Adam Holewinski

Single-atom alloys (SAAs) are an emerging class of heterogeneous catalysts that maximize atom efficiency by embedding isolated reactive metal atoms within a host metal surface, creating well-defined active sites. Atomic isolation imparts unique catalytic properties, including high selectivity, resistance to poisoning, coking, and sintering, and in some cases the ability to circumvent conventional scaling relationships that limit traditional catalysts. This article focuses on progress in the SAA field since our 2020 review (Chem. Rev.2020, 120, 12044−12088.), while incorporating earlier foundational studies where relevant. We first examine the fundamental principles governing SAA formation, stability, electronic structure, reactivity, and selectivity, drawing on insights from experimental single-crystal model systems and theoretical studies. We then review the application of SAAs in a wide range of industrially important reactions, including hydrogenation, dehydrogenation, selective oxidation, water–gas shift, Fischer–Tropsch synthesis, and dry reforming, among others. Beyond thermal catalysis, SAAs demonstrate great promise in electrocatalysis and plasmon-driven chemistry. Their structurally simple active sites have also enabled the rational design of novel catalysts and promoters that challenge conventional catalytic intuition. Looking ahead, recent advances in computing, including machine-learning potentials, offer the opportunity to explore vast compositional spaces and discover new alloy chemistries for challenging reactions using the SAA concept.

High-entropy oxides (HEOs) offer vast compositional design space for discovering emergent functionalities, yet their controlled nanoscale synthesis remains challenging. Here, we develop a generalizable colloidal strategy that enables precision synthesis of HEO nanocrystals with compositions spanning quinary to septenary systems. Mechanistic studies reveal that differences in precursor reactivity drive a multistage growth pathway and that cooperative multimetal chemistry─where one metal initiates single-phase nucleation and a vacancy-forming metal promotes cation redistribution during growth─enables homogeneous multication incorporation. The resulting rocksalt HEO nanocrystals can be transformed into spinel phases and exhibit excellent oxygen-evolution reaction activity. Machine-learning-accelerated theoretical analysis and operando characterization identify Co–Co bridge sites on spinel {111} facets as the dominant oxygen-evolution active motifs operating through a lattice oxygen-mediated mechanism. These findings establish guiding principles for controlling nucleation, cation mixing, and active-site formation in compositionally complex oxides, enabling the rational design and data-driven optimization of high-entropy materials.

Homogeneous electrocatalysts (“mediators”) are often used in organic electrosynthesis to overcome electron transfer inhibitions and to control coupled chemical reactions. However, separating the mediators after completed electrolysis poses a challenge that is typically solved by column chromatography. In this context, the polymediator concept was previously developed, allowing for separation of mediator‐modified polymers using scalable size‐exclusive membrane processes. A problem that remained to be solved is the loss of catalytic activity with increasing molecular weight of the polymediator due to curbed mass transport, resulting in prolonged electrolysis durations. In the present report, we describe an approach to overcome this dilemma by suspending conductive carbon particles in the electrolyte. Using the 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO)‐catalyzed alcohol oxidation as a test reaction, the starting material is electrolyzed in a carbon particle suspension in the presence of TEMPO‐modified polymethacrylate (TPMA), leading to a significantly accelerated reaction compared to particle‐free conversion while maintaining high selectivity. In addition to investigating the influence of various parameters such as particle type and loading, substrate scope, scalability, and recyclability were examined, whereby promising results were obtained. Possible explanations for the enhanced performance—such as improved mass transport and mixing effects, as well as the formation of conductive networks—are discussed in detail.

Multi-component electrocatalysts provide wide possibilities to tune reactivity, though the fundamental origins of modification, such as electronic perturbation or bifunctionality, are often ambiguous. Here we demonstrate methods to elucidate operative mechanisms on such materials with microkinetic analysis based on generalized degrees of rate control and second-order kinetic observables. CO electro-oxidation kinetics are analyzed on a series of carbon-supported Ag _x Pd _100-x alloys to determine apparent CO and OH ^– reaction orders, and apparent transfer coefficients (inverse Tafel slopes). Each of these kinetic observables (derivatives of rate) varies across operating conditions, providing a rich dataset with second-order dependencies. The analysis reveals that OH ^– interacts with Ag sites through only partial charge transfer before coupling to Pd-bound CO. Maximum-likelihood fitting of binding energies, corroborated by grand-canonical density functional theory, also supports electronic effects favoring CO-OH coupling. The observation of non-integer charge transfer steps underscores a broader need for rigorous kinetic characterization beyond Tafel slope analysis.

