Stabilization of Carboxylate Surface Species on Pd(111)

The stabilization of formate and acetate surface species on Pd(111) has been studied by temperature-programmed desorption and infrared reflection absorption spectroscopy (IRRAS). In addition to the previously reported stabilization of carboxylate species by the pre-adsorption of oxygen, an even bigger stabilizing effect was observed upon the adsorption of carbon monoxide onto formate- and acetate-covered surfaces. The presence of carbon monoxide delays the decomposition of carboxylate species by blocking surrounding sites. Proper use of co-adsorbed species could help to improve selectivity or activity in heterogeneous catalytic reactions

Kinetic parameters for the elementary steps in the palladium-catalyzed synthesis of vinyl acetate

The kinetics of the reaction between gas-phase ethylene and adsorbed acetate species to form vinyl acetate monomer (VAM) on a Pd(111) surface are measured using infrared spectroscopy to monitor the rate of acetate removal, as well as the rates of VAM and ethylidyne formation, at various temperatures. The results are fit using a kinetic model first proposed by Samanos in which ethylene reacts with acetate species to form an acetoxyethyl intermediate that forms VAM via β-hydride elimination. The results of the kinetic model agree well with the experimental data and Arrhenius plots of the rate constants yield activation energies that are in good agreement with those predicted by density functional theory (DFT) calculations. DFT also predicts that the reaction activation energies should depend on the acetate coverage while the experimental data can be fit by constant values of the rate constants, suggesting that the reaction activation energies are similar for a reaction center surrounded either by acetate species, ethylidynes, or a combination of both. Finally, the kinetic parameters for VAM desorption are in good agreement with the experimental peak temperature measured by temperature-programmed desorption for VAM desorbing from an ethylidyne-covered surface. © 2010 Springer Science+Business Media, LLC

Coverage effects on the palladium-catalyzed synthesis of vinyl acetate: Comparison between theory and experiment

The high adsorbate coverages that form on the surfaces of many heterogeneous catalysts under steady-state conditions can significantly lower the activation energies for reactions that involve the coupling of two adsorbed intermediates while increasing those which result in adsorbate bond-breaking reactions. The influence of the surface coverage on the kinetics of metal-catalyzed reactions is often ignored in theoretical and even in some ultrahigh vacuum experimental studies. Herein, first principle density functional theoretical calculations are combined with experimental surface titration studies carried out over well-defined Pd(111) surfaces to explicitly examine the influence of coverage on the acetoxylation of ethylene to form vinyl acetate over Pd. The activation energies calculated for elementary steps in the Sámanos and Moiseev pathways for vinyl acetate synthesis carried out on acetate-saturated palladium surfaces reveal that the reaction proceeds via the Sámanos mechanism which is consistent with experimental results carried out on acetate-saturated Pd(111) surfaces. The rate-limiting step involves a β3-hydride elimination from the adsorbed acetoxyethyl intermediate, which proceeds with an apparent calculated activation barrier of 53 kJ/mol which is in very good agreement with the experimental barrier of 55 ± 4 kJ/mol determined from kinetic measurements. © 2010 American Chemical Society

Total methanol oxidation studied by MS-DRIFT operando and MES on bimetallic Au-Pd catalysts

The mechanism of methanol oxidation was investigated by combining operando and modulation-excitation diffuse reflectance infrared spectroscopy (MES-DRIFT) over monometallic Au/CeO2-ZrO2(CZ) and Pd/CZ catalysts, and also over the corresponding Au-Pd bimetallic catalysts with Au:Pd ratios of 1:1, 3:1 and 10:1. Adsorbed intermediaries were identified spectroscopically on the support, e.g. methoxy, mono- and bidentate formate, carbonates, and possibly CO adsorbed on gold sites or Pd modified by Au. The catalytic activity was evaluated by mass spectrometry (MS) coupled to the exit of the reactor cell (DRIFT), observing that the reaction is favored over the bimetallic catalysts with respect to their monometallic counterparts, and furthermore showing the first ones a trend which follows their Au:Pd ratios being more active the ones with less gold. It is postulated that the region of the support close to the metal particle is active during the reaction and possibly charge transfer between Au and Pd could be the main reason for their synergetic effect on the reaction studied. The less energetically favorable step is the decomposition of formate groups.Fil: Calaza, Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Baldó, Alina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Olmos, Carol. Universidad de Cádiz; EspañaFil: Chen, Xiaowei. Universidad de Cádiz; EspañaFil: Collins, Sebastián Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaThe American Chemical Society 255New OrleansEstados UnidosAmerican Chemical Societ

Ordered Au Nanoparticle Array on Au(111) through Coverage Control of Precursor Metal−Organic Chains

Metal–organic overlayer structures formed by 1,4-phenylene-diisocyanide (PDI) and Au adatoms on Au(111) in UHV, their stability in air, and the tip-induced Au nanoparticle formation on PDI–Au(111) surfaces in air were investigated using scanning tunneling microscopy (STM) and vibrational spectroscopy. This study reveals that the distribution of Au nanoparticles created during tip-induced release of Au atoms from molecule-Au adatom complexes shows strong dependence on the PDI coverage. Ordered Au nanoparticle arrays form in the medium-coverage regime, while more disordered distributions are observed at low and saturation coverages. The different distributions of Au nanoparticles are a direct consequence of the coverage-dependent assembly of (PDI–Au)n chains, their different stability in air, and a templating effect of the Au(111) surface, which is most pronounced for medium coverage, where phases of densely packed (PDI–Au)n chains and disordered PDI–Au assemblies are confined, respectively, to the fcc and hcp regions of the (22 × √3) surface reconstruction of Au(111). The Au nanoparticles nucleate preferentially in the disordered or defective regions of the PDI–Au precursor overlayer, and their formation requires ambient air and high negative tip-bias, suggesting an electrochemical initiation of Au release from the molecule–Au adatom complexes.Fil: Ghalgaoui, Ahmed. University Of Graz. Institute Of Physics; AustriaFil: Doudin, Nassar. University Of Graz. Institute Of Physics; AustriaFil: Calaza, Florencia Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina. Fritz-Haber-Institut der Max-Planck-Gesellschaft; AlemaniaFil: Surnev, Svetlozar. University Of Graz. Institute Of Physics; AustriaFil: Sterrer, Martin. University Of Graz. Institute Of Physics; Austri

