General trends in CO dissociation on transition metal surfaces

Influence of aggregation, defects, and contaminant oxygen on water dissociation at Cu(110) surface: A theoretical study

The DFT-PW91 slab model approach is employed to investigate the influence of aggregation, surface defects, and contaminant oxygen on water dissociation on Cu(110) at low temperatures. The dissociation barriers of water in various aggregate states are calculated in the range of 60-75 kJ/mol on the clean surfaces, in nice agreement with the experimentally determined values. It is revealed that the aggregation of water shows no propensity to reduce the activation barrier for the O-H bond breaking on Cu(110), at variance with the water chemistry on Ru(0001). The calculated activation energy on Cu(211) which is the most active stepped surface investigated is equal to the value on the (110) surface, indicating that the hydroxyl groups observed on Cu(110) at low temperatures may not stem from surface defects. The coadsorbed oxygen, whether as a "spectator" or a "participant," facilitates the water dissociation both kinetically and thermodynamically.

Dissociative adsorption of carbon monoxide on Mo(110): first-principles theory

The adsorption and dissociation of carbon monoxide on Mo (110) surface is studied with density functional theory. The results at different sites (atop, short bridge, long bridge, and hollow) are presented. The hollow site is found to be the most stable adsorption site for CO. The CO molecule is found to adsorb in end-on configurations (alpha states) at high coverage and inclined configurations (beta states) at low coverage. The dissociation activation energy from beta states is found to be approximately 1 eV lower than from alpha state. The adsorption of dissociation products, C and O, on Mo(110) has also been studied. The most stable adsorption site for C and O is long bridge and hollow site, respectively. The adsorption of C and O at low coverage is, in general, stronger than at high coverage, which is partly responsible for the high reactivity of CO dissociation at low coverage, since the binding energy of CO is not very sensitive to the coverage.

The importance of hydrogen's potential-energy surface and the strength of the forming R-H bond in surface hydrogenation reactions

An understanding of surface hydrogenation reactivity is a prevailing issue in chemistry and vital to the rational design of future catalysts. In this density-functional theory study, we address hydrogenation reactivity by examining the reaction pathways for N+H-->NH and NH+H-->NH(2) over the close-packed surfaces of the 4d transition metals from Zr-Pd. It is found that the minimum-energy reaction pathway is dictated by the ease with which H can relocate between hollow-site and top-site adsorption geometries. A transition state where H is close to a top site reduces the instability associated with bond sharing of metal atoms by H and N (NH) (bonding competition). However, if the energy difference between hollow-site and top-site adsorption energies (DeltaE(H)) is large this type of transition state is unfavorable. Thus we have determined that hydrogenation reactivity is primarily controlled by the potential-energy surface of H on the metal, which is approximated by DeltaE(H), and that the strength of N (NH) chemisorption energy is of less importance. DeltaE(H) has also enabled us to make predictions regarding the structure sensitivity of these reactions. Furthermore, we have found that the degree of bonding competition at the transition state is responsible for the trend in reaction barriers (E(a)) across the transition series. When this effect is quantified a very good linear correlation is found with E(a). In addition, we find that when considering a particular type of reaction pathway, a good linear correlation is found between the destabilizing effects of bonding competition at the transition state and the strength of the forming N-H (HN-H) bond.

Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode

We present results of density functional theory calculations on a Pt(111) slab with a bilayer of water, solvated protons in the water layer, and excess electrons in the metal surface. In this way we model the electrochemical double layer at a platinum electrode. By varying the number of protons/electrons in the double layer we investigate the system as a function of the electrode potential. We study the elementary processes involved in the hydrogen evolution reaction, 2(H(+) + e(-)) --> H(2), and determine the activation energy and predominant reaction mechanism as a function of electrode potential. We confirm by explicit calculations the notion that the variation of the activation barrier with potential can be viewed as a manifestation of the Brønsted-Evans-Polanyi-type relationship between activation energy and reaction energy found throughout surface chemistry.

