Synthesis of diethylcarbonate by ethanolysis of urea catalysed by heterogeneous mixed oxides

New Zn- and Ca-based mixed oxides have been tested in the ethanolysis of urea. Cerium and magnesium have revealed to be able to stabilize and enhance the activity of Zn and Ca. All the used compounds act as heterogeneous catalysts in a batch reactor and can be easily recovered and re-used in several catalytic runs. However, although ZnO dissolves as Zn(NCO)2(NH3)2 in the reaction medium under the operative conditions and then partly precipitates at room temperature ensuring a modest immediate recoverability and recyclability, 2CaO/CeO2 is insoluble also at the reaction temperature that makes it well suited even for the use in a flow reactor. MgO-ZnO and SiO2-ZnO have also been tested. The former has an interesting performance, but still not equal to that of 2CaO-CeO2. Interestingly, the latter catalyst is able to convert urea and ethanol into DEC with 91% conversion of urea and 98% selectivity in the long term

Bimetallic catalysts for the Fischer-Tropsch reaction

This short critical review summarises and analyses the developments in Fischer-Tropsch catalysis using bimetallic alloys. We introduce a simple notation for such catalysts, and monitor the reports of synergistic effects and composition/performance relationships. Special attention is given to CoFe alloys on a variety of supports, and to the effects of catalyst preparation methods and pre-treatment conditions. The key drawbacks in comparing the large amount of data available on Fischer-Tropsch catalysis are the high dimensionality of the problem and the lack of long time-on-stream studies. Based on the new understanding coming from characterisation studies of supported bimetallic particles, we propose a structured approach for effectively studying Fischer-Tropsch catalysi

Adsorption and dissociation of CO on body-centered cubic transition metals and alloys: Effect of coverage and scaling relations

The adsorption and dissociation of CO have been calculated on the (100) surfaces of the body-centered cubic transition metals Fe, Mo, Cr, and W and the alloys Fe3Mo and Fe3Cr using density functional theory for two CO coverages, 0.25 and 0.5 ML. A complete analysis of the vibrational frequencies was performed to check whether the calculated structures are stable geometries or transition-state structures. For coverages up to 0.25 ML, carbon monoxide adsorbs molecularly onto all four metals at fourfold hollow sites with tilting angles with respect to the surface normal of 47°, 57°, 57°, and 58° and adsorption energies of -1.53, -2.64, -3.03, and -3.01 eV for Fe, Mo, Cr, and W, respectively. The calculated CO stretching frequencies at this coverage are 1211, 1062, 1037, and 926 cm-1. At higher coverages, CO adsorption does not exhibit significant changes in both adsorption energy and tilting angle on all four metals but leads to blue shifts of the CO frequency for Fe and Cr and red shifts for Mo and W. Furthermore, scaling relations apply to a bent CO species at a surface coverage of 0.25 ML of CO on all four transition metals as well as the metal alloys Fe3Mo and Fe3Cr, in the sense that the heat of adsorption of CO and the activation energy of CO dissociation scale linearly with the heat of adsorption of the carbon as well as both dissociation products. © 2009 American Chemical Society

Density functional theory study of CO adsorption and dissociation on molybdenum(100)

The adsorption of CO on Mo(100) has been calculated for several adsorption states at four surface coverages using density functional theory (DFT). Dissociation of CO on Mo(100) has been investigated for two surface coverages: 0.25 and 0.5 monolayer (ML). A full analysis of the vibrational frequencies of CO was performed, to determine whether structures are stable adsorption states or transition states. Results show that CO adsorbs molecularly on the Mo(100) surfaces up to coverages of 0.5 ML at 4-fold hollow sites with the molecular axis tilted away from the surface normal by 55-57 and dissociates easily with activation energies ranging from 0.45 to 0.56 eV, leading to energy gains of -1.71 and -0.59 eV at 0.25 and 0.5 ML, after dissociation, respectively. The adsorption energy of the CO molecule at 0.25 ML is -2.64 eV with a C-O stretching vibration of 1062 cm-1. Increasing the CO surface concentration leads to a lower C-O stretching frequency of 958 cm-1, which is remarkable, and it is in conflict with the Blyholder model and previous experimental observations for CO on transition-metal surfaces. Furthermore, calculations reveal that reported CO desorption peaks in literature, thought to be due to recombination of carbon and oxygen, are more likely due to molecular desorption of CO at the 4-fold hollow position with a tilted geometry. This conclusion is supported by the low recombination energies calculated (one-third of that described in literature)

