Atomic Kinks Research Articles

CO and O2 Interaction with Kinked Pt Surfaces.

We investigate the chemical interaction of carbon monoxide (CO) and oxygen (O2) with kink atoms on steps of platinum crystal surfaces using a specially designed Pt curved sample. We aim at describing the fundamental stages of the CO oxidation reaction, i.e., CO-covered/poisoned stage and O-covered/active stage, at the poorly known kinked Pt facets by probing CO uptake/saturation and O2 saturation, respectively. Based on the systematic analysis that the curved surface allows, and using high-resolution X-ray photoemission, a diversity of terrace and step/kink species are straightforwardly identified and accurately quantified, defining a smooth structural and chemical variation across different crystal planes. In the CO-saturated case, we observe a preferential adsorption at step edges, where the CO coverage reaches a CO molecule per step Pt atom, significantly higher than their close-packed analogous steps with straight terrace termination. For the O-saturated surface, a significantly higher O coverage is observed in kinked planes compared to the Pt(111) surface. While the strong adsorption of CO at the kinked edges points toward a higher ignition temperature of the CO oxidation at kinks as compared to terraces, the large O coverage at steps may lead to an increased reactivity of kinked surfaces during the active stage of the CO oxidation.

Electrochemical Synthesis of Rh Nanorods with High-Index Facets for the Oxidation of Formic Acid

Noble metal catalysts play an important role in developing efficient fuel cells for commercial application at a large scale. Among them, Pt-group metal nanocatalysts enclosed with high-index facets have been expected to exhibit high electrocatalytic activity due to the high density of the low-coordination step and kink atoms exposed on the surface. Many good studies on Pt and Pd nanocatalysts enclosed with high-index facets have been reported while Rh nanocatalysts with a high-index facet have rarely been prepared. In this work, Rh nanorods with high-index facets (310) were synthesized successfully through repeated oxygenated species adsorption/desorption mediated by square wave potential (SWP). Electrochemical tests showed that Rh nanorods with high-index facets exhibited excellent electrocatalytic activity for the formic acid oxidation reaction (FAOR), and the peak current densities in positive scan of Rh nanorods (4.48 mA·cm–2) was 4.7 times higher than that of Rh black catalysts (0.95 mA·cm–2). In addition, the Rh nanorods could maintain higher current density than that of the Rh black catalysts throughout the time domain with potential held at 0.45 V, indicating that Rh nanorods had good stability for FAOR. The enhanced electrocatalytic performance of the Rh nanorods could be mainly ascribed to high-index facets with step and kink atoms, which could provide sufficient active sites for FAOR.

Electrodeposition of shaped PtIr alloy nanocrystals with high–index facets for the electro–catalytic oxidation of alcohols

Pt–group metal nanocatalysts enclosed with high–index facets have sufficient kink and step atoms on the surface, which could act as efficient sites for fuel cell reactions. Besides surface structure, alloying was another efficient strategy to obtain nanoparticles with excellent catalytic performance. So how about combining the surface structure of high–index facet with alloying to prepare Pt–based nanocatalysts? In this work, we have synthesized alloyed PtIr tetrahexahedral (THH) nanocrystals (NCs) with high–index facets (730) by electrodeposition. And PtIr trioctahedral (TOH) NCs with high–index facets (441) could be obtained by simply decreasing the upper limit potential (EU) of square–wave potential (SWP) with other condition keeping the same as that for the synthesis of PtIr THH NCs. The successful synthesis of alloyed PtIr NCs with high–index facets mainly lied in the repetitive adsorption/desorption of oxygenated species on the surface of nanoparticles. The prepared PtIr THH NCs exhibited excellent electrocatalytic activity and stability towards ethylene glycol oxidation reaction (EGOR) and glycerol oxidation reaction (GOR). Among the nano–catalysts, the peak in the forward scan of PtIr THH NCs for EGOR and GOR were 19.9 mA∙cm−2 and 17.6 mA∙cm−2, respectively, which were 9.5 and 6.8 times higher than those of commercial Pt/C. The excellent electro–catalytic performance of alloyed PtIr NCs could mainly be attributed to the surface structure effect of high–index facets and synergetic effect of the alloy structure. This work could provide a new idea for designing highly efficient nanocatalysts for fuel cells by combing surface structure effect and alloying effect together.

Multiple Vacancies on (111) Facets of Single-Crystal NiFe2 O4 Spinel Boost Electrocatalytic Oxygen Evolution Reaction.

