Atomic and Molecular Adsorption on Cu(111)

Exploring the Adsorption and Reactions of Methyl Radicals on M(111) Surfaces (M=Cu, Ag, Au): A DFT Study

It was reported that adsorbed methyl radicals produce ethane with Ag0- and Au0-nanoparticles in aqueous media, whereas on Cu0-powders, the product is methanol. The source of these differences was explored computationally, using the DFT method. The results indicate that up to six radicals can be adsorbed on Ag(111) and Au(111), (top site), while only four can be adsorbed on Cu(111) (fcc site), each surface containing eight atoms. The diffusion of the radicals on the surface is very easy on silver and copper, as this is achieved with a very low barrier (0.06 eV and 0.15 eV for Ag(111) and Cu(111), respectively), while on Au(111), the barrier is higher, 0.51 eV. The formation of ethane via a reaction of two adsorbed radicals is thermodynamically plausible for all studied coverage ratios on the three surfaces, but kinetically, it is plausible at room temperature only on Au(111) and Ag(111) at full coverage. Ethane can also be produced on Au(111) and Ag(111) by a collision of a solvated radical and an adsorbed radical. This is a barrierless process. On Cu(111), the yield of such a process is CH4(aq), and an adsorbed CH2 which reacts further with a non-adsorbed water molecule to produce adsorbed CH3OH.

Adsorption Sites in the High-Coverage Limit of CO on Cu(111)

The development of a comprehensive theory describing gas-surface interactions requires accurate experimental data for benchmarking. The adsorption of CO to Cu(111) is such a benchmark system. While state-of-the-art calculations still erroneously predict the favored adsorption site at low coverage to be the 3-fold hollow site, experimental studies have not yet definitively identified adsorption sites for all overlayer structures. Using a new combination of well-established techniques, we have reinvestigated CO adsorption, its ordering, and desorption on Cu(111). Our results support earlier suggestions for various overlayer structures for different coverages. For the intermediate-coverage 1.5×1.5 structure, we show that only on-top sites are occupied. Bridge site adsorption occurs only beyond a coverage of 0.42 monolayer (ML) in the 1.4×1.4 structure. Occupancy of this site is associated with a shift of atop CO molecules to off-centered atop with a characteristic IR absorbance frequency. These findings indicate a complex balance of coverage-dependent adsorbate interactions and binding energies that results in nonintuitive ordering and requires improvements in theory to understand.

Resolution of Selectivity Steps of CO Reduction Reaction on Copper by Quantum Monte Carlo

Electrochemical reduction of carbon monoxide to valuable fuels and chemicals on copper surfaces remains a challenging area in catalysis due to a limited understanding of adsorption mechanisms and reaction pathways. Although density functional theory (DFT)-based studies have investigated these processes, their accuracy varies across different functionals. Here, we present the application of fixed-node diffusion Monte Carlo (FNDMC) to benchmark the adsorption energies of CO*, H*, and key CO reduction reaction (CORR) intermediates, COH* and CHO* on the Cu(111) surface. Our results for CO* and H* adsorption energies closely align with experimentally measured chemisorption reactions, highlighting the limitations of DFT and providing site-specific energy comparisons that are often not available experimentally. Additionally, we explore the effect of explicit solvation, demonstrating how water stabilizes the COH* over CHO*, thus suggesting a critical role of COH* in CORR. Finally, we release our high-accuracy FNDMC benchmarks for testing and developing new DFT functionals for electrocatalysis. Overall, this study underscores the potential of FNDMC for detailed surface chemistry studies and offers new insights into catalytic processes.

Defluorination Mechanisms and Real-Time Dynamics of Per- and Polyfluoroalkyl Substances on Electrified Surfaces

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants found in groundwater sources and a wide variety of consumer products. In recent years, electrochemical approaches for the degradation of these harmful contaminants have garnered a significant amount of attention due to their efficiency and chemical-free modular nature. However, these electrochemical processes occur in open, highly non-equilibrium systems, and a detailed understanding of PFAS degradation mechanisms in these promising technologies is still in its infancy. To shed mechanistic insight into these complex processes, we present the first constant-electrode potential (CEP) quantum calculations of PFAS degradation on electrified surfaces. These advanced CEP calculations provide new mechanistic details about the intricate electronic processes that occur during PFAS degradation in the presence of an electrochemical bias, which cannot be gleaned from conventional density functional theory calculations. We complement our CEP calculations with large-scale ab initio molecular dynamics simulations in the presence of an electrochemical bias to provide time scales for PFAS degradation on electrified surfaces. Taken together, our CEP-based quantum calculations provide critical reaction mechanisms for PFAS degradation in open electrochemical systems, which can be used to prescreen candidate material surfaces and optimal electrochemical conditions for remediating PFAS and other environmental contaminants.

