CO Oxidation Research Articles

Diving into the influence of phosphorus on ceria-supported copper manganese bimetallic oxides for catalytic removal of carbon monoxide

The application of ceria-supported catalysts as promising catalysts for air pollutant abatement via heterogeneous catalytic processes is still a challenge. Phosphorus (P) is a potential threat to catalyst deactivation but has not been sufficiently explored. Herein, a systematic study reveals the poisoning behaviors of P on Cu-Mn bimetallic oxides supported by ceria (Cu-Mn/CeO2) as a typical case in CO oxidation and provides new insights into the deactivation mechanism. The P impregnation decreases the surface area of Cu-Mn/CeO2 and leads to the emergence of a new sheet morphology. The interactions between P and catalyst components (support and active species) result in the generation of dominant P species of PO43− and PO3−. The P addition also weakens the oxygen storage capacity of ceria and decreases the proportions of low-valent Cu and Mn, hindering electron transfers among metals. Furthermore, the reducibility and the adsorption and activation of O2 are limited in P-loaded Cu-Mn/CeO2 catalysts as found in H2-TPR and O2-TPD when increasing the P concentration. In situ DRIFTS experiments demonstrate that the inhibition of the formation and transformation of active intermediates, namely carbonyls and carbonates, is responsible for the poisoning behaviors of P in the pathway of CO oxidation over Cu-Mn/CeO2 catalyst.

Significantly suppressing CO release achieved by the catalysis effect of nanostructured rare earth/manganese oxides: Application in flame retardant thermoplastic polyurethane

There is significant smoke and toxic volatiles generated from the combustion of thermoplastic polyurethane (TPU), which has compromised its application and posed a significant threat to human life. Here, the hydrothermal-citrate complexation method synthesised the rare earth Mn-based composite catalyst and blended with TPU to mitigate smoke release and toxic gas generation during TPU combustion. The results demonstrate that the inclusion of 3 wt% Mn-La and Mn-Ce catalysts into TPU leads to a 41.3% and 33.6% decrease in maximum smoke density (Ds max), respectively, along with a 52.4% and 50.5% reduction in peak CO production rate (pCOPR). The mechanism of rare earth Mn-based catalyst-based smoke suppression and toxicity reduction in TPU is explained at a microscopic scale based on density functional theory (DFT) research: the introduction of catalyst bolsters the adsorption of O2 and CO on the surface of TPU nanocomposites and facilitates the oxidation of CO. Additionally, it can expedite the formation of dense carbon layers and impede heat and mass transfer. The TPU nanocomposites exhibit excellent flame retardancy and effective smoke suppression. A feasible strategy for manufacturing fire-safety TPU nanocomposites with favorable comprehensive properties is proposed.

Restructuring the interfacial active sites to generalize the volcano curves for platinum-cobalt synergistic catalysis

Computationally derived volcano curve has become the gold standard in catalysis, whose practical application usually relies on empirical interpretations of composition or size effects by the identical active site assumption. Here, we present a proof-of-concept study on disclosing both the support- and adsorbate-induced restructuring of Pt-Co bimetallic catalysts, and the related interplays among different interfacial sites to propose the synergy-dependent volcano curves. Multiple characterizations, isotopic kinetic investigations, and multiscale simulations unravel that the progressive incorporation of Co into Pt catalysts, driven by strong Pt-C bonding (metal-support interfaces) and Co-O bonding (metal-adsorbate interfaces), initiates the formation of Pt-rich alloys accompanied by isolated Co species, then Co segregation to epitaxial CoOx overlayers and adjacent Co3O4 clusters, and ultimately structural collapse into amorphous alloys. Accordingly, three distinct synergies, involving lattice oxygen redox from Pt-Co alloy/Co3O4 clusters, dual-active sites engineering via Pt-rich alloy/CoOx overlayer, and electron coupling within exposed alloy, are identified and quantified for CO oxidation (gas-phase), ammonia borane hydrolysis (liquid-phase), and hydrogen evolution reaction (electrocatalysis), respectively. The resultant synergy-dependent volcano curves represent an advancement over traditional composition-/size-dependent ones, serving as a bridge between theoretical models and experimental observations in bimetallic catalysis.

