Kinetics Of CO Oxidation Research Articles

Homogeneous Catalysts of RedOx Processes Based on Heteropolyacid Solutions. VI: Developing a Process for Low-Temperature Oxidation of CO with Oxygen

Research on the development of a homogeneous process for low-temperature oxidation of carbon monoxide in the presence of a “platinum group metal + vanadium-containing heteropolyacid (HPA)” catalytic system has been presented. The optimal reaction conditions, ensuring the maximum rate of CO oxidation to CO2, have been determined; for this reaction the kinetic features have been established, and its mechanism has been proposed. It has been shown that homogeneous systems based on PdII complex exhibit high activity and productivity, but have low stability and operate only at pH below 1,5. The stability of the catalyst can be increased by simultaneous introduction of σ- and π-donor ligands into the system; however, it is more effective to use the PtIV complex in the presence of catalytic amounts of palladium salt at a ratio of 100/1. The transition from HPA solutions with a low content of vanadium atoms (H7PMo8V4O40) to solutions of modified compositions (H10P3Mo18V7O84) ensures an increase in the activity and productivity of the system, with the kinetics of CO oxidation being maintained. The combined homogeneous catalyst PtIV + PdII + H10P3Mo18V7O84 remains stable during multi-cycle use without reducing activity, operates without an induction period and can be used at pH range of 1,7–2,0, which simplifies the process equipment.

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.

CO oxidation on IrO2(110) surfaces

We investigated the oxidation of CO on stoichiometric and O-rich IrO2(110) surfaces using temperature programmed reaction spectroscopy (TPRS), density functional theory (DFT) calculations and microkinetic simulations. Adsorbed CO on the s-IrO2(110) surface generates CO and CO2 peaks near 545 K during TPRS, and only about 38 % of the CO oxidized to CO2 when the initial CO layer was saturated. Pre-adsorbed O-atoms, so-called on-top oxygen (Ot), promote the oxidation of CO adsorbed on IrO2(110). On the Ot-covered surface, CO oxidation by Ot atoms produces a CO2 TPRS peak at ∼370 K, and all of the initially adsorbed CO oxidizes to CO2 when the initial Ot coverage is greater than the CO coverage. In agreement with the TPRS results, DFT calculations predict that the barrier is about 100 kJ/mol lower for CO oxidation by an Ot atom than a lattice O-atom of IrO2(110). A microkinetic model, parameterized with energy barriers computed using DFT, accurately reproduces the CO and CO2 TPRS traces only after CO binding energies are lowered to values determined using a hybrid exchange-correlation functional and the barrier for CO molecules to fill bridging O-vacancies is lowered. The simulations predict that O-vacancies play an important role in mediating the CO oxidation kinetics on s-IrO2(110), and thereby demonstrate the importance of future spectroscopic studies aimed at characterizing the nature of the surface CO and O species involved in reaction. This study provides new insights for understanding CO oxidation on IrO2(110), and provides evidence that several elementary steps can be involved in governing this chemistry.

Site-Sensitivity of CO Oxidation Kinetics Catalyzed by Atomically Dispersed Pt1/TiO2

Site-Sensitivity of CO Oxidation Kinetics Catalyzed by Atomically Dispersed Pt₁/TiO₂

Transient kinetic analysis of CO oxidation over a Cu-SSZ-13 catalyst proves complete conversion of ZCu2+(OH)- cations to binuclear Cu2+ species

The nuclearity of Cu2+ species active in the low-temperature Reduction Half Cycle (RHC) of NOx Selective Catalytic Reduction with NH3 over Cu-CHA catalysts is controversial. In the past, transient CO to CO2 oxidation protocols have been used to titrate binuclear Cu2+ species, and identified NH3-solvated ZCu2+(OH)- ions as their precursors. However, the prior results relied on asymptotic extrapolation due to the very slow CO oxidation kinetics. We herein present the results from a prolonged (24 h) CO titration experiment under dry conditions over an industrial Cu-SSZ-13 catalyst: the results demonstrate the conversion of all ZCu2+(OH)- ions to binuclear Cu2+ complexes and agree well with the extrapolation of a shorter (90 min) experiment, based on the assumption of the CO oxidation rate being second order in ZCu2+(OH)-. These outcomes confirm the adequacy of short CO oxidation tests to titrate ZCu2+(OH)- ions and support a Cu2+ pair mediated RHC pathway.

