CO on Pt(111): A puzzle revisited

Capitalistic Chemistry

The concept of "bond strength" is of essence for modeling every kind of reactive chemistry. Particularly within the field of catalysis and surface science, the interaction strength of adsorbates to surfaces affects the activity, selectivity, and stability of intermediates and transition states. Here, we introduce a simple approach to chemical reactions through an analogy with business. We regard rehybridization as the investment a molecule makes to prepare its electronic and geometrical structure to form new bonds. The resulting bond strength is the total proceeds from bond formation, and the difference (exothermicity) is the profit. The predictive power lies in the fact that any change in the electronic structure to prepare for bond formation requires the involvement of specific excited states. Thus, with knowledge of the energy needed for this excitation (investment) and the strength of the resulting interaction one can predict whether a specific reaction or bonding mode will be favored. We apply this concept to rationalize observed binding modes at surfaces and the often observed large structural changes even for "weakly" chemisorbed systems and finally to justify using small metal clusters to correct chemisorption energies from periodic DFT calculations.

Correlating Optical and Structural Properties of CO on Transition Metal Surfaces

We present an optical study based on the quasiparticle self-consistent GW (QS GW ) approximation combining structural information taken from density functional theory (DFT) to elucidate spectral features of CO adsorbed on Pt(111) and Cu(111). Optical information and structural arrangement of the adsorbed CO are correlated by varying both site positions and CO coverage as compared to experimental studies (θ = 1/4 to θ = 1/2). This enables us to resolve key spectral features of both occupied and unoccupied molecular states at various adsorbate coverages, comparing theory to experiment. Using experimental data as benchmarks, we show the theory compares well with available data. Its predictive power provides a new path to infer information about the structure of CO from optical information and can help to predict the presence of other little understood adsorbates such as an OCCO dimer that may be relevant to mechanistic pathways for reduction of CO2 to high value C2 + products. This new approach complements total energy calculations and also fills a void in DFT-based theory that is known to be an unreliable predictor of the energetics of CO on transition metal surfaces.

Sublimation Transformation Synthesis of Dual-Atom Fe Catalysts for Efficient Oxygen Reduction Reaction

Dual-atom catalysts (DACs) have garnered significant interest due to their remarkable catalytic reactivity. However, achieving atomically precise control in the fabrication of DACs remains a major challenge. Herein, we developed a straightforward and direct sublimation transformation synthesis strategy for dual-atom Fe catalysts (Fe2/NC) by utilizing in situ generated Fe2Cl6(g) dimers from FeCl3(s). The structure of Fe2/NC was investigated by aberration-corrected transmission electron microscopy and X-ray absorption fine structure (XAFS) spectroscopy. As-obtained Fe2/NC, with a Fe-Fe distance of 0.3 nm inherited from Fe2Cl6, displayed superior oxygen reduction performance with a half-wave potential of 0.90 V (vs. RHE), surpassing commercial Pt/C catalysts, Fe single-atom catalyst (Fe1/NC), and its counterpart with a common and shorter Fe-Fe distance of ~0.25 nm (Fe2/NC-S). Density functional theory (DFT) calculations and microkinetic analysis revealed the extended Fe-Fe distance in Fe2/NC is crucial for the O2 adsorption on catalytic sites and facilitating the subsequent protonation process, thereby boosting catalytic performance. This work not only introduces a new approach for fabricating atomically precise DACs, but also offers a deeper understanding of the intermetallic distance effect on dual-site catalysis.

Reactivity of Ts and At oxides and oxyhydrides with a gold surface from periodic DFT calculations

Adsorption energies, Eads, of oxides and oxyhydrides of the superheavy element (SHEs) Ts and of its lighter homologue At on the gold surface are predicted on the basis of relativistic periodic density functional theory calculations via AMS BAND software. The following compounds were considered: MO, MO2, MOO, and MO(OH) (where M = At and Ts). The aim of this study is to support "one-atom-at-a-time" gas-phase chromatography experiments on reactivity/volatility of SHEs. The results obtained indicate that all the molecules investigated should interact fairly strongly with the gold surface, with those of Ts being more reactive than At ones. The similarity in the Eads values of all the considered At compounds would make it challenging to differentiate between them while measuring their adsorption enthalpies, given experimental uncertainty. However, the difference in Eads among Ts compounds is more pronounced, so that one should be able to differentiate between the species.

