Effect of the exchange-correlation potential and of surface relaxation on the description of the H2O dissociation on Cu(111)

Theoretical Calculations on Metal Catalysts Toward Water-Gas Shift Reaction: a Review

Water-gas shift (WGS) reaction offers a dominating path to hydrogen generation from fossil fuel, in which heterogeneous metal catalysts play a crucial part in this course. This review highlights and summarizes recent developments on theoretical calculations of metal catalysts developed to date, including surface structure (e. g., monometallic and polymetallic systems) and interface structure (e. g., supported catalysts and metal oxide composites), with special emphasis on the characteristics of crystal-face effect, alloying strategy, and metal-support interaction. A systematic summarization on reaction mechanism was performed, including redox mechanism, associative mechanism as well as hybrid mechanism; the development on chemical kinetics (e. g., molecular dynamics, kinetic Monte Carlo and microkinetic simulation) was then introduced. At the end, challenges associated with theoretical calculations on metal catalysts toward WGS reaction are discussed and some perspectives on the future advance of this field are provided.

First-principles-based kinetic Monte Carlo simulations of CO oxidation on catalytic Au(110) and Ag(110) surfaces

At the core of the development of more efficient and reliable fuel cells (FCs), there are several essential chemical reactions, namely carbon monoxide (CO) oxidation. This reaction is a keystone in the cleaning of hydrogen fuel used in fuel cells due to strong poisoning by this species of the platinum catalyst used in these devices. The present work aims to provide insight regarding the activation of CO oxidation by gold or silver microfacets possessing low coordinated atoms. To achieve this, density functional theory (DFT) quantum calculations, which determined two competing reaction pathways for CO oxidation, i.e., by molecularly adsorbed oxygen, and by dissociated oxygen, are combined with first-principles kinetic Monte Carlo (1p-kMC) simulations, which employed the resulting DFT parameters in order to address the effect of temperature and partial pressures and the interplay of the elementary reaction events. The use of 1p-kMC is a step further from available works regarding the CO oxidation on gold- and silver-based catalysts for cleansing of hydrogen that is used as a fuel in FCs. Indeed, this research contributes to the conclusion that CO oxidation should preferentially occur on silver microfacets, while the obtained turnover frequencies (TOFs) reinforced such a conclusion.

Implicit solvent effects in the determination of Brønsted–Evans–Polanyi relationships for heterogeneously catalyzed reactions

The Brønsted–Evans–Polanyi relationship derived for the water dissociation reaction within an implicit solvent approach is similar to that without such effects.

Experimental Study and Modeling of the UV-Vis and Infrared Spectra of the [VO(O2)Hheida]- Complex Dissolved in Water

Combined theoretical and experimental studies of the [VO(O₂)Hheida]^- anion dissolved in water that may serve as a functional model for vanadium haloperoxidase enzymes have been performed. The geometrical structure and absorption and vibrational spectra of this system have been evaluated within the framework of density functional theory (DFT). The obtained theoretical results on the equilibrium structure and optical spectra are in quite good agreement with the experimental data. With the aid of the combination of UV-visible spectroscopy and electronic structure calculations, it has been revealed that, in the apparent absorption spectra of the [VO(O₂)Hheida]^- anion, the highest in energy band corresponds to a ligand to metal electron excitation, while the band with a maximum at 430 nm arises from the peroxo group. The calculations also reproduce quite well the positions, intensities and the grouping of frequencies in the near-infrared (NIR) spectra. The visualization of the calculated vibrations in the energy range of 400-1100 cm^-1 has been presented.