Elucidation of site structures and CO oxidation kinetics of the Ir1/TiO2 single-atom catalyst

Elucidation of site structures and CO oxidation kinetics of the Ir1/TiO2 single-atom catalyst

Deposited PtGe Clusters as Active and Durable Catalysts for CO Oxidation**

AbstractControl of CO emissions raises serious environmental concerns in the current chemical industry, as well as in nascent technologies based on hydrogen such as electrolyzers and fuel cells. As for now, Pt remains one of the state‐of‐the‐art catalysts for the CO oxidation reaction, but unfortunately, it suffers from CO self‐poisoning. Recently, Pt−Ge alloys were proposed to be an excellent alternative to reduce CO poisoning. This work investigates the impact of Ge content on the CO oxidation kinetics of Pt4Gen subnanoclusters supported on MgO. A Ge concentration dependence of the reaction kinetics is found due to a strong synergy between Pt and Ge. Pt−Ge nanoalloys act as a bifunctional catalyst by displaying dual adsorption sites; i. e., CO is adsorbed on Pt whereas oxygen binds to Ge, forming an alternative oxygen source GeOx. Besides, Ge alloying modifies the electronic structure of Pt (ligand effects) and reduces the affinity to CO. In this way, the competition between CO and O2 adsorption and the overbinding of CO is alleviated, achieving a CO poisoning‐free kinetic regime. Our calculations suggest that Pt4Ge3 is the optimal catalyst, evidencing that alloying composition is a parameter of extreme importance in nanocatalyst design. The work relies on global optimization search techniques to determine the accessibility of multiple structures at different conditions, mechanistic studies and microkinetic modeling.

Size-Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at LowTemperature and the Consequences for CO Oxidation Activity.

Understanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al2O3using in situ/operando spectroscopy and ex situ aberration-corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperatureand the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Size‐Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at Low Temperature and the Consequences for CO Oxidation Activity

AbstractUnderstanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al2O3 using in situ/operando spectroscopy and ex situ aberration‐corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperature and the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Identifying Adsorbed OH Species on Pt and Ru Electrodes with Surface-Enhanced Infrared Absorption Spectroscopy through CO Displacement.

OH adspecies are involved in numerous electrocatalytic reactions, such as CO, H2, methanol, and ethanol oxidation and oxygen reduction reactions, as a reaction intermediate and/or reactant. In this work, we have, for the first time, identified the OH stretching band of OH adspecies on Pt, Ru, and Pt/Ru electrodes with surface-enhanced infrared absorption spectroscopy (SEIRAS) in a flow cell through potential modulation and CO displacement. We found that while Ru had a relatively constant OH coverage at potentials between 0.1 and 0.8 V, Pt had a maximum OH coverage at 0.6 V in 0.1 M HClO4 and 0.7 V in 0.1 M KOH. CO oxidation kinetics on Ru were sluggish, although adsorbed OH appeared on Ru at very low potentials. Binary Pt/Ru electrodes promote CO oxidation through a synergistic effect in which Ru promotes OH adsorption and Pt catalyzes the reaction between the CO and OH adspecies. In addition, water coadsorbed with CO at Ru sites of Pt/Ru also plays an important role. These new spectroscopic results about OH adspecies could advance the understanding of the mechanism of fuel cell related electrocatalysis.

Embedding oxophilic rare-earth single atom in platinum nanoclusters for efficient hydrogen electro-oxidation

Designing Pt-based electrocatalysts with high catalytic activity and CO tolerance is challenging but extremely desirable for alkaline hydrogen oxidation reaction. Herein we report the design of a series of single-atom lanthanide (La, Ce, Pr, Nd, and Lu)-embedded ultrasmall Pt nanoclusters for efficient alkaline hydrogen electro-oxidation catalysis based on vapor filling and spatially confined reduction/growth of metal species. Mechanism studies reveal that oxophilic single-atom lanthanide species in Pt nanoclusters can serve as the Lewis acid site for selective OH- adsorption and regulate the binding strength of intermediates on Pt sites, which promotes the kinetics of hydrogen oxidation and CO oxidation by accelerating the combination of OH− and *H/*CO in kinetics and thermodynamics, endowing the electrocatalyst with up to 14.3-times higher mass activity than commercial Pt/C and enhanced CO tolerance. This work may shed light on the design of metal nanocluster-based electrocatalysts for energy conversion.

Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation.

Catalysts can undergo structural changes during the reaction, affecting the number and/or the shape of active sites. For example, Rh can undergo interconversion between nanoparticles and single atoms when CO is present in the reaction mixture. Therefore, calculating a turnover frequency in such cases can be challenging as the number of active sites can change depending on the reaction conditions. Here, we use CO oxidation kinetics to track Rh structural changes occurring during the reaction. The apparent activation energy, considering the nanoparticles as the active sites, was constant in different temperature regimes. However, in a stoichiometric excess of O2, there were observed changes in the pre-exponential factor, which we link to changes in the number of active Rh sites. An excess of O2 enhanced CO-induced Rh nanoparticle disintegration into single atoms, affecting catalyst activity. The temperature at which these structural changes occur depend on Rh particle size, with small particle sizes disintegrating at higher temperature, relative to the temperature required to break apart bigger particles. Rh structural changes were also observed during in situ infrared spectroscopic studies. Combining CO oxidation kinetics and spectroscopic studies allowed us to calculate the turnover frequency before and after nanoparticle redispersion into single atoms.

