Publications
2025
Abstract
A precise understanding of heterogeneous catalyst structure and activity is required to design more efficient, greener, industrial chemical processes. While it is possible to resolve both the structure and activity of a catalyst, their innate complexity means it is nontrivial to unambiguously assign one to the other. Herein, we demonstrate how transient measurements, coupled with kinetic modeling, can be used to resolve kinetics with sufficient precision such that it is possible to resolve the identity of an “active site” over technical catalysts. We find that under CO rich conditions the transient activity and coverage dependencies measured over a Pd/γ-Al2O3 catalyst during pulsed flow and transient spectroscopy CO oxidation experiments can be quantitatively predicted using a kinetic model derived from ultrahigh vacuum surface science experiments over Pd(111) single crystals. Thus, we conclude that the state (i.e., structure and composition of the active site) of the Pd/γ-Al2O3 catalyst under reaction conditions is equivalent to that of a Pd(111) surface. Instead of determining a catalyst’s structure and assigning it to activity, we instead propose a new approach: using precise kinetic measurements to infer the catalyst’s state. This approach enables the identification of active sites that have been challenging to resolve using traditional spectroscopic or microscopic methods, offering a powerful and complementary tool for designing new catalysts.
Abstract
In this communication we report how to assemble a DRIFTS system that can precisely control the pressure in the cell from <10−6 Torr to atmospheric pressure using easily accessible components. As a proof-of-principle we adsorb and react CO over a Pd/Al2O3 catalyst where we can observe pressure, temperature, and reaction-dependent preferential binding to adsorption sites.
Abstract
The performance of bimetallic dilute alloy catalysts is largely determined by the size of minority metal ensembles on the nanoparticle surface. By analyzing the synthesis of catalysts comprising Pd8Au92 nanoparticles supported on silica using surface-sensitive techniques, we report that whether Pd overgrowth occurs before or after Au nanoparticle deposition onto the support controls the surface Pd ensemble size and abundance. These differences in Pd ensembles influence catalytic reactivity in H2–D2 isotope exchange and benzaldehyde hydrogenation, which, in correlation with theoretical calculations, is used to elucidate the active site(s) in each reaction. To clarify how the synthetic sequence controls the formation of Pd ensembles, we combine numerical wetting calculations and molecular dynamics simulations (with a machine-learned force field) to visualize Pd deposition and migration on the nanoparticle surface, respectively. Our results suggest that the nanoparticle–support interface restricts nanoparticle accessibility to Pd deposition, which consequently controls the Pd ensemble size, illustrating the critical role of nanoscale wetting phenomena during bimetallic catalyst preparation.
Abstract
Engineering colloidally stable multimetallic nanocrystals has many benefits in a wide range of applications and introduces the opportunity of manipulating physical, chemical, and electronic structure properties of materials at the nanoscale. Synthesis routes are challenged by the chemical complexity required to temporally and spatially coordinate the reduction and alloying of multiple metal species, which hampered the development of tunable libraries of colloidal materials to date. In this work, we demonstrate a synthesis method guided by machine learning-accelerated simulations to incorporate five or more metal elements in monodisperse, colloidally stable nanocrystals. Multiple seed materials can be used, leading to a library of multimetallic nanocrystals with tunable electronic, physical, and alloying structure. The advantage of this synthetic protocol is highlighted in the preparation of catalytic materials that showed two orders of magnitude higher reaction rates than monometallic catalysts and outstanding thermal stability, thus highlighting the promise of this approach for high-performance materials in many areas.
Abstract
Halide perovskite nanocrystals are promising materials for optoelectronic applications. Metal doping provides an avenue to boost their performance further, e.g., by enhancing light emission, or to provide additional functionalities, such as nanoscale magnetism and polarization control. However, the synthesis of widely size-tunable nanocrystals with controlled doping levels has been inaccessible using traditional hot injection synthesis, preventing systematic studies on dopant effects toward device applications. Here, we report a versatile synthesis method for metal-doped perovskite nanocrystals with precise control over size and doping concentration under ambient conditions. Our room temperature approach results in fully size-tunable isovalent doping of CsPbX3 nanocrystals (X = Cl, Br, I) with various transition metals M2+ tested (M = Mn, Ni, Zn). This gives for the first time access to small, yet precisely doped quantum dots beyond the weak confinement regime reported so far. It also enables a comparative study of the photophysics across multiple size and dopant regimes, where we show dopant-induced localization to dominate over quantum confinement effects. This generalizable, facile synthesis method thus provides a toolbox for engineering perovskite nanocrystals toward light-emitting technologies under industrially relevant conditions.
