Showing papers by "Jens K. Nørskov published in 1999"

Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals

Abstract: A simple formulation of a generalized gradient approximation for the exchange and correlation energy of electrons has been proposed by Perdew, Burke, and Ernzerhof (PBE) [Phys. Rev. Lett. 77, 3865 (1996)]. Subsequently Zhang and Yang [Phys. Rev. Lett. 80, 890 (1998)] have shown that a slight revision of the PBE functional systematically improves the atomization energies for a large database of small molecules. In the present work, we show that the Zhang and Yang functional (revPBE) also improves the chemisorption energetics of atoms and molecules on transition-metal surfaces. Our test systems comprise atomic and molecular adsorption of oxygen, CO, and NO on Ni(100), Ni(111), Rh(100), Pd(100), and Pd(111) surfaces. As the revPBE functional may locally violate the Lieb-Oxford criterion, we further develop an alternative revision of the PBE functional, RPBE, which gives the same improvement of the chemisorption energies as the revPBE functional at the same time as it fulfills the Lieb-Oxford criterion locally.

Surface segregation energies in transition-metal alloys

Abstract: We present a database of $24\ifmmode\times\else\texttimes\fi{}24$ surface segregation energies of single transition metal impurities in transition-metal hosts obtained by a Green's-function linear-muffin-tin-orbitals method in conjunction with the coherent potential and atomic sphere approximations including a multipole correction to the electrostatic potential and energy. We use the database to establish the major factors which govern surface segregation in transition metal alloys. We find that the calculated trends are well described by Friedel's rectangular state density model and that the few but significant deviations from the simple trends are caused by crystal structure effects.

Role of Steps in N 2 Activation on Ru(0001)

Abstract: Using adsorption experiments and density functional calculations we show that ${\mathrm{N}}_{2}$ dissociation on the Ru(0001) surface is totally dominated by steps. The measured adsorption rate at the steps is at least 9 orders of magnitude higher than on the terraces at 500 K, and the corresponding calculated difference in activation energy is 1.5 eV. The low barrier at the step is shown to be due to a combination of electronic and geometrical effects. The consequences for Ru as a catalyst for ammonia synthesis are discussed.

Enhancement of surface self-diffusion of platinum atoms by adsorbed hydrogen

Abstract: Surface diffusion of atoms is an important phenomenon in areas of materials processing such as thin-film growth and sintering. Self-diffusion (that is, diffusion of the atoms of which the surface is comprised) has been much studied on clean metal and semiconductor surfaces1,2. But in most cases of practical interest the diffusion happens on surfaces partly covered by atoms and molecules adsorbed from the gas phase. Adsorbed hydrogen atoms are known to be capable of both promoting and inhibiting self-diffusion3,4,5,6,7, offering the prospect of using adsorbed gases to control growth or sintering processes8,9,10,11. Here we derive mechanistic insights into this effect from observations, using the scanning tunnelling microscope, of hydrogen-promoted self-diffusion of platinum on the Pt(110) surface. We see an activated Pt–H complex which has a diffusivity enhanced by a factor of 500 at room temperature, relative to the other Pt adatoms. Our density-functional calculations indicate that the Pt–H complex consists of a hydrogen atom trapped on top of a platinum atom, and that the bound hydrogen atom decreases the diffusion barrier.

Theoretical analysis of hydrogen chemisorption on Pd(111), Re(0001) and PdML/Re(0001), ReML/Pd(111) pseudomorphic overlayers

