Catalysts For Low-temperature CO Oxidation Research Articles

Hierarchical N-Doped CuO/Cu Composites Derived from Dual-Ligand Metal-Organic Frameworks as Cost-Effective Catalysts for Low-Temperature CO Oxidation.

Development of multi-ligand metal–organic frameworks (MOFs) and derived heteroatom-doped composites as efficient non-noble metal-based catalysts is highly desirable. However, rational design of these materials with controllable composition and structure remains a challenge. In this study, novel hierarchical N-doped CuO/Cu composites were synthesized by assembling dual-ligand MOFs via a solvent-induced coordination modulation/low-temperature pyrolysis method. Different from a homogeneous system, our heterogeneous nucleation strategy provided more flexible and cost-effective MOF production and offered efficient direction/shape-controlled synthesis, resulting in a faster reaction and more complete conversion. After pyrolysis, they further transformed to a unique metal/carbon matrix with regular morphology and, as a hot template, guided the orderly generation of metal oxides, eliminating sintering and agglomeration of metal oxides and initiating a synergistic effect between the N-doped metal oxide/metal and carbon matrix. The prepared N-doped CuO/Cu catalysts held unique water resistance and superior catalytic activity (100% CO conversion at 140 °C).

Dynamic structure of highly disordered manganese oxide catalysts for low-temperature CO oxidation

The dynamic structures of a highly disordered manganese oxide catalyst (HDMO) and intermediates were thoroughly studied from precursor to working catalyst. Excellent CO oxidation activity at low temperatures (T50 = 83 °C) was performed over a HDMO in the presence of H2O and CO2. The specific reaction rate of 4.56 μmolCO·gcat−1·s−1 at 90 °C was approximately 25 times and 15 times higher than those of bixbyite-type α-Mn2O3 and cryptomelane-type α-MnO2, respectively. With X-ray photon electron spectroscopy (XPS), we reveal that more surface oxygen vacancies (Ov) and adsorbed oxygen species from the highly disordered structure could promote the redox property and oxygen release capability. In situ Raman and DRIFTS spectroscopy were used to identify a dynamic surface phase transformation during CO adsorption and oxidation. In combination with kinetic studies of CO oxidation, we conclude that there are two mixed mechanisms for CO oxidation over HDMO: the Langmuir-Hinshelwood (L-H) mechanism and the Mars-van Krevelen (MvK) mechanism.

Pt and Ir supported on mixed Ce0.97Ru0.03O2 oxide as low-temperature CO oxidation catalysts

Pt and Ir catalysts (3% w/w) supported on Ce0.97Ru0.03O2 were synthesized and successfully tested in a low-temperature CO oxidation process. The CO oxidation was followed in-operation, by FTIR spectroscopy. The catalysts were characterized by SEM-EDS, HTEM, XRD, DRS UV–vis, XPS techniques and BET isotherms. It was found that Pt and Ir nanoparticles on Ce0.97Ru0.03O2 drastically improved the CO oxidation in comparison to CeO2, showing the best performance the Pt/ Ce0.97Ru0.03O2 system. From the FTIR studies, a route for CO oxidation was proposed. The CO oxidation pathway in Ce0.97Ru0.03O2 considers that CO was adsorbed at the Ce and Ru sites, while O2 is adsorbed on the surface oxygen vacancies, being activated by nearby Ru species. Subsequently, the activated oxygen reacts with CO linked to Ce and Ru to produce CO2. Also, Pt and Ir promotes oxygen vacancies, increasing the activation-adsorption of O2 and, consequently, the activity of the catalyst was improved.

A Robust Cu Catalyst for Low-Temperature CO Oxidation in Flue Gas: Mitigating Deactivation via Co-Doping

CuCeTiOx (CCT) catalyst is considered as a promising prospect attributable to their high activity for low-temperature CO oxidation. However, rapid deactivation when treating humid flue gas hindered their industrial exploitation. The hydroxide ion (OH−) dissociated from H2O, and carbonate intermediates derived from CO/CO2 deposited on the catalyst surface of CCT catalyst, inhibits the CO oxidation by surface oxygen on active sites. In this study, the detrimental effect caused by H2O and CO2 were evaluated, and the performance of CCT catalysts were investigated and compared using in situ DRIFTs study. Further, intentional doping on the CCT using transition metal (e.g., Co and Mn) was performed to mitigate the catalyst deactivation caused by H2O and CO2. The incorporation of cobalt in Co-CCT altered the reaction pathway and mitigated the deactivation via enhancing the consumption of surface adsorbed OH- by CO, reducing the occupancy of active sites. Also, preferential adsorption of CO further suppressed the competition of OH- and CO2 towards active sites on catalyst attributable to the abundant oxygen vacancies and low coordinated metal (i.e., Cu+, Ce3+) in Co-CCT, which significantly enhanced the resistance to H2O and CO2 in the flue gas. This work thoroughly analyzed the mechanism of H2O and CO2 impacting the catalyst activity during low-temperature CO oxidation, is able to provide innovative insights for the design of highly-active and long-shelf life catalysts. The incorporation of cobalt in CuCeTiOx catalyst facilitates the formation of oxygen vacancies, the adsorption of CO, and the consumption of OH-, speeding up the CO oxidation to CO2 and promoting the resistance to deactivation caused by H2O and CO2 in the flue gas.