Abstract
The dynamic response of heterogeneous catalytic materials to their environment opens a wide variety of possible surface states which may have increased catalytic activity. In this work, we find that it is possible to generate a surface state with increased catalytic activity over metallic 2 nm Pt nanoparticles by performing a thermal treatment of the CO*-covered Pt catalyst. This state is characterised by its ability to oxidise CO to CO2 at room temperature. By combining pressure pulse experiments with in situ spectroscopy we correlate the formation of this high-activity state with the desorption of weakly bound CO* molecules from well-coordinated Pt sites. This high-activity state is metastable, degrading after elevated thermal treatments or upon readsorption of CO at room temperature. We conclude that this metastable state is highly localised to the surface of the nanoparticle, however its exact atomic structure remains open to speculation.
Abstract
Oxidative coupling reactions enable biomass-derived oxygenates to serve as sustainable platform molecules for a wide range of high-value chemicals. These catalytic reactions can be selectively triggered over alloys wherein a highly active dopant metal such as Pd is diluted into a sea of highly selective host metal atoms such as Au. Here, a range of supported Pd1Aux (x = 5–200) alloy nanoparticles were synthesized using a sequential reduction method with colloidal Au to achieve a high degree of compositional control and particle size uniformity. The promotional role of Pd was examined in the oxidation of ethanol to yield acetaldehyde and the coupling product ethyl acetate. Reactivity trends indicate that both the overall rate of ethanol oxidation and the selectivity toward coupling increase with Pd doping. Rate order and activation energy trends further suggest that the promotional role of Pd does not likely originate from simple O2 dissociation and spillover but rather from the stabilization of alkoxides at Pd-Au interfaces, disproportionately increasing coupling vs simple oxidation. Infrared spectroscopy and density functional theory calculations offer further insights into Pd microstructures in the presence of various key adsorbates, suggesting that Pd can lend this promotion in an isolated state. While this state is generally unstable in the surface due to preferences for segregation into the bulk, oxygen and pathway intermediates may aid in stabilizing surface structures. These findings lay groundwork to explain selectivity and activity control in a much wider range of oxidative functionalizations and to guide further catalyst development.
Abstract
Despite the broad catalytic relevance of metal–support interfaces, controlling their chemical nature, the interfacial contact perimeter (exposed to reactants), and consequently, their contributions to overall catalytic reactivity, remains challenging, as the nanoparticle and support characteristics are interdependent when catalysts are prepared by impregnation. Here, we decoupled both characteristics by using a raspberry-colloid-templating strategy that yields partially embedded PdAu nanoparticles within well-defined SiO2 or TiO2 supports, thereby increasing the metal–support interfacial contact compared to nonembedded catalysts that we prepared by attaching the same nanoparticles onto support surfaces. Between nonembedded PdAu/SiO2 and PdAu/TiO2, we identified a support effect resulting in a 1.4-fold higher activity of PdAu/TiO2 than PdAu/SiO2 for benzaldehyde hydrogenation. Notably, partial nanoparticle embedding in the TiO2 raspberry-colloid-templated support increased the metal–support interfacial perimeter and consequently, the number of Au/TiO2 interfacial sites by 5.4-fold, which further enhanced the activity of PdAu/TiO2 by an additional 4.1-fold. Theoretical calculations and in situ surface-sensitive desorption analyses reveal facile benzaldehyde binding at the Au/TiO2 interface and at Pd ensembles on the nanoparticle surface, explaining the connection between the number of Au/TiO2 interfacial sites (via the metal–support interfacial perimeter) and catalytic activity. Our results demonstrate partial nanoparticle embedding as a synthetic strategy to produce thermocatalytically stable catalysts and increase the number of catalytically active Au/TiO2 interfacial sites to augment catalytic contributions arising from metal–support interfaces.
