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

Facilely Fabricated Single-Site Ptδ+-O(OH)x- Species Associated with Alkali on Zirconia Exhibiting Superior Catalytic Oxidation Reactivity

Fabrication of robust isolated atom catalysts has been a research hotspot in the environment catalysis field for the removal of various contaminants, but there are still challenges in improving the reactivity and stability. Herein, through facile doping alkali metals in Pt catalyst on zirconia (Pt-Na/ZrO2), the atomically dispersed Ptδ+-O(OH)x- associated with alkali metal via oxygen bridge was successfully fabricated. This novel catalyst presented remarkably higher CO and hydrocarbon (HCs: C3H8, C7H8, C3H6, and CH4) oxidation activity than its counterpart (Pt/ZrO2). Systematically direct and solid evidence from experiments and density functional theory calculations demonstrated that the fabricated electron-rich Ptδ+-O(OH)x- related to Na species rather than the original Ptδ+-O(OH)x-, serving as the catalytically active species, can readily react with CO adsorbed on Ptδ+ to produce CO2 with significantly decreasing energy barrier in the rate-determining step from 1.97 to 0.93 eV. Additionally, owing to the strongly adsorbed and activated water by Na species, those fabricated single-site Ptδ+-O(OH)x- linked by Na species could be easily regenerated during the oxidation reaction, thus considerably boosting its oxidation reactivity and durability. Such facile construction of the alkali ion-linked active hydroxyl group was also realized by Li and K modification which could guide to the design of efficient catalysts for the removal of CO and HCs from industrial exhaust.

Decelerating catalyst aging of natural gas engines using organic Rankine cycle under road conditions

High exhaust temperature is an intrinsic nature of natural gas engines which underlies power de-rating and thermal aging of after-treatment system; therefore, this study integrates an organic Rankine cycle (ORC) system between engine and it's three-way catalyst (TWC) to address these challenges. ORC facilitates power output enhancement through exhaust energy recovery and alleviates thermal aging by reducing exhaust temperature. To estimate the effectiveness of this hypothesized system, a simulation-based investigation is performed. First, simulation models, including engine, TWC, and vehicle dynamic models, are built and validated by experimental data. According to the temperature characteristics of different TWCs, three scenarios, representing old, current, and prospective TWC technology, are formulated to estimate the ORC performance under Worldwide Harmonized Light Vehicles Test Cycle. Results show that ORC system can substantially alleviate the thermal damage caused by high exhaust temperature and extend TWC lifespan. It is estimated that over 98.5 % of thermal damage can be decreased by proper ORC setting, and the average TWC lifespan extension can be at least 55.4, making a reduced noble metal usage and cost of TWC. Meanwhile, with the decrease of the working temperature of TWC, ORC can recover exhaust energy under more road conditions, further improving the net power and shortening the payback period of extra ORC hardware costs. A reduction in the working temperature of TWC from 770.5 K to 618 K yields a 109 % enhancement in maximum power, coupled with a 62.30 % reduction in the payback period. These findings fully reflect the advantage of ORC-TWC coupling and indicate that ORC is supposed to be used more for the TWC with a low working temperature to maximize economic effectiveness. This study provides a novel pathway for thermal aging alleviation of TWC and a valuable reference for prospective studies on matching ORC with TWC under road conditions.

General and Scalable Synthesis of Mesoporous 2D MZrO2 (M = Co, Mn, Ni, Cu, Fe) Nanocatalysts by Amorphous-to-Crystalline Transformation

In modern heterogeneous catalysis, it remains highly challenging to create stable, low-cost, mesoporous 2D photo-/electro-catalysts that carry atomically dispersed active sites. In this work, a general shape-preserving amorphous-to-crystalline transformation (ACT) strategy is developed to dope various transition metal (TM) heteroatoms in ZrO2, which enabled the scalable synthesis of TMs/oxide with a mesoporous 2D structure and rich defects. During the ACT process, the amorphous MZrO2 nanoparticles (M = Fe, Ni, Cu, Co, Mn) are deposited within a confined space created by the NaCl template, and they transform to crystalline 2D ACT-MZrO2 nanosheets in a shape-preserving manner. The interconnected crystalline ACT-MZrO2 nanoparticles thus inherit the same structure as the original MZrO2 precursor. Owing to its rich active sites on the surface and abundant oxygen vacancies (OVs), ACT-CoZrO2 gives superior performance in catalyzing the CO2-to-syngas conversion as demonstrated by experiments and theoretical calculations. The ACT chemistry opens a general route for the scalable synthesis of advanced catalysts with precise microstructure by reconciliating the control of crystalline morphologies and the dispersion of heteroatoms.

Atomic Pt Sites Anchored in the Interface between Grains on Vacancy-Enriched CeO2 Nanosheets: One-Step Precursor Combustion Synthesis

Atomic metal catalysts have unique electronic, structural, and catalytic properties, which are widely used in the field of catalysis. However, designing new simple synthesis methods to fabricate atomic metal catalysts is a challenge in catalytic applications. Herein, a one-step precursor combustion strategy is presented that starts directly from precursors of metal salts, using a spontaneous combustion process convert platinum nitrate to atomic Pt sites. The atomic Pt sites with low valence are anchored in the formed interface between grains on vacancy-enriched CeO2 nanosheets. The obtained Pt/CeO2-2 catalyst exhibits much higher three-way catalytic activities at low temperatures than Pt/CeO2-C catalysts prepared using the traditional impregnation method. Density functional theory calculations show that the generated lower valent Pt atoms in the CeO2 interface promote catalytic activity through reducing the energy barrier, and lead to an overall improvement of three-way catalytic activities. This facile strategy provides new insights into the study of the properties and applications of atomic noble metal catalysts.

