Interface-Confined FeOx Adlayers Induced by Metal Support Interaction in Pt/FeOx Catalysts

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.

Disentangling Local Interfacial Confinement and Remote Spillover Effects in Oxide-Oxide Interactions

Supported oxides are widely used in many important catalytic reactions, in which the interaction between the oxide catalyst and oxide support is critical but still remains elusive. Here, we construct a chemically bonded oxide-oxide interface by chemical deposition of Co3O4 onto ZnO powder (Co3O4/ZnO), in which complete reduction of Co3O4 to Co0 has been strongly impeded. It was revealed that the local interfacial confinement effect between Co oxide and the ZnO support helps to maintain a metastable CoOx state in CO2 hydrogenation reaction, producing 93% CO. In contrast, a physically contacted oxide-oxide interface was formed by mechanically mixing Co3O4 and ZnO powders (Co3O4-ZnO), in which reduction of Co3O4 to Co0 was significantly promoted, demonstrating a quick increase of CO2 conversion to 45% and a high selectivity toward CH4 (92%) in the CO2 hydrogenation reaction. This interface effect is ascribed to unusual remote spillover of dissociated hydrogen species from ZnO nanoparticles to the neighboring Co oxide nanoparticles. This work clearly illustrates the equally important but opposite local and remote effects at the oxide-oxide interfaces. The distinct oxide-oxide interactions contribute to many diverse interface phenomena in oxide-oxide catalytic systems.

Micellar Nanoreactors Enabled Site-Selective Decoration of Pt Nanoparticles Functionalized Mesoporous SiO2 /WO3-x Composites for Improved CO Sensing

Site-selective and partial decoration of supported metal nanoparticles (NPs) with transition metal oxides (e.g., FeOx ) can remarkably improve its catalytic performance and maintain the functions of the carrier. However, it is challenging to selectively deposit transition metal oxides on the metal NPs embedded in the mesopores of supporting matrix through conventional deposition method. Herein, a restricted in situ site-selective modification strategy utilizing poly(ethylene oxide)-block-polystyrene (PEO-b-PS) micellar nanoreactors is proposed to overcome such an obstacle. The PEO shell of PEO-b-PS micelles interacts with the hydrolyzed tungsten salts and silica precursors, while the hydrophobic organoplatinum complex and ferrocene are confined in the hydrophobic PS core. The thermal treatment leads to mesoporous SiO2 /WO3-x framework, and meanwhile FeOx nanolayers are in situ partially deposited on the supported Pt NPs due to the strong metal-support interaction between FeOx and Pt. The selective modification of Pt NPs with FeOx makes the Pt NPs present an electron-deficient state, which promotes the mobility of CO and activates the oxidation of CO. Therefore, mesoporous SiO2 /WO3-x -FeOx /Pt based gas sensors show a high sensitivity (31 ± 2 in 50 ppm of CO), excellent selectivity, and fast response time (3.6 s to 25 ppm) to CO gas at low operating temperature (66 °C, 74% relative humidity).

Confinement chemistry of FeOx centers for activating molecular oxygen under ambient conditions

Activating molecular oxygen under mild conditions is highly important for developing advanced green technologies and for understanding the origin and running of life as well, which still remains a challenge. In this work, we report on the confinement chemistry for activating molecular oxygen over oxides under mild conditions by presenting the synthesis and characterization of FeO_x species confined to the pores of support CeO₂ nanospheres. Active catalytic materials are obtained by a controllable three-step method via the formation of porous CeO₂ nanospheres that have an average diameter of 120 nm and exhibit a large surface area of 168 m² g^-1 and a pore size of 18.7 nm, confining FeO_x in intimate contact with ultra-small Pt particles in pores. The optimized PtO_y-FeO_x/CeO₂-H catalyst showed an excellent performance in the preferential oxidation of CO reactions, as featured by 100% CO conversion at room temperature with almost no attenuation in a prolonged operation, which could not be accessible without pore-confined FeO_x centers. Mechanical studies prove that the reaction progresses via abnormal non-competitive adsorption associated with synergistic roles from uniform loading, stabilization of divalent Fe species, surface oxygen activation on CeO₂ supports, and the reduced H₂ spillover effect on Pt⁰, making the CO species adsorbed on Pt^δ+ easier to be desorbed. The methodology demonstrated here may inspire one to explore more advanced catalysts with high activity at room temperature essential for a wide range of applications.

Dynamic transformation between bilayer islands and dinuclear clusters of Cr oxide on Au(111) through environment and interface effects

SignificanceFor oxide catalysts, it is important to elucidate and further control their atomic structures. In this work, well-defined CrO2 bilayer islands and Cr2O7 dinuclear clusters have been grown on Au(111) and unambiguously identified by scanning tunneling microscopy and theoretical calculations. Upon cycled redox treatments, the two kinds of oxide nanostructures can be reversibly transformed. It is interesting to note that both Cr oxides do not exist in bulk but need to be stabilized by the metal surface and the specific environment. Our results suggest that both redox atmosphere and interface confinement effects can be used to construct an oxide nanostructure with the specific chemical state and structure.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions

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.

Synergetic effect dependence on activated oxygen in the interface of NiOx-modified Pt nanoparticles for the CO oxidation from first-principles

CO oxidation on NiO_x-modified Pt nanoparticles (NPs) was investigated by first-principles calculations and microkinetic methods. The binding energies of O₂ and CO on NiO_x/Pt suggest that CO adsorption is dominant and the CO oxidation mainly follows the Mars-van Krevelen (M-vK) mechanism. It was found that the interfacial O of NiO_x/Pt played a key role in the combination of adsorbed CO to O, as well as the O₂ dissociation. With a lower O vacancy formation energy, NiO_x/Pt_edge shows about four orders higher reaction rates than NiO_x/Pt₍₁₀₀₎. Microkinetic analysis suggests that the rate-determining step also depends on the active O at the interface. The calculations highlight the synergetic effect difference of NiO_x selectively deposited on the different sites of Pt NPs on the CO oxidation from the atomic reaction mechanism, and throws light on the high activity of CO oxidation on partially covered NiO_x/Pt_edge nanoparticles.

