Opportunities for Tailoring Catalytic Properties Through Metal-Support Interactions

Facile Synthesis of Highly Dispersed and Well-Alloyed Bimetallic Nanoparticles on Oxide Support

Supported alloy catalysts play a pivotal role in many heterogeneous catalytic processes of socioeconomic and environmental importance. But the controlled synthesis of supported alloy nanoparticles with consistent composition and tight size distribution remains a challenging issue. Herein, a simple yet effective method for preparation of highly dispersed, homogeneously alloyed bimetallic nanoparticles on oxide supports is reported. This method is based on solid solution of metal cations in parent oxide and strong electrostatic adsorption of a secondary metal species onto the oxide surface. In the reductive annealing process, hydrogen spillover occurs from the surface metal with a higher reduction potential to the solute metal in solid solution, leading to metal exsolution and homogenous alloying of the metals on the oxide surface. The ceria-supported Ni-Pt alloy is chosen as a model catalyst and hydrazine monohydrate decomposition is chosen as a probe reaction to demonstrate this method, and particularly its advantages over the conventional impregnation and galvanic replacement methods. A systematic application of this method using different oxides and base-noble metal pairs further elucidates its applicability and generality.

Nanoscale interface engineering for solid oxide fuel cells using atomic layer deposition

Atomic layer deposition (ALD), which is already actively used in the semiconductor industry, has been in the spotlight in various energy fields, such as batteries and fuel cells, given its unique ability to enable the nanoscale deposition of diverse materials with a variety of compositions onto complex 3D structures. In particular, with regard to ceramic fuel cells, ALD has attracted attention because it facilitates the manufacturing of thin and dense electrolytes. Furthermore, recently, electrode surfaces and electrode/electrolyte interface modification are arising as new research strategies to fabricate robust fuel cells. In this mini-review, we present a brief overview of ALD and recent studies that utilize ALD in ceramic fuel cells, such as manufacturing thin film electrolytes, stabilizing electrodes, functionalizing electrodes, and modifying the chemistry of electrode surfaces. We also propose research directions to expand the utility and functionality of the ALD techniques.

Dry Reforming of Methane with Ni Supported on Mechanically Mixed Yttria-Zirconia Support

This study focuses on CH4 reforming with CO2 over Ni supported on yttria mixed with zirconia support. Different loading of yttria was used to enhance the performance of Ni towards achieving the optimum activity. The physicochemical properties of both fresh calcined and used catalysts were studied using a range of characterization techniques. The specific surface area measurement by the BET method showed a progressive increase in the area with an increase in yttria loading. The monoclinic (m- ZrO2) and tetragonal (t- ZrO2) phases were identified on all the samples by the XRD analysis. A reduction in the intensity of m- ZrO2 was observed on adding Ni to the catalysts while the diffraction pattern of crystalline yttria was not identified. The reducibility analysis showed the influence of yttria. It induces the formation of NiYO3 species with stronger active metal-support interaction. From the catalytic test, 5Ni/10Y-Z-3215 had the highest feed conversion of about 68 and 88% for CH4 and CO2 respectively. The TEM analysis showed a uniform dispersion of NiO particles over the mixed yttria-zirconia support with no agglomeration of the active metal particles after the reaction. The measurement of the quantity of carbon deposits by the TGA revealed that the increase in yttria loading enhanced the gasification of carbon deposits with 5Ni/20Y-Z-3215 recording the lowest weight loss of about 28%.

Graphical Abstract

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

A new synthesis of highly active Rh–Co alloy nanoparticles supported on N-doped porous carbon for catalytic C–Se cross-coupling and p-nitrophenol hydrogenation reactions

New Rh–Co/NPC synthesized through a one-pot thermal reduction synthesis has a high catalytic activity for organic chemistry reactions.

Designing Nanoparticles and Nanoalloys for Gas-Phase Catalysis with Controlled Surface Reactivity Using Colloidal Synthesis and Atomic Layer Deposition

Supported nanoparticles are commonly applied in heterogeneous catalysis. The catalytic performance of these solid catalysts is, for a given support, dependent on the nanoparticle size, shape, and composition, thus necessitating synthesis techniques that allow for preparing these materials with fine control over those properties. Such control can be exploited to deconvolute their effects on the catalyst's performance, which is the basis for knowledge-driven catalyst design. In this regard, bottom-up synthesis procedures based on colloidal chemistry or atomic layer deposition (ALD) have proven successful in achieving the desired level of control for a variety of fundamental studies. This review aims to give an account of recent progress made in the two aforementioned synthesis techniques for the application of controlled catalytic materials in gas-phase catalysis. For each technique, the focus goes to mono- and bimetallic materials, as well as to recent efforts in enhancing their performance by embedding colloidal templates in porous oxide phases or by the deposition of oxide overlayers via ALD. As a recent extension to the latter, the concept of area-selective ALD for advanced atomic-scale catalyst design is discussed.