Synergistic growth of nickel and platinum nanoparticles via exsolution and surface reaction

Bimetallic catalysts combining precious and earth-abundant metals in well designed nanoparticle architectures can enable cost efficient and stable heterogeneous catalysis. Here, we present an interaction-driven in-situ approach to engineer finely dispersed Ni decorated Pt nanoparticles (1-6 nm) on perovskite nanofibres via reduction at high temperatures (600-800 oC). Deposition of Pt (0.5 wt%) enhances the reducibility of the perovskite support and promotes the nucleation of Ni cations via metal-support interaction, thereafter the Ni species react with Pt forming alloy nanoparticles, with the combined processes yielding smaller nanoparticles that either of the contributing processes. Tuneable uniform Pt-Ni nanoparticles are produced on the perovskite surface, yielding reactivity and stability surpassing 1 wt.% Pt/γ-Al2O3 catalysts for CO oxidation. This approach heralds the possibility of in-situ fabrication of supported bimetallic nanoparticles with engineered compositional distributions and performance.

Co3O4-Based Materials as Potential Catalysts for Methane Detection in Catalytic Gas Sensors

The present work deals with the development of Co3O4-based catalysts for potential application in catalytic gas sensors for methane (CH4) detection. Among the transition-metal oxide catalysts, Co3O4 exhibits the highest activity in catalytic combustion. Doping Co3O4 with another metal can further improve its catalytic performance. Despite their promising properties, Co3O4 materials have rarely been tested for use in catalytic gas sensors. In our study, the influence of catalyst morphology and Ni doping on the catalytic activity and thermal stability of Co3O4-based catalysts was analyzed by differential calorimetry by measuring the thermal response to 1% CH4. The morphology of two Co3O4 catalysts and two NixCo3-xO4 with a Ni:Co molar ratio of 1:2 and 1:5 was studied using scanning transmission electron microscopy and energy dispersive X-ray analysis. The catalysts were synthesized by (co)precipitation with KOH solution. The investigations showed that Ni doping can improve the catalytic activity of Co3O4 catalysts. The thermal response of Ni-doped catalysts was increased by more than 20% at 400 °C and 450 °C compared to one of the studied Co3O4 oxides. However, the thermal response of the other Co3O4 was even higher than that of NixCo3-xO4 catalysts (8% at 400 °C). Furthermore, the modification of Co3O4 with Ni simultaneously brings stability problems at higher operating temperatures (≥400 °C) due to the observed inhomogeneous Ni distribution in the structure of NixCo3-xO4. In particular, the NixCo3-xO4 with high Ni content (Ni:Co ratio 1:2) showed apparent NiO separation and thus a strong decrease in thermal response of 8% after 24 h of heat treatment at 400 °C. The reaction of the Co3O4 catalysts remained quite stable. Therefore, controlling the structure and morphology of Co3O4 achieved more promising results, demonstrating its applicability as a catalyst for gas sensing.

Water-assisted oxidative redispersion of Cu particles through formation of Cu hydroxide at room temperature

Sintering of active metal species often happens during catalytic reactions, which requires redispersion in a reactive atmosphere at elevated temperatures to recover the activity. Herein, we report a simple method to redisperse sintered Cu catalysts via O2-H2O treatment at room temperature. In-situ spectroscopic characterizations reveal that H2O induces the formation of hydroxylated Cu species in humid O2, pushing surface diffusion of Cu atoms at room temperature. Further, surface OH groups formed on most hydroxylable support surfaces such as γ-Al2O3, SiO2, and CeO2 in the humid atmosphere help to pull the mobile Cu species and enhance Cu redispersion. Both pushing and pulling effects of gaseous H2O promote the structural transformation of Cu aggregates into highly dispersed Cu species at room temperature, which exhibit enhanced activity in reverse water gas shift and preferential oxidation of carbon monoxide reactions. These findings highlight the important role of H2O in the dynamic structure evolution of supported metal nanocatalysts and lay the foundation for the regeneration of sintered catalysts under mild conditions.

Hydrothermal Aging Enhances Nitrogen Oxide Reduction over Iron-Exchanged Zeolites at 150 °C

Ammonia selective catalytic reduction (NH3-SCR) over copper- and iron-exchanged zeolites is a state-of-the-art technology for removal of nitrogen oxides (NOx, NO, and NO2) from exhaust emissions but suffers from poor low-temperature (i.e., 150 °C) activity. Here we show that hydrothermal aging of Fe-beta, Fe-ZSM-5, and Fe-ferrierite at 650 °C or higher leads to a remarkable increase in NOx conversion from ∼30 to ∼80% under fast NH3-SCR conditions at 150 °C. The practical relevance of this finding becomes more evident as an aged Fe-beta/fresh Cu-SSZ-13 composite catalyst exhibits ∼90% conversion. We propose that a neutral heteronuclear bis-μ-oxo ironaluminum dimer might be created within iron zeolites during hydrothermal aging and catalyze ammonium nitrate reduction by NO at 150 °C. Density functional theory calculations reveal that the activation free energy (125 versus 147 kJ mol-1) for the reaction of NO with adsorbed NO3- species, the rate-determining step of ammonium nitrate reduction, is considerably lower on the bis-μ-oxo ironaluminum site than on the well-known mononuclear iron-oxo cation site, thus greatly enhancing the overall SCR activity.

