Electronic Structure of Single-Atom Alloys and Its Impact on The Catalytic Activities

New Insights into the ORR Catalysis on Pt Alloy Nanoparticles from an Element Specific d-Band Analysis

Bolstered by their unique atomic structures and tailored compositions, nanoalloys exhibit extraordinary properties making them ideal materials to solve challenges in energy storage and conversion catalysis. However, a quantitative description of the structure-property relationships using an accurate descriptor-based model for nanoalloys, ranging from bimetallic to multimetallic compositions, is needed to drive efficient material design toward high-performance catalysis. In this work, we highlight the electronic property and catalytic activity relationship from an element specific d-band analysis of Pt-based alloy catalysts using X-ray absorption near-edge spectroscopy (XANES). Using a series of L10-MPt/Pt (M = Fe, Co, Ni) core/shell alloy catalysts with well-defined atomic structures, we quantified subtle differences in the Pt d-electron states and correlated the Pt d-band structure to their superior catalytic activity toward the oxygen reduction reaction (ORR). Our analysis used the upper d-band edge position as a predictive descriptor for the mass activity toward the ORR instead of the commonly used d-band center position. Together with density functional theory calculations and Nørskov d-band theory, the upper d-band edge position for the Pt states, derived from experimental measurements, elucidates new physical insights into the ORR performance of the L10-MPt/Pt core/shell catalysts. An element specific Pt d-band analysis using XANES overcomes challenges in traditional X-ray photoelectron spectroscopy-based valence d-band analysis, which cannot distinguish signals from independent elements in nanoalloys. Thus, the insights from the element specific d-band analysis presented in this work are a promising approach to determine structure-property relationships in a variety of transition metal nanoalloys and will be useful in the design of future high-performance catalysts.

Structural Analysis of Single-Atom Catalysts by X-ray Absorption Spectroscopy

ConspectusMetal nanoparticles (NPs) are one of the most frequently used heterogeneous catalysts. However, only the surface atoms in the NPs can participate in catalytic reactions. To maximize the atomic efficiency, the active sites can be reduced to single atoms. Generally, catalysts that have isolated metal atoms on the surface of a support are called single-atom catalysts (SACs). Many techniques have been developed and applied to probe the structures of SACs. Nevertheless, the structural characterization of SACs is still challenging as it requires the analysis of their structure and properties with atomic and sometimes even subatomic resolution. X-ray absorption spectroscopy (XAS) is a powerful tool in investigating the local coordination environment of SACs since it is element-specific and can provide accurate structural information at the subatomic level (∼0.01 Å).In this Account, we present our perspectives on the structural analysis of SACs from some unique features in the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). We first highlight the importance of the XANES peak features in the sensitive analysis of SAC structures. Such analysis is illustrated to be even more useful in the joint applications of experimental and theoretical XAS. The inspection of the metal-metal (M-M) peak in Fourier transformed EXAFS (FT-EXAFS) spectra is a widely used method to identify the single-atom structure, but this method is not always reliable. Thus, we point out the importance of fitting EXAFS and the thorough interpretation of structural parameters such as coordination numbers (CNs, the number of neighboring atoms next to a chosen atom), bond distances, and the Debye-Waller factor (σ2). The small FT-EXAFS peak for the M-M shell is often ignored in the structural analysis of SACs. Here, it is demonstrated that a careful analysis of these small peaks could help more reliably analyze the SAC structure, and it would be particularly useful in the analysis of a single-atom alloy (SAA). Next, the usefulness of bond distance and σ2 analysis is highlighted, and such analysis is shown to be particularly helpful for the analysis of SAAs, which is rarely discussed in the literature. Given the advantage that XAS data can be collected under various conditions, we show that in situ XAS can provide important information about the catalytic mechanism of the SAC catalyst. In particular, we emphasize the significance of using an advanced in situ technique to extract detailed structural information that is difficult to obtain from regular XAS experiments. Finally, we highlight the importance of jointly using XAS with other complementary methods in a more complete understanding of the structure and properties of SACs. It is anticipated that with further development of XAS techniques and improved data analysis, XAS will become even more powerful in providing insights into the structure-property relationships of SACs, which can advance their practical applications.

Bridging Together Theoretical and Experimental Perspectives in Single-Atom Alloys for Electrochemical Ammonia Production

Ammonia is an essential commodity in the food and chemical industry. Despite the energy-intensive nature, the Haber-Bosch process is the only player in ammonia production at large scales. Developing other strategies is highly desirable, as sustainable and decentralized ammonia production is crucial. Electrochemical ammonia production by directly reducing nitrogen and nitrogen-based moieties powered by renewable energy sources holds great potential. However, low ammonia production and selectivity rates hamper its utilization as a large-scale ammonia production process. Creating effective and selective catalysts for the electrochemical generation of ammonia is critical for long-term nitrogen fixation. Single-atom alloys (SAAs) have become a new class of materials with distinctive features that may be able to solve some of the problems with conventional heterogeneous catalysts. The design and optimization of SAAs for electrochemical ammonia generation have recently been significantly advanced. This comprehensive review discusses these advancements from theoretical and experimental research perspectives, offering a fundamental understanding of the development of SAAs for ammonia production.

