Catalytic Oxidation Of CO Research Articles

Silver-manganese mixed oxide with crednerite structure as a catalyst for low-temperature catalytic CO oxidation

The paper describes the conditions for the preparation of AgMnO2 particles with the crednerite structure without impurity crystallized phases. It was shown that catalytic properties of AgMnO2 in CO oxidation might be different in dependence on the nature of the alkali-containing precursor used. The highest catalytic activity was achieved when using a combination of K- and Na-containing precursors. It has been found that the low-temperature catalytic activity of AgMnO2 increases noticeably with a decrease in the relative silver amount. An in situ X-ray diffraction study revealed a partial release of silver from crednerite structure with the formation of Ag0 particles when AgMnO2 was heated in a CO+O2 mixture to 150–200 °C, whereas heating to 250 °C followed by cooling to room temperature led to the incorporation of silver back into crednerite structure. The low-temperature catalytic activity of AgMnO2 correlates with the amount of weakly bounded oxygen species on the surface.

Insights into the Contribution of Oxidation-Reduction Pretreatment for Mn0.2Zr0.8O2-δ Catalyst of CO Oxidation Reaction.

A Mn0.2Zr0.8O2-δ mixed oxide catalyst was synthesized via the co-precipitation method and studied in a CO oxidation reaction after different redox pretreatments. The surface and structural properties of the catalyst were studied before and after the pretreatment using XRD, XANES, XPS, and TEM techniques. Operando XRD was used to monitor the changes in the crystal structure under pretreatment and reaction conditions. The catalytic properties were found to depend on the activation procedure: reducing the CO atmosphere at 400-600 °C and the reaction mixture (O2 excess) or oxidative O2 atmosphere at 250-400 °C. A maximum catalytic effect characterized by decreasing T50 from 193 to 171 °C was observed after a reduction at 400 °C and further oxidation in the CO/O2 reaction mixture was observed at 250 °C. Operando XRD showed a reversible reduction-oxidation of Mn cations in the volume of Mn0.2Zr0.8O2-δ solid solution. XPS and TEM detected the segregation of manganese cations on the surface of the mixed oxide. TEM showed that Mn-rich regions have a structure of MnO2. The pretreatment caused the partial decomposition of the Mn0.2Zr0.8O2-δ solid solution and the formation of surface Mn-rich areas that are active in catalytic CO oxidation. In this work it was shown that the introduction of oxidation-reduction pretreatment cycles leads to an increase in catalytic activity due to changes in the origin of active states.

Novel insights into CO2 inhibition with additives in catalytic aerobic oxidation of biomass-derived carbohydrates to formic acid

Formic acid (FA) is considered an ideal energy carrier for hydrogen. Catalytic aerobic oxidation of biomass-derived carbohydrates is a promising method to produce renewable FA. Interestingly, the addition of some low-carbon alcohols or dimethyl sulfoxide remarkably improved FA selectivity in the popular vanadium-based catalytic system. This work unveils an advanced mechanism of the product improvement with additives. Control experiments of vanadium-substituted heteropolyacids (HPA-n) in different redox states and ESR spectroscopy verified that hydroxyl radicals were formed in the re-oxidation of reduced HPA-n. Different-type oxidations and model reactions revealed that the hydroxyl radicals caused overoxidation by oxidizing aldehydic groups to carboxylic groups as the precursors of CO2. The additives scavenged hydroxyl radicals so glucose was oxidized into FA with high selectivity. Furthermore, the hydroxyl radicals were quantified and the effect of isopropanol dosage on glucose oxidation was investigated. The CO2-inhibition effect of isopropanol was also exhibited for various carbohydrates and vicinal diols. Based on the mechanism, catalytic co-oxidation of glucose and methanol was proposed, showing an extremely high FA yield from glucose and high reactivity of methanol. The findings shed light into the mechanism of the product improvement with additives and pave an efficient way to produce a renewable energy carrier.

