High CO Oxidation Activity Research Articles

Integrated Cobalt Oxide Based Nanoarray Catalysts with Hierarchical Architectures: In Situ Raman Spectroscopy Investigation on the Carbon Monoxide Reaction Mechanism

AbstractHerein, a facile strategy for the in situ growth of a Co3O4‐based precursor with unique hierarchical architectures oriented diagonal or perpendicular to Ni surfaces is reported. This strategy to prepare grafted ZIF‐67@Co3O4 and MOF‐199@Co3O4 precursor structures is based on a simple hydrothermal synthesis method to obtain the Co3O4 precursor and the subsequent in situ growth of ZIF‐67 and MOF‐199, respectively. The morphologies of the Co3O4 products can be tailored by controlling the solvent polarity and concentration of precipitants. CO is chosen as a probe molecule to evaluate the catalytic performance of the as‐synthesized Co3O4‐based oxide catalysts, and the structure–activity relationships are confirmed by using TEM, H2 temperature‐programmed reduction, X‐ray photoelectron spectroscopy, Raman spectroscopy and in situ Raman spectroscopy, and extended X‐ray absorption fine structure analysis. These analysis results demonstrate that irislike Co3O4 exhibits a high catalytic activity for CO oxidation and contains an abundance of surface defect sites (Co3+ species) to result in an excellent low‐temperature reducibility, oxygen vacancies and unsaturated chemical bonds on the surface. Moreover, we used in situ Raman spectroscopy to record the structural transformation of Co3O4 directly during the reaction, which confirmed that CO oxidation on the surface of Co3O4 can proceed through the Langmuir–Hinshelwood mechanism (<200 °C) and the Mars–van Krevelen mechanism (>200 °C).

Evolution of Active Sites in Pt-Based Nanoalloy Catalysts for the Oxidation of Carbonaceous Species by Combined in Situ Infrared Spectroscopy and Total X-ray Scattering

We present results from combined in situ infrared spectroscopy and total X-ray scattering studies on the evolution of catalytically active sites in exemplary binary and ternary Pt-based nanoalloys during a sequence of CO oxidation-reactivation-CO oxidation reactions. We find that when within a particular compositional range, the fresh nanoalloys may exhibit high catalytic activity for low-temperature CO oxidation. Using surface-specific atomic pair distribution functions (PDFs) extracted from the in situ total X-ray scattering data, we find that, regardless of their chemical composition and initial catalytic activity, the fresh nanoalloys suffer a significant surface structural disorder during CO oxidation. Upon reactivation in oxygen atmosphere, the surface of used nanoalloy catalysts both partially oxidizes and orders. Remarkably, it largely retains its structural state when the nanoalloys are reused as CO oxidation catalysts. The seemingly inverse structural changes of studied nanoalloy catalysts occurring under CO oxidation and reactivation conditions affect the active sites on their surface significantly. In particular, through different mechanisms, both appear to reduce the CO binding strength to the nanoalloy's surface and thus increase the catalytic stability of the nanoalloys. The findings provide clues for further optimization of nanoalloy catalysts for the oxidation of carbonaceous species through optimizing their composition, activation, and reactivation. Besides, the findings demonstrate the usefulness of combined in situ infrared spectroscopy and total X-ray scattering coupled to surface-specific atomic PDF analysis to the ongoing effort to produce advanced catalysts for environmentally and technologically important applications.

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.

A systematic theoretical study on FeOx-supported single-atom catalysts: M1/FeOx for CO oxidation