Bringing Molecules Together: Synergistic Coadsorption at Dopant Sites of Single Atom Alloys

Bringing molecules together on a catalytic surface is a prerequisite for bimolecular and recombination reactions. However, in the absence of attractive interactions between reactants, such as hydrogen bonds, this poses a challenge. In contrast, based on density functional theory, we show that coadsorption at active sites of single-atom alloys (SAAs) is favored and that coadsorption is a general phenomenon observed for catalytically relevant adsorbates on a broad range of SAAs under temperature and pressure conditions commonly employed for catalysis. Dopants located in both terrace sites and in step edge defects exhibit a preference for coadsorption, displaying similar periodic trends. Using kinetic Monte Carlo simulations, we compare the reactivity of a model reaction on both a pure metal and an SAA and show that the preference for coadsorption significantly alters the overall reaction energy profile, even when the barriers for the rate-determining elementary step are identical. In our models, the coadsorption preference enhances the catalytic activity of the SAA surface by several orders of magnitude compared to the pure metal. We also report infrared (IR) spectroscopic signatures of coadsorption, which facilitate experimental detection. Analysis reveals that in these systems repulsive lateral interactions between nearby molecules are more than compensated for by the enhanced binding at dopant sites. Among the broad range of systems considered, SAAs containing early transition metals (TMs) exhibit the strongest coadsorption preference, which can be rationalized by assuming the existence of an optimal number of electrons involved in binding. The strong coadsorption preference, together with facile product desorption from early TMs, renders these systems attractive candidates for catalysis. Moreover, these SAAs could open new routes for reduction reactions because coadsorption with hydrogen is favored.

Deciphering the Factors Controlling Hydrogen and Methyl Spillover upon Methane Dissociation on Rh/Cu(111) Single-Atom Alloy

Spillover of adsorbed species from one active site to another is a key step in heterogeneous catalysis. However, the factors controlling this step, particularly the spillover of polyatomic species, have rarely been studied. Herein, we investigate the spillover dynamics of H* and CH3* species on a single-atom alloy surface (Rh/Cu(111)) upon the dissociative chemisorption of methane (CH4), using molecular dynamics that considers both surface phonons and electron-hole pairs. These dynamical calculations are made possible by a high-dimensional potential energy surface machine learned from density functional theory data. Our results provide compelling evidence that the H* and CH3* can spill over on the metal surface at experimental temperatures and reveal novel dynamical features involving an internal motion during diffusion for CH3*. Increasing surface temperature has a minor effect on promoting spillover, as geminate recombinative desorption becomes more prevalent. However, the poisoning of the active site can be mitigated by the frequent gaseous molecular collisions that occur under ambient pressure in real-world catalysis, which transfer energy to the trapped adsorbates. Interestingly, the bulky CH3* exhibits a significant spillover advantage over the light H* due to its larger size, which facilitates energy acquisition. These insights help to advance our understanding of spillover in heterogeneous catalysis.

Vacancy Ordering in Ultrathin Copper Oxide Films on Cu(111)

Oxide thin films grown on metal surfaces have wide applications in catalysis and beyond owing to their unique surface structures compared to their bulk counterparts. Despite extensive studies, the atomic structures of copper surface oxides on Cu(111), commonly referred to as "44" and "29", have remained elusive. In this work, we demonstrated an approach for the structural determination of oxide surfaces using element-specific scanning tunneling microscopy (STM) imaging enhanced by functionalized tips. This approach enabled us to resolve the atomic structures of "44" and "29" surface oxides, which were further corroborated by noncontact atomic force microscopy (nc-AFM) measurements and Monte Carlo (MC) simulations. The stoichiometry of the "44" and "29" frameworks was identified as Cu23O16 and Cu16O11, respectively. Contrary to the conventional hypothesis, we observed ordered Cu vacancies within the "44" structure manifesting as peanut-shaped cavities in the hexagonal lattice. Similarly, a combination of Cu and O vacancies within the "29" structure leads to bean-shaped cavities within the pentagonal lattice. Our study has thus resolved the decades-long controversy on the atomic structures of "44" and "29" surface oxides, advancing our understanding of copper oxidation processes and introducing a robust framework for the analysis of complex oxide surfaces.