Structure characterization of aged automobile exhaust catalysts using electron probe microanalysis

BACKGROUND

Driven by emission regulations, the technology of emission control catalysts has been under increasing need for development. Understanding the deactivation mechanism of aged or spent automobile exhaust catalysts is the key to extending their service lifetime. However, the lack of comprehensive microstructural characterization results in an incomplete understanding of their physicochemical properties. Deactivation mechanism of automobile exhaust catalysts is a considerably complex phenomenon, it can be classified into three groups based on its origin: thermal sintering, chemical poisoning and mechanical deactivation.

RESULTS

In this study, an aged high-mileage automobile exhaust catalyst with Pd and Rh active phases supported on a cerium zirconium oxide doped alumina coating on cordierite was analysed; six consecutive monolithic blocks along the inlet to the outlet of the aged catalyst were extracted, and the corresponding metallographic samples were fabricated using the vacuum impregnation resin method. The purpose of this study was to accurately characterize the different regions of the monolith via electron probe microanalysis and to infer potential causes of catalyst deactivation. Two major causes of deactivation were found: (1) aggregation and alloying of precious-metal particles caused by thermal sintering and (2) chemical poisoning caused by sulphur and phosphorus. Other mechanisms, such as mechanical degradation, which mainly manifests as the loss or wear of the washed coating, were also found to be involved in deactivation. Additionally, the catalytic activity tests showed a considerable decrease in the aged catalyst. The poison concentration trends in the washcoat indicated that P is detrimental to CO oxidation, while S accumulation affects propane oxidation.

SIGNIFICANCE

This analysis method can be of substantial practical significance in developing advanced washcoat materials. Meanwhile, it has great potential in the washcoat analysis of honeycomb-shaped monolithic catalyst, such as natural gas catalyst, diesel vehicle oxidation catalyst and other honeycomb catalysts applied in chemical industry.

Reactant-Induced Structural Evolution of Pt Catalysts Confined in Zeolite

Reactant-induced structural evolutions of heterogeneous metal catalysts are frequently observed in numerous catalytic systems, which can be associated with the formation or deactivation of active sites. In this work, we will show the structural transformation of subnanometer Pt clusters in pure-silica MFI zeolite structure in the presence of CO, O2, and/or H2O and the catalytic consequences of the Pt-zeolite materials derived from various treatment conditions. By applying the appropriate pretreatment under a reactant atmosphere, we can precisely modulate the size distribution of Pt species spanning from single Pt atoms to small Pt nanoparticles (1-5 nm) in the zeolite matrix, resulting in the desirably active and stable Pt species for CO oxidation. We also show the incorporation of Fe into the zeolite framework greatly promotes the stability of Pt species against undesired sintering under harsh conditions (up to 650 °C in the presence of CO, O2, and moisture).

Boosting the catalytic performance of Al2O3-supported Pd catalysts by introducing CeO2 promoters

Maintaining the stability of noble metals is the key to the long-term stability of supported catalysts. In response to the instability of noble metal species at high temperatures, we developed a synergistic strategy of dual oxide supports. By designing and constructing ceria components with small sizes, we have achieved unity in the ability of catalytic materials to supply oxygen and stabilize metal species. In this study, we prepared Al2O3-CeO2-Pd (AlCePd) catalysts containing trace amounts of Ce through the hydrolysis of cerium acetate, which achieved 100% CO conversion at 160 °C. More importantly, the activity remained at its initial 100% in the long-term durability testing, demonstrating the high stability of AlCePd. In contrast, the CO conversion of the CeO2-Pd (CePd) catalyst decreased from 100% to 54% within 3 h. Through comprehensive studies, we found that this excellent catalytic performance stems from the stabilizing effect of an alumina support and the possible reverse oxygen spillover effect of small-sized ceria components, where small-sized ceria components provide active oxygen for independent Pd species, making it possible for the CO adsorbed on Pd to react with this oxygen species.

Synthesis of core@shell catalysts guided by Tammann temperature

Designing high-performance thermal catalysts with stable catalytic sites is an important challenge. Conventional wisdom holds that strong metal-support interactions can benefit the catalyst performance, but there is a knowledge gap in generalizing this effect across different metals. Here, we have successfully developed a generalizable strong metal-support interaction strategy guided by Tammann temperatures of materials, enabling functional oxide encapsulation of transition metal nanocatalysts. As an illustrative example, Co@BaAl2O4 core@shell is synthesized and tracked in real-time through in-situ microscopy and spectroscopy, revealing an unconventional strong metal-support interaction encapsulation mechanism. Notably, Co@BaAl2O4 exhibits exceptional activity relative to previously reported core@shell catalysts, displaying excellent long-term stability during high-temperature chemical reactions and overcoming the durability and reusability limitations of conventional supported catalysts. This pioneering design and widely applicable approach has been validated to guide the encapsulation of various transition metal nanoparticles for environmental tolerance functionalities, offering great potential to advance energy, catalysis, and environmental fields.