A single-atom catalyst (SAC) that was first proposed by us in 2011 has aroused significant recent interest. Among the various SACs, FeOx-based ones including Pt1/FeOx, Ir1/FeOx, Au1/FeOx, Ni1/FeOx, and Fe1/FeOx have been investigated either experimentally or theoretically for CO oxidation. However, a systematic study of FeOx-based SACs has not been conducted. For a comprehensive understanding of FeOx-supported single-metal-atom catalysts, extensive density functional theory calculations were carried out on the activities and catalytic mechanisms of SACs with the 3d, 4d, and 5d metals of group VIII to IB, i.e., M1/FeOx (M = Fe, Co, Ni, Cu; Ru, Rh, Pd, Ag; Os, Ir, Pt, Au) for CO oxidation. Remarkably, a new noble metal SAC, Pd1/FeOx, with high activity in CO oxidation was found and is predicted to be even better than the previously reported Pt1/FeOx and Ni1/FeOx. In comparison, other M1/FeOx SACs (M = Fe, Co, Cu; Ru, Rh, Ag; Os, Ir, Au) showed only low activities in CO oxidation. Moreover, the adsorption strength of CO on the single-atom active sites was found to be the key in determining the catalytic activity of these SACs for CO oxidation, because it governs the recoverability of oxygen vacancies on their surfaces in the formation of a second CO2 during CO oxidation. Our systematic studies of FeOx-supported SACs will help in understanding the fundamental mechanisms of the interactions between singly dispersed surface metal atoms and FeOx substrate and in designing highly active FeOx-supported SACs.

Structures and properties of La0.8Sr0.2MnO3+σ-z SBA-15 (z=0, 1, 2, 4) perovskite catalysts

Mesoporous La0.8Sr0.2MnO3+σ/z SBA-15 (z = 1, 2, 4) perovskite oxides were synthesized via hard-templating with ordered mesoporous silica SBA-15 as the template. The as-prepared samples were characterized by XRD, SEM, AFM, BET, and XPS and the catalytic activity was tested for CO oxidation. The wide-angle XRD patterns showed that La0.8Sr0.2MnO3+σ perovskite was formed. The SEM and AFM analyses exhibited that La0.8Sr0.2MnO3+σ by hard-templating method had much smaller particle size (18 nm) than that (40 nm) by the sol-gel method. The perovskite-type oxides La0.8Sr0.2MnO3+σ/z SBA-15 (z = 1, 2, 4) also displayed a higher BET surface area from 70 to 143.7 m2/g and a disordered mesostructure from nitrogen sorption analysis, as well as a small-angle XRD pattern. Moreover, the La0.8Sr0.2MnO3+σ/z SBA-15 (z = 1, 2, 4) perovskite exhibited a much higher activity in CO oxidation than the conventional La0.8Sr0.2MnO3+σ perovskite. Further analysis by the means of XPS techniques indicated that the existence of high content of Oads/Olatt species contributed to the high activity.

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity, Stability, and Temperature‐ Dependent Activation Properties

AbstractNanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs4[α‐SiW12O40]⋅n H2O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was −67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Auδ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.

Carbon Monoxide Oxidation by Polyoxometalate-Supported Gold Nanoparticulate Catalysts: Activity, Stability, and Temperature- Dependent Activation Properties.

Nanoparticulate gold supported on a Keggin-type polyoxometalate (POM), Cs4 [α-SiW12 O40 ]⋅n H2 O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was -67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U-shaped curve. It was revealed by IR measurement that Auδ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas-phase reactions.

Engineering TiO2 supported Pt sub-nanoclusters via introducing variable valence Co ion in high-temperature flame for CO oxidation.

As a typical catalytic reaction model, CO catalytic oxidation has many practical applications in gas purification. TiO2 supported Pt sub-nanoclusters have been prepared by introducing variable valence Co ions into a one step flame spray pyrolysis process. Co2+ was oxidized to Co3+ in the high-temperature flame, and the released electrons were transferred to the surface of Pt and suppressed the aggregation of Pt nanoclusters supported on TiO2. As a result, the average size of Pt nanoclusters reduced from 2.47 nm to 0.72 nm with only 1% Co2+ ion doping. Moreover, due to the presence of Co, surface oxygen species were also affected, and these changes also led to a significant increase in the catalytic activity of CO oxidation. The temperature at 100% conversion was decreased from 120 °C to 70 °C, and the TOF increased by an order of magnitude. In addition, in situ DRIFTS was also used to investigate the cause of the significant increase in activity, and it was shown that adsorbed CO species on Pt could be desorbed more easily because of the electron transfer between Pt and Co species.