Potassium-incorporated manganese oxide enhances the activity and durability of platinum catalysts for low-temperature CO oxidation

Potassium-incorporated manganese oxide is demonstrated as an efficient support for fabricating highly active and stable Pt catalysts for low-temperature CO oxidation.

Integration of an Oxidation Catalyst with Pd/Zeolite-Based Passive NOx Adsorbers: Impacts on Degradation Resistance and Desorption Characteristics

Fully ion-exchanged Pd/SSZ-13 model passive NOx adsorbers (PNA) exhibit significant NOx storage capability, achieving NOx-to-Pd ratios of 1, and NOx desorption at higher temperatures, where downstream NOx reduction catalysts are active, under simulated exhaust conditions. However, CO has been found to induce PNA degradation, which limits the potential for practical application. In this study, in an attempt to understand the consequences of limiting CO exposure, we integrated a model Pt/Al2O3 diesel oxidation catalyst (DOC) with a Pd/SSZ-13 PNA and performed NOx adsorption and temperature-programmed desorption (TPD) cycles with the PNA, the DOC, and the DOC+PNA integrated system. Despite the high initial NOx-to-Pd ratio, Pd/SSZ-13 experienced significant degradation over 15 adsorption and desorption cycles when including CO, with the NOx-to-Pd ratio dropping from 0.98 to 0.75. CO oxidation over the DOC+PNA integrated system lights off at significantly lower temperature compared to the PNA, limiting the PNA CO exposure at temperatures above 200 °C. As a result, the durability of the DOC+PNA integrated system is enhanced, and only a 0.02 decrease in NOx-to-Pd ratio was observed over the 15 test cycles. A further benefit with integration of the PNA with the DOC was a lower temperature NOx release, within a more practical temperature window. This is due to the enhanced oxidation activity of the DOC+PNA integrated system and consequently an early onset of NO2 formation, which was found to trigger the low temperature NOx release. Low temperature NOx adsorption and TPD experiments with controlled exposure of CO and NO2 reveal two types of NOx storage mechanisms, one of which is destabilized by the presence of NO2, leading to the evolution of a lower temperature NOx release. Overall, integrating the PNA with a highly active low temperature CO oxidation catalyst was beneficial by lowering the NOx release temperature window and leading to significantly less NOx capacity loss.

CONCEPTUAL APPROACHES TO THE DEVELOPMENT OF CATALYSTS FOR LOW-TEMPERATURE CARBON MONOXIDE OXIDATION WITH AIR OXYGEN

Actual questions concerning the modern stage of development of catalysts for low-temperature carbon monoxide oxidation and their application in the personal respiratory protective equipment (PRPE) have been analyzed. Some approaches contributing to purposeful control of activity of Wacker type catalysts consisting of both palladium(II) and copper(II) salts and sorbents of different origin as well as their possible use in several environment protection devices have been considered. Despite some progress in the development of new catalysts for low-temperature CO oxidation, Hopcalite catalysts characterized by well-known disadvantages are as usual used in the series-produced PRPE. In our opinion, Au containing nanocatalysts and Pd(II)-Cu(II) catalysts based on modified natural sorbents such as Tripoli and clinoptilolite (Ukraine) are more perspective for this purpose. Available data concerning test conditions and activity parameters for some commercial catalysts at comparable CO inlet concentrations commonly used in PRPE, i.e. 130-300 mg/m3, were summarized. For comparison of their activity and evaluation of their applicability in PRPE, their air purifying ability (down to MPCCO in the air of work area) was used. It can be seen that the Au containing Nanoqold Cat (0.57 % Au/TiO2/AC) nanocatalyst demonstrates the highest activity providing the almost complete air purifying from CO: at CO inlet concentration of 625 mg/m3 and the residence time value of 0.04 s, the outlet concentrations are 1.25‑7.5 mg/m3 for 12 h. For NanAuCat (0.57 % Au/TiO2/AC) and AUROlite (1 % Au/TiO2), CO conversion values were equal to 48 % and 22 %, respectively, however, in this case the residence time value was lower by an order of magnitude. STC catalyst (Pt/SnO2) and AK-62 (2.4 % Pd/Al2O3) catalyst are less active than the above mentioned Au containing catalysts. The Wacker type catalysts are characterized by similar parameters however KNO-CO/MT is somewhat better showing the air purification down to MPC even at the residence time of 0.29 s. The residence time is a parameter determining the catalyst weight necessary to achieve a required CO conversion and dimensions of an applicable gas mask filter. Hopcalite catalysts used in the PRPE produced by the main companies over the world are the cause of large dimensions and weights of PRPE cartridges and canisters. Thus, low values of catalyst weights in “Platan” and “Suprovidnyk” devices, i.e. 50 g and 97 g, respectively, and high levels of protection from CO are their great advantages.