Highly efficient catalytic CO oxidation by balancing the crystal and mesoporous structure of Ta2O5 carrier combined with cobalt

Because the combination of Co and common support (Al, Ti, et al.) usually shows poor catalytic activity, tantalum has been introduced for CO conversion as a novel carrier due to the excellent performance. In this work, Co3O4 was loaded on four forms of tantalum support (amorphous, mesoporous, crystalline and crystalline mesoporous, abbreviated as Co/Ta-am, Co/Ta-me, Co/Ta-cry and Co/Ta-cm, respectively). And among these four states, Co/Ta-cm presents the highest CO conversion rate. The temperature of 90% CO conversion rate of Co/Ta-cm is approximately 130 ℃ at the high space velocity of 780,000 h−1, which is 70%, 82% and 88% higher than that of Co/Ta-cry, Co/Ta-me and Co/Ta-am, respectively. Moreover, the characteristics of the catalyst have been studied by XRD, SEM, BET, TGA, and H2-TPR, and the results reveal that Co/Ta-cm has the excellent crystallinity and stability, and the morphology is regular. X-ray photon spectroscopy and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) have been performed to investigate the reaction mechanism, which indicates that Co3+ is the active component and lattice oxygen plays a critical role in CO oxidation.

Tailoring the electronic states of Pt by atomic layer deposition of Al2O3 for enhanced CO oxidation performance: Experimental and theoretical investigations

Herein, atomic layer deposition (ALD)-Al2O3 films were applied as a medium to modulate the electronic states of the supported Pt. Based on the sacrificial template of ZIF-67, the ternary Co3O4/Pt/Al2O3 catalyst exhibited better performance in catalytic CO oxidation than the binary Co3O4/Pt catalyst. Both experimental and calculation results indicate the existence of charge transport between the Pt and the ALD-Al2O3 films, leading to electron-rich Pt, enhanced O2 affinity, and reduced activation energy. Interestingly, the oxidation resistance of Pt nanoparticles was visualized when annealing Co3O4/Pt/Al2O3 catalyst at high temperatures. Mechanism studies indicate the typical LH mechanism occurred in CO oxidation, showing that the co-adsorbed CO and O2 could be converted to carbonate-type *OCOO intermediates via low energy barriers. At low temperatures, CO2 was mainly produced by the lattice oxygen-mediated M-vK mechanism based on isotope labeling experiments. Our findings provide an alternative way to regulate the electronic states of noble metals by the ALD technique.

Dielectric barrier discharge plasma synthesis of Ag/γ-Al2O3 catalysts for catalytic oxidation of CO

In this study, Ag/γ-Al2O3 catalysts were synthesized by an Ar dielectric barrier discharge plasma using silver nitrate as the Ag source and γ-alumina (γ-Al2O3) as the support. It is revealed that plasma can reduce silver ions to generate crystalline silver nanoparticles (AgNPs) of good dispersion and uniformity on the alumina surface, leading to the formation of Ag/γ-Al2O3 catalysts in a green manner without traditional chemical reductants. Ag/γ-Al2O3 exhibited good catalytic activity and stability in CO oxidation reactions, and the activity increased with increase in the Ag content. For catalysts with more than 2 wt% Ag, 100% CO conversion can be achieved at 300 °C. The catalytic activity of the Ag/γ-Al2O3 catalysts is also closely related to the size of the γ-alumina, where Ag/nano-γ-Al2O3 catalysts demonstrate better performance than Ag/micro-γ-Al2O3 catalysts with the same Ag content. In addition, the catalytic properties of plasma-generated Ag/nano-γ-Al2O3 (Ag/γ-Al2O3-P) catalysts were compared with those of Ag/nano-γ-Al2O3 catalysts prepared by the traditional calcination approach (Ag/γ-Al2O3-C), with the plasma-generated samples demonstrating better overall performance. This simple, rapid and green plasma process is considered to be applicable for the synthesis of diverse noble metal-based catalysts.

Preparation of Cu0.1-xNixCe0.9O2-y catalyst by ball milling and its CO catalytic oxidation performance

Preparation of Cu0.1-xNixCe0.9O2-y catalyst by ball milling and its CO catalytic oxidation performance

Theoretical exploration of Rh1/CeO2 catalysts with high performance using CO oxidation as a probe reaction

Rh1/CeO2 attracts much attention in single-atom catalysis due to its unique catalytic performance and maximum atomic efficiency. Since different possible structures of Rh1/CeO2 coexist in real catalysts, it is hard to discriminate their catalytic roles and reaction mechanism due to the difficulty of preparing pure modeling catalyst. To solve this difficult problem, the adsorption and reaction behaviors of CO and O2 on four modeling catalysts of Rh1/CeO2 were systematically investigated by using density functional theory calculation in this work. The catalytic role and CO oxidation mechanism on Rh1/CeO2 catalysts were elucidated. On Rh1/CeO2(111), CO reacts with adsorbed O2 via Eley-Rideal mechanism; and the rate-determined step of CO oxidation cycle is CO reaction with the rest O atom of O2 with a barrier of 0.99 eV. On Rh1/CeO2-δ(111), CO reacts with activated O2 via Langmuir-Hinshelwood mechanism and continue the oxidation cycle via Rh1/CeO2(111). On RhxCe1-xO2, CO directly reacts with surface oxygen without barrier to form RhxCe1-xO2-δ; while the barrier of CO oxidation cycle on RhxCe1-xO2-δ is 1.18 eV. Upon the discrimination of the catalytic roles of Rh1/CeO2 catalysts with different structures, several important insights into design and development of Rh1/CeO2 catalysts with high oxidation performance were proposed.

Promotion effect of CO oxidation via activation of surface lattice oxygen by single atom Cu/MnO2 catalyst

The catalytic oxidation of CO by single-atom transition metal catalysts has always been the focus of researchers. Herein, we report a strategy for the preparation of Cu single-atom anchored on MnO2 by oxalate chelation-assisted hydrothermal method, and the obtained Cu/MnO2–0.25 catalyst with the optimal molar ratio achieved about 90% CO conversion at 80 °C. The strong electronic interaction between Cu and MnO2 not only effectively inhibits the aggregation of Cu single atoms, but also the formed Cu-O-Mn hybrid structure activates the adjacent surface lattice oxygen, further improving the catalytic oxidation performance of CO at low and medium temperatures. The elevated activity is achieved at the single-atom active site by the Mars-van Krevelen (MvK) mechanism. This work provides a simple and reliable method for the rational design of stable transition metal single-atom catalysts and deepens the understanding of metal-support strong interactions (SMSI).

Tailored synthesized Pt/ZSM-5 catalysts with excellent water vapor stability for low temperature oxidation of CO and C3H6

Pt supported on ZSM-5 catalysts are important for the catalytic oxidation of CO and volatile organic compounds (VOCs) due to their extraordinary performance at low temperatures and economic purposes. However, achieving complete oxidation at low temperatures and stability under water vapor remains challenging. Herein, we report the synthesis of several loading of Pt on a ZSM-5 catalyst via a flame synthesis technique. The Pt/ZSM-5 exhibited good activity towards complete CO and C3H6 oxidation at low temperatures due to the interaction between the Pt and ZSM-5. The combined effect of the PtOx (Pt2++Pt4+) and lattice oxygen from Pt (OPtOx) as the active centers in Pt/ZSM-5 favors the complete oxidation with high reaction rates, TOFs, and low activation energies (Ea), which is well consistent with the increasing concentration of the (Pt2++Pt4+)/Pt0. Pt/ZSM-5 presents a long-time tolerance to water vapor deactivation even at temperatures below 80 °C. The distributed Pt favor the reaction occurring through the Langmuir-Hinshelwood (L-H) route. The excellent catalytic results indicate an attractive approach for designing supported Pt on ZSM-5 catalysts for automobiles and industry exhausts.

Catalytic CO Oxidation by Single Atom Catalysts of Transition Metal-doped χ3-Borophene: A First Principles Study

Density functional theory was used to investigate the effect of χ3-borophene-based single atom catalysts for CO oxidation. Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms were investigated to explore the catalytic oxidation activity of CO molecules in the presence of single atom catalysts. Our calculations indicate that Fe@Bχ3 and Co@Bχ3 have good catalytic oxidation effects for CO oxidation with energy barriers of 0.64 eV and 0.62 eV, respectively.

Operando CO Infrared Spectroscopy and On-Line Mass Spectrometry for Studying the Active Phase of IrO2 in the Catalytic CO Oxidation Reaction

We combine operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with on-line mass spectrometry (MS) to study the correlation between the oxidation state of titania-supported IrO2 catalysts (IrO2@TiO2) and their catalytic activity in the prototypical CO oxidation reaction. Here, the stretching vibration of adsorbed COad serves as the probe. DRIFTS provides information on both surface and gas phase species. Partially reduced IrO2 is shown to be significantly more active than its fully oxidized counterpart, with onset and full conversion temperatures being about 50 °C lower for reduced IrO2. By operando DRIFTS, this increase in activity is traced to a partially reduced state of the catalysts, as evidenced by a broad IR band of adsorbed CO reaching from 2080 to 1800 cm−1.

