Active Sites For CO Oxidation Research Articles

Enhancing the Performance of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) Perovskites as Catalysts for CO Oxidation by Decreasing the Ba Content.

Mixed oxides featuring perovskite-type structures (ABO3) offer promising catalytic properties for applications focused on the control of atmospheric pollution. In this work, a series of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) samples have been synthesized, characterized and tested as catalysts for CO oxidation reaction in conditions close to that found in the exhausts of last-generation automotive internal combustion engines. All samples were observed to be active as catalysts for CO oxidation during CO-TPRe tests, with Ba0.7MnO3 (B0.7M) being the most active one, as it presents the highest amount of oxygen vacancies (which act as active sites for CO oxidation) and Mn (IV), which features the highest levels of reducibility and the best redox properties. B0.7M has also showcased a high stability during reactions at 300 °C, even though a slightly lower CO conversion is achieved during the second consecutive reaction cycle. This performance appears to be related to the decrease in the Mn (IV)/Mn (III) ratio.

SnO2-modified CuMnOx catalysts for humid CO oxidation at low temperature

Low-temperature CO oxidation has been widely applied in environmental pollution control, but challenges remain in CO purification in high-humidity environments. In this study, the CuMnSn8 catalyst exhibited excellent low-temperature CO oxidation performance, achieving complete CO oxidation at temperatures below 55 °C in 85 % relative humidity. A series of characterizations indicated that an appropriate amount of SnO2 doping could promote the formation of more amorphous oxides, increase the oxygen species content, and facilitate the dispersion of Cu and Mn elements. The catalytic active sites for CO oxidation and the influence of SnO2 doping on the CO oxidation mechanism were investigated through in-situ DRIFTS experiments. The results indicated that Sn enhanced the synergistic effect between Cu and Mn, altered the pathway of CO oxidation reaction, generated more easily decomposable bidentate carbonate intermediates. This work provides insight into the design of CO oxidation catalysts with high activity in high humidity.

Very Low Temperature CO Oxidation over Atomically Precise Au25 Nanoclusters on MnO2.

Atomically precise Au25 nanoclusters have garnered significant interest in the field of heterogeneous catalysis due to their remarkable activity and selectivity. However, for the extensively studied reaction of low-temperature CO oxidation, their performance has not been competitive compared to other known gold nanocatalysts. To address this, we deposited Au25(SR)18 (R = CH2CH2Ph) nanoclusters onto a manganese oxide support (Au25/MnO2), resulting in a very stable and highly active catalyst. By optimizing the pretreatment temperature, we were able to significantly enhance the performance of the Au25/MnO2 catalyst, which outperformed most other gold catalysts. Impressively, 100% conversion of CO was achieved at temperatures as low as -50 °C, with 50% conversion being reached below -70 °C. Furthermore, the existence of ligands could also influence the negative apparent activation energy observed at intermediate temperatures. Analysis using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD) techniques indicated that the Au25 nanoclusters remained stable on the catalyst surface even after pretreatment at high temperatures. In-situ modulation excitation spectroscopy (MES) spectra also confirmed that the Au cluster was the active site for CO oxidation, highlighting the potential of atomically precise Au25 nanoclusters as primary active sites at very low temperatures.

Effect of Surface Temperature on the Distribution and Reactivity of Rh Active Sites for CO Oxidation

AbstractSingle‐atom catalysts are a fast‐emerging area in which late‐transition metal atoms are supported on oxides, metals, and carbonaceous supports. They show great promise for selective chemical reactions due to their well‐defined active sites, and significantly reduce precious metal loading. However, oxide‐supported single‐atom catalysts have drawbacks such as deactivation due to sintering, and there are on‐going debates about the exact nature of the active sites. Herein we report a model system where the surface temperature during Rh deposition on a copper oxide support dictates the Rh size distribution. Our temperature‐programmed desorption experiments reveal that Rh clusters perform CO oxidation at lower temperature than single atoms, and isotopic labelling demonstrates differences in the mechanism in that the C−O bond is broken and reformed on clusters, as opposed to a pure Mars van Krevelen mechanism for CO oxidation at the single‐atom Rh sites. We further examine the cluster size distributions with scanning tunneling microscopy and X‐ray photoelectron spectroscopy which confirm the presence of both clusters and single atoms for the lower temperature deposition and only single atoms for the higher temperature deposition. Together, these results provide a fundamental understanding of the differences in the CO oxidation mechanism on Rh atoms and clusters.

A density functional theory study of the CO oxidation on Pt1 supported on PtX2 (X = S, Se, Te)

The geometric stability, electronic structure and catalytic properties towards CO oxidation of a single Pt atom supported on PtS2, PtSe2, PtTe2 with a S, Se, and Te vacancies (Pt1/PtX2) were systematically explored by density functional theory calculations. The results showed that the single Pt atom can be located at the vacancy site stably, providing accessible active sites for CO oxidation. Both the Langmuir-Hinshelwood (LH) mechanism and the termolecular Eley-Rideal (TER) mechanism were considered comprehensively. By comparing the rate-determining step (RDS) of CO oxidation process, we found that the LH mechanism is easier to achieve. Meanwhile, as the X atom changes from S to Te, the RDS energy barrier gradually decreases, which is attributed to promoted adsorption and activation of O2 molecules on Pt1/PtX2. The results show that Pt1/PtTe2 is the most active catalysts with a low RDS barrier of 0.41 eV. This work provides an important reference for the design of single atom catalysts (SACs) based on transition metal dichalcogenides (TMDs), and has profound implications for the use of supported SACs on defective TMDs for CO oxidation or other reactions.

Unique structure of active platinum-bismuth site for oxidation of carbon monoxide

As the technology development, the future advanced combustion engines must be designed to perform at a low temperature. Thus, it is a great challenge to synthesize high active and stable catalysts to resolve exhaust below 100 °C. Here, we report that bismuth as a dopant is added to form platinum-bismuth cluster on silica for CO oxidation. The highly reducible oxygen species provided by surface metal-oxide (M-O) interface could be activated by CO at low temperature (~50 °C) with a high CO2 production rate of 487 μmolCO2·gPt−1·s−1 at 110 °C. Experiment data combined with density functional calculation (DFT) results demonstrate that Pt cluster with surface Pt−O−Bi structure is the active site for CO oxidation via providing moderate CO adsorption and activating CO molecules with electron transformation between platinum atom and carbon monoxide. These findings provide a unique and general approach towards design of potential excellent performance catalysts for redox reaction.

Mechanisms and energetics of CO oxidation on MnO2-supported Pt13 clusters

The interface formed by metal clusters supported on (reducible) transition metal oxides (TMOs) is usually responsible for the excellent activity of the redox reaction in the heterogeneously catalytic field. In this work, the catalytic combustion of CO was selected as a typical probe reaction to theoretically reveal the features of Pt13/β-MnO2 (1 1 0) catalyst, which possesses four active sites i.e., interface, β-MnO2 (1 1 0) facet, metallicity (Pt0) and partial oxidized Pt (Ptx+). All active sites were carefully investigated by the characteristics of CO oxidation. For Pt0 and Ptx+ sites, the adsorption energies of CO are larger than those of O2, which might block O2 active sites and cause CO poison. Meanwhile, CO oxidation by the dissociated O atom has the relatively high reaction energy barrier. Thus, Pt0 and Ptx+ are not good active sites for CO oxidation. For β-MnO2 (1 1 0) facet, O2 dissociation is not easy with a high barrier for vacancies healing. In contrast, the most favorable mechanism of CO oxidation is the catalytic cycle at the interface, with low reaction energy barriers in the entire pathway. We hope this comprehensive work can provide a meaningful reference towards the design of heterogeneous catalysts.

