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X-ray absorption spectroscopy is utilized to study the distribution of Au and Ag within the metal core of the Au 104 Ag 40 (PET) 60 nanocluster, revealing a Ag single-atom alloy structure associated with unique bonding properties.
Single-atom alloy (SAA) catalysis research often reports that a SAA catalyst, in the general formulation of a single-atom metal M1 alloyed on the surface of the host metal M2, facilitates a probe reaction. However, for catalytic reactions that present decoupled rate- and selectivity-limiting steps, the alloying site density may significantly manipulate these independent steps, but it has rarely been explicitly examined for any SAA systems. Herein, using the electrocatalytic CO reduction as a probe reaction, we report that the nominal Pd 1 Cu cube SAA catalysts exhibit distinctive high reactivity toward ethylene or ethanol, respectively, depending on whether the Pd atoms are in dilute or crowded forms. Although the presence of single-atom Pd embedded on Cu uniformly promotes CHO* formation and C─C coupling, the dilute-Pd 1 Cu favors ethylene formation by enabling low-barrier C─O cleavage from a flat CH 2 CH 2 OH* intermediate, whereas the crowded-Pd 1 Cu promotes ethanol formation by stabilizing an upright hydrogenation transition state of the same intermediate. Furthermore, we present evidence that the catalytic chemistry of crowded Pd 1 species differs from that of the Pd 2 -dimer; the latter, albeit unstable, steers reaction selectivity to acetate instead. In conclusion, these results uncovered the underappreciated importance of controlling SAA catalytic chemistry from the perspective of single-atom site densities.
Single-atom alloys (SAAs) are promising materials for heterogeneous catalysis due to their unique structure and electronic properties. SAAs have active sites narrowed down to the single-atom level, which combines the advantages of alloy materials and single-site catalysts. Given the unique structural feature of SAAs, their electronic properties can be more flexibly tailored than for their monometallic counterparts, which can be used to effectively control their catalytic activities. One interesting feature commonly observed for SAAs is the lower density of state (DOS) near the Fermi level than their bulk references. Comparing with results for their monometallic bulk reference, the most noticeable electronic property change in SAAs is the narrowing of the valence band, which gives them free-atom-like character. Moreover, the d-band position of both single atoms and their host metals can show a pronounced shift. These changes of electronic structure in SAAs could largely affect the adsorption behavior of adsorbates during the catalytic processes. Close examination of the relationship between electronic structure and catalytic activity can provide useful guidance for rational design of new catalysts with improved performance.
Structurally ordered intermetallic nanocrystals (NCs) and single-atom catalysts (SACs) are two emerging catalytic motifs for sustainable chemical production and energy conversion. However, both have synthetic limitations which can lead to the aggregation of NCs or metal atoms. Single-atom alloys (SAAs), which contain isolated metal atoms in a host metal, can overcome the aggregation concern because of the thermodynamic stabilization of single atoms on host metal surfaces. We report a direct solution-phase synthesis of Cu/CuAu core/shell NCs with tunable SAA layers. This synthesis can be extended to other Cu/CuM (M = Pt, Pd) systems, in which M atoms are isolated in the copper host. Using this method, the density of SAAs on a copper surface can be controlled, resulting in both low and high densities of single atoms. Alloying gold into the copper matrix introduced ligand effects that optimized the chemisorption of *NO 3 and *N. As a result, the densely packed Cu/CuAu material demonstrated a high selectivity toward NH 3 from the electrocatalytic nitrate reduction reaction with an 85.5% Faradaic efficiency while maintaining a high yield rate of 8.47 mol h -1 g -1 . This work advances the design of atomically precise catalytic sites by creating core/shell NCs with SAA atomic layers, opening an avenue for broad catalytic applications.
We demonstrate that single-atom alloy catalysts can be made by exposing physical mixtures of monometallic supported Cu and Pd catalysts to vinyl acetate (VA) synthesis reaction conditions. This reaction induces the formation of mobile clusters of metal diacetate species that drive extensive metal nanoparticle restructuring, leading to atomic dispersion of the precious metal, smaller nanoparticle sizes than the parent catalysts, and high activity and selectivity for both VA synthesis and ethanol dehydrogenation reactions. As a result, this approach is scalable and appears to be generalizable to other alloy catalysts.
