Embedded phases: a way to active and stable catalysts

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

Methane (CH₄ ) is an attractive energy source and important greenhouse gas. Therefore, from the economic and environmental point of view, scientists are working hard to activate and convert CH₄ into various products or less harmful gas at low-temperature. Although the inert nature of CH bonds requires high dissociation energy at high temperatures, the efforts of researchers have demonstrated the feasibility of catalysts to activate CH₄ at low temperatures. In this review, the efficient catalysts designed to reduce the CH₄ oxidation temperature and improve conversion efficiencies are described. First, noble metals and transition metal-based catalysts are summarized for activating CH₄ in temperatures ranging from 50 to 500 °C. After that, the partial oxidation of CH₄ at relatively low temperatures, including thermocatalysis in the liquid phase, photocatalysis, electrocatalysis, and nonthermal plasma technologies, is briefly discussed. Finally, the challenges and perspectives are presented to provide a systematic guideline for designing and synthesizing the highly efficient catalysts in the complete/partial oxidation of CH₄ at low temperatures.

Effect of Precursor Status on the Transition from Complex to Carbon Shell in a Platinum Core-Carbon Shell Catalyst

Encapsulating platinum nanoparticles with a carbon shell can increase the stability of core platinum nanoparticles by preventing their dissolution and agglomeration. In this study, the synthesis mechanism of a platinum core-carbon shell catalyst via thermal reduction of a platinum-aniline complex was investigated to determine how the carbon shell forms and identify the key factor determining the properties of the Pt core-carbon shell catalyst. Three catalysts originating from the complexes with different platinum to carbon precursor ratios were synthesized through pyrolysis. Their structural characteristics were examined using various analysis techniques, and their electrochemical activity and stability were evaluated through half-cell and unit-cell tests. The relationship between the nitrogen to platinum ratio and structural characteristics was revealed, and the effects on the electrochemical activity and stability were discussed. The ratio of the carbon precursor to platinum was the decisive factor determining the properties of the platinum core-carbon shell catalyst.

One-Step Solvothermal Synthesis of Ni Nanoparticle Catalysts Embedded in ZrO2 Porous Spheres to Suppress Carbon Deposition in Low-Temperature Dry Reforming of Methane

Ni nanoparticle catalysts embedded in ZrO₂ porous spheres and ZrO₂ porous composite spheres, SiO₂-ZrO₂, MgO-ZrO₂, and Y₂O₃-ZrO₂, with 83-115 nm diameter and 167-269 m²/g specific surface area were prepared by a one-pot and one-step solvothermal reaction from precursor solutions consisting of Ni(NO₃)₂‧6H₂O, Zr(OⁿBu)₄, and acetylacetone in moist ethanol combined with either Si(OEt)₄, magnesium acetylacetate, or Y(OⁱPr)₃. The obtained Ni catalysts have high specific surface areas of 130-196 m²/g, even after high-temperature reduction by H₂ at 450 °C for 2 h. They were utilized as catalysts for low-temperature dry reforming of methane (DRM) at 550 °C to suppress carbon deposition on Ni nanoparticles. The Ni catalysts embedded in SiO₂-ZrO₂ and Y₂O₃-ZrO₂ demonstrated high catalytic activity and long stability in the reaction. Moreover, carbon deposition on Ni nanoparticles in the DRM reaction was effectively suppressed in when using the SiO₂-ZrO₂ and Y₂O₃-ZrO₂ composites.

Highly dispersed Ni-based catalysts derived from the LaNiO3 perovskite for dry methane reforming: promotional effect of the Ni0–Ni2+ dipole inlaid on the support

A hyperdispersed Ni-based catalyst from LaNiO3 performed well in dry methane reforming reaction, which was attributed to the promotional effect of the Ni0–Ni2+ dipole.

Two Tungstates Containing Platinum Nanoparticles Prepared by Air-Calcining Keggin-Type Polyoxotungstate-Coordinated Diplatinum(II) Complexes: Effect on Sintering-Resistance and Photocatalysis

