Unsaturated Zn Research Articles

A comparative study of the NH 3 chemisorption on ZnO(101̄0) and Cu 2O(111) non-polar surfaces

The chemisorption of NH 3 on the ZnO(101̄0) and Cu 2O(111) non-polar surfaces is theoretically investigated to look into the different Lewis acidity of exposed, unsaturated Zn(II) and Cu(I) ions. Both molecular and dissociative processes have been taken into account for NH 3 on ZnO(101̄0), whereas only the molecular chemisorption of NH 3 on Cu 2O(111) has been considered. Calculations include the evaluation of surface relaxation before/after chemisorption, optimized adsorbate geometries, chemisorption energies and adsorbate vibrational frequencies. Theoretical data indicate that the chemisorption of NH 3 on the defect free ZnO(101̄0) surface occurs only molecularly and that the adsorbate is more tightly bound to Cu(I) than to Zn(II) sites. The agreement with available experimental data is satisfactory.

The skeletal isomerization of 1-butene over Zn-silicoaluminophosphate molecular sieves

The catalytic transformations of 1-butene were performed over a zinc-substituted silicoaluminophosphate molecular sieve (ZnAPSO-11), over a Zn-supported SAPO-11 molecular sieve (Zn/SAPO-11) and over the unpromoted SAPO-11 solid. The ZnAPSO-11 catalyst showed the highest selectivity towards skeletal isomerization as well as the highest skeletal isomerization efficiency (SIE). In addition, the largest number of acid sites, particularly strong acid sites, were observed with temperature-programmed desorption of ammonia (NH3-TPD) over the ZnAPSO-11 catalyst. The acid strength over the latter was also the highest. The agreement between the catalytic and the acidity results obtained in the present work can be explained in terms of the model of Gielgens et al. In this model, the substitution of Al(III) ions by Zn(II) ions leads to partially unsaturated Zn(II) ions in the vicinity of structural P-OH groups. The Bronsted acidity of the latter may be enhanced by Bronsted-Lewis interaction, thus rendering catalysts with stronger acidity necessary for the skeletal isomerization. The lower molar fraction of Al(III) cations found in the ZnAPSO-11 solid, compared to the SAPO-11 sample, strongly supports the above. The results for the Zn-supported system were extremely different. Significant decreases in the strong acidity as well as in the skeletal isomerization selectivity, compared to the ZnAPSO-11 catalyst, were observed. These differences reinforce the idea of incorporation of Zn(II) into the silicoaluminate framework for the ZnAPSO-11 solid.

Coordination chemistry of CO and NH 3 on ZnO(0001): a molecular cluster study of the CO and NH 3 bonding interaction with a d 10 ion

A detailed investigation of the electronic structure of CO chemisorbed on the ZnO(0001) polar surface has been carried out by using the local-density-functional molecular-cluster approach. A Zn 22O 22/CO model cluster is used to evaluate internuclear distances, adsorption energies and vibrational frequencies of the surface-CO and C-O vibrational modes. The chaining to the rest of the solid has been simulated with pseudo-atoms carrying a not-integer nuclear charge. A realistic description of the electronic structure of the Lewis acid site is shown to be crucial to obtain a correct chemisorptive interaction. In particular, it appears that removing half an electron from each surface Zn allows an excellent agreement between experiment and theory as a consequence of the absence of occupied dangling bonds on ZnO(0001). Preliminary results relative to NH 3 chemisorption over the same surface are also presented and discussed. The bonding mechanism of CO to ZnO(0001) is dominated by a covalent interaction involving a donation from the highest occupied molecular orbital of CO into the empty levels of the coordinatively unsaturated Zn surface ions. The same kind of mechanism is active in NH 3 chemisorption, even though the electrostatic interaction between the nh 3 dipole moment and the field at ZnO(0001) is at least as important as the covalent one. Slight variations of the electronic structure of the surface greatly affect the bonding of CO to the surface itself as a consequence of a consistent donation into the CO lowest unoccupied molecular orbital from the partially occupied dangling bonds of the Zn unsaturated ions nearest to the Lewis acid site. Transition state calculations seem to indicate that the assignments of the ultraviolet photoelectron spectra of ZnO(0001) after exposure to CO or NH 3 need to be slightly modified.

Methanol synthesis on monovalent copper species stabilized on silica and zinc oxide

A set of exchanged Cu/SiO 2 and Cu Zn/SiO 2 solids were used as models of copper-based methanol synthesis catalysts. The conditions for the selective reduction of the supported Cu II ions into unsaturated Cu I species are given. A good correlation was established between methanol selectivity and catalyst stability under the reaction conditions and the concentration of Cu I species in the prereduced catalysts. The methanol selectivity and the catalyst stability are enhanced in the presence of unsaturated Zn 2+ cations. The effect of Zn 2+ ions is greater for Cu/ZnO than for Cu Zn/SiO 2 catalysts. Methanol selectivity and stability under the reaction conditions are greatly enhanced with a ZnO support. A model of the active sites is proposed.

Electronic Structure of Grain Boundaries and Interfaces in Polycrystalline Zinc Oxide

ABSTRACTPreliminary theoretical models for the electronic structure of grain boundaries and interfaces in polycrystalline ZnO have been constructed on the basis of self-consistent-field X-alpha scattered-wave (SCF-Xα-SW) cluster molecular-orbital calculations. The disposition and character of the interface states, relative to the valence and conduction bands of the otherwise perfect crystalline material, have been studied for clusters representing coordinatively unsaturated Zn surface sites and molecular O2 chemisorption thereon. The possible effects of the resuiting interface states on electron transport at grain boundaries in ZnO varistors have been addressed.