There is widespread interest in developing catalysts with uniform active sites that consist of single atoms, thereby simplifying the reaction mechanism and improving product selectivity. Here, we examine experimentally how CO can be used to modify the active sites on a strong binding single-atom alloy and examine how this in turn impacts product selectivity for a reaction that has two different pathways. Specifically, we find that CO can be used to selectively block isolated Rh atom active sites in a RhCu(111) model single-atom alloy catalyst surface and promote the dehydrogenation pathway for adsorbed ethyl groups by suppressing the hydrogenation pathway.
Ethanol is an important source of clean hydrogen, acetaldehyde, acetic acid, acetate esters, and light hydrocarbons. Controlling the divergent reaction pathways to these products requires understanding how different active sites influence the elementary steps involved. Herein, we present a combined surface science, theory, and nanoparticle catalysis study demonstrating how two single-atom dopants (Rh and Ni) in a Cu host can distinctively alter the selectivity of alcohol conversion. Specifically, our model studies reveal that ethanol reacts on Ni 1 Cu single-atom alloys to selectively produce acetaldehyde, whereas methane and CO are also formed on Rh 1 Cu single-atom alloys. Interestingly, these different reactivities are in contrast to the behavior of the pure metals as Ni(111) and Rh(111) surfaces favor methane/CO and surface carbon/CO, respectively. DFT calculations of reaction pathways and simulated product desorption based on microkinetic analyses explain these reactivity differences, demonstrating that C–C cleavage leading to methane formation has a lower barrier on Rh single-atom sites. To test the catalytic relevance of these fundamental results we synthesized and characterized supported Ni 1 Cu and Rh 1 Cu single-atom alloy nanoparticles with dopant:Cu ratios of 1:200. Flow reactor results revealed that both Ni and Rh increased ethanol conversion over Cu and that Ni 1 Cu catalysts were >99.9% selective to acetaldehyde, while Rh 1 Cu also produced 0.6%–2.6% of equimolar methane and CO between 433 and 493 K, demonstrating that C–C bond cleavage is enabled by isolated Rh sites. Furthermore, these catalytic results bridge the pressure and materials gaps, and together, this study provides insights into how different isolated dopant sites promote different catalytic pathways.
Electrocatalytic CO 2 reduction into value-added multicarbon products offers a means to close the anthropogenic carbon cycle using renewable electricity. However, the unsatisfactory catalytic selectivity for multicarbon products severely hinders the practical application of this technology. In this paper, we report a cascade AgCu single-atom and nanoparticle electrocatalyst, in which Ag nanoparticles produce CO and AgCu single-atom alloys promote C-C coupling kinetics. As a result, a Faradaic efficiency (FE) of 94 ± 4% toward multicarbon products is achieved with the as-prepared AgCu single-atom and nanoparticle catalyst under ~720 mA cm -2 working current density at -0.65 V in a flow cell with alkaline electrolyte. Density functional theory calculations further demonstrate that the high multicarbon product selectivity results from cooperation between AgCu single-atom alloys and Ag nanoparticles, wherein the Ag single-atom doping of Cu nanoparticles increases the adsorption energy of *CO on Cu sites due to the asymmetric bonding of the Cu atom to the adjacent Ag atom with a compressive strain.
