Catalytic systems based on supported noble metals are extensively studied because of their widespread application. Discussions remain about the nature of the active species, whether they are atomically dispersed or nanoparticles, and their reactivity. In this work, combining in situ/operando spectroscopy with theoretical modeling, we propose a phase diagram of atomically dispersed platinum on ceria, demonstrating that it reversibly changes from PtIVO2 to PtIIO as a function of temperature and oxygen partial pressure. The phase diagram helps identify the stability domain of each species, while spectroscopies provide a quantitative evaluation depending on the reaction conditions. Finally, our results show that high-temperature activation in the presence of steam of supported atomically dispersed platinum enhances the activity toward low-temperature carbon monoxide oxidation because it promotes aggregation into nanoparticles. This work highlights the structure-activity relationship in supported metal catalysts and proposes a suitable approach to determine the amount of each species before the investigation of the reaction mechanism.

The metal-support interaction plays a critical role in heterogeneous catalysis. Under reducing conditions, oxidic supports may interact with supported metal particles, by either forming an oxide overlayer or an alloy. The structure of both the support and the nanoparticle, as well as of the interface itself, changes in response to varying environmental conditions. Here, we present a fullyab initioapproach to predict the structures and energetics of such systems for a range of transition metals (Me = Cu, Ru, Pd, Ag, Rh, Os, Ir, Pt, Au) supported on titania surfaces as a function of gas atmosphere composition. The competing formation of a monolayer comprising fully oxidized titania (TiO₂), its reduced forms (Ti₂O₃, TiO), and the Ti-Me surface alloy, is investigated. The stability of each of these phases is found to be very sensitive to the environmental conditions and the supported metal. Encapsulation of metal, also known as classical strong metal-support interaction (SMSI), was predicted by thermodynamic driving force analysis. We show that a simple parameter, the Ti-Me alloy formation energy, is a good descriptor for the strength of the interaction between metal substrates and reduced titania monolayers and has predictive power towards the conditions under which an overlayer is stable. The presented thermochemical data and phase diagram analysis can be used to identify the structure and stability of supported metal catalysts under realistic conditions.

The direct conversion of methane to methanol using oxygen is a challenging but potentially rewarding pathway towards utilizing methane. By using a stepwise chemical looping approach, copper-exchanged zeolites can convert methane to methanol, but productivity is still too low for viable implementation. However, if the nature of the active site could be elucidated, that information could be used to design more effective catalysts. By employing anomalous X-ray powder diffraction with support from theory and other X-ray techniques, we have derived a quantitative and spatial description of the highly selective, active copper sites in zeolite omega (Cu-omega). This is the first comprehensive description of the structure of non-copper-oxo active species and will provide a pivotal model for future development for materials for methane to methanol conversion.

In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure of the catalyst active site, remains the subject of intensive debate. By using high-pressure operando techniques: steady-state isotope transient kinetic analysis coupled with infrared spectroscopy, together with time-resolved X-ray absorption spectroscopy and X-ray powder diffraction, and supported by electron microscopy and theoretical modeling, we present direct evidence that zinc formate is the principal observable reactive intermediate, which in the presence of hydrogen converts into methanol. Our results indicate that the copper–zinc alloy undergoes oxidation under reaction conditions into zinc formate, zinc oxide and metallic copper. The intimate contact between zinc and copper phases facilitates zinc formate formation and its hydrogenation by hydrogen to methanol.

Heterogeneous catalysts play a pivotal role in the chemical industry. The strong metal-support 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.

In this critical review we examine the current state of our knowledge in respect of the nature of the active sites in copper containing zeolites for the selective conversion of methane to methanol. We consider the varied experimental evidence arising from the application of X-ray diffraction, and vibrational, electronic, and X-ray spectroscopies that exist, along with the results of theory. We aim to establish both what is known regarding these elusive materials and how they function, and also where gaps in our knowledge still exist, and offer suggestions and strategies as to how these might be closed such that the rational design of more effective and efficient materials of this type for the selective conversion of methane might proceed further.