Facile synthesis of highly-dispersed Pt/CeO2 by a spontaneous surface redox chemical reaction for CO oxidation

A highly-dispersed Pt/CeO2 catalyst synthesized through a spontaneous surface redox reaction between metal ions and Ce(OH)3/CeO2 nanorods delivers high CO oxidation activity.

Litchi-peel-like hierarchical hollow copper-ceria microspheres: aerosol-assisted synthesis and high activity and stability for catalytic CO oxidation.

Copper-ceria is considered to be a promising system used in exhaust treatment due to its low cost and decent catalytic activity. Herein, we have developed novel litchi-peel-like hollow copper-ceria microspheres with varying Cu contents via an aerosol-assisted route. It is found that the dextrin in the spray solution plays a significant role as a sacrificial template and leads to the formation of this hierarchical hollow structure, in which higher surface area and active CuOx species with higher dispersion result in better catalytic activity compared to the usual hollow samples. The litchi-peel-like sample with 20% Cu exhibits the best reactivity for CO oxidation, namely 50% conversion at 83 °C and 100% conversion at 120 °C. Importantly, this novel copper-ceria sample displays outstanding catalytic stability involving cycle stability, long-term stability and thermal stability, which is attributed to step-stabilized strong interaction between CuOx species and CeO2. The superior catalytic activity and stability beyond commercial 5 wt% Pt/Al2O3 provides it with the potential to be a substitute for Pt-based catalysts in practical applications.

V-modified Co3O4nanorods with superior catalytic activity and thermostability for CO oxidation

Solid-state chemical strategy was applied to synthesize V-modified Co3O4nanorods with high catalytic activity and thermostability for CO oxidation.

A computational study of CO oxidation reactions on metal impurities in graphene divacancies.

Based on the density functional theory calculations, the formation geometry, electronic properties, and catalytic activity of metal impurities in divacancy graphene (M-DG, M = Mo, Fe, Co, and Ni) were systematically investigated. It has been found that the reactive gases have different stabilities on M-DG substrates, and these quite stable substrates exhibit high catalytic activity for CO oxidation by comparing the traditional Eley-Rideal (ER) and Langmuir-Hinshelwood (LH), as well as the new termolecular ER (TER) mechanisms. For the Co-DG substrate, the coadsorption of O2 and CO as a starting step is an energetically more favorable process, whereas the dissociation reaction of O2 molecules on Mo-DG substrate has a much smaller energy barrier, and the generation of atomic oxygen is active for CO oxidation. These results indicate that the varied adsorption behaviors of reactive gases on M-DG substrates can determine the catalytic pathways and energy barriers, which give us insight into the surface reactivity of graphene-metal composite catalysis in energy-related devices.

Copper based mixed oxide catalysts (CuMnCe, CuMnCo and CuCeZr) for the oxidation of CO at low temperature

The effect of surface enrichment in mixed metal oxides of CuMnCe, CuMnCo, and CuCeZr catalysts is highly effective for CO oxidation. The present work describes the preparation of CuMnCe, CuMnCo and CuCeZr catalysts and the characterization techniques applied to gain information about the catalyst structure and activity. Among the catalysts, CuMnCe and CuMnCo were prepared by the co-precipitation method and another catalyst CuCeZr was prepared by the reactive grinding method. In the variety of mixed metal oxide catalysts, CuMnCe was considered to be the most effective for high activity. The addition of ceria in CuMnOx catalyst has also improved the activity and stability due to increasing the meso/macropore volume and surface area after aging treatment. A novel route of reactive calcination (RC) of the precursor was the synthesis of highly active catalyst. The amazing activity of the novel catalysts over the conventional ones (obtained by calcination of the precursors in stagnant air and flowing air) in CO oxidation was associated with the unusual morphology as evidenced by XRD, SEM-EDX, XPS, BET and FTIR characterization. The catalysts obtained by RC of various precursors showed the highest activity for CO oxidation in the following order: CuMnCe > CuCeZr > CuMnCo and the traditional route of calcination also followed the same activity order. Further, the activity order of the catalysts obtained by various calcination conditions was as follows: RC > flowing-air > stagnant-air.

