Selective CO Oxidation Research Articles

The effect of SO2 on NH3-SCR reaction synergistic CO oxidation over a Mn-Co-Ti catalyst

Mn–Co–TiO2 prepared using an impregnation method was applied to synergistic NH3-based selective catalytic reduction (NH3-SCR) and CO oxidation in a simulated flue gas. The NO conversion, NO oxidation, and CO oxidation were substantially enhanced after adding 15% Co3O4 to the 20MnTi catalyst. In an NO or NO + NH3 atmosphere, CO oxidation over the Mn–Co–TiO2 catalyst was mainly restrained by the competitive adsorption and formation of sulfate species. In contrast, the NH3-SCR reaction progressed almost unimpeded in a CO atmosphere. A SO2 atmosphere drastically reduced the NO conversion of the catalyst at lower temperatures and almost completely suppressed the oxidation activities toward NO and CO. The amount of remaining NO oxidation (approximately 10%) was primarily contributed by gas-phase oxidation of NO by O2. Next, fresh and SO2-poisoned Mn–Co–TiO2 were characterized using various methods. Results indicated that no conspicuous change occurred in the crystal structure of the samples after SO2 poisoning, but numerous sulfate products were deposited on the catalyst surface. The deposition of sulfate-species byproducts decreased the specific surface area and pore volume of the catalyst and shifted the surface acid sites to a higher temperature range, further reducing the CO oxidation performance. Moreover, the catalytic reaction of CO and NO followed the Eley–Rideal mechanism. Furthermore, the mechanisms through which multicomponent flue gases influence the synergistic NH3-SCR and CO oxidation performance were proposed.

Gold oxide formation on Au(111) under CO oxidation conditions at room temperature.

Although gold-based catalysts are promising candidates for selective low-temperature CO oxidation, the reaction mechanism is not fully understood. On a Au(111) model catalyst, we observe the formation of gold oxide islands under exposure to atmospheric pressures of oxygen or CO oxidation reaction conditions in an in situ scanning tunneling microscope. The gold oxide formation is interpreted in line with the water-enabled dissociation of O2 on the step edges of Au(111). Contaminants on the gold surface can strongly promote the gold oxide formation even on the terraces. On the other hand, TiO2 nanoparticles on the Au(111) do not show any influence on the formation of the gold oxide and are thus not providing a significant amount of atomic oxygen to the gold at room temperature. Overall, the presence of gold oxide is likely under industrial conditions.

Molybdenum‐based Nanocatalysts for CO Oxidation Reactions in Direct Alcohol Fuel Cells: A Critical Review

AbstractCarbon monoxide (CO) oxidation is crucial in fuel cell anodes. Recent research has focused on electrocatalysts that synergistically enhance CO oxidation alongside alcohol/hydrogen oxidation. High sensitivity and selectivity for CO oxidation at lower onset potentials are the key objectives. Molybdenum (Mo) has emerged as a promising non‐noble transition metal co‐catalyst for CO oxidation. Mo versatility arises from its ability to alloy with Pt and mix with other non‐noble transition metals in various forms (MoOx, MoS2, MoC). Carbon‐supported Mo nanoparticles have shown potential in reducing Pt loading and improving CO tolerance due to Mo oxyphilic properties, effectively oxidizing weakly adsorbed CO and lowering onset and peak potentials. However, challenges persist, such as a limited potential window for CO oxidation and decreased CO adsorption affinity at higher potentials. Addressing these issues requires understanding factors affecting Mo‐based electrocatalysts (Mo‐ECs) activity, including PtMo alloy composition, Mo′s chemical state, cell temperature, and the role of carbon support. This article provides a comprehensive review of Mo‐ECs role in CO electrooxidation in direct alcohol fuel cells (DAFCs) over the past two decades.