Abstract: Gradient-corrected density-functional theory (DFT-GGA) periodic slab calculations have been used to analyze the binding of atomic hydrogen on monometallic Pd(111), Re(0001), and bimetallic ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001)$ [pseudomorphic monolayer of Pd(111) on Re(0001)] and ${\mathrm{Re}}_{\mathrm{ML}}/\mathrm{P}\mathrm{d}(111)$ surfaces. The computed binding energies of atomic hydrogen adsorbed in the fcc hollow site, at 100% surface coverage, on the Pd(111), Re(0001), ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001),$ and ${\mathrm{Re}}_{\mathrm{ML}}/\mathrm{P}\mathrm{d}(111)$ surfaces, are -2.66, -2.82, -2.25, and -2.78 eV, respectively. Formal chemisorption theory was used to correlate the predicted binding energy with the location of the d-band center of the bare metal surfaces, using a model developed by Hammer and N\o{}rskov. The DFT-computed adsorption energies were also analyzed on the basis of the density of states (DOS) at the Fermi level for the clean metal surfaces. The results indicate a clear correlation between the d-band center of the surface metal atoms and the hydrogen chemisorption energy. The further the d-band center is from the Fermi level, the weaker is the chemisorption bond of atomic hydrogen on the surface. Although the DOS at the Fermi level may be related to the location of the d-band, it does not appear to provide an independent parameter for assessing surface reactivity. The weak chemisorption of hydrogen on the ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001)$ surface relates to substantial lowering of the d-band center of Pd, when it is pseudomorphically deposited as a monolayer on a Re substrate.

Nitrogen Adsorption and Hydrogenation on a MoFe6S9 Complex

Abstract: The enzyme nitrogenase catalyzes the biological nitrogen fixation where ${\mathrm{N}}_{2}$ is reduced to ${\mathrm{NH}}_{3}$. Density functional calculations are presented of the bonding and hydrogenation of ${\mathrm{N}}_{2}$ on a ${\mathrm{MoFe}}_{6}{\mathrm{S}}_{9}$ complex constructed to model aspects of the active site of nitrogenase. ${\mathrm{N}}_{2}$ is found to bind end on to one of the Fe atoms. A complete energy diagram for the addition of hydrogen to the ${\mathrm{MoFe}}_{6}{\mathrm{S}}_{9}$ complex with and without ${\mathrm{N}}_{2}$ is given, and a mechanism for ammonia synthesis is proposed on this basis.

Molecular N2 chemisorption—specific adsorption on step defect sites on Pt surfaces

Abstract: Infrared reflection-absorption spectroscopy and density functional theory, within the generalized gradient approximation, were used to investigate both experimentally and theoretically N2 chemisorption on stepped and smooth Pt surfaces. N2 chemisorption was observed to occur only on the edge atoms of step defect sites in atop configuration by both methods. The calculated vibrational frequency of N2 chemisorbed on Pt(112) step sites (2244 cm−1) is in good agreement with the frequency observed experimentally (2231–2234 cm−1) at saturation coverage on Pt(335) and Pt(779). The predicted small N2 binding energy confirmed its weak chemisorption on Pt surfaces claimed in previous studies. The calculations indicate that N2 decreases and CO increases the work function of the Pt(112) surface. N2 could be coadsorbed with CO below saturation coverage of the steps with CO and there is a charge transfer between the two adspecies through the substrate.

Advances in deep desulfurization

Abstract: Hydrotreating and deep hydrodesulfurization (HDS) have attracted increased attention recently due to the introduction of new legislation regarding fuel specifications In order to meet these challenges, there is a need to modify and improve existing reactors and processes and to introduce more active and selective catalysts The removal of the so-called sterically hindered sulfur-containing molecules, like 4,6 dimethyldibenzothiophene, is observed to be a key issue for deep HDS Also the choice of reactor internals plays an important role for deep HDS In order to guide rational catalyst developments, structure-activity relationships are desired and the Co-Mo-S model and the Bond Energy Model have been useful for describing many activity parameters for promoted catalysts and transition metal sulfides It has also recently been shown that important new insight may be gained from density functional theory (DFT) calculations The present article will focus on some of the current practical and theoretical issues

Catalysis from first principles

Abstract: Recent progress in the theoretical description of elementary reactions on transition metal surfaces is discussed. Calculations based on density functional theory and a non-local description of exchange and correlation effects can now be used to predict changes in reactivity from one system to the next. On the basis of the calculations, models can be developed elucidating then “electronic factor” in catalysis.