Linear Scaling Relations Research Articles

Reconstructing the Coordination Environment of Fe/Co Dual-atom Sites towards Efficient Oxygen Electrocatalysis for Zn-Air Batteries.

Dual-atom catalysts with nitrogen-coordinated metal sites embedded in carbon can drive the oxygen reduction and evolution reactions (ORR/OER) in rechargeable zinc-air batteries (ZABs), and the further improvement is limited by the linear scaling relationship of intermediate binding energies in the absorbate evolution mechanism (AEM). Triggering the lattice oxygen mechanism (LOM) is promising to overcome this challenge, but has yet been verified since the lacking of bridge oxygen (O) in the rigid coordination environment of the metal centers. Here, we demonstrate that suitably tailored dual-atom catalysts of FeCo-N-C can undergo out-plane and in-plane reconstruction to form the both axial O and bridge O at the metal centers, and thus activate the LOM pathway. The tailored FeCo-N-C with shortened Fe-N bonds also favor the ORR process, therefore is a promising dual-atom oxygen catalyst. The assembled rechargeable ZABs demonstrate a peak power density of 332 mW cm-2, and exhibit no notable decline after ~ 720 h of continuous cycling.

Regulation of Oxide Pathway Mechanism for Sustainable Acidic Water Oxidation.

The advancement of acid-stable oxygen evolution reaction (OER) electrocatalysts is crucial for efficient hydrogen production through proton exchange membrane (PEM) water electrolysis. Unfortunately, the activity of electrocatalysts is constrained by a linear scaling relationship in the adsorbed evolution mechanism, while the lattice-oxygen-mediated mechanism undermines stability. Here, we propose a heterogeneous dual-site oxide pathway mechanism (OPM) that avoids these limitations through direct dioxygen radical coupling. A combination of Lewis acid (Cr) and Ru to form solid solution oxides (CrxRu1-xO2) promotes OH adsorption and shortens the dual-site distance, which facilitates the formation of *O radical and promotes the coupling of dioxygen radical, thereby altering the OER mechanism to a Cr-Ru dual-site OPM. The Cr0.6Ru0.4O2 catalyst demonstrates a lower overpotential than that of RuO2 and maintains stable operation for over 350 h in a PEM water electrolyzer at 300 mA cm-2. This mechanism regulation strategy paves the way for an optimal catalytic pathway, essential for large-scale green hydrogen production.

Propane Dehydrogenation on PtxZny Active Sites in Silicalite‐1

AbstractThe improvement of Pt‐based catalysts for propane dehydrogenation (PDH) has progressed by recent investigations that have identified Zn as a promising promoter for Pt subnanometer catalysts. It is desirable to gain insights into the structure, stability, and activity of such active sites and the factors that influence them, such as Zn : Pt ratio, Pt coordination and nuclearity. Here, we employ density functional theory and microkinetic simulations to investigate the stability of PtxZny (x=1–3, y=0–3) active sites grafted on silanols of Silicalite‐1 and the PDH activity of Pt. We find that the coordination of a Pt atom to a nest of grafted Zn(II) atoms increases the stability of the Pt1Zny sites, whose activity is similar for y=0–2 and drops dramatically for y>2. We further demonstrate, via linear scaling relations and microkinetic simulations, that the turnover frequency obeys a volcano law as a function of propylene binding strength. The Pt2Zn1 and Pt3Zn1 sites are stable and exhibit activity similar to Pt1Zn2, but only Pt1Zn2 manifests reaction kinetics consistent with experimental data, strongly suggesting the active site composition in the synthesized catalyst samples. The methodology presented here suggests a general strategy for deducing active site information such as composition through simple kinetic experiments.

