Step Of CO Oxidation Research Articles

CO Oxidation Promoted by NO Adsorption on RhMn2O3- Cluster Anions.

CO oxidation represents an important model reaction in the gas phase to provide a clear structure-reactivity relationship in related heterogeneous catalysis. Herein, in combination with mass spectrometry experiments and quantum-chemical calculations, we identified that the RhMn2O3- cluster cannot oxidize CO into gas-phase CO2 at room temperature, while the NO preadsorbed products RhMn2O3-[(NO)1,2] are highly reactive in CO oxidation. This discovery is helpful to get a fundamental understanding on the reaction behavior in real-world three-way catalytic conditions where different kinds of reactants coexist. Theoretical calculations were performed to rationalize the crucial roles of preadsorbed NO where the strongly attached NO on the Rh atom can greatly stabilize the products RhMn2O2-[(NO)1,2] during CO oxidation and at the same time works together with the Rh atom to store electrons that stay originally in the attached CO2- unit. The leading result is that the desorption of CO2, which is the rate-determining step of CO oxidation by RhMn2O3-, can be greatly facilitated on the reactions of RhMn2O3-[(NO)1,2] with CO.

Exploring the role of oxygen species on β- and γ-MnO2 catalyst under in-plasma catalysis configuration at different temperature in CO oxidation using operando TPR-DRIFTS-MS

Catalyst surface active oxygen species have an important influence on plasma catalytic oxidation. However, the reaction process of CO and O3 migration and transformation on the catalyst surface during discharge at different temperature is still unclear. In this study, β- and γ-MnO2 are selected to unveil the roles of different oxygen species under in-plasma catalysis configuration at different temperature in CO oxidation. Operando TPR-DRIFTS-MS is used to reveal CO and O3 oxidation pathways. The results show that O3 can react with CO at room temperature, and O3 is also converted to other reactive oxygen species on the catalyst surface. As the temperature increases, the oxygen species of the catalyst itself (MnO and Mn-O-Mn) can react with CO to form oxygen vacancies (Mn-□-Mn and Mn=□). Mn-□-Mn are beneficial for O2 decomposition and promotes the regeneration of MnO and Mn-O-Mn. The carbonate covered on the surface of the catalyst is the rate-determining step of CO oxidation. It is hoped that Mn-based catalysts with more reactive oxygen species and oxygen defects will be prepared to facilitate low-temperature CO oxidation under plasma conditions.

Kinetic and Computational Studies of CO Oxidation and PROX on Cu/CeO2 Nanospheres

As supported CuO is well-known for low temperature activity, CuO/CeO2 nanosphere catalysts were synthesized and tested for CO oxidation and preferential oxidation of CO (PROX) in excess H2. For the first reaction, ignition was observed at 95 °C, whereas selective PROX occurred in a temperature window from 50 to 100 °C. The catalytic performance was independent of the initial oxidation state of the catalyst (CuO vs. Cu0), suggesting that the same active phase is formed under reaction conditions. Density functional modeling was applied to elucidate the intermediate steps of CO oxidation, as well as those of the comparably less feasible H2 transformation. In the simulations, various Cu and vacancy sites were probed as reactive centers enabling specific pathways.

High activity of CuO/#-Fe2O3 for low temperature CO oxidation: Effect of support crystal types in catalyst design

A series of Fe2O3 supported CuO catalysts were synthesized to investigate the effect of crystal types on CO oxidation performance of CuO/Fe2O3. The catalysts were characterized with BET surface area, XRD, Raman, O2-TPD, H2-TPR, XPS and in situ DRIFTS. The results showed that the crystal types had significant impact on the performance of CuO/Fe2O3, and the CuO/γ-Fe2O3 catalyst exhibited higher catalytic activity, which corresponding to 98.5% CO conversion at 180 °C. Besides, CuO/γ-Fe2O3 displayed better reducibility, which improved the redox cycle between Fe2+ and Fe3+ species. Furthermore, the pathway of CO oxidation on CuO/Fe2O3 followed L-H mechanism. Large amount of Fe2+ species and oxygen vacancies on CuO/γ-Fe2O3 increased the oxygen activation ability, which provided more reactive oxygen species to promote the rate-determining step in CO oxidation. Moreover, carbonate species on CuO/α-Fe2O3 were prone to accumulate, which resulted in the covering of active sites on catalyst surface and inhibited the adsorption and activation of CO and O2.