Cu/CeO2 as efficient low-temperature CO oxidation catalysts: effects of morphological structure and Cu content

Cu loaded on different morphologies of CeO2 were synthesized and tested by SEM and CO catalytic oxidation experiment, the results indicated that with the same Cu load, nanorod-like Cu/CeO2 performed the best catalytic activity than 3D flower-like Cu/CeO2 and gear-like Cu/CeO2. Then nanorod-like Cu/CeO2 were chose to explore the best Cu load on CeO2 nanorods for CO oxidation. Cu/CeO2 nanorods were characterized by TEM, XRD, XPS as well as physical and chemical adsorption. The results indicate that 0.15Cu/CeO2 nanorods have the best catalytic activity because of more reducing copper species (Cu+), adsorbed oxygen (Oads) and Ce3+ species on the catalysts surface, which can achieve 99% CO conversion at 100 °C. The effect of CO2 and water vapor on catalytic activity was also examined.

Ultra-low loading of iron oxide on Pt/Al2O3 for enhanced catalytic activity of CO oxidation at room temperature: A simple method for applications

Fe-oxide particles were deposited on commercial Pt/Al2O3 in a highly dispersed manner via simple temperature regulated-chemical vapor deposition. We found that the catalytic activity of Pt/Al2O3 toward CO oxidation at room temperature can be enhanced by deposition of Fe-oxide on Pt/Al2O3. Particularly, as Fe-loading decreased from ∼10 to 0.34 wt%, catalytic activity toward CO oxidation gradually increased. Results of various analyses of catalysts attributed this Fe-oxide-induced enhancement effect to the boundary sites of Pt-Fe on Fe-Pt/Al2O3. It is worth noting that the enhanced catalytic activity of Pt/Al2O3 upon Fe-oxide loading was observed even in the presence of water vapor (40% of the relative humidity). In addition, Fe-oxide-loaded Pt/Al2O3 showed high thermal stability, and the catalytic activity at room temperature was retained after repeated exposure to high temperatures. Our results show the potential of Fe-oxide-loaded Pt/Al2O3 prepared by tr-CVD as a catalyst for low-temperature CO oxidation in practical applications.

Low temperature CO oxidation by doped cerium oxide electrospun fibers

We investigated CO oxidation behavior of doped cerium oxide fibers. Electrospinning technique was used to fabricate the inorganic fibers after burning off polymer component at 600 °C in air. Cu, Ni, Co, Mn, Fe, and La were doped at 10 and 30 mol% by dissolving metal salts into the polymeric electrospinning solution. 10 mol% Cu-doped ceria fiber showed excellent catalytic activity for low temperature CO oxidation with 50% CO conversion at just 52 °C. This 10 mol% Cu-doped sample showed unexpected regeneration behavior under simple ambient air annealing at 400 °C. From the CO oxidation behavior of the 12 samples, we conclude that absolute oxygen vacancy concentration estimated by Raman spectroscopy is not a good indicator for low temperature CO oxidation catalysts unless extra care is taken such that the Raman signal reflects oxide surface status. The experimental trend over the six dopants showed limited agreement with theoretically calculated oxygen vacancy formation energy in the literature.