Copper-exchanged zeolites are a class of redox-active materials that find application in the selective catalytic reduction of exhaust gases of diesel vehicles and, more recently, the selective oxidation of methane to methanol. However, the structure of the active copper-oxo species present in zeolites under oxidative environments is still a subject of debate. Herein, we make a comprehensive study of copper species in copper-exchanged zeolites with MOR, MFI, BEA, and FAU frameworks and for different Si/Al ratios and copper loadings using X-ray absorption spectroscopy. Only obtaining high quality EXAFS data, collected at largek-values and measured under cryogenic conditions, in combination with wavelet transform analysis enables the discrimination between the copper-oxo species having different structures. The zeolite topology strongly affects the copper speciation, ranging from monomeric copper species to copper-oxo clusters, hosted in zeolites of different topologies. In contrast, the variation of the Si/Al ratio or copper loading in mordenite does not lead to significant differences in XAS spectra, suggesting that a change, if any, in the structure of copper species in these materials is not distinguishable by EXAFS.

Through the combination of density functional theory calculations and ab initio atomistic thermodynamics modeling, we demonstrate that atomically dispersed platinum species on ceria can adopt a range of local coordination configurations and oxidation states that depend on the surface structure and environmental conditions. Unsaturated oxygen atoms on ceria surfaces play the leading role in stabilization of PtO_x species. Any mono-dispersed Pt⁰ species are thermodynamically unstable compared to bulk platinum, and oxidation of Pt⁰ to Pt²⁺ or Pt⁴⁺ is necessary to stabilize mono-dispersed platinum atoms. Reduction to Pt⁰ leads to sintering. Both Pt²⁺ and Pt⁴⁺ prefer to form the square-planar [PtO₄] configuration. The two most stable Pt²⁺ species on the (223) and (112) surfaces are thermodynamically favorable between 300 and 1200 K. The most stable Pt⁴⁺ species on the (100) surface tends to desorb from the surface as gas phase above 950 K. The resulting phase diagrams of the atomically dispersed platinum in PtO_x clusters on various ceria surfaces under a range of experimentally relevant conditions can be used to predict dynamic restructuring of atomically dispersed platinum catalysts and design new catalysts with engineered properties.

Development of a suitable mild-condition process for direct conversion of methane to methanol faces multiple challenges, the principal ones being the higher reactivity of the primary oxidation products and the need for temperature swings in the typically employed chemical looping procedures. To circumvent these problems, the use of water as a mild oxidant has been recently suggested, leading to the concurrent formation of molecular hydrogen. By means of ab initio calculations, we address the experimentally observed features of the reaction to identify possible reaction pathways of such hydrogen release. We propose that, along with a strong stabilizing effect of water, short-lived [Cu-H] intermediate species play a crucial role in the mechanism of the reaction. Proton transfer from the Brønsted acid site of the zeolite framework via an adsorbed water molecule to the Cu^I species generates a [Cu-H] intermediate, which then facilitates the release of molecular hydrogen. This allows the reaction to proceed over a relatively low-energy transition state configuration. At the same time, excess of water leads to increased complexity of the concerted transition state, which results in hindering of the hydrogen transfer and increase of the corresponding energy barrier.

Direct methane functionalization and, in particular, the selective partial oxidation to methanol, remains an eminent challenge and a field of competitive research. The conversion of methane to methanol over transition-metal-containing zeolites using molecular oxygen is a promising and extensively studied process. Herein, we scrutinize some oft-cited assumptions in this topic—which include the labelling of the process as biomimetic, the debate regarding the industrial viability of direct methane-oxidation systems and the claim that methane is difficult to activate—and delineate the extent to which these are scientifically robust. We highlight both the merits and pitfalls of such statements and point out the hazards associated with their improper use. By examining these misconceptions, we build an outlook for future research, highlighting the need to optimize materials and process conditions for the stepwise approach and to further explore catalytic processes that explicitly employ strategies for the preservation of methanol.

A direct route to convert methane into high-value commodities, such as methanol, with high selectivity is one of the primary challenges in modern chemistry. Copper-exchanged zeolites show remarkable selectivity in the chemical looping process. Although multiple copper species have been proposed as active, an in situ spectroscopic investigation is difficult, because of their similar fingerprints. We used ambient pressure X-ray photoelectron spectroscopy to investigate an actual powder sample. We could discriminate between different types of active species involved in the conversion of methane to methanol over two different copper-exchanged zeolites, namely, mordenite and mazzite. After activation at 400 °C in oxygen, we followed the reaction in situ at 200 °C, switching from methane to water, and followed by a second cycle with anaerobic activation. Our experimental results, combined with theoretical calculations, prove that Cu(II) sites bound to extra-framework oxygen are involved in the reaction, and that their structure, formation, and stabilization depend on the type of zeolite and on the Si/Al ratio. ©

Samples of the zeolite mordenite with different Si/Al ratios were used to synthesize materials with monomeric and oligomeric copper sites that are active in the direct conversion of methane into methanol. A comparison of two reactivation protocols with oxygen (aerobic oxidation) and water (anaerobic oxidation), respectively, revealed that such copper–oxo species possess different reactivity towards methane and water. We show for the first time that oligomeric copper species exhibit high activity under both aerobic and anaerobic activation conditions, whereas monomeric copper sites produce methanol only in aerobic processes.