Facile and Stable CuInO2 Nanoparticles for Efficient Electrochemical CO2 Reduction

Searching for electrocatalysts for the electrochemical CO2 reduction reaction (e-CO2RR) with high selectivity and stability remains a significant challenge. In this study, we design a Cu-CuInO2 composite with stable states of Cu0/Cu+ by electrochemically depositing indium onto CuCl-decorated Cu foil. The catalyst displays superior selectivity toward the CO product, with a maximal Faraday efficiency of 89% at -0.9 V vs the reversible hydrogen electrode, and maintains impressive stability up to 27 h with a retention rate of >76% in Faraday efficiency. Our systematical characterizations reveal that the catalyst's high performance is attributed to CuInO2 nanoparticles. First-principles calculations further confirm that CuInO2(012) is more conducive to CO generation than Cu(111) under applied potential and presents a higher energy barrier than Cu(111) for the hydrogen evolution reaction. These theoretical predictions are consistent with our experimental observations, suggesting that CuInO2 nanoparticles offer a facile catalyst with a high selectivity and stability for e-CO2RR.

Systematic DFT studies of CO-Tolerance and CO oxidation on Cu-doped Ni surfaces

Nickel-based surfaces have received significant attention as an efficient substrate for electrooxidation. This work studied doped nickel surfaces with Cu atoms to enhance the CO-Tolerance. A comparative study was performed for CO adsorption upon different cleavage facets of pristine and Cu-doped nickel surfaces, whereas the adsorption energy, charge transfer, and density of state for CO were estimated using GGA-RPBE calculation method. Several adsorption probabilities were considered, and the change in adsorption energy and bond lengths were used to explain the CO adsorption mechanism. Otherwise, the density of state was employed to study the 3σ and 1π orbital to demonstrate the adsorption of CO onto the different facets. According to our analysis, the Cu-doped nickel surface showed higher CO tolerance than the pristine nickel surface. Whereas the calculated CO adsorption energies of Cu-doped surfaces have more positive values than the non-doped counterparts. The catalytic ability of pristine and Cu-doped Ni(111) was studied to evaluate the ability of surface poisoning resistance. Thus, oxidation of CO to CO2 was studied using the Eley-Rideal mechanism upon the pristine and Cu-doped surfaces of Ni(100) where the rate-determining step for CO oxidation upon the reported surfaces was estimated as CO + O2* → CO2* + O* by an energy barrier of 1.05 and 0.9 eV for pristine, and Cu-doped Ni (100).

Synthesis of Cyano-Benzylidene Xanthene Synthons Using a Diprotic Brønsted Acid Catalyst, and Their Application as Efficient Inhibitors of Aluminum Corrosion in Alkaline Solutions

Novel cyano-benzylidene xanthene derivatives were synthesized using one-pot and condensation reactions. A diprotic Brønsted acid (i.e., oxalic acid) was used as an effective catalyst for the promotion of the synthesis process of the new starting xanthene–aldehyde compound. Different xanthene concentrations (ca. 0.1–2.0 mM) were applied as corrosion inhibitors to control the alkaline uniform corrosion of aluminum. Measurements were conducted in 1.0 M NaOH solution using Tafel extrapolation and linear polarization resistance (LPR) methods. The investigated xanthenes acted as mixed-type inhibitors that primarily affect the anodic process. Their inhibition efficiency values were enhanced with inhibitor concentration, and varied according to their chemical structures. At a concentration of 2.0 mM, the best-performing studied xanthene derivative recorded maximum inhibition efficiency values of 98.9% (calculated via the Tafel extrapolation method) and 98.4% (estimated via the LPR method). Scanning electron microscopy (SEM) was used to examine the morphology of the corroded and inhibited aluminum surfaces, revealing strong inhibitory action of each studied compound. High-resolution X-ray photoelectron spectroscopy (XPS) profiles validated the inhibitor compounds’ adsorption on the Al surface. Density functional theory (DFT) and Monte Carlo simulations were applied to investigate the distinction of the anticorrosive behavior among the studied xanthenes toward the Al (111) surface. The non-planarity of xanthenes and the presence of the nitrile group were the key players in the adsorption process. A match between the experimental and theoretical findings was evidenced.

