Collectivity effect in cluster catalysis under operational conditions.

Revisiting active sites in heterogeneous catalysis: Their structure and their dynamic behaviour

Interplay between remote single-atom active sites triggers speedy catalytic oxidation

The identification of the active sites in heterogeneous catalysis is important for a mechanistic understanding of the structure–reactivity relationship. Among others, the oxide/metal boundaries are expected to contain the active sites in various catalytic reactions. To reveal their nature and their chemical evolution under reaction conditions, the catalytic role of an oxide/metal system consisting of well-ordered ZnO nanoislands grown on Pt(111) in low-temperature CO oxidation was studied by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in operando conditions, and additionally by ultra-high vacuum scanning tunneling microscopy. To illustrate the special role played by the oxide/metal boundaries, a systematic comparative study of ZnO/Pt(111) with the pristine Pt(111) surface was undertaken. The regimes where mass transfer limitation starts to occur were identified using NAP-XPS and mass spectrometry measurements in combination, allowing a sound discussion on the relation between steady-state molar fractions of reactants/product and surface reactivity. Via the measurement of the steady-state CO2 molar fraction, we observed that the CO oxidation reaction rate over the ZnO/Pt(111) system is superior to that over Pt(111) in a temperature range extending to 410 K. The pivotal, albeit unexpected, role of ZnO-bound hydroxyls was clearly highlighted by the observation of the chemical signature of the CO + OH associative reaction at the ZnO/Pt boundaries. The carboxyl formed at low temperature (410 K) can be the intermediate species in the CO oxidation reaction, the OHs at the Pt/ZnO boundary being the cocatalyst, which explains the synergistic effect of ZnO and Pt. However, the species formed at higher temperature (from 445 K) are formates that would essentially be spectators.

Active Sites in Heterogeneous Catalysis

This Perspective explores the integration of machine learning potentials (MLPs) in the research of heterogeneous catalysis, focusing on their role in identifying in situ active sites and enhancing the understanding of catalytic processes. MLPs utilize extensive databases from high-throughput density functional theory (DFT) calculations to train models that predict atomic configurations, energies, and forces with near-DFT accuracy. These capabilities allow MLPs to handle significantly larger systems and extend simulation times beyond the limitations of traditional ab initio methods. Coupled with global optimization algorithms, MLPs enable systematic investigations across vast structural spaces, making substantial contributions to the modeling of catalyst surface structures under reactive conditions. The review aims to provide a broad introduction to recent advancements and practical guidance on employing MLPs and also showcases several exemplary cases of MLP-driven discoveries related to surface structure changes under reactive conditions and the nature of active sites in heterogeneous catalysis. The prevailing challenges faced by this approach are also discussed.

Identification of the active sites in heterogeneous catalysis is important for a mechanistic understanding of the structure–reactivity relationship and rationale of the design of new catalysts, but remains challenging. Among others, the boundaries at metal nanoparticles and supported oxides were found to be important and attributed to the active sites in various catalytic reactions. To reveal the nature of the active sites at the boundaries, the catalytic role of the inverse 3d transition-metal oxide nanoislands on Pt(111) for low-temperature CO oxidation was studied by density functional theory calculations. A characteristic Pt–cation ensemble at the oxide/metal boundaries as the active sites is identified. In Pt–cation ensembles, coordinate-unsaturated (CUS) cations exposed at the edges of oxide nanoislands are highly active for O2 adsorption and dissociation, and less-reactive Pt binds modestly with dissociated O responsible for the facile CO oxidation. Inverse VIIIB-oxide/Pt boundaries exhibit high ac...

