Strong Metal–Support Interactions between Nickel and Iron Oxide during CO2 Hydrogenation

Metal-support interactions in metal oxide-supported atomic, cluster, and nanoparticle catalysis

Supported metal catalysts are essential to a plethora of processes in the chemical industry. The overall performance of these catalysts depends strongly on the interaction of adsorbates at the atomic level, which can be manipulated and controlled by the different constituents of the active material (i.e., support and active metal). The description of catalyst activity and the relationship between active constituent and the support, or metal-support interactions (MSI), in heterogeneous (thermo)catalysts is a complex phenomenon with multivariate (dependent and independent) contributions that are difficult to disentangle, both experimentally and theoretically. So-called "strong metal-support interactions" have been reported for several decades and summarized in excellent review articles. However, in recent years, there has been a proliferation of new findings related to atomically dispersed metal sites, metal oxide defects, and, for example, the generation and evolution of MSI under reaction conditions, which has led to the designation of (sub)classifications of MSI deserving to be critically and systematically evaluated. These include dynamic restructuring under alternating redox and reaction conditions, adsorbate-induced MSI, and evidence of strong interactions in oxide-supported metal oxide catalysts. Here, we review recent literature on MSI in oxide-supported metal particles to provide an up-to-date understanding of the underlying physicochemical principles that dominate the observed effects in supported metal atomic, cluster, and nanoparticle catalysts. Critical evaluation of different subclassifications of MSI is provided, along with discussions on the formation mechanisms, theoretical and characterization advances, and tuning strategies to manipulate catalytic reaction performance. We also provide a perspective on the future of the field, and we discuss the analysis of different MSI effects on catalysis quantitatively.

Control of metal-support interaction for tunable CO hydrogenation performance over Ru/TiO2 nanocatalysts

The catalytic behavior of CO hydrogenation can be modulated by metal-support interactions, while the role of the support remains elusive. Herein, we demonstrate that the presence of strong metal-support interactions (SMSI) depends strongly on the crystal phase of TiO2 (rutile or anatase) and the treatment conditions for the TiO2 support, which could critically control the activity and selectivity of Ru-based nanocatalysts for CO hydrogenation. High CO conversion and olefin selectivity were observed for Ru/rutile-TiO2 (Ru/r-TiO2), while catalysts supported by anatase (a-TiO2) showed almost no activity. Characterization confirmed that the SMSI effect could be neglected for Ru/r-TiO2, while it is dominant on Ru/a-TiO2 after reduction at 300 °C, resulting in the coverage of Ru nanoparticles by TiOx overlayers. Such SMSI could be suppressed by H2 treatment of the a-TiO2 support and the catalytic activity of the as-obtained Ru/a-TiO2(H2) can be greatly elevated from almost inactive to >50% CO conversion with >60% olefin selectivity. Further results indicated that the surface reducibility of the TiO2 support determines the SMSI state and catalytic performance of Ru/TiO2 in the CO hydrogenation reaction. This work offers an effective strategy to design efficient catalysts for the FTO reaction by regulating the crystal phase of the support.

Niobium Modification of CeO2 Tuning Electron Density of Nickel-Ceria Interfacial Sites for Enhanced CO2 Methanation

CO2 methanation has attracted considerable attention as a promising strategy for recycling CO2 and generating valuable methane. This study presents a niobium-doped CeO2-supported Ni catalyst (Ni/NbCe), which demonstrates remarkable performance in terms of CO2 conversion and CH4 selectivity, even when operating at a low temperature of 250 °C. Structural analysis reveals the incorporation of Nb species into the CeO2 lattice, resulting in the formation of a Nb-Ce-O solid solution. Compared with the Ni/CeO2 catalyst, this solid solution demonstrates an improved spatial distribution. To comprehend the impact of the Nb-Ce-O solid solution on refining the electronic properties of the Ni-Ce interfacial sites, facilitating H2 activation, and accelerating the hydrogenation of CO2* into HCOO*, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis and density functional theory (DFT) calculations were conducted. These investigations shed light on the mechanism through which the activity of CO2 methanation is enhanced, which differs from the commonly observed CO* pathway triggered by oxygen vacancies (OV). Consequently, this study provides a comprehensive understanding of the intricate interplay between the electronic properties of the catalyst's active sites and the reaction pathway in CO2 methanation over Ni-based catalysts.