Thermal activation of Pd/CeO2-SnO2 catalysts for low-temperature CO oxidation

In this work, the counter precipitation method was used to synthesise Pd/CeO2-SnO2 catalysts, which possess excellent low-temperature activity and high thermal stability. It was revealed that calcination of Pd/CeO2-SnO2 catalysts at 800−1000 °C induces significant growth of catalytic activity in CO oxidation at T<150 °C. This effect of thermal activation for Pd/CeO2-SnO2 catalysts was enhanced when water was admitted to the reaction mixture. In the presence of water the T50 value for the Pd/CeO2-SnO2 catalyst calcined at 900 °C becomes 45 °C lower than for the Pd/CeO2 catalyst. It was found that calcination of the catalysts at T<600 °C leads to the formation of solid solutions based on the fluorite and rutile structures. As the calcination temperature is raised above 600 °C, the solid solutions decompose with formation of catalytically active PdxCe1-xO2-δdispersed phase on the surface of SnO2 nanoparticles. The formed nanoheterogeneous structure provides both high thermal stability and high water resistance of Pd/CeO2-SnO2 catalysts.

Surface Modification Strategy for Promoting the Performance of Non-noble Metal Single-Atom Catalysts in Low-Temperature CO Oxidation.

As a bridge between homogeneous and heterogeneous catalyses, single-atom catalysts (SACs), especially the noble metal atoms, have received extensive attention from both the fundamental and applied perspectives recently. High cost and difficulty in synthesis are considerable factors, however, limiting the development and practical applications of SACs. Thus, seeking for non-noble SACs for substituting the noble ones is not only of vital importance but also a long-standing challenge. Herein, a surface modification strategy by introducing an oppositely charged dopant and inducing the charge transfer between the SAC and the substrate was proposed to improve the stability and catalytic performance of the non-noble Cu SAC. Using first-principles density functional theory (DFT) calculations, it was demonstrated that the introduction of C in the MoS2 monolayer (C:MoS2, experimentally available) can assist in stabilizing Cu and make it more positively charged, which will facilitate the adsorption of the reactants and further enhance the activity for CO oxidation. Strikingly, our results show that CO oxidation over Cu-C:MoS2 is more favorable than over the Pt atom deposited on the pristine MoS2 (Pt-MoS2), exhibiting its potential in noble metal substitution and low-temperature CO oxidation. Additionally, Cu-C:MoS2 was observed to have a response to visible light, which manifests that it may be a promising photocatalyst. The strategy proposed here provides an efficient route to regulate the electronic structures of SACs through charge transfer, which further promotes the reactivity of the non-noble metal SACs. We hope that this strategy can contribute to design more SACs with low cost and high efficiency, which will be beneficial for their practical applications.

Synergism of Iron and Platinum Species for Low-Temperature CO Oxidation: From Two-Dimensional Surface to Nanoparticle and Single-Atom Catalysts.

CO oxidation is one of the most studied reactions in heterogeneous catalysis. It is present in air cleaning and automotive emission control. It also participates in the removal of CO from streams of hydrogen used in fuel cells. Because of the competitive adsorption of CO and O2 over active sites, the use of Pt-based catalysts for low-temperature CO oxidation remains a challenge. Recently, great progress has been made with catalysts containing Pt-Fe species because of the contribution of Fe species to O2 activation. The structure-activity relationship and reaction mechanisms have been investigated with various Pt-Fe catalysts. In this Perspective, we give a summary of the recent advances of low-temperature CO oxidation over Pt-Fe catalysts with a focus on the synergistic effect of Pt and Fe species in the CO and O2 activation of catalytic reactions. Future prospects for the preparation of highly effective Pt-Fe catalysts are also proposed.

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.

Low-Temperature Catalytic CO Oxidation Over Non-Noble, Efficient Chromia in Reduced Graphene Oxide and Graphene Oxide Nanocomposites

Herein, bare chromia nanoparticles (Cr2O3 NPs) and chromia supported on reduced graphene oxide (rGO) and graphene oxide (GO) hybrids were synthesized, followed by characterization by means of FESEM, Raman spectroscopy, TGA, XRD, TEM/HRTEM, XPS and N2 sorptiometry. The investigated bare Cr2O3 and the hybrids (Cr2O3/rGO and Cr2O3/GO) were employed as catalysts for low-temperature CO oxidation. Compared with the other catalysts, the results revealed efficient catalytic activity using Cr2O3/GO, which was attributed to its higher surface area together with the mixed oxidation state of chromium (Cr3+ and Cr&gt;3+). These are important oxidation sites that facilitate the electron mobility essential for CO oxidation. Moreover, the presence of carbon vacancy defects and functional groups facilitate the stabilizing of Cr2O3 NPs on its surface, forming a thermally stable hybrid material, which assists the CO oxidation process. The Cr2O3/GO hybrid is a promising low-cost and efficient catalyst for CO oxidation at low temperatures. The higher activity of graphene oxide supported Cr2O3 NPs can provide an efficient and cost-effective solution to a prominent environmental problem.