Designing plausible catalysts for synthesis of zingerone from vanillin and acetone via aldol condensation reaction on O-terminated M3C2-MXenes

Herein, high-performance O-terminated M3C2 MXene catalysts were screened for the synthesis of zingerone using vanillin and acetone. The electrochemical and electronic properties of O-terminated ordered MXenes in the form of M3C2O2(H) [M = Ti, V, and Cr (3d); Zr, Nb, and Mo (4d); and Hf, Ta, and W (5d)] were investigated using first principle density functional theory (DFT) calculations. These calculations revealed 37 stable candidates based on their negative binding energy values of ΔG*H and three O-terminated M3C2 MXene candidates with superior zingerone synthesis activity than pure precious metal (platinum), a UL > −0.4 V can guarantee high activity and low energy consumption during zingerone synthesis. Ti3C2O (FCC)), Hf3C2O2 (HCPFCC), and Nb3C2O2 (HCPHCP) exhibited higher catalytic activity than other candidates, and their performances were competitive for zingerone synthesis, with low limiting potentials of −0.37, −0.27, and −0.35 V, respectively. Zingerone synthesis via the protolysis reaction between vanillin and acetone includes two paths, i.e., induced protolysis and direct protolysis via the aldol condensation reaction. Multiple descriptors were used to evaluate and screen promising candidates for the synthesis of zingerone. For example, ΔG(*vanillin*acetone), and Bader charge, and d-band center analyses revealed the origin of zingerone synthesis activity based on binding energy and electronic structure. Importantly, binding energy and electronic structure have a strong intrinsic linear scaling relationship that can be used to screen for the optimal catalyst by controlling the scaling relationship between the intermediate binding energy and electronic properties. Indicating a scaling relationship and volcano plot were established based on ΔG(*vanillin+*acetone) and ΔG[*zingiberone] depending on Limiting potential (UL). Finally, we proposed a method for directly inducing protolysis during the aldol condensation reaction, which may be widely applicable to different biomass-derived carbonyl compounds, yielding chemicals and medicinal compounds through biomass valorization.

Single and dual-atom catalysts towards electrosynthesis of ammonia and urea: a review.

Ammonia and urea represent two important chemicals that have contributed to the rapid development of humanity. However, their industrial production requires harsh conditions, consuming excessive energy and resulting in significant greenhouse gas emission. Therefore, there is growing interest in the electrocatalytic synthesis of ammonia and urea as it can be carried out under ambient conditions. Recently, atomic catalysts (ACs) have gained increased attention for their superior catalytic properties, being able to outperform their micro and nano counterparts. This review examines the advantages and disadvantages of ACs and summarises the advancement of ACs in the electrocatalytic synthesis of ammonia and urea. The focus is on two types of AC - single-atom catalysts (SACs) and diatom catalysts (DACs). SACs offer various advantages, including the 100% atom utilization that allows for low material mass loading, suppression of competitive reactions such as hydrogen evolution reaction (HER), and alternative reaction pathways allowing for efficient synthesis of ammonia and urea. DACs inherit these advantages, possessing further benefits of synergistic effects between the two catalytic centers at close proximity, particularly matching the NN bond for N2 reduction and boosting C-N coupling for urea synthesis. DACs also possess the ability to break the linear scaling relation of adsorption energy of reactants and intermediates, allowing for tuning of intermediate adsorption energies. Finally, possible future research directions using ACs are proposed.

Integrating few-atom layer metal on high-entropy alloys to catalyze nitrate reduction in tandem

While high-entropy alloy (HEA) catalysts seem to have the potential to break linear scaling relationships (LSRs) due to their structural complexity, the weighted averaging of properties among multiple principal components actually makes it challenging to diverge from the symmetry dependencies imposed by the LSRs. Herein, we develop a ‘surface entropy reduction’ method to induce the exsolution of a component with weak affinity for others, resulting in the formation of few-atom-layer metal (FL-M) on the surface of HEAs. These exsolved FL-M surpass the confines of the original configurational space of conventional HEAs, and collaborate with the HEA substrate, serving as geometrically separated active sites for multiple intermediates in a complex reaction. This FL-M-covered HEA shows an outstanding performance for electrocatalytic reduction of nitrate to ammonia (NH3) with a Faradaic efficiency of 92.7%, an NH3 yield rate of 2.45 mmol h–1 mgcat.–1, and high long-term stability (>200 h). Our work achieves the precise manipulation of atomic arrangement, thereby expanding both the chemical space occupied by known HEA catalysts and their potential application scenarios.

Fluorine-Doped Carbon Support Enables Superfast Oxygen Reduction Kinetics by Breaking the Scaling Relationship.

