Low-temperature CO Oxidation Reaction Research Articles

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.

Constructing an oxygen vacancy- and hydroxyl-rich TiO2-supported Pd catalyst with improved Pd dispersion and catalytic stability.

Surface property modification of catalyst support is a straightforward approach to optimize the performance of supported noble metal catalysts. In particular, oxygen vacancies and hydroxyl groups play significant roles in promoting noble metal dispersion on catalysts as well as catalytic stability. In this study, we developed a nanoflower-like TiO2-supported Pd catalyst that has a higher concentration of oxygen vacancies and surface hydroxyl groups compared to that of commercial anatase and P25 support. Notably, due to the distinctive structure of the nanoflower-like TiO2, our catalyst exhibited improved dispersion and stabilization of Pd species and the formation of abundant reactive oxygen species, thereby facilitating the activation of CO and O2 molecules. As a result, the catalyst showed remarkable efficiency in catalyzing the low-temperature CO oxidation reaction with a complete CO conversion at 80 °C and stability for over 100h.

Adsorption and Oxidation of CO on Co3O4/Ir(100) Thin Films

The combination of noble metals and reducible metal oxides often displays superior catalytic properties in low-temperature CO oxidation reactions. In the present study, we investigated the adsorption and activation of CO on submonolayer and monolayer CoOx films deposited on Ir(100) at low temperature under ultra-high vacuum (UHV) conditions. Using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED), we found that the CoOx films have a (3 × 1) structure and consist of a Co–O bilayer and an O–Co–O trilayer associated with Co2+ and Co3+ states, respectively. A combination of XPS, temperature-programmed desorption (TPD), and infrared reflection absorption spectrum (IRAS) reveals that the presence of the interface of the submonolayer CoOx film and Ir significantly enhances the catalytic activity of cobalt oxide for CO oxidation. In contrast, a monolayer film of CoOx without exposing CoOx–Ir interface sites is hardly active for CO oxidation. CO adsorbs on the bridge sites of Ir atoms on the submonolayer CoOx/Ir(100) surface at 100 K. Upon heating, CO partially reacts with oxygen on the surface to produce CO2, which desorbs at 215 K, while unreacted CO molecules adsorbed on Ir bridge sites migrate to top sites. Increasing the temperature leads to CO2 desorption at 295 and 325 K, arising from the reaction of CO with chemisorbed oxygen on the Ir surface and oxygen of cobalt oxide, respectively. These results reveal the importance of the interface between Ir and cobalt oxide in the oxidation of CO.

Confining Gold Nanoparticles in Preformed Zeolites by Post-Synthetic Modification Enhances Stability and Catalytic Reactivity and Selectivity.

Confining Au nanoparticles (NPs) in a restricted space (e.g., zeolite micropores) is a promising way of overcoming their inherent thermal instability and susceptibility to aggregation, which limit catalytic applications. However, such approaches involve complex, multistep encapsulation processes. Here, we describe a successful strategy and its guiding principles for confining small (<2 nm) and monodisperse Au NPs within commercially available beta and MFI zeolites, which can oxidize CO at 40 °C and show size-selective catalysis. This protocol involves post-synthetic modification of the zeolite internal surface with thiol groups, which confines AuClx species inside microporous frameworks during the activation process whereby Au precursors are converted into Au nanoparticles. The resulting beta and MFI zeolites contain uniformly dispersed Au NPs throughout the void space, indicating that the intrinsic stability of the framework promotes resistance to sintering. By contrast, in situ scanning transmission electron microscopy (STEM) studies evidenced that Au precursors in bare zeolites migrate from the matrix to the external surface during activation, thereby forming large and poorly dispersed agglomerates. Furthermore, the resistance of confined Au NPs against sintering is likely relevant to the intrinsic stability of the framework, supported by extended X-ray absorption fine structure (EXAFS), H2 chemisorption, and CO Fourier transform infrared (FT-IR) studies. The Au NPs supported on commercial MFI maintain their uniform dispersity to a large extent after treatment at 700 °C that sinters Au clusters on mesoporous silicas or beta zeolites. Low-temperature CO oxidation and size-selective reactions highlight that most gold NPs are present inside the zeolite matrix with a diameter smaller than 2 nm. These findings illustrate how confinement favors small, uniquely stable, and monodisperse NPs, even for metals such as Au susceptible to cluster growth under conditions often required for catalytic use. Moreover, this strategy may be readily adapted to other zeolite frameworks that can be functionalized by thiol groups.

