Integration of metal species with zeolites catalyzes a wide range of molecular transformations. Among these, cobalt ions stabilized within aluminosilicate zeolites (Co-ZSM-5) catalyze oxidative activation of ethane at practical temperatures (e.g., 648 K). Rate measurement demonstrates that C2H6 and O2 react at isolated monomeric cobalt sites. The introduction of NH3, however, forms mobile cobalt-ammine complexes that persist even at high temperatures requisite for C-H activation. These complexes possess sufficient mobility to dimerize and form oxygen-bridged dicobalt-ammine centers, which activate C-H bonds and form C-N bonds at 648 K, evidenced by in situ UV-vis, XAS, Raman spectroscopies, and kinetic analysis. Quantum chemical and molecular dynamics simulations reveal that the zeolite framework atomically disperses the cationic cobalt atoms, facilitates the spontaneous formation of cobalt-ammine complexes in the presence of NH3, and then enables dimerization with O2 by screening repulsive forces between the cobalt-ammines. These findings demonstrate an unusual transformation of active sites during catalytic turnover in zeolites: dimeric organometallic complexes competent for multistep reactions may form and decompose during each catalytic turnover to bypass kinetic hurdles encountered at monomeric structures.

Metal-oxide-supported Pt-group metal catalysts are essential for industrial reactions, including CO oxidation in automotive emission control. Maximizing the utilization of these metals is crucial for designing efficient catalysts. Therefore, understanding how the metal electronic properties and metal–support interactions change with metal nuclearity, especially in the subnanometer regime, is essential for advancing the catalytic performance. This study investigates the reactivity of CO oxidation on Pt supported on anatase TiO2 with varying Pt nuclearity from single-atoms, 0.9 nm clusters to 1.8 nm nanoparticles. Kinetic measurements and in-situ infrared spectroscopy reveal that CO oxidation activity and the Pt resistance to oxidation increase with nuclearity. Additionally, in-situ infrared and X-ray absorption spectroscopy results show that the electron density on Pt under CO oxidation increases with Pt nuclearity. The reaction mechanism is shown to change as a function of nuclearity. For 0.9 nm clusters and 1.8 nm nanoparticles, TiO2 lattice oxygen is involved in the reaction, likely via the Mars–van Krevelen mechanism. In contrast, on single-atoms, oxygen from the lattice is not labile, and extra oxygen adatoms adsorbed on the Pt-Ti interface participate in CO2 formation, leading to a significantly lower activity than 0.9 nm and 1.8 nm Pt. Kinetic analysis coupled with temperature-programmed reduction by CO reveals that the higher activity is due to lower oxygen vacancy formation energy induced by larger Pt nuclearity. Overall, our study demonstrates that Pt nuclearity influences the reducibility of TiO2 through electronic interaction, which gives rise to the structure-sensitivity of CO oxidation on Pt/TiO2 catalyst.

In-situ and operando techniques in heterogeneous electrocatalysis are a powerful tool used to elucidate reaction mechanisms. Ultimately, they are key in determining concrete links between a catalyst's physical/electronic structure and its activity en route to designing next-generation systems. To this end, the exact execution and interpretation of these lines of experiments is critical as this determines the strength of conclusions that can be drawn and what uncertainties remain. Instead of focusing on how techniques were used to understand systems, as is the case with most reviews on the topic, this work instead initiates a nuanced discussion of 1) how to best carry out each technique and 2) initiate a nuanced analysis of which level of insights can be drawn from the set of in-situ or operando experiments/controls carried out. We focus on several commonly used techniques, including vibrational (IR, Raman) spectroscopy, X-ray absorption spectroscopy and electrochemical mass spectrometry. In addition to this, we include sections of reactor design and the link with theoretical modelling that are applicable across all techniques. While we focus on heterogeneous electrocatalysis, we make links when appropriate to the areas of photo- and thermo-catalytic systems. We highlight common pitfalls in the field, how to avoid them, and what sets of complementary experiments may be used to strengthen the analysis. We end with an overview of what gaps remain in in-situ and operando techniques and what innovations must be made to overcome them.

SnO 2 -supported Pd single atoms coupled with small cluster structures, incorporated into sensing platforms, exhibit exceptional performance and selectivity toward CO.

