CO Oxidation Reaction Research Articles

Microscopic Investigation of CO Oxidation Reaction by Copper–Manganese Oxide Catalysts

Microscopic Investigation of CO Oxidation Reaction by Copper–Manganese Oxide Catalysts

Staggered distribution structure Cu-Mn catalysts for mitigating smoke and gas toxicity in combustion: Unravelling mechanistic insight through operando studies

The integration of flame retardancy and environmental friendliness can be achieved by designing cost-effective, stable, and efficient catalysts to reduce smoke and toxicity during the combustion of polymeric materials. This work reports the development of non-precious metal catalysts for thermoplastic polyurethanes (TPU), which exhibit a significant reduction in smoke and gases toxicity while providing flame retardancy. The use of MgB2 as a support enables the in-situ growth of highly efficient Cu-Mn based catalysts, resulting in a remarkable 51.5 % decrease in total smoke release and a 49.1 % reduction in peak rate of CO generation of TPU according to cone calorimeter test results. Furthermore, Cu-Mn/MgB2 demonstrates an impressive 83.1 % reduction in total CO production rate during the steady-state tube furnace test. Additionally, density functional theory is employed to analyze the binding energy between catalysts and TPU as well as the adsorption energy of gases. This elucidates the rational reaction mechanism behind the catalyst and smoke inhibiting process. By combining transition metal oxide catalyzed CO oxidation reaction with polymeric material combustion, this study presents a promising approach for efficient smoke suppression with potential applications.

Electronic Structure of Rh and Ir Single Atom Catalysts Supported on Defective and Doped ZnO: Assessment of Their Activity Towards CO Oxidation.

This study investigated the electronic structure of single-atom Rhodium (Rh) and Iridium (Ir) adsorbed on defective and impurity-doped ZnO(0001) surfaces, and assessed their activity towards the CO oxidation reaction. Our findings reveal that surface impurities significantly influence the binding energies and electronic properties of the metal atoms, with Al and Cr serving as particularly effective promoters. While Rh and Ir acquire a positive charge upon incorporation on the unpromoted Zn(0001) surface, adsorption directly on the promoter results in a net negative charge, thus facilitating the activation of both CO and O2 species. These results highlight the potential of impurity-promoted ZnO surfaces in modulating and tailoring the electronic properties of SACs, which can be used for a rational design of active single-atom catalysts.

High-Temperature Behavior of Pd/MgO Catalysts Prepared via Various Sol–Gel Approaches

A series of Pd/MgO catalysts based on nanocrystalline MgO were prepared via different sol–gel approaches. In the first two cases, palladium was introduced during the gel preparation, followed by drying it in supercritical or ambient conditions. In the third case, aerogel-prepared MgO was impregnated with an ethanol solution of Pd(NO3)2. The prepared catalysts differ in particle size and oxidation state of palladium. The catalytic performance and thermal stability of the samples were examined in a model reaction of CO oxidation at prompt thermal aging conditions. The as-prepared and aged materials were characterized by low-temperature nitrogen adsorption, UV-vis spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and ethane hydrogenolysis testing reaction. The highest initial activity (T50 = 103 °C) was demonstrated by the impregnated sample, containing Pd0 particles of 3 nm in size. The lowest T50 value (215 °C) after aging at 1000 °C was demonstrated by the impregnated Pd/MgO-WI sample. The high-temperature behavior of the catalysts was found to be affected by the initial oxidation state and dispersion of Pd. Two deactivation mechanisms, such as the agglomeration of Pd particles and migration of small Pd species into the bulk of the MgO support with the formation of Pd-MgO solid solutions, were discussed.

D-Electrons of Platinum Alloy Steering CO Pathway for Low-Charge Potential Li-CO2 Batteries.

Aprotic Li-CO2 batteries suffer from sluggish solid-solid co-oxidation kinetics of C and Li2CO3, requiring extremely high charging potentials and leading to serious side reactions and poor energy efficiency. Herein, we introduce a novel approach to address these challenges by modulating the reaction pathway with tailored Pt d-electrons and develop an aprotic Li-CO2 battery with CO and Li2CO3 as the main discharge products. Note that the gas-solid co-oxidation reaction between CO and Li2CO3 is both kinetically and thermodynamically more favorable. Consequently, the Li-CO2 batteries with CoPt alloy-supported on nitrogen-doped carbon nanofiber (CoPt@NCNF) cathode exhibit a charging potential of 2.89 V at 50 μA cm-2, which is the lowest charging potential to date. Moreover, the CoPt@NCNF cathode also shows exceptional cycling stability (218 cycles at 50 μA cm-2) and high energy efficiency up to 74.6 %. Comprehensive experiments and theoretical calculations reveal that the lowered d-band center of CoPt alloy effectively promotes CO desorption and inhibits further CO reduction to C. This work provides promising insights into developing efficient and CO-selective Li-CO2 batteries.