It is well-established that Pt-based catalysts suffer from the unfavorable linear scaling relationship (LSR) between *OOH and *OH (ΔG(*OOH)=ΔG(*OH)+3.2±0.2 eV) for the oxygen reduction reaction (ORR), resulting in a great challenge to significantly reduced ORR overpotentials. Herein, we propose a universal and feasible strategy of fluorine-doped carbon supports, which optimize interfacial microenvironment of Pt-based catalysts and thus significantly enhance their reactive kinetics. The introduction of C-F bonds not only weakens the *OH binding energy, but also stabilizes the *OOH intermediate, resulting in a break of LSR. Furthermore, fluorine-doped carbon constructs a local super-hydrophobic interface that facilitates the diffusion of H2O and the mass transfer of O2. Electrochemical tests show that the F-doped carbon-supported Pt catalysts exhibit over 2-fold higher mass activities than those without F modification. More importantly, those catalysts also demonstrate excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests. This study not only validates the feasibility of tuning the electrocatalytic microenvironment to improve mass transport and to break the scaling relationship, but also provides a universal catalyst design paradigm for other gas-involving electrocatalytic reactions.

Fluorine‐Doped Carbon Support Enables Superfast Oxygen Reduction Kinetics by Breaking the Scaling Relationship

AbstractIt is well‐established that Pt‐based catalysts suffer from the unfavorable linear scaling relationship (LSR) between *OOH and *OH (ΔG(*OOH)=ΔG(*OH)+3.2±0.2 eV) for the oxygen reduction reaction (ORR), resulting in a great challenge to significantly reduced ORR overpotentials. Herein, we propose a universal and feasible strategy of fluorine‐doped carbon supports, which optimize interfacial microenvironment of Pt‐based catalysts and thus significantly enhance their reactive kinetics. The introduction of C−F bonds not only weakens the *OH binding energy, but also stabilizes the *OOH intermediate, resulting in a break of LSR. Furthermore, fluorine‐doped carbon constructs a local super‐hydrophobic interface that facilitates the diffusion of H2O and the mass transfer of O2. Electrochemical tests show that the F‐doped carbon‐supported Pt catalysts exhibit over 2‐fold higher mass activities than those without F modification. More importantly, those catalysts also demonstrate excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) tests. This study not only validates the feasibility of tuning the electrocatalytic microenvironment to improve mass transport and to break the scaling relationship, but also provides a universal catalyst design paradigm for other gas‐involving electrocatalytic reactions.

Regulating Local Coordination Sphere of Ir Single Atoms at the Atomic Interface for Efficient Oxygen Evolution Reaction.

Single-atom catalysts dispersed on an oxide support are essential for overcoming the sluggishness of the oxygen evolution reaction (OER). However, the durability of most metal single-atoms is compromised under harsh OER conditions due to their low coordination (weak metal-support interactions) and excessive disruption of metal-Olattice bonds to enable lattice oxygen participation, leading to metal dissolution and hindering their practical applicability. Herein, we systematically regulate the local coordination of Irsingle-atoms at the atomic level to enhance the performance of the OER by precisely modulating their steric localization on the NiO surface. Compared to conventional Irsingle-atoms adsorbed on NiO surface, the atomic Ir atoms partially embedded within the NiO surface (Iremb-NiO) exhibit a 2-fold increase in Ir-Ni second-shell interaction revealed by X-ray absorption spectroscopy (XAS), suggesting stronger metal-support interactions. Remarkably, Iremb-NiO with tailored coordination sphere exhibits excellent alkaline OER mass activity and long-term durability (degradation rate: ∼1 mV/h), outperforming commercial IrO2 (∼26 mV/h) and conventional Irsingle-atoms on NiO (∼7 mV/h). Comprehensive operando X-ray absorption and Raman spectroscopies, along with pH-dependence activity tests, identified high-valence atomic Ir sites embedded on the NiOOH surface during the OER followed the lattice oxygen mechanism, thereby circumventing the traditional linear scaling relationships. Moreover, the enhanced Ir-Ni second-shell interaction in Iremb-NiO plays a crucial role in imparting structural rigidity to Ir single-atoms, thereby mitigating Ir-dissolution and ensuring superior OER kinetics alongside sustained durability.