Low-temperature CO oxidation over Ag/SiO2 catalysts: Effect of OH/Ag ratio

Combined application of TPx methods, H2‐O2 titration, UV–vis DRS, TGA‐DSC‐MS, TEM, XRD, N2 adsorption at −196 oC allowed proving the OH/Ag molar ratio as the key parameter defining the catalytic properties of silica-supported silver (Ag/SiO2) in low‐temperature CO oxidation. A new insight into the formation of active species on the catalyst surface is presented. In this study, Ag/SiO2 catalysts with Ag loading of 5 and 8 wt.% were prepared by incipient wetness impregnation method on the basis of commercial silicas preliminary calcined at 500, 700 and 900°C. Detailed characterization of catalysts by physicochemical methods revealed that molar ratio between the concentration of surface OH groups (normalized to support mass) and silver amount in the prepared catalysts (OH/Ag ratio) affects the silver dispersion, structure of silver nanoparticles (NPs) and their catalytic properties. Only at optimal value of OH/Ag ratio the silver NPs are stated to possess both high dispersion and defective multidomain structure (that consists of several nanodomains) providing the adsorption of weakly bound oxygen species responsible for high catalytic activity in CO oxidation at low‐temperature. Additionally, the co-existence of two types of active sites reacting with CO at room temperature with and without formation of adsorbed carbonate species is discussed. The first type of active sites was catalytically active in low-temperature CO oxidation with CO2 release at room temperature (RT). The second type may retain surface carbonate species up to ∼40–50°C. The balance between these species shifts towards the first type for the active catalyst.

Atomically Dispersed Pd–O Species on CeO2(111) as Highly Active Sites for Low-Temperature CO Oxidation

Ceria-supported Pd is a promising heterogeneous catalyst for CO oxidation relevant to environmental cleanup reactions. Pd loaded onto a nanorod form of ceria exposing predominantly (111) facets is already active at 50 °C. Here we report a combination of CO-FTIR spectroscopy and theoretical calculations that allows assigning different forms of Pd on the CeO2(111) surface during reaction conditions. Single Pd atoms stabilized in the form of PdO and PdO2 in a CO/O2 atmosphere participate in a catalytic cycle involving very low activation barriers for CO oxidation. The presence of single Pd atoms on the Pd/CeO2-nanorod, corroborated by aberration-corrected TEM and CO-FTIR spectroscopy, is considered pivotal to its high CO oxidation activity.

Atomically dispersed Co atoms on MoS2 monolayer: A promising high-activity catalyst for CO oxidation

We systematic investigated the CO oxidation reaction over a single Co atom supported by MoS2 monolayer (Co/MoS2) on the basis of density functional theory calculations and ab initio molecular dynamics simulations. The binding energy of a single Co atom onto MoS2 monolayer is up to 2.80eV, making the diffusion and aggregation of supported Co atom difficult. For CO oxidation reaction, both Eley–Rideal and Langmuir–Hinshelwood mechanisms are considered. The energy barriers of CO oxidation through the Langmuir–Hinshelwood mechanism for the rate-limiting step of formation the peroxide-like O-C-O-O intermediate is only 0.57eV. The relatively high CO oxidation activity of Co/MoS2 may be attributed to the strong electronic resonance among the 3d orbital of the Co atoms and the antibonding 2π* orbitals of adsorbed CO and O2. We proposal that the MoS2 monolayer can serve as a defect-free two dimensional single atom catalysts substrate for CO oxidation reaction. It provides not only a large surface ratio, but also a uniform distribution anchoring points for fixing metal atoms as activation sites. We hope that this work contributes to design more effective nonprecious-metal single-atom catalysts for CO oxidation and widen the applications of monolayer MoS2 materials.