Effect of temperature on CO oxidation over Pt(111) in two-dimensional confinement

Confined catalysis between a two-dimensional (2D) cover and metal surfaces has provided a unique environment with enhanced activity compared to uncovered metal surfaces. Within this 2D confinement, weakened adsorption and lowered activation energies were observed using surface science experiments and density functional theory (DFT) calculations. Computationally, the role of electronic and mechanical factors responsible for the improved activity was deduced only from static DFT calculations. This demands a detailed investigation on the dynamics of reactions under 2D confinement, including temperature effects. In this work, we study CO oxidation on a 2D graphene covered Pt(111) surface at 90 and 593 K using DFT-based ab initio molecular dynamics simulations starting from the transition state configuration. We show that CO oxidation in the presence of a graphene cover is substantially enhanced (2.3 times) at 90 K. Our findings suggest that 2D confined spaces can be used to enhance the activity of chemical reactions, especially at low temperatures.

Reactivity of Group 13 Elements Tl and Element 113, Nh, and of Their Hydroxides with Respect to Various Quartz Surfaces from Periodic Relativistic DFT Calculations

Adsorption properties of group 13 element Tl and the superheavy element Nh, as well of their hydroxides on various modified quartz surfaces, are predicted on the basis of relativistic periodic DFT calculations using the BAND software. The obtained adsorption energies, E_ads, of the MOH (M = Tl and Nh) molecules are indicative of the relatively strong interaction of the hydroxides with all the considered quartz surfaces. In contrast, adsorption of the Tl and Nh atoms was found to be significantly weaker. The adsorption strength of both M and MOH (M = Tl and Nh) was shown to increase with the dehydroxylation of the quartz surface. Very good agreement is reached between the calculated E_ads(TlOH) of 133 kJ/mol on the fully hydroxylated quartz surface and of 157 kJ/mol on the partially dehydroxylated quartz surface on the one hand and experimental adsorption enthalpies, -ΔH_ads, of 134/137 ± 5 kJ/mol (at ∼300 °C) and 158 ± 3 kJ/mol (at ∼500 °C), respectively, on the other hand. Thus, we suggest that all the experimental ΔH_ads values for Tl should be assigned to the adsorption/desorption of the TlOH molecule. For NhOH, its adsorption properties on various quartz surfaces should be very similar to those of TlOH, with slightly smaller E_ads values. Adsorption of the Nh atom should, however, be much weaker than that of the Tl atom due to stronger spin-orbit effects in Nh.

Finite-Size Error Cancellation in Diffusion Monte Carlo Calculations of Surface Chemistry

The accurate prediction of reaction mechanisms in heterogeneous (surface) catalysis is one of the central challenges in computational chemistry. Quantum Monte Carlo methods─Diffusion Monte Carlo (DMC) in particular─are being recognized as higher-accuracy, albeit more computationally expensive, alternatives to Density Functional Theory (DFT) for energy predictions of catalytic systems. A major computational bottleneck in the broader adoption of DMC for catalysis is the need to perform finite-size extrapolations by simulating increasingly large periodic cells (supercells) to eliminate many-body finite-size effects and obtain energies in the thermodynamic limit. Here, we show that it is possible to significantly reduce this computational cost by leveraging the cancellation of many-body finite-size errors that accompanies the evaluation of energy differences when calculating quantities like adsorption (binding) energies and mapping potential energy surfaces. We analyze the cancellation and convergence of many-body finite-size errors in two well-known adsorbate/slab systems, H₂O/LiH(001) and CO/Pt(111). Based on this analysis, we identify strategies for obtaining binding energies in the thermodynamic limit that optimally utilize error cancellation to balance accuracy and computational efficiency. Using one such strategy, we then predict the correct order of adsorption site preference on CO/Pt(111), a challenging problem for a wide range of density functionals. Our accurate and inexpensive DMC calculations are found to unambiguously recover the top > bridge > hollow site order, in agreement with experimental observations. We proceed to use this DMC method to map the potential energy surface of CO hopping between Pt(111) adsorption sites. This reveals the existence of an L-shaped top-bridge-hollow diffusion trajectory characterized by energy barriers that provide an additional kinetic justification for experimental observations of CO/Pt(111) adsorption. Overall, this work demonstrates that it is routinely possible to achieve order-of-magnitude speedups and memory savings in DMC calculations by taking advantage of error cancellation in the calculation of energy differences that are ubiquitous in heterogeneous catalysis and surface chemistry more broadly.