A comparative study of CO oxidation on Cu-doped C3N monolayer with N and C vacancies

In this study, we employ density functional theory based on first-principles calculations to investigate the CO oxidation performance of a Cu-doped C3N monolayer with N and C vacancies, which is a promising low-temperature and high-activity catalyst. We find that Cu dopants are more stably anchored at both the N and C vacancies of the C3N monolayer. To comprehensively understand the effect of N and C vacancies on CO oxidation, we investigate both the Eley–Rideal and Langmuir–Hinshelwood (LH) mechanisms. Our results indicate that the LH mechanism exhibits much lower barriers for the rate-limiting step of CO oxidation, suggesting its excellent performance in this process. Moreover, the charge state of Cu atoms and the asymmetric atomic interface at the C vacancy lead to a lower rate-limiting barrier for CO oxidation compared to that at the N vacancy (0.5 vs 0.71 eV). This study represents a promising strategy to enhance the activity and selectivity of CO oxidation by controlling the asymmetric atomic interface of single-atom catalysts.

One-dimensional 3d-TM–solophene polymers as high-activity single‐atom catalysts for CO oxidation

Inspired by the successful synthesized one-dimensional (1D) 3d-transition metal solophene polymer (TM-SP), we systematically investigate CO oxidation reactions on TM-SP as single atom catalysts (SACs) by density functional theory (DFT). The stabilities, adsorptive profiles of reactants and chemical kinetics as three descriptors are taken into account to efficiently select promising SACs among nine candidates (TM-SP, TM = Sc-Cu). Our calculations reveal that Sc-SP shows the highest catalytic activity for CO oxidation. After considering the Langmuir Hinshelwood (LH), Eley Rideal (ER) and tri-molecular Eley-Rideal (TER) mechanisms, we found the rate-determining step of CO oxidation on Sc-SP prefers to follow the LH mechanism whose energy barrier is 0.58 eV. The second CO2 generated fast via either LH or ER mechanisms with no activation barrier. Moreover, our results verify that the catalytic behavior of transition metals is intrinsic, because the kinetic characteristic is independent by changes in the different macrocyclic ligands (N2O2, N3O and N4) of the active sites in the kinetically-controlled CO oxidation reaction. The high catalytic performance of Sc-SP may be attributed to the proper strengths of adsorbates, high stabities and fast kinetics. These results provide atomic-level insights into CO oxidation which opens up a novel strategy for rational design of efficient SACs.

Identifying the surface active sites of FeOx-modified Pt/Nb2O5 catalysts in CO and propane oxidation

A comparative study on FeOx-modified Pt/Nb2O5 catalyzing CO and propane oxidation has been conducted to differentiate the surface active sites demanded for these two important reactions. Pt-Fe/Nb2O5 displays improved activity for CO oxidation but decreased activity for propane oxidation compared to Pt/Nb2O5. Detailed characterizations reveal the alteration of the geometrical and electronic states of Pt species and surface acidity induced upon the addition of FeOx, thus clarifying the divergent catalytic responses in CO and propane oxidation. Pt-FeOx interface sites promotes oxygen activation that is a crucial step in CO oxidation, thereby accelerating the CO oxidation. However, the rate-determining step in propane oxidation is propane activation that requires the synergistic catalysis of acid sites and Pt0, the critical active sites for propane oxidation. The inferior activity of Pt-Fe/Nb2O5 in propane oxidation is explained by the coverage of acid sites and Pt species.

C20 fullerene and its boron- and nitrogen-doped counterparts as an efficient catalyst for CO oxidation

DFT calculations are utilised to investigate the CO oxidation on the C20, BC19, and NC19 clusters. For CO oxidation over considered clusters, two continuous steps are proposed that in each step one CO2 molecule is released from clusters surface. The calculations demonstrate that in the case of the C20 cluster, the first step of CO oxidation takes place through the ER mechanism on two routes with a barrier height of 1.06 eV and 2.57 eV for the rate-limiting step. Also, in the cases of BC19 and NC19 clusters, both reaction paths occur via the ER mechanism. The activation energy of the first reaction step is about 0.53 and 0.46 eV, while it is negligible for the second step that is 0.04 and 0.18 eV for BC19 and NC19 clusters, respectively. Based on the present theoretical results, the catalytic activity of BC19 and NC19 clusters toward the CO oxidation is more than that of the C20 cluster. These results show that the BC19 and NC19 clusters can be recommended as an efficient and metal-free catalyst for CO oxidation at near ambient temperatures. Research Highlights CO oxidation over C20 fullerene has been investigated. The effect of N/B-doing on the CO oxidation reaction is also studied. The N/B-doping increases the catalytic activity of C20 fullerene. Boron/Nitrogen-doped C20 fullerene can be applied as an efficient catalyst for CO oxidation.