Oxygen evolution reaction (OER) as the rate-determining reaction of water splitting has been attracting enormous attention. At present, only some noble-metal oxide materials (IrO2 and RuO2 ) have been reported as efficient OER electrocatalysts for OER. However, the high cost and scarcity of these noble-metal oxide materials greatly hamper their large-scale practical application. Herein, we synthesize 100% (111) faceted NiFe2 O4 single crystals with multiple vacancies (cation vacancies and O vacancies). The (111) facets can supply enough platform to break chemical bonds and enhance electrocatalytic activity, due to its high density of atomic steps and kink atoms. Compared to NiFe2 O4 (without vacancies), the as-synthesized NiFe2 O4 -Ar (with vacancies) exhibits a dramatically improved OER activity. The NiFe2 O4 -Ar-30 shows the lowest onset potential (1.45 V vs RHE) and the best electrocatalytic OER activity with the lowest overpotential of 234 mV at 50 mA cm-2 . Furthermore, based on the theoretical calculations that the introduction of multiple vacancies can effectively modulate the electronic structure of active centers to accelerate charge transfer and reaction intermediates adsorption,which can reduce the reaction energy barrier and enhance the activity of electrochemical OER.

Scaling Platinum-Catalyzed Hydrogen Dissociation on Corrugated Surfaces.

We determine absolute reactivities for dissociation at low coordinated Pt sites. Two curved Pt(111) single‐crystal surfaces allow us to probe either straight or highly kinked step edges with molecules impinging at a low impact energy. A model extracts the average reactivity of inner and outer kink atoms, which is compared to the reactivity of straight A‐ and B‐type steps. Local surface coordination numbers do not adequately capture reactivity trends for H2 dissociation. We utilize the increase of reactivity with step density to determine the area over which a step causes increased dissociation. This step‐type specific reactive area extends beyond the step edge onto the (111) terrace. It defines the reaction cross‐section for H2 dissociation at the step, bypassing assumptions about contributions of individual types of surface atoms. Our results stress the non‐local nature of H2 interaction with a surface and provide insight into reactivity differences for nearly identical step sites.

Scaling Platinum‐Catalyzed Hydrogen Dissociation on Corrugated Surfaces

AbstractWe determine absolute reactivities for dissociation at low coordinated Pt sites. Two curved Pt(111) single‐crystal surfaces allow us to probe either straight or highly kinked step edges with molecules impinging at a low impact energy. A model extracts the average reactivity of inner and outer kink atoms, which is compared to the reactivity of straight A‐ and B‐type steps. Local surface coordination numbers do not adequately capture reactivity trends for H2 dissociation. We utilize the increase of reactivity with step density to determine the area over which a step causes increased dissociation. This step‐type specific reactive area extends beyond the step edge onto the (111) terrace. It defines the reaction cross‐section for H2 dissociation at the step, bypassing assumptions about contributions of individual types of surface atoms. Our results stress the non‐local nature of H2 interaction with a surface and provide insight into reactivity differences for nearly identical step sites.

Self-growing Cu/Sn bimetallic electrocatalysts on nitrogen-doped porous carbon cloth with 3D-hierarchical honeycomb structure for highly active carbon dioxide reduction

The electrochemical CO2 reduction reaction (CO2RR) into liquid fuels is a promising avenue to both store intermittent renewable energy and reduce global CO2 emission, but daunting bottlenecks owing to poor selectivity, low activity and stability of the electrocatalysts. Herein, we report that bimetallic copper and tin are simultaneously deposited on the nitrogen doped porous carbon cloth (N-CC) via an efficient and facile co-electroplating strategy. The highly conductive N-doped porous carbon cloth as substrate enables a largely uniform distribution of N-doped defects, which can provide more active sites for the nucleation and growth of anchoring nano-sized Cn/Sn (3.39 nm). The as-prepared electrode (Cu(1)Sn(4)-N-CC), with a 3D-hierarchical porous honeycomb structure, could not only favor the diffusion of electrolyte and serve as “transfer posts” to confine the intermediates of reduced CO2, but also provide abundant terrace, ledge and kink atoms to function as active site for highly efficient CO2RR. As such, the obtained Cu(1)Sn(4)-N-CC electrode convert CO2 to formate with allured faradic efficiency (90.24 %), partial current density (15.56 mA cm−2) and production rate (173 μmol h-1 cm−2) at −0.97 VRHE, along with a long-term tolerance in CO2RR. This study may provide a new design for self-growing bimetallic catalyst on N-doped porous substrate to regulate advanced catalysts/electrodes for different applications.