NiPt Nanoparticles Anchored onto Hierarchical Nanoporous N-Doped Carbon as an Efficient Catalyst for Hydrogen Generation from Hydrazine Monohydrate

Catalytic decomposition of the hydrogen-rich hydrazine monohydrate (N₂H₄·H₂O) represents a promising hydrogen storage/production technology. A rational design of advanced N₂H₄·H₂O decomposition catalysts requires an overall consideration of intrinsic activity, number, and accessibility of active sites. We herein report the synthesis of a hierarchically nanostructured NiPt/N-doped carbon catalyst using a three-step method that can simultaneously address these issues. The chelation of metal precursors with polydopamine and thermolysis of the resulting complexes under reductive atmosphere resulted in a concurrent formation of N-doped carbon substrate and catalytically active NiPt alloy nanoparticles. Thanks to the usage of a silica nanosphere template and dopamine precursor, the N-doped carbon substrate possesses a hierarchical macroporous-mesoporous architecture. This, together with the uniform dispersion of tiny NiPt nanoparticles on the carbon substrate, offers opportunity for creating abundant and accessible active sites. Benefiting from these favorable attributes, the NiPt/N-doped carbon catalyst enables a complete and rapid hydrogen production from alkaline N₂H₄·H₂O solution with a rate of 1602 h^-1 at 50 °C, which outperforms most existing catalysts for N₂H₄·H₂O decomposition.

Gas-Permeable Inorganic Shell Improves the Coking Stability and Electrochemical Reactivity of Pt toward Methane Oxidation

Solid oxide fuel cells produce electricity directly by oxidizing methane, which is the most attractive natural gas fuel, and metal nanocatalysts are a promising means of overcoming the poor catalytic activity of conventional ceramic electrodes. However, the lack of thermal and chemical stability of nanocatalysts is a major bottleneck in the effort to ensure the lifetime of metal-decorated electrodes for methane oxidation. Here, for the first time, this issue is addressed by encapsulating metal nanoparticles with gas-permeable inorganic shells. Pt particles approximately 10 nm in size are dispersed on the surface of a porous La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) electrode via wet infiltration and are then coated with an ultrathin Al₂O₃ layer via atomic layer deposition. The Al₂O₃ overcoat, despite being an insulator, significantly enhances the immunity to carbon coking and provides high activity for the electrochemical oxidation of methane, thereby reducing the reaction impedance of the Pt-decorated electrode by more than 2 orders of magnitude and making the electrode activity of the Pt-decorated sample at 650 °C comparable with those reported at 800 °C for pristine LSCM electrodes. These observations provide a new perspective on strategies to lower the operation temperature, which has long been a challenge related to hydrocarbon-fueled solid oxide fuel cells.

CO2 Hydrogenation over Nanoceria-Supported Transition Metal Catalysts: Role of Ceria Morphology (Nanorods versus Nanocubes) and Active Phase Nature (Co versus Cu)

In this work we report on the combined impact of active phase nature (M: Co or Cu) and ceria nanoparticles support morphology (nanorods (NR) or nanocubes (NC)) on the physicochemical characteristics and CO2 hydrogenation performance of M/CeO2 composites at atmospheric pressure. It was found that CO2 conversion followed the order: Co/CeO2 > Cu/CeO2 > CeO2, independently of the support morphology. Co/CeO2 catalysts demonstrated the highest CO2 conversion (92% at 450 °C), accompanied by 93% CH4 selectivity. On the other hand, Cu/CeO2 samples were very selective for CO production, exhibiting 52% CO2 conversion and 95% CO selectivity at 380 °C. The results obtained in a wide range of H2:CO2 ratios (1-9) and temperatures (200-500 °C) are reaching in both cases the corresponding thermodynamic equilibrium conversions, revealing the superiority of Co- and Cu-based samples in methanation and reverse water-gas shift (rWGS) reactions, respectively. Moreover, samples supported on ceria nanocubes exhibited higher specific activity (µmol CO2·m-2·s-1) compared to samples of rod-like shape, disclosing the significant role of support morphology, besides that of metal nature (Co or Cu). Results are interpreted on the basis of different textural and redox properties of as-prepared samples in conjunction to the different impact of metal entity (Co or Cu) on CO2 hydrogenation process.