Precise control of Pt encapsulation in zeolite-based catalysts for a stable low-temperature CO oxidation reaction

Precise control of Pt encapsulation in zeolite-based catalysts for a stable low-temperature CO oxidation reaction

Tunable Electronic Metal–Support Interactions on Ceria-Supported Noble-Metal Nanocatalysts in Controlling the Low-Temperature CO Oxidation Activity

A fundamental study on the metal-support interactions of supported metal catalysts is of great importance for developing heterogeneous catalysts with high performance, is still attracting and challenging in many heterogeneous catalytic reactions. In this work, we report the catalytic performances of CeO2-supported noble-metal catalysts among single atoms, subnanoclusters (∼1 nm), and nanoparticles (2.2-2.7 nm) upon low-temperature CO oxidation reaction between 50 and 250 °C. The subnanoclusters and nanoparticles of Ru, Rh, and Ir showed much higher activities than those of the single atoms, while a Pd single-atom catalyst was more active than Pd subnanoclusters and nanoparticles. According to the results of multiple ex situ and in situ characterizations, the much different activities of Ru, Rh, Ir, and Pd were derived from the alterable electronic metal-support interactions (EMSI), which determine the concurrent reaction pathway including the famous Mars van Krevelen mechanism and carbonate-intermediate route on the most active metal sites of Mδ+ (0 < δ < 1) for Ru, Rh, and Ir and Pd2+ for Pd. Also, the moderate EMSI of CeO2-supported Rh subnanoclusters furthest benefited activation of the adsorbed CO molecule and ensured it the highest activity among CeO2-supported Ru, Rh, and Ir catalysts with similar metal deposit sizes.

Ultrastable titania-supported Au nanoparticles covered by chromia via photo-induced strategy for low-temperature CO oxidation

Here we report the design and preparation of an ultrastable Au nanocatalyst through a photo-deposition strategy at room temperature, in which Au nanoparticles were covered by an island-like dispersed Cr2O3 overlayer. The latter was induced by the excited electrons in the TiO2 conduction band transferred to the Au nanoparticle. The resulting titania-supported Au catalyst promoted by chromia is resistant to sintering and exhibits excellent activity and stability in the low-temperature CO oxidation reaction and air atmosphere.

Preparation of the Mn/Co mixed oxide catalysts for low-temperature CO oxidation reaction.

The Mn/Co mixed powders with various Mn/Co molar ratios were prepared by the coprecipitation method and used in low-temperature CO oxidation. The physicochemical characteristics of these powders were characterized using the Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature-programmed reduction (TPR), and scanning electron microscopy (SEM) analyses. The results demonstrated that the Mn/Co molar ratio significantly affected both the textural and catalytic properties and the sample with a Mn/Co = 1:1 possessed a BET area of 123.7 m2g-1 with a small mean pore size of 6.44 nm. The catalytic results revealed that the pure cobalt and manganese catalysts possessed the low catalytic activity and the pure Co catalyst is not active at temperatures lower than 140 °C. The highest catalytic activity was observed for the catalyst with a Mn/Co = 1. The obtained results showed that the incorporation of Pd into the Mn/Co catalyst significantly enhanced the catalytic activity for oxidation of carbon monoxide and the highest CO conversion was observed for the catalyst with 1 wt.% Pd and this catalyst exhibited a CO conversion of 100% at 80 °C.

Highly Efficient CuO/α-MnO2 Catalyst for Low-Temperature CO Oxidation.

Copper manganese composite (hopcalite) catalyst has been widely explored for low-temperature CO oxidation reactions. However, the previous reports on the stabilization of such composite catalysts have shown that they deactivated severely under moist conditions. Herein, we developed an α-MnO2 nanorod-supported copper oxide catalyst that is very active and stable for the conditions with or without moisture by the deposition precipitation (DP) method. Incredibly, the CuO/MnO2 DP catalyst (with 5 wt % copper loading) achieves superior activity with a reaction rate of 9.472 μmol-1·gcat-1·s-1 even at ambient temperatures, which is at least double times of that for the reported copper-based catalyst. Additionally, the CuO/MnO2 DP catalyst is significantly more stable than the copper manganese composite catalysts reported in the literature under the presence of 3% water vapor as well as without moisture. A correlation between the catalytic CO oxidation activity and textural characteristics was derived via multitechnique analyses. The results imply that the superior activity of the CuO/MnO2 DP catalyst is associated with the proper adsorption of CO on partially reduced copper oxide as Cu(I)-CO and more surface oxygen species at the interfacial site of the catalyst.