2024
Abstract
The catalytic and plasmonic properties of bimetallic gold–palladium (Au-Pd) nanoparticles (NPs) critically depend on the distribution of the Au and Pd atoms inside the nanoparticle bulk and at the surface. Under operating conditions, the atomic distribution is highly dynamic. Analyzing gas induced redistribution kinetics at operating temperatures is therefore key in designing and understanding the behavior of Au-Pd nanoparticles for applications in thermal and light-driven catalysis, but requires advanced in situ characterization strategies. In this work, we achieve the in situ analysis of the gas dependent alloying kinetics in bimetallic Au-Pd nanoparticles at elevated temperatures through a combination of CO-DRIFTS and gas-phase in situ transmission electron microscopy (TEM), providing direct insight in both the surface- and nanoparticle bulk redistribution dynamics. Specifically, we employ a well-defined model system consisting of colloidal Au-core Pd-shell NPs, monodisperse in size and uniform in composition, and quantify the alloying dynamics of these NPs in H2 and O2 under isothermal conditions. By extracting the alloying kinetics from in situ TEM measurements, we show that the alloying behavior in Au-Pd NPs can be described by a numerical diffusion model based on Fick's second law. Overall, our results indicate that exposure to reactive gasses strongly affects the surface composition and surface alloying kinetics, but has a smaller effect on the alloying dynamics of the nanoparticle bulk. Both our in situ methodology as well as the quantitative insights on restructuring phenomena can be extended to a wider range of bimetallic nanoparticle systems and are relevant in understanding the behavior of nanoparticle catalysts under operating conditions.
Abstract
Supported metal nanoparticle (NP) catalysts are vital for the sustainable production of chemicals, but their design and implementation are limited by the ability to identify and characterize their structures and atomic sites that are correlated with high catalytic activity. Identification of these ''active sites'' has relied heavily on extrapolation to supported NP systems from investigation of idealized surfaces, experimentally using single crystals or supported NPs which are always modelled computationally using slab or regular polyhedra models. However, the ability of these methods to predict catalytic activity remains qualitative at best, as the structure of metal NPs in reactive environments has only been speculated from indirect experimental observations, or otherwise remains wholly unknown. Here, by circumventing these limitations for highly accurate simulation methods, we provide direct atomistic insight into the dynamic restructuring of metal NPs by combining in situ spectroscopy with molecular dynamics simulations powered by a machine learned force field. We find that in reactive environments, NP surfaces evolve to a state with poorly defined atomic order, while the core of the NP may remain bulk-like. These insights prove that long-standing conceptual models based on idealized faceting for small metal NP systems are not representative of real systems under exposure to reactive environments. We show that the resultant structure can be elucidated by combining advanced spectroscopy and computational tools. This discovery exemplifies that to enable faithful quantitative predictions of catalyst function and stability, we must move beyond idealized-facet experimental and theoretical models and instead employ systems which include realistic surface structures that respond to relevant physical and chemical conditions.
Abstract
Nanoparticle (NP) size and proximity are two physical descriptors applicable to practically all NP-supported catalysts. However, with conventional catalyst design, independent variation of these descriptors to investigate their individual effects on thermocatalysis remains challenging. Using a raspberry-colloid-templating approach, we synthesized a well-defined catalyst series comprising Pd12Au88 alloy NPs of three distinct sizes and at two different interparticle distances. We show that NP size and interparticle distance independently control activity and selectivity, respectively, in the hydrogenation of benzaldehyde to benzyl alcohol and toluene. Surface-sensitive spectroscopic analysis indicates that the surfaces of smaller NPs expose a greater fraction of reactive Pd dimers, compared to inactive Pd single atoms, thereby increasing intrinsic catalytic activity. Computational simulations reveal how a larger interparticle distance improves catalytic selectivity by diminishing the local benzyl alcohol concentration profile between NPs, thus suppressing its readsorption and consequently, undesired formation of toluene. Accordingly, benzyl alcohol yield is maximized using catalysts with smaller NPs separated by larger interparticle distances, overcoming activity–selectivity trade-offs. This work exemplifies the high suitability of the modular raspberry-colloid-templating method as a model catalyst platform to isolate individual descriptors and establish clear structure–property relationships, thereby bridging the materials gap between surface science and technical catalysts.