The selective oxidation of methanol to formaldehyde on Cu surfaces is an important and well-studied reaction. However, a systematic analysis of product selectivity as a function of oxygen coverage on Cu(111) and Cu-based single-atom alloys (SAAs) has not been previously reported. In this work, we present a comprehensive investigation of deuterated methanol (CD 3 OH) partial oxidation on Cu(111) and 1% PtCu(111) SAA surfaces as a function of preadsorbed oxygen coverage. Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) reveal that isolated Pt atoms inhibit the initial surface oxidation of Cu(111) under low oxygen exposures. Despite this difference in oxidation kinetics, the product selectivity remains largely unaffected: on both Cu(111) and PtCu(111), formaldehyde (CD 2 O) is the predominant partial oxidation product over a broad range of oxygen coverages. The selectivity toward formaldehyde peaks at intermediate oxygen coverages (∼0.3 monolayers, ML), indicating the existence of an optimal oxygen loading for partial oxidation. Notably, the similar product selectivities on Cu(111) and PtCu(111) over a range of surface oxygen coverage indicate that Pt single atoms do not significantly alter the reaction pathway or shift the optimal oxygen coverage for formaldehyde formation. Control experiments confirm that Cu(111) is unreactive toward methanol in the absence of oxygen, while PtCu(111) surfaces produce a small amount of formaldehyde even when oxygen is not preadsorbed, indicating that isolated Pt atoms facilitate O−H activation at below 150 K, leading to H 2 desorption, followed by C−D activation at higher temperatures (∼350 K). Density functional theory (DFT)-based calculations show that Pt atoms increase the O 2 dissociation barrier relative to Cu(111), consistent with the observed inhibition of oxidation at low exposures. Overall, this work provides the first detailed selectivity map for methanol oxidation on oxidized Cu(111) and PtCu(111) SAA surfaces. By linking classical mechanistic insights such as methoxy- and formate-mediated pathways with single-atom alloy catalyst design, this work demonstrates that while Pt substitution modulates the oxidation kinetics and oxygen binding, the overall selectivity toward formaldehyde is governed primarily by oxygen coverage. These findings underscore the potential of isolated dopants to tune surface oxidation behavior without compromising the intrinsic partial oxidation selectivity of copper-based catalysts.
Ethanol dehydrogenation presents a promising pathway towards the production of acetaldehyde, a valuable building block in chemicals production. Under non-oxidative conditions, the reaction is facilitated by supported Cu nanoparticles which afford reasonable activity and high selectivity. The stability issues associated with Cu nanoparticle sintering can be addressed by the addition of small amounts of Ni, which further boost reactivity while retaining selectivity. Despite the promise of NiCu single-atom alloys for non-oxidative ethanol dehydrogenation, little is known about the role of each component and the pathway of this mechanistically complex process. Herein, kinetic investigations from reactor tests identify C-H bond scission as the rate limiting step, while 1-hydroxyethyl is detected as the intermediate via IR spectroscopy. Temperature program desorption studies are employed to examine the effect of Ni coverage and to demonstrate that Ni atoms activate ethanol selectively at lower temperatures, resulting in higher acetaldehyde yield than pure Cu. Temperature program desorption experiments also reveal the spillover of intermediates from the Ni atom to neighboring Cu sites as a relevant step in the reaction pathway. Density functional theory calculations are used to investigate the 2 reaction energetics and to confirm that C-H bond scission is the initial reaction step, while a clear effect of H 2 partial pressure on the reaction pathway is realized. Further, counter to the expected behavior that all reaction steps take place on the Ni atoms, our degree of rate control analysis reveals that a mechanism involving spillover of the 1-hydroxyethyl intermediate from the Ni atom to the Cu surface, where it will dehydrogenate further, is more likely. Furthermore, our combined kinetic, spectroscopic, and theoretical approach sheds light on this complex reaction mechanism and represents a promising method for the understanding and designing of highly active, selective, and stable single-atom alloys for other multistep catalytic processes.