N-Coordinated Dual-Metal Single-Site Catalyst for Low-Temperature CO Oxidation

Catalysts for CO oxidation reaction are mainly based on oxide/hydroxide materials with multicomponent active sites. Here, we report a nonoxide/hydroxide material, atomically dispersed dual-metal single sites (Fe–Co sites) on N-doped carbon support, as a highly active catalyst for CO oxidation. It can greatly lower the temperature for complete CO conversion as low as −73 °C with a turnover frequency of 0.096 s–1. X-ray absorption near-edge structure spectra, pulse-adsorption microcalorimetry, and density functional theory studies show that the Fe–Co sites synergistically catalyze CO oxidation facilely following the Langmuir–Hinshelwood (L-H) mechanism with CO preferentially adsorbing at the Co sites and O2 adsorbing at the Fe sites. These results, for the first time, reveal that the dual-metal single site on N-doped carbon can efficiency catalyze low-temperature CO oxidation reaction without the involvement of supports, such as oxygen vacancies and surface hydroxyl groups.

The synthesis of hollow In2O3 @ Pd-Co3O4 core/shell nanofibers with ultra-thin shell for the low-temperature CO oxidation reaction

In this paper, a series of hollow In2O3 @ Pd-Co3O4 core/shell nanofibers with ultra-thin shell about 20–30 nm have been successfully fabricated by coaxial electrospinning, which used in CO oxidation. Desirably, the In2O3 @ Pd-Co3O4 exhibited the higher CO activity with T90 = 56 °C and lower activation energy. Through some characterizations, it is verified that ultra-thin shell structures, stronger redox ability, structure defects, higher proportion of Pd2+ species and smaller proportion of Co3+ species and higher chemical adsorbed oxygen species of In2O3 @ Pd-Co3O4 were preponderant to the increase of CO activity. Therefore, the In2O3 @ Pd-Co3O4 catalyst could be considered a prospective candidate for the CO oxidation.

Effect of Zr Content on the Activity of 5%СuO/Ce1– xZrxO2 Catalysts in CO Oxidation by Oxygen in the Excess of Hydrogen

The article reports the synthesis of 5%CuO/Ce1– xZrxO2 catalysts based on CeO2, ZrO2 oxides and Ce1– xZrxO2 solid solutions with х = 0.2, 0.5, and 0.8. It is found that copper oxide is present in the catalysts in a highly dispersed form. In strong interaction with supports, it forms active oxygen, which participates in CO chemisorption and low-temperature oxidation of CO in the presence of hydrogen. In selective CO oxidation, the highest conversion (γmах = 100%) was obtained at temperatures of 120–160°С in the presence of 5%CuO/CeO2. In the modification of CeO2 by zirconium cations, the conversion on 5%Ce0.5Zr0.5O2 decreases to 92% at 160°С because oxygen binding strengthens on copper-containing sites. On the 5%CuO/ZrO2 sample, the maximum conversion is 67% at 180°С. The modification of ZrO2 by cerium cations leads to an increase in the conversion to 87% at 160°С on the 5%CuO/Ce0.2Zr0.8O2 sample as a result of increasing the amount of oxygen vacancies in the support. Taking into account the properties of CO complexes formed on copper-containing oxidation and adsorption sites, and the interaction of these complexes with adsorbed oxygen, their participation in the reaction of low-temperature CO oxidation by oxygen in excess of hydrogen on 5%CuO/CeO2 and 5%CuO/ZrO2 catalysts is considered.

Preparation and optimization of the MnCo2O4 powders for low temperature CO oxidation using the Taguchi method of experimental design

The MnCo2O4 powders were prepared using a coprecipitation method and employed in low temperature CO oxidation. The preparation parameters in the coprecipitation method were optimized by the Taguchi method of experimental design to synthesize the MnCo2O4 catalyst with high catalytic performance in a low temperature CO oxidation reaction. The results showed that the optimized sample indicated a mesoporous structure with a high specific surface area of 108.42 m2/g and a small pore size of 9.44 nm. The obtained results confirmed that the Taguchi method was effective in optimization of the preparation factors. Moreover, the highest CO conversion was observed on the optimized sample by the Taguchi method and this sample possessed a CO conversion of 100% at 150 °C, which was higher than those observed for other samples at the same temperature.

