Critical Review of Low-Temperature CO Oxidation and Hysteresis Phenomenon on Heterogeneous Catalysts

Methane Catalytic Combustion under Lean Conditions over Pristine and Ir-Loaded La1-xSrxMnO3 Perovskites: Efficiency, Hysteresis, and Time-on-Stream and Thermal Aging Stabilities

The increasing use of natural gas as an efficient, reliable, affordable, and cleaner energy source, compared with other fossil fuels, has brought the catalytic CH4 complete oxidation reaction into the spotlight as a simple and economic way to control the amount of unconverted methane escaping into the atmosphere. CH4 emissions are a major contributor to the 'greenhouse effect', and therefore, they need to be effectively reduced. Catalytic CH4 oxidation is a promising method that can be used for this purpose. Detailed studies of the activity, oxidative thermal aging, and the time-on-stream (TOS) stability of pristine La1-xSrxMnO3 perovskites (LSXM; X = % substitution of La with Sr = 0, 30, 50 and 70%) and iridium-loaded Ir/La1-xSrxMnO3 (Ir/LSXM) perovskite catalysts were conducted in a temperature range of 400-970 °C to achieve complete methane oxidation under excess oxygen (lean) conditions. The effect of X on the properties of the perovskites, and thus, their catalytic performance during heating/cooling cycles, was studied using samples that were subjected to various pretreatment conditions in order to gain an in-depth understanding of the structure-activity/stability correlations. Large (up to ca. 300 °C in terms of T50) inverted volcano-type differences in catalytic activity were found as a function of X, with the most active catalysts being those where X = 0%, and the least active were those where X = 50%. Inverse hysteresis phenomena (steady-state rate multiplicities) were revealed in heating/cooling cycles under reaction conditions, the occurrence of which was found to depend strongly on the employed catalyst pre-treatment (pre-reduction or pre-oxidation), while their shape and the loop amplitude were found to depend on X and the presence of Ir. All findings were consistently interpreted, which involved a two-term mechanistic model that utilized the synergy of Eley-Rideal and Mars-van Krevelen kinetics.

Investigation of palladium catalysts in mesoporous silica support for CO oxidation and CO2 adsorption

The oxidation of Carbon monoxide (CO) to Carbon dioxide (CO2) is one of the most extensively investigated reactions in the field of heterogeneous catalysis, and it occurs via molecular rearrangements induced by catalytic metal atoms with oxygen intermediates. CO oxidation and CO2 capture are instrumental processes in the reduction of green-house gas emissions, both of which are used in low-temperature CO oxidation in the catalytic converters of vehicles. CO oxidation and CO2 adsorption at different temperatures are evaluated for palladium-supported silica aerogel (Pd/SiO2). The synthesized catalyst was active and stable for low-temperature CO oxidation. The catalytic activity was enhanced after the first cycle due to the reconditioning of the catalyst's pores. It was found that the presence of oxide forms of palladium in the SiO2 microstructure, influences the performance of the catalysts due to oxygen vacancies that increases the frequency of active sites. CO2 gas adsorption onto Pd/SiO2 was investigated at a wide-ranging temperature from 16 to 120 °C and pressures ∼1 MPa as determined from the isotherms that were evaluated, where CO2 showed the highest equilibrium adsorption capacity at 16 °C. The Langmuir model was employed to study the equilibrium adsorption behavior. Finally, the effect of moisture on CO oxidation and CO2 adsorption was considered to account for usage in real-world applications. Overall, mesoporous Pd/SiO2 aerogel shows potential as a material capable of removing CO from the environment and capturing CO2 at low temperatures.

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.