Water at the Interface Between Defective Graphene and Cu or Pt (111) Surfaces

The presence of defects in the graphenic layers deposited on metal surfaces modifies the nature of the interaction. Unsaturated carbon atoms, due to vacancies in the lattice, form strong organometallic bonds with surface metal atoms that highly enhance the binding energy between the two materials. We investigate by means of a wide set of dispersion-corrected density functional theory calculations how such strong chemical bonds affect both the electronic properties of these hybrid interfaces and the chemical reactivity with water, which is commonly present in the working conditions. We compare different metal substrates (Cu vs Pt) that present a different type of interaction with graphene and with defective graphene. This comparative analysis allows us to unravel the controlling factors of water reactivity, the role played by the carbon vacancies and by the confinement or "graphene cover effect". Water is capable of breaking the C-Cu bond by dissociating at the undercoordinated carbon atom of the vacancy, restoring the weak van der Waals type of interaction between the two materials that allows for an easy detachment of graphene from the metal, but the same is not true in the case of Pt, where C-Pt bonds are much stronger. These conclusions can be used to rationalize water reactivity at other defective graphene/metal interfaces.

Prediction of metallic nanotube reactivity for H2O activation

The reactivity of metallic nanotubes toward the catalysis of water dissociation, a key step in the water gas shift reaction (WGSR), was analyzed through density functional theory (DFT) calculations. Water dissociation was studied on surfaces of nanotubes based on copper, gold and platinum, and also on platinum doped copper and gold nanotubes. Gold and copper nanotubes present activities that are similar to those of their corresponding extended surfaces but, in the case of the Pt(5,3) nanotube, a significant improvement in the activity is found when compared with the extended surfaces. In fact, the calculations predict the water dissociation to be spontaneous on Pt(5,3) with a low activation energy barrier. The platinum doping of gold and copper nanotubes leads to contrasting effects, i.e., with a slight increase of activity found on gold and a slight decrease of activity in the case of copper. The consideration of a Brönsted-Evans-Polanyi (BEP) relationship to estimate the activation energy barriers for the O-H bond break leads to a satisfactory agreement between estimated and explicitly calculated values which suggests the validity of the BEP relationship for qualitative predictions of the activities of metal nanotubes towards the water dissociation reaction.

Highly active Au/δ-MoC and Au/β-Mo2C catalysts for the low-temperature water gas shift reaction: effects of the carbide metal/carbon ratio on the catalyst performance

Water gas shift reaction catalyzed by Mo carbides surfaces and on Au supported thereon is studied by experiments and computational methods.

Molecular and dissociative adsorption of water and hydrogen sulfide at perfect and defective Cu(110) surfaces

The adsorption of H₂O and H₂S onto Cu(110) surfaces lead to considerably different geometries and surface reconstruction and relaxation mechanisms.

Effect of the Exchange-Correlation Potential on the Transferability of Brønsted-Evans-Polanyi Relationships in Heterogeneous Catalysis

As more and more accurate density functional methods emerge, the transferability of Brønsted-Evans-Polanyi (BEP) relationships obtained with previous models is an open question. In this work, BEP relationships derived from different density functional theory based calculations are analyzed to answer this question. In particular, BEP relationships linking the activation energy of O-H bond breaking reactions taking place on metallic surfaces with the adsorption energy of the reaction products are chosen as a case study. These relationships are obtained with the widely used Perdew-Wang (PW91) generalized gradient approximation (GGA) exchange-correlation functional and with the more accurate meta-GGA Tao-Perdew-Staroverov-Scuseria (TPSS) one. We provide compelling evidence that BEP relationships derived from PW91 and TPSS functionals are essentially coincidental. This finding validates previously published BEP relationships and indicates that the reaction activation energy barrier can be obtained by the determination of the energy reaction descriptor value at the less computationally demanding GGA level; an important aspect to consider in future studies aimed at the computational design of catalysts with improved characteristics.

Methanol dissociation on bimetallic surfaces: validity of the general Brønsted–Evans–Polanyi relationship for O–H bond cleavage

Density functional theory calculations were used to study the dissociation of the O–H bond in methanol on several bimetallic transition metal surfaces, composed of elements showing high or moderate activity towards this reaction, namely, Ni, Rh, Ru, Ir, Pd, Au, Zn and Cu.