Trends in C–O and C–N bond formations over transition metal surfaces: An insight into kinetic sensitivity in catalytic reactions

Transition metal catalyzed bond formation is a fundamental process in catalysis and is of general interest throughout chemistry. To date, however, the knowledge of association reactions is rather limited, relative to what is known about dissociative processes. For example, surprisingly little is known about how the bond-forming ability of a metal, in general, varies across the Periodic Table. In particular, the effect of reactant valency on such trends is poorly understood. Herein, the authors examine these key issues by using density functional theory calculations to study CO and CN formations over the 4d metals. The calculations reveal that the chemistries differ in a fundamental way. In the case of CO formation, the reaction enthalpies span a much greater range than those of CN formation. Moreover, CO formation is found to be kinetically sensitive to the metal; here the reaction barriers (E(a)) are found to be influenced by the reaction enthalpy. CN formation, conversely, is found to be relatively kinetically insensitive to the metal, and there is no correlation found between the reaction barriers and the reaction enthalpy. Analysis has shown that at the final adsorbed state, the interaction between N and the surface is relatively greater than that of O. Furthermore, in comparison with O, relatively less bonding between the surface and N is observed to be lost during transition state formation. These greater interactions between N and the surface, which can be related to the larger valency of N, are found to be responsible for the relatively smaller enthalpy range and limited variation in E(a) for CN formation.

Slab model studies of water adsorption and decomposition on clean and X- (X = C, N and O) contaminated Pd(111) surfaces

To explore the effect of surface contaminants on water chemistry at metallic surfaces, adsorption and decomposition of water monomers on clean and X/Pd(111)(X = C, N and O) surfaces are investigated based on density functional theory calculations. It is revealed that H(2)O binds to Pd(111) surface primarily through the mixing of its 1b(1) with the Pd 4d(z(2)) state. A charge accumulation between the oxygen atom of water and the bound Pd atom is calculated, which is found to be relevant to the H(2)O-Pd interaction. Water adsorption results in a reduction of surface work function and the polarization of the X 2p states. The O-H bond scission of H(2)O on the clean Pd(111) is an energy unfavorable process. In the case of X-assisted O-H bond breaking on X/Pd(111) surfaces, however, the reaction barrier tends to be lower than that on the clean surface and decreases from C/Pd(111) to O/Pd(111). In particular, water decomposition is found to become feasible on O/Pd(111), in agreement with the experimental observations. The calculated barrier is demonstrated to be correlated linearly with the density of X 2p states at the Fermi level. A thorough energy analysis demonstrates that the following geometrical and electronic factors favor the barrier reduction on X/Pd(111) with respect to water decomposition on clean Pd(111): (i) the less deformed structure of water in TS; (ii) the decreased bonding competition between the fragments OH and H. The remarkable decrease of the barrier on O/Pd(111) is revealed to be due to the largest stabilization of the split H atom and the least deformation of water in the TS.

Adsorption and Activation of CO over Flat and Stepped Co Surfaces: A First Principles Analysis

The adsorption and activation of CO over flat Co{0001}, corrugated Co{1120}, and stepped Co{1012} and Co{1124} surfaces have been analyzed using periodic density functional theory calculations. CO strongly chemisorbs on all these surfaces but does not show a strong dependence on the surface structure. The calculated structure of adsorbed CO on Co{0001} at 1/3 monolayer (ML) of coverage was found to be in good agreement with the experiment. The barrier for CO dissociation over Co{0001} was found to decrease with decreasing CO coverage, taking on a value of 232 kJ/mol at 1/4 ML and 218 kJ/mol at 1/9 ML. The presence of the "zigzag" channel on Co{1120} enhances the reactivity slightly by reducing the barrier for CO dissociation to 195 kJ/mol. In contrast, the stepped Co{1012} and Co{1124} surfaces are much more active than the flat and corrugated surfaces. Both stepped surfaces provide direct channels for CO dissociation that do not have barriers with respect to gas-phase CO. In general the activation barriers lower as the reaction energies become more exothermic. Reconstruction of the step edges that occur in the product state, however, prevents a linear correlation between the reaction energy and the activation energy.

Catalytic Oxidation Activity of Pt3O4 Surfaces and Thin Films

The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of platinum oxide surfaces and thin films and their reactivities toward oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive toward either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favored at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than that on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.

C–O bond scission of methoxide on Pd nanoparticles: A density functional study

C-O bond scission of methoxide species adsorbed at the surface of Pd nanoparticle was studied by DF calculations for the example of cuboctahedral Pd(79). To investigate different locations of adsorbed intermediates as well as the transition state of C-O bond scission, a substrate model was used, which allows one to consider adsorbates without any local geometry restrictions. In contrast to reaction sites on the flat Pd(111) surface and on extended facets, scission of the C-O bond of methoxide at cluster edges is exothermic by approximately 40 kJ mol(-1) and the decomposition product CH(3) is found to be stabilized there. However, the high calculated activation barrier, approximately 140 mol(-1), implies only a very slow reaction compared to dehydrogenation of CH(3)O.

