Step Edge Sites Research Articles

Unravelling CO Activation on Flat and Stepped Co Surfaces: A Molecular Orbital Analysis.

Structure sensitivity in heterogeneous catalysis dictates the overall activity and selectivity of a catalyst whose origins lie in the atomic configurations of the active sites. We explored the influence of the active site geometry on the dissociation activity of CO by investigating the electronic structure of CO adsorbed on 12 different Co sites and correlating its electronic structure features to the corresponding C-O dissociation barrier. By including the electronic structure analyses of CO adsorbed on step-edge sites, we expand upon the current models that primarily pertain to flat sites. The most important descriptors for activation of the C-O bond are the decrease in electron density in CO's 1π orbital , the occupation of 2π anti-bonding orbitals and the redistribution of electrons in the 3σ orbital. The enhanced weakening of the C-O bond that occurs when CO adsorbs on sites with a step-edge motif as compared to flat sites is caused by a distancing of the 1π orbital with respect to Co. This distancing reduces the electron-electron repulsion with the Co d-band. These results deepen our understanding of the electronic phenomena that enable the breaking of a molecular bond on a metal surface.

Valorization to hydrogen of bio-oil aqueous fractions from lignocellulosic biomass pyrolysis by aqueous phase reforming over Pt/C catalyst

Aqueous phase reforming was studied for the valorization to hydrogen of the aqueous fraction of bio-oil from lignocellulosic biomass pyrolysis. Aqueous phase reforming was carried out at 220 °C with a 3 % wt. Pt/carbon catalyst and an aqueous fraction of bio-oil feed containing 1 % wt. of organic matter. Binary and multicomponent mixtures representing aqueous fractions of bio-oil were studied. Strong hydrogen production dependence on aqueous fraction of bio-oil composition was evidenced. Higher processability was obtained for aqueous fractions of bio-oil rich in levoglucosan and hydroxyacetone, reaching a hydrogen production close to 40 mmol per gram of organic carbon, whereas furfural and acetic acid hampered reforming. A significant influence of minority components such as methanol and, particularly, formic acid, was observed. These components improved reforming of the whole multicomponent mixture, showing that blending with biorefinery fractions rich in them can improve the applicability of aqueous phase reforming. The catalyst exhibited stable activity and hydrogen production after 5 reaction cycles and more than 20 h. An increase in selectivity was observed during cycles 2 and 3, which was studied by characterization of fresh and used catalysts to show structure-selectivity relationships. No significant variation of Pt2+/Pt0 and metal particle mean diameter were observed, but Pt nanoparticles underwent morphological changes leading to higher prevalence of low coordination sites, in particular step-edge sites on the particle surface, resulting in higher hydrogen production. Temperature programmed desorption and oxidation analysis showed a cumulative deposition of reaction byproducts on the catalyst surface that contributed to loss of activity.

Direct Observation of Atomic Step-Assisted Stabilization of Polar Surface.

Polar surface is intrinsically unstable and thus highly reactive due to the uncompensated surface charges. The charge compensation is accompanied by various surface reconstructions, establishing novel functionality for its applications. Our in-situ atomic-scale electron microscopy study directly shows that the atomic step and step-assisted reconstruction play central roles in the charge compensation of polar oxide surface. The flat (LaO)+-terminated LaAlO3 (001) polar surface, when annealed at high temperature in vacuum, transits to the (015) vicinal surface via the dynamic motion and interaction of atomic steps. While the (015) vicinal surface possesses zero polarization along the surface normal, the thermodynamic ground state is achieved when the in-plane polarization is fully compensated via the reconstruction of step edge atoms; the step edge La atoms are displaced from their ordinary atomic sites toward the adjacent Al step edge site, resulting in the formation of negatively charged La vacancy at the corresponding step edges. As confirmed by first-principles calculations, the observed step reconstruction of (015) vicinal surface can completely cancel both out-of-plane and in-plane electric fields. This hitherto unknown mechanism reveals the central role of step reconstruction in stabilizing polar surfaces, providing valuable insights for understanding the novel charge compensation mechanism accompanied by the step reconstruction. This article is protected by copyright. All rights reserved.