Fine-Tuning of Pt Dispersion on Al2O3 and Understanding the Nature of Active Pt Sites for Efficient CO and NH3 Oxidation Reactions

Fine-tuning the dispersion of active metal species on widely used supports is a research hotspot in the catalysis community, which is vital for achieving a balance between the atomic utilization efficiency and the intrinsic activity of active sites. In this work, using bayerite Al(OH)3 as support directly or after precalcination at 200 or 550 °C, Pt/Al2O3 catalysts with distinct Pt dispersions from single atoms to clusters (ca. 2 nm) were prepared and evaluated for CO and NH3 removal. Richer surface hydroxyl groups on AlOx(OH)y support were proved to better facilitate the dispersion of Pt. However, Pt/Al2O3 with relatively lower Pt dispersion could exhibit better activity in CO/NH3 oxidation reactions. Further reaction mechanism study revealed that the Pt sites on Pt/Al2O3 with lower Pt dispersion could be activated to Pt0 species much easier under the CO oxidation condition, on which a higher CO adsorption capacity and more efficient O2 activation were achieved simultaneously. Compared to Pt single atoms, PtOx clusters could also better activate NH3 into -NH2 and -HNO species. The higher CO adsorption capacity and the more efficient NH3/O2 activation ability on Pt/Al2O3 with relatively lower Pt dispersion well explained its higher CO/NH3 oxidation activity. This study emphasizes the importance of avoiding a singular pursuit of single-atom catalyst synthesis and instead focusing on achieving the most effective Pt species on Al2O3 support for targeted reactions. This approach avoids unnecessary limitations and enables a more practical and efficient strategy for Pt catalyst fabrication in emission control applications.

Catalytic Structure Design by AI Generating with Spectroscopic Descriptors

Generative artificial intelligence has depicted a beautiful blueprint for on-demand design in chemical research. However, the few successful chemical generations have only been able to implement a few special property values because most chemical descriptors are mathematically discrete or discontinuously adjustable. Herein, we use spectroscopic descriptors with machine learning to establish a quantitative spectral structure-property relationship for adsorbed molecules on metal monatomic catalysts. Besides catalytic properties such as adsorption energy and charge transfer, the complete spatial relative coordinates of the adsorbed molecule were successfully inverted. The spectroscopic descriptors and prediction models are generalized, allowing them to be transferred to several different systems. Due to the continuous tunability of the spectroscopic descriptors, the design of catalytic structures with continuous adsorption states generated by AI in the catalytic process has been achieved. This work paves the way for using spectroscopy to enable real-time monitoring of the catalytic process and continuous customization of catalytic performance, which will lead to profound changes in catalytic research.

Covalent organic frameworks: linkage types, synthetic methods and bio-related applications

Covalent organic frameworks (COFs) are composed of small organic molecules linked via covalent bonds, which have tunable mesoporous structure, good biocompatibility and functional diversities. These excellent properties make COFs a promising candidate for constructing biomedical nanoplatforms and provide ample opportunities for nanomedicine development. A systematic review of the linkage types and synthesis methods of COFs is of indispensable value for their biomedical applications. In this review, we first summarize the types of various linkages of COFs and their corresponding properties. Then, we highlight the reaction temperature, solvent and reaction time required by different synthesis methods and show the most suitable synthesis method by comparing the merits and demerits of various methods. To appreciate the cutting-edge research on COFs in bioscience technology, we also summarize the bio-related applications of COFs, including drug delivery, tumor therapy, bioimaging, biosensing and antimicrobial applications. We hope to provide insight into the interdisciplinary research on COFs and promote the development of COF nanomaterials for biomedical applications and their future clinical translations.

Selective Reduction of NO into N2 Catalyzed by Rh1-Doped Cluster Anions RhCe2O3-5. J Am Chem Soc 2023;145:18658-18667. [PMID: 37572057 DOI: 10.1021/jacs.3c06565]

Catalytic conversion of toxic nitrogen oxide (NO) and carbon monoxide (CO) into nitrogen (N2) and carbon dioxide (CO2) is imperative under the weight of the increasingly stringent emission regulations, while a fundamental understanding of the nature of the active site to selectively drive N2 generation is elusive. Herein, in combination with state-of-the-art mass-spectrometric experiments and quantum-chemical calculations, we demonstrated that the rhodium-cerium oxide clusters RhCe2O3-5- can catalytically drive NO reduction by CO and give rise to N2 and CO2. This finding represents a sharp improvement in cluster science where N2O is commonly produced in the rarely established examples of catalytic NO reduction mediated with gas-phase clusters. We demonstrated the importance of the unique chemical environment in the RhCe2O3- cluster to guide the substantially improved N2 selectivity: a triatomic Lewis "acid-base-acid" Ceδ+-Rhδ--Ceδ+ site is proposed to strongly adsorb two NO molecules as well as the N2O intermediate that is attached on the Rh atom and can facilely dissociate to form N2 assisted by both Ce atoms.

Surface Lattice-Embedded Pt Single-Atom Catalyst on Ceria-Zirconia with Superior Catalytic Performance for Propane Oxidation

Tuning the metal-support interaction and coordination environment of single-atom catalysts can help achieve satisfactory catalytic performance for targeted reactions. Herein, via the facile control of calcination temperatures for Pt catalysts on pre-stabilized Ce0.9Zr0.1O2 (CZO) support, Pt single atoms (Pt1) with different strengths of Pt-CeO2 interaction and coordination environment were successfully constructed. With the increase in calcination temperature from 350 to 750 °C, a stronger Pt-CeO2 interaction and higher Pt-O-Ce coordination number were achieved due to the reaction between PtOx and surface Ce3+ species as well as the migration of Pt1 into the surface lattice of CZO. The Pt/CZO catalyst calcined at 750 °C (Pt/CZO-750) exhibited a surprisingly higher C3H8 oxidation activity than that calcined at 550 °C (Pt/CZO-550). Through systematic characterizations and reaction mechanism study, it was revealed that the higher concentration of surface Ce3+ species/oxygen vacancies and the stronger Pt-CeO2 interaction on Pt/CZO-750 could better facilitate the activation of oxygen to oxidize C3H8 into reactive carbonate/carboxyl species and further promote the transformation of these intermediates into gaseous CO2. The Pt/CZO-750 catalyst can be a potential candidate for the catalytic removal of hydrocarbons from vehicle exhaust.