Highly Water-Resistant Ce-Doped CuMnOx Catalyst for Durable CO Oxidation by Inhibiting Byproducts

Highly Water-Resistant Ce-Doped CuMnO_<i>x</i> Catalyst for Durable CO Oxidation by Inhibiting Byproducts

Two Different Atomically Dispersed Pt Atoms Supported on Ceria.

Atomically dispersed metals on oxide supports with different distribution positions or coordination environments can dictate the reactivity; they have therefore attracted tremendous attention recently. Nonetheless, the acknowledging and understanding of different single atoms remain challenging due to the reactivity controversy of the supported single atoms and clusters or nanoparticles, particularly on the widely used ceria supports. Herein, by modulating the loading amount of Pt single atoms carefully with strong electrostatic adsorption on conventionally synthesized ceria supports, we obtained two different atomically dispersed Pt atoms with similar Pt-O coordination environments and CO adsorption characteristics. One is anchored on the surface of ceria, and it can migrate and aggregate once activated with reduction-reoxidation treatments. The other may be trapped by the surface defects or vacancies in ceria and would be fixed on the ceria support firmly in isolated states during activation. Despite the similar CO adsorption during the reaction, the former can catalyze CO oxidation in both the status of single atoms and aggregated PtOx clusters. However, the latter is inactive for the reaction and would not be affected by the activation treatment. It cannot involve the CO oxidation, resulting in the waste of supported Pt atoms.

Facet-Dependent Diversity of Pt-O Coordination for Pt1/CeO2 Catalysts Achieved by Oriented Atomic Deposition.

The surface chemistry of CeO2 is dictated by the well-defined facets, which exert great influence on the supported metal species and the catalytic performance. Here we report Pt1/CeO2 catalysts exhibiting specific structures of Pt-O coordination on different facets by using adequate preparation methods. The simple impregnation method results in Pt-O3 coordination on the predominantly exposed {111} facets, while the photo-deposition method achieves oriented atomic deposition for Pt-O4 coordination into the "nano-pocket" structure of {100} facets at the top. Compared to the impregnated Pt1/CeO2 catalyst showing normal redox properties and low-temperature activity for CO oxidation, the photo-deposited Pt1/CeO2 exhibits uncustomary strong metal-support interaction and extraordinary high-temperature stability. The preparation methods dictate the facet-dependent diversity of Pt-O coordination, resulting in the further activity-selectivity trade-off. By applying specific preparation routes, our work provides an example of disentangling the effects of support facets and coordination environments for nano-catalysts.

Facet‐Dependent Diversity of Pt−O Coordination for Pt1/CeO2 Catalysts Achieved by Oriented Atomic Deposition

AbstractThe surface chemistry of CeO2 is dictated by the well‐defined facets, which exert great influence on the supported metal species and the catalytic performance. Here we report Pt1/CeO2 catalysts exhibiting specific structures of Pt−O coordination on different facets by using adequate preparation methods. The simple impregnation method results in Pt−O3 coordination on the predominantly exposed {111} facets, while the photo‐deposition method achieves oriented atomic deposition for Pt−O4 coordination into the “nano‐pocket” structure of {100} facets at the top. Compared to the impregnated Pt1/CeO2 catalyst showing normal redox properties and low‐temperature activity for CO oxidation, the photo‐deposited Pt1/CeO2 exhibits uncustomary strong metal‐support interaction and extraordinary high‐temperature stability. The preparation methods dictate the facet‐dependent diversity of Pt−O coordination, resulting in the further activity‐selectivity trade‐off. By applying specific preparation routes, our work provides an example of disentangling the effects of support facets and coordination environments for nano‐catalysts.

Metal-decorated opal-like silica catalysts: High-temperature reconstructions in cobalt-ceria active layer during simultaneous oxidation of CO and hydrocarbons

Opal-like silica, which represents a new class of SiO2-based nanomaterials, was used as a support for the deposition of cobalt and cerium oxides as active components. A series of samples was prepared and tested in the oxidation of CO and hydrocarbons in a prompt thermal aging regime. As revealed by a set of physicochemical methods, the initial catalytic performance of the samples is high (T50 = 214 °С) due to the emergence of new catalytic sites, which appeared as a result of interaction between Co and Ce oxides located between silica nanospheres in a close proximity to each other. At 800 °C, these particles undergo agglomeration, which is expressed in a slight deactivation of the catalysts (T50 ≥ 260 °С). The bulk silicate phase is formed at 900 °C, thus leading to further deactivation. Recovering of the catalytic activity (T50 = 252 °С) takes place at 1000 °С.