Analysis of reported substrate-misorientation and growth-temperature dependences of alloy composition in metalorganic vapor-phase epitaxially grown c-plane InGaN

Reported substrate-misorientation and growth-temperature dependences of alloy composition in metalorganic vapor-phase epitaxially grown c-plane InGaN were successfully analyzed by applying least-squares regression to relationships derived in the kink segregation model. The analysis gave a reasonable result that the equilibrium indium content at the kink site decreased with growth temperature T due to increased desorption of indium adatoms. The analysis also revealed that the relaxation time for the indium content at the step-edge site (i.e., the length of time before the indium content at the step-edge site reaches its equilibrium value) was reduced with increasing T (i.e., 34 ms at 780 °C and 21 ms at 820 °C). The analysis can be improved by more experimental results for various T and contribute to practical predictions of compositional variation in alloy semiconductors.

Enhanced catalytic activity for methanol synthesis from CO2 hydrogenation by doping indium into the step edge of Rh(211): A theoretical study

The doping of foreign metals can modify the electronic properties of the original catalyst surfaces, thereby changing the catalytic activity. Realistic surfaces possess many defects, such as step sites. Herein, the first-principles calculation is implemented to research how indium doping on stepped Rh(211) alters the catalytic activity for methanol synthesis. The calculated substitution energy shows that the indium atom tends to be doped at the step edge of Rh(211). Almost all species prefer to adsorb at the step-edge sites because the surface atoms at the step-edge possess the lowest coordination number. The electronic structure analysis demonstrates that the activation degree of CO2* chiefly relies on the charge transfer between CO2* and catalyst. Furthermore, the doped indium atoms make In/Rh(211) expose more H* adsorption sites and promote the relative stability of adsorbed species on In/Rh(211). Besides, the doping of indium significantly inhibits the generation of by-product CO*, and enables the activation barrier to be as low as 0.98 eV for the rate-determining step on In/Rh(211), which is much lower than that of some reported catalysts. The ultrahigh catalytic activity of In/Rh(211) might be attributed to the decrease of the work function induced by the doping of indium, enhancing the relative stability of reaction species and CO2* activation degree.

Boosting the H2–D2 Exchange Activity of Dilute Nanoporous Ti–Cu Catalysts through Oxidation–Reduction Cycle–Induced Restructuring

The use of nanoporous metals as catalysts has attracted significant interest in recent years. Their high‐curvature, nanoscale ligaments provide not only high surface area but also a high density of undercoordinated step edge and kink sites. However, their long‐term stability, especially at higher temperatures, is often limited by thermal coarsening and the associated loss of surface area. Herein, it is demonstrated that the nanoscale morphology of nanoporous Cu can be regenerated by applying oxidation/reduction cycles at 250 °C. Specifically, the morphological evolution and H2 dissociation activity of hierarchical nanoporous Cu catalysts doped with Ti during structural rearrangement triggered by oxidative and reductive atmospheres at elevated temperatures are studied. In addition to coarsening of the structure at elevated temperatures, oxidation at 400 °C causes an expansion of the ligaments. Subsequent reduction at 400 °C leads to the formation of particles and a drop in the H2 dissociation activity compared the fresh catalyst. However, performing the redox cycle at 250 °C reverses coarsening and boosts the H2 dissociation activity for the hydrogen–deuterium (H2–D2) reaction. Herein, the possibility to reverse coarsening is demonstrated, thereby mitigating the loss of activity frequently observed in nanoporous catalysts.

