Decrease In Adsorption Energy Research Articles

Interaction of oxygen with the Pt(1 1 1) surface in wide conditions range. A DFT-based thermodynamical simulation

We present the results of ab initio calculations of oxygen atomic adsorption in a wide range of coverage on Pt(1 1 1). At θ = 0.25 ML, the O adsorption at fcc hollow site is clearly favoured over the hcp site. At θ = 0.5 ML, the O adsorption energy decreases but the same site is favoured. When experimental or theoretical previously reported data are available, the calculated adsorption energies and site preferences are in good agreement. Among the various configurations and coverages investigated in the present work, no adsorption is stable beyond θ = 0.5 ML, except by occupation of a subsurface tetrahedral site. In that case, a total O coverage of 0.75 ML could be achieved, which is only slightly less stable than the θ = 0.5 ML configuration. The use of thermodynamics permitted to explore the temperature–pressure stability domain corresponding to 0.25 ML, 0.5 ML and 0.75 ML. From this, we conclude that subsurface O species could be stable at temperatures lower than 700 K, with O 2 pressures of 1 bar or less.

Reactivity of the Oxygen Sites in the V2O5/TiO2 Anatase Catalyst

We present for the first time a theoretical investigation on the reactivity of the different oxygen sites in a dispersed V2O5/TiO2 slab model. Our model represents anhydrous conditions and contains all the potentially active oxygen sites proposed to date in the literature: vanadyl VO, bridging V−O−V, interface V−O−Ti, and surface Ti−O−Ti oxygen sites. Geometric features are analyzed in terms of bond distances and site accessibility. Projected density of states diagrams show a decrease of the band gap for the supported system with respect to pure anatase (001). The most energetic levels of the valence band correspond to the oxygen atoms in the V2O5 unit, although a further decomposition in terms of mono- and dicoordinated oxygen is not obvious. Atomic hydrogen adsorption on the different sites is tracked. The most exothermic systems are those where H adsorbs on the V2O5 oxygen sites due to their higher basicity regarding pure anatase. The heats of adsorption referred to atomic H are V−O−Ti (2.95 eV) > VO (2.60 eV) > V−O−V (2.17 eV) > Ti−O−Ti (2.07 eV). This means that the most reactive sites are those located at the interface between the V2O5 and the TiO2 unit, while the vanadyl VO bonds are more stable and would react at a lower extent in hydrogen atomic adsorption. Relaxation makes the adsorption energy decrease: the support relaxes less when hydrogen adsorbs than for the bare V2O5/TiO2 system. H adsorption forms hydroxyl OH bonds by reduction of the vanadium atom, even when the adsorption site is located on the TiO2 surface sites.

Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals.

The modification of the electronic and chemical properties of Pt(111) surfaces by subsurface 3d transition metals was studied using density-functional theory. In each case investigated, the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals, resulting in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces. The magnitude of the decrease in adsorption energy was largest for the early 3d transition metals and smallest for the late 3d transition metals. In some cases, dissociative adsorption was calculated to be endothermic. The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties. The implications of these findings are discussed in the context of catalyst design, particularly for fuel cell electrocatalysts.

Local adsorption sites and bondlength changes in Ni [formula omitted]/H/CO and Ni [formula omitted]/CO

The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni–C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 ± 0.04 Å in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 ± 0.04 Å. These changes in the Ni–CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni–C bondlength by only 0.06 ± 0.04 Å, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding.

Energy barriers and chemical properties in the coadsorption of carbon monoxide and oxygen on Ru(0001)

Using density-functional theory we investigate the interactions and chemical properties of the coadsorption of carbon monoxide and oxygen on ruthenium (0001 ). For the adsorption phases that occur in nature, where CO occupies the top site, we find that with increasing oxygen coverage, the adsorption energy of CO can remain practically unchanged or even exhibit a slight increase. We attribute the increase to an O-induced lateral weakening of Ru-Ru bonds of non-O-bonded surface Ru atoms. Thus, these non-O-bonded Ru atoms can form stronger bonds to an on-top CO adsorbate. In contrast, a more expected behavior of a notable decrease in CO adsorption energy with increasing O coverage is observed only if the O atoms bond to the same Ru atoms as CO as, for example, is the case when CO occupies hollow sites, Furthermore, for some of the structures, we find that there is a manifestation of small activation energy barriers for CO adsorption well above the surface.