Uncertainty Quantification of Linear Scaling, Machine Learning, and Density Functional Theory Derived Thermodynamics for the Catalytic Partial Oxidation of Methane on Rhodium.

Accurate and complete microkinetic models (MKMs) are powerful for anticipating the behavior of complex chemical systems at different operating conditions. In heterogeneous catalysis, they can be further used for the rapid development and screening of new catalysts. Density functional theory (DFT) is often used to calculate the parameters used in MKMs with relatively high fidelity. However, given the high cost of DFT calculations for adsorbates in heterogeneous catalysis, linear scaling relations (LSRs) and machine learning (ML) models were developed to give rapid estimates of the parameters in MKM. Regardless of the method, few studies have attempted to quantify the uncertainty in catalytic MKMs, as the uncertainties are often orders of magnitude larger than those for gas phase models. This study explores uncertainty quantification and Bayesian Parameter Estimation for thermodynamic parameters calculated by DFT, LSRs, and GemNet-OC, a ML model developed under the Open Catalyst Project. A model for catalytic partial oxidation of methane (CPOX) on Rhodium was chosen as a case study, in which the model's thermodynamic parameters and their associated uncertainties were determined using DFT, LSR, and GemNet-OC. Markov Chain Monte Carlo coupled with Ensemble Slice Sampling was used to sample the highest probability density (HPD) region of the posterior and determine the maximum of the a posteriori (MAP) for each thermodynamic parameter included. The optimized microkinetic models for each of the three estimation methods had quite similar mechanisms and agreed well with the experimental data for gas phase mole fractions. Exploration of the HPD region of the posterior further revealed that adsorbed hydroxide and oxygen likely bind on facets other than Rhodium 111. The demonstrated workflow addresses the issue of inaccuracies arising from the integration of data from multiple sources by considering both experimental and computational uncertainties, and further reveals information about the active site that would not have been discovered without considering the posterior.

Integrating Active Learning and DFT for Fast-Tracking Single-Atom Alloy Catalysts in CO2-to-Fuel Conversion.

Electrocatalytic carbon dioxide reduction (CO2RR) technology enables the conversion of excessive CO2 into high-value fuels and chemicals, thereby mitigating atmospheric CO2 concentrations and addressing energy scarcity. Single-atom alloys (SAAs) possess the potential to enhance the CO2RR performance by full utilization of atoms and breaking linear scaling relationships. However, quickly screening high-performance metal portfolios of SAAs remains a formidable challenge. In this study, we proposed an active learning (AL) framework to screen high-performance catalysts for CO2RR to yield fuels such as CH4 and CH3OH. After four rounds of AL iterations, the ML model attained optimal prediction performance with the test set R2 of approximately 0.94 and successful prediction was achieved for the binding free energy of *CHO, *COH, *CO, and *H on 380 catalyst surfaces with an accuracy within 0.20 eV. Subsequent analysis of the SAA catalysts' activity, selectivity, and stability led to the discovery of eight previously unexplored SAA catalysts for CO2RR. Notably, the surface activity of Ti@Cu(100), Au@Pt(100), and Ag@Pt(100) shines prominently. Utilizing DFT calculations, we elucidated the complete reaction pathway of the CO2RR on the surfaces of these catalysts, confirming their high catalytic activity with limiting potentials of -0.11, -0.34, and -0.46 eV, respectively, which are significantly lower than those of pure Cu catalysts. The results showcase the exceptional predictive prowess of AL, providing a valuable reference for the design of CO2RR catalysts.

Transport of a comb-like polymer across a nanochannel subject to a pulling force.