Harvesting recyclable ammonia (NH 3 ) from the electrocatalytic reduction of nitrate (NO 3 RR) offers a sustainable strategy to close the ecological nitrogen cycle from nitration contamination in an energy-efficient and environmentally friendly manner. The emerging intermetallic single-atom alloys (ISAAs) are recognized to achieve the highest site density of single atoms by isolating contiguous metalatoms into single sites stabilized by another metal within the intermetallic structure, which holds promise to couple the catalytic benefits from intermetallic nanocrystals and single-atom catalysts for promoting NO 3 RR. Herein, ISAA In-Pd bimetallene, in which the Pd single atoms are isolated by surrounding In atoms, is reported to boost neutral NO 3 RR with a NH 3 Faradaic efficiency(FE) of 87.2%, a yield rate of 28.06 mg h -1 mg Pd -1 , and an exceptional electrocatalytic stability with increased activity/selectivity over 100 h and 20 cycles. The ISAA structure induces substantially diminished overlap of Pdd-orbitals and narrowed p-d hybridization of In-p and Pd-d states around the Fermi level, resulting in a stronger NO 3 - adsorption and a depressed energy barrier of the potential-determining step for NO 3 RR. Finally, further integrating the NO 3 RR catalyst into a Zn-NO 3 - flow battery as the cathode delivers a power density of 12.64 mW cm -2 and a FE of 93.4% for NH 3 production.
The development of cost-effective and highly efficient electrocatalysts is critical to help electrochemical non-enzymatic sensors achieve high performance. Here, in this work, a new class of catalyst, Ru single atoms confined on Cu nanotubes as a single-atom alloy (Ru 1 Cu NTs), with a unique electronic structure and property, was developed to construct a novel electrochemical non-enzymatic glucose sensor for the first time. The Ru 1 Cu NTs with a diameter of about 24.0 nm showed a much lower oxidation potential (0.38 V) and 9.0-fold higher response (66.5 μA) current than Cu nanowires (Cu NWs, oxidation potential 0.47 V and current 7.4 μA) for glucose electrocatalysis. Moreover, as an electrochemical non-enzymatic glucose sensor, Ru1Cu NTs not only exhibited twofold higher sensitivity (54.9 μA mM −1 cm −2 ) and wider linear range (0.5–8 mM) than Cu NWs, but also showed a low detection limit (5.0 μM), excellent selectivity, and great stability. According to theoretical calculation results, the outstanding catalytic and sensing performance of Ru 1 Cu NTs could be ascribed to the upshift of the d-band center that helped promote glucose adsorption. This work presents a new avenue for developing highly active catalysts for electrochemical non-enzymatic sensors.
The identification of thermodynamic descriptors of catalytic performance is essential for the rational design of heterogeneous catalysts. Here, we investigate how spillover energy, a descriptor quantifying whether intermediates are more stable at the dopant or host metal sites, can be used to design single-atom alloys (SAAs) for formic acid dehydrogenation. Further, using theoretical calculations, we identify NiCu as a SAA with favorable spillover energy and demonstrate that formate intermediates produced after the initial O–H activation are more stable at Ni sites where rate-determining C–H activation occurs. Surface science experiments demonstrated that NiCu(111) SAAs are more reactive than Cu(111) while they still follow the formate reaction pathway. However, reactor studies of silica-supported NiCu SAA nanoparticles showed only a modest improvement over Cu resulting from surface coverage effects. Overall, this study demonstrates the potential of engineering SAAs using spillover energy as a design parameter and highlights the importance of adsorbate–adsorbate interactions under steady-state operation.
Electrocatalytic nitrate reduction reaction (NO 3 – RR) technology provides a promising solution to recover the nitrate nutrition from wastewater through catalyzing nitrate reduction into value-added NH 3 . However, the selectivity and efficiency of electrocatalysts are frustrated due to the imbalance of *H adsorption (for NO 3 hydrogenation) and unavoidable adjacent *H self-coupling on active sites, resulting in competitive hydrogen evolution reaction (HER). Here, we report a PdCu single-atom alloy (SAA) catalyst that allows isolated Pd sites to produce *H for the hydrogenation process of *NO 3 on neighboring Cu sites, which can restrain the *H self-coupling through extending the distance between two *H and thus effectively suppress competitive HER. Consequently, the PdCu SAA catalyst exhibits an ultrahigh NH 3 Faraday efficiency (FE) of 97.1% with a yield of 15.4 μmol cm –2 h –1 from the electrocatalytic NO 3 – RR in the neutral electrolyte, outperforming most of the reported catalysts. In conclusion, single-crystal experiments and theoretical calculations further prove that the introduction of atomic Pd on the Cu (100) surface could serve as the main active site and greatly decrease the energy barrier of the rate-determining step (RDS) on Cu from ΔG = 0.39 eV (*NOO → *NOOH) to ΔG = 0.10 eV of *NOH → *NHOH on PdCu SAA.
