The effect of surface relaxation on the N-2 dissociation rate on stepped Ru: A Transition State Theory Study

van Harrevelt R, Honkala K, Norskov JK, Manthe U. The effect of surface relaxation on the N2 dissociation rate on stepped Ru: A Transition State Theory Study. Journal of Chemical Physics. 2006;124(2):026102: 026102

The reaction rate for dissociative adsorption of N-2 on stepped Ru(0001): Six-dimensional quantum calculations

van Harrevelt R, Honkala K, Norskov JK, Manthe U. The reaction rate for dissociative adsorption of N2 on stepped Ru(0001): Six-dimensional quantum calculations. Journal of Chemical Physics. 2005;122(23): 234702.Quantum-mechanical calculations of the reaction rate for dissociative adsorption of N-2 on stepped Ru(0001) are presented. Converged six-dimensional quantum calculations for this heavy-atom reaction have been performed using the multiconfiguration time-dependent Hartree method. A potential-energy surface for the transition-state region is constructed from density-functional theory calculations using Shepard interpolation. The quantum results are in very good agreement with the results of the harmonic transition-state theory. In contrast to the findings of previous model calculations on similar systems, the tunneling effect is found to be small. (C) 2005 American Institute of Physics

Spectroscopic link between adsorption site occupation and local surface chemical reactivity

In this Letter we show that sequences of adsorbate-induced shifts of surface core level (SCL) x-ray photoelectron spectra contain profound information on surface changes of electronic structure and reactivity. Energy shifts and intensity changes of time-lapsed spectral components follow simple rules, from which adsorption sites are directly determined. Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the d-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity

Multidimensional effects on dissociation of N-2 on Ru(0001)

Theoretical Chemistr

Effect of atomic layer deposited zinc promoter on the activity of copper-on-zirconia catalysts in the hydrogenation of carbon dioxide to methanol

The development of active catalysts for carbon dioxide (CO2) hydrogenation to methanol is intimately related to the creation of effective metal-oxide interfaces. In this work, we investigated how the order of addition of copper and zinc on zirconia influences the catalytic properties, the catalytic activity and selectivity toward methanol. Regarding the carbon dioxide conversion and methanol production, the catalysts on which the promoter (zinc) was atomically deposited after copper impregnation (i.e., ZnO/Cu/ZrO2 and ZnO/Cu/ZnO/ZrO2) were superior catalysts compared to the reverse copper-after-zinc catalyst (Cu/ZnO/ZrO2). Temperature-programmed experiments and in situ diffuse reflectance infrared Fourier transform-spectroscopy (DRIFTS) experiments allowed us to elucidate the benefits of the zinc-after-copper pair to store CO2 as carbonate species and further convert them into formate species, key intermediates in the formation of methanol. This research provides insights into the potential of atomic layer deposition in the development of tailored heterogeneous catalysts for efficient CO2 valorization to methanol.Peer reviewe

Metal ammine complexes for hydrogen storage

The hopes of using hydrogen as an energy carrier are severely dampened by the fact that there is still no safe, high-density method available for storing hydrogen. We investigate the possibility of using metal ammine complexes as a solid form of hydrogen storage. Using Mg(NH 3 ) 6 Cl 2 as the example, we show that it can store 9.1% hydrogen by weight in the form of ammonia. The storage is completely reversible, and by combining it with an ammonia decomposition catalyst, hydrogen can be delivered at temperatures below 620 K. Storing hydrogen in a safe, high-density, condensed phase is a notoriously difficult problem. 1 Storage in the form of metal hydrides has been studied for decades, 2 and most recently attention has been focused on the so-called complex hydrides based on alanates 3 or borates. properties, but most of them also suffer from problems relating to the density of hydrogen being too low, the kinetics of hydrogen release being too slow, or the regeneration of the hydride being too difficult. 5 In the present paper we explore a new way of storing hydrogen in the form of metal ammine complexes. They decompose thermally by evolving ammonia at a temperature which can be varied by changing the composition of the complex. By combining such complexes with an ammonia decomposition catalyst one obtains a very versatile hydrogen source. We consider Mg(NH 3 ) 6 Cl 2 Metal ammine complexes of the form M(NH 3 ) n X m , where M is a metal cation like Mg, Ca, Cr, Ni, and Zn, and X is an anion like Cl or SO 4 , have been know for more than a century. 9 Mg(NH 3 ) 6 Cl 2 was prepared by leading 1 bar of ammonia (Hede Nielsen, .99.9%) over anhydrous MgCl 2 (Merck, .98%) at 300 K. The purity and phase composition was verified by X-ray powder diffraction. pellet to obtain a high volumetric density. The saturated salt was then put in a small cell, where the rate of ammonia desorption from the sample could be measured quantitatively. This was done by absorbing the ammonia immediately at the outlet of the cell in a miniaturized scrubber using a small flow of distilled water. The ammonia content was then determined using an on-line conductivity cell

Tailoring oxide properties: An impact on adsorption characteristics of molecules and metals