Nanocrystalline pristine and Pd-loaded tin (IV) oxide (SnO2) nanocomposites with different loadings were synthesized via facile impregnation and in-situ reduction, followed by annealing. The crystal structure and morphology of the samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. X-ray photoelectron spectroscopy, Raman spectroscopy and ex-situ extended X-ray absorption fine structure (EXAFS) confirm PdO nanoclusters stabilized on SnO2 surface. Results revealed that Pd/ SnO2 with 2.8 wt% loading exhibits the best sensing performance, including high sensitivity to CO with a low detection limit, fast response, and good selectivity to CO against interfering gases. Its enhanced sensing performance is attributed to both fine structure of PdO, and the synergy between PdO and SnO2 as well as dissimilar defect structures and concentrations. In-situ FTIR measurements unraveled CO adsorption kinetics on Pd/SnO2 under reaction conditions, based on which a possible sensing mechanism is put forth. Namely, Pd and PdO on edges, steps, and terraces of (100) and (111) facets provide favorable adsorption and activation sites for CO, from which activated fragments are spilled over onto SnO2 to react with ionosorbed oxygen, locally decreasing depletion layer and sensor resistance; and concurrently, carbon-related species are formed and decomposed into CO2.

Current reverse water-gas shift (RWGS) technologies require extreme temperatures of >900°C. The ability to perform RWGS at lower temperatures could open new opportunities for sustainable chemical and fuel production, but most catalyst materials produce methane and coke at lower temperatures, especially at elevated pressures targeted for industrial processes. Here we show that transition metal-free catalysts composed of K2CO3 or Na2CO3 dispersed in commercial γ-Al2O3 supports (K2CO3/γ-Al2O3 and Na2CO3/γ-Al2O3) are highly effective RWGS catalysts in the intermediate-temperature regime. At a high gas hourly space velocity (GHSV) of 30,000 h−1 and operating pressure of 10 bar, K2CO3/γ-Al2O3 reached RWGS equilibrium-limited CO2 conversion at 550°C and was 100% selective for CO at all temperatures tested (up to 700°C). Na2CO3/γ-Al2O3 was also 100% CO-selective and only slightly less active. Both catalysts were stable for 100s of hours on stream at 525°C and tolerated large quantities of hydrocarbon impurity in the CO2/H2 feed. The unique performance attributes, combined with the low-cost components and extremely simple synthesis, make dispersed carbonate RWGS catalysts attractive options for industrial application.

The requirement for C2H2 concentrations below 2 parts per million (ppm) in gas streams for C2H4 polymerization necessitates its semihydrogenation to C2H4. We demonstrate selective chemical looping combustion of C2H2 in C2H4-rich streams by Bi2O3 as an alternative catalytic pathway to reduce C2H2 concentration below 2 ppm. Bi2O3 combusts C2H2 with a first-order rate constant that is 3000 times greater than the rate constant for C2H4 combustion. In successive redox cycles, the lattice O of Bi2O3 can be fully replenished without discernible changes in local Bi coordination or C2H2 combustion selectivity. Heterolytic activation of C-H bonds across Bi-O sites and the higher acidity of C2H2 results in lower barriers for C2H2 activation than C2H4, enabling selective catalytic hydrocarbon combustion leveraging differences in molecular deprotonation energies.

A sustainable and robust supply chain of rare earth elements (REEs) is necessary to meet our consumer, national security and clean energy goals. However, current intra-REE separation technologies (e.g. solvent extraction) are costly and carry a heavy environmental burden. Therefore, the development of new aqueous based ligands that are selective for individual REEs will be integral in future REE production systems. To develop these ligands, an understanding of how ligand coordination structure relates to selectivity is imperative. We used X-ray absorption spectroscopy (XAS) to observe the local structure around four lanthanide (Ln) ions (La, Ce, Pr and Nd) complexed by water and several relevant chelating ligands [lanmodulin EF-hand 1 peptides (LanM1), ethylenediaminetetraacetic acid (EDTA), aminotris(methylenephosphonic acid) (ATMP) and citric acid]. To collect these liquid-phase XAS spectra, we developed a new flow cell that prevents bubble interference and beam damage to the samples. In the X-ray absorption near-edge structure (XANES), we observed energy shifts in the white line, white line broadening and differences in the white line intensity of different Ln-ligand complexes between ligands. In the extended X-ray absorption fine structure (EXAFS), we distinguished differences in peak intensity and distance between coordinating ligands. Differences in the local coordination structure between Ln-LanM1 peptide complexes were more subtle compared with the other ligands (La-water, La-EDTA, La-ATMP and La-citric acid complexes). Further XANES and EXAFS studies, in combination with modelling and other techniques, could greatly improve our structural knowledge of how these aqueous ligands bind Ln ions and how they can be used to design more selective ligands for more efficient and sustainable REE separations.