Reactivity of superheavy elements Cn and Fl and of their oxides in comparison with homologous species of Hg and Pb, respectively, towards gold and hydroxylated quartz surfaces

Adsorption energies, E_ads, and other properties of atoms and oxides of the superheavy elements (SHEs) Cn and Fl, as well as of the homologous species of Hg and Pb, on Au(111) and fully hydroxylated quartz surfaces are predicted on the basis of 2c-DFT calculations and a periodic slab model using BAND software. The ambition of the work is to interpret the outcome of "one-atom-at-a-time" gas-phase chromatography experiments on the reactivity/volatility of SHEs. The present results with an improved (dispersion corrected) exchange-correlation functional show that, in agreement with our earlier predictions and experimental results on Pb, Hg and Cn, the sequence of the E_ads values of the atoms on the gold surface should be Pb ≫ Hg > Fl > Cn, with rather moderate E_ads values smaller than 90 kJ mol^-1 (except for that for Pb). Oxides of Hg, Cn and Fl should be much more reactive with the gold surface than the corresponding atoms, with E_ads values of about 200 kJ mol^-1. A striking difference in the geometry of the deposited oxides was found between group 12 and group 14. An analysis of the E_ads values for M and MO (M = Hg/Cn and Pb/Fl) on the hydroxylated α-quartz surface enables one to conclude that atoms of Hg, Cn and Fl should not interact with such a surface at room temperature, while Pb should adsorb on it. Oxides of these elements, on the contrary, should strongly adsorb on quartz with E_ads ≥ 100 kJ mol^-1. The present theoretical data agree with the experimental results on the elemental species of Hg, Cn and Fl.

Interplaying coordination and ligand effects to break or make adsorption-energy scaling relations

The linear relations between adsorption energies are one of the cornerstones of contemporary catalysis in view of the simplicity and predictive power of the computational models built upon them. Despite their extensive use, the exact nature of scaling relations is not yet fully understood, and a comprehensive theory of scaling relations is yet to be elaborated. So far, it is known that scalability is dictated by the degree of resemblance of the adsorbed species. In this work, density functional theory calculations show that CO and OH, two dissimilar species, scale or not depending on the surface facet where they adsorb at Pt alloys. This peculiar behavior arises from an interplay of ligand and geometric effects that can be used to modulate adsorption-energy scalability. This study opens new possibilities in catalysis, as it shows that surface coordination is a versatile tool to either balance or unbalance the similarities among adsorbates at alloy surfaces.