Structure sensitivity of ethanol steam reforming over the Rh catalyst: Reaction kinetics and deactivation mechanisms

Rh catalyst exhibits high activity and selectivity of hydrogen production for ethanol steam reforming (ESR). In this work, we used density functional theory calculations and microkinetic modeling to study the structure sensitivity of ESR over the Rh catalyst, with stepped Rh(211) and terraced Rh(111) as models. The dominant reaction paths on the two surfaces were determined by pruning a complex network under different conditions. Further microkinetic analysis of the dominant reaction pathways suggested that Rh(211) shows higher activity and CO2 selectivity than Rh(111), but lower stability due to carbon deposition. High steam/ethanol ratio was found to be important to the reaction which can reduce carbon accumulation and improve CO2 selectivity. The transition state of CH3CH2OH dissociation is rate-controlling over both surfaces at high temperatures, while the removal processes of C* is rate controlling on Rh(211) at low temperatures, resulting in deactivation of the step edge sites. After screening several metal dopants over Rh(211), we found that the d-band center is well correlated with carbon adsorption energy, and stability trend is consistent with experiments. Our work could provide understanding of structure sensitivity and guidelines of rational Rh-based catalyst design for the ESR reaction.

Density functional theory study of active sites and reaction mechanism of ORR on Pt surfaces under anhydrous conditions

AbstractIdentifying active sites and catalytic mechanism of the oxygen reduction reaction under anhydrous conditions are crucial for the development of next generation proton exchange membrane fuel cells (PEMFCs) operated at a temperature>100 °C. Here, by employing density functional theory calculations, we studied ORR on flat and stepped Pt(111) surfaces with both (110) and (100) type of steps. We found that, in contrast to ORR under hydrous conditions, (111) terrace sites are not active for ORR under anhydrous conditions, because of weakened binding of ORR intermediates induced by O* accumulation on the surface. On the other hand, step edges, which are generally not active for ORR under hydrous conditions, are predicted to be the active sites for ORR under anhydrous conditions. Among them, (110) type step edge with a unique configuration of accumulated O stabilizes O2 adsorption and facilitates O2 dissociation, which lead an overpotential < 0.4 V. To improve ORR catalysts in high-temperature PEMFCs, it is desirable to maximize (110) step edge sites that present between two (111) facets of nanoparticles.

Selective Hydration of Rutile TiO2 as a Strategy for Site-Selective Atomic Layer Deposition.

The feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO2 facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H2O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO2 single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

Desorption trends of small alcohols and the disruption of intermolecular interactions at defect sites on Au(111).

Gold-based catalysts have received tremendous attention as supports and nanoparticles for heterogeneous catalysis, in part due to the ability of nanoscale Au to catalyze reactions at low temperatures in oxidative environments. Surface defects are known active sites for low temperature Au chemistry, so a full understanding of the interplay between intermolecular interactions and surface morphology is essential to an advanced understanding of catalytic behavior and efficiency. In a systematic study to better understand the adsorption and intermolecular behavior of small alcohols (C1-C4) on Au(111) defect sites, coverage studies of methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, and isobutanol have been conducted on Au(111) using ultrahigh vacuum temperature programmed desorption (UHV-TPD). These small alcohols molecularly adsorb on the Au(111) surface and high resolution experiments reveal distinct terrace, step edge, and kink adsorption features for each molecule. The hydrogen-bonded (H-bonded) networks of small alcohols on Au(111), except for 1-butanol and isobutanol, have been previously imaged on the molecular level at low temperatures by scanning tunneling microscopy. Primary C1-C3 alcohols exhibit planar H-bonded long extended zigzag chain networks while 2-butanol arranges in tetramer clusters of H-bonded molecules due to steric hindrance inhibiting the proximity of molecules on Au(111). Herein, the desorption energy of small primary alcohols was shown to trend linearly with increasing C1-C4 carbon chain length, indicating that the H-bonded molecular packing of 1-butanol resembles that of methanol, ethanol, and 1-propanol, while isobutanol and 2-butanol deviate from the trend. Butanol isomer studies allow the prediction of isobutanol long extended chains in contrast to tetramers. The distinction between the desorption of butanol isomers highlights the role of intermolecular interactions due to the difference in molecular packing structures on Au(111). Furthermore, by studying the energetics of terrace H-bonded networks in comparison with molecular adsorption at undercoordinated step edge and kink defect sites, it is shown that the contribution of stabilizing van der Waals forces to the overall adsorption energy is less for small alcohols adsorbed at kink sites (3.1 kJ mol-1 per CH2) and similar for those adsorbed at step edge (4.8 kJ mol-1 per CH2) and Au terrace sites (4.9 kJ mol-1 per CH2).