We investigate the dynamics of comb-like polymer translocation through a nanochannel using three-dimensional Langevin dynamics simulations based on a coarse-grained chain model. A comprehensive set of simulations are performed to examine the effects of system parameters such as the grafting densityρof the side chains, the polymer chain length, the nanochannel dimensions, and the magnitude of the pulling force on the translocation dynamics. For a given polymer chain length, keeping the backbone length is constant while varyingρ, we have found that the dependence of the mean translocation time⟨τ⟩onρis non-monotonic, with a maximum translocation time for a specificρat which the translocation is the slowest. The simulation results also show that⟨τ⟩is not significantly affected by the channel width above a certain radius, while the comb-like polymer translocation is hindered by a narrower channel due to increased interactions between the chain monomers and the channel. In addition,⟨τ⟩increases linearly with the nanochannel length. A linear scaling relationship between the mean translocation time⟨τ⟩and the chain lengthNof polymer is obtained,⟨τ⟩∼N. Similarly, the dependence of⟨τ⟩on the backbone chain sizeNbbhas a quasi-linear dependence,⟨τ⟩∼Nbb. On the other hand, the translocation velocityvfollows a power-law relationship with the polymer chain lengthNasv∼N-1. The mean translocation time also shows an inverse linear relationship with the magnitude of the pulling forceF,⟨τ⟩∼F-1. The power-law relationships discovered in this study contribute to the fundamental understanding of the comb polymer translocation dynamics and to establishing a framework for further investigations in this field.

Propane Dehydrogenation on PtxZny Active Sites in Silicalite-1.

The improvement of Pt-based catalysts for propane dehydrogenation (PDH) has progressed by recent investigations that have identified Zn as a promising promoter for Pt subnanometer catalysts. It is desirable to gain insights into the structure, stability, and activity of such active sites and the factors that influence them, such as Zn:Pt ratio, Pt coordination and nuclearity. Here, we employ density functional theory and microkinetic simulations to investigate the stability of PtxZny (x=1-3, y=0-3) active sites grafted on silanols of Silicalite-1 and the PDH activity of Pt. We find that the coordination of a Pt atom to a nest of grafted Zn(II) atoms increases the stability of the Pt1Zny sites, whose activity is similar for y=0-2 and drops dramatically for y>2. We further demonstrate, via linear scaling relations and microkinetic simulations, that the turnover frequency obeys a volcano law as a function of propylene binding strength. The Pt2Zn1 and Pt3Zn1 sites are stable and exhibit activity similar to Pt1Zn2, but only Pt1Zn2 manifests reaction kinetics consistent with experimental data, strongly suggesting the active site composition in the synthesized catalyst samples. The methodology presented here suggests a general strategy for deducing active site information such as composition through simple kinetic experiments.

Investigating episodic mass loss in evolved massive stars

Mass loss during the red supergiant (RSG) phase plays a crucial role in the evolution of an intermediate-mass star; however, the underlying mechanism remains unknown. We aim to increase the sample of well-characterized RSGs at subsolar metallicity by deriving the physical properties of 127 RSGs in nine nearby southern galaxies. For each RSG, we provide spectral types and used MARCS atmospheric models to measure stellar properties from their optical spectra, such as the effective temperature, extinction, and radial velocity. By fitting the spectral energy distribution, we obtained the stellar luminosity and radius for 92 RSGs, finding that ~50% of them have log(L/L⊙) ≥ 5.0 and six RSGs have R ≳ 1400 R⊙. We also find a correlation between the stellar luminosity and mid-IR excess of 33 dusty variable sources. Three of these dusty RSGs have luminosities exceeding the revised Humphreys-Davidson limit. We then derived a metallicity-dependent J − Ks color versus temperature relation from synthetic photometry and two new empirical J − Ks color versus temperature relations calibrated on literature TiO and J-band temperatures. To scale our derived cool TiO temperatures to values that are in agreement with the evolutionary tracks, we derived two linear scaling relations calibrated on J-band and i-band temperatures. We find that the TiO temperatures are more discrepant as a function of the mass-loss rate, and discuss future prospects of the TiO bands as a mass-loss probe. Finally, we speculate that three hot dusty RSGs may have experienced a recent mass ejection (12% of the K-type sample) and classify them as candidate Levesque-Massey variables.