Bimetallic Nanoparticles Associating Noble Metals and First-Row Transition Metals in Catalysis

Bimetallic nanoparticles (NPs) are complex systems with properties that far exceed those of the individual constituents. In particular, association of a noble metal and a first-row transition metal are attracting increasing interest for applications in catalysis, electrocatalysis, and magnetism, among others. Such objects display a rich structural chemistry thanks to their ability to form intermetallic phases, random alloys, or core-shell species. However, under reaction conditions, the surface of these nanostructures may be modified due to migration, segregation, or isolation of single atoms, leading to the formation of original structures with enhanced catalytic activity. In this respect, Zakhtser et al. report in this issue of ACS Nano the synthesis and study of the chemical evolution of the surface of a series of PtZn nanostructured alloys. In this Perspective, we report some selected examples of bimetallic nanocatalysts and their increased activity compared to that of the corresponding pure noble metal, with a special focus on Pt-based systems. We also discuss the mobility of the species present on the catalyst surface and the electronic influence of one metal to the other.

Morphology-dependent interfacial interactions of Fe2O3 with Ag nanoparticles for determining the catalytic reduction of p-nitrophenol

In this work, we fabricated three kinds of Ag/Fe₂O₃ model catalysts with different morphologies to study the interfacial interactions between Ag and Fe₂O₃, and how they affected the catalytic activity in hydrogenation of p-nitrophenol was explored. The hydrothermal method was used to synthesize the metal oxide supported silver catalyst, with various morphologies including nanoplates (NPs), nanospheres (NSs), and nanocubes (NCs). The crystal structure, morphology and surface elements of the composite were investigated by various measurements, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The catalytic activity was also evaluated by the reduction of p-nitrophenol to p-aminophenol. It was found that the activities of the above catalysts varied with the morphology of the support. Among them, Ag/Fe₂O₃ NPs promoted the highest performance, Ag/Fe₂O₃ NSs were slightly inferior, and Ag/Fe₂O₃ NCs were the worst. At last, we ascribed the remarkable activity of Ag/Fe₂O₃ NPs to the strong metal-support interactions between Ag and Fe₂O₃.

Large-Pore Mesoporous CeO2 -ZrO2 Solid Solutions with In-Pore Confined Pt Nanoparticles for Enhanced CO Oxidation

Active and stable catalysts are highly desired for converting harmful substances (e.g., CO, NO_x ) in exhaust gases of vehicles into safe gases at low exhaust temperatures. Here, a solvent evaporation-induced co-assembly process is employed to design ordered mesoporous Ce_x Zr_{1- x} O₂ (0 ≤ x ≤ 1) solid solutions by using high-molecular-weight poly(ethylene oxide)-block-polystyrene as the template. The obtained mesoporous Ce_x Zr_{1- x} O₂ possesses high surface area (60-100 m² g^-1 ) and large pore size (12-15 nm), enabling its great capacity in stably immobilizing Pt nanoparticles (4.0 nm) without blocking pore channels. The obtained mesoporous Pt/Ce_0.8 Zr_0.2 O₂ catalyst exhibits superior CO oxidation activity with a very low T₁₀₀ value of 130 °C (temperature of 100% CO conversion) and excellent stability due to the rich lattice oxygen vacancies in the Ce_0.8 Zr_0.2 O₂ framework. The simulated catalytic evaluations of CO oxidation combined with various characterizations reveal that the intrinsic high surface oxygen mobility and well-interconnected pore structure of the mesoporous Pt/Ce_0.8 Zr_0.2 O₂ catalyst are responsible for the remarkable catalytic efficiency. Additionally, compared with mesoporous Pt/Ce_x Zr_{1- x} O₂ -s with small pore size (3.8 nm), ordered mesoporous Pt/Ce_x Zr_{1- x} O₂ not only facilitates the mass diffusion of reactants and products, but also provides abundant anchoring sites for Pt nanoparticles and numerous exposed catalytically active interfaces for efficient heterogeneous catalysis.

Biochar-supported magnetic noble metallic nanoparticles for the fast recovery of excessive reductant during pollutant reduction

The catalytic reduction of diverse pollutants by noble metal catalysts in the presence of reductants is a highly effective and widely used method. However, the considerable cost of noble metal catalysts impedes the practical application of this method, and the recovery of excessive reductants has not been reported previously. In this work, we prepared inexpensive biochar-supported magnetic noble metallic nanoparticles (NPs) and efficiently recovered the excessive reductants in the form of H₂. The as-synthesized biochar-supported noble metallic NPs exhibited high H₂ recovery during the 4-nitrophenol reduction reaction. Results showed that the catalysts with low noble metallic content have higher H₂ recovery rate than commercial Pd/C, Ag/C, and Pt/C. The catalytic mechanism of magnetic biochar-supported noble metallic NPs was demonstrated to be a "synergetic effect", where biochar and Fe₃O₄ acted as accelerants that enable noble metallic NPs to produce active hydrogen for the reduction reaction, and the excess active hydrogen atoms combined to form H₂.

2D oxides on metal materials: concepts, status, and perspectives

Two-dimensional oxide-on-metal materials: concepts, methods, and link to technological applications, with 5 subtopics: structural motifs, robustness, catalysis, ternaries, and nanopatterning.