Creating Atomically Iridium-Doped PdOx Nanoparticles for Efficient and Durable Methane Abatement

The urgent environmental concern of methane abatement, attributed to its high global warming potential, necessitates the development of methane oxidation catalysts (MOC) with enhanced low-temperature activity and durability. Herein, an iridium-doped PdOx nanoparticle supported on silicalite-1 zeolite (PdIr/S-1) catalyst was synthesized and applied for methane catalytic combustion. Comprehensive characterizations confirmed the atomically dispersed nature of iridium on the surface of PdOx nanoparticles, creating an Ir4f-O-Pdcus microstructure. The atomically doped Ir transferred more electrons to adjacent oxygen atoms, modifying the electronic structure of PdOx and thus enhancing the redox ability of the PdIr/S-1 catalysts. This electronic modulation facilitated methane adsorption on the Pd site of Ir4f-O-Pdcus, reducing the energy barrier for C-H bond cleavage and thereby increasing the reaction rate for methane oxidation. Consequently, the optimized PdIr0.1/S-1 showed outstanding low-temperature activity for methane combustion (T50 = 276 °C) after aging and maintained long-term stability over 100 h under simulated exhaust conditions. Remarkably, the novel PdIr0.1/S-1 catalyst demonstrated significantly enhanced activity even after undergoing harsh hydrothermal aging at 750 °C for 16 h, significantly outperforming the conventional Pd/Al2O3 catalyst. This work provides valuable insights for designing efficient and durable MOC catalysts, addressing the critical issue of methane abatement.

Low-Temperature Methane Activation Reaction Pathways over Mechanochemically-Generated Ce4+/Cu+ Interfacial Sites

Methane is a valuable resource and its valorization is an important challenge in heterogeneous catalysis. Here it is shown that CeO2/CuO composite prepared by ball milling activates methane at a temperature as low as 250 °C. In contrast to conventionally prepared catalysts, the formation of partial oxidation products such as methanol and formaldehyde is also observed. Through an in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and operando Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS) approach, it can be established that this unusual reactivity can be attributed to the presence of Ce4+/Cu+ interfaces generated through a redox exchange between Ce3+ and Cu2+ atoms facilitated by the mechanical energy supplied during milling. DFT modeling of the electronic properties confirms the existence of a charge transfer mechanism. These results demonstrate the effectiveness and distinctiveness of the mechanical approach in creating unique and resilient interfaces thereby enabling the optimization and refining of CeO2/CuO catalysts in methane activation reactions.

Redox dynamics and surface structures of an active palladium catalyst during methane oxidation

Catalysts based on palladium are among the most effective in the complete oxidation of methane. Despite extensive studies and notable advances, the nature of their catalytically active species and conceivable structural dynamics remains only partially understood. Here, we combine operando transmission electron microscopy (TEM) with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations to investigate the active state and catalytic function of Pd nanoparticles (NPs) under methane oxidation conditions. We show that the particle size, phase composition and dynamics respond appreciably to changes in the gas-phase chemical potential. In combination with mass spectrometry (MS) conducted simultaneously with in situ observations, we uncover that the catalytically active state exhibits phase coexistence and oscillatory phase transitions between Pd and PdO. Aided by DFT calculations, we provide a rationale for the observed redox dynamics and demonstrate that the emergence of catalytic activity is related to the dynamic interplay between coexisting phases, with the resulting strained PdO having more favorable energetics for methane oxidation.

Facilitating the dry reforming of methane with interfacial synergistic catalysis in an Ir@CeO2-x catalyst

The dry reforming of methane provides an attractive route to convert greenhouse gases (CH4 and CO2) into valuable syngas, so as to resolve the carbon cycle and environmental issues. However, the development of high-performance catalysts remains a huge challenge. Herein, we report a 0.6% Ir/CeO2-x catalyst with a metal-support interface structure which exhibits high CH4 (~72%) and CO2 (~82%) conversion and a CH4 reaction rate of ~973 μmolCH4 gcat-1 s-1 which is stable over 100 h at 700 °C. The performance of the catalyst is close to the state-of-the-art in this area of research. A combination of in situ spectroscopic characterization and theoretical calculations highlight the importance of the interfacial structure as an intrinsic active center to facilitate the CH4 dissociation (the rate-determining step) and the CH2* oxidation to CH2O* without coke formation, which accounts for the long-term stability. The catalyst in this work has a potential application prospect in the field of high-value utilization of carbon resources.