Catalytic oxidation of CO on Pd38 cluster and Pd slab, a computational study

The catalytic oxidation of CO was comparatively investigated on the Pd slab and Pd38 cluster models by density functional theory (DFT) calculation, in order to reveal the mechanism of CO oxidation over Pd catalysts. The results show that the rate-determining step of CO oxidation on the Pd38 cluster is the dissociation of O2, with the energy barrier of 0.65 eV, whereas the oxidation of CO turns to be the rate-determining step on Pd slab, with the energy barrier of 0.87 eV. Obviously, the oxidation of CO on the Pd38 cluster is much easier than that on the Pd slab, suggesting that the activity of Pd catalysts is related to the dispersion of active Pd species; the Pd catalyst with higher Pd dispersion also exhibits higher activity in CO oxidation.

A promising and new single-atom catalyst for CO oxidation: Si-embedded MoS2 monolayer

Using dispersion-corrected density functional theory calculations, we introduce a silicon atom embedded MoS2 (Si–MoS2) as new and robust catalyst for oxidation of CO to CO2. According to our results, CO and O2 molecules are largely activated over Si–MoS2 due to considerable hybridization between the Si-3d and O2-2π* or CO-2π* states. The first step of CO oxidation (i.e., CO + O2 → CO2 + O*) preferably proceeds via the Langmuir-Hinshelwood (LH) mechanism. This includes the formation of a peroxo-type OOCO intermediate by overcoming an activation energy of 0.38 eV. The results show that the energy barrier for the removing the O* moiety left on the Si atom is only 0.32 eV. The calculated energy barriers are comparable with other theoretical reports of some noble metals. The results of the present study suggest that Si–MoS2 could be considered as a promising and low-cost catalyst for high performance oxidation of CO.

Metal-support interaction in catalysis: The influence of the morphology of a nano-oxide domain on catalytic activity

Using wet chemistry synthesis methods we prepared nanodumbbell structures as a model oxide supported metal catalyst. In this peculiar configuration, a single metallic domain (M) is connected to a single metal oxide (MOx) one. The size, composition and morphology of each domain can be carefully controlled, allowing us to investigate the effects resulting from a hollow morphology of the MOx domains, while all other material’s properties were kept constant. We chose the CO oxidation as a model oxidation reaction and increasing the population of nanocrystals (NCs) with hollow oxide domains resulted in a decrease in catalytic activity. Despite the manipulation of oxide morphology affected the surface charge of the Au domain, the bulk oxide reducibility and the crystallinity of the nanosized oxide support, the rate limiting step of CO oxidation was not affected. The same apparent activation energy was indeed measured independently from the population of NCs with hollow oxide domains. The difference in catalytic performance was thus ascribed to a different number of interfacial active sites when the morphology evolved from full to hollow.

A simplified model on carbon monoxide yield in burning of polymeric solids containing flame retardants

Recent advances in Microscale Combustion Calorimetry (MCC) allow polymeric solids to be pyrolyzed and oxidized separately at the milligram scale, and the combustion gases, O2, CO, and CO2 to be measured using in-line gas analyzers. A simplified two-step reaction mechanism, which involves a CO generation step and a CO oxidation step, was developed to better describe the chemical reaction scheme that generates/oxidizes these products within the premixed combustor. Different incomplete combustion conditions were attained by varying the MCC combustor temperature and a numerical model of the combustor was developed to simulate the species evolution. Kinetics parameters were found by minimizing the errors in fitting the simulated results with the experiments. The two-step method was employed to examine halogen- and phosphorous-containing polymers of known compositions. The halogen additives were found to decrease gas phase reaction rate, whereas the phosphorous additives do not show significant effect. In cone calorimeter fire tests, both additives inhibit combustion efficiency by promoting soot formation.

A Linear Scaling Relation for CO Oxidation on CeO2-Supported Pd.