Computational investigation of van der Waals corrections in the adsorption properties of molecules on the Cu(111) surface

Here, we report a computational investigation on the role of the most common van der Waals (vdW) corrections (D2, D3, D3(BJ), TS, TS+SCS, TS+HI, and dDsC) employed in density functional theory (DFT) calculations within local and semilocal exchange-correlation functionals to improve the description of the interaction between molecular species and solid surfaces. For this, we selected several molecular model systems, namely, the adsorption of small molecules (CH₃, CH₄, CO, CO₂, H₂O, and OH) on the close-packed Cu(111) surface, which bind via chemisorption or physisorption mechanisms. As expected, we found that the addition of the vdW corrections enhances the energetic stability of the Cu bulk in the face-centered cubic structure, which contributes to increasing the magnitude of the mechanical properties (elastic constants, bulk, Young, and shear modulus). Except for the TS+SCS correction, all vdW corrections substantially increase the surface energy, while the work function changes by about 0.05 eV (largest change). However, we found substantial differences among the vdW corrections when comparing its effects on interlayer spacing relaxations. Based on bulk and surface results, we selected only the D3 and dDsC vdW corrections for the study of the adsorption properties of the selected molecules on the Cu(111) surface. Overall, the addition of these vdW corrections has a greater effect on weakly interacting systems (CH₄, CO₂, H₂O), while the chemisorption systems (CH₃, CO, OH) are less affected.

Quantification of Hydride Coverage on Cu(111) by Electrochemical Mass Spectrometry

Electrochemical mass spectrometry (EC-MS) is combined with chronoamperometry to quantify H coverage associated with the surface hydride phase on Cu(111) in 0.1 mol/L H2SO4. A two-step potential pulse program is used to examine anion desorption and hydride formation, and the inverse, by tracking the 2 atomic mass unit (amu) signal for H2 production in comparison to the charge passed. On the negative potential step, the reduction current is partitioned between anion desorption, hydride formation, and the hydrogen evolution reaction (HER). For modest overpotentials, variations in partial processes are evident as inflections in the chronoamperometry and EC-MS signal. On the return step to positive potentials, hydride decomposition by H recombination to H2 occurs in parallel with sulfate adsorption. The challenge associated with the inherent diffusional delay in the EC-MS response is mitigated by total H2 collection and steady-state analysis facilitated by the thin-layer EC-MS cell geometry as demonstrated for the HER on a non-hydride forming Ag electrode. Analysis of the respective transients and steady-state response on Cu(111) reveals a saturated hydride fractional coverage of 0.67 at negative potentials with an upper bound charge of 106 μC/cm2 (average electrosorption valency of ≈1.76) associated with adsorption of the (√ 3 × √ 7 ) mixed sulfate-water adlayer at positive potentials.

Surface Hydride Formation on Cu(111) and Its Decomposition to Form H2 in Acid Electrolytes

Mass spectrometry and Raman vibrational spectroscopy were used to follow competitive dynamics between adsorption and desorption of H and anions during potential cycling of three low-index Cu surfaces in acid electrolytes. Unique to Cu(111) is a redox wave for surface hydride formation coincident with anion desorption, while the reverse reaction of hydride decomposition with anion adsorption yields H₂ by recombination rather than oxidation to H₃O⁺. Charge imbalance between the reactions accounts for the asymmetric voltammetry in SO₄^2-, ClO₄^-, PO₄^3-, and Cl^- electrolytes with pH 0.68-4.5. Two-dimensional hydride formation is evidenced by the reduction wave prior to H₂ evolution and vibrational bands between 995 and 1130 cm^-1. In contrast to Cu(111), no distinct voltammetric signature of surface hydride formation is observed on Cu(110) and Cu(100). The Cu(111) hydride surface phase may serve to catalyze hydrofunctionalization reactions such as CO₂ reduction to CH₄ and should be broadly useful in electro-organic synthesis.

