calculated by high-level DFT based calculations.

The three dimensional structures of Cu(PVET)₂ and Ni(PVET)₂ are presented and show a planar coordination around the metal in both molecules.

Coauthored with M. A. V. Ribeiro da Silva, M. D. M. C. Ribeiro da Silva, L. C. M. Silva, A. M. Damas, F. Dietze, E. Hoyer

Inorganica Chimica Acta, 356 95 (2003).

18- Adsorption of small palladium clusters on the relaxed alpha-Al₂O₃(0001) surface

The interaction of small Pdn clusters (n=3,4) with the relaxed Al-terminated alpha-Al₂O₃(0001) surface has been investigated using embedded cluster and periodic slab models within a first principles density functional approach. From the present study it is concluded that the structure of supported Pd3 is largely distorted from the gas phase equilibrium geometry whereas the structure of supported Pd4 is less distorted and reminiscent of the most stable gas phase isomer. Consequently, the adhesion energy of Pd3 on the relaxed alpha-Al₂O₃(0001) surface is smaller than that of Pd4. The presence of Pd atoms induces a rather large reorganization of the atomic structure of the surface. The results obtained for the different structures of supported Pd4 suggest that there is a competition between three- and two-dimensional growth of the supported crystallites. Also, in contrast with results obtained for Pd adsorption on other oxide surfaces, there are no preferred adsorption sites for Pd deposited on the corundum surface.

Coauthored with Zbigniew Lodziana and Francesc Illas

Journal of Physical Chemistry B, 107 6411 (2003).

17- Gas-phase thermodynamic properties of dichlorophenols from density functional theory calculations

Density Functional Theory has been used to investigate gas-phase thermodynamic properties of phenol and dichlorophenols. Molecular geometries, energies and vibrational frequencies were computed at the B3LYP and BP86 levels of theory. At T>=298.15 K, calculated standard enthalpies of formation are in excellent agreement with experimental data. The average deviation between calculated and experimental values is of about 2.3 kJ/mol, and, in some cases theoretical values fall within experimental uncertainty. Other properties for which only a few experimental results were available in the literature were also calculated, namely, O-H homolytic bond dissociation energies, gas-phase acidities, ionization energies and proton and electron affinities.

Coauthored with Manuel A. V. Ribeiro da Silva.

Journal of Physical Chemistry A, 107 869 (2003).

16- Density functional theory study on the structure of bis(1,2-diethyl-3-benzoyl-thioureato) copper(II). Planar or distorted tetrahedral CuS₂O₂ conformation?

Density functional theory calculations at the B3LYP level of theory combined with several basis sets were carried out on the structure of bis(1,2-diethyl-3-benzoyl-thioureato) copper(II). The results obtained clearly show two minima with local cis-CuS₂O₂ geometry, one resembling the X-ray experimental structure, in which the coordination polyhedron is a distorted tetrahedron, and the other with the CuS₂O₂ moiety nearly planar, resembling the corresponding ureato adn a similar mercapto-β-diketone. A third minimum was found for a trans-CuS₂O₂ conformation. The calculated energy gap between the three structures is lower than 5 kJ/mol.

Coauthored with Manuel A. V. Ribeiro da Silva.

Inorganic Chemistry Communications, 6 149 (2003).

15- Surface model and exchange-correlation functional effects on the description of Pd/alpha-Al₂O₃ (0001)

The interaction of Pd with the Al-terminated alpha-Al₂O₃(0001) surface has been investigated using an embedded cluster model and periodic-supercell approaches. Furthermore, several treatments of electronic exchange and correlation within Density Functional (DF) Theory have been employed including generalized gradient approximation (GGA) and hybrid exchange functionals. In the periodic calculations the influence of pseudopotentials and basis sets have also been investigated by comparing GGA results obtained using all electron basis set and pseudopotential plane-wave approaches. For a given choice of the exchange-correlation functional and for a fixed substrate, the cluster and slab models predict nearly the same structural parameters and adsorption energies. All structural models reproduce the general trend for the interaction of Pd with the alpha-Al₂O₃ (0001) surface which is that there is a slight preference for adsorption above surface sites sitting directly above oxygen atoms either from the 2^nd ot 5^th layer. However, significantly larger differences exist when comparing different DF methods within a given surface model. The cluster and periodic slab models predict a large adsorbate- induced relaxation with a similar description of the metal-oxide interface provided a minimum number of surface layers is included in the optimization procedure.