Studies on High-Temperature Evolution of Low-Loaded Pd Three-Way Catalysts Prepared by Laser Electrodispersion

Pd/Al₂O₃ catalyst of the "crust" type with Pd loading of 0.03 wt.% was prepared by the deposition of 2 nm Pd particles on the outer surface of the alumina support using laser electrodispersion (LED). This technique differs from a standard laser ablation into a liquid in that the formation of monodisperse nanoparticles occurs in the laser torch plasma in a vacuum. As is found, the LED-prepared catalyst surpasses Pd-containing three-way catalysts, obtained by conventional chemical synthesis, in activity and stability in CO oxidation under prompt thermal aging conditions. Thus, the LED-prepared Pd/Al₂O₃ catalyst showed the best thermal stability up to 1000 °C. The present research is focused on the study of the high-temperature evolution of the Pd/Al₂O₃ catalyst in two reaction mixtures by a set of physicochemical methods (transmission electron microscopy, X-ray photoelectron spectroscopy, and diffuse reflectance UV-vis spectroscopy). In order to follow the dispersion of the Pd nanoparticles during the thermal aging procedure, the testing reaction of ethane hydrogenolysis was also applied. The possible reasons for the high stability of LED-prepared catalysts are suggested.

The Dynamic Nature of Graphene Active Sites in the H2O Gasification process: A ReaxFF and DFT Study

CONTEXT

A steam-rich environment is a more promising application scenario for future coal-fired processes, while active sites are the key factor that determines the reactivity of carbonaceous fuels. The steam gasification process of carbon surfaces with different numbers of active sites (0, 12, 24, 36) was simulated using reactive molecular dynamics in the present study. The temperature for the decomposition of H₂O and the gasification of carbon is determined using temperature-increasing simulation. The decomposition of H₂O was influenced by two driving forces, thermodynamics and active sites on the carbon surface, which dominated the different reaction stages, leading to the observed segmentation phenomenon of the H₂ production rate. The existence and number of initial active sites have a positive correlation with both two stages of the reaction, greatly reducing the activation energy. Residual OH groups play an important role in the gasification of carbon surfaces. The supply of OH groups through the cleavage of OH bonds in H₂O is the rate-limiting step in the carbon gasification reaction. The adsorption preference at carbon defect sites was calculated using density functional theory. Two stable configurations (ether & semiquinone groups) can be formed with O atoms adsorbed on the carbon surface according to the number of active sites. This study will provide further insights into the tuning of active sites for advanced carbonaceous fuels or materials.

METHODS

The large-scale atomic/molecule massively parallel simulator (LAMMPS) code combined with the reaction force-field method was used to carry out the ReaxFF molecular dynamics simulation, where the ReaxFF potentials were taken from Castro-Marcano, Weismiller and William. The initial configuration was built using Packmol, and the visualization of the calculation results was realized through Visual Molecular Dynamics (VMD). The timestep was set to 0.1 fs to detect the oxidation process with high precision. PWscf code in QUANTUM ESPRESSO (QE) package, was used to evaluate the relative stability of different possible intermediate configurations and the thermodynamic stability of gasification reactions. The projector augmented wave (PAW) and the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE-GGA) were adopted. Kinetic energy cutoffs of 50 Ry and 600 Ry, and a uniform mesh of 4 × 4 × 1 k-points were used.

Rational design of trimetallic AgPt-Fe3O4 nanozyme for catalyst poisoning-mediated CO colorimetric detection

Carbon monoxide (CO) is not only a highly poisonous gas that brings great health risk, but also a significant signaling molecule in body. However, it is still challengeable for development of alternative colorimetric probes to traditional organic chromophores for simple, sensitive and convenient CO sensing. Here, for the first time, we rationally design a novel hydrophilic AgPt-Fe₃O₄ nanozyme with a unique heterodimeric nanostructure for colorimetric sensing of CO based on the excellent peroxidase-like catalytic activity as well as highly poisonous effect of CO on the nanozyme's catalytic activity. Both experimental evidence and theoretical calculations reveal the trimetallic AgPt-Fe₃O₄ nanozyme is susceptible to poisoning with the strongest affinity towards CO compared to individual Fe₃O₄ or Ag-Fe₃O₄, which is attributed to the adequate exposure of the active metallic sites and efficient interfacial synergy of unique heterodimeric nanostructure. Accordingly, a novel nanozyme-based colorimetric strategy is developed for CO detection with a low detection limit of 5.6 ppb in solution. Furthermore, the probe can be prepared as very convenient test strips and integrated with the portable smartphone platforms for detecting CO gas samples with a low detection limit of 8.9 ppm. Overall, our work proposes guidelines for the rational design of metallic heterogeneous nanostructure to expand the analytical application of nanozyme.