Spectroscopic and reactive characterization of mixed-metal Fe-Cr trimer nodes in metal-organic framework MIL-100.

Metal-organic framework MIL-100 is synthesized featuring heterometallic Fe and Cr M3O nodes where mixing of the metals within the nodes is evidenced using a combination of in situ IR spectroscopy, NO titrations, and CO oxidation kinetics. Reactivity data indicate distinctive properties of the bimetallic nodes.

Surface Oxygen Vacancies Confined by Ferroelectric Polarization for Tunable CO Oxidation Kinetics.

Surface oxygen vacancies have been widely discussed to be crucial for tailoring the activity of various chemical reactions from CO, NO, to water oxidation by using oxide-supported catalysts. However, the real role and potential function of surface oxygen vacancies in the reaction remains unclear because of their very short lifetime. Here, it is reported that surface oxygen vacancies can be well confined electrostatically for a polarization screening near the perimeter interface between Pt {111} nanocrystals and the negative polar surface (001) of ferroelectric PbTiO3. Strikingly, such a catalyst demonstrates a tunable catalytic CO oxidation kinetics from 200°C to near room temperature by increasing the O2 gas pressure, accompanied by the conversion curve from a hysteresis-free loop to one with hysteresis. The combination of reaction kinetics, electronic energy loss spectroscopy (EELS) analysis, and density functional theory (DFT) calculations, indicates that the oxygen vacancies stabilized by the negative polar surface are the active sites for O2 adsorption as a rate-determining step, and then dissociated O moves to the surface of the Pt nanocrystals for oxidizing adsorbed CO. The results open a new pathway for tunable catalytic activity of CO oxidation.

Non-noble metal single-atom catalyst with MXene support: Fe1/Ti2CO2 for CO oxidation

MXenes have attracted considerable attention owing to their versatile and excellent physicochemical properties. Especially, they have potential applications as robust support for single atom catalysts. Here, quantum chemical studies with density functional theory are carried out to systematically investigate the geometries, stability, electronic properties of oxygen functionalized Ti 2 C (Ti 2 CO 2 ) supported single-atom catalysts M 1 /Ti 2 CO 2 (M = Fe, Co, Ni, Cu Ru, Rh, Pd, Ag Os, Ir, Pt, Au). A new non-noble metal SAC Fe 1 /Ti 2 CO 2 has been found to show excellent catalytic performance for low-temperature CO oxidation after screening the group 8-11 transition metals. We find that O 2 and CO adsorption on Fe 1 atom of Fe 1 /Ti 2 CO 2 is favorable. Accordingly, five possible mechanisms for CO oxidation on this catalyst are evaluated, including Eley-Rideal, Langmuir-Hinshelwood, Mars–van Krevelen, Termolecular Eley-Rideal, and Termolecular Langmuir-Hinshelwood (TLH) mechanisms. Based on the calculated reaction energies for different pathways, Fe 1 /Ti 2 CO 2 shows excellent kinetics for CO oxidation via TLH mechanism, with distinct low-energy barrier (0.20 eV) for the rate-determining step. These results demonstrate that Fe 1 /Ti 2 CO 2 MXene is highly promising 2D materials for building robust non-noble metal catalysts. MXene supported non-noble metal single-atom catalyst Fe 1 /Ti 2 CO 2 is found to exhibit excellent activity for low-temperature CO oxidation through first-principles computational modelling.

Comparative investigation of structure and operating parameters on the performance and reaction dynamic of CO conversion on silica aerogel and fumed-silica-supported Pd catalysts

The catalyst morphology, metal-support interaction, and reaction conditions greatly influence the catalytic performance and reaction dynamics. Similarly, the dispersion of the metal within the support plays a crucial role in the thermal stability and sintering of the catalyst. Furthermore, temperature-dependent conversion hysteresis is well-known to occur during ignition and extinction of exothermic CO oxidation over supported Pd catalysts due to the variation of CO adsorption on the surface or bulk oxidation of Pd and the ability of the catalyst to regenerate the active sites. Herein, the catalytic performance and the hysteresis behavior of mesoporous silica aerogel supported Pd (Pd/a-SiO2), and commercial fumed silica-supported Pd (Pd/f-SiO2) were investigated and compared using CO oxidation as a probe reaction under different reaction conditions and operating parameters (i.e., catalyst weight, ramp rate, and flow rate). Surface and morphologic examination using XPS, FTIR, and TEM of Pd/a-SiO2 and Pd/f-SiO2 reveal a strong correlation between the catalyst surface and structure and its catalytic performance and stability under different reaction parameters. Moreover, this study presents the effect of surface area, particle size, and size distribution on diffusion and mass transport of reactants (CO, O2) and products (CO2) and active sites accessibility.This study showed that Pd/f-SiO2 had better efficiency under high (turbulence) flow. Moreover, intrinsic apparent activation energy (Ea) and the number of active sites were calculated from the Kinetics of CO oxidation fitted using Arrhenius plots indicate that the ramp rate has less effect on Pd/f-SiO2 catalytic behavior. Even though, Pd/f-SiO2 had higher relative active sites than Pd/a-SiO2, (Ea) was lower. Cyclic stability and long-term stabilities showed that both catalysts are stable and can regenerate the active sites. The current study contributes to understanding the catalysts' surface, structural and morphological properties on the catalysts' performance toward CO oxidation and other reactions under dynamic conditions.