Atomic Level Understanding of the Structural Stability and Catalytic Activity of Nanoporous Gold/Titania Cluster Inverse Catalysts at Ambient and High Temperatures.

Nanoporous gold (NPG) exhibits exceptional catalytic performance at low temperatures, but its activity declines at elevated temperatures due to structural coarsening. Loading metal oxide nanoparticles onto NPG can enhance its catalytic activity at high temperatures. In this work, we used NPG-supported titania nanoparticles as a model system (denoted as Ti2O4/NPG) to study their catalytic activity at ambient and high temperatures with CO oxidation as a probe reaction by density functional theory (DFT) calculation and ab initio molecular dynamics (AIMD) simulations. The possible factors that may affect the CO oxidation reaction pathways and energy profiles on the Ti2O4/NPG, such as oxygen vacancies; silver impurities; Mars-van Krevelen (MvK), Eley-Rideal (ER), or trimolecular Eley-Rideal (TER) mechanisms; and catalytic active sites, were comprehensively investigated. The results showed that reaction energy barriers on Ti2O4/NPG were not significantly decreased compared to the pristine NPG, indicating that their catalytic activities at ambient temperature were comparable. At the evaluated temperature (400 °C), the Ti2O4/NPG exhibited superior thermal stability and maintained its active sites, while the NPG reduced active sites due to surface coarsening. The strong oxide-metal interaction (SOMI) effect between the NPG and Ti2O4 nanoparticles is found to be a main factor for the high structural stability and catalytic activity at high temperatures.

Selective Reduction of NO into N2 Catalyzed by the RhM2O3- Clusters (M = Ta, V, and Al): Importance of the Triatomic Lewis Acid-Base-Acid Mδ+-Rhδ--Mδ+ Site.

Catalytic NO reduction by CO into N2 and CO2 is imperative owing to the increasingly rigorous emission regulation. Identifying the nature of active sites that govern the reactivity and selectivity of NO reduction is pivotal to tailor catalysts, while it is extremely challenging because of the complexity of real-life systems. Guided by our newly discovered triatomic Lewis acid-base-acid (LABA, Ceδ+-Rhδ--Ceδ+) site that accounts for the selective reduction of NO into N2 catalyzed by the RhCe2O3- cluster in gas-phase experiments, the reactivity of the RhM2O3- (M = Ta, V, and Al) clusters in catalytic NO reduction by CO was explored. We determined theoretically that the LABA site still prevails to reduce NO to N2 mediated by RhTa2O3- and RhV2O3-, and the strong M-oxygen affinity was emphasized to construct the LABA site. An overall assessment highlights that RhV2O3- functions as a more promising catalyst because the well-fitting V-O bonding strength facilitates both elementary reactions of NO reduction and CO oxidation.

Al‐Decorated C20 Fullerene as a Promising Noble Metal‐Free Catalyst for CO Oxidation: A DFT Study

AbstractTo remove the toxic carbon monoxide (CO) gas from the environment, recently, researchers have taken great interest in single‐atom catalysts (SACs). In this study, we investigated various reaction pathways and barrier energies for the CO oxidation process onto Al‐decorated C20 fullerene (Al@C20) employing density functional theory (DFT). The outcomes validate that Al decoration on C20 is energetically stable while the corresponding electronic properties show that the Al atom acts as the reactive site for catalytic activity. The bond distances and larger negative adsorption energies (−0.65 and −2.53 eV) evince that CO and O2 species adsorbed strongly to the Al atom. For the oxidation of CO, two popular mechanisms are examined: Eley Rideal (ER) and Langmuir Hinshelwood (LH). The assessment of energy barriers discloses that the CO oxidation reaction progresses via the LH mechanism. Additionally, the CO + O* → CO2 reaction proceeds very quickly onto the Al@C20 catalyst, with a very small energy barrier (0.13 eV), indicating the excellent catalytic reactivity of the studied catalyst. These results propose that the designed catalyst can be valuable in the progress of novel noble metal‐free catalysts for harmful CO elimination from the environment.