Reinforcing the Efficiency of Photothermal Catalytic CO2 Methanation through Integration of Ru Nanoparticles with Photothermal MnCo2O4 Nanosheets

Carbon dioxide (CO2) hydrogenation to methane (CH4) is regarded as a promising approach for CO2 utilization, whereas achieving desirable conversion efficiency under mild conditions remains a significant challenge. Herein, we have identified ultrasmall Ru nanoparticles (∼2.5 nm) anchored on MnCo2O4 nanosheets as prospective photothermal catalysts for CO2 methanation at ambient pressure with light irradiation. Our findings revealed that MnCo2O4 nanosheets exhibit dual functionality as photothermal substrates for localized temperature enhancement and photocatalysts for electron donation. As such, the optimized Ru/MnCo2O4-2 gave a high CH4 production rate of 66.3 mmol gcat-1 h-1 (corresponding to 5.1 mol gRu-1 h-1) with 96% CH4 selectivity at 230 °C under ambient pressure and light irradiation (420-780 nm, 1.25 W cm-2), outperforming most reported plasmonic metal-based catalysts. The mechanisms behind the intriguing photothermal catalytic performance improvement were substantiated through a comprehensive investigation involving experimental characterizations, numerical simulations and density functional theory (DFT) calculations, which unveiled the synergistic effects of enhanced charge separation efficiency, improved reaction kinetics, facilitated reactant adsorption/activation and accelerated intermediate conversion under light irradiation over Ru/MnCo2O4. A comparison study showed that, with identical external input energy during the reaction, Ru/MnCo2O4-2 had a much higher catalytic efficiency compared to Ru/TiO2 and Ru/Al2O3. This study underscores the pivotal role played by photothermal supports and is believed to engender a heightened interest in plasmonic metal nanoparticles anchored on photothermal substrates for CO2 methanation under mild conditions.

Architecture Design and Catalytic Activity: Non-Noble Bimetallic CoFe/fe3 O4 Core-Shell Structures for CO2 Hydrogenation

Non-noble metal catalysts now play a key role in promoting efficiently and economically catalytic reduction of CO₂ into clean energy, which is an important strategy to ameliorate global warming and resource shortage issues. Here, a non-noble bimetallic catalyst of CoFe/Fe₃ O₄ nanoparticles is successfully designed with a core-shell structure that is well dispersed on the defect-rich carbon substrate for the hydrogenation of CO₂ under mild conditions. The catalysts exhibit a high CO₂ conversion activity with the rate of 30% and CO selectivity of 99%, and extremely robust stability without performance decay over 90 h in the reverse water gas shift reaction process. Notably, it is found that the reversible exsolution/dissolution of cobalt in the Fe₃ O₄ shell will lead to a dynamic and reversible deactivation/regeneration of the catalysts, accompanying by shell thickness breathing during the repeated cycles, via atomic structure study of the catalysts at different reaction stages. Combined with density functional theory calculations, the catalytic activity reversible regeneration mechanism is proposed. This work reveals the structure-property relationship for rational structure design of the advanced non-noble metallic catalyst materials with much improved performance.

In situ study of the evolution of NiFe nanocatalysts in reductive and oxidative environments upon thermal treatments