Effects of Ce Content in Precursor Alloys on Catalytic Properties of CeO2 Nanorods

In this study, Al-Ce alloy ribbons with Ce content (at.%) ranging from 6% to 12% were prepared by a simple melt-spinning method. The microstructure, phase structure, porosity and catalytic CO oxidation activity of dealloyed and calcined ribbons were analyzed. The results revealed that all the CeO2 catalysts showed a nanorod framework structure, and the size of the CeO2 nanorods became finer with the decrease of Ce content in the precursors. The specific surface area was the largest and porous size was the smallest for dealloyed and calcined Al92Ce8 ribbons. The CeO2 nanorod prepared from the Al92Ce8 precursor exhibited the highest catalytic activity, with total CO conversion at 320°C. The larger specific surface area could provide more gas diffusion paths and richer reaction active sites for CO oxidation.

New insights into the role of active copper species in CuO/Cryptomelane catalysts for the CO-PROX reaction

Nominally different Cu contents have been loaded on synthetic Cryptomelane oxide and the resulting CuO/Cryptomelane catalysts have been tested in the CO-PROX reaction. Catalytic activity was assessed in terms of the tunable synergistic interaction between copper and cryptomelane. This work entails the use of conventional characterization techniques such as XRD, Raman, FE-SEM, XPS with complementary DFT simulation. Additionally, it is reported for the first time, in-situ Work Function measurements of CO and O2 interaction in CuO/Cryptomelane catalysts. Results reveal the strong influence of the interfacial properties on CuO/Cryptomelane reducibility, where Cu+ species are formed by means of redox interplay between Cu+2/Cu+ and Mn3+/ Mn4+. The optimal performance is achieved by the efficient participation of two active species: (1) Cu+ from an intimate MnOx contact, providing the active sites for CO oxidation; and (2) surface Cu+ species stabilized in CuO bulky particles for the efficient O2 refilling.

CO Oxidation over Metal Oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2) Doped CuO-Based Catalysts Supported on Mesoporous Ce0.8Zr0.2O2 with Intensified Low-Temperature Activity

CuO-based catalysts are usually used for CO oxidation owing to their low cost and excellent catalytic activities. In this study, a series of metal oxide (La2O3, Fe2O3, PrO2, Sm2O3, and MnO2)-doped CuO-based catalysts with mesoporous Ce0.8Zr0.2O2 support were simply prepared by the incipient impregnation method and used directly as catalysts for CO catalytic oxidation. These mesoporous catalysts were systematically characterized by X-ray powder diffraction (XRD), N2 physisorption, transmission electron microscopy (TEM), energy-dispersed spectroscopy (EDS) mapping, X-ray photoelectron spectroscopy (XPS), and H2 temperature programmed reduction (H2-TPR). It was found that the CuO and the dopants were highly dispersed among the mesoporous framework via the incipient impregnation method, and the strong metal framework interaction had been formed. The effects of the types of the dopants and the loading amounts of the dopants on the low-temperature catalytic performances were carefully studied. It was concluded that doped transition metal oxides could regulate the oxygen mobility and reduction ability of catalysts, further improving the catalytic activity. It was also found that the high dispersion of rare earth metal oxides (PrO2, Sm2O3) was able to prevent the thermal sintering and aggregation of CuO-based catalysts during the process of calcination. In addition, their presence also evidently improved the reducibility and significantly reduced the particle size of the CuO active sites for CO oxidation. The results demonstrated that the 15CuO-3Fe2O3/M-Ce80Zr20 catalyst with 3 wt. % of Fe2O3 showed the best low-temperature catalytic activity toward CO oxidation. Overall, the present Fe2O3-doped CuO-based catalysts with mesoporous nanocrystalline Ce0.8Zr0.2O2 solid solution as support were considered a promising series of catalysts for low-temperature CO oxidation.

Eye-Readable Detection and Oxidation of CO with a Platinum-Based Catalyst and a Binuclear Rhodium Complex.

Toxic gases that are colorless and odorless, such as CO, are a major environmental concern and require early detection to prevent serious toxicological effects. In this study, a unique system (Pt/HMSs-BRC) was fabricated by combining a catalyst (Pt/hollow mesoporous silica spheres, Pt/HMSs) with a silica gel containing an adsorbed chromogenic probe (binuclear rhodium complex, BRC). The process is a simple method to prepare well-dispersed and uniform Pt nanoparticles. The Pt/HMSs-BRC materials demonstrated early CO detection and excellent catalytic performance for CO oxidation. The probe exhibited remarkable color modulation from gray-violet to light-yellow when exposed to CO concentration levels above 50 ppm, and the color of the chromogenic probe was fully recoverable. By a kinetics-assisted discrimination method and DFT calculations, it was found that the corner Pt sites are the dominant active sites for CO oxidation.

Eye‐Readable Detection and Oxidation of CO with a Platinum‐Based Catalyst and a Binuclear Rhodium Complex

AbstractToxic gases that are colorless and odorless, such as CO, are a major environmental concern and require early detection to prevent serious toxicological effects. In this study, a unique system (Pt/HMSs‐BRC) was fabricated by combining a catalyst (Pt/hollow mesoporous silica spheres, Pt/HMSs) with a silica gel containing an adsorbed chromogenic probe (binuclear rhodium complex, BRC). The process is a simple method to prepare well‐dispersed and uniform Pt nanoparticles. The Pt/HMSs‐BRC materials demonstrated early CO detection and excellent catalytic performance for CO oxidation. The probe exhibited remarkable color modulation from gray‐violet to light‐yellow when exposed to CO concentration levels above 50 ppm, and the color of the chromogenic probe was fully recoverable. By a kinetics‐assisted discrimination method and DFT calculations, it was found that the corner Pt sites are the dominant active sites for CO oxidation.

Influence of {111} nanofaceting on the dynamics of CO adsorption and oxidation over Au supported on CeO2 nanocubes: An operando DRIFT insight

Abstract CO adsorption and further reaction with O2 on two Au/CeO2 model systems have been investigated by operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) in order to unveil the active sites for CO oxidation. One of the ceria supports was constituted by crystallites depicting a nanocube shape bounded mostly by {100} planes, whilst the second one was consisted of nanocubes whose {110} surfaces were reconstructed into {111}-nanofacets with a zig-zag shape. The deconvolution of the CO adsorption bands and a detailed analysis of the time-evolution of these bands were also carried out. These studies evidenced a fast adsorption of CO over Au° and then Auδ--CO bands appeared during exposure to CO, in parallel with the appearance of Ce3+ features. Such changes suggest that CO adsorption process starts on Au sites on the low coordinated sites of the surface of the Au nanoparticles and their transfer to the interface sites. By comparing the nanostructure of the two catalysts, the Auδ--CO species could be fruitfully related to CO adsorption on sites located at the Au||CeO2 perimeter. The sample containing gold nanoparticles dispersed over the {111}-nanofaceted CeO2 nanocubes, which retained a higher amount of gold nanoparticles, depicts higher contribution of Auδ−−CO bands than the other Au catalyst in the DRIFT spectra. The dynamic DRIFT study clearly revealed that the CO species adsorbed on Auδ- sites were the most reactive ones in both gold supported systems for CO oxidation. This explains much higher TOF values observed on the catalyst prepared on the surface reconstructed support. A detailed analysis on the reactivity was performed by modulation excitation spectroscopy (MES). The presence of peroxide surface species (O22-) in nanofaceted {111} could lead to higher catalytic activity than that of gold on the non-reconstructed catalyst.

Pinpointing the active sites and reaction mechanism of CO oxidation on NiO.