Catalytic behaviour of Mn2.94M0.06O4-δ (M=Pt, Ru and Pd) catalysts for low temperature water gas shift (WGS) and CO oxidation

Noble metal (Pt, Ru and Pd) substituted Mn3O4 catalysts have been synthesized in this work by a sonochemical route. The catalysts were characterised by XRD, XPS, TEM, H2-TPR and BET surface area analyser and the activity of these catalysts was tested towards low temperature water gas shift reaction (WGS) and CO oxidation reaction. It was observed that these catalysts have the tetragonal crystalline structure of Mn3O4 and the average particle size was found in the range of 12 nm–22 nm. H2-TPR results show that the strong metal support interaction between substituted metal and Mn3O4 leads to high reducibility and makes these catalysts active for WGS and CO oxidation. Pt substituted Mn3O4 showed higher activity towards WGS compared to other synthesized catalysts and 99.9% conversion was observed at 260 °C without methane formation. The activation energy of Mn2.94Pt0.06O4-δ was found to be 59 ± 0.6 kJ/mol. DRIFTS analysis was carried out to propose the reaction mechanism for water gas shift and CO oxidation. Redox mechanism was hypothesized for WGS and used to correlate the experimental data over Pt substituted Mn3O4. Similarly, kinetic parameters were estimated based on Langmuir-Hinshelwood mechanism for CO oxidation over Pd substituted Mn3O4 which showed better activity compare to other synthesized catalysts and 99.9% conversion was observed at 175 °C. The activation energy was calculated from Arrhenius plot which was found to be 30 ± 0.4 kJ/mol.

Tetraalkylammonium Salts of Platinum Nitrato Complexes: Isolation, Structure, and Relevance to the Preparation of PtOx/CeO2 Catalysts for Low-Temperature CO Oxidation

A series of tetraalkylammonium salts with anionic platinum nitrato complexes (Me4N)2[Pt2(μ-OH)2(NO3)8] (1), (Et4N)2[Pt2(μ-OH)2(NO3)8] (2), ( n-Pr4N)2[Pt2(μ-OH)2(NO3)8] (3b), ( n-Pr4N)2[Pt(NO3)6] (3a), and ( n-Bu4N)2[Pt(NO3)6] (4) were isolated from nitric acid solutions of [Pt(H2O)2(OH)4] in high yield. The structures of salts 2, 3a, 3b, and 4, prepared for the first time, were characterized by X-ray diffraction. The sorption of [Pt(NO3)6]2- and [Pt2(μ-OH)2(NO3)8]2- complexes onto the ceria surface from acetone solutions of salts 4 and 1 was examined. The dimeric anion was shown to quickly and irreversibly chemisorb onto the CeO2 carrier, selectively transforming into Pt(II) centers after thermal treatment, becoming active in the low-temperature CO oxidation reaction ( T50% = 110 °C at a space velocity of 240 000 h-1). By contrast, the homoleptic complex [Pt(NO3)6]2- did not interact with the ceria, which may be attributed to the substitutional inertness of the [Pt(NO3)6]2- anion. We believe that the strategy based on the sorption of polynuclear platinum nitrato complexes is an effective route to prepare ionic platinum species uniformly distributed on an oxide carrier for various catalytic applications.

Controlled synthesis of Pd/CoOx–InOx nanofibers for low-temperature CO oxidation reaction

Herein, we report a series of Pd/CoOx–InOx nanofibers with different morphologies (such as nanofibers, porous nanofibers and bead-like nanofibers) via electrospinning, annealing and impregnation methods.