Investigation into geometric configurations and electronic properties of CeO2-supported gold clusters

A detailed understanding of the geometric structure and electronic properties of gold nanoparticles on the ceria surface is crucial for comprehending their unique catalytic activity. Using the first-principles method based on density functional theory, the adsorption of Aux (x = 1–4) clusters on the CeO2(111) surface was studied. It was discovered that the standing configurations of Au2 and Au3, as well as the tetrahedral structure of Au4, are the most stable adsorption structures. The stability of these configurations is jointly determined by the number and strength of Au-Au bonds, the Au-O bonding energy, and the interaction dynamics between the clusters and the substrate. The analysis of Bader charge, difference charge density and density of states suggested that lattice relaxation and electronic localization occur in the reduced Ce3+. The reduced amount and location of Ce3+ are significantly influenced by the position and charge transfer amount of Aux cluster. The adsorption of CO on Au4/CeO2(111) indicated that stronger Au-C bonding energy due to the hybridization of Au-5d and C-2p, thereby enhancing the catalytic activity for CO oxidation reactions.

Study of NO and CO Formation Pathways in Jet Flames with CH4/H2 Fuel Blends

The existing natural gas transportation pipelines can withstand a hydrogen content of 0 to 50%, but further research is still needed on the pathways of NO and CO production under moderate or intense low oxygen dilution (MILD) combustion within this range of hydrogen blending. In this paper, we present a computational fluid dynamics (CFD) simulation of hydrogen-doped jet flame combustion in a jet in a hot coflow (JHC) burner. We conducted an in-depth study of the mechanisms by which NO and CO are produced at different locations within hydrogen-doped flames. Additionally, we established a chemical reaction network (CRN) model specifically for the JHC burner and calculated the detailed influence of hydrogen content on the mechanisms of NO and CO formation. The findings indicate that an increase in hydrogen content leads to an expansion of the main NO production region and a contraction of the main NO consumption region within the jet flame. This phenomenon is accompanied by a decline in the sub-reaction rates associated with both the prompt route and NO-reburning pathway via CHi=0–3 radicals, alongside an increase in N2O and thermal NO production rates. Consequently, this results in an overall enhancement of NO production and a reduction in NO consumption. In the context of MILD combustion, CO production primarily arises from the reduction of CO2 through the reaction CH2(S) + CO2 ⇔ CO + CH2O, the introduction of hydrogen into the system exerts an inhibitory effect on this reduction reaction while simultaneously enhancing the CO oxidation reaction, OH + CO ⇔ H + CO2, this dual influence ultimately results in a reduction of CO production.

Role of H2O Adsorption in CO Oxidation over Cerium-Oxide Cluster Anions (CeO2)nO- (n = 1-4).

Water (H2O) is ubiquitous in the environment and inevitably participates in many surface reactions, including CO oxidation. Acquiring a fundamental understanding of the roles of H2O molecules in CO oxidation poses a challenging but pivotal task in real-life catalysis. Herein, benefiting from state-of-the-art mass-spectrometric experiments and quantum chemical calculations, we identified that the dissociation of a H2O molecule on each of the cerium oxide cluster anions (CeO2)nO- (n = 1-4) at room temperature can create a new atomic oxygen radical (O•-) that then oxidizes a CO molecule. The size-dependent reactivity of H2O-mediated CO oxidation on (CeO2)nO- clusters was rationalized by the orbital compositions (O2p) and energies of the lowest unoccupied molecular orbitals of active O•- radicals modified by H2O dissociation. Our findings not only provide new insights into H2O-mediated CO oxidation but also demonstrate the importance of H2O in modulating the reactivity of the O•- radicals.

Electronic Oxide-Metal Strong Interactions (EOMSI) Localized at CeOx-Ag Interface.