The addition of palladium (Pd) to preformed gold nanoparticles (Au NPs) enables the formation of core-shell structures with enhanced catalytic performance in oxidation reactions. However, predicting the precise palladium content required to achieve maximum catalytic activity remains difficult based on current understanding. Herein, Pd was systematically introduced onto titania-supported Au NPs (2, 6, and 10 nm) to evaluate their performance in benzyl alcohol oxidation. A volcano-like trend in catalytic activity was observed, where activity increased with Pd addition, peaked, and then declined. The Pd loading required for maximum activity depended on Au NP size: ≈40 at% Pd/Au for 2.6 nm, ≈20 at% Pd/Au for 6.4 nm, and ≈12.5 at% Pd/Au for 10.6 nm. For Au NPs > 6 nm, peak activity aligned with monolayer Pd coverage, while for smaller NPs (2-3 nm), optimal Pd content was below monolayer predictions. X-ray absorption spectroscopy revealed a core-shell structure at low Pd content, but higher Pd loadings led to Pd diffusion into the Au core. This structural transformation likely caused activity decline, indicating that AuPd alloying negatively impacts catalysis. These results highlight that core-shell Au@Pd catalysts outperform AuPd alloys and provide crucial insights for designing highly active bimetallic catalysts.

An emerging design heuristic for electrochemical nitrate reduction (NO3RR) catalysts is synthesizing electron-deficient sites to facilitate binding of electron-rich NO3-. However, this rule has rarely been applied to metal-, nitrogen-doped carbon (MNC) catalysts. Titanium (Ti), with low electronegativity and high NO3RR reactivity, is a compelling MNC candidate. To date, atomically dispersed TiNx motifs have eluded synthesis due to the strong oxophilicity of Ti. Here, we leverage nitrogen-rich carbon flowers (CF) to overcome synthetic challenges and produce Ti-, N-doped carbon flower (TiCF) catalysts. Advanced materials characterization demonstrates that TiCF catalysts are a mixed phase material with 3/4 of Ti atoms in TiO2-like nanoparticles and 1/4 of Ti atoms in novel, atomically dispersed TiNx sites. TiCF achieves 61 ± 7% NH3-selectivity at -0.70 V vs RHE and 14 ± 5 mA/cm2 to NH3 formation (|jNH3|) at -0.85 V vs RHE in (0.1 M NaOH + 0.1 M NaNO3 + 0.45 M Na2SO4) electrolyte. Control studies show both CF morphology and Ti sites are essential for high NO3RR activity. Density functional theory calculations attribute the NO3RR reactivity to TiNx, which facilitates multiple bond formation with surface intermediates to promote favorable NH3 synthesis pathways. Thus, TiCF exhibits 60× higher |jNH3| values than bulk Ti and NH3 yield rates (>0.06 mmol NH3/h/cm2) that are competitive with state-of-the-art MNC catalysts (e.g., FeNC, CuNC). TiCF introduces a new class of Ti electrocatalysts, advancing the MNC design space and sustainable NH3 production.

Titanium dioxide (TiO2) is one of the most extensively studied oxides as an active catalyst or catalyst support, particularly in energy and environmental applications, but the atomistic mechanisms governing its dynamic response to reactive environments and their correlation to reactivity remain largely elusive. Using in situ environmental transmission electron microscopy (ETEM), synchrotron X-ray diffraction (XRD), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), temperature-programmed reduction (TPR), reactivity measurements, and theoretical modeling, we reveal the dynamic interplay between oxygen loss and replenishment of anatase TiO2 under varying reactive conditions. Under H2 exposure, anatase TiO2 undergoes surface reduction via lattice oxygen loss, forming Ti3O5. In contrast, CO2 exposure induces oxygen replenishment, reversing stoichiometry. In mixed H2/CO2 environments, the reverse water-gas shift (RWGS) reaction proceeds selectively on stepped and high-indexed TiO2 surfaces, whereas the thermodynamically stable TiO2(101) surface remains inactive and intact. Critically, H2 pretreatment generates oxygen vacancies on TiO2(101), transforming it into an active Ti3O5 or defect-rich surface that catalyzes RWGS. By correlating surface structure, defect dynamics, and gas-phase interactions, this work deciphers the competition between H2-driven reduction and CO2-driven oxidation pathways at the atomic scale. These insights establish defect engineering as a strategic lever to activate inert TiO2 facets, advancing the design of adaptive catalysts for sustainable fuel synthesis technologies.