A General Dual-Metal Nanocrystal Dissociation Strategy to Generate Robust High-Temperature-Stable Alumina-Supported Single-Atom Catalysts

Designing new synthesis routes to fabricate highly thermally durable precious metal single-atom catalysts (SACs) is challenging in industrial applications. Herein, a general strategy is presented that starts from dual-metal nanocrystals (NCs), using bimetallic NCs as a facilitator to spontaneously convert a series of noble metals to single atoms on aluminum oxide. The metal single atoms are captured by cation defects in situ formed on the surface of the inverse spinel (AB₂O₄) structure, which process provides numerous anchoring sites, thus facilitating generation of the isolated metal atoms that contributes to the extraordinary thermodynamic stability. The Pd₁/AlCo₂O₄-Al₂O₃ shows not only improved low-temperature activity but also unprecedented (hydro)thermal stability for CO and propane oxidation under harsh aging conditions. Furthermore, our strategy exhibits a small scaling-up effect by the simple physical mixing of commercial metal oxide aggregates with Al₂O₃. The good regeneration between oxidative and reductive atmospheres of these ionic palladium species makes this catalyst system of potential interest for emissions control.

In Situ and Emerging Transmission Electron Microscopy for Catalysis Research

Catalysts are the primary facilitator in many dynamic processes. Therefore, a thorough understanding of these processes has vast implications for a myriad of energy systems. The scanning/transmission electron microscope (S/TEM) is a powerful tool not only for atomic-scale characterization but also in situ catalytic experimentation. Techniques such as liquid and gas phase electron microscopy allow the observation of catalysts in an environment conducive to catalytic reactions. Correlated algorithms can greatly improve microscopy data processing and expand multidimensional data handling. Furthermore, new techniques including 4D-STEM, atomic electron tomography, cryogenic electron microscopy, and monochromated electron energy loss spectroscopy (EELS) push the boundaries of our comprehension of catalyst behavior. In this review, we discuss the existing and emergent techniques for observing catalysts using S/TEM. Challenges and opportunities highlighted aim to inspire and accelerate the use of electron microscopy to further investigate the complex interplay of catalytic systems.

Fabricating Robust Pt Clusters on Sn-Doped CeO2 for CO Oxidation: A Deep Insight into Support Engineering and Surface Structural Evolution

The size effect on nanoparticles, which affects the catalysis performance in a significant way, is crucial. The tuning of oxygen vacancies on metal-oxide support can help reduce the size of the particles in active clusters of Pt, thus improving catalysis performance of the supported catalyst. Herein, Ce-Sn solid solutions (CSO) with abundant oxygen vacancies have been synthesized. Activated by simple CO reduction after loading Pt species, the catalytic CO oxidation performance of Pt/CSO was significantly better than that of Pt/CeO₂ . The reasons for the elevated activity were further explored regarding ionic Pt single sites being transformed into active Pt clusters after CO reduction. Due to more exposed oxygen vacancies, much smaller Pt clusters were created on CSO (ca. 1.2 nm) than on CeO₂ (ca. 1.8 nm). Consequently, more exposed active Pt clusters significantly improved the ability to activate oxygen and directly translated to the higher catalytic oxidation performance of activated Pt/CSO catalysts in vehicle emission control applications.

Photocatalytic formation of a gas permeable layer selectively deposited on supported metal nanoparticles for sintering-resistant thermal catalysis

Nanoparticle aggregation of supported metal catalysts at high temperatures is a serious problem that causes a drop in catalytic performance. This study investigates the protection of metal nanoparticles from sintering by selectively forming nanoscale SiO₂ shells on Pd supported on TiO₂ by ultraviolet (UV) light irradiation. The proton-coupled reduction reaction increases the local pH around Pd nanoparticles, resulting in hydrolysis of tetraethoxyorthosilicate (TEOS) in only the vicinity of the metal. An apparent quantum efficiency of only 0.6% is obtained for the Pd/TiO₂ catalyst in H₂ evolution from ethanol-containing water under 370 nm excitation light. Therefore, the pH of raw slurry solution should be precisely controlled to that slightly below the threshold value for the TEOS hydrolysis reaction before the photodeposition. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) clearly show that the particle size of the Pd nanoparticles (∼40 nm) with the SiO₂ shell (∼20 nm) was almost unchanged by the high-temperature treatment at 900 °C in air, suggesting that the SiO₂ shell prevented thermal aggregation of Pd nanoparticles. The Pd/TiO₂ without SiO₂ shell decoration exhibited a drop in the number of active sites, which was likely due to aggregation of the Pd catalysts. However, the number of active sites on the Pd@SiO₂/TiO₂ catalyst was maintained even after the catalyst was calcined at 900 °C. Consequently, the Pd@SiO₂/TiO₂ catalyst maintained its catalytic performance for simulated exhaust gas purification even after treatment at 900 °C. This study presents a methodology to produce sintering-tolerant supported metal nanoparticles using the photocatalytic gas permeable layer fabrication method.

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.

The Extent of Platinum-Induced Hydrogen Spillover on Cerium Dioxide

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum-ceria catalyst. At low temperatures (25-100 °C and 1 mbar H2) surface O-H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce4+ to Ce3+. This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H2), O-H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring.