CHD3 Dissociation on the Kinked Pt(210) Surface: A Comparison of Experiment and Theory

To be able to simulate activated heterogeneously catalyzed reactions on the edge and corner sites of nanoparticles, a method for calculating accurate activation barriers for the reactions is required. We have recently demonstrated that a semiempirical specific reaction parameter (SRP) density functional developed to describe CHD3 dissociation on a flat Ni(111) surface is transferable to describing the same reaction on a stepped Pt(211) surface. In the current work, we compare initial sticking coefficients measured using the King and Wells beam reflectivity technique and calculated from ab initio molecular dynamics trajectories using the same SRP functional for CHD3 dissociation on a kinked Pt(210) surface at a temperature of 650 K. The calculated sticking coefficients overestimate those determined experimentally, with an average energy shift between the two curves of 13.6 kJ/mol, which is over a factor of 3 times higher than the 4.2 kJ/mol limit that defines chemical accuracy. This suggests the SRP functional predicts an activation barrier that is too low for the dissociation on the least coordinated kink atom, which is the site of the lowest energy transition state and where most of the dissociation occurs in the calculations.

Ultrathin Co3O4 Nanosheets with Edge-Enriched {111} Planes as Efficient Catalysts for Lithium–Oxygen Batteries

Li-O2 batteries have aroused great interest as promising chemical power sources due to their high energy density. However, enhancement of the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the cathodes is still a great challenge. Herein, we design and prepare Co3O4 single-crystal nanosheets with edge-enriched {111} facets via a simple pyrolysis of α-Co(OH)2, which show a high activity as cathode catalysts for Li-O2. The catalytic activity and stability have been significantly improved in comparison with Co3O4 poor-edge {111} facets. This enhancement can be largely attributed to the edge-enriched {111} facets. Co3O4 nanosheets with edge-enriched {111} facets expose more metal atoms to boost the catalytic activity. Moreover, the edge-enriched {111} facets contain a high density of atomic steps and kink atoms which supply a wide space for breaking chemical bonds and increasing the reaction activity. This study also presents some insights into designing high-pe...

Designing Pt-Based Electrocatalysts with High Surface Energy

The reactivity of an electrocatalyst depends strongly on its surface structure. Pt-based electrocatalysts of nanocrystals (NCs) enclosed with high-index facets contain a large density of catalytically active sites formed from step and kink atoms on the facets and exhibit intrinsically superior activity. However, the Pt-based NCs of high-index facets do possess a high surface energy and are thermodynamically metastable, leading to a big challenge in their shape-controlled synthesis. To overcome the challenge, kinetic–thermodynamic control of crystal growth is indispensable and is currently realized mainly by electrochemical methods and surfactant-based wet chemical approaches. This Perspective reviews recent progresses in Pt-based electrocatalysts of monometallic and bimetallic NCs of high surface energy with different morphologies of convex or concave tetrahexahedron, trapezohedron, trisoctahedron, hexoctahedron, etc. Remarkable electrocatalytic performance of these NCs has been demonstrated. Despite the ...

The effect of crystal size variation on the rate of dissolution – A kinetic Monte Carlo study

Crystal size is an important parameter for many geochemical processes, and is often observed to have a significant influence on crystal dissolution rate. Although size versus solubility relationships are often cast in thermodynamic terms (the critical radius), the mechanistic basis for size versus dissolution rate is not understood in any detail. Here we examine the influence of size on dissolution rate and the mechanistic origin of this relationship isolated from other parameters. We use a kinetic Monte Carlo model approach to observe atomistic changes in the dissolution of four simple cubic crystals (edge dimension d=25, 64, 125, 187 unit cells). These simulations maintain constant solution boundary conditions representing an under-saturated, far-from-equilibrium state. We observe that surface area-normalized dissolution rates increase non-linearly with decreasing initial crystal size. This relationship can be understood mechanistically, in part as a coupling of kink- and step atom-site formation. Under conditions where these two constraints are in rough balance, the overall dissolution rate is maximized and mass removal is most efficient. This relationship reflects the mutual dependence of the formation of kink and step atoms, whose high detachment frequencies exert the dominant control over the dissolution process.