Ceria Nanoparticles’ Morphological Effects on the N2O Decomposition Performance of Co3O4/CeO2 Mixed Oxides

Ceria-based oxides have been widely explored recently in the direct decomposition of N2O (deN2O) due to their unique redox/surface properties and lower cost as compared to noble metal-based catalysts. Cobalt oxide dispersed on ceria is among the most active mixed oxides with its efficiency strongly affected by counterpart features, such as particle size and morphology. In this work, the morphological effect of ceria nanostructures (nanorods (ΝR), nanocubes (NC), nanopolyhedra (NP)) on the solid-state properties and the deN2O performance of the Co3O4/CeO2 binary system is investigated. Several characterization methods involving N2 adsorption at −196 °C, X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (ΤΕΜ) were carried out to disclose structure–property relationships. The results revealed the importance of support morphology on the physicochemical properties and the N2O conversion performance of bare ceria samples, following the order nanorods (NR) > nanopolyhedra (NP) > nanocubes (NC). More importantly, Co3O4 impregnation to different carriers towards the formation of Co3O4/CeO2 mixed oxides greatly enhanced the deN2O performance as compared to bare ceria samples, without, however, affecting the conversion sequence, implying the pivotal role of ceria support. The Co3O4/CeO2 sample with the rod-like morphology exhibited the best deN2O performance (100% N2O conversion at 500 °C) due to its abundance in Co2+ active sites and Ce3+ species in conjunction to its improved reducibility, oxygen kinetics and surface area.

Monolayer supported CuOx/Co3O4 as an active and selective low temperature NOx decomposition catalyst

A new NO decomposition catalyst, monolayer CuO_x/Co₃O₄, is reported with areal activity and N₂ selectivity exceeding Cu-ZSM5 at 350 °C.

Simultaneous tuning porosity and basicity of nickel@nickel-magnesium phyllosilicate core-shell catalysts for CO₂ reforming of CH₄

Ni@Ni-Mgphy (Ni-Mgphy = Ni-Mg phyllosilicate) core-shell catalysts were designed by hydrothermally treating Ni@SiO2 nanoparticles with magnesium nitrate salt. The porosity and basicity of the catalysts were easily tuned by forming Ni-Mgphy shell using Ni originating from Ni@SiO2 during the hydrothermal treatment process and Mg(NO3)2 as the Ni and Mg sources, respectively. Among Ni@Ni-Mgphy core-shell catalysts synthesized under different hydrothermal durations, the catalyst treated for 10 h achieved the best catalytic performance for CO2 reforming of CH4 reaction with stable CO2 and CH4 conversions of around 81% and 78%, respectively, within 95 h reaction duration at 700 °C. The high Ni accessibility, strong basicity, and high structural stability for Ni@Ni-Mgphy core-shell catalyst with 10 h treatment time accounted for its superb catalytic performance. This method to simultaneously tune the porosity and basicity of Ni@SiO2 core-shell nanoparticles demonstrates a general way to modify the properties of other silica based core-shell nanoparticles through treating them with different metal salts.

Chemistry of precious metal oxides relevant to heterogeneous catalysis

The platinum group metals (PGMs) are widely employed as catalysts, especially for the mitigation of automotive exhaust pollutants. The low natural abundance of PGMs and increasing demand from the expanding automotive sector necessitates strategies to improve the efficiency of PGM use. Conventional catalysts typically consist of PGM nanoparticles dispersed on high surface area oxide supports. However, high PGM loadings must be used to counter sintering, ablation, and deactivation of the catalyst such that sufficient activity is maintained over the operating lifetime. An appealing strategy for reducing metal loading is the substitution of PGM ions into oxide hosts: the use of single atoms (ions) as catalytic active sites represents a highly atom-efficient alternative to the use of nanoparticles. This review addresses the crystal chemistry and reactivity of oxide compounds of precious metals that are, or could be relevant to developing an understanding of the role of precious metal ions in heterogeneous catalysis. We review the chemical conditions that facilitate stabilization of the notoriously oxophobic precious metals in oxide environments, and survey complex oxide hosts that have proven to be amenable to reversible redox cycling of PGMs.