Mesostructured manganese oxides for efficient catalytic oxidation of CO, ethylene, and propylene at mild temperatures: Insight into the role of crystalline phases and physico-chemical properties

Removing pollutants for indoor air purification is a key point to ensure the health and well-being of people in confined environments. This work aims to provide new insight into the development of promising manganese oxide catalysts. The effects of different crystalline phases (MnO2 and Mn2O3) and the role of redox properties and structural defects were investigated to abate indoor pollutants at mild temperatures. These materials were extensively characterized through complementary techniques and catalytic tests were performed to oxidize 100 ppm of CO, ethylene, or propylene. The most promising catalyst was obtained through solution combustion synthesis, achieving total removal at 118, 222, and 172 °C, respectively, with the highest oxidation rate (2.41, 0.88, and 2.47 μmolg−1s−1) and lowest activation energy (50, 32, and 45 kJmol−1) for the three molecules. The synergy between crystalline phases enhanced the catalytic performance and their distribution in the structure was a crucial parameter affecting the number of structural defects.

Construction of CuCoCe ternary catalyst via a porous organic polymer template approach for efficient CO-PROX with wide operating temperature window

Preferential CO oxidation (CO-PROX) is a promising approach for removing trace CO from hydrogen-rich streams, effectively preventing Pt anode poisoning in proton-exchange membrane fuel cells (PEMFCs). To effectively eliminate CO within the PEMFC operating temperature range, catalysts must exhibit high CO conversion and O2 selectivity. Expanding the operating temperature window of these catalysts is crucial. This study presents the synthesis of a CuCoCe ternary catalyst (CuCo/Ce-P) using an imidazole-functionalized porous organic polymer as a sacrificial template. Compared to the CuCoCe ternary catalyst prepared without a template (CuCo/Ce), CuCo/Ce-P exhibits significantly enhanced catalytic activity, with a T50 reduction from 85 °C to 55 °C and an expanded operating temperature window of approximately 40 ℃. Characterization results indicate that the imidazole-functionalized porous organic polymer facilitates improved dispersion of active species by anchoring metal ions through electron-rich groups, enhances intermetallic interactions, promotes electron transfer, and significantly improves the oxygen-deficient capacity of the CuCoCe ternary catalyst. Consequently, the catalyst exhibits a greater number of active interfaces, leading to enhanced catalytic activity and a wider operating temperature window.

Enhancing Stability of Surface Au under Oxidizing Conditions through Reduced Bulk Au Content.

Contrary to the common assumption that a higher bulk content of precious metals facilitates the preservation of more surface noble metal by serving as a reservoir for surface enrichment, we demonstrate that a lower bulk content of Au results in a more stable arrangement of Au atoms at the surface of Cu-Au nanoparticles when exposed to an O2 atmosphere. Using ambient pressure X-ray photoelectron spectroscopy, we investigate the surface segregation and oxidation behavior of Cu-Au nanoparticles across various compositions. Our results reveal that in Au-rich nanoparticles exposed to an H2 atmosphere, surface segregation prompts the formation of a continuous Au-enriched shell, which subsequently oxidizes into a complete CuOx shell upon transitioning to an O2 atmosphere. Conversely, in Au-poor nanoparticles during H2 treatment, segregation results in the emergence of Au clusters embedded within the surface layer, persisting upon exposure to O2. This unexpected phenomenon shows that reducing the bulk content of precious metals can enhance the surface stability of noble atoms under oxidizing conditions, as further demonstrated by comparing the catalytic performance of Cu-Au nanoparticles with varying Au bulk contents in CO oxidation.