Gold single-atoms confined at the CeOx-TiO2interfaces with enhanced low-temperature activity toward CO oxidation

We use CeO_x-TiO₂hetero-interfaces generated on the surface of CeO_x-TiO₂hybrid oxide supporting powders to stabilize Au single-atoms (SAs) with excellent low-temperature activity toward CO oxidation. Based on intriguing density functional theory calculation results on the preferential formation of Au-SAs at the CeO_x-TiO₂interfaces and the high activity of Au-SAs toward the Mars-van Krevelen type CO oxidation, we synthesized a Au/CeO_x-TiO₂(ACT) catalyst with 0.05 wt.% of Au content. The Au-SAs stabilized at the CeO_x-TiO₂interfaces by electronic coupling between Au and Ce showed improved low-temperature CO oxidation activity than the conventional Au/TiO₂control group catalyst. However, the light-off profile of ACT showed that the early activated Au-SAs are not vigorously participating in CO oxidation. The large portion of the positive effect on the overall catalytic activity from the low activation energy barrier of ACT was retarded by the negative impact from the decreasing active site density at high temperatures. We anticipate that the low-temperature activity and high-temperature stability of Au-SAs that stand against each other can be optimized by controlling the electronic coupling strength between Au-SAs and oxide clusters at the Au-oxide-TiO₂interfaces. Our results show that atomic-precision interface modulation could fine-tune the catalytic activity and stability of Au-SAs.

Templated encapsulation of platinum-based catalysts promotes high-temperature stability to 1,100 °C

Stable catalysts are essential to address energy and environmental challenges, especially for applications in harsh environments (for example, high temperature, oxidizing atmosphere and steam). In such conditions, supported metal catalysts deactivate due to sintering-a process where initially small nanoparticles grow into larger ones with reduced active surface area-but strategies to stabilize them can lead to decreased performance. Here we report stable catalysts prepared through the encapsulation of platinum nanoparticles inside an alumina framework, which was formed by depositing an alumina precursor within a separately prepared porous organic framework impregnated with platinum nanoparticles. These catalysts do not sinter at 800 °C in the presence of oxygen and steam, conditions in which conventional catalysts sinter to a large extent, while showing similar reaction rates. Extending this approach to Pd-Pt bimetallic catalysts led to the small particle size being maintained at temperatures as high as 1,100 °C in air and 10% steam. This strategy can be broadly applied to other metal and metal oxides for applications where sintering is a major cause of material deactivation.

Periodic structural changes in Pd nanoparticles during oscillatory CO oxidation reaction

Nanoparticle (NP) catalysts are ubiquitous in energy systems, chemical production, and reducing the environmental impact of many industrial processes. Under reactive environments, the availability of catalytically active sites on the NP surface is determined by its dynamic structure. However, atomic-scale insights into how a NP surface reconstructs under reaction conditions and the impact of the reconstruction on catalytic activity are still lacking. Using operando transmission electron microscopy, we show that Pd NPs exhibit periodic round-to-flat transitions altering their facets during CO oxidation reaction at atmospheric pressure and elevated temperatures. This restructuring causes spontaneous oscillations in the conversion of CO to CO₂ under constant reaction conditions. Our study reveals that the oscillatory behavior stems from the CO-adsorption-mediated periodic restructuring of the nanocatalysts between high-index-faceted round and low-index-faceted flat shapes. These atomic-scale insights into the dynamic surface properties of NPs under reactive conditions play an important role in the design of high-performance catalysts.

Review of the application of Cu-containing SSZ-13 in NH3-SCR-DeNO x and NH3-SCO

The reduction of NO x emissions has become one of the most important subjects in environmental protection. Cu-containing SSZ-13 is currently the state-of-the-art catalyst for the selective catalytic reduction of NO x with ammonia (NH3-SCR-DeNO x ). Although the current-generation catalysts reveal enhanced activity and remarkable hydrothermal stability, still open challenges appear. Thus, this review focuses on the progress of Cu-containing SSZ-13 regarding preparation methods, hydrothermal resistance and poisoning as well as reaction mechanisms in NH3-SCR-DeNO x . Remarkably, the paper reviews also the progress of Cu-containing SSZ-13 in the selective ammonia oxidation into nitrogen and water vapor (NH3-SCO). The dynamics in the NH3-SCR-DeNO x and NH3-SCO fields make this review timely.

Oxidation behavior of layered FenGeTe2 (n = 3, 4, 5) and Cr2Ge2Te6 governed by interlayer coupling

Layered magnetic materials have recently received tremendous attention due to an attractive combination of functional properties suitable for nanoelectronics and spintronic applications. Enhancing the air stability of the material is a prerequisite for long-term durability of devices. However, the oxidation mechanism of layered magnetic materials is yet to be revealed. Herein we explore the oxidation behavior of monolayer and multilayer Fe_nGeTe₂ (n = 3, 4, 5) and Cr₂Ge₂Te₆ using first-principles calculations. The results show that these monolayer systems are prone to be oxidized in ambient air. With increasing thickness, however, multilayer Fe_nGeTe₂ exhibits distinct oxidation behavior from its monolayer counterparts, originating from its unexpected strong interlayer coupling characterized by wavefunction overlapping of adjacent Te atoms between Fe_nGeTe₂ layers. Moreover, O₂ adsorption does not severely deteriorate the magnetism of layered Fe_nGeTe₂ and Cr₂Ge₂Te₆. Given the fact that oxidation properties can be altered by interlayer coupling, our work opens a paradigm for obtaining oxidation-resistant layered materials.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)-induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal-oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.