Two tungstates containing platinum nanoparticles (Pt Npts) were obtained by air-calcining α-Keggin-type diplatinum(II)-coordinated polyoxotungstates, Cs3[α-PW11O39{cis-Pt(NH3)2}2]⋅8H2O (Cs-P-Pt) and Cs4[α-SiW11O39{cis-Pt(NH3)2}2]⋅11H2O (Cs-Si-Pt), at 700–900 °C for 5 h. The polyoxotungstate Cs-P-Pt was transformed to a mixture of Pt Npts and Cs3PW12O40 upon calcination, while the Cs-Si-Pt structures were transformed to Pt Npts and Cs4W11O35. The Pt Npts generated by air-calcining Cs-P-Pt at 700 °C for 5 h were uniform with an average particle size of 3.6 ± 1.1 nm, which was much smaller than that of the Pt Npts obtained by calcining Cs-Si-Pt (19.9 ± 9.9 nm) under identical conditions. This demonstrated the significant inhibitory effect of Cs-P-Pt on aggregation during high-temperature air-calcination at a high platinum content (10.6 wt.%) and in the absence of a support. During calcination at 700–900 °C, Cs-P-Pt exhibited higher activities than Cs-Si-Pt with respect to hydrogen evolution from aqueous triethanolamine solutions under visible light irradiation in the presence of Eosin Y, α-Keggin-type mono-aluminum-substituted polyoxotungstate, and titanium dioxide. When Cs-P-Pt was calcined at 800 °C for 100 h, no decrease in activity was observed in comparison with that upon calcination for 5 h.

Graphical Abstract

Opportunities and challenges in the development of advanced materials for emission control catalysts

Advances in engine technologies are placing additional demands on emission control catalysts, which must now perform at lower temperatures, but at the same time be robust enough to survive harsh conditions encountered in engine exhaust. In this Review, we explore some of the materials concepts that could revolutionize the technology of emission control systems. These include single-atom catalysts, two-dimensional materials, three-dimensional architectures, core@shell nanoparticles derived via atomic layer deposition and via colloidal synthesis methods, and microporous oxides. While these materials provide enhanced performance, they will need to overcome many challenges before they can be deployed for treating exhaust from cars and trucks. We assess the state of the art for catalysing reactions related to emission control and also consider radical breakthroughs that could potentially completely transform this field.

Interpenetrating polysaccharide-based hydrogel: A dynamically responsive versatile medium for precisely controlled synthesis of nanometals

Herein, we reported an interpenetrating polysaccharide-based hydrogel in which carboxymethyl chitosan (CMC) chains were physically dispersed throughout the thermoplastic elastomer gel network has been developed as a versatile platform for precisely controlled synthesis of nanometals. Results indicated the interpenetrated CMC chains could serve as multifunctional fillers for metal ions adsorption and stabilization while the thermally reconfigurable agarose (Agar) gel medium provides three-dimensional semi-solid framework for entrapping and recollecting of the fabricated nanometals. Specifically, the CMC chains were found to strongly coordinate with silver ions as a dynamically responsive metal-biopolymer complex within the bulk gel network as confirmed by the enhanced mechanical properties and regulated shape memory performances. Moreover, by varying CMC concentrations and coupling with a layer-stacking approach, multiple biochemical gradients could be facilely generated for in-situ synthesis of silver nanoparticles, achieving a narrow size of ~7 nm, confined sphere-shape and high concentrations. The monodispersed nanometals are confirmed to be highly active (e.g., considerable catalytic performance), and which could be easily recycled from the bulk gel system via a heating treatment. Thus, this work would provide a generic methodology for the multifunctional metallogel assembly and great possibility for controllable and largescale synthesis of noble nanometals toward biomedical applications.

Systematic Incorporation of Gold Nanoparticles onto Mesoporous Titanium Oxide Particles for Green Catalysts

This report describes the systematic incorporation of gold nanoparticles (AuNPs) onto mesoporous TiO2 (MPT) particles without strong attractive forces to efficiently serve as reactive and recyclable catalysts in the homocoupling of arylboronic acid in green reaction conditions. Unlike using nonporous TiO2 particles and conventional SiO2 particles as supporting materials, the employment of MPT particles significantly improves the loading efficiency of AuNPs. The incorporated AuNPs are less than 10 nm in diameter, regardless of the amount of applied gold ions, and their surfaces, free from any modifiers, act as highly reactive catalytic sites to notably improve the yields in the homocoupling reaction. The overall physical properties of the AuNPs integrated onto the MPT particles are thoroughly examined as functions of the gold content, and their catalytic functions, including the rate of reaction, activation energy, and recyclability, are also evaluated. While the rate of reaction slightly increases with the improved loading efficiency of AuNPs, the apparent activation energies do not clearly show any correlation with the size or distribution of the AuNPs under our reaction conditions. Understanding the formation of these types of composite particles and their catalytic functions could lead to the development of highly practical, quasi-homogeneous catalysts in environmentally friendly reaction conditions.