The Oxygen Reduction Reaction on Kinked Stepped Surfaces of Pt

The oxygen reduction reaction (ORR) has been studied on kinked stepped surfaces of Pt inside the stereographic triangle with the use of rotating disk electrode (RDE) in 0.1 M HClO4. The kinked stepped surfaces are composed of (331) = 3(111)-(111) structure of which step line contains (100) kink: 3(111) − [m(111) + (100)], where m denotes the number of straight step atoms. The activity for the ORR decreases with increasing the kink atom density d k of the surface with m = 1, 3, 6, and 21, whereas the surface with m = 11 has specifically high activity for the ORR. The activity on Pt(16 15 5) = 3(111) − [11(111)-(100)] exceeds that of Pt(331) that gives the highest activity in stepped surfaces. The electricity of Pt oxide formation on Pt(16 15 5) m = 11 is the lowest in the kinked stepped surfaces examined.

Self-Assembly of Protein Nanofibrils Orchestrates Calcite Step Movement through Selective Nonchiral Interactions.

The recognition of atomically distinct surface features by adsorbed biomolecules is central to the formation of surface-templated peptide or protein nanostructures. On mineral surfaces such as calcite, biomolecular recognition of, and self-assembly on, distinct atomic kinks and steps could additionally orchestrate changes to the overall shape and symmetry of a bulk crystal. In this work, we show through in situ atomic force microscopy (AFM) experiments that an acidic 20 kDa cement protein from the barnacle Megabalanus rosa (MRCP20) binds specifically to step edge atoms on {101̅4} calcite surfaces, remains bound and further assembles over time to form one-dimensional nanofibrils. Protein nanofibrils are continuous and organized at the nanoscale, exhibiting striations with a period of ca. 45 nm. These fibrils, templated by surface steps of a preferred geometry, in turn selectively dissolve underlying calcite features displaying the same atomic arrangement. To demonstrate this, we expose the protein solution to bare and fibril-associated rhombohedral etch pits to reveal that nanofibrils accelerate only the movement of fibril-forming steps when compared to undecorated steps exposed to the same solution conditions. Calcite mineralized in the presence of MRCP20 results in asymmetric crystals defined by frustrated faces with shared mirror symmetry, suggesting a similar step-selective behavior by MRCP20 in crystal growth. As shown here, selective surface interactions with step edge atoms lead to a cooperative regime of calcite modification, where templated long-range protein nanostructures shape crystals.

Adsorption and ring-opening of lactide on the chiral metal surface Pt(321)(S) studied by density functional theory.

We study the adsorption and ring-opening of lactide on the naturally chiral metal surface Pt(321)(S). Lactide is a precursor for polylactic acid ring-opening polymerization, and Pt is a well known catalyst surface. We study, here, the energetics of the ring-opening of lactide on a surface that has a high density of kink atoms. These sites are expected to be present on a realistic Pt surface and show enhanced catalytic activity. The use of a naturally chiral surface also enables us to study potential chiral selectivity effects of the reaction at the same time. Using density functional theory with a functional that includes the van der Waals forces in a first-principles manner, we find modest adsorption energies of around 1.4 eV for the pristine molecule and different ring-opened states. The energy barrier to be overcome in the ring-opening reaction is found to be very small at 0.32 eV and 0.30 eV for LL- and its chiral partner DD-lactide, respectively. These energies are much smaller than the activation energy for a dehydrogenation reaction of 0.78 eV. Our results thus indicate that (a) ring-opening reactions of lactide on Pt(321) can be expected already at very low temperatures, and Pt might be a very effective catalyst for this reaction; (b) the ring-opening reaction rate shows noticeable enantioselectivity.

Synthesis of Convex Hexoctahedral Pt Micro/Nanocrystals with High-Index Facets and Electrochemistry-Mediated Shape Evolution

Systematic manipulation of nanocrystal shapes is prerequisite for revealing their shape-dependent physical and chemical properties. Here we successfully prepared a complex shape of Pt micro/nanocrystals: convex hexoctahedron (HOH) enclosed with 48 {15 5 3} high-index facets by electrochemical square-wave-potential (SWP) method. This shape is the last crystal single form that had not been achieved previously for face-centered-cubic (fcc) metals. We further realized the shape evolution of Pt nanocrystals with high-index facets from tetrahexahedron (THH) to the HOH, and finally to trapezohedron (TPH) by increasing either the upper (EU) or lower potential (EL). The shape evolution, accompanied by the decrease of low-coordinated kink atoms, can be correlated with the competitive interactions between preferentially oxidative dissolution of kink atoms at high EU and the redeposition of Pt atoms at the EL.

ChemInform Abstract: Nanomaterials of High Surface Energy with Exceptional Properties in Catalysis and Energy Storage

AbstractReview: 139 refs.

Nanomaterials of high surface energy with exceptional properties in catalysis and energy storage

The properties of nanomaterials for use in catalytic and energy storage applications strongly depends on the nature of their surfaces. Nanocrystals with high surface energy have an open surface structure and possess a high density of low-coordinated step and kink atoms. Possession of such features can lead to exceptional catalytic properties. The current barrier for widespread industrial use is found in the difficulty to synthesise nanocrystals with high-energy surfaces. In this critical review we present a review of the progress made for producing shape-controlled synthesis of nanomaterials of high surface energy using electrochemical and wet chemistry techniques. Important nanomaterials such as nanocrystal catalysts based on Pt, Pd, Au and Fe, metal oxides TiO(2) and SnO(2), as well as lithium Mn-rich metal oxides are covered. Emphasis of current applications in electrocatalysis, photocatalysis, gas sensor and lithium ion batteries are extensively discussed. Finally, a future synopsis about emerging applications is given (139 references).

Surface X-ray scattering of high index plane of platinum containing kink atoms in solid–liquid interface: Pt(3 1 0) = 3(1 0 0)–(1 1 0)

Surface structure of Pt(3 1 0) = 3(1 0 0)–(1 1 0), which contains kink atoms in the step, has been determined with the use of in situ surface X-ray scattering (SXS) in the double layer region (0.50 V(RHE)) in 0.1 M HClO 4. Clean Pt(3 1 0) surface has pseudo (1 × 1) structure on which lateral displacements of 2–9% and 0.3–1% are found along a and b directions, respectively, whereas the surfaces of Pt(1 1 0) = 2(1 1 1)–(1 1 1) and Pt(3 1 1) = 2(1 0 0)–(1 1 1) are reconstructed to (1 × 2) according to previous reports. Interlayer spacing between the first and the second layers d 12 is contracted about 5% compared with the bulk spacing, whereas those between underlying layers are expanded down to fourth layer. Fully adsorbed CO has no effect on the surface structure of Pt(3 1 0). This result differs from that on Pt(1 1 1), where d 12 is expanded after CO adsorption.

In Situ Infrared Reflection Absorption Spectroscopy of Carbon Monoxide Adsorbed on Pt(S)-[n(100)×(110)] Electrodes

Structural effects on the adsorption of CO have been studied using infrared reflection absorption spectroscopy (IRAS) on Pt(S)-[n(100)x(110)] surfaces (n = 2, 5, 9) that have densely packed kink atoms in the step. Coverage and potential dependence of the IRAS spectra are scrutinized. On-top and bridge-bonded CO are found on all of the surfaces examined. CO is adsorbed on only kink at low coverage (thetaCO < or = 0.2). Adsorbed CO on kink gives an IR band at lower frequency than that on step. CO is adsorbed on both kink and terrace at 0.3 < or = thetaCO. Water is adsorbed on the terrace of Pt(510) n = 5 and Pt(910) n = 9 at low CO coverage, but water is not found on Pt(210) n = 2 of which the first layer is composed of only kink atoms. It is suggested that coadsorbed water on the terrace enhances the activity for the oxidation of adsorbed CO on the kink remarkably.

Structural effects on the oxidation of formic acid on the high index planes of palladium

Electrochemical oxidation of formic acid has been studied on the stepped and kinked-stepped surfaces of Pd in 0.1 M HClO 4 containing 0.1 M formic acid with the use of voltammetry. The surfaces examined are Pd(S)-[ n(1 0 0) × (1 1 0)], Pd(S)-[ n(1 1 1) × (1 0 0)] and Pd(S)-[ n(1 1 1) × (1 1 1)] series ( n = 2–9). The results are compared with those of Pd(S)-[ n(1 0 0) × (1 1 1)] series reported previously. All the electrodes give maximum currents of formic acid oxidation j P between 0.5 and 0.8 V (RHE). The values of j P plotted against the density of step (kink) atoms d S depend on the surface structure remarkably. Pd(S)-[ n(1 1 1) × (1 0 0)] surfaces provide maximum of j P at n = 5, whereas Pd(S)-[ n(1 0 0) × (1 1 0)] and Pd(S)-[ n(1 1 1) × (1 1 1)] do not give maximum of j P. The values of j P have the following order: Pd(S)-[ n(1 1 1) × (1 1 1)] < Pd(S)-[ n(1 1 1) × (1 0 0)] < Pd(S)-[ n(1 0 0) × (1 1 0)] < Pd(S)-[ n(1 0 0) × (1 1 1)]. The anodic current at more negative potential 0.20 V (RHE) shows different activity series: Pd(1 1 1) and Pd(1 1 0) have the highest rate for formic acid oxidation at 0.20 V (RHE).