Catalysts research for electrocatalytic CO 2 reduction reactions (CO 2 R) has undergone rapid growth in the last decade. Single-atom alloy catalysts (SAAs) featuring atomically dispersed metal dopants on host metal surfaces have shown promises in boosting CO 2 R yield by optimizing the structure and synergy of the catalytic metals at the atomic scale. Despite the exciting development of SAAs for CO 2 R in fundamental science, dedicated studies for its engineering implementation have been absent. We use this perspective to discuss our non-exhaustive engineering considerations for implementing SAAs for CO 2 R. Here, the perspective starts with a brief overview of the current research status for SAAs in CO 2 R, followed by focal points on structure uncertainties associated with catalyst manufacturing, catalyst layer degradation during reaction, and possibilities for SAAs to mitigate the salt precipitation issue at the device level. We hope our opinions will engage increasing attention toward the engineering catalysis research for applying SAAs to CO 2 R at scale.
Reactive separations of CO/CO 2 mixtures are a promising pathway to lower the energy requirement of CO 2 hydrogenation to chemicals and fuels, with applications in the U.S. Navy’s seawater-to-fuel process. With the CO/CO 2 feedstock, a challenge is activating CO to produce heavier hydrocarbons while preventing CO 2 methanation, requiring low-temperature Fischer-Tropsch synthesis (FTS) catalysts. In this work, we demonstrate that a Ru–Co single atom alloy (SAA) catalyst produces C 5+ hydrocarbons at a rate of 11.7 μmol/s/g-cobalt (hexane basis) in a 50/50 CO/CO 2 stream with ≤1% CO 2 conversion. The reaction operates at a relatively low temperature (200 °C) and high gas hourly space velocity (GHSV: 84,000 mL/g/h) that is compatible with the upstream reverse water-gas shift reaction. Detailed experiments, catalyst characterizations, and density functional theory (DFT) calculations have been conducted to understand the active phase, the role of the Ru dopant, and catalyst restructuring that occurs at elevated temperatures (>200 °C). Ru dopants are found to promote the reduction of Co species, enabling catalytic activity for CO hydrogenation without pre-reduction, but may not enhance the FTS activity or desired C 5+ hydrocarbon selectivity.
Here, picturing the atomic migration pathways of catalysts in a reactive atmosphere is of central significance for uncovering the underlying catalytic mechanisms and directing the design of high-performance catalysts. Here, we describe a reduction-controlled atomic migration pathway that converts nanoparticles to single atom alloys (SAAs), which has remained synthetically challenging in prior attempts due to the elusive mechanism. We achieved this by thermally treating the noble-metal nanoparticles M (M = Ru, Rh, Pd, Ag, Ir, Pt, and Au) on metal oxide (CuO) supports with H2/Ar. Atomic-level characterization revealed such conversion as the synergistic consequence of noble metal-promoted H2 dissociation and concomitant CuO reduction. The observed atomic migration pathway offers an understanding of the dynamic mechanisms study of nanomaterials formation and catalyst design.
Throughout its relatively short lifetime, ultra-high vacuum (UHV) surface chemistry has progressed quickly. In the 1960's, pioneers like Ertl and Somorjai started the field using single crystals and gained significant insight into catalytic processes by relating surface structure to reactivity. The more recent proliferation of scanning probes has significantly increased the power of the single crystal approach by enabling the atomic-scale structure of active sites to be correlated with their reactivity. In this perspective we briefly discuss how the field developed, identify some challenges, and highlight Single-Atom Alloys (SAAs), a new class of heterogeneous catalyst that was developed from a fundamental surface science approach. However, despite recent successes, funding for fundamental surface science has declined. Academic hires in the discipline are also declining in part due to the start-up costs. We make the case that fundamental UHV surface chemistry is still too young a field to be in recession.