Selective oxidation and nickel enrichment hinders the repassivation kinetics of multi-principal element alloy surfaces

Robust and sustained corrosion resistance in multi-principal element alloys (MPEA) requires rapid repassivation, i.e. regrowth of the passive layer once it is damaged or destroyed at the surface. In this study, we show that the repassivation of Al0.1CrCoFeNi MPEA in 0.6 M NaCl solution is hindered at pH of ∼ 2.4 - 6.8 due to the formation of a Ni-enriched subsurface layer as a result of selective oxidation and dissolution of several principal elements, which can be fully restored at pH of ∼ 14 from the oxidation of all principal elements. Specifically, surface characterization via X-ray photoelectron spectroscopy (XPS), high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy (STEM), and atom probe tomography (APT), are coupled with density functional theory (DFT) calculations to determine surface composition, oxidation state, and electron work function to uncover the structural origin of the pH-dependent repassivation mechanisms. It was found that selective oxidation of Cr, Co, and Fe in the acidic to neutral solutions altered the surface composition to be significantly enriched in Ni as compared to the bulk. Once the original passive film is destroyed locally by either pitting or tribocorrosion, this altered surface composition exhibited a much poorer repassivation capability due to the increased electron work function and reduced surface reactivity at higher Ni concentration. These understandings could shed light on the future compositional design of non-equiatomic MPEAs towards sustained repassivation and corrosion resistance over a wide pH range.

Spatially Separated Active Sites Enable Selective CO Oxidation Reaction on Oxide Catalyst.

The search for efficient non-noble-metal catalysts able to perform selective oxidation reactions is of great importance, with relevance to many catalytic processes. However, this is often hampered because the origin of the selectivity remains controversial, particularly for reactions catalyzed by oxides. Here, combining high-pressure surface imaging techniques and theoretical calculations, we identify that spatially separated active sites for O2 activation and H2 adsorption on an ultrathin Mn3O4 surface enable selective oxidation of CO over H2. Theoretical calculations reveal that Mn-O pairs for H2 dissociation are separated from Mn-Mn pairs for the formation of adsorbed O2* so that H2 has to surmount much higher barriers for both H2 dissociation and H diffusion while CO can directly react with O2* following the Eley-Rideal process. Our study sheds light on the atomic-level understanding of the surface structure-dependent selective oxidation reaction on oxide catalysts.

Chemical looping preferential oxidation of CO over ceria-supported γ-Fe2O3

A novel chemical looping preferential oxidation (CL-PROX) process is proposed and demonstrated for eliminating trace CO from a H2-rich stream. The proposed process exhibited an excellent CO removal performance (CO concentration < 100 ppm) and a high H2 recovery (>∼96 %) with a ceria-supported γ-Fe2O3 oxygen carrier. It was obtained by activating the fresh sample that was synthesized through a co-precipitation method and the innovative activation process comprising a partial reduction step at 400 °C followed by an oxidation process at 260 °C. The structure and evolution of iron species in the oxygen carrier were investigated through experimental characterizations during activation and redox cycles. TPR characterizations illustrated a significant difference in the reducibility of the active species, γ-Fe2O3, under H2 and CO atmospheres, which could be the evidence for its relatively high selectivity to CO oxidation. In-situ FTIR characterization also pointed out that the desorption of reaction intermediates to CO2 is facilitated via the reduction of the surface γ-Fe2O3, which might contribute to the outstanding CO oxidation performance of the oxygen carrier. The successful application of chemical looping process to selective oxidation of CO in H2-rich gas not only illustrates a new strategy for H2 purification, but also provides insights into the redox reaction mechanism with the newly-prepared γ-Fe2O3-based oxygen carrier.

Effect of the Pore Structure of Nanosilicas Decorated with Cobalt and Cerium Oxides on Catalytic Activity in the Selective Oxidation of Carbon Monoxide

Two types of nanosilicas with different pore structures are synthesized and decorated via impregnation with cobalt and cerium oxides. Meso–microporous spherical silica particles with thin walls of SiO2 nanochannels having specific surface area and pore volume of up to 1400 m2/g and 0.8 cm3/g, respectively, are used. Macroporous three-dimensionally ordered structures based on SiO2 (so-called synthetic opals) consisting of close-packed submicron spherical silica particles with respective porosity characteristics of 11 m2/g and 0.2 cm3/g are also used. The synthesized materials are characterized via low-temperature nitrogen adsorption, X-ray diffraction, SEM, XPS, and Fourier transform IR spectroscopy, and tested as catalysts for the selective oxidation of CO in excess H2 (CO-PROX). The effect of silica, the ratio of introduced oxides, and the order of their introduction on the structure and catalytic properties of Co–Ce/SiO2 are revealed. The catalytic behavior of the synthesized materials is determined from the specificity of interactions among the metal oxides and with the silica surface.