Unexpectedly large impact of van der Waals interactions on the description of heterogeneously catalyzed reactions: the water gas shift reaction on Cu(321) as a case example

The molecular mechanisms of the water gas shift reaction on Cu(321) have been chosen to investigate the effect of dispersion terms on the description of the energy profile and reaction rates.

Interaction of H2O and H2S with Cu(111) and the impact of the electric field: the rotating & translating adsorbate, and the rippled surface

The interactions of H₂O and H₂S monomers with Cu(111) in the absence and presence of an external electric field are studied using density functional theory.

Morphological effects of the nanostructured ceria support on the activity and stability of CuO/CeO2 catalysts for the water-gas shift reaction

Three CuO/CeO2 catalyst with different morphologies of ceria, namely nanospheres, nanorods and nanocubes, were synthesized and used to catalyze the water-gas shift (WGS) reaction. The reactivity tests showed that the Cu supported on the ceria nanospheres exhibited both the highest activity and superior stability when compared with the nanocube and nanorod ceria catalysts. Operando X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) methods were used to characterize these catalysts in their working state. High resolution electron microscopy (HRTEM, STEM) was used to look at the local atomic structure and nano-scale morphology. Our results show that the morphology of the ceria support, which can involve different crystal faces and concentrations of defects and imperfections, has a critical impact on the catalytic properties and influences: (1) the dispersion of CuO in the as-synthesized catalyst; (2) the particle size of metallic Cu upon reduction during the WGS reaction, (3) the stability of the metallic Cu upon variations of temperature, and (4) the dissociation of water on the ceria support. The nanosphere ceria catalyst showed an excellent water dissociation capability, the best dispersion of Cu and a strong Cu-Ce interaction, therefore delivering the best performance among the three WGS catalysts. The metallic Cu, which is the active species during the WGS reaction, was more stabilized on the nanospheres than on the nanorods and nanocubes and thus led to a better stability of the nanosphere catalyst than the other two architectures. Each catalyst exhibited a distinctive line-shape in the 800-1600 cm(-1) region of the DRIFTS spectra, pointing to the existence of different types of carbonate or carboxylate species as surface intermediates for the WGS.

Vibrationally promoted dissociation of water on Ni(111)

Water dissociation on transition-metal catalysts is an important step in steam reforming and the water-gas shift reaction. To probe the effect of translational and vibrational activation on this important heterogeneous reaction, we performed state-resolved gas/surface reactivity measurements for the dissociative chemisorption of D2O on Ni(111), using molecular beam techniques. The reaction occurs via a direct pathway, because both the translational and vibrational energies promote the dissociation. The experimentally measured initial sticking probabilities were used to calibrate a first-principles potential energy surface based on density functional theory. Quantum dynamical calculations on the scaled potential energy surface reproduced the experimental results semiquantitatively. The larger increase of the dissociation probability by vibrational excitation than by translation per unit of energy is consistent with a late barrier along the O-D stretch reaction coordinate.

Nanopattering in CeOx/Cu(111): A New Type of Surface Reconstruction and Enhancement of Catalytic Activity

Our results indicate that small amounts of an oxide deposited on a stable metal surface can trigger a massive surface reconstruction under reaction conditions. In low-energy electron microscopy (LEEM) experiments, no reconstruction of Cu(111) is observed after chemisorbing oxygen or after reducing O/Cu(111) in a CO atmosphere. On the other hand, LEEM images taken in situ during the reduction of CeO2/CuO1-x/Cu(111) show a complex nonuniform transformation of the surface morphology. Ceria particles act as nucleation sites for the growth of copper microterraces once CuO1-x is reduced. Can this reconstructed surface be used to enhance the catalytic activity of inverse oxide/metal catalysts? Indeed, CeOx on reconstructed Cu(111) is an extremely active catalyst for the water-gas shift process (CO + H2O → H2 + CO2), with the Cu microterraces providing very efficient sites for the dissociation of water and subsequent reaction with CO.