Tuning Chemical and Morphological Properties of Ceria Nanopowders by Mechanochemistry

Cerium oxide powders are widely used and are of fundamental importance in catalytic pollution control and energy production due to the unique chemical properties of CeO2. Processing steps involved in catalyst preparation, such as high-temperature calcination or mechanical milling processes, can alter the morphological and chemical properties of ceria, heavily affecting its final properties. Here, we focus on the tuning of CeO2 nanopowder properties by mild- and high-energy milling processes, as the mechanochemical synthesis is gaining increasing attention as a green synthesis method for catalyst production. The textural and redox properties were analyzed by an array of techniques to follow the aggregation and comminution mechanisms induced by mechanical stresses, which are more prominent under high-energy conditions but strongly depend on the starting properties of the ceria powders. Simultaneously, the evolution of surface defects and chemical properties was followed by Raman spectroscopy and H2 reduction tests, ultimately revealing a trade-off effect between structural and redox properties induced by the mechanochemical action. The mild-energy process appears to induce the largest enhancement in surface properties while maintaining bulk properties of the starting materials, hence confirming its effectiveness for its exploitation in catalysis.

Toward three-dimensionally ordered nanoporous graphene materials: template synthesis, structure, and applications

Precise template synthesis will realize three-dimensionally ordered nanoporous graphenes (NPGs) with a spatially controlled seamless graphene structure and fewer edges. These structural features result in superelastic nature, high electrochemical stability, high electrical conductivity, and fast diffusion of gases and ions at the same time. Such innovative 3D graphene materials are conducive to solving energy-related issues for a better future. To further improve the attractive properties of NPGs, we review the template synthesis and its mechanism by chemical vapor deposition of hydrocarbons, analysis of the nanoporous graphene structure, and applications in electrochemical and mechanical devices.

In Situ Synthesis of Encapsulated Pd@silicalite-2 for Highly Stable Methane Catalytic Combustion

Methane emissions from vehicles have made a significant contribution to the greenhouse effect, primarily due to its high global warming potential. Supported noble metal catalysts are widely employed in catalytic combustion of methane in vehicles, but they still face challenges such as inadequate low-temperature activity and deactivation due to sintering under harsh operating conditions. In the present work, a series of encapsulated structured catalysts with palladium nanoparticles confined in hydrophobic silicalite-2 were prepared by an in situ synthesis method. Based on various characterization methods, including XRD, HR-TEM, XPS, H2-TPR, O2-TPD, H2O-TPD, CH4-TPR, Raman, and in situ DRIFTS-MS, it was confirmed that PdOx nanoparticles were mainly encapsulated inside the silicalite-2 zeolite, which further maintained the stability of the nanoparticles under harsh conditions. Specifically, the 3Pd@S-2 sample exhibited high catalytic activity for methane oxidation even after harsh hydrothermal aging at 750 °C for 16 h and maintained long-term stability at 400 °C for 130 h during wet methane combustion. In situ Raman spectroscopy has confirmed that PdOx species act as active species for methane oxidation. During this reaction, methane reacts with PdOx to produce CO2 and H2O, while simultaneously reducing PdOx to metallic Pd species, which is further reoxidized by oxygen to replenish the PdOx catalyst.

Surface defect healing in annealing from nanoporous carbons to nanoporous graphenes

Nanoporous graphene (NPG) materials have the pronounced electrochemical stability of the seamless graphene structures developed over the 3D space. We revisited the Raman spectra of nanoporous carbons (NPCs) synthesized using θ-/γ-Al2O3 templates and NPGs converted from NPCs by annealing at 1800 °C to identify the type and density of defects. We found that both the NPCs and NPGs mostly consist of single-layered graphene with a few single vacancies and Stone-Wales defects. The density of vacancy defect per hexagon in the graphene sheet is estimated to be 10-2 for NPCs, while the annealing reduced the value to 10-3-10-4 for NPGs. This supports the outstanding chemical and electrochemical stability of the novel porous carbon materials.