Progress of Exsolved Metal Nanoparticles on Oxides as High Performance (Electro)Catalysts for the Conversion of Small Molecules

Utilizing electricity and heat from renewable energy to convert small molecules into value-added chemicals through electro/thermal catalytic processes has enormous socioeconomic and environmental benefits. However, the lack of catalysts with high activity, good long-term stability, and low cost strongly inhibits the practical implementation of these processes. Oxides with exsolved metal nanoparticles have recently been emerging as promising catalysts with outstanding activity and stability for the conversion of small molecules, which provides new possibilities for application of the processes. In this review, it starts with an introduction on the mechanism of exsolution, discussing representative exsolution materials, the impacts of intrinsic material properties and external environmental conditions on the exsolution behavior, and the driving forces for exsolution. The performances of exsolution materials in various reactions, such as alkane reforming reaction, carbon monoxide oxidation, carbon dioxide utilization, high temperature steam electrolysis, and low temperature electrocatalysis, are then summarized. Finally, the challenges and future perspectives for the development of exsolution materials as high-performance catalysts are discussed.

Size-controlled nanocrystals reveal spatial dependence and severity of nanoparticle coalescence and Ostwald ripening in sintering phenomena

A major aim in the synthesis of nanomaterials is the development of stable materials for high-temperature applications. Although the thermal coarsening of small and active nanocrystals into less active aggregates is universal in material deactivation, the atomic mechanisms governing nanocrystal growth remain elusive. By utilizing colloidally synthesized Pd/SiO2 powder nanocomposites with controlled nanocrystal sizes and spatial arrangements, we unravel the competing contributions of particle coalescence and atomic ripening processes in nanocrystal growth. Through the study of size-controlled nanocrystals, we can uniquely identify the presence of either nanocrystal dimers or smaller nanoclusters, which indicate the relative contributions of these two processes. By controlling and tracking the nanocrystal density, we demonstrate the spatial dependence of nanocrystal coalescence and the spatial independence of Ostwald (atomic) ripening. Overall, we prove that the most significant loss of the nanocrystal surface area is due to high-temperature atomic ripening. This observation is in quantitative agreement with changes in the nanocrystal density produced by simulations of atomic exchange. Using well-defined colloidal materials, we extend our analysis to explain the unusual high-temperature stability of Au/SiO2 materials up to 800 °C.

Comparative Catalytic Properties of Supported and Encapsulated Gold Nanoparticles in Homocoupling Reactions

This report describes strategies to increase the reactive surfaces of integrated gold nanoparticles (AuNPs) by employing two different types of host materials that do not possess strong electrostatic and/or covalent interactive forces. These composite particles are then utilized as highly reactive and recyclable quasi-homogeneous catalysts in a C-C bond forming reaction. The use of mesoporous TiO₂ and poly(N-isopropylacrylamide), PNIPAM, particles allows for the formation of relatively small and large guest AuNPs and provides the greatly improved stability of the resulting composite particles. As these AuNPs are physically incorporated into the mesoporous TiO₂ (i.e., supported AuNPs) and PNIPAM particles (i.e., encapsulated AuNPs), their surfaces are maximized to serve as highly reactive catalytic sites. Given their increased physicochemical properties (e.g., stability, dispersity, and surface area), these composite particles exhibit notably high catalytic activity, selectivity, and recyclability in the homocoupling of phenylboronic acid in water and EtOH. Although the small supported AuNPs display slightly faster reaction rates than the large encapsulated AuNPs, the apparent activation energies (E_a) of both composite particles are comparable, implying no obvious correlation with the size of guest AuNPs under the reaction conditions. Investigating the overall physical properties of various composite particles and their catalytic functions, including the reactivity, selectivity, and E_a, can lead to the development of highly practical quasi-homogeneous catalysts in green reaction conditions.

Spatial Ensembles of Copper-Silica with Carbon Nanotubes as Ultrastable Nanostructured Catalysts for Selective Hydrogenation

Catalyst deactivation is one of the most important issues in heterogeneous catalysis. Constructing a stable nanoscale structure that maintains efficient activity and prolonged stability under redox conditions for catalysis, particularly hydrogenation reactions, remains attractive albeit the flourishing nanoscience. This work presents a facile route to synthesize a semi-encapsulated transition metal by assembling three-dimensional transition metal silicate nanotubes onto carbon nanotubes (CNTs) as precursors. The obtained materials expose an active surface of the transition metal for efficient catalysis and form a specific structure to inhibit the migration of metal nanoparticles (NPs) by establishing strong metal-support interactions. Cu@SiO₂ prepared by common precipitation shows an inferior activity, and its performance is easily attenuated because of the aggregation of Cu NPs. The addition of CNTs as a carrier doubles the intrinsic activity of Cu catalysts. This hybrid catalyst, which consists of Cu species, SiO₂, and CNTs, is among the best catalysts for dimethyl oxalate hydrogenation with boosting activity of 25 h^-1 and enhanced stability of more than 200 h.