Electrocatalytic reactions involve interfacial interactions between the surfaces of electrodes and reactive species at an electrolyte interface. There are presently no universal or unambiguous methods to directly assay the active top atomic layer composition that influences the reactivity of these electrodes under relevant operating conditions. Low-energy ion scattering (LEIS) spectroscopy is a surface characterization technique that yields compositional analysis of the outermost atomic layer of a material, but it must be performed in ultrahigh vacuum (UHV). Application of LEIS measurements to electrochemical materials that are removed from ambient liquid-phase environments thus leaves an open question as to whether the surface that is transferred to UHV is truly the surface that manifested during the electrochemical reaction. Toward the goal of preserving the active surface state, we developed a sample transfer workflow for LEIS enabling air-free removal and drying of an electrode from an electrochemical cell while maintaining control of the potential using an auxiliary electrode. The potential-controlled emersion method was demonstrated to give distinct potential-dependent surface compositions for a Cu–Pd alloy relative to removal after uncontrolled return to open-circuit potential. A Cu-enriched surface was found at anodic potential and a Pd-enriched surface at cathodic potential, suggesting that the approach can be used to retain representative atomic configurations during transfer. Since adsorbates will often persist from the reaction environment, conventional sample pretreatment methods for removal, including atomic O and atomic H exposure, were also contrasted. Both methods were found to differ with results from incidental low-dose depth profiling by the LEIS primary ion source, which removes adventitious species and surface atoms during the course of repeated measurements. These depth profiles were found to be sensitive to sample history and thus qualitatively informative, despite the possible changes induced by ion damage. The results exhibit (i) the need for complete control over the polarization state of the sample at all times (no excursions to open circuit during transfer) and (ii) the utility of low-dose depth profiling to capture changes in the near-surface composition.

Multi-component electrocatalysts provide wide possibilities to tune reactivity, though the fundamental origins of modification, such as electronic perturbation or bifunctionality, are often ambiguous. Here we demonstrate methods to elucidate operative mechanisms on such materials with microkinetic analysis based on generalized degrees of rate control and second-order kinetic observables. CO electro-oxidation kinetics are analyzed on a series of carbon-supported Ag _x Pd _100-x alloys to determine apparent CO and OH ^– reaction orders, and apparent transfer coefficients (inverse Tafel slopes). Each of these kinetic observables (derivatives of rate) varies across operating conditions, providing a rich dataset with second-order dependencies. The analysis reveals that OH ^– interacts with Ag sites through only partial charge transfer before coupling to Pd-bound CO. Maximum-likelihood fitting of binding energies, corroborated by grand-canonical density functional theory, also supports electronic effects favoring CO-OH coupling. The observation of non-integer charge transfer steps underscores a broader need for rigorous kinetic characterization beyond Tafel slope analysis.

Electrochemical valorization of biomass-derived substrates has become a prominent area of research due to its potential to produce value-added products from renewable feedstocks in a more sustainable way. First-row transition-metal electrodes are compelling candidates for these conversions due to their stability, abundance, and cost-effectiveness. Herein, we report on the colloidal synthesis of Mn_xZn_1-xO (x = 0.3-0.7) nanoparticles and their electrocatalytic activity toward furfural oxidation. We find that the hindrance of a MnO impurity can be achieved by leveraging the oxidation state of the Mn precursor. The Mn_xZn_1-xO composition closely follows the ratio of precursors, with all of the nanoparticles having a wurtzite structure as determined by ICP-MS and PXRD, respectively. XANES and XPS revealed the presence of Mn in different oxidation states, with the ratio of these varying based on the composition. When comparing the electrocatalytic activity of the monometallic and bimetallic oxides for furfural oxidation, a decrease in current density was observed with increasing Zn content. We find that the Mn_xZn_1-xO nanoparticles favor the formation of the 6 e^- oxidation product 5-hydroxy-2(5H)-furanone, while both monometallic oxides primarily yield CO₂ and other deeply oxidized products. These findings can contribute toward the design and synthesis of more active and selective electrocatalysts.