Changes of surface, fine pore and variable charge properties of a brown forest soil under various tillage practices

Effects of 6 years no-tillage (NT), ploughing, disking and the two last treatments combined with loosening on surface area, water vapor adsorption energy, variable charge and fine pore properties of a brown forest soils were studied using water vapor adsorption–desorption, back-titration and mercury intrusion measurements. The studied soil properties altered markedly under mechanical tillage treatment as compared to NT soil. The radii and the volumes of cryptopores (sizes from 1 to a few tens of nanometers) decreased and the opposite was found for ultramicropores (sizes from a few tens of nanometers to around 10 μm). However, fractal dimension of cryptopores and ultramicropores had changed very slightly, indicating that general geometrical structure of the fine pore system in the studied range (ca 1 nm–10 μm) remained unaltered despite pore size-shift. Surface areas and the amount of variable surface charge were markedly lower in mechanically tilled soil. A decrease of organic matter content was observed as well. Decrease of water vapor adsorption energy and increase of the fraction of strongly acidic surface functional groups accompanied mechanical tillage treatments.

Effect of extreme acid and alkali treatment on surface properties of soils

Effects of natural or anthropogenic soil acidification and alkalization on chemical or biological properties have been studied extensively while little is known about changes in physicochemical characteristics, such as surface area or adsorption energy. To investigate this, samples of six Polish soils (Inceptisols, Mollisols) and three Korean soils (Alfisols, Ultisols) of different origin and mineral composition were acidified and alkalized with elevated concentrations of hydrochloric acid and sodium hydroxide ranging from 0.001 to I mol dm -3 . Surface properties of these soils were studied. The surface area and average adsorption energy decreased in general in both treatments. The treatments induced the decrease in amount of high and medium energy centers, however the fraction of low energy centers increased. The behavior of surface properties differed from the above at treatments at highest reagent concentrations, especially for Korean soils, rich in clay and iron oxides. The general pattern of the adsorption energy decrease observed in most of the soils indicates that the overall water binding forces become lower after treatments. In this case the soil water may be more available for plants, despite its amount decreases.

Hydrogen, sulphur and chlorine coadsorption on Pd(111): a theoretical study of poisoning and promotion

First principles calculations for the coadsorption of hydrogen with sulphur and chlorine on Pd(111) are presented. Individually, both sulphur and chlorine poison hydrogen adsorption, sulphur being the more efficient poison. The observed sulphur poisoning is a structural effect. The adsorption energy decreases and the diffusion barrier increases for hydrogen atoms in the vicinity of sulphur adatoms. A sulphur coverage of 0.33 ML is sufficient to completely poison hydrogen adsorption on Pd(111). The presence of chlorine adatoms on the sulphur-poisoned surface is found to stabilise localised hydrogen adsorption. The possible promotional effects of chlorine on sulphur-poisoned catalysts are discussed.

Oxygen adsorption on the Ru(101¯0) surface: Anomalous coverage dependence

Oxygen adsorption onto Ru (10 bar 1 0) results in the formation of two ordered overlayers, i.e. a c(2 times 4)-2O and a (2 times 1)pg-2O phase, which were analyzed by low-energy electron diffraction (LEED) and density functional theory (DFT) calculation. In addition, the vibrational properties of these overlayers were studied by high-resolution electron loss spectroscopy. In both phases, oxygen occupies the threefold coordinated hcp site along the densely packed rows on an otherwise unreconstructed surface, i.e. the O atoms are attached to two atoms in the first Ru layer Ru(1) and to one Ru atom in the second layer Ru(2), forming zigzag chains along the troughs. While in the low-coverage c(2 times 4)-O phase, the bond lengths of O to Ru(1) and Ru(2) are 2.08 A and 2.03 A, respectively, corresponding bond lengths in the high-coverage (2 times 1)-2O phase are 2.01 A and 2.04 A (LEED). Although the adsorption energy decreases by 220 meV with O coverage (DFT calculations), we observe experimentally a shortening of the Ru(1)-O bond length with O coverage. This effect could not be reconciled with the present DFT-GGA calculations. The nu(Ru-O) stretch mode is found at 67 meV [c(2 times 4)-2O] and 64 meV [(2 times 1)pg-2O].

Chemisorption of ethylene, propylene and isobutylene on ordered Sn/Pt(111) surface alloys