Bayesian Optimization-guided Discovery of High-performance Methane Combustion Catalysts based on Multi-component PtPd@CeZrOx Core-Shell Nanospheres

Formula regulation of multi-component catalysts by manual search is undoubtedly a time-consuming task, which has severely impeded the development efficiency of high-performance catalysts. In this work, PtPd@CeZrOx core-shell nanospheres, as a successful case study, is explicitly demonstrated how Bayesian optimization (BO) accelerates the discovery of methane combustion catalysts with the optimal formula ratio (the Pt/Pd mole ratio ranges from 1/2.33-1/9.09, and Ce/Zr from 1/0.22-1/0.35), which directly results in a lower conversion temperature (T50 approaching to 330 °C) than ones reported hitherto. Consequently, the best sample obtained could be efficiently developed after two rounds of iterations, containing only 18 experiments in all that is far less than the common human workload via the traditional trial-and-error search for optimal compositions. Further, this BO-based machine learning strategy can be straightforward extended to serve the autonomous discovery in multi-component material systems, for other desired properties, showing promising opportunities to practical applications in future.

Facile and Scalable Mechanochemical Synthesis of Defective MoS2 with Ru Single Atoms Toward High-Current-Density Hydrogen Evolution

Designing a facile strategy to prepare catalysts with highly active sites are challenging for large-scale implementation of electrochemical hydrogen production. Herein, a straightforward and eco-friendly method by high-energy mechanochemical ball milling for mass production of atomic Ru dispersive in defective MoS2 catalysts (Ru1 @D-MoS2 ) is developed. It is found that single atomic Ru doping induces the generation of S vacancies, which can break the electronic neutrality around Ru atoms, leading to an asymmetrical distribution of electrons. It is also demonstrated that the Ru1 @D-MoS2 exhibits superb alkaline hydrogen evolution enhancement, possibly attributing to this electronic asymmetry. The overpotential required to deliver a current density of 10 mA cm-2 is as low as 107 mV, which is much lower than that of commercial MoS2 (C-MoS2 , 364 mV). Further density functional theory (DFT) calculations also support that the vacancy-coupled single Ru enables much higher electronic distribution asymmetry degree, which could regulate the adsorption energy of intermediates, favoring the water dissociation and the adsorption/desorption of H*. Besides, the long-term stability test under 500 mA cm-2 further confirms the robust performance of Ru1 @D-MoS2 . Our strategy provides a promising and practical way towards large-scale preparation of advanced HER catalysts for commercial applications.

Pd-Ceria/CNMs Composites as Catalysts for CO and CH4 Oxidation

The application of composite materials as catalysts for the oxidation of CO and other toxic compounds is a promising approach for air purification. In this work, the composites comprising palladium and ceria components supported on multiwall carbon nanotubes, carbon nanofibers and Sibunit were studied in the reactions of CO and CH₄ oxidation. The instrumental methods showed that the defective sites of carbon nanomaterials (CNMs) successfully stabilize the deposited components in a highly-dispersed state: PdO and CeO₂ nanoparticles, subnanosized PdO_x and Pd_xCe_1-xO_2-δ clusters with an amorphous structure, as well as single Pd and Ce atoms, are formed. It was shown that the reactant activation process occurs on palladium species with the participation of oxygen from the ceria lattice. The presence of interblock contacts between PdO and CeO₂ nanoparticles has an important effect on oxygen transfer, which consequently affects the catalytic activity. The morphological features of the CNMs, as well as the defect structure, have a strong influence on the particle size and mutual stabilization of the deposited PdO and CeO₂ components. The optimal combination of highly dispersed PdO_x and Pd_xCe_1-xO_2-δ species, as well as PdO nanoparticles in the CNTs-based catalyst, makes it highly effective in both studied oxidation reactions.

Bimetallic NiFe Nanoparticles Supported on CeO2 as Catalysts for Methane Steam Reforming

Ni-Fe nanocatalysts supported on CeO₂ have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts' nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H₂-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L g_cat^-1 h^-1 and with the reactant flow varying between 54 and 415 L g_cat^-1 h^-1 at 700 °C. Hydrogen production rates of 67 mol g_met^-1 h^-1 have been achieved. The performance of the ball-milled Fe_0.1Ni_0.9/CeO₂ catalyst was similar to that of Ni/CeO₂ at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO₂ has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al₂O₃ catalysts.