A Sulfur-Tolerant MOF-Based Single-Atom Fe Catalyst for Efficient Oxidation of NO and Hg0

Catalytic oxidation of NO and Hg⁰ is a crucial step to eliminate multiple pollutants from emissions from coal-fired power plants. However, traditional catalysts exhibit low catalytic activity and poor sulfur resistance due to low activation ability and poor adsorption selectivity. Herein, a single-atom Fe decorated N-doped carbon catalyst (Fe₁ -N₄ -C), with abundant Fe₁ -N₄ sites, based on a Fe-doped metal-organic framework is developed to oxidize NO and Hg⁰ . The results demonstrate that the Fe₁ -N₄ -C has ultrahigh catalytic activity for oxidizing NO and Hg⁰ at low and room temperature. More importantly, Fe₁ -N₄ -C exhibits robust sulfur resistance as it preferably adsorbs reactants over sulfur oxides, which has never been achieved before with traditional catalysts. Furthermore, SO₂ boosts the catalytic oxidation of NO over Fe₁ -N₄ -C through accelerating the circulation of active sites. Density functional theory calculations reveal that the Fe₁ -N₄ active sites result in a low energy barrier and high adsorption selectivity, providing detailed molecular-level understanding for its excellent catalytic performance. This is the first report on NO and Hg⁰ oxidation over single-atom catalysts with strong sulfur tolerance. The outcomes demonstrate that single-atom catalysts are promising candidates for catalytic oxidation of NO and Hg⁰ enabling cleaner coal-fired power plant operations.

In Situ Small-Angle Neutron Scattering Analysis of Water Evaporation from Porous Exhaust-Gas-Catalyst Supports

Porous media as catalyst supports are key to developing automotive exhaust purification systems. In particular, the water content of these porous media is attracting research attention because catalyst supports containing condensed water vapor at the early stage of cold start require a longer warm-up period. In this regard, water isotherms and evaporation in porous Al₂O₃ were investigated in this study using in situ small-angle neutron scattering (SANS) experiments. Unlike conventional evaluation methods, such as weighing and X-ray tomography, SANS distinguishes water in the primary and secondary pores using a contrast-matching method. Time-resolved measurements showed that water started to evaporate from the secondary pores in tens of seconds and subsequently from the primary pores in a hundred seconds. Exhaustive experiments conducted using nine alumina-based samples revealed that the drying rate depended on the secondary pore size of the porous Al₂O₃. The proposed approach can enable the evaluation of controlling factors to additionally optimize the performance of automotive exhaust gas catalysts, especially during cold start.

Like Cures like: Detoxification Effect between Alkali Metals and Sulfur over the V2O5/TiO2 deNOx Catalyst

The V₂O₅/TiO₂ (VTi) catalyst has been widely employed for the NH₃ selective catalytic reduction (NH₃-SCR) reaction, and sulfur (S) and alkali metals (K) were usually considered as poisons during this reaction. In this work, the synergistic effect of S and K over the VTi catalyst for the NH₃-SCR reaction was analyzed and discussed. It is surprisingly observed that the synergistic effects of S and K exhibited a detoxification effect on the NH₃-SCR reaction. That is, although the VTi catalyst exhibited moderate resistance to S poisoning and unsatisfactory resistance to K deactivation, the SCR activity was restored to close to fresh VTi when K and S coexisted. This detoxification effect also could occur between other alkali metals (e.g., Ca and Na) and sulfur. X-ray photoelectron spectroscopy and charge density difference studies both indicate that the introduction of K could significantly affect the electronic structure of V, but this toxic effect was recovered by the further addition of S because of the strong interaction between S and K. Therefore, this detoxification effect can occur in the practical reaction atmosphere, which alleviates the alkali metal poisoning of commercial catalysts.

Lattice Strain and Surface Activity of Ternary Nanoalloys under the Propane Oxidation Condition

The ability to harness the catalytic oxidation of hydrocarbons is critical for both clean energy production and air pollutant elimination, which requires a detailed understanding of the dynamic role of the nanophase structure and surface reactivity under the reaction conditions. We report here findings of an in situ/operando study of such details of a ternary nanoalloy under the propane oxidation condition using high-energy synchrotron X-ray diffraction coupled to atomic pair distribution function (HE-XRD/PDF) analysis and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The catalysts are derived by alloying Pt with different combinations of second (Pd) and third (Ni) transition metals, showing a strong dependence of the catalytic activity on the Ni content. The evolution of the phase structure of the nanoalloy is characterized by HE-XRD/PDF probing of the lattice strain, whereas the surface activity is monitored by DRIFTS detection of the surface intermediate formation during the oxidation of propane by oxygen. The results reveal the dominance of the surface intermediate species featuring a lower degree of oxygenation upon the first C-C bond cleavage on the lower-Ni-content nanoalloy and a higher degree of oxygenation upon the second C-C bond cleavage on the higher-Ni-content nanoalloy. The face-centered-cubic-type phase structures of the nanoalloys under the oxidation condition are shown to exhibit Ni-content-dependent changes of lattice strains, featuring the strongest strain with little variation for the higher-Ni-content nanoalloy, in contrast to the weaker strains with oscillatory variation for the lower-Ni-content nanoalloys. This process is also accompanied by oxygenation of the metal components in the nanoalloy, showing a higher degree of oxygenation for the higher-Ni-content nanoalloy. These subtle differences in phase structure and surface activity changes correlate with the Ni-composition-dependent catalytic activity of the nanoalloys, which sheds a fresh light on the correlation between the dynamic change of atomic strains and the surface reactivity and has significant implications for the design of oxidation catalysts with enhanced activities.