Coauthored with Francesc Illas, N. Cruz Hernández, J. F. Sanz, Adrian Wander and Nic M. Harrison

Journal of Chemical Physics, 116 1684 (2002).

14- The interaction of Pd with alpha-Al₂O₃(0001): A case study of modeling the metal-oxide interface on complex substrates

The Pd/alpha-Al₂O₃(0001) interface at low Pd coverage has been studied by a variety of theoretical methods and different models of this metal-oxide interface. All results are consistent and predict a noticeable interaction dominated by the metal polarization in response to the presence of the substrate. A significant contribution of the charge transfer from the transition metal to the surface is also observed. The periodic fully relaxed calculations show that the most favorable adsorption site for the interaction of Pd with corundum involves the anionic surface sites, in particular the on-top oxygen site. It is also shown that adsorption of Pd atoms on the surface induces a significant relaxation of the aluminum layer. The small differences observed in the adsorption energies near the oxygen atoms sugest a high mobility of Pd atoms on the surface.

Coauthored with Francesc Illas, N. Cruz Hernández, A. Márquez, J. F. Sanz.

Physical Review B, 65 125414 (2002).

13- The adsorption of nitromethane on the Au(111) surface

The density functional theory (DFT) based hybrid-method B3LYP has been used to study the interaction of the nitromethane molecule (CH₃NO₂) with the Au(111) surface. The perfect Au(111) surface has been represented by a rather large cluster model, Au₂₂, that was in turn used to extract information about the preferred adsorption geometry of the CH₃NO₂ species. In order to extract energetic information about the stability of adsorbed nitromethane, calculations were also performed for the gas-phase CH₃NO₂ molecule. The results obtained here are used to interpret experimental data. The computed geometry for adsorbed CH₃NO₂ agrees with the structure proposed from a previous experimental work.

Coauthored with Francesc Illas.

International Journal of Molecular Sciences 2 211 (2001).

12- The Structural Relaxation of the alpha-Al₂O₃(0001) - An Investigation of Potential Errors

All electron, first principles, periodic calculations have been carried out to investigate the relaxation of the Al-terminated corundum surface. Different theoretical formalisms, Hartree-Fock and Density Functional Theory using either the LDA or the hybrid B3LYP exchange-correlation functional, have been employed. In the past, many ab initio studies of surface structure have been performed using the experimental, rather that the theoretically optimized bulk structures. In the current study, we show that this leads to significant differences in the predicted geometry. In addition, we demonstrate that optimizing a limited sub-set of surface parameters also has a significant effect on the accuracy of the final geometry. The extent of the effect associated with each of these sources is shown to be strongly dependant on the choice of exchange-correlation potential.

Coauthored with Ibério. de P.R. Moreira, Peter Reinhardt, Adrian Wander, Barry G. Searle, Nic M. Harrison, and Francesc Illas.

Chemical Physics Letters 341 412 (2001).

11- First-principles study of the adsorption of formaldehyde on the clean and atomic oxygen covered Cu(111) surface

The interaction of formaldehyde with the clean and atomic oxygen-covered Cu(111) surfaces has been studied by means of cluster model density functional calculations in which Cu₂₂ (14,8) is used to represent the perfect Cu(111) surface. The calculations point towards a n¹- H₂CO-O orientation with the oxygen atom almost on top of a copper surface atom. The formaldehyde adsorption energy is of 22-25 kJ/mol and the internal geometry of adsorbed formaldehyde is almost identical to that of the molecule in the gas-phase. The C-O bond is almost parallel to the surface and the conformation with the molecular plane normal to the surface is slightly preferred to the conformation with the molecular plane nearly parallel to the surface. A Cu₂₂-O model where atomic oxygen is adsorbed on a fcc hollow site was used to study the co-adsorption and reaction of formaldehyde with atomic oxygen. Oxygen co-adsorption has a dramatic effect on the formaldehyde adsorption energy which is increased by 50%. The calculated energy barrier for the formation of the dioxymethylene intermediate species through the H₂CO+O -> H₂CO₂ reaction is of 36 kJ/mol.

Coauthored with José A. N. Ferreira Gomes and Francesc Illas.