Facile Hydrogenolysis of Sugars to 1,2-Glycols by Ru@PPh3/OPPh3 Confined Large-Pore Mesoporous Silica

Tandem hydrogenation vis-à-vis hydrogenolysis of xylose to 1,2-glycols remains a major challenge. Although one-pot conversion of xylose to 1,2-glycols requires stringent conditions, a sustainable approach would be quite noteworthy. We have developed a microwave route for the one-pot conversion of pentose (C5) and hexose (C6) sugars into glycol and hexitol, without pressurized hydrogen reactors. A pronounced hydrogenolysis of sugars to glycols is observed by Ru single atom (SA) on triphenylphosphine/phosphine oxide-modified silica (Ru@SiP), in contrast to Ru SA on pristine (Ru@SiC) and 3-aminopropyl-modified silica (Ru@SiN). A promising "ligand effect" was observed through phosphine modification of silica that presents a 70% overall yield of all reduced sugars (xylitol + glycols) from a 99% conversion of xylose with Ru@SiP. A theoretical study by DFT depicts an electronic effect on Ru-SA by triphenylphosphine that promotes the catalytic hydrogenolysis of sugars under mild conditions. Hence, this research represents an important step for glycols from biomass-derived sources.

Insights into the mechanism in electrochemical CO2 reduction over single-atom copper alloy catalysts: A DFT study

Copper single-atom alloy catalysts (M@Cu SAAs) have shown great promise for electrochemical CO2 reduction reaction (CO2RR). However, a clear understanding of the CO2RR process on M@Cu SAAs is still lacking. This study uses density functional theoretical (DFT) calculations to obtain a comprehensive mechanism and the origin of activity of M@Cu SAAs. The importance of the adsorption mode of M@Cu is revealed: key intermediates either adsorbed in the adjacent hollow site around Cu atoms (AD mode) or adsorbed directly on the top site of M (SE mode). AD mode generally exhibits finely tuned binding strengths of key intermediates, which significantly enhances the activity of the catalysts. Increasing the coverage of ∗CO on the M@Cu with SE mode leads to relocation of the active site, resulting in improved activity of C2 products. The insights gained in this work have significant implications for rational design strategy toward efficient CO2RR electrocatalysts.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Challenges and Opportunities in Engineering the Electronic Structure of Single-Atom Catalysts

Controlling the electronic structure of transition-metal single-atom heterogeneous catalysts (SACs) is crucial to unlocking their full potential. The ability to do this with increasing precision offers a rational strategy to optimize processes associated with the adsorption and activation of reactive intermediates, charge transfer dynamics, and light absorption. While several methods have been proposed to alter the electronic characteristics of SACs, such as the oxidation state, band structure, orbital occupancy, and associated spin, the lack of a systematic approach to their application makes it difficult to control their effects. In this Perspective, we examine how the electronic configuration of SACs can be engineered for thermochemical, electrochemical, and photochemical applications, exploring the relationship with their activity, selectivity, and stability. We discuss synthetic and analytical challenges in controlling and discriminating the electronic structure of SACs and possible directions toward closing the gap between computational and experimental efforts. By bringing this topic to the center, we hope to stimulate research to understand, control, and exploit electronic effects in SACs and ultimately spur technological developments.

Free-atom-like d states beyond the dilute limit of single-atom alloys

Through a data-mining and high-throughput density functional theory approach, we identify a diverse range of metallic compounds that are predicted to have transition metals with "free-atom-like" d states that are highly localized in terms of their energetic distribution. Design principles that favor the formation of localized d states are uncovered, among which we note that site isolation is often necessary but that the dilute limit, as in most single-atom alloys, is not a pre-requisite. Additionally, the majority of localized d state transition metals identified from the computational screening study exhibit partial anionic character due to charge transfer from neighboring metal species. Using CO as a representative probe molecule, we show that localized d states for Rh, Ir, Pd, and Pt tend to reduce the binding strength of CO compared to their pure elemental analogues, whereas this does not occur as consistently for the Cu binding sites. These trends are rationalized through the d-band model, which suggests that the significantly reduced d-band width results in an increased orthogonalization energy penalty upon CO chemisorption. With the multitude of inorganic solids that are predicted to have highly localized d states, the results of the screening study are likely to result in new avenues for heterogeneous catalyst design from an electronic structure perspective.

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, E_PCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy ε_LUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, ε_LUS is obtained from only a single DFT calculation but can predict E_PCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts.