High CO Oxidation Research Articles

Effect of surface Fe-S hybrid structure on the activity of the perfect and reduced α-Fe2O3(001) for chemical looping combustion

Sulfurization of the gradually reduced Fe2O3 surfaces is inevitable while Fe2O3 is used as an oxygen carrier (OC) for coal chemical looping combustion (CLC), which will result in formation of Fe-S hybrid structure on the surfaces. The Fe-S hybrid structure will directly alter the reactivity of the surfaces. Therefore, detailed properties of Fe-S hybrid structure over the perfect and reduced Fe2O3(001) surfaces, and its effect on the interfacial interactions, including CO oxidization and decomposition on the surfaces, were investigated by using density functional theory (DFT) calculations. The S atom prefers to chemically bind to Fe site with electron transfer from the surfaces to the S atom, and a deeper reduction of Fe2O3(001) leads to an increasing interaction between S and Fe. The formation of Fe-S hybrid structure alters the electronic properties of the gradually reduced Fe2O3(001) surfaces, promoting CO oxidation on the surfaces ranging from Fe2O3 to FeO, but depressing carbon deposition on the surfaces ranging from FeO to Fe. The sulfurized FeO acts as a watershed to realize relatively high CO oxidation rate and low carbon deposition. Results provided a fundamental understanding for controlling and optimizing the CLC processes.

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.

Zoning and Trapping Effects on CO and Hydrocarbon Light-Off in Diesel Oxidation Catalysts

Monometallic Pt and bimetallic Pt/Pd oxidation catalysts, as model diesel oxidation catalysts, were tested in the oxidation of CO, C2H4, C2H6, C7H8, and C6H14. These single monolith catalysts were then compared to a dual-zoned system, where two monoliths were placed in series and the zones were comprised of different Pt/Pd ratios. Finally, a dual-layered system was also evaluated, where the added layer was made of Ag/BEA as a hydrocarbon trap. In terms of trends observed, for the single-zoned system, increasing the Pd content led to increased CO oxidation activity, while the best hydrocarbon oxidation was observed with increasing Pt content, except in the case of C2H6. An improvement in overall oxidation performance was achieved with the dual-zoned system, where a catalyst with a 1:5 Pt:Pd ratio upstream and monometallic Pt downstream proved best of those tested. This is attributed to a combination of relatively high CO oxidation in the upstream high-Pd content piece and the relatively high hydrocarbon ox...

Tailoring nanopores within nanoparticles of PtCo networks as catalysts for methanol oxidation reaction

It has been recently reported that engineering the pores within alloy nanoparticles leads to improvement in the electrochemical activity of nanoparticle catalysts due to the enhanced electrochemical active surface areas. However, to date, few works have reported the tailoring intraporosity within alloy nanoparticle networks. In this study, a different and innovative approach was adopted to yield a network-like PtCo catalyst composed of intraporous nanoparticles used a cotton-like PtCo precursor material. It was found that the network-like structure and intrapores within the nanoparticles could co-evolve after careful controlled electrochemical dealloying, whereby Pt-rich surface was formed during the leaching out of Co in the first 13th potentiostatic cyclic voltammetry cycles from +0.056 to +1.256V vs. RHE. Electrochemical data also showed that the mass and area activity of the obtained PtCo networks toward methanol oxidation reaction (MOR) was nearly 3.9, 2.0 and 2.1 times higher than that of commercial Pt/C, PtRu/C catalyst respectively, and much higher than that of Pt3Co networks made of only solid nanoparticles. Moreover, it was observed that such networks exhibited high CO oxidation ability whilst maintaining high catalytic durability under an applied potential of +0.756V vs. RHE. It was found that developing network-like catalysts composed of porous nanoparticles can be an efficient strategy to improve the catalytic activity and durability of fuel cell catalysts.

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.

Identifying the adsorbed active intermediates on Pt surface and promotion of activity through the redox CeO2 in preferential oxidation of CO in H2

A series of non- and modified with niobia and ceria alumina-supported Pt catalysts with different Pt content (0.3–2wt%) were prepared by wetness impregnation method and characterized by N2 adsorption-desorption isotherms, XRD, TPR, DRIFT, in situ EXAFS and ΔXANES. The effect of the content of Pt and promoter on the change of catalyst electronic structure and behaviors in the reaction of preferential oxidation of CO in the presence of H2 was studied. A strong interaction of Pt with CO and O2 in atmospheres of H2/CO and H2/CO/O2 mixtures was detected by ΔXANES. Different steps of activation of reagents molecules were observed. The increase of activity with increasing the Pt content was related to the presence of Pt nanoparticles with high electron density. While, the highest metal dispersion of the low metal-loaded Pt catalyst was responsible for its high CO oxidation selectivity. Addition of ceria to Pt catalysts caused significantly increase of activity and CO oxidation selectivity at lower reaction temperatures. Niobia addition did not improve the catalysts behaviors. The promotion effect of ceria was related to the oxygen mobility at PtOCe interface. The ceria-promoted Pt catalysts with the highest ceria loading of 12wt% exhibited the highest activity and CO oxidation selectivity, but at higher reaction temperatures of about 110°–130°C it was begin slowly to decrease. This was attributed to going on the reverse of WGS reaction and high O2 consumption to H2 oxidation during the PROX-CO reaction on ceria alumina-supported Pt catalysts at high temperatures.

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.

Pollen-structured hierarchically meso/macroporous silica spheres with supported gold nanoparticles for high-performance catalytic CO oxidation

Pollen-structured hierarchically meso/macroporous silica spheres (PHMSs) are synthesized using rape pollen grains and a triblock copolymer poly(ethylene oxide-block-propylene oxide-block-ethylene oxide) (P123) as templates. The PHMSs are comprised of mesoporous walls that form a macroporous structure replicating the net-like morphology of the pollen grain exine. The unique structure has a high BET surface area of 626m2g−1 and uniform mesopores with average pore size below 5nm. With the Au content fixed at 6.8wt%, the Au nanoparticles supported on the PHMSs achieved a higher CO conversion at an ambient temperature (30°C) than those supported on typical mesoporous silica SBA-15. Au nanoparticles supported on PHMSs also reached 100% CO conversion at a lower temperature than those supported on the SBA-15. The effects of silica support on the catalytic performances are investigated through calculation of the diffusion coefficients and simulation of gas diffusion in PHMSs and SBA-15. It is shown that the hierarchical PHMSs facilitate gas diffusion better than the non-hierarchical SBA-15, hence leading to higher CO oxidation catalytic performance.

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.

Density Functional Theory Studies of Oxygen Affinity of Small Au Nanoparticles

Through density functional theory calculations, to provide insight into the origins of the catalytic activity of Au nanoparticles (NPs) toward oxidation reactions, we have scrutinized the oxygen adsorption chemistry of 9 types of small unsupported Au NPs of around 1 nm in size (Au13, Au19, Au20, Au25, Au38, and Au55) looking at several factors (size, shape, and coordination number). We found that these NPs, except for the icosahedral Au13, do not strongly bind to O2 molecules. Energetically most feasible O2 adsorption that potentially provides high CO oxidation activity is observed in the icosahedral Au13, our smallest Au NP. In spite of the chemical inertness of bulk Au, the structural fluxionality of such very small Au NP enables strong O2 adsorption. Our results can support recent experimental findings that the exceptional catalytic activity of Au NPs comes from very small Au species consisting of around 10 atoms each.

Solid-Solution Alloying of Immiscible Ru and Cu with Enhanced CO Oxidation Activity.

We report on novel solid-solution alloy nanoparticles (NPs) of Ru and Cu that are completely immiscible even above melting point in bulk phase. Powder X-ray diffraction, scanning transmission electron microscopy, and energy-dispersive X-ray measurements demonstrated that Ru and Cu atoms were homogeneously distributed in the alloy NPs. Ru0.5Cu0.5 NPs demonstrated higher CO oxidation activity than fcc-Ru NPs, which are known as one of the best monometallic CO oxidation catalysts.

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.

Low Temperature CO oxidation over Iron Oxide Nanoparticles Decorating Internal Structures of a Mesoporous Alumina

Using a chemical vapor deposition method with regulated sample temperatures under ambient pressure conditions, we were able to fully decorate the internal structure of a mesoporous Al2O3 bead (~1 mm in particle diameter) with iron oxide nanoparticles (with a mean lateral size of less than 1 nm). The iron oxide-decorated Al2O3 showed a high CO oxidation reactivity, even at room temperature. Very little deactivation of the CO oxidation activity was observed with increasing reaction time at ~100 °C. Additionally, this catalyst showed high CO oxidation activity, even after annealing at ~900 °C under atmospheric conditions (i.e., the structure of the catalysts could be maintained under very harsh treatment conditions). We show that our catalysts have potential for application as oxidation catalysts in industrial processes due to the simplicity of their fabrication process as well as the high and stable catalytic performance.

Efficient CO Oxidation by 50-Facet Cu2O Nanocrystals Coated with CuO Nanoparticles.

As carbon monoxide oxidation is widely used for various chemical processes (such as methanol synthesis and water-gas shift reactions H2O + CO ⇄ CO2 + H2) as well as in industry, it is essential to develop highly energy efficient, inexpensive, and eco-friendly catalysts for CO oxidation. Here we report green synthesis of ∼10 nm sized CuO nanoparticles (NPs) aggregated on ∼400 nm sized 50-facet Cu2O polyhedral nanocrystals. This CuO-NPs/50-facet Cu2O shows remarkable CO oxidation reactivity with very high specific CO oxidation activity (4.5 μmolCO m-2 s-1 at 130 °C) and near-complete 99.5% CO conversion efficiency at ∼175 °C. This outstanding catalytic performance by CuO NPs over the pristine multifaceted Cu2O nanocrystals is attributed to the surface oxygen defects present in CuO NPs which facilitate binding of CO and O2 on their surfaces. This new material opens up new possibilities of replacing the usage of expensive CO oxidation materials.

Enhanced thermal stability of palladium oxidation catalysts using phosphate-modified alumina supports

Alumina with 5.0 μmol m−2 phosphorus additives could retain a higher Pd dispersion after aging, resulting in much higher CO oxidation activity.

UV-LED Photocatalytic Oxidation of CO over the Pd/TiO2 Catalysts Synthesized by the Decomposition of Pd(acac)2

The Pd nanoparticles were deposited on the TiO2 surface via the thermal decomposition of palladium acetylacetonate (Pd(acac)2). The catalytic activity of Pd/TiO2 catalysts was investigated in the CO oxidation under ambient conditions both in the absence and presence of UV-LED irradiation. The effect of additional UV pretreatment of the catalyst on the activity in CO oxidation was also studied. The rate of CO oxidation under UV irradiation was 3.5 times higher than the rate of dark catalytic oxidation. The dark catalytic and photocatalytic oxidation rates were monotonically increased as the Pd content was increased up to 2 wt.%. A small size and narrow size distribution of Pd nanoparticles on the TiO2 surface were the reasons for a high activity in CO oxidation. Al2O3 and SiO2 were also employed for the Pd deposition to investigate the effect of semiconducting support. Pd/TiO2 had much higher CO oxidation rate under UV irradiation than the Pd/Al2O3 and Pd/SiO2 catalysts, which do not have photocatalytic activity. It indicates that in the case of the Pd/TiO2 catalysts UV light energy can be used instead of thermal energy to increase the rate of CO oxidation.

An oxidized magnetic Au single atom on doped TiO2(110) becomes a high performance CO oxidation catalyst due to the charge effect

Catalysis using gold nanoparticles supported on oxides has been under extensive investigation for many important application processes.

CO oxidation catalyzed by RuO2 nanoparticles supported on mesoporous Al2O3 prepared via atomic layer deposition

Using atomic layer deposition (ALD), RuO2 was deposited on mesoporous Al2O3 with a mean pore size of ∼12 nm and particle diameter of ∼1 mm. The entire internal structure of mesoporous Al2O3 was coated by RuO2 nanostructures using ALD, forming a mesoporous bead of RuO2/Al2O3. Substantially high CO oxidation activity at reaction temperatures below 100 °C is also shown. There was a decrease in CO oxidation activity with time at 100 °C, and post-annealing at 700 °C fully recovered the catalytic activity of fresh RuO2/Al2O3. The RuO2/Al2O3 can be useful as an oxidation catalyst for eliminating harmful molecules.

Application and Improvement of NOx Storage and Reduction Technology to Meet Real Driving Emissions

Strategies to achieve RDE requirement for diesel passenger car are introduced in this paper focusing on NOx storage and reduction (NSR) technologies. High NOx conversion performance with advanced NSR technologies which make use of reducing agents like NH3 or intermediate reductants created from HC (Bisaiji et al. in SAE Int J Fuels Lubr 5:380–388, 2012) dosing, is demonstrated by vehicle tests. The mechanisms to create reducing agents are discussed in detail. MS analysis identified the key elements to produce reductant species for diesel NOx after-treatment by adsorbed intermediate reductants (DiAir), focusing on the effect of oxygen on the catalyst surface and the catalyst formulation. It was found that surface oxygen has an important role to make intermediate reductants during HC dosing for DiAir. Moreover, the role of Ce component in oxidizing the injected HC and create intermediate reductants effectively became clear. Furthermore, high CO oxidation performance of NSR after ageing in comparison with diesel oxidation catalyst is shown and the mechanism is discussed. Oxidation performance is also important for diesel vehicles because the catalyst temperature is further reducing due to CO2 reduction. The potential to meet next stringent legislation is shown in this paper from the view point of not only NOx reduction but also CO oxidation by an advanced NSR system.