Mechanochemically activated Au/CeO2 for enhanced CO oxidation and COPrOx reaction

COPrOx reaction is one of the particularly appealing and cost-effective solutions for the selective oxidation of CO in hydrogen-rich gas streams to meet the stringent requirement of the current electrocatalysts employed in low-temperature fuel cells. Herein, we synthesized ultrasmall Au clusters supported on ceria by one-step solvent-free mechanochemical method. These catalysts exhibited higher activity for COPrOx and CO oxidation compared to the sample prepared by conventional incipient wetness impregnation. The unique Au-Ce interaction caused by impact and friction between the ball, vessel, and powders, greatly promotes the generation of positive charged Auδ+ active sites and, at the same time, the reducibility of the catalyst. Interestingly, an aging treatment of the ball-milled samples resulted in a significant superior performance of the catalytic activity. This enhancement has been attributed to a change in the oxidation state of Au between the fresh and the aged catalysts prepared by ball milling.

A comparative study of CO oxidation on Cu-doped C3N monolayer with N and C vacancies

In this study, we employ density functional theory based on first-principles calculations to investigate the CO oxidation performance of a Cu-doped C3N monolayer with N and C vacancies, which is a promising low-temperature and high-activity catalyst. We find that Cu dopants are more stably anchored at both the N and C vacancies of the C3N monolayer. To comprehensively understand the effect of N and C vacancies on CO oxidation, we investigate both the Eley–Rideal and Langmuir–Hinshelwood (LH) mechanisms. Our results indicate that the LH mechanism exhibits much lower barriers for the rate-limiting step of CO oxidation, suggesting its excellent performance in this process. Moreover, the charge state of Cu atoms and the asymmetric atomic interface at the C vacancy lead to a lower rate-limiting barrier for CO oxidation compared to that at the N vacancy (0.5 vs 0.71 eV). This study represents a promising strategy to enhance the activity and selectivity of CO oxidation by controlling the asymmetric atomic interface of single-atom catalysts.

Selective Visible-Light Photocatalytic Oxidation of Sulfides and Catalytic CO2 Fixation by Two-Interpenetrated Photoresponsive MOF-150

Sulfides and CO2 are the main pollutants that cause environmental acidification and the greenhouse effect. However, the rational use of these pollutants can not only save resources but also improve the environment. The two-interpenetrated MOF-150 was prepared by the photoresponsive 4,4′,4″-nitrilotribenzoic acid (H3NTB) ligand and Zn4O clusters, and it exhibits excellent optical activity derived from H3NTB. Especially as the substrate, thioanisole is oxidized to sulfoxide with medicinal value after 10 h of Xe light irradiation, and the conversion rate of 98% and the selectivity of 100% are obtained. Higher photocatalytic performance is also observed when the substrate is extended to other sulfides (7 examples). At the same time, MOF-150 has an excellent performance in the capture and recycling catalytic fixation of CO2. With epichlorohydrin as the reaction substrate, the cycloaddition efficiency of CO2 can reach over 99%. When it is extended to other epoxides, satisfactory cycloaddition efficiency was also obtained (7 examples). Therefore, MOF-150 is a promising heterogeneous catalyst with high catalytic efficiency and selectivity for a wide range of substrates.

Simultaneous removal of NO and CO over Ni-Ce bifunctional catalyst supported by modified activated coke at oxygen-rich condition

The modified activated coke (AC-N) supported Ni-Ce transition metal catalyst prepared by incipient-wetness impregnation method can simultaneously catalyze the ammonia selective catalytic reduction (NH3-SCR) reaction and CO oxidation reaction, realizing the removal of NO and CO under low temperature and oxygen-rich conditions. The Ni-Ce/AC-N catalyst can achieve high-efficiency conversion of NO and CO at 175–250 °C, and the conversion rates of NO and CO are both above 95% in this temperature range. After modified by nitric acid, the active coke support has a stronger interaction with metal components, which is conducive to better dispersion of the active components on the catalyst surface, and improves the specific surface area and redox capacity of the catalyst. The synergistic effect between Ni and Ce results in more Ni2+ and Ce3+ species on the catalyst surface, which is beneficial to the improvement of catalytic activity.