Journal of Molecular Catalysis A - Chemical 170 187 (2001).

10- The adsorption of methyl nitrite on the Au (111) surface

The interaction of the methyl nitrite molecule (CH₃ONO) with the gold (111) surface has been studied by means of density functional calculations. The perfect Au(111) surface has been represented by a rather large cluster model, Au₂₂, that was in turn used to extract information about the preferred adsorption geometry of the CH₃ONO species. Vibrational frequencies and adsorption energy are also reported. The calculated adsorption energies are 31.2 kJ/mol with respect to the gas phase cis-conformer and 35.1 kJ/mol with respect to trans-- methyl nitrite, very close to the experimental adsorption energy of 33.5 kJ/mol. From the analysis of vibrational frequencies of gas-phase and adsorbed species it is concluded that only the cis-conformer is present at the Au(111) surface.

Coauthored with Francesc Illas.

Catalysis Letters 71 31 (2001).

phd

9- A DFT study of the methanol oxidation catalyzed by a copper surface

The energy variation in the overall catalytic oxidation of methanol to carbon dioxide on a copper surface is analyzed using the density functional theory. Adsorption energies, geometries and charges for methanol and the various intermediates were computed from geometry optimized calculations on a large cluster containing 22 copper atoms used to model the Cu (111) surface. The calculated parameters are reported for the most stable adsorption modes of the various species considered: CH₃OH, CH₃O, H₂CO, H₂CO₂, HCO₂, H and O. The results obtained are in good agreement with available experimental data. The adsorption energies obtained were then used to plot the energy variation along the decomposition of methanol. The energy profile shows that the several reaction steps are lower in energy than the initial state: CH₃OH _(g) + O _(ads). It is also shown that the stability of the adsorbed formaldehyde species is responsible for getting H₂CO or CO₂ as the final oxidation products. The decomposition of formaldehyde to formate is highly exothermic and this is the main reason for the high reactivity of the dioxymethylene intermediate. This prevents, therefore, the experimental verification of its existence. The total energy of the final products is 210 kJ/mol lower than that of the initial state.

Coauthored with José A. N. Ferreira Gomes.

Surface Science 471 59 (2001)..

8- Aplicação da química teórica ao estudo de reacções catalíticas sobre superfícies