Synergistic Effects of a CeO2/SmMn2O5-H Diesel Oxidation Catalyst Induced by Acid-Selective Dissolution Drive the Catalytic Oxidation Reaction

A diesel oxidation catalyst (DOC) is installed upstream of an exhaust after-treatment line to remove CO and hydrocarbons and generate NO₂. The catalyst should possess both good oxidation ability and thermal stability because it sits after the engine. We present a novel high-performance DOC with high steam resistance and thermal stability. A selective dissolution method is adopted to modify the surface physicochemical environment of CeO₂-SmMn₂O₅. The X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Raman, electron paramagnetic resonance, hydrogen temperature-programmed reduction, and temperature-programmed desorption results reveal that surface Sm cations are partially removed with the exposure of more Mn⁴⁺ and Ce³⁺ cations and the presence of active surface oxygen species. This mechanism benefits the oxygen transformation from Ce to Mn and promotes the Ce³⁺ + Mn⁴⁺ ↔ Ce⁴⁺ + Mn³⁺ redox cycle according to the in situ near-ambient pressure X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transformation spectroscopy results. Under laboratory-simulated diesel combustion conditions, the catalyst demonstrates excellent low-temperature oxidation catalytic activity (CO and C₃H₆ conversion: T₁₀₀ = 250 °C) compared to a Pt-based catalyst (CO and C₃H₆ conversion: T₁₀₀ = 310 °C) with a WHSV of 120,000 mL g^-1 h^-1. Specifically, NO conversion reaches 68% when the temperature is approximately 300 °C.

Effect of oxygen storage materials on the performance of Pt-based three-way catalysts

Pt supported on oxygen storage materials (CeO2 and CeO2–ZrO2) as effective three-way catalysts.

Direct Conversion of Methane to C2 Hydrocarbons in Solid-State Membrane Reactors at High Temperatures

Direct conversion of methane to C₂ compounds by oxidative and nonoxidative coupling reactions has been intensively studied in the past four decades; however, because these reactions have intrinsic severe thermodynamic constraints, they have not become viable industrially. Recently, with the increasing availability of inexpensive "green electrons" coming from renewable sources, electrochemical technologies are gaining momentum for reactions that have been challenging for more conventional catalysis. Using solid-state membranes to control the reacting species and separate products in a single step is a crucial advantage. Devices using ionic or mixed ionic-electronic conductors can be explored for methane coupling reactions with great potential to increase selectivity. Although these technologies are still in the early scaling stages, they offer a sustainable path for the utilization of methane and benefit from the advances in both solid oxide fuel cells and electrolyzers. This review identifies promising developments for solid-state methane conversion reactors by assessing multifunctional layers with microstructural control; combining solid electrolytes (proton and oxygen ion conductors) with active and selective electrodes/catalysts; applying more efficient reactor designs; understanding the reaction/degradation mechanisms; defining standards for performance evaluation; and carrying techno-economic analysis.

Sabatier principle of metal-support interaction for design of ultrastable metal nanocatalysts

[Figure: see text].

A Hydrothermally Stable Single-Atom Catalyst of Pt Supported on High-Entropy Oxide/Al2O3: Structural Optimization and Enhanced Catalytic Activity

A catalyst with high-entropy oxide (HEO)-stabilized single-atom Pt can afford low-temperature activity for catalytic oxidation and remarkable durability even under harsh conditions. However, HEO is easy to harden during sintering, which results in a few defective sites for anchoring single-atom metals. Herein, we present a sol-gel-assisted mechanical milling strategy to achieve a single-atom catalyst of Pt-HEO/Al₂O₃. The strong interaction between HEO and Al₂O₃ effectively inhibits the growth of HEO microparticles, which leads to generation of more surface defects because of the nanoscale effect. Meanwhile, another strong interaction between Pt and HEO stabilizes single-atom Pt on HEO. Temperature-programmed techniques further verify that the reactivity of surface lattice oxygen species is enhanced because of the Pt-O-M bonds on the surface of HEO. Unlike conventional single-atom Pt catalysts, Pt-HEO/Al₂O₃ as a heterogeneous catalyst not only exhibits superior stability against hydrothermal aging but also presents long-term reaction stability for CO catalytic oxidation, which exceeds 540 h. The present work opens a new door for rational design of hydrothermally stable single-atom Pt catalysts, which are highly promising in practical applications.

Transformation of Highly Stable Pt Single Sites on Defect Engineered Ceria into Robust Pt Clusters for Vehicle Emission Control

Engineering surface defects on metal oxide supports could help promote the dispersion of active sites and catalytic performance of supported catalysts. Herein, a strategy of ZrO₂ doping was proposed to create rich surface defects on CeO₂ (CZO) and, with these defects, to improve Pt dispersion and enhance its affinity as single sites to the CZO support (Pt/CZO). The strongly anchored Pt single sites on CZO support were initially not efficient for catalytic oxidation of CO/C₃H₆. However, after a simple activation by H₂ reduction, the catalytic oxidation performance over Pt/CZO catalyst was significantly boosted and better than Pt/CeO₂. Pt/CZO catalyst also exhibited much higher thermal stability. The structural evolution of Pt active sites by H₂ treatment was systematically investigated on aged Pt/CZO and Pt/CeO₂ catalysts. With H₂ reduction, ionic Pt single sites were transformed into active Pt clusters. Much smaller Pt clusters were created on CZO (ca. 1.2 nm) than on CeO₂ (ca. 1.8 nm) due to stronger Pt-CeO₂ interaction on aged Pt/CZO. Consequently, more exposed active Pt sites were obtained on the smaller clusters surrounded by more oxygen defects and Ce³⁺ species, which directly translated to the higher catalytic oxidation performance of activated Pt/CZO catalyst in vehicle emission control applications.