Supported Binuclear Gold Phosphine Complexes as CO Oxidation Catalysts: Insights into the Formation of Surface‐Stabilized Au Particles

Atomically precise gold phosphine complexes as precursors for supported Au catalysts tested in CO oxidation are presented. Using a variety of analytical techniques, including in situ and operando X‐ray absorption spectroscopy, it is discovered that minor changes in the ligand of the molecular complexes result in significantly different activation behaviors of supported Au catalysts under reaction conditions. When using [Au2(μ2‐POP)2]OTf2 (POP = tetraphenylphosphoxane) as single‐source precursor, an active supported oxidation catalyst in second light‐off is obtained, outperforming a commercial Au/TiO2 and a P‐free Au/Al2O3 reference catalyst. Conversely, using [Au2(μ2‐dppe)2]OTf2 (dppe = diphenylphosphinoethane) on alumina leads to a significant decrease in CO oxidation activity. This difference is attributed to the formation of P‐containing ligand residues on the support in the case of [Au2(μ2‐POP)2]OTf2/Al2O3, which enhances the thermal stability of the Au particles and affects the particle's electronic properties through charge transfer processes. This work provides insights into the dynamic ligand decomposition of molecular gold complexes under reaction conditions and demonstrates the delicate balance between the stabilization of Au particles, clusters, and complexes using ligands and the blocking of active sites. This knowledge will pave the way for the targeted use of molecular transition metal complexes as precursors in synthesizing surface‐stabilized nanoparticles.

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.

Embedding Reverse Electron Transfer Between Stably Bare Cu Nanoparticles and Cation-Vacancy CuWO4.

Cu nanoparticles (NPs) have attracted widespread attention in electronics, energy, and catalysis. However, conventionally synthesized Cu NPs face some challenges such as surface passivation and agglomeration in applications, which impairs their functionalities in the physicochemical properties. Here, the issues above by engineering an embedded interface of stably bare Cu NPs on the cation-vacancy CuWO4 support is addressed, which induces the strong metal-support interactions and reverse electron transfer. Various atomic-scale analyses directly demonstrate the unique electronic structure of the embedded Cu NPs with negative charge and anion oxygen protective layer, which mitigates the typical degradation pathways such as oxidation in ambient air, high-temperature agglomeration, and CO poisoning adsorption. Kinetics and in situ spectroscopic studies unveil that the embedded electron-enriched Cu NPs follow the typical Eley-Rideal mechanism in CO oxidation, contrasting the Langmuir-Hinshelwood mechanism on the traditional Cu NPs. This mechanistic shift is driven by the Coulombic repulsion in anion oxygen layer, enabling its direct reaction with gaseous CO to form the easily desorbed monodentate carbonate.

Deciphering the structural origin for the remarkable CO oxidation activities of the CuO-based catalysts with diverse morphological CeO2 supports

In this work, octahedral, cubic, particle-like, spherical and rod-shaped nano-CeO2 supports were synthesized by hydrothermal method. CuO/CeO2 catalysts were then prepared via an ultrasonic-assisted impregnation method, employing these differently shaped CeO2 supports. The resulting catalysts were tested for their performance in CO oxidation. The findings revealed that the CuO active sites significantly influenced the oxygen storage and release capacity of CeO2, thereby improving its catalytic activity for low-temperature CO oxidation. The key factors affecting the CO oxidation performance of CuO/CeO2 catalysts with different CeO2 morphologies were investigated through various characterizations. It was found that the specific surface area was not the primary factor in determining catalytic activity. Instead, the exposed crystal planes played a crucial role. By adjusting the crystal plane exposure of CeO2, the supported 1CuO/CeO2 catalysts exhibited different crystal surface configurations. Notably, the 1CuO/CeO2-S catalyst, with exposed (100) and (110) crystal planes, and the 1CuO/CeO2-P catalyst, with exposed (111), (100), and (110) planes, exhibited higher surface defect concentrations. This increased defect concentration facilitated the formation of Cu+ species, enhancing the redox properties and further improving the overall catalytic performance of CO oxidation.

Efficient low-temperature CO removal from high-humidity sintering flue gas by combination Cu and OMS-2

The non-precious metal manganese-based catalysts currently show insufficient CO catalytic oxidation performance in actual sintering flue gas conditions, with increased water vapor levels causing catalyst deactivation. This study utilized a high-performance manganese dioxide octahedral molecular sieve (OMS-2). The effect of metal doping on the catalytic CO oxidation performance was investigated in preparing OMS-2 using the co-precipitation method. The experiments showed that Cu doping increased CO conversion efficiency more than other metals (Co, Ag, Zn, and Fe), with optimal performance achieved at a 1.91 wt% doping level. Besides, Cu doping significantly enhanced water resistance of the catalyst, enabling effective CO removal in high-humidity conditions. The study observed that Cu ions infiltrated the catalyst framework by substituting some of the Mn ions, creating additional active sites in the form of oxygen vacancies and improving surface oxygen mobility, thereby enhancing the performance of CO catalytic oxidation. Furthermore, Cu doping demonstrated selective absorption of water vapor, with CuxO in the catalyst, effectively adsorbing water vapor and protecting the initial active sites, thereby mitigating water vapor-induced poisoning. Even in 15 vol% H2O at 150 °C, 1.91%Cu-OMS-2 maintained total catalytic activity. Therefore, the co-precipitation method-prepared 1.91%Cu-OMS-2 catalyst holds excellent potential for CO removal in sintering flue gas and shows promise for practical applications.