A detailed study on the synthesis of Pt₃Co intermetallic nanoparticles supported on ceria via preferential chemical vapor deposition was conducted, leading to a fundamental understanding of the deposition process and the Co-Pt alloying. This understanding helps us to develop a facile and scalable method for preferential adding of a metal on the surface of another metal already supported on an oxide which facilitates the design of novel structured nanoparticles. The fluidized flow reactor eliminated the deposition profile and resulted in Pt₃Co nanoparticles uniformly and homogeneously distributed on ceria. The kinetic study of cobalt deposition on the platinum surface, in accordance with DFT calculations, demonstrates that the platinum surface catalyzes the deposition reaction; while the ceria surface is inert in the preferential deposition temperature window between 150 °C and 180 °C. The obtained sample was characterized by in situ XRD, HAADF-STEM, FT-IR, and XAS methods. The results indicate the formation of uniform Pt₃Co nanoparticles with an average size of 1.1 nm. This sample showed superior catalytic activity in preferential oxidation of CO with an almost twice higher CO conversion rate and CO₂ selectivity compared to a classically synthesized sample with successive impregnation.

Ostwald ripening is a leading cause of the degradation of platinum group catalysts at high temperature in an oxidizing atmosphere. Recent experiments suggested that volatile species can be trapped on ceria, forming atomically dispersed active catalytic sites instead of large nanoparticles. Here we present a comparative density functional theory study of the interaction of PtO₂(g), the most likely volatile species responsible for the process of Ostwald ripening, with various surfaces. Defect-free CeO₂(111) and Al₂O₃(100) surfaces have a very small binding energy towards PtO₂(g) compared to the platinum surface, indicative of particle growth. However, the stepped edge of the CeO₂(111) surface effectively traps the mobile species, generating atomically dispersed catalysts. Such trapped single-atom platinum-on-ceria catalysts are predicted to have a square-planar [PtO₄] structure, with the platinum atom strongly binding to the surface, preventing platinum atoms from aggregating into larger nanoparticles. These results provide an atomic insight into the single atom trapping and suggest a route for the development of sinter-resistant catalysts.

Copper-containing zeolites exhibit high activity in the direct partial oxidation of methane into methanol at relatively low temperatures. Di- and tricopper species have been proposed as active catalytic sites, with recent experimental evidence also suggesting the possibility of the formation of larger copper oxide species. Using density functional theory based global geometry optimization, we were able to identify a general trend of the copper oxide cluster stability increasing with size. For instance, the identified ground-state structures of tetra- and pentamer copper clusters of Cu_nO_n²⁺ and Cu_nO_n-1²⁺ stoichiometries embedded in an 8-ring channel of mordenite exhibit higher relative stability compared to smaller clusters. Moreover, the aluminium content and localization in the zeolite pore influence the cluster's stability and its geometrical motif, which offers a perspective of tuning the properties of copper-exchanged zeolites by creating copper oxide clusters of a given structure and size. With the activity of the cluster towards methane being connected to its stability, such tuning will potentially allow the design of catalysts with engineered properties.

Periana argues that the stepwise reaction of methane with water is thermodynamically unfavorable and therefore impractical. We reply by presenting an in-depth thermodynamic analysis of each step in the process and show that the surface concentrations of the reactants and products as well as the stabilizing effect of additional water molecules, as discussed in the original paper, fully support the feasibility of the proposed reaction.

Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (∼97%) over a copper-containing zeolite, based on partial oxidation with water. The activation in helium at 673 kelvin (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mole of CH₃OH per mole of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, infrared spectroscopy, and density functional theory calculations, we propose a mechanism involving methane oxidation at Cu^II oxide active centers, followed by Cu^I reoxidation by water with concurrent formation of hydrogen.