Both density functional theory calculations and numerous experimental studies demonstrate a variety of unique features in metal supported oxide films and transition metal doped simple oxides, which are markedly different from their unmodified counterparts. This review highlights, from the computational perspective, recent literature on the properties of the above mentioned surfaces and how they adsorb and activate different species, support metal aggregates, and even catalyse reactions. The adsorption of Au atoms and clusters on metal-supported MgO films are reviewed together with the cluster׳s theoretically predicted ability to activate and dissociate O2 at the Au–MgO(100)/Ag(100) interface, as well as the impact of an interface vacancy to the binding of an Au atom. In contrast to a bulk MgO surface, an Au atom binds strongly on a metal-supported ultra-thin MgO film and becomes negatively charged. Similarly, Au clusters bind strongly on a supported MgO(100) film and are negatively charged favouring 2D planar structures. The adsorption of other metal atoms is briefly considered and compared to that of Au. Existing computational literature of adsorption and reactivity of simple molecules including O2, CO, NO2, and H2O on mainly metal-supported MgO(100) films is discussed. Chemical reactions such as CO oxidation and O2 dissociation are discussed on the bare thin MgO film and on selected Au clusters supported on MgO(100)/metal surfaces. The Au atoms at the perimeter of the cluster are responsible for catalytic activity and calculations predict that they facilitate dissociative adsorption of oxygen even at ambient conditions. The interaction of H2O with a flat and stepped Ag-supported MgO film is summarized and compared to bulk MgO. The computational results highlight spontaneous dissociation on MgO steps. Furthermore, the impact of water coverage on adsorption and dissociation is addressed. The modifications, such as oxygen vacancies and dopants, at the oxide–metal interface and their effect on the adsorption characteristics of water and Au are summarized. Finally, more limited computational literature on transition metal (TM) doped CaO(100) and MgO(100) surfaces is presented. Again, Au is used as a probe species. Similar to metal-supported MgO films, Au binds more strongly than on undoped CaO(100) and becomes negatively charged. The discussion focuses on rationalization of Au adsorption with the help of Born–Haber cycle, which reveals that the so-called redox energy including the electron transfer from the dopant to the Au atom together with the simultaneous structural relaxation of lattice atoms is responsible for enhanced binding. In addition, adsorption energy dependence on the position and type of the dopant is summarized.peerReviewe

Ligand assisted hydrogenation of levulinic acid on Pt(111) from first principles calculations

In this study, we investigate the hydrogenation reaction of levulinic acid to 4-hydroxypentanovic acidon a ligand-modified Pt(111) using DFT. Modifying nanoparticle surfaces with ligands can havebeneficial effects on the desired reaction such as improved selectivity or lower activation energies.The N3,N3-dimethyl-N2-(quinolin-2-yl)propane-1,2-diamine (AQ) ligand was selected to modify thesurface, since it combines good surface adsorption properties with functional groups that can influencethe reaction. The adsorption geometry of the AQ ligand was studied as well as the co-adsorptionof a second AQ for the possibility of self-assembly. We find that dissociated hydrogen from thePt(111) surface can protonate the AQ ligand and discuss the role this plays on the mechanism ofthe hydrogenation reaction of levulinic acid (LA). By comparing the ligand-modified Pt(111) surfaceto the bare Pt(111) surface we show that the reaction changes from a step-wise to a concertedmechanism due to the influence of the ligand molecule. This demonstrates the effect ligand-modifiedsurfaces can have in catalyzing reactions and shows that desired reactions can be achieved by tuningthe reaction environment

Understanding Structure and Stability of Monoclinic Zirconia Surfaces from First-Principles Calculations

Under the water-rich pre-treatment and/or reaction conditions, structure and chemistry of the monoclinic zirconia surfaces are strongly influenced by oxygen vacancies and incorporated water. Here, we report a combined first-principles and atomistic thermodynamics study on the structure and stability of selected surfaces of the monoclinic zirconia. Our results indicate that among the studied surfaces, the most stable (111) surface is the least vulnerable towards oxygen vacancies in contrast to the less stable (011) and (101) surfaces, where formation of oxygen vacancies is energetically more favorable. Furthermore, we present a vigorous, systematic screening of water incorporation onto the studied surfaces. We observe that the greatest stabilization of the surfaces is achieved when a part of the adsorbed water molecules is dissociated. Nevertheless, the importance of water dissociation for achieving the greatest stabilization is high for the less stable (011) and (101) surfaces, while completely hydrated (111) surface is stabilized equally regardless of the water dissociation state. Analysis of the constructed phase diagrams reveals that the (111) surface remains preferably clean and the (011) and (101) surfaces have dissociated water at low coverage under the reactive conditions of T = 600–900 K and p(H2O) < 1 bar. Upon temperature decrease and/or pressure increase, all studied surfaces gradually uptake water until fully hydrated. All in all, our findings complement and broaden the existing picture of the structure and stability of the monoclinic zirconia surfaces under the pre-treatment and/or reaction conditions, enabling rationalization of the potential roles of zirconia as a heterogeneous support and a catalyst component.peerReviewe

Globally Optimized Equilibrium Shapes of Zirconia-Supported Rh and Pt Nanoclusters : Insights into Site Assembly and Reactivity

Metal−support interfaces form an active site for many important catalytic reactions. The modeling of these interfacial sites calls for approximations to set up a structure model, which in turn may significantly have an impact on studied chemistry and obtained atomistic understanding. Herein, we have employed a density functional theory-based genetic approach to obtain globally optimized nanostructures for Rh and Pt clusters on a ZrO2 support. The analysis of the obtained structures shows that Rh clusters take more compact shapes, whereas Pt prefers elongated and low-symmetry structures. We find that metal−oxide perimeter sites are structurally different, presenting varying Pt and Rh coordinations and CO adsorption energies. Our analysis shows that the presence of a support always destabilizes CO adsorption at the cluster edge, but the magnitude of destabilization varies substantially from site to site. The complexity of catalyst−support interactions demonstrates that even an inert support can intricately influence the reactivity of interfacial sites.peerReviewe