Abstract
This study introduces a novel iterative Bragg peak removal with automatic intensity correction (IBR-AIC) methodology for X-ray absorption spectroscopy (XAS), specifically addressing the challenge of Bragg peak interference in the analysis of crystalline materials. The approach integrates experimental adjustments and sophisticated post-processing, including an iterative algorithm for robust calculation of the scaling factor of the absorption coefficients and efficient elimination of the Bragg peaks, a common obstacle in accurately interpreting XAS data, particularly in crystalline samples. The method was thoroughly evaluated on dilute catalysts and thin films, with fluorescence mode and large-angle rotation. The results underscore the technique's effectiveness, adaptability and substantial potential in improving the precision of XAS data analysis. While demonstrating significant promise, the method does have limitations related to signal-to-noise ratio sensitivity and the necessity for meticulous angle selection during experimentation. Overall, IBR-AIC represents a significant advancement in XAS, offering a pragmatic solution to Bragg peak contamination challenges, thereby expanding the applications of XAS in understanding complex materials under diverse experimental conditions.
Abstract
A detailed knowledge of reaction kinetics is key to the development of new more efficient heterogeneous catalytic processes. However, the ability to resolve site dependent kinetics has been largely limited to surface science experiments on model systems. Herein, we can bypass the pressure, materials, and temperature gaps, resolving and quantifying two distinct pathways for CO oxidation over SiO2-supported 2 nm Pt nanoparticles using transient pressure pulse experiments. We find that the pathway distribution directly correlates with the distribution of well-coordinated (e.g., terrace) and under-coordinated (e.g., edge, vertex) CO adsorption sites on the 2 nm Pt nanoparticles as measured by in situ DRIFTS. We conclude that well-coordinated sites follow classic Langmuir-Hinshelwood kinetics, but under-coordinated sites follow non-standard kinetics with CO oxidation being barrierless but conversely also slow. This fundamental method of kinetic site deconvolution is broadly applicable to other catalytic systems, affording bridging of the complexity gap in heterogeneous catalysis.
Abstract
Our ability to rationalise the activity of heterogeneous catalytic materials relies on a precise understanding of both the structure and composition of the catalyst under reaction conditions to resolve active sites. Their highly dynamic nature means that catalysis suffers from an observer effect, where the act of measuring activity and/or structure will modify the structure itself. Herein, we present a well-defined catalyst consisting of 2 nm Pt nanoparticles supported on SiO2 that can be dynamically modified and regenerated without irreversibly changing the underlying structure or activity of the catalyst. Using in situ diffuse reflectance infrared Fourier transform spectroscopy and transmission electron microscopy we identify that the catalyst is resistant to oxidative and reductive treatments, but irreversibly restructures in reactive environments (CO + O2) at moderate pressures and temperatures. We show that by using transient pressure pulse experiments it is possible to minimise the restructuring in reactive environments, which when coupled with kinetic modelling can be used to identify site and state dependent activity while simultaneously quantifying the number of active sites. The well-defined nature of this 2 nm Pt/SiO2 catalyst means it can be utilised to gain molecular insight into catalytic processes and act as a new material for use in heterogeneous catalytic surface science. Further, the rapid restructuring of the catalyst during CO oxidation demonstrates that it is not a benign characterisation tool for comparison of catalysts.
Abstract
Kinetic modelling has been key to developing a mechanistic understanding of the epoxidation of ethylene to ethylene oxide over silver catalysts. However, models of varying active site and mechanistic complexity have all been able to recreate steady state activity and selectivity, leading to ambiguity about the exact mechanism and nature of the active site. Herein, we validate three leading kinetic models for ethylene epoxidation over metallic silver catalysts by numerically recreating non-steady state temporal analysis of products experiments. We find all of the models are able to very generally recreate the trends observed in the pulse experiments, but that only a two-site model modified to mimic the presence of a subsurface oxygen reservoir is able to accurately recreate the trends observed in a state-altering experiment over oxidised silver. Specific to this model is the inclusion of a electrophilic oxygen species adsorbed on top of the surface oxide which acts as the active site for the selective oxidation of ethylene. This work exemplifies that while simplified single-site models for ethylene epoxidation are useful tools for broad screening, more complex models are required to capture the precise activity of the catalyst.