Synthesis of Pd decorated flower-like NiO composite and its high catalytic performance for CO oxidation under ambient conditions

Hierarchically porous Pd/NiO composites were successfully prepared through a hydrothermal and in-situ reduction protocol. NiO nanoparticles less than 10nm aggregated together to form irregular NiO nanosheets and finally composed the flower-like NiO sphere. When Pd nanoparticles were evenly dispersed and loaded, the resulting materials showed high catalytic activity for low temperature CO oxidation especially under ambient conditions. For 3wt% Pd loaded catalyst, T50 for 1 vol% CO conversion was as low as −15°C under a space velocity of 15 000mlh−1gcat−1. The kinetic study revealed the presence of water molecules can drastically lower the activation energy and finally promote the CO oxidation process.

A Theoretical Insight into Enhanced Catalytic Activity of Au by Multiple Twin Nanoparticles

Recently, it has been reported that the morphology of Au nanoparticles (NPs) affects the catalytic activity of CO oxidation; twin crystal NPs show higher activity for CO oxidation than single-crystal NPs. In this study, density functional calculations have been carried out to investigate the morphology effect of Au NPs using CO as a probe molecule. In the case of Au NPs with a size of more than 2 nm, CO adsorption energy on the Au NPs is mainly determined by a coordination number (CN) of adsorption sites. CO binding to a multiple twin NP with a size of about 1 nm is stronger than that on a single-crystal NP with the same size. A simple CN explanation cannot be applied to the enhancement of CO binding to the small multiple twin NP. This enhancement is related to a deformation of the NP structure before and after CO adsorption. It is suggested that the multiple twin NP with a size of less than 1 nm, which shows the deformation upon CO adsorption, contributes to the higher activity for CO oxidation.

Cold Plasma for Synthesizing High Performance Bimetallic PdCu catalysts: Effect of Reduction Sequence and Pd/Cu Atomic Ratios

Alumina supported bimetallic PdCu catalysts were prepared by simple incipient wetness impregnation followed by the process of atmospheric-pressure cold plasma reduction using Pd(NO3)2 and Cu(NO3)2 as Pd and Cu precursors, respectively. The influence of reduction sequence and Pd/Cu atomic ratios on the structure and performance of the catalysts were investigated. The highest CO oxidation activity was obtained for the PdCu catalyst (PdCu/Al2O3–P) prepared by co-impregnation and reduction of the Pd and Cu precursors with a Pd/Cu atomic ratio of 1:1. Meantime, the PdCu/Al2O3–P catalyst prepared by cold plasma exhibits much higher performance than that prepared by conventional thermal reduction. The optimum results from the alloying degree, crystallite size and dispersion of PdCu nanoparticles, and the chemisorbed oxygen species on the surface of the support. As a result of the interaction between Pd and Cu species, high alloying degree, small size and high dispersion of PdCu nanoparticles in PdCu/Al2O3–P were obtained. In addition, due to the fast (6 min reduction) and low temperature reduction process, more active chemisorbed oxygen species were formed on the surface of PdCu/Al2O3–P. The results of diffuse reflectance fourier transform infrared and catalytic stability show that the interaction between Pd and carbon species is weakened, which may facilitate the adsorption and activation of O2. These make PdCu/Al2O3–P an excellent catalyst for CO oxidation. Atmospheric-pressure cold plasma is proved to be a facile and efficient method for fabricating high performance bimetallic PdCu catalysts, which may have potential applications for preparing other bimetallic catalysts.

Copper oxide catalyst supported on niobium oxide for CO oxidation at low temperatures

CuOx/Nb2O5 shows higher CO oxidation activity than CuOx/Al2O3. Analysis of X-ray photoelectron spectra shows the presence of Cu1+ species on Nb2O5, which is known to be the active species for CO oxidation. In contrast, only Cu2+ is found to be present on Al2O3. Enhanced activity of CuOx/Nb2O5 is consistent with its lower activation energy for CO oxidation compared with CuOx/Al2O3. It is argued that the formation of stable Cu1+ species on Nb2O5 is a result of presence of deep copper acceptor states within the band gap of Nb2O5. 6%CuOx/Nb2O5 shows comparable activity to 1%Pt/Al2O3 but loses its advantage above 500°C.