Enumerating Active Sites on Metal Nanoparticles: Understanding the Size Dependence of Cobalt Particles for CO Dissociation.

Detailed understanding of structure sensitivity, a central theme in heterogeneous catalysis, is important to guide the synthesis of improved catalysts. Progress is hampered by our inability to accurately enumerate specific active sites on ubiquitous metal nanoparticle catalysts. We employ herein atomistic simulations based on a force field trained with quantum-chemical data to sample the shape of cobalt particles as a function of their size. Algorithms rooted in pattern recognition are used to identify surface atom arrangements relevant to CO dissociation, the key step in the Fischer–Tropsch (FT) reaction. The number of step-edge sites that can catalyze C–O bond scission with a low barrier strongly increases for larger nanoparticles in the range of 1–6 nm. Combined with microkinetics of the FT reaction, we can reproduce experimental FT activity trends. The stabilization of step-edge sites correlates with increasing stability of terrace nanoislands on larger nanoparticles.

The Vital Role of Step-Edge Sites for Both CO Activation and Chain Growth on Cobalt Fischer–Tropsch Catalysts Revealed through First-Principles-Based Microkinetic Modeling Including Lateral Interactions

Microkinetic modeling is employed to predict catalytic turnover rates, product distributions, preferred mechanistic pathways, and rate- and selectivity-controlling elementary reaction steps for the Fischer-Tropsch (FT) reaction. We considered all relevant elementary reaction steps on Co(112̅1) step-edge and Co(0001) terrace sites as well as such important aspects as coverage-related lateral interactions, different chain-growth mechanisms, and the migration of adsorbed species between the two surfaces in the dual-site model. CHx–CHy coupling pathways relevant to the carbide mechanism have favorable barriers in comparison to the overall barriers for the CO insertion mechanism. A comparison of reaction barriers indicates why cobalt is such a good FT catalyst: CO bond scission and chain growth compete, while termination to olefins has a slightly higher barrier. The predicted kinetic parameters correspond well with experimental kinetic data. The Co(112̅1) model surface is highly active and selective for the FT reaction. Adding terrace Co(0001) sites in a dual-site model approach leads to a substantially higher CH4 selectivity at the expense of the C2+-hydrocarbons selectivity. The chain-growth probability decreases with increasing temperature and H2/CO ratio, caused by faster hydrogenation of the hydrocarbon chains. The elementary reaction steps for O removal and CO dissociation significantly control the overall CO consumption rate. Chain growth occurs almost exclusively at step-edge sites, while additional CH4 stems from CH and CH3 migration from step-edge to terrace sites. Replacing CO by CO2 as the reactant shifts the product distribution nearly completely to CH4, which is related to the much higher H/CO coverage ratio during CO2 hydrogenation in comparison to CO hydrogenation. These findings highlight the importance of a proper balance of CO and H surface species during the FT reaction and pinpoint step-edge sites as the locus of the FT reaction with low-reactive terrace sites near step-edge sites being the origin of unwanted CH4.

Revealing the structure of a catalytic combustion active-site ensemble combining uniform nanocrystal catalysts and theory insights

Supported metal catalysts are extensively used in industrial and environmental applications. To improve their performance, it is crucial to identify the most active sites. This identification is, however, made challenging by the presence of a large number of potential surface structures that complicate such an assignment. Often, the active site is formed by an ensemble of atoms, thus introducing further complications in its identification. Being able to produce uniform structures and identify the ones that are responsible for the catalyst performance is a crucial goal. In this work, we utilize a combination of uniform Pd/Pt nanocrystal catalysts and theory to reveal the catalytic active-site ensemble in highly active propene combustion materials. Using colloidal chemistry to exquisitely control nanoparticle size, we find that intrinsic rates for propene combustion in the presence of water increase monotonically with particle size on Pt-rich catalysts, suggesting that the reaction is structure dependent. We also reveal that water has a near-zero or mildly positive reaction rate order over Pd/Pt catalysts. Theory insights allow us to determine that the interaction of water with extended terraces present in large particles leads to the formation of step sites on metallic surfaces. These specific step-edge sites are responsible for the efficient combustion of propene at low temperature. This work reveals an elusive geometric ensemble, thus clearly identifying the active site in alkene combustion catalysts. These insights demonstrate how the combination of uniform catalysts and theory can provide a much deeper understanding of active-site geometry for many applications.