Nonclassical Strong Metal-Support Interactions for Enhanced Catalysis

Strong metal-support interaction (SMSI), which encompasses reversible encapsulation and de-encapsulation and modulation of surface adsorption properties, imposes great impacts on the performance of heterogeneous catalysts. Recent development of SMSI has surpassed the prototypical encapsulated Pt-TiO₂ catalyst, affording a series of conceptually novel and practically advantageous catalytic systems. Here we provide our perspective on recent progress in nonclassical SMSIs for enhanced catalysis. Unravelling the structural complexity of SMSI necessitates the combination of multiple characterization techniques at different scales. Synthesis strategies leveraging chemical, photonic, and mechanochemical driving forces further expand the definition and application scope of SMSI. Exquisite structure engineering permits elucidation of the interface, entropy, and size effect on the geometric and electronic characteristics. Materials innovation places the atomically thin two-dimensional materials at the forefront of interfacial active site control. A broader space is awaiting exploration, where exploitation of metal-support interactions brings compelling catalytic activity, selectivity, and stability.

Pd-Ce-Ox/MWCNTs and Pt-Ce-Ox/MWCNTs Composite Materials: Morphology, Microstructure, and Catalytic Properties

The composite nanomaterials based on noble metals, reducible oxides, and nanostructured carbon are considered to be perspective catalysts for many useful reactions. In the present work, multi-walled carbon nanotubes (MWCNTs) were used for the preparation of Pd-Ce-Ox/MWCNTs and Pt-Ce-Ox/MWCNTs catalysts comprising the active components (6 wt%Pd, 6 wt%Pt, 20 wt%CeO2) as highly dispersed nanoparticles, clusters, and single atoms. The application of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) provided analysis of the samples’ morphology and structure at the atomic level. For Pd-Ce-Ox/MWCNTs samples, the formation of PdO nanoparticles with an average crystallite size of ~8 nm was shown. Pt-Ce-Ox/MWCNTs catalysts comprised single Pt2+ ions and PtOx clusters less than 1 nm. A comparison of the catalytic properties of the samples showed higher activity of Pd-based catalysts in CO and CH4 oxidation reactions in a low-temperature range (T50 = 100 °C and T50 = 295 °C, respectively). However, oxidative pretreatment of the samples resulted in a remarkable enhancement of CO oxidation activity of Pt-Ce-Ox/MWCNTs catalyst at T < 20 °C (33% of CO conversion at T = 0 °C), while no changes were detected for the Pd-Ce-Ox/MWCNTs sample. The revealed catalytic effect was discussed in terms of the capability of the Pt-Ce-Ox/MWCNTs system to form unique PtOx clusters providing high catalytic activity in low-temperature CO oxidation.

Identification of Highly Selective Surface Pathways for Methane Dry Reforming Using Mechanochemical Synthesis of Pd–CeO2

The methane dry reforming (DRM) reaction mechanism was explored via mechanochemically prepared Pd/CeO₂ catalysts (PdAcCeO₂M), which yield unique Pd–Ce interfaces, where PdAcCeO₂M has a distinct reaction mechanism and higher reactivity for DRM relative to traditionally synthesized impregnated Pd/CeO₂ (PdCeO₂IW). In situ characterization and density functional theory calculations revealed that the enhanced chemistry of PdAcCeO₂M can be attributed to the presence of a carbon-modified Pd⁰ and Ce^4+/3+ surface arrangement, where distinct Pd–CO intermediate species and strong Pd–CeO₂ interactions are activated and sustained exclusively under reaction conditions. This unique arrangement leads to highly selective and distinct surface reaction pathways that prefer the direct oxidation of CH_x to CO, identified on PdAcCeO₂M using isotope labeled diffuse reflectance infrared Fourier transform spectroscopy and highlighting linear Pd–CO species bound on metallic and C-modified Pd, leading to adsorbed HCOO [1595 cm^–1] species as key DRM intermediates, stemming from associative CO₂ reduction. The milled materials contrast strikingly with surface processes observed on IW samples (PdCeO₂IW) where the competing reverse water gas shift reaction predominates.