Resolving the structure and composition of supported nanoparticles under reaction conditions remains a challenge in heterogeneous catalysis. Advanced configurational sampling methods at the density functional theory level are used to identify stable structures of a Pd8 cluster on ceria (CeO2) in the absence and presence of O2. A Monte Carlo method in the Gibbs ensemble predicts Pd-oxide particles to be stable on CeO2 during CO oxidation. Computed potential energy diagrams for CO oxidation reaction cycles are used as input for microkinetics simulations. Pd-oxide exhibits a much higher CO oxidation activity than metallic Pd on CeO2. This work presents for the first time a scaling relation for a CeO2-supported metal nanoparticle catalyst in CO oxidation: a higher oxidation degree of the Pd cluster weakens CO binding and facilitates the rate-determining CO oxidation step with a ceria O atom. Our approach provides a new strategy to model supported nanoparticle catalysts.

Probing reaction pathways for oxidation of CO by O2 molecule over P-doped divacancy graphene: A DFT study

Using the dispersion-corrected density functional theory calculations, the catalytic performance of P-doped graphene with divacancy (P-GDV) is examined for the oxidation of CO by O2 molecule. According to our results, the first step of CO oxidation (CO + O2 → CO2 + O∗) proceeds via Langmuir–Hinshelwood mechanism with a small activation energy (0.28 eV). The remaining O atom (O∗) is then eliminated by another incoming CO molecule with an energy barrier of 0.59 eV. It is found that the orbital hybridization between the O2-2π and CO-2π∗ states in the coadsorbed structure plays an important role for the first step of the CO oxidation. Our result indicates that the low-cost P-GDV can be regarded as an active metal-free catalyst for CO oxidation at room temperature.

Combinatorial selection of a two-dimensional 3d-TM-tetracyanoquinodimethane (TM-TCNQ) monolayer as a high-activity nanocatalyst for CO oxidation.

The CO oxidation reaction on single 3d-transition metal catalytic sites in experimentally realized tetracyanoquinodimethane (TM-TCNQ) monolayers (TM = Sc-Zn) is systematically investigated by means of first-principles calculations. Considering the stabilities, adsorption characteristics and thermodynamics of all the ten candidates (Sc-Zn), Sc-TCNQ is found to display the lowest activation energies and yield the highest catalytic activity for room temperature CO oxidation. Exploring the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms, we find that the rate-limiting step of CO oxidation catalyzed by Sc-TCNQ (CO + O2* → OOCO*) can follow the LH mechanism with free energy barriers as low as 0.73 eV at 300 K. The second step of CO + O* → CO2 can occur with rather small energy barriers via either LH or ER mechanisms. The high activity of Sc-TCNQ can be attributed to its unique structural and electronic features by possessing high stability, optimum adsorption energies with adsorbates, and fast reaction kinetics. These results have significant implications for the synthesis of two-dimensional single atom catalysis for CO oxidation with low-cost and high activity at low temperature.

CO oxidation on Mn-N4 porphyrin-like carbon nanotube: A DFT-D study

First-principle calculations based on dispersion-corrected density functional theory (DFT-D) are carried out to reveal the elementary steps of CO oxidation on Mn-N4 porphyrin-like carbon nanotube (MnN4-CNT). The stability of MnN4-CNT is confirmed by the first-principles molecular dynamics simulation. According to the energetic calculations, CO and O2 prefer to anchor at the MnN4 site and the adsorption of CO is slightly more favorable than O2. The three reaction mechanisms of CO oxidation on the MnN4-CNT are explored, namely, Eley-Rideal, Langmuir–Hinshelwood, and a “new” termolecular Eley-Rideal, respectively. It is found that the termolecular Eley-Rideal reaction mechanism is the most favorable one and the energy barrier for the rate-limiting step is merely 0.69eV. Our results suggest the high catalytic activity of MnN4-CNT for CO oxidation at room temperature.

Effect of supports over Pd/Fe2O3 on CO oxidation at low temperature

Pd/Fe2O3 catalysts were prepared by deposition-precipitation method and investigated for CO oxidation. Compared with Pd/α-Fe2O3, Pd/γ-Fe2O3 exhibited the higher CO oxidation activity, and CO completely oxidation temperature was obtained at 0°C. CO oxidation occurred through the dual sites mechanism, namely CO adsorbed on Pd species and O activation on the support. The close contact between Pd and γ-Fe2O3 enhanced the redox recycle between Fe3+and Fe2+ species, which played a decisive role in oxygen activation. The excellent performance in oxygen activation efficiently accelerated the rate-determining step in CO oxidation. The accumulated carbonate and hydrocarbonate species on α-Fe2O3 blocked the oxygen activation which resulted in the low activity of Pd/α-Fe2O3 in CO oxidation.