Unraveling the evolution of exsolved Fe–Ni alloy nanoparticles in Ni-doped La0.3Ca0.7Fe0.7Cr0.3O3−δ and their role in enhancing CO2–CO electrocatalysis

In situ exsolved Fe–Ni nanoparticles anchored on a (La0.3Ca0.7)0.95Fe0.7Cr0.25Ni0.05O3−δ parent perovskite surface significantly enhance the CO oxidation kinetics, thus reaching excellent activity of the CO2 reduction reaction.

Porous stainless-steel fibers supported CuCeFeOx/Zeolite catalysts for the enhanced CO oxidation: Experimental and kinetic studies

To develop novel catalysts of high-performance and cost-effectiveness, and to investigate the reaction kinetics of CO oxidation, ternary CuCeFeOx catalysts supported on zeolite/PSF (porous stainless-steel fibers) were synthesized for the first time. Effects of different Ce/Fe ratios, loading amounts, calcination temperatures, and reaction kinetics were investigated. Remarkably improved catalytic performance was achieved in the PSF-supported catalysts compared to the granular ones, owing to the increased mass/heat transfer efficiency and the high dispersion of active metal oxide species anchored on the zeolite layer. The Cu3Ce12Fe4-400 sample exhibited the best catalytic activity with a temperature difference in T90 of almost 40 °C lower than the worst one. Characterization results from XRD, FTIR, TEM, XPS, H2-TPR, etc. revealed that the promoted reducibility of metal oxides and formation of more oxygen vacancies significantly contributed to the enhanced catalytic activity. Furthermore, a generalized rate expression was derived from intrinsic and macro kinetic studies by assuming the conversion of CO to CO2 as the rate-determining step, in which CO oxidation over the PSF-supported catalysts followed the pseudo-first-order kinetic established by the Langmuir-Hinshelwood type mechanism.

Calcium and copper substitution in stoichiometric and La-deficient LaFeO3 compositions: A starting point in next generation of Three-Way-Catalysts for gasoline engines

Calcium and copper substitutions, in A-site and B-site respectively, of parent stoichiometric LaFeO3 and La-deficient La0.7FeO3 perovskites led to significant improvements in the kinetics of CO and propene oxidation in typical three-way operating conditions. La-deficient La0.7Fe1-yCuyO3 perovskites were found more prone to surface copper oxide segregation leading to more active extra-framework copper oxide species in CO oxidation. Optimal performances were obtained on La0.7Fe0.8Cu0.2O3 composition. At higher Cu content, strong copper agglomeration leads to deactivation. More stable systems were obtained on dual substituted samples thanks to calcium substitution stabilizing copper inside the perovskite lattice and slowing down subsequent surface agglomeration. Rate enhancements in propene oxidation is observed on A-site deficient La0.6CaxFe0.8Cu0.2O3 with x ≤ 0.2 but a sharp loss in rate is observed on stoichiometric La0.6Ca0.4Fe0.8Cu0.2O3 perovskite explained by a progressive shift from suprafacial to intrafacial mechanism involving in this latter case the redox Fe4+/Fe3+ couple and lattice oxygen species.

Wolkenstein’s Model of Size Effects in CO Oxidation by Gold Nanoparticles

The Wolkenstein’s theory of catalysis and the d-band theory of formation chemical bonds between transition metal catalysts and adsorbates were used to develop the approach applied to the kinetics of CO oxidation by gold nanoparticles. In the model, within the framework of the mechanism of the reaction going through dissociative adsorption of oxygen molecules and reaction with gas-phase CO molecules, weak and strong chemisorption states of intermediates (O, CO2) were taken into account in the kinetic equations by introducing reversible electronic steps corresponding to electron transfers between the intermediates and the catalyst. As a result, we obtain the expression for the reaction rate, which exhibits a volcano-shape dependence upon the size of the gold nanoparticles at the conditions when the intermediates fractions are not small compared to the empty active sites of the catalyst. It is supposed that the approach can be also applied to the Langmuir-Hinshelwood mechanism.