vdW-DF-ahcx: a range-separated van der Waals density functional hybrid

Hybrid density functionals replace a fraction of an underlying generalized-gradient approximation (GGA) exchange description with a Fock-exchange component. Range-separated hybrids (RSHs) also effectively screen the Fock-exchange component and thus open the door for characterizations of metals and adsorption at metal surfaces. The RSHs are traditionally based on a robust GGA, such as PBE (Perdew J Pet al1996Phys. Rev. Lett.773865), for example, as implemented in the HSE design (Heyd Jet al2003J. Chem. Phys.1188207). Here we define an analytical-hole (Henderson T Met al2008J. Chem. Phys.128194105) consistent-exchange RSH extension to the van der Waals density functional (vdW-DF) method (Berland Ket al2015Rep. Prog. Phys.78066501), launching vdW-DF-ahcx. We characterize the GGA-type exchange in the vdW-DF-cx version (Berland K and Hyldgaard P 2014Phys. Rev. B89035412), isolate the short-ranged exchange component, and define the new vdW-DF hybrid. We find that the performance vdW-DF-ahcx compares favorably to (dispersion-corrected) HSE for descriptions of bulk (broad molecular) properties. We also find that it provides accurate descriptions of noble-metal surface properties, including CO adsorption.

Porous Fluorocarbon from Rice Husk for the Efficient Separation of Gases

A porous fluorocarbon sorbent is synthesized from rice husk (RH) in a microwave reactor and then evaluated for the adsorption of different gases (CH4, CO2, and N2). The fluorocarbon is characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), Raman spectroscopy, Thermal gravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Significant enhancement in the surface area of activated carbon material is obtained from 29 to 531 m2 g-1 after removing naturally present silica in RH. Results reveal that rice husk fluorocarbon (RHF) has a higher adsorption affinity for CO2 (1.8 mmol g-1) than that of the sulfonated rice husk (RHS) (1.4 mmol g-1) at 298 K while the corresponding separation factor of CO2/CH4 is 4 and 3; respectively. Higher separation factors of 12 and 10 are observed for the binary system of CO2/N2, respectively. Quantum chemical density functional theory (DFT) calculations agree with the experimental observations. They reveal that RHF exhibits strong columbic interactions with considerable interaction energies of -87.85, -76.75, and -55.65 kcal mol-1 with CO2, CH4, and N2 gases; respectively. Finally, the adsorption process results are highly reproducible, with a small decrease in the adsorption capacity of less than 5% after repeated trials.

Reactivity of the Superheavy Element 115, Mc, and Its Lighter Homologue, Bi, with Respect to Gold and Hydroxylated Quartz Surfaces from Periodic Relativistic DFT Calculations: A Comparison with Element 113, Nh

Adsorption energies (E_ads) of the superheavy element (SHE) Mc, its lighter homologue (Bi), as well as of another superheavy element Nh and some lighter homologues of SHEs on gold and hydroxylated quartz surfaces are predicted via periodic relativistic density functional theory calculations. The aim of this study is to support "one-atom-at-a-time" gas-phase chromatography experiments that are examining the reactivity and volatility of Mc. The obtained E_ads values of the Bi and Mc atoms on the Au(111) surface are >200 kJ/mol. On the hydroxylated quartz surface, Mc should adsorb with a minimal energy of 58 kJ/mol. On both types of surfaces, E_ads(Mc) should be ∼100 kJ/mol smaller than E_ads(Bi) due to strong relativistic effects on its valence 7p electrons. A comparison with other SHEs under investigation shows that Mc should adsorb on gold more strongly than Cn, Nh, and Fl, while on quartz, Mc should adsorb like Nh, with both of them absorbing more strongly than volatile Cn and Fl. The highest reactivity of Mc in the row of the 7p elements is caused by the largest orbital and relativistic destabilization and expansion of the 7p_3/2 atomic orbital. Using the calculated E_ads, the distribution of the Nh and Mc events in the gas-phase chromatography column with quartz and gold-plated detectors is predicted via Monte Carlo simulations. As a result, Mc atoms should be almost 100% adsorbed in the first section of the chromatography column on quartz, while a few atoms of Nh can reach the second part of the column with gold-plated detectors.