Step-edge and kink segregation models for analysis of reported step-velocity dependences of carbon concentration in GaN

The correlation between step velocity and carbon incorporation during the step-flow growth of GaN was analyzed from the standpoints of the meantime until a carbon atom incorporated at the kink site moves through the step-edge site to the surface site, τ, and the length of time before the carbon concentration at the step-edge site reaches its equilibrium value, τstep. Reported step-velocity dependences of carbon concentration in GaN were successfully reproduced with the assumption of τ ≫ τstep or τ < τstep. Due to the difficulty in estimating τstep, additional experiments are needed to tell which segregation is dominant for carbon in GaN.

Chloroform Hydrodechlorination on Palladium Surfaces: A Comparative DFT Study on Pd(111), Pd(100), and Pd(211)

Palladium has been shown to be an effective catalyst for chloroform hydrodechlorination, which serves as a promising treatment method for industrial chloroform waste. To investigate the structure sensitivity of this chemistry on Pd surfaces, we performed a density functional theory (DFT, GGA-PW91) study of the chloroform hydrodechlorination reaction on three distinct facets: Pd(111), Pd(100), and Pd(211). Based on the DFT results, the binding strengths of most surface intermediates generally increase in the following order: Pd(111) < Pd(100) < Pd(211). On all three Pd facets, methane is formed as the preferred reaction product through a pathway in which CHCl3* is fully dechlorinated to CH* first, and then hydrogenated to CH4. We constructed potential energy diagrams (PED) and compared the reaction energetics for chloroform hydrodechlorination on the three Pd facets. We propose that the competition between the desorption of chloroform and its initial dechlorination to form CHCl2* likely determines the hydrodechlorination activity of the catalyst. On Pd(111), the desorption of chloroform is energetically favored over its dechlorination while the dechlorination barriers are lower than the desorption barriers on Pd(100) and Pd(211). On the other hand, Pd(100) and Pd(211) bind atomic chlorine stronger and also catalyze the formation of atomic carbon effectively; both are potential site-blocking species. Our results suggest that the more open facets and step edge sites of a Pd nanoparticle may carry higher intrinsic activity towards chloroform hydrodechlorination than the close-packed facets, yet these under-coordinated sites are more prone to catalyst poisoning.

Energy barriers for Pb adatom diffusion on stepped ultrathin Pb(111) quantum nanofilms: First-principles calculations

Various properties of Pb(111) nanofilms, prototypical quantum films, have been studied extensively. However, key ab initio-level energy barriers for Pb adatom diffusion on stepped Pb(111) nanofilm surfaces are still not available. Using first-principles density functional theory, we calculate these barriers for films with thicknesses of few monolayers (ML). We find that two-atom exchange is always much more favorable than direct hopping to cross A- or B-type steps. Ehrlich-Schwoebel (ES) barriers for downward transport to a higher-coordination step-edge site depend strongly on the film thickness. For such transport from 2- to 1-ML terraces, or from 4- to 3-ML terraces, there is no an ES barrier, but large ES barriers of more than 100 meV are found from 3- to 2-ML terraces. We also obtain the barriers for diffusion along the step edges and find that these step-edge barriers are significantly larger than terrace diffusion barriers. In addition, we analyze energetics for diffusion on the top flat surface of a nanofilm supported on a vicinal surface, and thus having underlying buried or ghost steps. We quantify the tilted potential energy surface in both ghost A- and B-step regions separating 2- and 3-ML (as well as 3- and 4-ML) terraces. Consequences are discussed for the growth kinetics of supported Pb nanofilms, where the support does not strongly affect electronic states within the film.

