The locations and energies of Cu ions in a Cu/SSZ-13 zeolite catalyst were investigated by density functional theory (DFT) calculations. For "naked" Cu 2+ ions (i.e., Cu 2+ ions with no ligands in their coordination spheres other than zeolite lattice oxygen atoms), the more energetically favorable sites are within a 6-membered ring. However, with the presence of various adsorbates, the energy difference between 6-and 8-membered ring locations greatly diminishes. Specifically, Cu 2+ ions are substantially stabilized by −OH ligands (as [Cu II (OH)] + ), making the extra-framework sites in an 8-membered ring energetically more favorable than 6-membered ring sites. Under fully dehydrated high vacuum conditions with different Si/Al and Cu/Al ratios, three chemisorbed NO species coexist upon exposure of NO to Cu/SSZ-13: NO + , Cu 2+ −NO, and Cu + −NO. The relative signal intensities for these bands vary greatly with Si/Al ratios. The vibrational frequency of chemisorbed NO was found to be very sensitive to the location of Cu 2+ ions. On the one hand, with the aid from DFT calculations, the nature for these vibrations can be assigned in detail. On the other hand, the relative intensities for various Cu 2+ −NO species provide a good measure of the nature of Cu 2+ ions as functions of Si/Al and Cu/Al ratios and the presence of humidity. These new findings cast doubt on the generally accepted proposal that only Cu 2+ ions located in 6-membered rings are catalytically active for NH 3 −SCR.
Copper is a common catalyst for many important chemical reactions including low-temperature water gas shift, selective catalytic reduction of NO x , methanol synthesis, methanol steam reforming, and partial oxidation of methanol. The degree of surface oxidation, or the oxidation state of the active site, during these reactions has been debated and is known to have a large influence on the reaction rates. Therefore, elucidating the atomic-scale structure of copper surface oxides is an important step toward a fuller understanding of reaction mechanisms in heterogeneous catalysis. The so-called “29” monolayer oxide film is a common intermediate in the oxidation of Cu(111). The large size of its unit cell has thus far prevented the development of a definitive model for its structure. Using high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we arrive at a model for the “29” Cu x O film on Cu(111). There is very good agreement between experimental and computational STM images over a range of biases. Through the construction of a phase diagram from first-principles, we further find that the “29” structure derived from the DFT calculations is indeed the most stable structure under the experimental conditions considered. This work yields an accurate picture of the atomic scale structure of the “29” oxide film and therefore a basis for beginning to understand adsorption sites and reaction mechanisms on this catalytically relevant surface.
The synergistic catalysis in the hydrodeoxygenation of phenolic compounds over a Pd/Fe bimetallic surface has been well established. However, the nature of this synergy is still in part a mystery. In this work, we used a combined experimental and theoretical approach to understand a potential function of the surface Pd in the reduction of Pd/Fe2O3. This function of Pd was investigated via the comparison of the reduction properties as well as other physicochemical properties of samples synthesized by the reduction of Fe2O3 nanoparticles with and without surface Pd. Temperature-programmed reduction studies demonstrated the remarkable facilitation of reduction by addition of Pd, evidenced by a 150 °C shift toward lower temperature of the reduction peak of Fe3+. From X-ray photoelectron spectroscopy and theoretical calculation results, the interaction between Pd and the Fe2O3 surface occurs through the exchange of electrons with both the surface Fe and O atoms. This bonding between the Pd and surface oxide elements causes the Pd to partially donate electrons to the oxide surface, making the surface electrons more delocalized. This electron delocalization stabilizes the reduced oxide surfaces, as suggested by the TPR results and theoretical prediction. Therefore, the stabilization of the reduced Fe surface as well as the facilitated water formation by introduction of Pd is expected to significantly contribute to the Pd–Fe synergy in hydrodeoxygenation catalysis.
Abstract:A promising material for hydrogen storage at room temperature−Al doped graphene was proposed theoretically by using density functional theory calculation. Hydrogen storage capacity of 5.13 wt% was predicted at T = 300 K and P = 0.1 Gpa with adsorption energy E b = -0.260 eV/H 2 . This is close to the target of 6 wt% and satisfies the requirement of immobilization hydrogen with E b of -0.2 ~ -0.4 eV/H 2 at ambient temperature and modest pressure for commercial applications specified by U.S. Department of Energy. It is believed that the doped Al varies the electronic structures of both C and H 2 . The bands of H 2 overlapping with those of Al and C synchronously are the underlying mechanism of hydrogen storage capacity enhancement.
Bimetallic surfaces have been found to greatly improve the performance of numerous chemical processes due to synergistic interactions between the metal components. To be able to tailor the adsorption of aromatic molecules of these surfaces, the synergistic interactions within the surface and their effects on adsorbates must be elucidated. In this work, we examine the energetic and electronic interaction between benzene and several model PdFe bimetallic surfaces, with low and high Pd coverage limits, using density functional theory and compare our results with the adsorption of benzene on a pure Fe (110) surface. The adsorption energy trends on these model surfaces show that the interactions between the Pd and Fe significantly decrease the strength of benzene's adsorption on surface Pd without significantly weakening its adsorption on the adjacent surface Fe. From the electronic analyses, the decreased adsorption strength of benzene on the model PdFe surfaces is due to the shift in the Pd's dband center away from the Fermi level. These results show that aromatic compounds will preferentially adsorb onto any exposed Fe in a PdFe surface due to the greater availability of electronic states near the Fermi level. Therefore, under typical catalytic conditions, the strength of the adsorption of benzene can be tailored on the basis of the amount of Pd added to an Fe surface because increasing the concentration of Pd in the surface will increase the amount of interaction between the adsorbate and the modified surface Pd.
As a first step towards a microscopic understanding of single-Pt atom-dispersed catalysts on non-conventional TiN supports, we present density-functional theory (DFT) calculations to investigate the adsorption properties of Pt atoms on the pristine TiN(100) surface, as well as the dominant influence of surface defects on the thermodynamic stability of platinized TiN. Optimized atomic geometries, energetics, and analysis of the electronic structure of the Pt/TiN system are reported for various surface coverages of Pt. We find that atomic Pt does not bind preferably to the clean TiN surface, but under typical PEM fuel cell operating conditions, i.e. strongly oxidizing conditions, TiN surface vacancies play a crucial role in anchoring the Pt atom for its catalytic function. Whilst considering the energetic stability of the Pt/TiN structures under varying N conditions, embedding Pt at the surface N-vacancy site is found to be the most favorable under N-lean conditions. Thus, the system of embedding Pt at the surface N-vacancy sites on TiN(100) surfaces could be promising catalysts for PEM fuel cells.
To provide a basis for understanding the reactive processes on nickel surfaces at fuel cell anodes, we investigate the influence of an external electric field on the dehydrogenation of methyl species on a Ni(111) surface using density functional theory calculations. The structures, adsorption energies and reaction barriers for all methyl species dissociation on the Ni(111) surface are identified. Our results show that the presence of an external electric field does not affect the structures and favorable adsorption sites of the adsorbed species, but causes the adsorption energies of the CHx species at the stable site to fluctuate around 0.2 eV. Calculations give an energy barrier of 0.692 eV for CH3* → CH2* + H*, 0.323 eV for CH2* → CH* + H* and 1.373 eV for CH* → C* + H*. Finally, we conclude that the presence of a large positive electric field significantly increases the energy barrier of the CH* → C* + H* reaction more than the other two reactions, suggesting that the presence of pure C atoms on Ni(111) are impeded in the presence of an external positive electric field.
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