Designing new SRP density functionals including non-local vdW-DF2 correlation for H2 + Cu(111) and their transferability to H2 + Ag(111), Au(111) and Pt(111)

Specific reaction parameter density functionals (SRP-DFs) that can describe dissociative chemisorption molecular beam experiments of hydrogen (H₂) on cold transition metal surfaces with chemical accuracy have so far been shown to be only transferable among different facets of the same metal, but not among different metals. We design new SRP-DFs that include non-local vdW-DF2 correlation for the H₂ + Cu(111) system, and evaluate their transferability to the highly activated H₂ + Ag(111) and H₂ + Au(111) systems and the non-activated H₂ + Pt(111) system. We design our functionals for the H₂ + Cu(111) system since it is the best studied system both theoretically and experimentally. Here we demonstrate that a SRP-DF fitted to reproduce molecular beam sticking experiments for H₂ + Cu(111) with chemical accuracy can also describe such experiments for H₂ + Pt(111) with chemical accuracy, and vice versa. Chemically accurate functionals have been obtained that perform very well with respect to reported van der Waals well geometries, and which improve the description of the metal over current generalized gradient approximation (GGA) based SRP-DFs. From a systematic comparison of our new SRP-DFs that include non-local correlation to previously developed SRP-DFs, for both activated and non-activated systems, we identify non-local correlation as a key ingredient in the construction of transferable SRP-DFs for H₂ interacting with transition metals. Our results are in excellent agreement with experiment when accurately measured observables are available. It is however clear from our analysis that, except for the H₂ + Cu(111) system, there is a need for more, more varied, and more accurately described experiments in order to further improve the design of SRP-DFs. Additionally, we confirm that, when including non-local correlation, the sticking of H₂ on Cu(111) is still well described quasi-classically.

Rice husk derived Aminated Silica for the efficient adsorption of different gases

In this present work, we successfully prepared aminated silica (ASiO2) from rice husk ash (RHA) and functionalized with 3-aminopropyltriethoxysilane (APTES). Physical and chemical properties of the synthesized material were investigated by various techniques SEM-EDX, XPS, FTIR, TGA. The surface area of RHA was 223 m2/g, while for ASiO2 was 101 m2/g. Molecular level DFT calculations revealed that the functionalization of ASiO2 resulted in a significant decrease in the HOMO-LUMO energy gap, a reduction in hardness, and a consequent increase in charge transfer characteristics. The adsorption behavior at low pressure (1 atm.) of aminated silica on different gases CO2, CH4, H2, and N2 at temperatures 77, 273, 298 K was studied. The adsorption of hydrogen was reported for the first time on aminated silica with an excellent adsorption capacity of 1.2 mmol/g. The ASiO2 exhibited excellent performance in terms of gas separation in binary mixtures of CO2/CH4, CO2/N2 and CO2/H2 at 273, and 298 K, respectively. The catalyst further exhibits high stability during three cycles with less than 10% variation in the separation capacity.

Relativistic Effects on the Properties of Lr: A Periodic DFT Study of the Adsorption of Lr on Surfaces of Ta in Comparison with Lu and Tl

With the aim of finding out whether the predicted 7s²7p_1/2 ground-state electron configuration of Lr will have an effect on its reactivity, calculations of the adsorption properties of Lr(7s²7p), its homologue Lu(6s²5d), and the related p element Tl(6s²6p) on the surface of Ta were performed using the relativistic periodic ADF BAND suite. The obtained adsorption energies, E_ads(M), are in excellent agreement with the measured adsorption enthalpies of Lu and Tl, showing that Lr adsorbs on the surface of Ta similarly to Lu and much differently (215 kJ/mol more strongly) from Tl. An AO population analysis reveals that Lr interacts with the Ta surface preferentially via the 7s AO, with some participation of the 6d as well as 7p_1/2 and 7p_3/2 AOs. In contrast, E_ads(Tl) is governed mainly by the 6p(Tl) AOs. Thus, the present investigations show that Lr should behave like Lu but not like the p element Tl on transition-metal surfaces.