O modo de adsorção de uma vasta gama de moléculas em superfícies metálicas tem sido estudado nas duas últimas décadas usando várias técnicas experimentais e também metodologias teóricas [1,2]. Este tipo de estudos fundamentais sobre a decomposição e reacção de pequenas moléculas em superfícies catalíticas assume grande importância no esclarecimento dos mecanismos e dos intermediários presentes em reacções de catálise heterogénea. A reacção do metanol sobre superfícies de metais de transição é um dos processos catalíticos mais estudados pela ciência das superfícies [1]. As razões principais resultam da importância do metanol na indústria química (síntese do álcool a partir do "syn-gas" e oxidação do álcool a formaldeído) e também devido ao facto do metanol ser tomado como um modelo para explicar a adsorção de álcoois com maior peso molecular. O metanol pode ser adsorvido molecularmente sobre superfícies de metais de transição ou, dependendo da natureza do metal ao qual está ligado e da presença de espécies co-adsorvidas, sofrer uma reacção de decomposição originando uma grande variedade de produtos. Para a reacção do metanol catalisada por superfícies limpas de metais de transição, os produtos de decomposição dependem da reactividade inerente do metal para a activação das ligações C-H ou da ligação C-O. Nas superfícies de metais nobres (cobre, prata e ouro) ocorre apenas uma desidrogenação parcial do metanol originando formaldeído e hidrogénio. Nas superfícies de prata e de ouro é necessária a presença de oxigénio atómico pré-adsorvido de modo a ocorrer a cisão da ligação O-H. O formato de metilo é também um dos produtos formados nestas superfícies e deverá ser originado pela reacção do formato com o radical metoxilo. O formato e o metoxilo são dois dos intermediários propostos ao longo da reacção de oxidação. Pelo facto de estas duas espécies químicas serem bastante estáveis, o seu modo de adsorção foi estudado intensivamente. Foram propostos outros intermediários tais como o dioximetileno mas devido à sua baixa estabilidade ainda não foi possível a sua confirmação através de estudos experimentais. Para a reacção de oxidação do metanol catalisada por superfícies de metais de transição mais reactivos (Ni, Pt, Pd, Ru, Rh) foi também observado o monóxido de carbono como um dos produtos de reacção. Nas superfícies de ferro e de molibdénio foi detectado experimentalmente algum metano o que indica para estas superfícies a quebra da ligação C-O. Ao longo dos últimos anos têm surgido novas técnicas experimentais para o estudo destes fenómenos próximos das superfícies catalíticas como por exemplo a microscopia de varrimento por efeito de túnel (STM), espectroscopias de alta resolução (HREELS), espectroscopias de raios infra-vermelhos (FTIR, RAIRS), espectroscopias de raios X (XPS, NEXAFS, SEXAFS), etc. Apesar deste grande aumento na quantidade e qualidade dos dados experimentais. Este tipo de dificuldades pode ser facilmente exemplificado pelos inúmeros trabalhos experimentais realizados até se ter confirmado o modo de adsorção do formato (HCO₂) nas superfícies de cobre (100) e (110). Foram publicados 7 estudos estruturais realizados pelo grupo liderado por D. P. Woodruff [3] e os resultados distintos obtidos por cada um deles mostram a dificuldade na determinação da forma e sítio de adsorção do formato nestas superfícies. É nestas situações que o uso de métodos teóricos adquire extrema utilidade. Usando pequenos agregados metálicos pode ser estudada por cálculo quântico a interacção química de moléculas com estes agregados metálicos de modo a obter os sítios e a forma de adsorção destas moléculas na superfície metálica. Podem também ser obtidos os números de onda relativos a cada um dos modos normais de vibração das espécies adsorvidas de modo a ajudar a interpretação dos espectros vibracionais. De modo e entender ao nível atómico a oxidação do metanol foi usada a teoria dos funcionais de densidade (DFT) para estudar a interacção das espécies químicas referidas anteriormente e que surgem como intermediários ou produtos ao longo do processo catalítico. O efeito de espécies co-adsorvidas foi também analisado.

Coauthored with José A. N. Ferreira Gomes.

Proceedings of the XVII Simpósio Ibero-americano de Catálise.

7- Comparative study of geometry and bonding character for methoxy radical adsorption on noble metals

Results are reported of quantum density functional theory calculations for methoxy radical adsorption on the (111) surfaces of copper, silver and gold. The metallic surfaces are modeled by clusters of seven atoms chosen to simulate the top, bridge, hcp hollow and fcc hollow sites. A control test on the use of these small clusters was performed by running a calculation on a 22-atoms cluster. A comparison between the energetics of adsorption on the different sites identifies the fcc hollow position as the preferred one for CH₃O adsorption on the metal surfaces considered and the methoxy C-O axis is found to be perpendicular to the surface. It is shown how the methoxy radical binds on the hollow sites of copper, silver and gold metallic surfaces; the three oxygen p orbitals are found to interact strongly with the nearest neighbour metallic atoms except for the top position where only the p orbital aligned along the direction of the oxygen-metal atom is involved. In all cases, there is a charge transfer of approximately 0.6-0.8 e from the metal atoms to the methoxy radical. The bonding of the methoxy radical on the surfaces studied has a largely ionic character. The calculated adsorption parameters and vibrational frequencies of the adsorbed species are in good agreement with available experimental data.

Coauthored with José A. N. Ferreira Gomes.

Journal of Molecular Structure (Theochem), 503 189 (2000).

6- Adsorption of the formyl species on transition metal surfaces

The density functional theory (DFT) and the cluster approach were used to get information concerning the adsorption of the formyl species on transition metal surfaces. At first, the adsorption of HCO on four different sites of the Cu (111) surface was studied. An initial h¹-HCO-C conformation was considered in all cases. After optimisation, obtained results point towards a preferred h ¹-HCO-C adsorption conformation with the carbon atom located between a bridge and a hcp hollow site. The calculated adsorption energy is 140 kJmol^-1, approximately. The internal geometrical parameters of the adsorbed species lie in between the calculated bond lengths and angles of the gas phase HCO and HCO^- species. Adsorption of the formyl species on the Au (111) and Pt (111) surfaces was also studied. Starting from the optimised internal geometry of the adsorbed HCO species, a small adsorption energy on the gold surface, 52 kJmol^-1, and a large adsorption energy on the platinum surface, 252 kJmol^-1 were calculated. The distance from the carbon atom to the metallic surface is 1.9 Å on copper, 2.2 Å on gold and 1.5 Å on platinum. Small C-Surface distances led to large C-O bond lengths. The short carbon to surface distance in HCO adsorption on platinum when compared to copper and gold may be responsible for the distinct methanol oxidation on this surface. Calculated vibrational frequencies are in good agreement with previous theoretical and experimental data. The bonding to the surface has a character mainly ionic on copper and more covalent on platinum.