Electronic oxide-metal strong interactions (EOMSI) refer to the electronic oxide-metal interactions (EOMI) between oxide adlayers and underlying metal substrate that is strong enough to stabilize supported oxide adlayers in a low-oxidation state, which individually is not stable under an ambient condition, from high temperature oxidation in air to a certain extent. Herein we report the deposition and electronic structure of CeOx adlayers on capping ligand-free cubic Ag nanocrystals, i.e., CeOx/Ag inverse catalysts. The EOMI occur via the charge transfer from Ag substrate to CeOx adlayers in the CeOx/Ag inverse catalyst, and the EOMSI are observed in the CeOx/Ag inverse catalyst with the average thickness of CeOx adlayers about 0.9 nm to exclusively form Ce2O3 adlayers stable against oxidation at 400 °C. As the thickness of CeOx adlayers increases, ceria adlayers with oxygen vacancies (CeO2-x) emerge and grow in the CeOx/Ag inverse catalysts, and the Ce3+/Ce4+ ratio decreases. Catalytic performance of CeOx/Ag inverse catalysts in the CO oxidation reaction is closely linked with the thickness and electronic structure of CeOx adlayers. These results demonstrate that the EOMSI and EOMI in the oxide/metal inverse catalysts are localized at the oxide-metal interface and sensitively vary with the thickness of oxide adlayers, offering a strategy of thickness engineering to tune electronic structures of oxide adlayers in oxide/metal inverse catalysts.

Partial substitution of rhodium with ruthenium in Pd-Rh nanoalloys and its impact on catalytic characteristics

In the present work, alumina-supported trimetallic Pd-Rh-Ru catalysts were synthesized and studied in comparison with the bimetallic Pd-Rh reference sample. The alloy nanoparticles were formed on the surface of the alumina support by thermolysis of the complex salts [Rh(NH3)5Cl][Pd(NO2)4] and [Rh(NH3)5Cl]0.5[Ru(NH3)5Cl]0.5[Pd(NO2)4], preliminary deposited via an incipient wet impregnation method. The influence of the conditions of the thermolysis process on the phase composition of the final products and the size of the trimetallic particles was established. Thus, nanoscale trimetallic (Rh-Ru-Pd) alloy particles of a given composition were obtained. The catalytic performance of the alumina-supported samples was examined in a CO oxidation reaction under prompt thermal aging conditions. It was ascertained that the addition of ruthenium only improves both the initial activity and thermal stability of the catalytic system. The state of each metal in the alloy nanoparticles was characterized by diffuse reflectance UV–vis spectroscopy and X-ray photoelectron spectroscopy.

Exploring the Facet-Dependent Structural Evolution of Pt/CeO2 Catalysts Induced by Typical Pretreatments for CO Oxidation.

Atomic-scale insights into the interactions between metals and supports play a crucial role in optimizing catalyst design, understanding catalytic mechanisms, and enhancing chemical conversion processes. The effects of oxide support on the dynamic behavior of supported metal species during pretreatments or reactions have been attracting a lot of attention; however, very less systematic integrations are carried out experimentally using real catalysts. In this study, we here utilized facet-controlled CeO2 as examples to explore their influence on the supported Pt species (1.0 wt %) during the reducing and oxidizing pretreatments that are typically applied in heterogeneous catalysts. By employing a combination of microscopy, spectroscopy, and first-principles calculations, it is demonstrated that the exposed crystal facets of CeO2 govern the evolution behavior of supported Pt species under different environmental conditions. This leads to distinct local coordinations and charge states of the Pt species, which directly influence the catalytic reactivity and can be leveraged to control the catalytic performance for CO oxidation reactions.

Enhancing the Performance of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) Perovskites as Catalysts for CO Oxidation by Decreasing the Ba Content.

Mixed oxides featuring perovskite-type structures (ABO3) offer promising catalytic properties for applications focused on the control of atmospheric pollution. In this work, a series of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) samples have been synthesized, characterized and tested as catalysts for CO oxidation reaction in conditions close to that found in the exhausts of last-generation automotive internal combustion engines. All samples were observed to be active as catalysts for CO oxidation during CO-TPRe tests, with Ba0.7MnO3 (B0.7M) being the most active one, as it presents the highest amount of oxygen vacancies (which act as active sites for CO oxidation) and Mn (IV), which features the highest levels of reducibility and the best redox properties. B0.7M has also showcased a high stability during reactions at 300 °C, even though a slightly lower CO conversion is achieved during the second consecutive reaction cycle. This performance appears to be related to the decrease in the Mn (IV)/Mn (III) ratio.

Understanding of CO oxidation reaction by molecular and atomic oxygen on Cu19 cluster using the density functional theory

Understanding of CO oxidation reaction by molecular and atomic oxygen on Cu19 cluster using the density functional theory

Can Subsurface Oxygen Species in Oxides Participate in Catalytic Reactions? An 17O Solid-State Nuclear Magnetic Resonance Study.