Properties and Reactivity of Hydroxides of Group 13 Elements In, Tl, and Nh from Molecular and Periodic DFT Calculations

Adsorption energies, E_ads, of gaseous hydroxides of In, Tl, and the superheavy element Nh on surfaces of Teflon and gold are predicted using molecular and periodic relativistic DFT calculations. The ambition of the work is to assist related "one atom at a time" gas-phase chromatography experiments on the volatility of NhOH. The obtained low values of E_ads(MOH), where M = In, Tl, Nh, on Teflon should guarantee easy transportation of the molecules through the Teflon capillaries from the accelerator to the chemistry setup. Straightforward band-structure DFT calculations using the revPBE-D3(BJ) functional have given an E_ads(MOH) value of 161.4 kJ/mol on the Au(111) surface, being indicative of significant molecule-surface interaction. The MOH-gold surface binding is shown to take place via the oxygen atom of the hydroxide, with the oxygen-gold charge density transfer increasing from InOH to NhOH. The trend in E_ads(MOH) is shown to be InOH < TlOH < NhOH, caused by increasing molecular dipole moments and decreasing stability of the hydroxides in this row. A trend in E_ads of the atoms of these elements on gold is, however, opposite, In > Tl > Nh, caused by the increasing relativistic contraction and stabilization of the np_1/2 AO with Z. These opposite trends in E_ads(MOH) and E_ads(M) in group 13 lead to almost equal E_ads(Nh) and E_ads(NhOH) values, making identification of Nh, as a type of species, difficult by measuring its adsorption enthalpy on gold.

A relativistic periodic DFT study on interaction of superheavy elements 112 (Cn) and 114 (Fl) and their homologs Hg and Pb, respectively, with a quartz surface

Relativistic periodic calculations of adsorption energies of group-12 elements Hg and Cn and group-14 elements Pb and Fl on a hydroxylated (001) α-quartz surface at different adsorbate coverage have been performed using the ADF-BAND program.

Size-dependent properties of transition metal clusters: from molecules to crystals and surfaces--computational studies with the program ParaGauss

In the so-called scalable regime the size-dependent behavior of the physical and chemical properties of transition metal clusters is described by scaling relationships. For most quantities this scalable regime is reached for cluster sizes between a few tens and a few hundreds of atoms, hence for systems for which an accurate treatment by density functional theory is still feasible. Thus, by invoking scaling relations one is able to obtain properties of very large nanoparticles and even the bulk limit from the results of a series of smaller cluster models. In this invited review we illustrate this strategy by exploiting results from computational studies that mostly were carried out with the density functional theory software ParaGauss. We address mainly the size-dependent behavior of the properties of transition metal clusters. To this end, we first present benchmark studies probing various approximations that are used in such density functional calculations. Subsequently we show how physical insight may be gained by exploring less understood types of systems. These applications range from bare clusters to nanoislands and nanoalloys to adsorption complexes.

Kinetic modelling of heterogeneous catalytic systems

The importance of heterogeneous catalysis in modern life is evidenced by the fact that numerous products and technologies routinely used nowadays involve catalysts in their synthesis or function. The discovery of catalytic materials is, however, a non-trivial procedure, requiring tedious trial-and-error experimentation. First-principles-based kinetic modelling methods have recently emerged as a promising way to understand catalytic function and aid in materials discovery. In particular, kinetic Monte Carlo (KMC) simulation is increasingly becoming more popular, as it can integrate several sources of complexity encountered in catalytic systems, and has already been used to successfully unravel the underlying physics of several systems of interest. After a short discussion of the different scales involved in catalysis, we summarize the theory behind KMC simulation, and present the latest KMC computational implementations in the field. Early achievements that transformed the way we think about catalysts are subsequently reviewed in connection to latest studies of realistic systems, in an attempt to highlight how the field has evolved over the last few decades. Present challenges and future directions and opportunities in computational catalysis are finally discussed.