Multiple CO Oxidation Promoted by Au2 Dimers in Au2TiO4− Cluster Anions

Time-of-flight mass spectrometry experiments demonstrated that laser ablation generated and mass selected Au2 TiO4 (-) cluster anions can unexpectedly oxidize three CO molecules in an ion trap reactor. This is an improvement on the more commonly observed oxidation of at most two CO molecules by a doped cluster. Quantum chemistry calculations were performed to rationalize the reactions. The lowest energy isomer of Au2 TiO4 (-) contains a superoxide unit, the participation of which in CO oxidation can be promoted by the Au2 dimer. The Au2 dimer functions as a rather flexible electron reservoir in each CO oxidation step in terms of the release and storage of electrons to drive the dissociation of superoxide to peroxide and then to lattice oxygen atoms, which can be removed by reaction with CO molecules. This gas-phase study enriches our understanding toward the nature of reactive oxygen species involved in gold-catalyzed oxidation reactions.

Interactions of Metal Oxo Species with Platinum-Based Catalysts Toward More Efficient Electrooxidation of Dimethyl Ether: Comparison to Methanol

Methanol is so far the most commonly considered fuel as an alternative to hydrogen. However, the practical performance is limited by several factors, which include the slow kinetics of methanol oxidation and crossover of methanol from the anode to the cathode side of the cell. More recently, there has been growing interest in dimethyl ether (CH3OCH3), a gas commonly used in aerosol propellants, solvents and coolants, as a possible alternative fuel for low temperature fuel cells. Dimethyl ether (DME) is non-toxic, non-teratogenic, non-mutagenic and non-carcinogenic gas. When oxidized to CO2, a CH3OCH3 molecule releases 12 electrons. The fuel has several advantages over typical organic fuels. Unlike ethanol (CH3CH2OH), the CH3OCH3 ether does not require breaking of the C-C bond. A fairly low CH3OCH3 dipole moment results in the lower ether crossover (through the membrane) in comparison to methanol. Dimethyl ether is much less toxic than methanol and can be readily stored and transported using existing technologies. In addition, the CH3OCH3 ether has potentially higher energy density relative to methanol (namely, 8.2 vs. 6.1 kWh kg-1). Platinum is one of the typical catalysts for the oxidation of organic molecules. It is quite active toward the dehydrogenation step, but suffers from the CO oxidation step. The bimetalic catalysts (e.g. PtRu, PtSn) can lower the onset potential of DME oxidation peak from 0.5 V for Pt/C to 0.4 V vs. Normal Hydrogen Electrode (NHE), but the maximum current densities are obtained on Pt/C. We propose here to study the electrooxidation of DME at Pt and PtRu nanoparticles that have been deposited onto nanostructured zirconium(IV) oxide matrix (thin film on electrode surface). Zirconia is known to act as a catalytic component capable of enhancing the reactivity of the electrocatalytic system by increasing the overall acidity at the reaction interface. When combined (as a support) with Pt or PtRu centers, the existence of large population of hydroxyl groups on ZrO2 surface is believed to facilitate desorption of CO adsorbates from Pt and to induce their oxidation to CO2. In the presence of nanostructured zirconia (existing in the vicinity of the catalytic metal), the transport of protons at the electrocatalytic interface is also favored. Synergistic interactions between ruthenium and zirconia components are also feasible. In the present report, we will not limit our results to DME, but similar effects have been observed during oxidation of methanol. We are also going to consider combination of Pt or PtRu with the other metal (Pd) or metal (W, Mo) oxo nanostructures. The support from National Research Center (NCN), Poland under the MAESTRO Project No 2012/04/A/ST4/00287 is highly appreciated.

Catalytically Active and Spectator Ce(3+) in Ceria-Supported Metal Catalysts.

Identification of active species and the rate-determining reaction steps are crucial for optimizing the performance of oxygen-storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce(3+) species in a ceria-supported platinum catalyst during CO oxidation are short-lived and therefore cannot be observed under steady-state conditions. Using time-resolved resonant X-ray emission spectroscopy, we quantitatively correlated the initial rate of Ce(3+) formation under transient conditions to the overall rate of CO oxidation under steady-state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce(3+) was present as spectators. This approach can be applied to various catalytic processes involving oxygen-storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.