Effects of the synthetic condition on the stability, particle size and redox chemistry of nanoporous CoAlPO-34

This study focuses on the effect of the synthetic conditions on the stability, particle size, redox chemistry of cobalt into the framework of CoAlPO-34. It seems that the most sufficient pH for the substitution of Co into the framework of CoAlPO-34 was pH around 7.5 when the as-synthesized bifunctional catalyst has the best redox property. The pH of the initial gel has strong effect on the particle size of CoAlPO-34. The substitution of cobalt and redox chemistry were determined by: EXAFS combined with XRD, XANES, IR. Stability of the nanoporous catalyst studied by in situ XRD were also reported

Chemical bonding in zeolites

A review is given with 28 refs. on the current status of chem. bonding of zeolites and AlPO's. Short range covalent bonding dominates, the tetrahedra have to be considered relatively rigid and the Si-O-Si bond angle flexible. Differences in energy of the SiO2 or AlPO4 polymorphs are small. The relative stability depends on a proper accounting of the small changes in electrostatic energy. The deprotonation energy is also mainly detd. by short range covalent interactions. These are only properly accounted for when full lattice relaxation is included in the calcns. Isomorphous substitution effects are also dominated by changes in covalent interaction energ

A computational study of the heterogeneous synthesis of hydrazine on Co3Mo3N

Periodic and molecular density functional theory calculations have been applied to elucidate the associative mechanism for hydrazine and ammonia synthesis in the gas phase and hydrazine formation on Co3Mo3N. We find that there are two activation barriers for the associative gas phase mechanism with barriers of 730 and 658 kJ/mol, corresponding to a hydrogenation step from N2 to NNH2 and H2NNH2 to H3NNH3, respectively. The second step of the mechanism is barrierless and an important intermediate, NNH2, can also readily form on Co3Mo3N surfaces via the Eley–Rideal chemisorption of H2 on a pre-adsorbed N2 at nitrogen vacancies. Based on this intermediate a new heterogeneous mechanism for hydrazine synthesis is studied. The highest relative barrier for this heterogeneous catalysed process is 213 kJ/mol for Co3Mo3N containing nitrogen vacancies, clearly pointing towards a low-energy process for the synthesis of hydrazine via a heterogeneous catalysis route

Interfacial chemistry in the electrocatalytic hydrogenation of CO2 over C-Supported Cu-Based systems

Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)–O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe–Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)–O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu–Zn system represents the optimal active ensembles with stabilized Cu(I)–O; DFT simulations rationalize this observation by indicating that Cu–Zn–O neighboring atoms are able to activate CO2, whereas Cu–Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies

Preface

Micro- and mesoporous materials are the topic of intensive study due to both the challenges they pose to fundamental science and to their wide ranging applications in key industrial processes. The complexity of the systems requires input from many different approaches and techniques; and for over 30 years an increasingly powerful range of computational modeling techniques has been successfully applied to these materials. Indeed modeling is widely recognized as an indispensable tool in studying both the structures and properties of these materials and of the behavior of molecules within them. Moreover, with the continuing growth of computer power, the field is now able to investigate in detail the real systems probed by experiment and is gaining a growing predictive power.\u3cbr/\u3e\u3cbr/\u3eThis book reviews some of the most important developments in the field over recent years. It will illustrate the increased range of techniques that are now available and the growing diversity of materials within the family of micro- and mesoporous solids. It follows earlier books on the topic including “Modelling of structure and reactivity in zeolites” (Academic Press, London, 1992) and “Computer Modelling of Microporous Materials” (Elsevier, Amsterdam, 2004) which charted the earlier stages of the application of modeling techniques in micro- and mesoporous science. We hope that it illustrates the current scope, achievements, and excitement of the field

Quantum-chemical studies of alkene chemisorption in chabazite: A comparison of cluster and embedded-cluster models

Quantum-chemical studies of ethene, propene and isobutene chemisorption at an aluminosilicate Bronsted-acid site in the zeolite chabazite are reported. Comparison of the results using different cluster models and a qm/mm (quantum mechanical/molecular mechanical) embedded cluster approach are compared and contrasted. As in previous studies, the activation barriers for the chemisorption process leading to a surface alkoxide are found to follow a carbenium ion trend, i.e. ethene > propene > isobutene. In contrast to previous studies, however, results indicate that the stability of the alkoxide is also very sensitive to a number of factors, the dominant one being steric interactions with the acid site, i.e, the stability order is ethene > propene > isobutene. This steric effect and other, less dominant, contributions are only observed when host environment effects are included in the model, in the present case via constraints on the cluster boundaries and via the qm/mm embedded-cluster approach. The possible formation of stable carbenium ions in the pores of acidic zeolites is discussed