The conversion of biomass as a sustainable path to access valuable chemicals and fuels is very attractive for the chemical industry, but catalytic conversions still often rely on the use of noble metals. Sustainability constraints require developing alternative catalysts from abundant and low-cost metals. In this context, NiFe nanoparticles are interesting candidates. In their reduced and supported form, they have been reported to be more active and selective than monometallic Ni in the hydrogenation of the polar functions of organic molecules, and the two metals are very abundant. However, unlike noble metals, Ni and Fe are easily oxidized in ambient conditions, and understanding their transformation in both oxidative and reductive atmospheres is an important though seldom investigated issue to be addressed before their application in catalysis. Three types of NiFe nanoparticles were prepared by an organometallic approach to ensure the formation of ultrasmall nanoparticles (<3.5 nm) with a narrow size distribution, controlled composition and chemical order, while working in mild conditions: Ni2Fe1 and Ni1Fe1, both with a Ni rich core and Fe rich surface, and an alloy with a Ni1Fe9 composition. Supported systems were obtained by the impregnation of silica with a colloidal solution of the preformed nanoparticles. Using advanced characterization techniques, such as wide-angle X-ray scattering (WAXS) and X-ray absorption spectroscopy (XAS) in in situ conditions, this study reports on the evolution of the chemical order and of the oxidation state of the metals upon exposure to air, hydrogen, and/or increasing temperature, all factors that may affect their degree of reduction and subsequent performance in catalysis. We show that if oxidation readily occurs upon exposure to air, the metals can revert to their initial state upon heating in the presence of H₂ but with a change in structure and chemical ordering.

Engineering Heterogeneous Catalysis with Strong Metal-Support Interactions: Characterization, Theory and Manipulation

Strong metal-support interactions (SMSI) represent a classic yet fast-growing area in catalysis research. The SMSI phenomenon results in the encapsulation and stabilization of metal nanoparticles (NPs) with the support material that significantly impacts the catalytic performance through regulation of the interfacial interactions. Engineering SMSI provides a promising approach to steer catalytic performance in various chemical processes, which serves as an effective tool to tackle energy and environmental challenges. Our Minireview covers characterization, theory, catalytic activity, dependence on the catalytic structure and inducing environment of SMSI phenomena. By providing an overview and outlook on the cutting-edge techniques in this multidisciplinary research field, we not only want to provide insights into the further exploitation of SMSI in catalysis, but we also hope to inspire rational designs and characterization in the broad field of material science and physical chemistry.

Hydrogen Interaction with Oxide Supports in the Presence and Absence of Platinum

Oxides are essential catalysts and supports for noble metal catalysts. Their interaction with hydrogen enables, e.g., their use as a hydrogenation catalyst. Among the oxides considered reducible, substantial differences exist in their capability to activate hydrogen and how the oxide structure transforms due to this interaction. Noble metals, like platinum, generally enhance the oxide reduction by hydrogen spillover. This work presents a systematic temperature-programmed reduction study (300 to 873 K) of iron oxide, ceria, titania, zirconia, and alumina, with and without supported platinum. For all catalysts, platinum enhances the reducibility of the oxide. However, there are pronounced differences among all catalysts.

Interaction between Nickel Oxide and Support Promotes Selective Catalytic Reduction of NOx with C3H6

Selective catalytic reduction of NO x by C 3 H 6 (C 3 H 6 -SCR) was investigated over NiO catalysts supported on different metaloxides. A NiAlO x mixed oxide phase was formed over NiO/γ-Al 2 O 3 catalyst, inducing an immediate interaction between NiO x and AlO x species. Such interaction resulted in a charge transfer from Ni to Al site and the formation of Ni species in high oxidation state. In comparison to other NiO-loaded catalysts, NiO/γ-Al 2 O 3 catalyst exhibited the highest NO x conversion at temperature higher than 450 °C, but a poor C 3 H 6 oxidation activity due to the decreased nucleophilicity for surface oxygen species. By temperatureprogramed NO oxidation, it is indicated that nitrate species were rapidly formed and stably maintained at high temperature over NiO/γ-Al 2 O 3 catalyst. In situ transient reactions further verified the LangmuirHinshelwood mechanism for C 3 H 6 -SCR, where both gaseous NO and C 3 H 6 were adsorbed and activated on catalyst surface and reacted to generate N 2 . Due to the strong metal-support interaction over NiO/γ-Al 2 O 3 catalyst, both nitrate and C x H y O z intermediates were well preserved to attain high C 3 H 6 -SCR activity.

Controlling CO2 hydrogenation selectivity by Rh-based catalysts with different crystalline phases of TiO2

A series of Rh-based catalysts with various crystalline phases (p25, anatase, and rutile) were prepared via the incipient-wetness impregnation method. It was found that these catalysts had different metal-support interactions. Hence, 1%Rh/p, 1%Rh/r, and 1%Rh/a exhibited methane, CO, and methanol selectivity, respectively.