Heterolytic Hydrogen Activation: Understanding Support Effects in Water–Gas Shift, Hydrodeoxygenation, and CO Oxidation Catalysis

Renaissance of Strong Metal-Support Interactions

Strong metal-support interactions (SMSIs) have emerged as a significant and cutting-edge area of research in heterogeneous catalysis. They play crucial roles in modifying the chemisorption properties, interfacial structure, and electronic characteristics of supported metals, thereby exerting a profound influence on the catalytic properties. This Perspective aims to provide a comprehensive summary of the latest advancements and insights into SMSIs, with a focus on state-of-the-art in situ/operando characterization techniques. This overview also identifies innovative designs and applications of new types of SMSI systems in catalytic chemistry and highlights their pivotal role in enhancing catalytic performance, selectivity, and stability in specific cases. Particularly notable is the discovery of SMSI between active metals and metal carbides, which opens up a new era in the field of SMSI. Additionally, the strong interactions between atomically dispersed metals and supports are discussed, with an emphasis on the electronic effects of the support. The chemical nature of SMSI and its underlying catalytic mechanisms are also elaborated upon. It is evident that SMSI modification has become a powerful tool for enhancing catalytic performance in various catalytic applications.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ∼1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ∼5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H₂. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Unlocking the Catalytic Potential of TiO2-Supported Pt Single Atoms for the Reverse Water-Gas Shift Reaction by Altering Their Chemical Environment

Single-atom catalysts (SACs) often exhibit dynamic responses to the reaction and pretreatment environment that affect their activity. The lack of understanding of these behaviors hinders the development of effective, stable SACs, and makes their investigations rather difficult. Here we report a reduction-oxidation cycle that induces nearly 5-fold activity enhancement on Pt/TiO2 SACs for the reverse water-gas shift (rWGS) reaction. We combine microscopy (STEM) and spectroscopy (XAS and IR) studies with kinetic measurements, to convincingly show that the low activity on the fresh SAC is a result of limited accessibility of Pt single atoms (Pt1) due to high Pt-O coordination. The reduction step mobilizes Pt1, forming small, amorphous, and unstable Pt aggregates. The reoxidation step redisperses Pt into Pt1, but in a new, less O-coordinated chemical environment that makes the single metal atoms more accessible and, consequently, more active. After the cycle, the SAC exhibits superior rWGS activity to nonatomically dispersed Pt/TiO2. During the rWGS, the activated Pt1 experience slow deactivation, but can be reactivated by mild oxidation. This work demonstrates a clear picture of how the structural evolution of Pt/TiO2 SACs leads to ultimate catalytic efficiency, offering desired understanding on the rarely explored dynamic chemical environment of supported single metal atoms and its catalytic consequences.

Kinetics of H2 Adsorption at the Metal-Support Interface of Au/TiO2 Catalysts Probed by Broad Background IR Absorbance

H₂ adsorption on Au catalysts is weak and reversible, making it difficult to quantitatively study. We demonstrate H₂ adsorption on Au/TiO₂ catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H₂ adsorption; adsorption equilibrium constants are comparable to volumetric adsorption measurements. H₂ adsorption kinetics measured with the BBA show a lower E_app value (23 kJ mol^-1 ) for H₂ adsorption than previously reported from proxy H/D exchange (33 kJ mol^-1 ). We also identify a previously unreported H-O-H bending vibration associated with proton adsorption on electronically distinct Ti-OH metal-support interface sites, providing new insight into the nature and dynamics of H₂ adsorption at the Au/TiO₂ interface.

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

Supports, ligands and additives can promote heterolytic H₂ splitting by a cooperative mechanism with metal nanoparticles.

Reverse water-gas shift reaction over Pt/MoOx/TiO2: reverse Mars–van Krevelen mechanism via redox of supported MoOx

Pt/MoO_x/TiO₂ shows excellent catalytic performance for the reverse water-gas shift reaction at 250 °C via reverse Mars–van Krevelen mechanism.

Water–Gas Shift Activity of Pt Catalysts Prepared by Different Methods

Platinum supported on ceria and zirconia was prepared through different preparation methods: Coprecipitation (CP), spray drying (SD), and flame spray pyrolysis (FSP). The catalysts were characterized by XRD, TPR, N2 adsorption, and H2 chemisorption, and the water–gas shift activity in the range 190–310 °C and initial stability at 300–310 °C were tested. Although the spray-dried Pt/CeO2/ZrO2 catalyst shows the highest initial activity, it deactivates rapidly at 300 °C and levels out at similar activity as the coprecipitated Pt/CeO2 and Pt/CeO2/ZrO2 within a few hours. Flame spray pyrolysis appears to be a promising preparation method concerning the stability of catalysts, although the initial activity is rather poor. High activity is related to high Pt dispersion, low reduction temperature, and small support particles. The support particle size is also much affected by the preparation method.

In Situ Dispersion of Palladium on TiO2 During Reverse Water-Gas Shift Reaction: Formation of Atomically Dispersed Palladium

The application of single-atom catalysts (SACs) to high-temperature hydrogenation requires materials that thermodynamically favor metal atom isolation over cluster formation. We demonstrate that Pd can be predominantly dispersed as isolated atoms onto TiO₂ during the reverse water-gas shift (rWGS) reaction at 400 °C. Achieving atomic dispersion requires an artificial increase of the absolute TiO₂ surface area by an order of magnitude and can be accomplished by physically mixing a precatalyst (Pd/TiO₂ ) with neat TiO₂ prior to the rWGS reaction. The in situ dispersion of Pd was reflected through a continuous increase of rWGS activity over 92 h and supported by kinetic analysis, infrared and X-ray absorption spectroscopies and scanning transmission electron microscopy. The thermodynamic stability of Pd under high-temperature rWGS conditions is associated with Pd-Ti coordination, which manifests upon O-vacancy formation, and the artificial increase in TiO₂ surface area.