The adsorption and decomposition of a series of alkenes – ethylene, propylene and isobutylene – on Pt(111) and the (2×2) and ( 3 × 3 ) R30° Sn/Pt(111) surface alloys was investigated with temperature programmed desorption (TPD), low energy electron diffraction (LEED) and sticking coefficient measurements. In the case of ethylene, this study extends a previous investigation [M.T. Paffett, S.C. Gebhard, R.G. Windham and B.E. Koel. Surf. Sci. 223 (1989) 449]. We report for the first time a (2 3 ×2 3 ) R30° ordered structure for ethylene chemisorbed on the (2×2)Sn/Pt(111) surface alloy. In general for these small alkenes, sticking coefficient and TPD measurements show little or no effect of alloyed Sn on either the initial sticking coefficient or the saturation coverage of these molecules on the two Sn/Pt(111) ordered surface alloys when compared with the clean Pt(111) surface at 100 K. As was reported previously for ethylene, these Pt–Sn surface alloys have weaker propylene and isobutylene chemisorption bonding and strongly suppressed decomposition. Considering propylene and isobutylene as methyl-substituted ethylene reveals a small trend toward decreasing adsorption energy and on all of these surfaces as the amount of methyl substitution increases. The propylene adsorption energy decreases from 17.4 to 14.8 and then to 11.7 kcal mol −1 as the substrate is changed from Pt(111) to the (2×2) and the ( 3 × 3 ) R30° alloys, as estimated by TPD peak temperatures. The isobutylene adsorption energy decreases from 17.1 to 14.7 and then to 10.8 kcal mol −1 for the same series. This implicates that the “Pt-only” three-fold hollow sites are very important for strong alkene chemisorption, since removal of these sites on the ( 3 × 3 ) R30° alloy causes a sharper decrease in the adsorption energy than expected based upon the changes observed for the (2×2) alloy. Even though propylene and isobutylene contain allylic C–H bonds that are much weaker than the vinylic C–H bonds in ethylene, only ca. 5–7% as much propylene and isobutylene decomposes on the (2×2) alloy compared to the Pt(111) surface and no decomposition occurs on the ( 3 × 3 ) R30° alloy. This result shows the importance of adjacent “Pt-only” three-fold hollow sites in alkene decomposition.

Trends in the bonding of CO to the surfaces of Pt3M alloys (M=Ti, Co, and Sn)

Studies of the surface composition and CO chemisorption on the [111] and [100] oriented single crystals of the isostructural [face-centered-cubic (fcc)] alloys of bulk stoichiometry Pt3M (M=Ti, Co, and Sn) are reviewed. In principle, the study of these alloys is ideal for separating ensemble effects from electronic effects in adsorbate bonding, since the surfaces of these ordered alloys formed by bulk truncation have the M atoms in fixed positions without any M–M pair sites. Also by varying M from Ti to Co to Sn one can examine the effect of the d and s–p orbital contributions to the intermetallic bond on the binding energy of CO. In practice, one must account for surface segregation, and the formation of surfaces that do not have the ideal structure and composition. The strength of the intermetallic bond is found to play a major role in determining the composition of the surface. In the very exothermic alloys Pt3Ti and Pt3Sn, the thermodynamic tendency for the atom with the lower surface energy to be at the surface determines which of the two bulk planes forms the surface plane in the [100] orientation, either pure Pt(Pt3Ti) or 50% M(Pt3Sn). In CoPt3 where the intermetallic bonding is relatively weak, the lower surface energy of Pt produces pure Pt planes on both the [111] and [100] orientations. Photoemission and thermal desorption studies show that the intermetallic bonding in all three alloys has only a small effect on the binding energy of CO, a decrease in adsorption energy of 20 kJ/mol. The surprising invariance of adsorption energy on Pt3M surfaces having such a large variation in the d-orbital configuration of the M atom is attributed to the predominance of s–p orbital contributions to the intermetallic bond in these alloys.

Surface area and heat of adsorption measurements of a microporous carbon

Surface areas, micropore volumes, and isosteric heats of adsorption are reported for a microporous carbon as a function of conversion. The isosteric heats of adsorption were calculated from the temperature dependence of the equilibrium adsorption pressure at temperatures between 112 and 184 K, and are reported for a surface coverage (relative to the BET surface area) up to θ = 0.33 The isosteric heats show a decrease with coverage that is characteristic of an energetically heterogeneous surface. This decrease in adsorption energy with coverage was found to be more marked for the higher conversion chars. The Henry's Law regime isosteric heat, however, showed no significant variation with respect to carbon conversion or the presence of carbon-oxygen surface complexes. The average value for the isosteric heat in the Henry's Law regime is 15.3 kj/mole. Both the heat of adsorption and the surface area data are consistent with a structural model for microporous carbons in which the ultramicropores are formed by slit-like pores of two adjacent lamellae. These carbons have a distribution of pore widths, but the smallest accessible pore is approximately 6.2Å wide.