The shape of supported metal particles regulates their catalytic reactivity and is determined by the degree of wetting between the metal particle and the support surface. Flattened particles that wet support surfaces were reported in various catalytic systems, particularly in the subnanometer size regime. Such consequential metal-support wetting phenomena are poorly understood, and methods to study them on powder catalysts under realistic conditions are lacking. Here, we investigate the size-dependent wetting behaviors of Ru particles on two reducible-oxide supports, anatase TiO2 (TiO2-A) and CeO2, under reducing catalytic conditions. X-ray absorption spectroscopy (XAS), low-energy ion scattering (LEIS), and density functional theory (DFT) are combined to determine the shape of Ru particles. Ru particles remain three-dimensional without wetting the TiO2-A support within the coverage range studied (0.06-0.98 Ru nm-2). In contrast, at low coverages (<0.25 Ru nm-2), Ru wets the CeO2 support to form flat, disordered structures. The higher wettability of CeO2 than TiO2-A is attributed to oxygen vacancies in the near-surface region. The shape difference between small Ru particles or clusters on the two supports leads to drastically contrasting catalytic reactivities in polyolefin hydrogenolysis, despite similar diameters. This work highlights the implications of metal-support wetting, or cluster shape, on catalytic behaviors of small metal clusters, while establishing the foundation for future systematic studies of such a phenomenon in realistic systems, by delivering a multitechnique methodology and revealing governing fundamental principles.

In situ and operando X-ray absorption spectroscopy (XAS) provides fundamental insight into the working principles of electrocatalysts and is an important tool for future catalyst development. However, the design of an operando XAS electrocatalytic cell is not facile, and researchers designing cells, whether new cells or modifications to previous cells, often spend many hours on cell design before obtaining high-quality XAS data. Here, we describe the design, with engineering drawings, and operation of a versatile XAS cell with options for gas flow, electrolyte flow, pH monitoring, temperature monitoring, and the ability to handle many catalyst forms (any catalyst that can be deposited onto a conductive X-ray transparent substrate). We benchmarked XAS spectra collected using the new experimental cell to a previous cell design showing its ability to produce quality XAS data. We demonstrate the viability of this cell by providing insight into electrocatalysts by studying cation effects and show the tetrabutylammonium cation prevents bulk oxidation of copper. We hope the availability of this cell allows researchers to convert time typically spent on cell design to time spent on breakthroughs in electrocatalysis.

Multicomponent catalysts can be designed to synergistically combine reaction intermediates at interfacial active sites, but restructuring makes systematic control and understanding of such dynamics challenging. We here unveil how reducibility and mobility of indium oxide species in Ru-based catalysts crucially control the direct, selective conversion of CO2 to ethanol. When uncontrolled, reduced indium oxide species occupy the Ru surface, leading to deactivation. With the addition of steam as a mild oxidant and using porous polymer layers to control In mobility, Ru-In2O3 interface sites are stabilized, and ethanol can be produced with superior overall selectivity (70%, rest CO). Our work highlights how engineering of bifunctional active ensembles enables cooperativity and synergy at tailored interfaces, which unlocks unprecedented performance in heterogeneous catalysts.

Nickel phosphides are of particular interest because they are highly active and stable catalysts for petroleum/biorefinery and hydrogen production. Despite their significant catalytic potential, synthesizing various phase-pure nickel phosphide nanoparticles of uniform size remains a challenge. In this work, we develop a robust trioctylphosphine (TOP)-mediated route to make highly uniform phase-pure Ni12P5, Ni2P, and Ni5P4 nanoparticles. The synthetic route forms amorphous Ni70P30 nanoparticle intermediates. The reactions can be stopped at the amorphous stage when amorphous particles are desired. The amount of P incorporation can be controlled by varying the ratio of TOP to Ni(II). The mechanism for composition control involves the competition of the kinetics of two processes: the addition of the reduced Ni and the incorporation of P into Ni. Uniform Ni70P30 amorphous nanoparticles can be generated at a high TOP-to-Ni(II) ratio, where the P incorporation kinetics is made to dominate. Ni70P30 can later be transformed into phase-pure Ni12P5, Ni2P, and Ni5P4 nanocrystals of uniform size. The transformation can be controlled precisely by modulating the temperature. A UV-vis study coupled with theoretical modeling reveals Ni(0)-TOPx complexes along the synthetic path. This approach may be expanded to create other metal compounds, potentially enabling the synthesis of uniform nanoparticles of a greater variety.