Three-Dimensional Mesoporous Ni-CeO2 Catalysts with Ni Embedded in the Pore Walls for CO2 Methanation

Mesoporous Ni-based catalysts with Ni confined in nanochannels are widely used in CO2 methanation. However, when Ni loadings are high, the nanochannels are easily blocked by nickel particles, which reduces the catalytic performance. In this work, three-dimensional mesoporous Ni-CeO2-CSC catalysts with high Ni loadings (20−80 wt %) were prepared using a colloidal solution combustion method, and characterized by nitrogen adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and H2 temperature programmed reduction (H2-TPR). Among the catalysts with different Ni loadings, the 50% Ni-CeO2-CSC with 50 wt % Ni loading exhibited the best catalytic performance in CO2 methanation. Furthermore, the 50% Ni-CeO2-CSC catalyst was stable for 50 h at 300° and 350 °C in CO2 methanation. The characterization results illustrate that the 50% Ni-CeO2-CSC catalyst has Ni particles smaller than 5 nm embedded in the pore walls, and the Ni particles interact with CeO2. On the contrary, the 50% Ni-CeO2-CP catalyst, prepared using the traditional coprecipitation method, is less active and selective for CO2 methanation due to the larger size of the Ni and CeO2 particles. The special three-dimensional mesoporous embedded structure in the 50% Ni-CeO2-CSC can provide more metal–oxide interface and stabilize small Ni particles in pore walls, which makes the catalyst more active and stable in CO2 methanation.

Endogenous Nanoparticles Strain Perovskite Host Lattice Providing Oxygen Capacity and Driving Oxygen Exchange and CH4 Conversion to Syngas

Particles dispersed on the surface of oxide supports have enabled a wealth of applications in electrocatalysis, photocatalysis, and heterogeneous catalysis. Dispersing nanoparticles within the bulk of oxides is, however, synthetically much more challenging and therefore less explored, but could open new dimensions to control material properties analogous to substitutional doping of ions in crystal lattices. Here we demonstrate such a concept allowing extensive, controlled growth of metallic nanoparticles, at nanoscale proximity, within a perovskite oxide lattice as well as on its surface. By employing operando techniques, we show that in the emergent nanostructure, the endogenous nanoparticles and the perovskite lattice become reciprocally strained and seamlessly connected, enabling enhanced oxygen exchange. Additionally, even deeply embedded nanoparticles can reversibly exchange oxygen with a methane stream, driving its redox conversion to syngas with remarkable selectivity and long term cyclability while surface particles are present. These results not only exemplify the means to create extensive, self-strained nanoarchitectures with enhanced oxygen transport and storage capabilities, but also demonstrate that deeply submerged, redox-active nanoparticles could be entirely accessible to reaction environments, driving redox transformations and thus offering intriguing new alternatives to design materials underpinning several energy conversion technologies.

Recent advances in three-way catalysts of natural gas vehicles

This review presents recent advances in TWCs for NGVs, particularly for Pd-based catalysts and potential alternatives.

Pd/Fe3O4 Nanofibers for the Catalytic Conversion of Lignin-Derived Benzyl Phenyl Ether under Transfer Hydrogenolysis Conditions

Novel magnetite-supported palladium catalysts, in the form of nanofiber materials, were prepared by using the electrospinning process. Two different synthetic techniques were used to add palladium to the nanofibers: (i) the wet impregnation of palladium on the Fe3O4 electrospun support forming the Pd/Fe3O4[wnf] catalyst or (ii) the direct co-electrospinning of a solution containing both metal precursor specimens leading to a Pd/Fe3O4[cnf] sample. The obtained Pd-based Fe3O4 nanofibers were tested in the transfer hydrogenolysis of benzyl phenyl ether (BPE), one of the simplest lignin-derived aromatic ethers, by using 2-propanol as H-donor/solvent, and their performances were compared with the analogous impregnated Pd/Fe3O4 catalyst and a commercial Pd/C. A morphological and structural characterization of the investigated catalysts was performed by means of SEM-EDX, TGA-DSC, XRD, TEM, H2-TPR, and N2 isotherm at 77 K analysis. Pd/Fe3O4[wnf] was found to be the best catalytic system allowing a complete BPE conversion after 360 min at 240 °C and a good reusability in up to six consecutive recycling tests.