Mechanism of the acetonitrile hydrogenation to amines over a platinum catalyst investigated by in situ infrared spectroscopy and DFT modeling

The mechanism of the hydrogenation of acetonitrile (CH3CN and CD3CN) to amines over a platinum filmand Pt/Al2O3 catalyst was investigated by in situ infrared spectroscopy. The reaction was studied under realisticconditions -liquid phase with toluene as solvent- using a flow-through cell-microreactor in attenuated total reflection(ATR) mode developed in our research group. Reaction intermediates were identified combining infrared spectra withtheoretical modeling by DFT. A sequential hydrogenation mechanism is proposed. Acetonitrile linearly and 2-foldchemisorbed on platinum sites is hydrogenated to form an imine surface intermediate (CH3CH=NH), which ishydrogenated to ethylamine. In turn, this imine intermediate can condense producing diethylamine and triethylamineand ammonia as a by-product. The time-evolution of the IR signals were modeled using a proposed microkineticmechanism and intrinsic kinetic constants were obtained under chemical control.Fil: Vogt, Lautaro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Quaino, Paola Monica. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Calaza, Florencia Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Collins, Sebastián Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina255th American Chemical Society National Meeting and ExpositionNew OrleansEstados UnidosAmerican Chemical Societ

Adsorption and Decomposition of Glycerol on Pristine and Oxygen Modified Au(111) Surfaces

Research on biomass derived raw materials for conventional catalytic processes, especially those directed to replace human dependence on fossil-based energy, is a high priority academic topic worldwide. Glycerol, the ubiquitous by-product of biodiesel manufacture, is seen as a promising building block due to its versatile functionality. Hence, research efforts to valorize it by selective partial oxidation are widespread. Fundamental knowledge of the interaction of glycerol with metal surfaces in the presence of oxygen is of extreme importance to rationally design new catalytic materials. In this work, a complete study of glycerol interaction with pristine and oxygen modified Au(111) surfaces is presented, by means of X-ray photoelectron and infrared reflection absorption spectroscopies, aided by temperature programmed desorption (TPD) experiments using mass spectrometry. On the clean Au(111) surface, glycerol adsorbs at 150 K through weak interactions between the oxygen atoms from OH groups and gold atoms. No thermal activation is observed and only molecular desorption is detected in TPD at 293 K. On the other hand, when the Au surface is precovered with oxygen atoms in the form of chemisorbed oxygen, glycerol adsorbs in a slightly different geometry and is activated even at low temperatures. The observation of spectral features related to C=O bonds clearly corroborates the activation of the alcohol groups toward partial oxidation intermediates. Possible products desorbing from the surface due to this activation are identified as dihydroxyacetone, hydroxypiruvic, tartronic and formic acid, as well as H2O and CO2.Fil: Calaza, Florencia Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Baltanas, Miguel Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Sterrer, Martin. Institute Of Physics, University Of Graz; AustriaFil: Freund, Hans-Joachim. Dept Of Chemical Physics, Fritz-haber-institut Der Mpg; Alemani

An Infrared Spectroscopic and Temperature-Programmed Desorption Study of 1,1-Difluoroethylene on Clean and Hydrogen-Covered Pd(111)

The surface chemistry of 1,1-difluoroethylene was studied on clean and hydrogen-covered Pd(111) using a combination of temperature-programmed desorption and reflection absorption infrared spectroscopy (RAIRS) to explore whether the larger infrared absorbance of 1,1-difluoroethylene than ethylene may be used to examine reactions under realistic catalytic conditions using RAIRS. It was found that the chemistry of 1,1-difluoroethylene on Pd(111) surfaces is similar to that of ethylene with bonding occurring in both the π- and di-σ-forms. However, due to the presence of C–F bonds in the molecule, the infrared absorbances for 1,1-difluoroethylene were much larger than those for ethylene. This provides the potential for using RAIRS for in situ studies of catalytic reactions that involve alkenes

Molecular Adsorption Changes the Quantum Structure of Oxide-Supported Gold Nanoparticles: Chemisorption versus Physisorption

STM conductance spectroscopy and mapping has been used to analyze the impact of molecular adsorption on the quantized electronic structure of individual metal nanoparticles. For this purpose, isophorone and CO2, as prototype molecules for physisorptive and chemisorptive binding, were dosed onto monolayer Au islands grown on MgO thin films. The molecules attach exclusively to the metal-oxide boundary, while the interior of the islands remains pristine. The Au quantum well states are perturbed due to the adsorption process and increase their mutual energy spacing in the CO2 case but move together in isophorone-covered islands. The shifts disclose the nature of the molecule-Au interaction, which relies on electron exchange for the CO2 ligands but on dispersive forces for the organic species. Our experiments reveal how molecular adsorption affects individual quantum systems, a topic of utmost relevance for heterogeneous catalysis