Activity and Thermal Aging Stability of La1-xSrxMnO3 (x = 0.0, 0.3, 0.5, 0.7) and Ir/La1-xSrxMnO3 Catalysts for CO Oxidation with Excess O2

The catalytic oxidation of CO is probably the most investigated reaction in the literature, for decades, because of its extended environmental and fundamental importance. In this paper, the oxidation of CO on La_1-xSr_xMnO₃ perovskites (LSMx), either unloaded or loaded with dispersed Ir nanoparticles (Ir/LSMx), was studied in the temperature range 100-450 °C under excess O₂ conditions (1% CO + 5% O₂). The perovskites, of the type La_1-xSr_xMnO₃ (x = 0.0, 0.3, 0.5 and 0.7), were prepared by the coprecipitation method. The physicochemical and structural properties of both the LSMx and the homologous Ir/LSMx catalysts were evaluated by various techniques (XRD, N₂ sorption-desorption by BET-BJH, H₂-TPR and H₂-Chem), in order to better understand the structure-activity-stability correlations. The effect of preoxidation/prereduction/aging of the catalysts on their activity and stability was also investigated. Results revealed that both LSMx and Ir/LSMx are effective for CO oxidation, with the latter being superior to the former. In both series of materials, increasing the substitution of La by Sr in the composition of the perovskite resulted to a gradual suppression of their CO oxidation activity when these were prereduced; the opposite was true for preoxidized samples. Inverse hysteresis phenomena in activity were observed during heating/cooling cycles on the prereduced Ir/LSMx catalysts with the loop amplitude narrowing with increasing Sr-content in LSMx. Oxidative thermal sintering experiments at high temperatures revealed excellent antisintering behavior of Ir nanoparticles supported on LSMx, resulting from perovskite's favorable antisintering properties of high oxygen storage capacity and surface oxygen vacancies.

Synergistic Catalytic Effect of Ag and MgO Nanoparticles Supported on Defective BN Surface in CO Oxidation Reaction

Micron-sized supports of catalytically active nanoparticles (NPs) can become a good alternative to nanocarriers if their structure is properly tuned. Here, we show that a combination of simple and easily scalable methods, such as defect engineering and polyol synthesis, makes it possible to obtain Ag and MgO nanoparticles supported on defective hexagonal BN (h-BN) support with high catalytic activity in the CO oxidation reaction. High-temperature annealing in air of Mg-containing (<0.2 at.%) h-BN micropellets led to surface oxidation, the formation of hexagonal-shaped surface defects, and defect-related MgO NPs. The enhanced catalytic activity of Ag/MgO/h-BN materials is attributed to the synergistic effect of h-BN surface defects, ultrafine Ag and MgO NPs anchored at the defect edges, and MgO/Ag heterostructures. In addition, theoretical simulations show a shift in the electron density from metallic Ag towards MgO and the associated decrease in the negative charge of oxygen adsorbed on the Ag surface, which positively affects the catalytic activity of the Ag/MgO/h-BN material.

Estudio teórico de la oxidación de CO con O2 usando catalizadores de Au-Pd y Au-Pt

En el presente estudio se realizaron cálculos con base en la Teoría del Funcional de la Densidad Electrónica (DFT) con la aproximación B3PW91/LANL2DZ para optimizar los sistemas monometálicos y bimetálicos Au9, Au8Pd, Au8Pt, AuPd8, AuPt8, Pd9 y Pt9. Los materiales fueron teóricamente evaluados como catalizadores para la oxidación de monóxido de carbono (CO) y se determinó el sistema más favorable para la adsorción de esta molécula. La sustitución de átomos de Pt y Pd por átomos de Au en los nonámeros generó un cambio en la estructura tridimensional del sistema. El análisis de reactividad global mostró que el clúster más reactivo es 𝑃𝑡9, seguido por 𝐴𝑢𝑃𝑡8. Los índices de Fukui identificaron los sitios más susceptibles para un ataque nucleofílico de ambos clústeres. La adsorción de CO generó una cascada de oxidación que liberó ~4,5 eV, indicando que la reacción es altamente exotérmica y exergónica. Los clústeres 𝐴𝑢𝑃𝑡8 y 𝑃𝑡9 mostraron los valores más bajos de energía de activación de la etapa determinante del mecanismo. En general, la sustitución de un átomo de platino (o paladio) por un átomo de oro no afecta la reactividad de los nonámeros y, por tanto, se infiere que el clúster 𝐴𝑢𝑃𝑡8 podría ser un catalizador promisorio en la oxidación de CO.