Direct Dry Synthesis of Supported Bimetallic Catalysts: A Study on Comminution and Alloying of Metal Nanoparticles

Ball milling is growing increasingly important as an alternative synthetic tool to prepare catalytic materials. It was recently observed that supported metal catalysts could be directly obtained upon ball milling from the coarse powders of metal and oxide support. Moreover, when two compatible metal sources are simultaneously subjected to the mechanochemical treatment, bimetallic nanoparticles are obtained. A systematic investigation was extended to different metals and supports to understand better the mechanisms involved in the comminution and alloying of metal nanoparticles. Based on this, a model describing the role of metal-support interactions in the synthesis was developed. The findings will be helpful for the future rational design of supported metal catalysts via dry ball milling.

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Optimizing the Microstructure of SnO2-CeO2 Binary Oxide Supported Palladium Catalysts for Efficient and Stable Methane Combustion

The preparation of palladium-based catalysts with both high catalytic activity and hydrothermal stability currently appears as a critical topic in methane combustion. Herein, we propose a facile strategy to boost the performance of SnO₂-CeO₂ binary oxide supported palladium catalysts by tuning the composition of supports. The coexistence of SnO₂ and CeO₂ phases in an appropriate ratio is favorable for the formation of both Pd_xCe_1-xO_2-δ and Pd_xSn_1-xO_2-δ solid solutions due to the reduced crystallite size. This unique microstructure could enhance the metal-support interaction to stabilize the active PdO phase and promote its reoxidation, meanwhile generating more oxygen vacancies to improve the reducibility of PdO. On account of the facilitated conversion of PdO ↔ Pd, coupled with the low-temperature dissociation of methane promoted by abundant active oxygen species, the Pd/5Sn5Ce catalyst exhibits a superior catalytic activity with a T₉₉ of ca. 360 °C, a robust stability under both dry and wet conditions, and an excellent thermal stability during heating-cooling light-off tests.

Mechanochemical Strategies for the Preparation of SiO2-Supported AgAu Nanoalloy Catalysts

Silver-gold nanoalloys were prepared from their metal salts precursors through bottom-up mechanochemical synthesis, using one-pot or galvanic replacement reaction strategies. The nanostructures were prepared over amorphous SiO₂ as an inert supporting material, facilitating their stabilization without the use of any stabilizing agent. The nanomaterials were extensively characterized, confirming the formation of the bimetallic nanostructures. The nanoalloys were tested as catalysts in the hydrogenation of 2-nitroaniline and exhibited up to 4-fold the rate constant and up to 37% increased conversion compared to the respective single metal nanoparticles. Our approach is advantageous to produce nanoparticles with clean surfaces with available catalytic sites, directly in the solid-state and in an environmentally friendly manner.

Nano-Confinement of Insulating Sulfur in the Cathode Composite of All-Solid-State Li-S Batteries Using Flexible Carbon Materials with Large Pore Volumes

Durable nanostructured cathode materials for efficient all-solid-state Li-S batteries were prepared using a conductive single-walled 3D graphene with a large pore volume as the cathode support material. At high loadings of the active material (50-60 wt %), microscale phase segregation was observed with a conventional cathode support material during the charging/discharging processes but this was suppressed by the confinement of insulating sulfur into the mesopores of the elastic and flexible nanoporous graphene with a large pore volume of 5.3 mL g^-1. As such, durable three-phase contact was achieved among the solid electrolyte, insulating sulfur, and the electrically conductive carbon. Consequently, the electrochemical performances of the assembled all-solid-state batteries were significantly improved and feasible under the harsh conditions of operation at 353 K, and improved cycling stability as well as the highest specific capacity of 716 mA h per gram of cathode (4.6 mA h cm^-2, 0.2 C) was achieved with high sulfur loading (50 wt %).

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.

Structural Evolution of Bimetallic PtPd/CeO2 Methane Oxidation Catalysts Prepared by Dry Milling

Bimetallic Pt-Pd catalysts supported on ceria have been prepared by mechanochemical synthesis and tested for lean methane oxidation in dry and wet atmosphere. Results show that the addition of platinum has a negative effect on transient light-off activity, but for Pd/Pt molar ratios between 1:1 and 8:1 an improvement during time-on-stream experiments in wet conditions is observed. The bimetallic samples undergo a complex restructuring during operation, starting from the alloying of Pt and Pd and resulting in the formation of unprecedented "mushroom-like" structures consisting of PdO bases with Pt heads as revealed by high-resolution transmission electron microscopy (HRTEM) analysis. On milled samples, these structures are well-defined and observed at the interface between palladium and ceria, whereas those on the impregnated catalyst appear less ordered and are located randomly on the surface of ceria and of large PdPt clusters. The milled catalyst prepared by first milling Pd metal and ceria followed by the addition of Pt shows better performances compared to a conventional impregnated sample and also to a sample obtained by inverting the Pd-Pt milling order. This has been ascribed to the intimate contact between Pd and CeO₂ generated at the nanoscale during the milling process.