Inverse gas chromatographic investigation of the effect of hydrogen in carbon monoxide adsorption over silica supported Rh and Pt–Rh alloy catalysts, under hydrogen-rich conditions

Selective CO oxidation (SCO) has attracted scientific and technological interest due to its application to the operation of proton electrolyte membrane fuel cells (PEM-FCs). CO adsorption, being an elementary step of SCO, is studied over silica supported monometallic Rh and Rh0.50 + Pt0.50 alloy catalysts, under various hydrogen atmospheres, namely: 25% H2 + 75% He, 50% H2 + 50% He and 75% H2 + 25% He carrier gas mixture compositions. The investigation of CO adsorption is done by utilizing reversed-flow gas chromatography (RF-GC). As a result rate constants for the adsorption (k1), desorption (k(-1)) and irreversible CO binding (k2) over the studied catalysts as well as the respective activation energies are determined. The variation of the rate constants and the activation energies against the nature of the used catalyst (monometalic or alloy) and the amount of hydrogen in the carrier gas gives useful information for the selectivity as well as the activity of CO oxidation over group VIII noble metals. At low temperatures and under H2-rich conditions compatible with the operation of PEM fuel cells the activity of the monometallic and the alloy catalysts is expected to be similar, however the selectivity of Rh0.50 + Pt0.50 alloy catalyst is expected to be higher, making Pt-Rh alloy catalyst as a better candidate for CO preferential oxidation (PROX). The low energy barrier values found in the present work, most likely are referred to high surface amounts of CO. The desorption barriers determined are in any case much lower than the respective activation energies found for CO desorption in the absence of hydrogen indicating a H2-induced desorption, which can explain the observed in the literature rate enhancement of SCO oxidation.

Structure Dependence of NO Adsorption and Dissociation on Platinum Surfaces

The influence of surface structure on NO chemisorption and dissociation on Pt[100]-(1x1), Pt[211], and Pt[410] has been studied using density functional theory slab calculations with the generalized gradient corrections. The presence of steps on Pt[211] strengthens the NO-surface chemisorption bond, but the barrier for NO dissociation remains high. On the other hand, the steps on Pt[410] help to stabilize the N and O adatoms that form upon dissociation and the transition state. The calculated barrier of 80.2 kJ/mol on Pt[410] is in good agreement with experiment. These results show that both the presence of steps and the nature of the steps are important to activate NO. An ensemble of square-arranged Pt atoms has been identified as an important feature in activating the N-O bond.

Identification of general linear relationships between activation energies and enthalpy changes for dissociation reactions at surfaces

The activation energy to reaction is a key quantity that controls catalytic activity. Having used ab inito calculations to determine an extensive and broad ranging set of activation energies and enthalpy changes for surface-catalyzed reactions, we show that linear relationships exist between dissociation activation energies and enthalpy changes. Known in the literature as empirical Brønsted-Evans-Polanyi (BEP) relationships, we identify and discuss the physical origin of their presence in heterogeneous catalysis. The key implication is that merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.

General rules for predicting where a catalytic reaction should occur on metal surfaces: a density functional theory study of C-H and C-O bond breaking/making on flat, stepped, and kinked metal surfaces

To predict where a catalytic reaction should occur is a fundamental issue scientifically. Technologically, it is also important because it can facilitate the catalyst's design. However, to date, the understanding of this issue is rather limited. In this work, two types of reactions, CH(4) <--> CH(3) + H and CO <--> C + O on two transition metal surfaces, were chosen as model systems aiming to address in general where a catalytic reaction should occur. The dissociations of CH(4) --> CH(3) + H and CO --> C + O and their reverse reactions on flat, stepped, and kinked Rh and Pd surfaces were studied in detail. We find the following: First, for the CH(4) <--> Ch(3) + H reaction, the dissociation barrier is reduced by approximately 0.3 eV on steps and kinks as compared to that on flat surfaces. On the other hand, there is essentially no difference in barrier for the association reaction of CH(3) + H on the flat surfaces and the defects. Second, for the CO <--> C + O reaction, the dissociation barrier decreases dramatically (more than 0.8 eV on Rh and Pd) on steps and kinks as compared to that on flat surfaces. In contrast to the CH(3) + H reaction, the C + O association reaction also preferentially occurs on steps and kinks. We also present a detailed analysis of the reaction barriers in which each barrier is decomposed quantitatively into a local electronic effect and a geometrical effect. Our DFT calculations show that surface defects such as steps and kinks can largely facilitate bond breaking, while whether the surface defects could promote bond formation depends on the individual reaction as well as the particular metal. The physical origin of these trends is identified and discussed. On the basis of our results, we arrive at some simple rules with respect to where a reaction should occur: (i) defects such as steps are always favored for dissociation reactions as compared to flat surfaces; and (ii) the reaction site of the association reactions is largely related to the magnitude of the bonding competition effect, which is determined by the reactant and metal valency. Reactions with high valency reactants are more likely to occur on defects (more structure-sensitive), as compared to reactions with low valency reactants. Moreover, the reactions on late transition metals are more likely to proceed on defects than those on the early transition metals.

Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold

Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO(2) has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O(2) dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O(2) dissociation on Au (O(2)-->2O(ad)) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O(2) on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O(2) can adsorb on the interface of Au/TiO(2) with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O(2) dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O(2)-->CO(2)+O, and CO+O-->CO(2); and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.

A New Insight into Fischer−Tropsch Synthesis

Fischer-Tropsch (FT) reaction is an important synthetic route to convert CO and H(2) to fuels and chemicals in industry. To date, its reaction mechanism remains uncertain. With extensive density functional theory studies on FT reactions on Ru, we compare quantitatively several C/C coupling mechanisms that are likely to be involved. We found that a well-regarded CH(2) + CH(2)R (R = H or alkyl) mechanism possesses high reaction barriers, and a stepwise C + CR mechanism has been identified that may be relevant to FT synthesis.

A density functional theory study on the active center of Fe-only hydrogenase: characterization and electronic structure of the redox states

We have carried out extensive density functional theory (DFT) calculations for possible redox states of the active center in Fe-only hydrogenases. The active center is modeled by [(H(CH(3))S)(CO)(CN(-))Fe(p)(mu-DTN)(mu-CO)Fe(d)(CO)(CN(-))(L)](z)() (z is the net charge in the complex; Fe(p)= the proximal Fe, Fe(d) = the distal Fe, DTN = (-SCH(2)NHCH(2)S-), L is the ligand that bonds with the Fe(d) at the trans position to the bridging CO). Structures of possible redox states are optimized, and CO stretching frequencies are calculated. By a detailed comparison of all the calculated structures and the vibrational frequencies with the available experimental data, we find that (i) the fully oxidized, inactive state is an Fe(II)-Fe(II) state with a hydroxyl (OH(-)) group bonded at the Fe(d), (ii) the oxidized, active state is an Fe(II)-Fe(I) complex which is consistent with the assignment of Cao and Hall (J. Am. Chem. Soc. 2001, 123, 3734), and (iii) the fully reduced state is a mixture with the major component being a protonated Fe(I)-Fe(I) complex and the other component being its self-arranged form, Fe(II)-Fe(II) hydride. Our calculations also show that the exogenous CO can strongly bond with the Fe(II)-Fe(I) species, but cannot bond with the Fe(I)-Fe(I) complex. This result is consistent with experiments that CO tends to inhibit the oxidized, active state, but not the fully reduced state. The electronic structures of all the redox states have been analyzed. It is found that a frontier orbital which is a mixing state between the e(g) of Fe and the 2 pi of the bridging CO plays a key role concerning the reactivity of Fe-only hydrogenases: (i) it is unoccupied in the fully oxidized, inactive state, half-occupied in the oxidized, active state, and fully occupied in the fully reduced state; (ii) the e(g)-2 pi orbital is a bonding state, and this is the key reason for stability of the low oxidation states, such as Fe(I)-Fe(I) complexes; and (iii) in the e(g)-2 pi orbital more charge accumulates between the bridging CO and the Fe(d) than between the bridging CO and the Fe(p), and the occupation increase in this orbital will enhance the bonding between the bridging CO and the Fe(d), leading to the bridging-CO shift toward the Fe(d).

An insight into alkali promotion: a density functional theory study of CO dissociation on K/Rh(111)

The important role of alkali additives in heterogeneous catalysis is, to a large extent, related to the high promotion effect they have on many fundamental reactions. The wide application of alkali additives in industry does not, however, reflect a thorough understanding of the mechanism of their promotional abilities. To investigate the physical origin of the alkali promotion effect, we have studied CO dissociation on clean Rh(111) and K-covered Rh(111) surfaces using density functional theory. By varying the position of potassium atoms relative to a dissociating CO, we have mapped out the importance of different K effects on the CO dissociation reactions. The K-induced changes in the reaction pathways and reaction barriers have been determined; in particular, a large reduction of the CO dissociation barrier has been identified. A thorough analysis of this promotion effect allows us to rationalize both the electronic and the geometrical factors that govern alkali promotion effect: (i) The extent of barrier reductions depends strongly on how close K is to the dissociating CO. (ii) Direct K-O bonding that is in a very short range plays a crucial role in reducing the barrier. (iii) K can have a rather long-range effect on the TS structure, which could reduce slightly the barriers.