Nanoengineering of Catalysts for Enhanced Hydrogen Production

Hydrogen (H2) has emerged as a sustainable energy carrier capable of replacing/complementing the global carbon-based energy matrix. Although studies in this area have often focused on the fundamental understanding of catalytic processes and the demonstration of their activities towards different strategies, much effort is still needed to develop high-performance technologies and advanced materials to accomplish widespread utilization. The main goal of this review is to discuss the recent contributions in the H2 production field by employing nanomaterials with well-defined and controllable physicochemical features. Nanoengineering approaches at the sub-nano or atomic scale are especially interesting, as they allow us to unravel how activity varies as a function of these parameters (shape, size, composition, structure, electronic, and support interaction) and obtain insights into structure–performance relationships in the field of H2 production, allowing not only the optimization of performances but also enabling the rational design of nanocatalysts with desired activities and selectivity for H2 production. Herein, we start with a brief description of preparing such materials, emphasizing the importance of accomplishing the physicochemical control of nanostructures. The review finally culminates in the leading technologies for H2 production, identifying the promising applications of controlled nanomaterials.

Promotion Effect of the Keggin Structure on the Sulfur and Water Resistance of Pt/CeTi Catalysts for CO Oxidation

Developing a catalyst with high SO2 and H2O resistance to achieve high-performance CO oxidation for specific industrial applications is highly desirable. Here, three catalysts were prepared using cerium titanium composite oxide (CeTi), molybdophosphate with Keggin structure-modified CeTi (Keg-CeTi), and molybdophosphate without Keggin structure-modified CeTi (MoP-CeTi) as supports, and their sulfur and water resistance in CO oxidation were tested. The characterization of XRD, BET, SO2/H2O-DRIFTS, XPS, TEM, SEM, NH3/SO2-TPD, H2-TPR, and ICP techniques revealed that the high SO2 and H2O resistance of Pt/Keg-CeTi in CO oxidation was related to its stronger surface acidity, better reduction of surface cerium and molybdenum species, and lower SO2 adsorption and transformation compared to Pt/CeTi and Pt/MoP-CeTi.

Hierarchical N-Doped CuO/Cu Composites Derived from Dual-Ligand Metal-Organic Frameworks as Cost-Effective Catalysts for Low-Temperature CO Oxidation

Development of multi-ligand metal-organic frameworks (MOFs) and derived heteroatom-doped composites as efficient non-noble metal-based catalysts is highly desirable. However, rational design of these materials with controllable composition and structure remains a challenge. In this study, novel hierarchical N-doped CuO/Cu composites were synthesized by assembling dual-ligand MOFs via a solvent-induced coordination modulation/low-temperature pyrolysis method. Different from a homogeneous system, our heterogeneous nucleation strategy provided more flexible and cost-effective MOF production and offered efficient direction/shape-controlled synthesis, resulting in a faster reaction and more complete conversion. After pyrolysis, they further transformed to a unique metal/carbon matrix with regular morphology and, as a hot template, guided the orderly generation of metal oxides, eliminating sintering and agglomeration of metal oxides and initiating a synergistic effect between the N-doped metal oxide/metal and carbon matrix. The prepared N-doped CuO/Cu catalysts held unique water resistance and superior catalytic activity (100% CO conversion at 140 °C).