Coauthored with José A. N. Ferreira Gomes.

Journal of Electroanalytical Chemistry, 446 283 (2000).

5- A theoretical study of dioxymethylene, proposed as intermediate in the oxidation of formaldehyde to formate over copper

The density functional theory based hybrid method B3LYP was used to study the interaction of the dioxymethylene species with the copper (111) surface. This species has been proposed experimentally as one possible intermediate in the oxidation of methanol catalysed by metal surfaces. The H₂CO₂ species is very unstable and this makes the experimental study too difficult. As far as we know there is not in the literature any direct theoretical or experimental study of H₂CO₂ adsorption on metal surfaces. The experimental knowledge is limited to the IR frequencies obtained for adsorption on metal oxides. In this study, two different clusters and two different adsorption sites have been studied. A two layer Cu₇ cluster was used to model the H₂CO₂ and [H₂CO₂] ^2- adsorption on a small copper island and a large three layer Cu₃₀ cluster was used to model the H₂CO₂ adsorption on a copper (111) surface. These clusters were used to extract information concerning the energetics, geometry and IR frequencies for the dioxymethylene adsorption. When compared with a similar species, formate, dioxymethylene is stabilised more efficiently on the cross-bridge site than on the aligned-bridge site which is the preferred orientation for formate. However, the oxygen to surface distances are similar and the same is observed for the bonding type which is mainly ionic. A bridge bonded conformation is predicted for adsorption on the two sites considered. The comparison of the adsorption energy of the dioxymethylene species and the adsorption energy of atomic oxygen and free formaldehyde yields an interesting result: H₂CO_{2 (ads)} is energetically more stable than adsorbed O _(ads) and H2CO _(g). The IR frequencies are in good agreement with experimental data obtained for dioxymethylene adsorption on several oxide surfaces.

Coauthored with José A. N. Ferreira Gomes.

Surface Science, 446 283 (2000).

4- Methoxy radical reaction to formaldehyde on clean and hydroxy radical covered copper (111) surfaces: a DFT study

The ab initio cluster approach has been used to investigate the decomposition reaction of methoxy radical to formaldehyde catalysed by a copper (111) surface and also to study the influence of co-adsorbed hydroxy species on the same reaction. Previous experimental studies support that the former reaction occurs with C-H bond cleavage without the interference of other co-adsorbed species on the metal surface. However, it was observed in theoretical works that the tilting of the methoxy C-O axis causes a large destabilisation effect in the methoxy adsorption energy. This tilting of the methoxy C-O axis is important in the decomposition reaction since it allows one of the methyl hydrogens to approach the surface. From the results reported in the present paper, we can predict a lower energy barrier for this reaction when hydroxy species are adsorbed on the surface. These findings also indicate that defects such as steps on the surface near to adsorbed methoxy islands should lower the energy barrier since the tilting of the methoxy C-O axis should be smaller.

Coauthored with José A. N. Ferreira Gomes and Francesc Illas.

Surface Science, 443 165 (1999).

3- A DFT study of dioxymethylene adsorption on the copper (111) surface

Quantum density functional theory results are reported for the adsorption of H₂CO₂ on the Cu (111) metallic surface as modelled by a Cu₃₀ cluster. The calculations suggest that the H₂CO₂ species stabilises in the cross-bridge site with the two oxygen atoms located at different distances from the surface. The H₂CO₂ species prefers an orientation with the OCO plane normal to the surface. Mulliken population analysis in the adsorbed species shows that there is charge donation from the copper surface to the double radical. The internal geometry of the adsorbed species is very different from that of the free molecules and this is the result of the strong interaction with the metal surface. The bonding of this adsorbate to the copper surface is essentially ionic.