N-doped titanium oxide/carbon composite as supports for platinum catalysts in highly efficient methanol oxidation

The commercialization of direct methanol fuel cells (DMFCs) faces significant challenges due to the susceptibility of conventional platinum-based catalysts to poisoning by intermediates such as CO and HCOO− generated during the methanol oxidation reaction (MOR). This study details the synthesis of a highly porous titanium dioxide/carbon composite (TCC) using NH2-MIL-125(Ti) (MIL = Matérial Institut Lavoisier) as a precursor. To enhance the conductivity and catalytic properties of TCC, a nitriding reaction is conducted in an NH3 atmosphere, producing a nitrogen-doped titanium oxide/carbon composite (N-TCC). The resulting N-TCC is employed as a support for a platinum catalyst to improve MOR performance in DMFCs. The Pt/N-TCC catalyst demonstrates high catalytic performance, achieving a mass activity of 537 mA mg−1 for the MOR, marking a 39 % increase over the commercial Pt/C catalyst. Furthermore, the Pt/N-TCC catalyst exhibits a low onset potential for CO oxidation, a higher If/Ib ratio, and greater stability in chronoamperometry test, indicating high resistance to CO poisoning and enhanced chemical stability. Therefore, N-TCC can be used as a promising support for Pt-based catalysts in DMFC applications.

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.

CO oxidation on single Pt atom supported by two-dimensional ZnO monolayer: Reaction mechanism and precursor activation

CO oxidation is an effective solution for controlling CO emissions, and supported single-atom catalysts have shown excellent catalytic reactivity for chemical reactions. CO oxidation on a single Pt atom supported by a two-dimensional ZnO monolayer is investigated by combing ab initio molecular dynamics and density functional theory calculations. Seven reaction mechanisms are identified, including two new trimolecular Elie-Riedel mechanisms; six of them, except for the Mars-van Krevelen mechanism, have rate-limiting step energy barriers between 0.31 and 0.47 eV, which is lower than the experimental activation barrier of 0.81 eV on single Pt atoms supported by bulk ZnO, and is much lower than that of about 1.0 eV on Pt(111) surface. Furthermore, two preferred reaction mechanisms are discovered, an Eley-Rideal mechanism and a trimolecular Eley-Rideal mechanism, both of which have rate-limiting step energy barriers as low as 0.31 eV. Our results show that the more antibonding orbitals are filled in the reaction precursor, the higher its activation level. Activation of precursors plays an important role in CO oxidation and leads to different mechanisms. These results provide insights into the design of efficient single-atom catalysts supported by two-dimensional materials for the removal of CO pollutants.

Mesoporous La0.8Sr0.2CoO3/H-COK-12 catalyst synthesized by template method: Highly active for the complete oxidation of CO

Using cheap sodium silicate as a silica source and surfactant P123 as a soft template, an environment-friendly mesoporous material, COK-12, was prepared in a near-neutral environment. COK-12 was modified with n-hexane to obtain an H-COK-12 template with a compact and ordered pore structure. In this study, the modified mesoporous material H-COK-12 was used as the template for the first time, and the mesoporous La0.8Sr0.2CoO3 (LSC) catalyst was prepared by nano-casting method combined with sol–gel method. The results showed that the LSC-650 catalyst synthesized at 650 °C for 2 h possessed a large specific surface area (103.46 m2/g) and the highest surface Co4+/Co3+ and Oads/Olat molar ratio. In addition, the CO conversion with the LSC-T (T = 550, 600, 650, 700 °C) samples indicated that the LSC-650 sample had the best catalytic activity (T50 = 145 °C, T90 = 149 °C) and stability.