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

M(II)Al4 Type Layered Double Hydroxides-Preparation Using Mechanochemical Route, Structural Characterization and Catalytic Application

The synthesis of the copper-poor and aluminum-rich layered double hydroxides (LDHs) of the CuAl₄ type was optimized in detail in this work, by applying an intense mechanochemical treatment to activate the gibbsite starting reagent. The phase-pure forms of these LDHs were prepared for the first time; using copper nitrate and perchlorate salts during the syntheses turned out to be the key to avoiding the formation of copper hydroxide sideproducts. Based on the use of the optimized syntheses parameters, the preparation of layered triple and multiple hydroxides was also attempted using Ni(II), Co(II), Zn(II) and even Mg(II) ions. These studies let us identify the relative positions of the incorporating cations in the well-known selectivity series as Ni²⁺ >> Cu²⁺ >> Zn²⁺ > Co²⁺ >> Mg²⁺. The solids formed were characterized by using powder X-ray diffractometry, UV–Vis diffuse reflectance spectroscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. The catalytic potential of the samples was investigated in carbon monoxide oxidation reactions at atmospheric pressure, supported by an in situ diffuse reflectance infrared spectroscopy probe. All solids proved to be active and the combination of the nickel and cobalt incorporation (which resulted in a NiCoAl₈ layered triple hydroxide) brought outstanding benefits regarding low-temperature oxidation and increased carbon monoxide conversion values.

Effect of Ag doping on Pd/Ag-CeO2 catalysts for CO and C3H6 oxidation

To achieve high fuel efficiency and low emission in automobiles, it is necessary to develop highly active diesel oxidation catalysts (DOCs). Pd/CeO₂ catalysts have been widely used as active catalysts for CO and C₃H₆ oxidation reactions. Additionally, Ag has been reported to enhance the oxygen storage capacity (OSC) of CeO₂, which contributes to the oxidation ability of Pd/CeO₂. In this study, Pd/Ag-CeO₂ catalysts were used for CO and C₃H₆ oxidation reactions. When CeO₂ was doped with appropriate amounts of Ag, reducibility and CO desorption rate were increased, which confirmed the high OSCs of Ag-doped catalysts. However, Ag particles were formed and the Ce³⁺/Ce⁴⁺ ratio decreased when CeO₂ was doped with excess amounts of Ag. In addition, reduced Pd (Pd⁰), which is an active species for C₃H₆ oxidation, was formed and maintained even under oxidative reaction conditions. Since the removal of C₃H₆ is important for the oxidation of CO and C₃H₆, the catalyst with the highest Pd⁰ fraction (Pd/0.1Ag-CeO₂ and Pd/0.3Ag-CeO₂) presented improved catalytic activity. Consequently, the optimal amount of Ag enhanced the OSC of Pd/Ag-CeO₂ catalysts and formed active Pd⁰ species under oxidative conditions, which resulted in the excellent catalytic activity of Pd/Ag-CeO₂ for the CO and C₃H₆ oxidation reaction.

Recent advances in catalytic systems in the prism of physicochemical properties to remediate toxic CO pollutants: A state-of-the-art review

Carbon monoxide (CO) is the most harmful pollutant in the air, causing environmental issues and adversely affecting humans and the vegetation and then raises global warming indirectly. CO oxidation is one of the most effective methods of reducing CO by converting it into carbon dioxide (CO₂) using a suitable catalytic system, due to its simplicity and great value for pollution control. The CO oxidation reaction has been widely studied in various applications, including proton-exchange membrane fuel cell technology and catalytic converters. CO oxidation has also been of great academic interest over the last few decades as a model reaction. Many review studies have been produced on catalysts development for CO oxidation, emphasizing noble metal catalysts, the configuration of catalysts, process parameter influence, and the deactivation of catalysts. Nevertheless, there is still some gap in a state of the art knowledge devoted exclusively to synergistic interactions between catalytic activity and physicochemical properties. In an effort to fill this gap, this analysis updates and clarifies innovations for various latest developed catalytic CO oxidation systems with contemporary evaluation and the synergistic relationship between oxygen vacancies, strong metal-support interaction, particle size, metal dispersion, chemical composition acidity/basicity, reducibility, porosity, and surface area. This review study is useful for environmentalists, scientists, and experts working on mitigating the harmful effects of CO on both academic and commercial levels in the research and development sectors.

Porous Silicon Carbide (SiC): A Chance for Improving Catalysts or Just Another Active-Phase Carrier?