Mechanisms for CO oxidation on Fe(iii)–OH–Pt interface: a DFT study

The full catalytic cycle that involves the oxidation of two CO molecules is investigated here by using periodic density functional calculations. To simulate the nature of Fe(OH)_x/Pt nanoparticles, three possible structural models, i.e., Fe(OH)_x/Pt(111), Fe(OH)_x/Pt(332) and Fe(OH)_x/Pt(322), are built. We demonstrate that Fe(iii)–OH–Pt stepped sites readily react with CO adsorbed nearby to directly yield CO₂ and simultaneously produce coordinatively unsaturated iron sites for O₂ activation. By contrast, the created interfacial vacancy on Fe(OH)_x/Pt(111) prefers to adsorb CO rather than O₂, thus inhabiting the catalytic cycles of CO oxidation. We suggest that such structure sensitivity can be understood in terms of the bond strengths of Fe(iii)–OH.

Orthogonal Intermolecular Interactions of CO Molecules on a One-Dimensional Substrate

By low-temperature scanning tunneling microscopy, we study CO molecule chemisorption on a quasi–one-dimensional Cu(110)-(2×1)-O surface. Atom-resolved images reveal how the interaction of CO with the surface Cu-O- chains gives rise to orthogonal attractive and repulsive intermolecular interactions. First-principles calculations show that CO molecules induce unprecedented lifting of the host Cu atoms by 1 Å from the Cu-O- chains, enabling the Cu-CO unit to tilt by 45° from the surface normal. Contrary to the behavior of CO on metal surfaces, this structural distortion enables unprecedented, orthogonal, short-range intermolecular dipole-dipole attraction and long-range, surface-mediated repulsion. These interactions lead to self-assembly into molecular nanograting structures consisting of arrays of single-molecule-wide CO rows. The origin of the novel behavior of CO molecules in the electronic and geometrical properties of the quasi–one-dimensional substrate suggests that similar molecule-molecule and molecule-substrate interactions could play an important role at catalytic sites on reactive surfaces.

FIRST-PRINCIPLES STUDY OF ADSORPTION OF CN ON Cu(111)

The adsorption of CN on Cu (111) has been investigated using density functional theory calculations based on plane-wave expansion and pseudo-potential treatment. Calculations within the generalized gradient approximation predicted a preference for CN in the fcc C -down site. No stationary points corresponding to pure parallel mode were found. But the tilted mode was found to be achievable. The calculated vibrational frequencies of CN were used to correctly discriminate between the adsorption sites.

Vibrational Recognition of Adsorption Sites for CO on Platinum and Platinum−Ruthenium Surfaces

We have studied the vibrational properties of CO adsorbed on platinum and platinum-ruthenium surfaces using density-functional perturbation theory within the Perdew-Burke-Ernzerhof generalized-gradient approximation. The calculated C-O stretching frequencies are found to be in excellent agreement with spectroscopic measurements. The frequency shifts that take place when the surface is covered with ruthenium monolayers are also correctly predicted. This agreement for both shifts and absolute vibrational frequencies is made more remarkable by the frequent failure of local and semilocal exchange-correlation functionals in predicting the stability of the different adsorption sites for CO on transition metal surfaces. We have investigated the chemical origin of the C-O frequency shifts introducing an orbital-resolved analysis of the force and frequency density of states, and assessed the effect of donation and backdonation on the CO vibrational frequency using a GGA+molecular U approach. These findings rationalize and establish the accuracy of density-functional calculations in predicting absolute vibrational frequencies, notwithstanding the failure in determining relative adsorption energies, in the strong chemisorption regime.

Towards an exact treatment of exchange and correlation in materials: application to the "CO adsorption puzzle" and other systems

It is shown that the errors of present-day exchange-correlation (XC) functionals are rather short ranged. For extended systems, the correction can therefore be evaluated by analyzing properly chosen clusters and employing highest-quality quantum chemistry methods. The XC correction rapidly approaches a universal dependence with cluster size. The method is applicable to bulk systems as well as to defects in the bulk and at surfaces. It is demonstrated here for CO adsorption at transition-metal surfaces, where present-day XC functionals dramatically fail to predict the correct adsorption site, and for the crystal bulk cohesive energy.