CatS: A predictive and user-friendly framework based on machine learning models for the screening of heterogeneous catalysts

With the machine learning evolution around the world, the application of these technologies has spread to several multidisciplinary areas. In heterogeneous catalysis it was no different. Studies that cover multiple fields have been developed applying these techniques; however, they are not widely employed to experiments due to mutual communication and understanding of problems between these two areas. In this work, a new way of developing, optimizing and presenting convolutional neural networks to the scientific community is proposed in addition to discussions about the problematic use of these tools. With accuracies above 90%, and surpassing previous works, the user will be able to infer results regarding usual chemical reaction systems, such as water-gas shift, selective CO oxidation, and steam reforming, without an exhaustive bibliographic or experimental research to evaluate the best combination of reaction parameters and catalyst features.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

Titania-decorated hybrid nano-architectures and their preliminary assessment in catalytic applications

Gold nanomaterials have been recognized as considerably active and effective catalysts toward several industrial and environmentally relevant reactions, among which are hydrocarbon selective oxidation, CO oxidation and propylene epoxidation. Engineered nanomaterials may be able to suppress some of the more severe issues that block their industrial applicability, such as recyclability, catalytic activity reduction during time due to sintering effects or leakage of the active material. In this regard, the composition of hybrid nanomaterials comprising encapsulated ultrasmall gold nanoparticles, also called ultrasmall-in-nano, will broaden and enhance their potential catalytic applications by including synergic features. Here, we report a systematic investigation on the design and production of optimized titania-decorated gold nano-architectures together with their efficiency evaluation on both formic acid (thermo/photo-induced) decomposition and cyclic carbonates formation by carbon dioxide fixation for carbon capture and storage technology. This work paves the way for the next development of ultrasmall-in-nano catalysts bearing an early–late transition metal combination for sustainable processes.

Novel Nonradical Oxidation of Sulfonamide Antibiotics with Co(II)-Doped g-C3N4-Activated Peracetic Acid: Role of High-Valent Cobalt-Oxo Species.

Herein, we report that Co(II)-doped g-C3N4 can efficiently trigger peracetic acid (PAA) oxidation of various sulfonamides (SAs) in a wide pH range. Quite different from the traditional radical-generating or typical nonradical-involved (i.e., singlet oxygenation and mediated electron transfer) catalytic systems, the PAA activation follows a novel nonradical pathway with unprecedented high-valent cobalt-oxo species [Co(IV)] as the dominant reactive species. Our experiments and density functional theory calculations indicate that the Co atom fixated into the nitrogen pots of g-C3N4 serves as the main active site, enabling dissociation of the adsorbed PAA and conversion of the coordinated Co(II) to Co(IV) via a unique two-electron transfer mechanism. Considering Co(IV) to be highly electrophilic in nature, different substituents (i.e., five-membered and six-membered heterocyclic moieties) on the SAs could affect their nucleophilicity, thus leading to the differences in degradation efficiency and transformation pathway. Also, benefiting from the selective oxidation of Co(IV), the established oxidative system exhibits excellent anti-interference capacity and achieves satisfactory decontamination performance under actual water conditions. This study provides a new nonradical approach to degrade SAs by efficiently activating PAA via heterogeneous cobalt-complexed catalysts.