Exploring the active sites and reaction mechanisms of CO oxidation on metal oxides is of great significance in the field of heterogeneous catalysis. NiO has been attracting increasing attention in this field due to its high performance and low cost. Nevertheless, the active sites and reaction mechanism of NiO still remain controversial due to the complexity of the experiments involved, the limitations of characterization techniques, and the difficulty in searching for the global reaction pathway in theory. In this work, terraced and stepped NiO(100) surfaces, with and without oxygen vacancies, were established based on the Wulff construction, and the active sites and reaction mechanism were revealed at the atomic level using a novel global pathway searching method. Theoretical results indicate that the coordination-unsaturated Ni ions are the active sites for CO oxidation; O2 interacts with the low-coordinated Ni ions to form reactive oxygen species; then they react with CO to form CO2; oxygen species on stepped NiO(100) have a low barrier and sustain a catalytic cycle. The present work reveals that the best direction for the design and development of NiO-based catalysts with high performance is to prepare NiO catalysts with more defects and low-coordinated Ni ions. We anticipate that the approach adopted in this work can be applied to a wide range of heterogeneous catalysts for exploring the active sites and mechanisms.

CO Oxidation Properties and Scanning Transmission Electron Microscopy Observation of Au/SrTiO3 Catalysts

Gold-on-SrTiO3 catalysts were prepared by deposition precipitation. These Au/SrTiO3 catalysts exhibited activity for low-temperature CO oxidation. Their catalytic activity indicated that the active sites for CO oxidation were located on the perimeter interface of Au particles because the reaction rate was proportional to the total length of the perimeter interfaces of all the Au particles on the SrTiO3 supports. The interface structure between the Au nanoparticles and SrTiO3 supports was observed by aberration-corrected scanning transmission electron microscopy (STEM). The preferential orientation relationship observed between Au and the SrTiO3 crystals was (001)[010]Au//(001)[010]SrTiO3. The interface structure between Au(001) and SrTiO3(001) was revealed by STEM at the atomic scale and a model structure was proposed for the interface.

ZIF-67-derived hollow nanocages with layered double oxides shell as high-Efficiency catalysts for CO oxidation

Constructing non-precious hybrid metal oxides with specific morphology as cost-effective and highly efficient catalysts is a promising way for the automotive exhaust purification. In this work, we report a facile strategy for the fabrication of a unique hollow Co-Ni layered double oxides (HLDO) nanocages by using zeolitic imidazole frameworks (ZIFs) as template. The synthesis of intermediate core-shell and hollow Co-Ni layered double hydroxides (HLDH) nanoflakes as well as the corresponding Co-Ni oxides products were successfully controlled, and the formation process was also explained. Among ZIF-67-derived oxides, HLDO exhibits excellent catalytic activities (complete conversion of CO into CO2 at 118 °C) and long-term stability for CO oxidation. The remarkable catalytic activities of HLDO can be attributed to high surface area (258 m2 g−1) inherited from the HLDH, which could provide more active sites for CO oxidation. In addition, active oxygen species indicated by the O 1 s XPS spectrum and improved synergistic effect between NiO and Co3O4 reflected by H2-TPR, further explain the enhanced performance of the HLDO catalysts. The presented strategy for controlled design and synthesis of hollow multicomponent metal oxides will provide prospects in developing highly effective catalysts.

Characterization and catalytic activity of CuO/TiO2-ZrO2 for low temperature CO oxidation

In this study, catalysts with different loadings of CuO supported on TZ20 (TiO2:ZrO2=80:20) composite oxides were prepared by the co-precipitation method and characterized by XRD, TEM, and H2-TPR measurements, and N2O decomposition experiments were conducted to determine the number of Cu sites. In addition, the relationship between the CuO states and CO oxidation activities of CuO/TZ20 was revealed. In our previous study, the CO oxidation activities of the CuO catalysts were as high as those observed for precious metal catalysts after simulated diesel exhaust aging tests. The TZ20 surface is covered with three types of Cu species. These species change with increasing CuO loading, going from isolated Cu2+, CuO layer, and finally to CuO particles at high CuO loadings. The CuO layer was the most readily reduced, as well as the most active site for CO oxidation. The amount of Cu forming CuO layers increased with increasing CuO loading; however, at CuO loadings larger than 5wt%, the amount of Cu forming CuO layers and the CO oxidation activity both remained constant. Additionally we indicated that adjusting the calcination condition caused the redispersion of CuO particles to the CuO layer and had the potential to improve the CO oxidation activities of CuO/TZ20.