Cryo-EM Structures of Atomic Surfaces and Host-Guest Chemistry in Metal-Organic Frameworks

Host-guest interactions govern the chemistry of a broad range of functional materials, but direct imaging using conventional transmission electron microscopy (TEM) has not been possible. This problem is exacerbated in metal-organic framework (MOF) materials, which are easily damaged by the electron beam. Here, we use cryogenic-electron microscopy (cryo-EM) to stabilize the host-guest structure and resolve the atomic surface of zeolitic imidazolate framework (ZIF-8) and its interaction with guest CO2 molecules. We image step-edge sites on the ZIF-8 surface that provides insight to its growth behavior. Furthermore, we observe two distinct binding sites for CO2 within the ZIF-8 pore, which are predicted by density functional theory (DFT) to be energetically favorable. This CO2 insertion induces an apparent ~3% lattice expansion along the <002> and <011> directions of the ZIF-8 unit cell. The ability to stabilize and preserve host-guest chemistry opens a rich materials space for scientific exploration and discovery using cryo-EM.

Similarities and differences for atomic and diatomic molecule adsorption on the B-5 type sites of the HCP(101̅6) surfaces of Co, Os, and Ru from DFT calculations

The differences in relative adsorption energies for mono-atomic and diatomic prototype species (C,N,O,S,H,CO,NO,SO,CH,NH,H2,O2) relevant to catalytic processes such as Fischer-Tropsch and Ammonia Synthesis chemistry are investigated on the previously un-studied (101¯6) surface(s) of Co, Os, and Ru. Recent work in the literature has confirmed that catalytically relevant nanoparticles of HCP elements such as Co, Os, and Ru typically possess highly active ‘B5’ sites; unfortunately many early and extant theory and model-ing treatments of "stepped HCP surfaces" use ad-hoc created steps via manual deletion of atoms from an ideal HCP(0001) slab model. To date the differences in adsorption energies at various B5 step edge types, and any possible trends across the same type of B5 sites on various HCP catalyst species has not been thoroughly characterized. Our work in this manuscript uses the low energy (101¯6) Miller Index surface of Co, Os, and Ru which exposes 2 distinct and strongly adsorbing step edge sites, the B5B and B5A step edge which have been reported as relevant in the literature for Cobalt nanoparticle catalysis applications. Results from this study should be used to help further understand atomistic processes on the stepped surfaces of catalytically active HCP elements.

Insight of the stability and activity of platinum single atoms on ceria

Single-atom catalysts (SACs) have recently attracted broad attention in the catalysis field due to their maximized atom efficiency and unique catalytic properties. An atomic-level understanding of the interaction between the metal atoms and support is vital for developing stable and high-performance SACs. In this work, Pt1 single atoms with loadings up to 4 wt.% were fabricated on ceria nanorods using the atomic layer deposition technique. To understand the Pt–O–Ce bond interfacial interactions, the stability of Pt1 single atoms in the hydrogen reducing environment was extensively investigated by using in situ diffuse reflectance infrared Fourier transform spectroscopy CO chemisorption measurements. It was found that ceria defect sites, metal loadings and high-temperature calcination are effective ways to tune the stability of Pt1 single atoms in the hydrogen environment. X-ray photoemission spectroscopy further showed that Pt1 single atoms on ceria are dominantly at a +2 valence state at the defect and step edge sites, while those on terrace sites are at a +4 state. The above tailored stability and electronic properties of Pt1 single atoms are found to be strongly correlated with the catalytic activity in the dry and water-mediated CO oxidation reactions.

Organic Molecule Adsorption on Stepped Si–Au Surfaces: Role of Functional Group on Geometry and Electronic Structure

Stepped gold‐stabilized Si(111) surfaces offer much potential for self‐organized assembly of one‐ and two‐dimensional nanostructured arrays of organic molecules due to the presence of regular atomic chains of Au and Si on the surface, parallel to the step edges. In this theoretical study, the adsorption of various simple organic molecules with different functional groups on the Si(553)–Au surface has been investigated. It has been found that adsorption at the step edge site is favored by all species. If the step edge is first passivated by hydrogen, adsorption occurs on Si atoms on the terraces. Instead, adsorption on the gold chains is completely unfavored. The influence of organic molecule adsorption on the electronic band structure and surface metallicity is investigated.