There is an obvious gap between efforts dedicated to the control of chemicophysical and morphological properties of catalyst active phases and the attention paid to the search of new materials to be employed as functional carriers in the upgrading of heterogeneous catalysts. Economic constraints and common habits in preparing heterogeneous catalysts have narrowed the selection of active-phase carriers to a handful of materials: oxide-based ceramics (e.g. Al₂O₃, SiO₂, TiO₂, and aluminosilicates-zeolites) and carbon. However, these carriers occasionally face chemicophysical constraints that limit their application in catalysis. For instance, oxides are easily corroded by acids or bases, and carbon is not resistant to oxidation. Therefore, these carriers cannot be recycled. Moreover, the poor thermal conductivity of metal oxide carriers often translates into permanent alterations of the catalyst active sites (i.e. metal active-phase sintering) that compromise the catalyst performance and its lifetime on run. Therefore, the development of new carriers for the design and synthesis of advanced functional catalytic materials and processes is an urgent priority for the heterogeneous catalysis of the future. Silicon carbide (SiC) is a non-oxide semiconductor with unique chemicophysical properties that make it highly attractive in several branches of catalysis. Accordingly, the past decade has witnessed a large increase of reports dedicated to the design of SiC-based catalysts, also in light of a steadily growing portfolio of porous SiC materials covering a wide range of well-controlled pore structure and surface properties. This review article provides a comprehensive overview on the synthesis and use of macro/mesoporous SiC materials in catalysis, stressing their unique features for the design of efficient, cost-effective, and easy to scale-up heterogeneous catalysts, outlining their success where other and more classical oxide-based supports failed. All applications of SiC in catalysis will be reviewed from the perspective of a given chemical reaction, highlighting all improvements rising from the use of SiC in terms of activity, selectivity, and process sustainability. We feel that the experienced viewpoint of SiC-based catalyst producers and end users (these authors) and their critical presentation of a comprehensive overview on the applications of SiC in catalysis will help the readership to create its own opinion on the central role of SiC for the future of heterogeneous catalysis.

Pulsed Laser in Liquids Made Nanomaterials for Catalysis

Catalysis is essential to modern life and has a huge economic impact. The development of new catalysts critically depends on synthetic methods that enable the preparation of tailored nanomaterials. Pulsed laser in liquids synthesis can produce uniform, multicomponent, nonequilibrium nanomaterials with independently and precisely controlled properties, such as size, composition, morphology, defect density, and atomistic structure within the nanoparticle and at its surface. We cover the fundamentals, unique advantages, challenges, and experimental solutions of this powerful technique and review the state-of-the-art of laser-made electrocatalysts for water oxidation, oxygen reduction, hydrogen evolution, nitrogen reduction, carbon dioxide reduction, and organic oxidations, followed by laser-made nanomaterials for light-driven catalytic processes and heterogeneous catalysis of thermochemical processes. We also highlight laser-synthesized nanomaterials for which proposed catalytic applications exist. This review provides a practical guide to how the catalysis community can capitalize on pulsed laser in liquids synthesis to advance catalyst development, by leveraging the synergies of two fields of intensive research.

CO Oxidation Efficiency and Hysteresis Behavior over Mesoporous Pd/SiO2 Catalyst

Carbon monoxide (CO) oxidation is considered an important reaction in heterogeneous industrial catalysis and has been extensively studied. Pd supported on SiO2 aerogel catalysts exhibit good catalytic activity toward this reaction owing to their CO bond activation capability and thermal stability. Pd/SiO2 catalysts were investigated using carbon monoxide (CO) oxidation as a model reaction. The catalyst becomes active, and the conversion increases after the temperature reaches the ignition temperature (Tig). A normal hysteresis in carbon monoxide (CO) oxidation has been observed, where the catalysts continue to exhibit high catalytic activity (CO conversion remains at 100%) during the extinction even at temperatures lower than Tig. The catalyst was characterized using BET, TEM, XPS, TGA-DSC, and FTIR. In this work, the influence of pretreatment conditions and stability of the active sites on the catalytic activity and hysteresis is presented. The CO oxidation on the Pd/SiO2 catalyst has been attributed to the dissociative adsorption of molecular oxygen and the activation of the C-O bond, followed by diffusion of adsorbates at Tig to form CO2. Whereas, the hysteresis has been explained by the enhanced stability of the active site caused by thermal effects, pretreatment conditions, Pd-SiO2 support interaction, and PdO formation and decomposition.

