Heterogeneous catalysts play a pivotal role in the chemical industry. The strong metalsupport interaction (SMSI), which affects the catalytic activity, is a phenomenon researched for decades. However, detailed mechanistic understanding on real catalytic systems is lacking. Here, this surface phenomenon was studied on an actual platinum-titania catalyst by state-of-the-art in situ electron microscopy, in situ X-ray photoemission spectroscopy and in situ X-ray diffraction, aided by density functional theory calculations, providing a novel real time view on how the phenomenon occurs. The migration of reduced titanium oxide, limited in thickness, and the formation of an alloy are competing mechanisms during high temperature reduction. Subsequent exposure to oxygen segregates the titanium from the alloy, and a thicker titania overlayer forms. This role of oxygen in the formation process and stabilization of the overlayer was not recognized before. It provides new application potential in catalysis and materials science.
Copper-zinc-alumina catalysts are the industrially-used formulation for methanol synthesis from carbon monoxide and carbon dioxide containing feedstock. Its high performance stems from synergies that develop between its components. This important catalytic system has been investigated with a myriad of approaches, however, no comprehensive agreement has emerged as to the fundamental source of its high activity. One potential source of the disagreements is the considerable variation in pressure used in studies to understand a process that is industrially performed at pressures above 20 bar. Here, by systematically studying the catalyst state during temperature-programmed reduction and under carbon dioxide hydrogenation with in situ and operando X-ray absorption spectroscopy over four orders of magnitude in pressure, we show how the state and evolution of the catalyst is defined by its environment. Especially below 1 bar, the structure of the catalyst shows a strong pressure dependence. As pressure gaps are a general problem in catalysis, these observations have wide-ranging ramifications.The improvement of heterogeneous catalysts is central to a sustainable development of energy conversion and the production of chemicals. Historically, such development relied heavily on trial-and-error based research. More recently, advances in characterization methods allowed the study of catalysts under pretreatment and catalytic conditions, thus in situ and operando. This has permitted the possibility to derive fundamental understanding of the state of the catalyst whilst it is actually working 1 . Ideally, a detailed comprehension of the reaction mechanisms of the desired catalytic reaction emerges. Many of these methods, such as electron microscopy and X-ray photoelectron spectroscopy, remain limited in their routine application to pressure regimes in the millibar range  ; others, however, such as X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), suffer no
The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)–induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal–oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.
Dependent on the application or characterization method catalysts are exposed to different gas pressures, which results in different structures. The quantitative determination of the structure and composition of a catalyst as a function of its gas environment allows the establishment of structure−performance relationships. Herein, we determine the structure of a platinum− titania catalyst under hydrogen during temperature-programmed reduction over 3 orders of magnitude in pressure, from 1 to 950 mbar. The pressure significantly influences the hydrogen uptake kinetics and the consecutive structural transformations of the platinum−titania catalyst. The reduction of the platinum precursor becomes pressure-independent above 30 mbar. Yet, the related spillover and stability of adsorbed hydrogen on the titania are a function of pressure. Higher pressures promote higher hydrogen uptake and prevent desorption of hydrogen from the catalyst. The hydrogen uptake triggers a phase transformation of anatase to rutile which is, as a result, pressure dependent. The presented systematic approach establishes a pressure−structure relation which can be applied for the catalyst treatment and to frame existing results on the catalytic system. Treating the same material at two different pressures will lead to different structures.
During the CO oxidation over metallic Pt clusters and Pt nanoparticles in Pt/CeO 2 catalysts, we found that the Pt surface concentration is a key descriptor for the reaction rate. By increasing the surface noble metal concentration (SNMC) of a Pt/CeO 2 catalyst by a factor of ∼4, while keeping the weight hourly space velocity constant, the ignition temperature of CO oxidation was decreased by ∼200 °C in the as-prepared state. Moreover, the stability was enhanced at higher SNMC. Complementary characterization and theoretical calculations unraveled that the origin of this improved reaction rate at higher Pt surface concentrations can be traced back to the formation of larger oxidized Pt-clusters and the SNMC-dependent aggregation rate of highly dispersed Pt species. The Pt diffusion barriers for cluster formation were found to decrease with increasing SNMC, promoting more facile agglomeration of active, metallic Pt particles. In contrast, when Pt particle formation was forced with a reductive pretreatment, the influence of the SNMC was temporarily diminished, and all catalysts showed a similar CO oxidation activity. The work shows the general relevance of the proximity influence in the formation and stabilization of active centers in heterogeneous catalysis.
Zeolite-supported metal catalysts are widely employed in a number of chemical processes, and the stability of the catalytically active species is one of the most critical factors determining the reaction performance. A good example is the Pd/zeolite catalyst, which provides high activity for methane oxidation but deactivates rapidly under the reaction conditions due to palladium nanoparticle sintering. Although coating the metals with thin shells of porous materials is a promising strategy to address the sintering of metals, it is still challenging to fix small metal particles completely inside zeolite crystals. Here, using an aminebased ligand to stabilize palladium during the zeolite synthesis, we realize the exclusive encapsulation of highly dispersed palladium oxide clusters (1.8−2.8 nm) in the microporous channels and voids of the nanosized silicalite-1 crystals. The synthesis conditions of the zeolite-supported catalyst influence the encapsulation degree and the size distribution of metal particles. Thanks to the encapsulation effect of small palladium oxide clusters, together with the inherent properties of silicalite-1 such as low acidity, high hydrophobicity, and high hydrothermal stability, the optimized Pd@silicalite-1 catalyst outperforms the traditional Pd-based catalysts prepared by wetness impregnation, exhibiting both high activity and better stability in the lean methane oxidation reaction.
Using ab initio modelling, we demonstrate that a simple parameter – alloy formation energy – is a good descriptor of an interaction strength between metal substrates and oxide monolayers, which allows constructing structure–material–environment maps.
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