Catalytic Membrane Reactors: The Industrial Applications Perspective

Catalytic membrane reactors have been widely used in different production industries around the world. Applying a catalytic membrane reactor (CMR) reduces waste generation from a cleaner process perspective and reduces energy consumption in line with the process intensification strategy. A CMR combines a chemical or biochemical reaction with a membrane separation process in a single unit by improving the performance of the process in terms of conversion and selectivity. The core of the CMR is the membrane which can be polymeric or inorganic depending on the operating conditions of the catalytic process. Besides, the membrane can be inert or catalytically active. The number of studies devoted to applying CMR with higher membrane area per unit volume in multi-phase reactions remains very limited for both catalytic polymeric and inorganic membranes. The various bio-based catalytic membrane system is also used in a different commercial application. The opportunities and advantages offered by applying catalytic membrane reactors to multi-phase systems need to be further explored. In this review, the preparation and the application of inorganic membrane reactors in the different catalytic processes as water gas shift (WGS), Fisher Tropsch synthesis (FTS), selective CO oxidation (CO SeLox), and so on, have been discussed.

Effects of hydrothermal oxidation time of Al on the catalytic performance of Ru/Al@Al2O3 for selective oxidation of CO in H2

The thermal-conducting support is highly desirable for endothermic and exothermic reactions as long as highly dispersed active sites can be maintained. The core-shell Al@Al2O3 supports with different aluminum (Al) contents were prepared from Al particle by controlling the reaction time during hydrothermal surface oxidation and applied as a support to the supported Ru catalysts for preferential CO oxidation in H2 (PROX). The prepared catalysts were characterized by N2-physisorption, X-ray diffraction, CO chemisorption, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), in situ diffuse reflectance infrared Fourier transform spectroscopy after CO adsorption (CO-DRIFTS), and temperature-programmed desorption of ammonia (NH3-TPD) and ethanol (ethanol-TPD). The catalytic activity was dependent on the Al content in the Al@Al2O3 support. The most active Ru/Al@γ-Al2O3 catalyst oxidized CO selectively in H2 over a wide reaction temperature. The surface property of the outermost exterior alumina layer in the Al@γ-Al2O3 support, determined with ethanol TPD, was beneficial for formation of Ru nanoparticles with weak adsorption of CO, probed with CO-DRIFTS, results in the high catalytic performance for PROX.

Calcination Temperature Effect in Catalyst Reactivity for the CO SELOX Reaction Using Perovskite-like LaBO3 (B: Mn, Fe, Co, Ni) Oxides

H2 production by conventional processes such as reforming reactions involves formation of some undesired products such as CO. Selective CO oxidation (SELOX) is a good alternative to reduce its concentration because it is a thermodynamically favorable process. Perovskite-like oxides (LaMO3) are used in this work to catalyze the CO-SELOX process and they were synthetized by the self-combustion method. All catalysts were calcined at different temperatures (400 °C–800 °C) to analyze its impact on the material physicochemical properties. Different analytical techniques were used to study changes in structural and chemical properties. Temperature-programmed reaction (TPRe), using CO oxidation and CO-SELOX, was used to measure the catalytic reactivity of perovskite-like oxides. It was found that catalytic activity decreases when calcination temperature (CT) increases and that is correlated with oxygen mobility, surface defects, and surface concentration of manganese or cobalt species. It is also found that there is a crystalline phase change in the solids when the CT increases from 500 to 600 °C. The high catalytic activity observed at low CT remained even if they were calcined below 600 °C, which implies the possibility of amorphous species having catalytic activity that do not require a well-structured perovskite.

Structural and morphological characterization of the perovskite LaFe0.2Cr0.8-xCoxO3 (x = 0.0, 0.2, 0.4, 0.6, 0.8) for selective oxidation of CO

Structural and morphological characterization of the perovskite LaFe0.2Cr0.8-xCoxO3 (x = 0.0, 0.2, 0.4, 0.6, 0.8) for selective oxidation of CO

The Influence of Yttrium Stabilized Zirconia as Support of Copper-Ceria Systems on their Catalytic Properties in the Prox Process

It is shown that with the same quantitative composition of the copper-ceria-zirconia catalyst, varying only the structural and dimensional characteristics of the yttrium stabilized zirconia (YSZr), it is possible to improve the activity and selectivity of the catalyst and expand the «temperature window» of its effective operation. It has been established that the low surface area of YSZr support, due to the high degree of agglomeration of its nanoparticles, contributes to the formation of a larger number of interaction zones of copper and cerium oxides that are active sites of the process of selective oxidation of CO in hydrogen (PROX – Preferential CO oxidation).