Testing the pairwise additive potential approximation using DFT: coadsorption of CO and N on Rh(100)

The interaction between adsorbates is a key issue in surface science, because these interactions can influence strongly the properties of chemisorbed species with consequences for the thermodn. and kinetics of surface processes. The simplest representation of adsorbate-adsorbate interactions is based on the assumption that all interactions are pairwise additive. This approach has been satisfactorily used in the modeling of temp.-programmed desorption (TPD) spectra using both continuum and Monte Carlo methods. However, the energies estd. within the pairwise approxn. have never been compared to the energies calcd. using d. functional theory (DFT) methods. We demonstrate that the pairwise additive potential approxn. is indeed a good representation of the adsorbate-adsorbate interactions, and that we do not need to include three-body interactions or higher-order terms to est. the perturbation of the adsorption energy of an adsorbate by the presence of other coadsorbates. Moreover, we show for the first time how DFT can be used to explain the desorption features that one finds in TPD expts., thus linking the TPD desorption features with actual microscopic configurations. [on SciFinder (R)

Mechanism and microkinetics of methanol synthesis via CO2 hydrogenation on indium oxide

Indium oxide has emerged as a highly effective catalyst for methanol synthesis by direct CO2 hydrogenation. Aiming at gathering a deeper fundamental understanding, mechanistic and (micro)kinetic aspects of this catalytic system were investigated. Steady-state evaluation at 5 MPa and variable temperature indicated a lower apparent activation energy for CO2 hydrogenation than for the reverse watergas shift reaction (103 versus 117 kJ mol!1), which is in line with the high methanol selectivity observed. Upon changing the partial pressure of reactants and products, apparent reaction orders of !0.1, 0.5, !0.2, and !0.9 were determined for CO2, H2, methanol, and water, respectively, which highlight a strong inhibition by the latter. Co-feeding of H2O led to catalyst deactivation by sintering for partial pressures exceeding 0.125 MPa, while addition of the byproduct CO to the gas stream could be favorable at a total pressure below 4 MPa but was detrimental at higher pressures. Density Functional Theory simulations conducted on In2O3(1 1 1), which was experimentally and theoretically shown to be the most exposed surface termination, indicated that oxygen vacancies surrounded by three indium atoms enable the activation of CO2 and split hydrogen heterolytically, stabilizing the polarized species formed. The most energetically favored path to methanol comprises three consecutive additions of hydrides and protons and features CH2OOH and CH2(OH)2 as intermediates. Microkinetic modeling based on the DFT results provided values for temperature and concentration-dependent parameters, which are in good agreement with the empirically obtained data. These results are expected to drive further optimization of In2O3-based materials and serve as a solid basis for reactor and process design, thus fostering advances towards a potential large-scale methanol synthesis from CO2

Applying Topological and Economical Principles in Catalyst Design: New Alumina-​Cobalt Core-​Shell Catalysts

Designing new and effective catalysts may be an art, but its consequences are very real and pragmatic. That said, chemists often build designs on ideal systems, whereas the manufg. of chems. requires catalysts that withstand varied feeds, harsh conditions and long exposure times. Moreover, economical considerations are often underestimated at the catalyst design stage. Here we discuss the inclusion of economical and topol. considerations early on in the catalyst design process, giving as an example the synthesis and testing of a new type of alumina​/cobalt Fischer-​Tropsch catalysts