Electronic tuning of active sites in heterogeneous catalysis with organic ligands remains challenging since the ligands are often bound to the most active sites on the catalysts' surfaces. In this work, gold nanoparticles, which are on average less than 2 nm in diameter, are synthesized with strongly binding thiol and phosphine ligands and have measurable quantities of accessible sites on their surfaces in both cases. Triphenylphosphine (TPP) is used as the phosphine ligand, while triphenylmethyl mercaptan (TPMT) serves as the thiol ligand. Phosphines are chosen because they are electron-donating ligands when bound to Au, and thiols are selected because they are electron-withdrawing on the Au surface. X-ray photoelectron spectroscopy (XPS) results show differences in the Au 4f binding energies between the TPP- and TPMT-bound Au nanoparticles. Fourier transform infrared spectroscopy (FTIR) measurements of bound CO indicate that the TPP-bound Au nanoparticles are more electron-rich than the TPMT-bound Au nanoparticles. The number of binding sites on the surface is quantified using 2-naphthalenethiol titration experiments. It is observed that the number of binding sites on the thiol and phosphine-bound Au nanoparticles is similar in both cases. The Au nanoparticles are used for three different reactions: resazurin reduction, CO oxidation, and benzyl alcohol oxidation. For both CO oxidation and benzyl alcohol oxidation, which are performed with the ligands attached, TPP- and TPMT-bound nanoparticles are both catalytically active. However, for resazurin reduction, the TPMT-bound Au nanoparticles are not active, while the TPP-bound Au nanoparticles are catalytically active. These results illustrate that the catalytic activity can be tuned using bound organic ligands with different electronic properties for reduction reactions using Au nanoparticle catalysts.

Experiments on the dependence of catalytic activity of nickel foils as a function of deformation and annealing are interpreted as evidence that dislocations are active sites in heterogeneous catalysis.

For Abstract see ChemInform Abstract in Full Text.

The concept that catalytic turnover occurs at a small fraction of the surface sites dates back to the 1920s. The application of modern surface science techniques and model catalysts confirmed the presence of active sites and identified their structures in some cases. Low coordination defect sites on transition metals, steps and kinks, or open “rough” crystal faces that make high coordination metal sites available have been uniquely active for breaking H–H, C–H, C–C, C=O, O=O and N≡N bonds. Oxide–metal interfaces provide highly active sites for reactions of C–H and C=O bonds. Electron acceptor and proton donor sites are implicated in hydrocarbon conversion (acid–base catalysis), and sites where metal ion–carbon bonds can form are active for polymerization. The observations of dynamic restructuring of catalytic surfaces upon adsorption of reactants indicate that many catalytic sites are created during the chemical reaction. Similar restructuring is detected for enzyme catalysts. The high mobility of both surface metal atoms and adsorbed molecules during the catalytic process observed recently bring into focus the dynamic nature of active sites that may have a finite lifetime as they form and disassemble. The development of techniques that provide improved time resolution and spatial resolution, and can be employed under catalytic reaction conditions will provide information about the time dependent changes of active site structure and molecular intermediates at these active sites as the reaction products form.

In the last century, progress in electrochemistry and electrocatalysis was very spectacular due to the remarkable evolution in surface science, chemistry, and physics. Electrochemical study of perfect smooth or bulk materials was the usual way to understand the interaction between the surfaces of such materials with their close environment. Therefore, any modification of the surface structure or composition provides change in the material behavior and the nature of the adsorbed species or near. Usually, the modification of smooth surface consists in the increase of its roughness factor through the deposition of sublayer or layer of metal particles. The deposition can be done on a well-defined surface (model electrode with a known crystallographic structure) [1]. Then, surface modification becomes a way of creating new active sites to enhance the reactivity of molecules. The development of nanoscale materials has changed the approach of studying and identifying active sites in heterogeneous catalysis, and particularly in electrocatalysis. Indeed, electrocatalysis uses the surface of a material, which is submitted to an electrode potential, as the reaction site. Therefore, the material structure, morphology and its composition are the key parameters to control the electrochemical process [2]. The nature of the active site depends on these parameters. Furthermore, the assessment of the nature of the active site before, during, and after the electrocatalytic reaction becomes a huge challenge. Thereby, electrochemical tools like cyclic voltammetry, underpotential deposition of a monolayer of a species [3–5], the specific adsorption of species or molecule, and CO stripping [6] were the first approaches. It is the basic measurement of the electrons flow through the surface per unit of time during the reaction at the surface. Therefore, the electric current per area unit represents the charge transfer reaction that occurs at a metal-solution interface. Since the middle of the last century, an increase in the development of several in situ spectroscopic techniques was observed due to the need of understanding the structure of the interface between electrodes and solutions. Indeed, coupling the electrochemistry measurements to other techniques such as Fourier Transform Infrared Spectroscopy (FTIRS), X-Ray Diffraction (XRD) [7, 8], Transmission Electron Microscopy (TEM) [9], Scanning Tunneling Microscopy (STM) [10], Surface-Enhanced Raman Scattering (SERS) [11] becomes a suitable approach to assess in real time at the electrified interface electrode-solution; some relevant data on electrocatalysts structure, morphology, composition, and stability of materials; and on the reaction intermediates and products. The identification of the surface state in addition to that of adsorbed species, intermediates, and products of the reaction process have permitted to determine a mechanistic pathway which is essential for enhancing the material performance and selectivity. It appears obvious that the identification of surface state of a nanomaterial under realistic electrochemical reaction conditions represents a noble scientific breakthrough. In the present chapter, for the first time some techniques coupled with electrochemistry able to characterize nanomaterials as electrodes will be extensively addressed. This chapter will also show the progress in in situ electrochemical approaches. One motivated approach is to be able to characterize electrochemically and experimentally the surface of the nanoparticle. Therefore, in the first part of the chapter, an example of a pure electrochemical tool, which permits to probe the nanoelectrocatalyst surface, is discussed. Although the progress in nanotechnology increases rapidly, various tools have been developed in electrochemistry for understanding the reaction pathway, intermediates, and products formation.