Catalytic Performance of Nanoporous Metal Skeleton Catalysts for Molecular Transformations

Nanoporous metal (MNPore) skeleton catalysts have attracted increasing attention in the field of green and sustainable heterogeneous catalysis owing to their unique three-dimensional nanopore structural features. In general, MNPores are fabricated through chemical or electrochemical corrosive dealloying of monolithic alloys. The dealloying process produces various MNPores with an open nanoporous network structure by formation of concave and convex hyperboloid-like ligaments. The large surface-to-volume ratio compared to bulk metals and high density of steps and kinks on ligaments of the unsupported MNPores make them promising heterogeneous catalyst candidates for highly active and selective molecular transformations. In this context, a variety of heterogeneous catalytic reactions using MNPores as nanocatalysts under gas- and liquid-phase conditions were developed over the last decade. In addition, the bulk metallic shape and mechanistic rigidity of the MNPore catalysts make the processes of catalyst recovery and reuse more facile and greener. This Minireview mainly focuses on the catalytic performance of nanoporous Au, Pd, Cu, and AuPd with respect to the achievements on catalytic applications in various molecular transformations.

Taming the stability of Pd active phases through a compartmentalizing strategy toward nanostructured catalyst supports

The design and synthesis of robust sintering-resistant nanocatalysts for high-temperature oxidation reactions is ubiquitous in many industrial catalytic processes and still a big challenge in implementing nanostructured metal catalyst systems. Herein, we demonstrate a strategy for designing robust nanocatalysts through a sintering-resistant support via compartmentalization. Ultrafine palladium active phases can be highly dispersed and thermally stabilized by nanosheet-assembled γ-Al₂O₃ (NA-Al₂O₃) architectures. The NA-Al₂O₃ architectures with unique flowerlike morphologies not only efficiently suppress the lamellar aggregation and irreversible phase transformation of γ-Al₂O₃ nanosheets at elevated temperatures to avoid the sintering and encapsulation of metal phases, but also exhibit significant structural advantages for heterogeneous reactions, such as fast mass transport and easy access to active sites. This is a facile stabilization strategy that can be further extended to improve the thermal stability of other Al₂O₃-supported nanocatalysts for industrial catalytic applications, in particular for those involving high-temperature reactions.

The design and synthesis of robust sintering-resistant nanocatalysts for high-temperature oxidation reactions remains challenging, even though the strategy of metal-support interactions has been extensively used. Here, the authors demonstrate an alternative strategy for designing robust nanocatalysts through a sintering-resistant support.

Selective Oxidation of HMF via Catalytic and Photocatalytic Processes Using Metal-Supported Catalysts

In this study, 5-hydroxymethylfurfural (HMF) oxidation was carried out via both the catalytic and the photocatalytic approach. Special attention was devoted to the preparation of the TiO₂-based catalysts, since this oxide has been widely used for catalytic and photocatalytic application in alcohol oxidation reactions. Thus, in the catalytic process, the colloidal heterocoagulation of very stable sols, followed by the spray-freeze-drying (SFD) approach, was successfully applied for the preparation of nanostructured porous TiO₂-SiO₂ mixed-oxides with high surface areas. The versatility of the process made it possible to encapsulate Pt particles and use this material in the liquid-phase oxidation of HMF. The photocatalytic activity of a commercial titania and a homemade oxide prepared with the microemulsion technique was then compared. The influence of gold, base addition, and oxygen content on product distribution in the photocatalytic process was evaluated.

One-pot organometallic synthesis of alumina-embedded Pd nanoparticles

Herein we report a one pot organometallic strategy to access alumina-embedded Pd nanoparticles. Unexpectedly, the decomposition of the organometallic complex tris(dibenzylideneacetone)dipalladium(0), Pd₂(dba)₃, by dihydrogen in the presence of aluminum isopropoxide, Al(iPrO)₃, and without extra stabilizers, was found to be an efficient method to generate a Pd colloidal solution. Careful characterization studies revealed that the so-obtained Pd nanoparticles were stabilized by an aluminum isopropoxide tetramer and 1,5-diphenyl-pentan-3-one, which was produced after reduction of the dba ligand from the organometallic precursor. Moreover, calcination of the obtained nanomaterial in air at 773 K for 2 h resulted in a nanocomposite material containing Pd nanoparticles embedded in Al₂O₃. This stabilization strategy opens new possibilities for the preparation of transition metal nanoparticles embedded in oxides.