Physical and Chemical Synthesis of Au/CeO2 Nanoparticle Catalysts for Room Temperature CO Oxidation: A Comparative Study

In many heterogeneous catalytic reactions, such as low-temperature CO oxidation, the preparation conditions, and the role of the CeO2 support (oxygen vacancies and redox properties) in the dispersion and the chemical state of Au, are considered critical factors for obtaining gold nanoparticle catalysts with high catalytic performance. In this work, the physical and chemical preparation methods were compared, aiming at understanding how the preparation method influences the catalytic activity. The Au/CeO2 nanoparticle catalysts with 5% Au loading were prepared via the Physical Laser Vaporization Controlled Condensation method (LVCC), and the chemical Deposition-Precipitation method (DP) was used to investigate the effect of synthesis methods on the structure and the catalytic activity toward the CO oxidation. In this manuscript, we compare the activity of nanostructured Au/CeO2 catalysts. The structure and the redox properties of the catalysts were investigated by the XRD, SEM, TEM, TPR, and XPS. The catalytic activity for low-temperature CO oxidation was studied using a custom-built quartz tube flow reactor coupled with an infrared detector system at atmospheric pressure. The study reveals that the prepared CeO2-supported Au nanoparticles’ catalytic activity was highly dependent on the preparation methods. It showed that the sample prepared by the DP method exhibits higher catalytic efficiency toward CO oxidation when compared with the sample prepared by the LVCC method. The high catalytic activity could be attributed to the small particle size and shape, slightly higher Au concentration at the surface, surface-active Au species such as Au1+, along with the large interface between Au and CeO2. This study suggests that the stability, dispersion of Au nanoparticles on CeO2, and strong interaction between Au and CeO2 lead to strong oxidation ability even below room temperature. Considering the universal character of different physical and chemical methods for Au/CeO2 preparation, this study may also provide a base for supported Au-based catalysts for many oxidation reactions in energy and environmental applications.

The Origin of Au/Ce1-xZrxO2 Catalyst’s Active Sites in Low-Temperature CO Oxidation

Gold catalysts have found applications in many reactions of both industrial and environmental importance. Great interest has been paid to the development of new processes that reduce energy consumption and minimize pollution. Among these reactions, the catalytic oxidation of carbon monoxide (CO) is an important one, considering that a high concentration of CO in the atmosphere creates serious health and environmental problems. This paper examines the most important achievements and conclusions arising from the own authorship contributions concerning (2 wt. % Au)/Ce1−xZrxO2 catalyst’s active sites in low-temperature CO oxidation. The main findings of the present review are: (1) The effect of preparing conditions on Au crystallite size, highlighting some of the fundamental underpinnings of gold catalysis: the Au surface composition and the poisoning effect of residual chloride on the catalytic activity of (2 wt. % Au)/Ce1−xZrxO2 catalysts in CO oxidation; (2) The identification of ion clusters related to gold and their effect on catalyst’ surface composition; (3) The importance of physicochemical properties of oxide support (e.g., its particle size, oxygen mobility at low temperature and redox properties) in the creation of catalytic performance of Au catalysts; (4) The importance of oxygen vacancies, on the support surface, as the centers for oxygen molecule activation in CO reaction; (5) The role of moisture (200–1000 ppm) in the generation of enhanced CO conversion; (6) The Au-assisted Mars-van Krevelen (MvK) adsorption–reaction model was pertinent to describe CO oxidation mechanism. The principal role of Au in CO oxidation over (2 wt. % Au)/Ce1−xZrxO2 catalysts was related to the promotion in the transformation process of reversibly adsorbed or inactive surface oxygen into irreversibly adsorbed active species; (7) Combination of metallic gold (Au0) and Au-OH species was proposed as active sites for CO adsorption. These findings can help in the optimization of Au-containing catalysts.