Coauthored with José A. N. Ferreira Gomes.

Electrochimica Acta, 45 653 (1999).

2- Adsorption of the formate species on copper surfaces: a DFT study

The Density Functional Theory and the cluster model approach have been applied to study the interaction of the formate species with the copper (100), (110) and (111) surfaces. The short-bridge, long-bridge and cross-bridge sites of the copper surface have been modelled by Cu₇ and Cu₈ metal clusters after checking the validity of the results against those obtained with much larger clusters. The results show that the formate species is stabilised strongly on the short-bridge site of the surfaces considered and this is in agreement with available experimental data. For adsorption on the short-bridge site, and for the three surfaces considered, the Cu_surf-O is close to 2.02 Å, the C-O bond length is 1.26 Å and the O-C-O angle is 128^o. On this adsorption site, formate is bridge-bonded with the two oxygens almost in a top position on two copper atoms. For the long-bridge site the oxygen atoms of the adsorbate are not located above the two copper atoms. The two O-C bonds are equivalent when formate is adsorbed on these two sites. On the cross-bridge site the formate species is bonded to the surface in a monodentate conformation. The two O-C bonds are different with two clearly different bond lengths. The difference is larger for adsorption on the Cu (100) surface. In this case the bond lengths are typical of a bond order of one and two. The bonding to the surface is essentially ionic and the total charge in the adsorbate is essentially ionic and the total charge in the adsorbate is 0.75 +/- 0.05 e, approximately.

Coauthored with José A. N. Ferreira Gomes.

Appeared in Surface Science, 432 279 (1999).

1- Cluster model study of methoxy radical adsorption on the Cu (111) surface

Results of quantum density functional theory calculations on the adsorption of the methoxy radical on the Cu (111) metallic surface are reported. The metal surface is modeled by clusters of up to 7 atoms and the cluster size effect is discussed. It is found that the hollow site on the surface is the preferred site for adsorption and the C-O bond is quasi perpendicular to the surface. The methyl group has a small barrier (5.5 kJ/mol) for rotation around the C-O bond. The calculated vibrational frequencies of the adsorbed species are in good agreement with the experimental data available.

Coauthored with José A. N. Ferreira Gomes.

Appeared in Journal of Molecular Structure (Theochem), 463 163 (1999).

Oral Presentations:

6^th Mediterranean Conference on Calorimetry and Thermal Analysis, Porto, Portugal, July 2003.

5^o Encontro Nacional da Divisão de Catálise, Évora, Portugal, Maio 2003.

5^o Encontro Nacional da Divisão de Catálise, Leiria, Portugal, Maio 2001.

Poster Presentations:

10^th International Conference on the Applications of Density Functional Theory in Chemistry and Physics, Brussels, Belgium, September 2003.

6^o Encontro Nacional de Química-Física da Sociedade Portuguesa de Química, Lisbon, Portugal, September 2003.

6^th Mediterranean Conference on Calorimetry and Thermal Analysis, Porto, Portugal, July 2003.

6^th Mediterranean Conference on Calorimetry and Thermal Analysis, Porto, Portugal, July 2003.

6^th Mediterranean Conference on Calorimetry and Thermal Analysis, Porto, Portugal, July 2003.

5^o Encontro Nacional da Divisão de Catálise, Évora, Portugal, Maio 2003.

XXXV^th International Conference on Coordination Chemistry, Heidelberg, Germany, July 2002.

Structure and Reactivity of Oxide Surfaces, Acquafredda di Maratea, Italy, June 2002.

XVIII Encontro Nacional da Sociedade Portuguesa de Química, Aveiro, Portugal, Março 2002.

5^o Encontro Nacional de Química-Física, Faro, Portugal, Setembro 2001.

56^th Annual Calorimetry Conference, Colorado Springs, USA, July-August 2001.

Euroconference on Molecular Mechanisms of Heterogeneous Catalysis, San Feliu de Guíxols, Spain, June 2001.

Catalysis from first principles (CECAM - PsiK workshops), Lyon, France, July 2000.

Conferences:

"Química computacional como ferramenta em estudos de catálise heterogénea", Departamento de Química da Universidade de Aveiro a convite do Prof. Doutor João Rocha, Aveiro, Portugal, Abril de 2001.