Metal oxides are highly important as components in heterogeneous catalysts. It is also well established that as the building blocks of nanoscaled materials get smaller, surfaces become increasingly important in determining their properties. Very early on, researchers found that catalytic activity is only indirectly related to the surface area; in fact it depends on the density of active sites. Although solid-state defects have been proposed to be the active sites in heterogeneous catalysis, active centers have rarely been conclusively identified and the rational design of catalysts is still out of reach. Processes in heterogeneous catalysis associated with the production of energy-storage molecules, for example, methanol as a storage molecule for hydrogen, is presently of great interest. Low-pressure production of methanol is conducted industrially with catalysts containing Cu and ZnO as the active phases and alumina as the support. Composites (Cu + ZnO), however, exhibit higher activities than that expected based on the performance of the single components. This was attributed to a strong metal–support interaction (SMSI) effect. Nevertheless, the active sites in Cu/ZnO and even in the pure ZnO have not yet been identified experimentally. Some important information concerning pure ZnO catalysts is already available: Generally, the catalytic activity of ZnO does not increase linearly with the increasing BET surface area. It was concluded that the catalytic formation of methanol over ZnO is a structure-sensitive reaction requiring the presence of polar ZnO facets. According to the most recent models of the methanol-synthesis reaction on the polar ZnO(0001) surface, oxygen vacancies formed on this surface of ZnO crystals serve as the active sites (Scheme 1). However, there has been no definite experimental proof for this assumption.

Catalytic activity of metal particles is reported to originate from the appearance of nonmetallic states, but conductive metallic particles, as an electron reservoir, should render electron delivery between reactants more favorably so as to have higher activity. We present that metallic rhodium particle catalysts are highly active in the low-temperature oxidation of carbon monoxide, whereas nonmetallic rhodium clusters or monoatoms on alumina remain catalytically inert. Experimental and theoretical results evidence the presence of electronic communications in between vertex atom active sites of individual metallic particles in the reaction. The electronic communications dramatically lower apparent activation energies via coupling two electrochemical-like half-reactions occurring on different active sites, which enable the metallic particles to show turnover frequencies at least four orders of magnitude higher than the nonmetallic clusters or monoatoms. Similar results are found for other metallic particle catalysts, implying the importance of electronic communications between active sites in heterogeneous catalysis.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

The microenvironment is recognized to be as crucial as active sites in heterogeneous catalysis. It was found that the catalytic activity of a set of chemical reactions can be significantly influenced by the confined space of carbon nanotubes (CNTs), with some reactions showing superior activity, while others experience a negative impact. The rational design of confined catalysis must rely on the accurate insights of confined microenvironment. However, the structural complexity of confined catalysts and the interaction with microenvironment hinders the deciphering of chemical origins behind experiments. In this work, Grand-canonical Monte Carlo (GCMC) simulations are conducted for confined catalysis in CNTs at various reaction atmospheres, accelerated by machine learning potentials. The statistical outcomes of GCMC simulations corroborate a general feature that the electronic interaction (binding energy) inside CNTs is weaker than outside cases. By using Random Forest (RF) model, we ascertain that the shortening of the bond lengths of catalysts within the confined space is the dominant factor, resulting in the weakened binding energy and the downshift of the d-band center. Using the bond length variation as a simplified descriptor, our microkinetic models successfully reproduced the seemingly contradictive experiments, namely, the observed enhancement and suppression for the same reaction.