Facile sintering of atomically dispersed supported noble metal catalysts at catalytically relevant temperatures, particularly under reducing conditions, poses a challenge for their practical applications. Some ligands, such as carbonyls, aid in improving the stability at the expense of severely suppressing the catalytic activity. Here, we demonstrate that substitution of the carbonyl ligands with reactive acetylene ligands can maintain the atomic dispersion of the supported mononuclear rhodium complex under harsh reducing conditions (>573 K), as confirmed by in -situ X-ray absorption near -edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. In contrast, the supported rhodium carbonyl complex aggregates into nanoclusters under identical conditions. Furthermore, our results indicate that the acetylene ligands provide this anti -sintering ability while retaining the hydrogenation activity.

Supported noble metal catalysts, ubiquitous in chemical technology, often undergo dynamic transformations between reduced and oxidized states-which influence the metal nuclearities, oxidation states, and catalytic properties. In this investigation, we report the results of in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, and other physical characterization techniques, bolstered by density functional theory, to elucidate the structural transformations of a set of MgO-supported palladium catalysts under oxidative treatment conditions. As the calcination temperature increased, the as-synthesized supported metallic palladium nanoparticles underwent oxidation to form palladium oxides (at approximately 400 °C), which, at approximately 500 °C, were oxidatively fragmented to form mixtures of atomically dispersed palladium cations. The data indicate two distinct types of atomically dispersed species: palladium cations located at MgO steps and those embedded in the first subsurface layer of MgO. The former exhibit significantly higher (>500 times) catalytic activity for ethylene hydrogenation than the latter. The results pave the way for designing highly active and stable supported palladium hydrogenation catalysts with optimized metal utilization.

The reduction of CO2 is known to promote increased alkene yields from alkane dehydrogenations when the reactions are cocatalyzed. The mechanism of this promotion is not understood in the context of catalyst active-site environments because CO2 is amphoteric, and even general aspects of the chemistry, including the significance of competing side reactions, differ significantly across catalysts. Atomically dispersed chromium cations stabilized in highly siliceous MFI zeolite are shown here to enable the study of the role of parallel CO2 reduction during ethylene-selective ethane dehydrogenation. Based on infrared spectroscopy and X-ray absorption spectroscopy data interpreted through calculations using density functional theory (DFT), the synthesized catalyst contains atomically dispersed Cr cations stabilized by silanol nests in micropores. Reactor studies show that cofeeding CO2 increases stable ethylene-selective ethane dehydrogenation rates over a wide range of partial pressures. Operando X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra indicate that during reaction at 650 degrees C the Cr cations maintain a nominal 2+ charge and a total Cr-O coordination number of approximately 2. However, CO2 reduction induces a change, correlated with the CO2 partial pressure, in the population of two distinct Cr-O scattering paths. This indicates that the promotional effect of parallel CO2 reduction can be attributed to a subtle change in Cr-O bond lengths in the local coordination environment of the active site. These insights are made possible by simultaneously fitting multiple EXAFS spectra recorded in different reaction conditions; this novel procedure is expected to be generally applicable for interpreting operando catalysis EXAFS data.

We present the Multi-site (MS) QuantEXAFS approach, designed to model the EXAFS data from samples containing an element in different local bonding geometries. Building upon our QuantEXAFS method, which maps experimental extended X-ray absorption fine structures (EXAFS) data to DFT-optimized structures, MS-QuantEXAFS introduces the key capability to probe fractional contributions of multiple sites that may be present in an experimental sample. Specifically, we demonstrate effectiveness of this technique by investigating mixed samples containing known fractions of site-isolated subsurface Pt/MgO with Pt nanoparticles uniformly supported on MgO. The 'site-fractions' obtained through MS-QuantEXAFS closely match (i. e., +/- 6%) the known fractions of the physically mixed samples. This approach has been generalized to other oxides, and thus represents an important advance in quantifying the speciation of non-uniform catalyst samples.