The Role of Surface Roughening in Improving the Selectivity of Copper for CO2 Electroreduction

Electroreduction of CO2 (eCO2RR) has emerged as a promising alternative for production of renewable fuels and important commodity chemicals such as ethylene and ethanol. Simultaneously, it opens an economically attractive path to mitigate point sources of CO2, a potent greenhouse gas. Despite decades of research efforts, Cu remains the only catalyst capable of producing significant amounts of desirable C2+ products, containing more than one carbon atom. However, further improvement is needed for industrial applications. It is known that roughened copper-based electrocatalysts surfaces exhibit enhanced selectivity towards C2+ products during electroreduction of CO2. This poses a challenge for modeling efforts aiming to find a rationale for the enhanced selectivity. In this work, we use an innovative modeling approach to investigate roughened copper surfaces derived from cuprous oxide on structures of 100x100 nm2 size with atomic resolution and accuracy compared to density functional theory (DFT). We combine semilocal DFT (RPBE) in VASP, effective medium theory (EMT), and linear scaling relationships via the recently proposed alpha parameter scheme to investigate the site-resolved catalytic selectivity of copper sites. Electrode potential-dependence is included via the use of the VASPsol implicit solvation and a grand canonical potential approach. The approach allows us to investigate the effect of varied macroscopic roughness on the intrinsic catalytic activity of copper by comparing surfaces exposed to simulated sputtering and electropolishing, respectively. The study highlights i) the challenge for designing improved copper catalysts for CO2RR with monodisperse active site selectivity, ii) the need for careful experimentation when comparing to theory, and iii) the limitations of macroscopic roughness in controlling atomic scale activity.

Selective electroreduction of CO2 to C2+ products on cobalt decorated copper catalysts

Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR) to multi-carbon (C2+) products is often plagued by low selectivity because the adsorption energies of different reaction intermediates are in a linear scaling relationship. Development of Cu-based bimetallic catalysts has been considered as an attractive strategy to address this issue; however, conventional bimetallic catalysts often avoid metals with strong CO adsorption energies to prevent surface poisoning. Herein, we demonstrated that limiting the amount of Co in CuCo bimetallic catalysts can enhance C2+ product selectivity. Specifically, we synthesized a series of CuCox catalysts with trace amounts of Co (0.07-1.8 at%) decorated on the surface of Cu nanowires using a simple dip coating method. Our results revealed a volcano-shaped correlation between Co loading and C2+ selectivity, with the CuCo0.4% catalyst exhibiting a 2-fold increase in C2+ selectivity compared to the Cu nanowire sample. In situ Raman and Infrared spectroscopies suggested that an optimal amount of Co could stabilize the Cu oxide/hydroxide species under the CO2RR condition and promote the adsorption of CO, thus enhancing the C2+ selectivity. This work expands the potential for developing Cu-based bimetallic catalysts for CO2RR.

Integrative catalytic pairs for efficient multi-intermediate catalysis.

Single-atom catalysts (SACs) have attracted considerable research interest owing to their combined merits of homogeneous and heterogeneous catalysts. However, the uniform and isolated active sites of SACs fall short in catalysing complex chemical processes that simultaneously involve multiple intermediates. In this Review, we highlight an emerging class of catalysts with adjacent binary active centres, which is called integrative catalytic pairs (ICPs), showing not only atomic-scale site-to-site electronic interactions but also synergistic catalytic effects. Compared with SACs or their derivative dual-atom catalysts (DACs), multi-interactive intermediates on ICPs can overcome kinetic barriers, adjust reaction pathways and break the universal linear scaling relations as the smallest active units. Starting from this active-site design principle, each single active atom can be considered as a brick to further build integrative catalytic clusters (ICCs) with desirable configurations, towards trimer or even larger multi-atom units depending on the requirement of a given reaction.

Fault size-dependent fracture energy explains multiscale seismicity and cascading earthquakes.

Earthquakes vary in size over many orders of magnitude, often rupturing in complex multifault and multievent sequences. Despite the large number of observed earthquakes, the scaling of the earthquake energy budget remains enigmatic. We propose that fundamentally different fracture processes govern small and large earthquakes. We combined seismological observations with physics-based earthquake models, finding that both dynamic weakening and restrengthening effects are non-negligible in the energy budget of small earthquakes. We established a linear scaling relationship between fracture energy and fault size and a break in scaling with slip. We applied this scaling using supercomputing and unveiled large dynamic rupture earthquake cascades involving >700 multiscale fractures within a fault damage zone. We provide a simple explanation for seismicity across all scales with implications for comprehending earthquake genesis and multifault rupture cascades.

Dual-Intermetallic Heterostructure on Hierarchical Nanoporous Metal for Highly Efficient Alkaline Hydrogen Electrocatalysis.

Constructing well-defined active multisites is an effective strategy to break linear scaling relationships to develop high-efficiency catalysts toward multiple-intermediate reactions. Here, dual-intermetallic heterostructure composed of tungsten-bridged Co3W and WNi4 intermetallic compounds seamlessly integrated on hierarchical nanoporous nickel skeleton is reported as a high-performance nonprecious electrocatalyst for alkaline hydrogen evolution and oxidation reactions. By virtue of interfacial tungsten atoms configuring contiguous multisites with proper adsorptions of hydrogen and hydroxyl intermediates to accelerate water dissociation/combination and column-nanostructured nickel skeleton facilitating electron and ion/molecule transportations, nanoporous nickel-supported Co3W-WNi4 heterostructure exhibits exceptional hydrogen electrocatalysis in alkaline media, with outstanding durability and impressive catalytic activities for hydrogen oxidation reaction (geometric exchange current density of ≈6.62 mA cm-2) and hydrogen evolution reaction (current density of ≈1.45 A cm-2 at overpotential of 200 mV). Such atom-ordered intermetallic heterostructure alternative to platinum group metals shows genuine potential for hydrogen production and utilization in hydroxide-exchange-membrane water electrolyzers and fuel cells.

H2-D2 Exchange Activity and Electronic Structure of Ag x Pd1-x Alloy Catalysts Spanning Composition Space.

Many computational studies of catalytic surface reaction kinetics have demonstrated the existence of linear scaling relationships between physical descriptors of catalysts and reaction barriers on their surfaces. In this work, the relationship between catalyst activity, electronic structure, and alloy composition was investigated experimentally using a Ag x Pd1-x Composition Spread Alloy Film (CSAF) and a multichannel reactor array that allows measurement of steady-state reaction kinetics at 100 alloy compositions simultaneously. Steady-state H2-D2 exchange kinetics were measured at atmospheric pressure on Ag x Pd1-x catalysts over a temperature range of 333-593 K and a range of inlet H2 and D2 partial pressures. X-ray photoelectron spectroscopy (XPS) was used to characterize the CSAF by determining the local surface compositions and the valence band electronic structure at each composition. The valence band photoemission spectra showed that the average energy of the valence band, ε̅v, shifts linearly with composition from -6.2 eV for pure Ag to -3.4 eV for pure Pd. At all reaction conditions, the H2-D2 exchange activity was found to be highest on pure Pd and gradually decreased as the alloy was diluted with Ag until no activity was observed for compositions with x Pd < 0.58. Measured H2-D2 exchange rates across the CSAF were fit using the Dual Subsurface Hydrogen (2H') mechanism to extract estimates for the activation energy barriers to dissociative adsorption, ΔE ads ‡, associative desorption, ΔE des ‡, and the surface-to-subsurface diffusion energy, ΔE ss, as a function of alloy composition, x Pd. The 2H' mechanism predicts ΔE ads ‡ = 0-10 kJ/mol, ΔE des ‡ = 30-65 kJ/mol, and ΔE ss = 20-30 kJ/mol for all alloy compositions with x Pd ≥ 0.64, including for the pure Pd catalyst (i.e., x Pd = 1). For these Pd-rich catalysts, ΔE des ‡ and ΔE ss appeared to increase by ∼5 kJ/mol with decreasing x Pd. However, due to the coupling of kinetic parameters in the 2H' mechanism, we are unable to exclude the possibility that the kinetic parameters predicted when x Pd ≥ 0.64 are identical to those predicted for pure Pd. This suggests that H2-D2 exchange occurs only on bulk-like Pd domains, presumably due to the strong interactions between H2 and Pd. In this case, the decrease in catalytic activity with decreasing x Pd can be explained by a reduction in the availability of surface Pd at high Ag compositions.