The effects of potassium on ammonia synthesis over iron single-crystal surfaces

The effects of potassium on ammonia synthesis over model iron single-crystal catalysts of (111), (100), and (110) orientation have been studied under high-pressure reaction conditions (20 atm reactant pressure of nitrogen and hydrogen). Under these conditions, no more than 0.15 monolayers (ML) of potassium can be stabilized on the iron surfaces. The Fe(110) surface shows no activity for ammonia synthesis in this study with or without adsorbed potassium. The presence of potassium on the Fe(111) and Fe(100) surfaces increases the rate of ammonia synthesis markedly. At a low reaction conversion of 0.3% the rate over Fe(111) and Fe(100) is enhanced by a factor of two in the presence of potassium. The effect of potassium on Fe(111) and Fe(100) is enhanced as higher reaction conversions (i.e., increasing ammonia partial pressures) are achieved because potassium induces changes in the reaction orders for both ammonia and hydrogen. No change in the activation energy for the reaction is observed with potassium, suggesting that the reaction mechanism has not been altered. Temperature-programmed desorption shows that the adsorption energy of ammonia is significantly reduced when coadsorbed with potassium. A model is proposed in which the decrease of ammonia adsorption energy, induced by potassium, reduces the concentration of ammonia on the iron surface. This effect decreases the number of active sites blocked by the ammonia product, thereby increasing the rate of ammonia synthesis. The model also suggests that an additional effect of potassium is to increase the rate of nitrogen dissociative chemisorption by about 30% over Fe(111) and Fe(100) under ammonia synthesis conditions.

Structure and electronic factors in benzene coordination to Cr(CO)3 and to cluster models of Ni, Pt, and Ag (111) surfaces

Abstract Bonding of benzene to a chromium tricarbonyl fragment and to cluster models of silver, nickel, and platinum (111) surfaces is found by means of molecular orbital calculations to be dominated by a benzene donation bond involving its π (e 1g ) orbitals and metal d orbitals. Three-fold hollow sites on the clusters are calculated to be most stable for benzene coordination, a conclusion reached in a number of experimental studies of benzene on metal surfaces. On the Ag, Ni, and pt clusters, the benzene CH bonds are found to bend away from the surface by −2°, 8° and 19°, respectively, a result of carbon atom hybridization to maximize overlap with metal orbitals. For benzene chromium tricarbonyl, the CH bonds are calculated to bend 3° toward the metal, compared to a 1.7° bend reported in a diffraction study. The direction and magnitude of the CH bending are shown to depend on the metal d orbital occupancy (an electronic factor) and the proximity of metal atoms in the adsorption site (a structure factor). Small Kekule distortions are calculated for the chromium complex and for the C 3v sites on Pt(111). Finally, recent experimental studies showing a decrease in benzene adsorption energy when potassium is coadsorbed on Pt(111) may be understood to result from decreased π donation which accompanies the shift up of the metal d band with cathodic charging.

Structure and electronic factors in benzene coordination to Cr(CO) 3 and to cluster models of Ni, Pt, and Ag (111) surfaces

Bonding of benzene to a chromium tricarbonyl fragment and to cluster models of silver, nickel, and platinum (111) surfaces is found by means of molecular orbital calculations to be dominated by a benzene donation bond involving its π (e 1g) orbitals and metal d orbitals. Three-fold hollow sites on the clusters are calculated to be most stable for benzene coordination, a conclusion reached in a number of experimental studies of benzene on metal surfaces. On the Ag, Ni, and pt clusters, the benzene CH bonds are found to bend away from the surface by −2°, 8° and 19°, respectively, a result of carbon atom hybridization to maximize overlap with metal orbitals. For benzene chromium tricarbonyl, the CH bonds are calculated to bend 3° toward the metal, compared to a 1.7° bend reported in a diffraction study. The direction and magnitude of the CH bending are shown to depend on the metal d orbital occupancy (an electronic factor) and the proximity of metal atoms in the adsorption site (a structure factor). Small Kekulé distortions are calculated for the chromium complex and for the C 3v sites on Pt(111). Finally, recent experimental studies showing a decrease in benzene adsorption energy when potassium is coadsorbed on Pt(111) may be understood to result from decreased π donation which accompanies the shift up of the metal d band with cathodic charging.

Adsorption of polypeptides on solid surfaces. I. Effect of chain stiffness

AbstractThe general method of obtaining the partition function and thermodynamic characteristics of polymer chains near an adsorbing surface, simulated by random walks on a lattice, is developed. The method takes into account the effect of short‐range interactions in polymer chains, in particular, the chain stiffness and secondary structure. The theory of adsorption of chains of different stiffness is developed, and the process of adsorption which occurs when the external conditions change is shown to be always a second‐order phase transition. The critical adsorption energy decreases and the sharpness of transition grows when the chain stiffness increases. A simple model of a chain with “virtual” steps is proposed which simplifies the treatment; the results obtained are in good agreement with exact theories. A general scheme of analysis of adsorption of chains with a given secondary structure is set forth and the analogy between the stiffness of a noncooperative chain and the presence of helical segments in a polypeptide chain is discussed.