Synthesis of Highly Active Pd@Cu-Pt/C Methanol Oxidation Electrocatalysts via Continuous, Co-Electroless Deposition

Controlled deposition of metals is essential for the creation of bimetallic catalysts having predictable composition and character. Continuous co-electroless deposition (co-ED) permits the creation of bimetallic catalysts with predictive control over composition. This method was applied to create a suite of Cu–Pt mixed-metal shell catalysts for use in methanol electrooxidation in direct methanol fuel cell applications (DMFCs). Enhanced performance of Cu–Pt compositions over Pt alone was predicted by existing computational studies in the literature. Experimental evidence from this study supports the bifunctional catalyst explanation for enhanced activity and confirms the optimum Cu:Pt ratio as Cu₃Pt for this methanol electrooxidation. This ability to control the composition of a bimetallic shell can be extended to other systems where the ratio of two metals is critical for catalytic performance.

Biomass hydrodeoxygenation catalysts innovation from atomistic activity predictors

Circular economy emphasizes the idea of transforming products involving economic growth and improving the ecological system to reduce the negative consequences caused by the excessive use of raw materials. This can be achieved with the use of second-generation biomass that converts industrial and agricultural wastes into bulk chemicals. The use of catalytic processes is essential to achieve a viable upgrade of biofuels from the lignocellulosic biomass. We carried out density functional theory calculations to explore the relationship between 13 transition metals (TMs) properties, as catalysts, and their affinity for hydrogen and oxygen, as key species in the valourization of biomass. The relation of these parameters will define the trends of the hydrodeoxygenation (HDO) process on biomass-derived compounds. We found the hydrogen and oxygen adsorption energies in the most stable site have a linear relation with electronic properties of these metals that will rationalize the surface's ability to bind the biomass-derived compounds and break the C-O bonds. This will accelerate the catalyst innovation for low temperature and efficient HDO processes on biomass derivates, e.g. guaiacol and anisole, among others. Among the monometallic catalysts explored, the scaling relationship pointed out that Ni has a promising balance between hydrogen and oxygen affinities according to the d-band centre and d-band width models. The comparison of the calculated descriptors to the adsorption strength of guaiacol on the investigated surfaces indicates that the d-band properties alone are not best suited to describe the trend. Instead, we found that a linear combination of work function and d-band properties gives significantly better correlation. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

The roles of step-site and zinc in surface chemistry of formic acid on clean and Zn-modified Cu(111) and Cu(997) surfaces studied by HR-XPS, TPD, and IRAS

The adsorption, desorption, and decomposition of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resolution x-ray photoelectron spectroscopy, temperature programmed desorption, and infrared reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid molecules adsorbed at 83 K were dissociated to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH molecules were dissociated into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH molecules were already dissociated into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid molecules were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissociation at low temperature. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissociation of formic acid. The desorption peak maxima of the formate decomposition products (CO₂ and H₂) on Zn-Cu(997) were shifted to higher temperatures than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface.

Promoting effect of tungsten carbide on the catalytic activity of Cu for CO2 reduction

Supporting Cu atoms on WC(0001) surfaces stabilizes CO₂ molecules relative to Cu(111), promoting the CO₂ catalytic activity on Cu/WC(0001).

Electronic effects due to organic linker-metal surface interactions: implications on screening of MOF-encapsulated catalysts

Using approximated NP/MOF interface models, DFT was used to investigate MOF-originated electronic effects on encapsulated NPs in NP@MOF hybrid catalysts.

DFT and microkinetic investigation of methanol synthesis via CO2 hydrogenation on Ni(111)-based surfaces

A DFT study of methanol production via CO2 hydrogenation reactions on clean Ni(111) and Ni(111)-M (M = Cu, Pd, Pt, or Rh) surfaces has been performed. The reaction network of this synthesis reaction has been determined using energy profiles. The competing reaction network between the formate-mediated route and the carboxyl-mediated route is also presented. Both routes are equally possible in mediating the overall synthesis reactions. A simple selectivity analysis based on the energy barrier shows that methanol synthesis is more preferred rather than formic acid (HCOOH) or carbon monoxide (CO) production. A mean-field kinetic analysis is also employed to determine the kinetic performance of all catalytic surfaces. The formate-mediated route is found to be energetically and kinetically more dominant than the carboxyl-mediated route. Cu, Pd, and Pt dopants are successful in increasing the kinetic performance of the clean Ni(111) surface in the formate route and Cu, Pt, and Rh dopants in the carboxyl route.