Synthesis of γ Phase and Amorphous Solid Dispersion of Glycine from α-Glycine During the Solvent-Free Ball Milling Process

Nano α-glycine crystals, γ-glycine crystals, and amorphous solid dispersion (ASD) of glycine were prepared through solvent-free ball milling of commercial α-glycine. The solid-state polymorph conversion of glycine from α to γ was completely realized by ball milling with 0.2 wt.% NaCl for 1 h or by ball milling with 0.02 wt.% NaCl for 1 h with subsequent storage for one week. The ASD of glycine was prepared by ball milling α-glycine with an equal amount of CaCl₂ for 1 h. We studied the effect of inorganic salt types and their concentrations on the extent of polymorph conversion and amorphization of glycine in our experiments. This solvent-free ball milling method could be used for the synthesis of polymorphs and amorphous phase of drugs and other organic materials.

Preparation of Pd/SiO2 Catalysts by a Simple Dry Ball-Milling Method for Lean Methane Oxidation and Probe of the State of Active Pd Species

A series of Pd/SiO2 catalysts were prepared with different Pd precursors by a dry ball-milling method and used in the catalytic oxidation of lean methane at low temperature. The effect of Pd precursors on the catalytic performance was investigated and the state of the most active Pd species was probed. The results indicate that dry ball-milling is a simple but rather effective method to prepare the Pd/SiO2 catalysts for lean methane oxidation, and palladium acetylacetonate is an ideal precursor to obtain a highly active Pd/SiO2-Acac catalyst with well- and stably dispersed Pd species, owing to the tight contact between acetylacetonate and Si–OH on the SiO2 support. Besides the size and dispersion of Pd particles, the oxidation state of Pd species also plays a crucial role in determining the catalytic activity of Pd/SiO2 in lean methane oxidation at low temperature. A non-monotonic dependence of the catalytic activity on the Pd oxidation state is observed. The activity of various Pd species follows the order of PdOx >> Pd > PdO; the PdOx/SiO2-Acac catalysts (in particular for PdO0.82/SiO2-Acac when x = 0.82) exhibit much higher activity in lean methane oxidation at low temperature than Pd/SiO2-Acac and PdO/SiO2-Acac. The catalytic activity of PdOx/SiO2 may degrade during the methane oxidation due to the gradual transformation of PdOx to PdO in the oxygen-rich ambiance; however, such degradation is reversible and the activity of a degraded Pd/SiO2 catalyst can be recovered through a redox treatment to regain the PdOx species. This work helps to foster a better understanding of the relationship between the structure and performance of supported Pd catalysts by clarifying the state of active Pd species, which should be beneficial to the design of an active catalyst in lean methane oxidation at low temperature.

Onset of High Methane Combustion Rates over Supported Palladium Catalysts: From Isolated Pd Cations to PdO Nanoparticles

Industrial low-temperature methane combustion catalyst Pd/Al₂O₃ suffers from H₂O-induced deactivation. It is imperative to design Pd catalysts free from this deactivation and with high atomic efficiency. Using a small-pore zeolite SSZ-13 as support, herein we report well-defined Pd catalysts with dominant active species as finely dispersed Pd cations, uniform PdO particles embedded inside the zeolite framework, or PdO particles decorating the zeolite external surface. Through detailed reaction kinetics and spectroscopic and microscopic studies, we show that finely dispersed sites are much less active than PdO nanoparticles. We further demonstrate that H₂O-induced deactivation can be readily circumvented by using zeolite supports with high Si/Al ratios. Finally, we provide a few rational catalyst design suggestions for methane oxidation based on the new knowledge learned in this study.