Three-dimensional strain dynamics govern the hysteresis in heterogeneous catalysis

Understanding catalysts strain dynamic behaviours is crucial for the development of cost-effective, efficient, stable and long-lasting catalysts. Here, we reveal in situ three-dimensional strain evolution of single gold nanocrystals during a catalytic CO oxidation reaction under operando conditions with coherent X-ray diffractive imaging. We report direct observation of anisotropic strain dynamics at the nanoscale, where identically crystallographically-oriented facets are qualitatively differently affected by strain leading to preferential active sites formation. Interestingly, the single nanoparticle elastic energy landscape, which we map with attojoule precision, depends on heating versus cooling cycles. The hysteresis observed at the single particle level is following the normal/inverse hysteresis loops of the catalytic performances. This approach opens a powerful avenue for studying, at the single particle level, catalytic nanomaterials and deactivation processes under operando conditions that will enable profound insights into nanoscale catalytic mechanisms.

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

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

Infrared Thermography as a Powerful, Versatile, and Elegant Research Tool in Chemistry: Principles and Application to Catalysis and Adsorption

In this Review, diverse chemical problems that have been approached by means of infrared thermography (IRT) are covered in depth. Moreover, some novel steps forward in this field are made, described and discussed. Namely, the latest-generation IRT performance capabilities are harnessed in full; the initial phase of catalytic CO oxidation (called "fast ignition") is presented at the 0.01 s temporal resolution; at the same resolution, the thermal manifestation of the adsorption-desorption wave propagation after the gaseous reactant pulsed (0.6 s) wetting is exhibited. Furthermore, a radical difference in the thermal behavior of differently calcined γ-Al₂ O₃ supported Au catalysts, which underwent successive H₂ O and CO attacks, is demonstrated, and the generally accepted fact that the catalyst temperature reflects the catalytic activity is validated experimentally. It is shown that latest-generation IRT may serve as unique and highly informative research tool in chemistry.

Mild Preoxidation Treatment of Pt/TiO2 Catalyst and Its Enhanced Low Temperature Formaldehyde Decomposition

The typical platinum nanoparticles loaded on titania (Pt/TiO2) were pretreated with mild oxidation (<300 °C) in pure oxygen to enhance the low-temperature formaldehyde (HCHO) decomposition performance. The structural properties of support and platinum nanoparticles were characterized by X-ray diffraction (XRD), physical adsorption/desorption, high-resolution transmission electron microscopy (HRTEM), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFITS), and temperature-programmed reduction and oxidation (TPR and TPO). The catalytic results showed that the low temperature HCHO decomposition activity of mild pre-oxidized Pt/TiO2 was around three times that of the pristine one. According to the characterization results, the structure of the Pt/TiO2 support and their Pt particle sizes had negligible change after pre-oxidation treatment. The cationic Pt content of Pt/TiO2 and surface roughness of Pt nanoparticles gradually increased with the increasing temperature of the pre-oxidation treatment. Mild pre-oxidation treatment was beneficial to the oxygen activation and water dissociation of Pt/TiO2. In situ HCHO-DFIRTS results showed that the mild pre-oxidation treatment could enhance the dehydrogenation of formate.

Recent Progress on Transition Metal Nitrides Nanoparticles as Heterogeneous Catalysts

This short review aims at providing an overview of the most recent literature regarding transition metal nitrides (TMN) applied in heterogeneous catalysis. These materials have received renewed attention in the last decade due to its potential to substitute noble metals mainly in biomass and energy transformations, the decomposition of ammonia being one of the most studied reactions. The reactions considered in this review are limited to thermal catalysis. However the potential of these materials spreads to other key applications as photo- and electrocatalysis in hydrogen and oxygen evolution reactions. Mono, binary and exceptionally ternary metal nitrides have been synthetized and evaluated as catalysts and, in some cases, promoters are added to the structure in an attempt to improve their catalytic performance. The objective of the latest research is finding new synthesis methods that allow to obtain smaller metal nanoparticles and increase the surface area to improve their activity, selectivity and stability under reaction conditions. After a brief introduction and description of the most employed synthetic methods, the review has been divided in the application of transition metal nitrides in the following reactions: hydrotreatment, oxidation and ammonia synthesis and decomposition.