The impacts of subsurface species of catalysts on reaction processes are still under debate, largely due to a lack of characterization methods for distinguishing these species from the surface species and the bulk. By using 17O solid-state nuclear magnetic resonance (NMR) spectroscopy, which can distinguish subsurface oxygen ions in CeO2 (111) nanorods, we explore the effects of subsurface species of oxides in CO oxidation reactions. The intensities of the 17O NMR signals due to surface and subsurface oxygen ions decrease after the introduction of CO into CeO2 nanorods, with a more significant decrease observed for the latter, confirming the participation of subsurface oxygen species. Density functional theory calculations show that the reaction involves subsurface oxygen ions filling the surface oxygen vacancies created by the direct contact of surface oxygen with CO. This new approach can be extended to the study of the role of oxygen species in other catalytic reactions.

CO formation characteristics in a centrally staged swirl combustor

Based on a validated numerical simulation methodology, this study focused on a centrally staged swirl combustor to investigate the combustion flow field inside the chamber under typical operating conditions. The CO formation characteristics of each region in the combustor were analyzed. In terms of elementary reactions, this study analyzed crucial reaction pathways contributing to CO formation in distinct regions utilizing the eddy dissipation concept model. Results reveal the occurrence of CO formation in the flame front and its rapid oxidation to form CO2 in the post-flame zone. CO2 partially dissociates into CO at high temperatures. Furthermore, air film quenching substantially impacts CO formation and emission, and air film greatly affects CO oxidation more than its formation. Local cooling and dilution effects from the air film contribute to the freezing of CO oxidation reactions. Therefore, a substantial quantity of CO remains unreacted, leading to increased CO emissions at the combustion chamber outlet.

Highly efficient Rh-doped MnO2 nanocatalysts for complete elimination of CO at room temperature.

Obliteration of carbon monoxide is significant due to its hazardous effect on human health and potential application in different fields. Catalytic CO oxidation at lower temperature is the most convenient method to diminish the toxicity of CO. The low-cost catalysts which are exhibiting higher activity at lower temperature with good stability are in demand. The nanosized Rh-doped MnO2 catalysts have been prepared by dextrose-assisted co-precipitation method. Catalytic CO oxidation reaction was carried out over these prepared nanocatalysts under environmentally suitable conditions. XRD confirms the phase formation of prepared catalysts. These samples exhibit rod-like morphology with thickness of rods of less than 10nm which is substantiated from electron microscopy images. XPS data reveals the oxidation state of Mn (+ 4) and Rh (+ 3). These catalysts are highly active for CO oxidation reaction at lower temperature, and one showed complete CO conversion at room temperature. The time-on-stream studies revealed that these catalysts are highly stable for CO oxidation for several hours. These catalysts are decidedly stable in moist condition and also showed higher activity in the presence of moisture, indicating participation of moisture in the oxidation reaction at above room temperatures.

CO2‐Mediated Hydrogen Energy Release‐Storage Enabled by Arc‐Discharge‐Synthesized High‐Dispersion Platinum Catalysts

AbstractDeveloping and fabricating a heterogeneous material for efficient dehydrogenation of formic acid (FA) to H2 coupled with hydrogenation of CO2 back to FA is a promising strategy to forming a carbon‐neutral CO2‐mediated hydrogen energy release‐storage system, which remains challenging. Herein, a facile one‐step ultrahigh‐temperature method of arc‐discharge route for synthesizing efficient single‐crystal α‐MoC supported platinum catalysts with high dispersion of Pt single atoms (SAs), nanoclusters (NCs), and nanoparticles (NPs) is reported. These catalysts exhibit remarkable performance for the adsorption properties and chemical conversion of small molecules including room‐temperature FA dehydrogenation, CO2 hydrogenation to FA (formate), and low‐temperature CO oxidation reactions. The enhanced electron transfer from MoC to active Pt species improves catalyst's surface electron density, thus modulating the chemical adsorption of these small molecules. The engineered 0.1Pt/MoC with Pt SAs shows an outstanding FA dehydrogenation efficiency, with ca. 100% FA conversion and an initial TOF of up to 7739 h−1 at 25 °C. The versatile 0.1Pt/MoC also displays the ability for converting CO2 hydrogenation back to the hydrogen storage carrier FA (formate) with 88.3% yield under mild conditions. Moreover, in‐depth insights into the material microstructure, Pt−MoC electronic interactions, reaction‐dependent particle size effect, and adsorption energy are comprehensively investigated.