2023
Abstract
The Temporal Analysis of Products (TAP) experiment provides an unparalleled level of kinetic insight into heterogenous catalytic materials, but due to the complex and expensive instrumentation required, its application has been limited to a small group of dedicated researchers. Herein we demonstrate through a series of designs that precisely defined TAP experiments can be performed on systems far smaller and simpler than previously imagined. The pulse reactors described in this work utilise readily available components and so can be assembled, operated, and maintained with minimal training. Using the case study of CO oxidation over a Pt/SiO2 catalyst we show that precise kinetic, mechanistic, and surface composition information is feasible using a single-valve design. With the developments outlined in this work we aim to decrease the activation barrier to TAP and open up the technique to a new generation of researchers.
Abstract
Herein, we present a design for a transient flow reactor system with high detection sensitivity and minimal dead volume, such that it is capable of sub-second switching of the gas stream flowing through a catalytic bed. We demonstrate the reactor's capabilities for step transient, pulse, and stream oscillation experiments using the model system of CO oxidation over Pd catalysts, and we find that we are able to precisely model step transients for CO oxidation using a pseudo-homogenous-packed bed reactor model. The design principles leading to minimal gas hold-up time and increased sensitivity that are described in this paper can be implemented into existing flow reactor designs with minimal cost, providing a readily accessible alternative to the existing transient instrumentation.
2021
Abstract
Dilute PdAu alloys are promising catalysts for selective oxidation and hydrogenation reactions. However, the surface composition and active site density of the minority metal, Pd, is unknown. In this study, we quantitatively determine a three-fold increase in the Pd site density on the surface of a dilute Pd0.08Au0.92 alloy catalyst after oxygen activation by titrating the oxidized surface with quantified pulses of CO.
Abstract
Catalysis is defined by kinetics, but after over a century of catalyst development our kinetic and mechanistic knowledge governing real-world catalysis is still severely lacking. This Perspective considers how we can use the precise knowledge available from both theoretical and fundamental experimental techniques to understand and even predict catalytic performance under operational conditions. We describe advances that link fundamental and “real-world” measurements, crucial elements required to successfully recreate a reaction network, and avenues that appear to offer viable routes to success.
2020
Abstract
Catalysis is defined by kinetics, but after over a century of catalyst development our kinetic and mechanistic knowledge governing real-world catalysis is still severely lacking. This Perspective considers how we can use the precise knowledge available from both theoretical and fundamental experimental techniques to understand and even predict catalytic performance under operational conditions. We describe advances that link fundamental and “real-world” measurements, crucial elements required to successfully recreate a reaction network, and avenues that appear to offer viable routes to success.
2019
Abstract
The utility of the surface reactivity observed for model systems under ultrahigh vacuum for predicting the performance of catalytic materials under ambient flow conditions is a highly debated topic in heterogeneous catalysis. Herein we show that vast differences in selectivity observed for methanol self-coupling across wide ranges of temperature and reactant pressure can be accurately predicted utilizing the kinetics and mechanism obtained from model studies on gold single crystals in ultrahigh vacuum regressed to fit transient pulse responses over nanoporous gold (Ag0.03Au0.97) at low pressures. Specifically, microkinetic modeling of the complex sequence of elementary steps governing this reaction predicts the dramatic effect of reactant partial pressure on the product distribution and leads to conclusion that the gas phase partial pressures of both reactants and the reaction temperature determine the changes in selectivity to methyl formate formation. Moreover, thorough analysis of the reaction network indicates that the product distribution becomes increasingly insensitive to kinetic effects at pressures approaching 1 bar, leading toward 100% selectivity methyl formate. A rigorous kinetic sensitivity analysis also demonstrates the complex interplay of the kinetics of the elementary steps and the overall catalytic behavior.
2018
Abstract
The quantitative prediction of catalyst selectivity is essential to the design of efficient catalytic processes and requires a detailed knowledge of the reaction mechanism and rate constants. Here we present a study that accurately predicts, using the kinetics and a mechanism derived from fundamental studies on single-crystal gold, the product distribution resulting from the complex reaction network that governs the oxidative coupling of methanol, catalysed by nanoporous gold between 360 and 425 K and for a vast range of pressures. Analysis of the transient product responses to micropulses of methanol over nanoporous gold yields a precise understanding of the marked dependence of selectivity on pressure, surface oxygen coverage and temperature. The key to a high selectivity for methyl formate is the surface lifetime and abundance of the methoxy. This successful microkinetic modelling of catalytic reactions across a wide set of reaction conditions is broadly applicable to predicting catalytic selectivity and provides a pathway to designing more efficient catalytic processes.