Mechanochemical Synthesis of Catalytic Materials

The mechanochemical synthesis of nanomaterials for catalytic applications is a growing research field due to its simplicity, scalability, and eco-friendliness. Besides, it provides materials with distinct features, such as nanocrystallinity, high defect concentration, and close interaction of the components in a system, which are, in most cases, unattainable by conventional routes. Consequently, this research field has recently become highly popular, particularly for the preparation of catalytic materials for various applications, ranging from chemical production over energy conversion catalysis to environmental protection. In this Review, recent studies on mechanochemistry for the synthesis of catalytic materials are discussed. Emphasis is placed on the straightforwardness of the mechanochemical route-in contrast to more conventional synthesis-in fabricating the materials, which otherwise often require harsh conditions. Distinct material properties achieved by mechanochemistry are related to their improved catalytic performance.

Relationship between penta-coordinated Al3+ sites in the Al2O3 supports and CH4 combustion activity of Pd/Al2O3 catalysts

Pd/Al₂O₃ catalysts were prepared using various Al₂O₃ supports with different structural features, revealing a significant insight into the methane (CH₄) combustion activity of Pd nanoparticles with the fraction of penta-coordinated Al³⁺ sites in the Al₂O₃ supports.

Catalytic Oxidation of Methane to Oxygenated Products: Recent Advancements and Prospects for Electrocatalytic and Photocatalytic Conversion at Low Temperatures

Methane is an important fossil fuel and widely available on the earth's crust. It is a greenhouse gas that has more severe warming effect than CO2. Unfortunately, the emission of methane into the atmosphere has long been ignored and considered as a trivial matter. Therefore, emphatic effort must be put into decreasing the concentration of methane in the atmosphere of the earth. At the same time, the conversion of less valuable methane into value-added chemicals is of significant importance in the chemical and pharmaceutical industries. Although, the transformation of methane to valuable chemicals and fuels is considered the "holy grail," the low intrinsic reactivity of its C-H bonds is still a major challenge. This review discusses the advancements in the electrocatalytic and photocatalytic oxidation of methane at low temperatures with products containing oxygen atom(s). Additionally, the future research direction is noted that may be adopted for methane oxidation via electrocatalysis and photocatalysis at low temperatures.

Ultrasonic-Ball Milling: A Novel Strategy to Prepare Large-Size Ultrathin 2D Materials

Large-size ultrathin 2D materials, with extensive applications in optics, medicine, biology, and semiconductor fields, can be prepared through an existing common physical and chemical process. However, the current exfoliation technologies still need to be improved upon with urgency. Herein, a novel and simple "ultrasonic-ball milling" strategy is reported to effectively obtain high quality and large size ultrathin 2D materials with complete lattice structure through the introduction of moderate sapphire (Al₂ O₃ ) abrasives in a liquid phase system. Ultimately numerous high-quality ultrathin h-BN, graphene, MoS₂ , WS₂ , and BCN nanosheets are obtained with large sizes ranging from 1-20 µm, small thickness of ≈1-3 nm and a high yield of over 20%. Utilizing shear and friction force synergistically, this strategy provides a new method and alternative for preparing and optimizing large size ultrathin 2D materials.

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox properties.

Oxidative Methane Conversion to Ethane on Highly Oxidized Pd/CeO2 Catalysts Below 400 °C

Methane upgrading into more valuable chemicals has received much attention. Herein, we report oxidative methane conversion to ethane using gaseous O₂ at low temperatures (<400 °C) and atmospheric pressure in a continuous reactor. A highly oxidized Pd deposited on ceria could produce ethane with a productivity as high as 0.84 mmol g_cat ^-1  h^-1 . The Pd-O-Pd sites, not Pd-O-Ce, were the active sites for the selective ethane production at low temperatures. Density functional theory calculations confirmed that the Pd-O-Pd site is energetically more advantageous for C-C coupling, whereas Pd-O-Ce promotes CH₄ dehydrogenation. The ceria helped Pd maintain a highly oxidic state despite reductive CH₄ flow. This work can provide new insight for methane upgrading into C₂ species.

Facile construction of ultrastable alumina anchored palladium catalysts via a designed one pot strategy for enhanced methane oxidation

A highly stable Pd–Al₂O₃ catalyst with anchored palladium species was facilely prepared through a one pot strategy for efficient methane oxidation.