Cu-Ce-O catalyst revisited for exceptional activity at low temperature CO oxidation reaction

In the present study, CexCu1−xO2−δ (x = 0, 0.3, 0.5, 0.8, 0.9, and 1) catalysts were synthesized using a facile and reproducible combustion method and were evaluated towards the CO oxidation (CO-OX) and CO preferential oxidation (CO-PROX) reactions as inexpensive and active catalytic materials. Pure ceria and copper oxide were compared to CexCu1−xO2−δ mixed metal oxides for benchmarking purposes. The catalysts were prepared using citric acid (C6H8O7) as fuel and nitrate salts of cerium and copper as oxidizers under fuel rich conditions. A variety of techniques including XRD, EDX, BET, SEM, Raman, XPS, H2-TPR and CO2-TPD, were used to analyze the microstructural, thermal and redox properties that may influence CO oxidation performance. A profound effect of Cu content was revealed that not only impacts the structural and redox properties of the catalysts but also affects the catalytic activity. The Ce0.8Cu0.2O2−δ catalyst presented the most promising performance among many similar (Cu-Ce-based catalysts) as well as noble supported catalysts published in the literature with T50 = 62 °C and T100 = 78 °C, an activity that coincides with the availability of labile oxygen species at low temperature (T < 100 °C) and the enhanced CO2 desorption at low temperature (low CO2-philicity) of this catalyst.

Improved performance of Mn ion substituted Ceria nanospheres for water gas shift reaction: Influence of preparation conditions

The present study reports the development of noble metal free nano-catalysts for low-temperature CO oxidation and Water gas shift reaction. Mn-substituted CeO2 solid solution catalysts were synthesized by co-precipitation, combustion and hydrothermal methods and characterized by various physicochemical techniques. The formation of the solid solution was confirmed by XRD. Raman spectroscopic confirmed the oxygen vacancies. The surface area, pore volume and pore size distribution confirmed by N2 physisorption analysis, whereas, XPS, UV–vis diffuse reflectance spectroscopy confirmed the oxidation state of the Mn ion. The particle size and morphology (spherical shape) of the material was confirmed using FESEM and HRTEM analysis. The resultant Ce1-xMnxO2-δ catalysts were subjected to CO oxidation and influence of various parameters was examined. Typical results indicated that the co-precipitation favored the best active catalyst.

Oxygen Adsorption and Low-Temperature CO Oxidation on a Nanoporous Au Catalyst: Reaction Mechanism and Foreign Metal Effects

To further our understanding of the role of trace impurities of the second metal in the catalytic performance of unsupported, nanoporous Au (NPG) catalysts, in particular for the activation of O2, we have prepared a NPG catalyst by electrochemical leaching of Cu from a AuCu alloy and investigated its behavior in the CO oxidation reaction. The structural and chemical properties of the as-prepared catalyst as well as that after reaction for 1000 min were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The nature of the surface oxygen species and the oxygen storage capacity were investigated and quantified by multi-pulse experiments in a temporal analysis of products (TAP) reactor. The catalytic behavior in the low-temperature CO oxidation reaction was evaluated both in a TAP reactor under dynamic vacuum conditions and in a conventional micro-reactor under atmospheric pressure. We discuss implications of these results and of similar data obtained previously on a Ag-containing NPG catalyst on the reaction mechanism and on the role of the second metal in the reaction and its impact on the reaction characteristics.

Effect of titania surface modification of mesoporous silica SBA-15 supported Au catalysts: Activity and stability in the CO oxidation reaction

As part of an ongoing effort to understand the deactivation and improve the stability of metal oxide-supported Au catalysts in the low-temperature CO oxidation reaction while maintaining their high activity, we have investigated the influence of a mesoporous silica SBA-15 substrate on the activity and stability of Au/TiO2 catalysts, which consist of a SBA-15 support surface modified by a monolayer of TiOx with Au nanoparticles on top. The extent of the TiOx surface modification was systematically increased, while the Au loading and the Au particle sizes were largely kept constant. Employing kinetic measurements at three different temperatures (30°C, 80°C, 180°C) and a number of ex situ methods as well as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for catalyst characterization, we found that the activity of these catalysts increases significantly with the Ti concentration and with reaction temperature. The tendency for deactivation remains essentially unchanged. Detailed in situ DRIFTS measurements reveal that the Au nanoparticles are largely formed on the TiOx surface-modified areas of the SBA-15 support and that the tendency for surface carbonate formation is very low. The observed deactivation may at least partly be related to the accumulation of molecularly adsorbed H2O species, in particular at low temperatures (30°C). These are likely to be formed from surface hydroxyl groups, they may affect the reaction either by blocking of active sites or by blocking the adsorption of reactants on the substrate. Other effects, such as reaction induced changes in the titania layer, must however, play a role as well, both at 80°C and in particular at 180°C, where accumulation of adsorbed species is negligible. The mechanistic ideas are supported by reactivation tests subsequent to calcination at 400°C, which were found to fully restore the initial activity.