Pt/Mo2C/C-cp as a highly active and stable catalyst for ethanol electrooxidation

Abstract A Pt/Mo2C/C-cp electrocatalyst with optimized Pt Mo2C chemical bonding is synthesized and evaluated for the ethanol oxidation reaction (EOR). The chemical bonding of Mo2C to Pt particles renders exceptional EOR activity at low potentials, which is 15 and 2.5 times higher than Pt/C and commercial 40% PtRu/C, respectively, at 0.6 V (vs. RHE). The stability of the Pt/Mo2C/C-cp electrocatalyst is comparable to the commercial 40% PtRu/C catalyst. CO stripping test demonstrates the existence of highly active sites for CO oxidation on the Pt/Mo2C/C-cp catalyst. In-situ infrared spectroscopic studies of EOR reveal that the excellent anti-poisoning ability of the Pt/Mo2C/C-cp catalyst is related to the relatively weak binding of carbonyl intermediates over the Pt/Mo2C/C-cp catalysts.

Mechanism and active sites of CO oxidation over single-crystal Au surfaces and a Au/TiO2(110) model surface

We describe the reaction mechanism and active sites for CO oxidation over a Au/TiO2(110) model surface and Au single-crystal surfaces, along with the role of H2O, on a molecular scale. At low temperature (< 320 K), H2O played an essential role in promoting CO oxidation, and the active site for CO oxidation was the perimeter of the interface between the gold nanoparticles and the TiO2 support (Auδ+−Oδ−−Ti). We believe that the O−O bond was activated by the formation of OOH, which was produced directly from O2 and H2O at the perimeter of the interface between the gold nanoparticles and the TiO2 support, and consequently OOH reacted with CO to form CO2. This reaction mechanism explains the dependence of the CO2 formation rate on O2 pressure at 300 K. In contrast, at high temperature (> 320 K), low-coordinated gold atoms built up on the surface as a result of surface reconstruction due to exposure to CO. The low-coordinated gold atoms adsorbed O2, which then dissociated and oxidized CO on the metallic gold surface.

Effects of the Electrodeposition Time in the Synthesis of Carbon-Supported Pt(Cu) and Pt-Ru(Cu) Core-Shell Electrocatalysts for Polymer Electrolye Fuel Cells

Pt(Cu)/C and Pt-Ru(Cu)/C electrocatalysts with core-shell structure supported on Vulcan Carbon XC72R have been synthesized by potentiostatic deposition of Cu nanoparticles on the support, galvanic exchange with Pt and spontaneous deposition of Ru species. The duration of the electrodeposition time of the different species has been modified and the obtained electrocatalysts have been characterized using electrochemical and structural techniques. The High Resolution Transmission Electron Microscopy (HRTEM), Fast Fourier Transform (FFT) and Energy Dispersive X-ray (EDX) microanalyses allowed the determining of the effects of the electrodeposition time on the nanoparticle size and composition. The best conditions identified from Cyclic Voltammetry (CV) corresponded to onset potentials for CO and methanol oxidation on Pt-Ru(Cu)/C of 0.41 and 0.32 V vs. the Reversible Hydrogen Electrode (RHE), respectively, which were smaller by about 0.05 V than those determined for Ru-decorated commercial Pt/C. The CO oxidation peak potentials were about 0.1 V smaller when compared to commercial Pt/C and Pt-Ru/C. The positive effect of Cu was related to its electronic effect on the Pt shells and also to the generation of new active sites for CO oxidation. The synthesis conditions to obtain the best performance for CO and methanol oxidation on the core-shell Pt-Ru(Cu)/C electrocatalysts were identified. When compared to previous results in literature for methanol, ethanol and formic acid oxidation on Pt(Cu)/C catalysts, the present results suggest an additional positive effect of the deposited Ru species due to the introduction of the bifunctional mechanism for CO oxidation.