Facile preparation of highly active and stable CuO–CeO2 catalysts for low-temperature CO oxidation via a direct solvothermal method

CuO–CeO2 catalysts prepared by a direct solvothermal method exhibit high activity and stability for low-temperature CO oxidation.

A highly active Rh1/CeO2 single-atom catalyst for low-temperature CO oxidation.

Rh1/CeO2 single-atom catalyst is highly active for CO oxidation and has the potential to serve as a multifunctional catalyst to save the usage of other noble metals in three-way catalysts. The high activity is achieved on single-atom active sites via the Mars-van Krevelen mechanism, thus avoiding CO poisoning at low temperatures.

Front Cover: Design of Ceria Catalysts for Low‐Temperature CO Oxidation (ChemCatChem 1/2020)

The Front Cover shows the design of a ceria catalysts for low-temperature CO oxidation. In their Review, Jeong Woo Han and co-workers review doping and loading systems are suggested as strategies for improving the CO oxidation activity of CeO2 catalysts. Transition metal (TM) and rare-earth metal (RE) dopants dramatically improve the activity and stability of the CeO2 catalysts. Besides, the origin of the activity due to the metal-support interaction is clearly presented. Based on experimental and theoretical approaches, metal supported on (RE, TM) co-doped CeO2 is expected to benefit environment by suppressing the vehicular emission of CO at low temperature. More information can be found in the Review by Hyung Jun Kim, Myeong Gon Jang et al. on page 11 in Issue 1, 2020 (DOI: 10.1002/cctc.201901787).

Linear Activation Energy-Reaction Energy Relations for LaBO3 (B = Mn, Fe, Co, Ni) Supported Single-Atom Platinum Group Metal Catalysts for CO Oxidation.

Single-atom catalysts are at the center of attention of the heterogeneous catalysis community because they exhibit unique electronic structures distinct from nanoparticulate forms, resulting in very different catalytic performance combined with increased usage of often costly transition metals. Proper selection of a support that can stably keep the metal in a high dispersion is crucial. Here, we employ spin-polarized density functional theory and microkinetics simulations to identify optimum LaBO3 (B = Mn, Fe, Co, Ni) supported catalysts dispersing platinum group metals as atoms on their surface. We identify a strong correlation between the CO adsorption energy and the d-band center of the doped metal atom. These CO adsorption strength differences are explained in terms of the electronic structure. In general, Pd-doped surfaces exhibit substantially lower activation barriers for CO2 formation than the Rh- and Pt-doped surfaces. Strong Brønsted–Evans–Polanyi correlations are found for CO oxidation on these single-atom catalysts, providing a tool to predict promising compositions. Microkinetics simulations show that Pd-doped LaCoO3 is the most active catalyst for low-temperature CO oxidation. Moderate CO adsorption strength and low reaction barriers explain the high activity of this composition. Our approach provides guidelines for the design of highly active and cost-effective perovskite supported single-atom catalysts.

Tailoring the surface structures of iron oxide nanorods to support Au nanoparticles for CO oxidation

AbstractIron oxide supported Au nanomaterials are one of the most studied catalysts for low-temperature CO oxidation. Catalytic performance not only critically depends on the size of the supported Au nanoparticles (NPs) but also strongly on the chemical nature of the iron oxide. In this study, Au NPs supported on iron oxide nanorods with different surface properties through β-FeOOH annealing, at varying temperatures, were synthesized, and applied in the CO oxidation. Detailed characterizations of the interactions between Au NPs and iron oxides were obtained by X-ray diffraction, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy. The results indicate that the surface hydroxyl group on the Au/FeOOH catalyst, before calcination (Au/FeOOH-fresh), could facilitate the oxygen adsorption and dissociation on positively charged Au, thereby contributing to the low-temperature CO oxidation reactivity. After calcination at 200 °C, under air exposure, the chemical state of the supported Au NP on varied iron oxides partly changed from metal cation to Au0 along with the disappearance of the surface OH species. Au/FeOOH with the highest Au0 content exhibits the highest activity in CO oxidation, among the as-synthesized catalysts. Furthermore, good durability in CO oxidation was achieved over the Au/FeOOH catalyst for 12 h without observable deactivation. In addition, the advanced identical-location TEM method was applied to the gas phase reaction to probe the structure evolution of the Au/iron oxide series of the catalysts and support structure. A Au NP size-dependent Ostwald ripening process mediated by the transport of Au(CO)x mobile species under certain reaction conditions is proposed, which offers a new insight into the validity of the structure-performance relationship.