Highly Efficient CO Oxidation on Atomically Thin Pt Plates Supported on Irreducible Si 7 × 7

Electronic interaction between metal nanoparticles and irreducible supports is an important aspect of heterogeneous catalysis due to its capability of controlling the associated catalytic performance. We elucidate the relationship between metal–support electronic interactions and catalytic activity on size-selected singe layered platinum plates Ptn (n = 10, 20, 30, and 45) supported on a Si (111)-7 × 7 reconstructed surface (Si-RS). Our first-principles density functional theory study shows that CO oxidation catalysis over Pt supported on the nonreducible Si-RS is affected by variations in charge transfer, cluster size, and electronic interactions between clusters and support. CO adsorbs on Ptn adsorption sites in two distinct ranges of adsorption energies, which explains the experimentally observed two different desorption temperatures of CO. Pt30/Si-RS is more active than other Ptn/Si-RS under the same conditions, showing markedly enhanced catalytic activity in terms of high turnover frequency of CO2 production. This superior activity originates from the optimum CO and O2 adsorption on Pt30 arising from the position of the d-band center being midway between their π orbitals. Experimental observations convey that the TOFs of CO2 production on Pt30(45)/Si-RS are very high, and these are tolerant to CO poisoning between 450 and 550 K. DFT computations of TOF on Pt30(45)/Si-RS for CO2 validate the experimentally observed trends at 350, 450, and 550 K and exhibit that the CO oxidation activity exceeds that of the conventional catalysts between 450 and 550 K. Our results depict the importance of size control and electronic structure tailoring while designing Pt-based catalysts and will motivate the effective utilization of rare elements on irreducible substrates.

Stable single iron atom within two-dimensional porphyrin sheet catalysis for CO oxidation: A computational investigation

CO produced by incomplete combustion of fuel poses a great threat to the environment and human health. Single atom catalysis can increase the active site of the catalyst, but it is unstable and easy to agglomerate at the higher temperatures. A stable single iron atom two-dimensional porphyrin sheet (Fe–TDPS) was designed, which was as CO oxidation catalyst to reduce costs and increase catalytic activity, and the reaction mechanism was investigated by density functional theory. Fe–TDPS has good stability at 1000 K with the ab initio molecular dynamics simulation. Absorption energies suggested that both CO and O2 preferred to anchor at the Fe–N4 site. The mechanisms of CO oxidation, which have reacted on the Fe–TDPS, were explored by Langmuir–Hinshelwood, Eley–Rideal, and ter-molecular Eley–Rideal mechanisms. Ter-molecular Eley–Rideal mechanism was feasible because of the small energy barrier. Results indicated that Fe–TDPS can be used as a catalyst for CO oxidation in the mild condition.

Mechanisms for Catalytic CO Oxidation on SiAun (n = 1-5) Cluster.

Significant progress has been made in understanding the reactivity and catalytic activity of gas-phase and loaded gold clusters for CO oxidation. However, little research has focused on mixed silicon/gold clusters (SiAun) for CO oxidation. In the present work, we performed density function theory (DFT) calculations for a SiAun (n = 1-5) cluster at the CAM-B3LYP/aug-cc-pVDZ-PP level and investigated the effects on the reactivity and catalytic activity of the SiAun cluster for CO oxidation. The calculated results show that the effect is very low for the activation barriers for the formation of OOCO intermediates on SiAu clusters, SiAu3 clusters, and SiAu5 clusters in the catalytic oxidation of CO and the activation energy barriers for the formation of OCO intermediates on OSiAu3, OSiAu4, and OSiAu5. Our calculations show that, compared with the conventional small Au cluster, the incorporation of Si enhances the catalytic performance towards CO oxidation.

CeO2-Supported TiO2-Pt Nanorod Composites as Efficient Catalysts for CO Oxidation.

Supported Pt-based catalysts have been identified as highly selective catalysts for CO oxidation, but their potential for applications has been hampered by the high cost and scarcity of Pt metals as well as aggregation problems at relatively high temperatures. In this work, nanorod structured (TiO2-Pt)/CeO2 catalysts with the addition of 0.3 at% Pt and different atomic ratios of Ti were prepared through a combined dealloying and calcination method. XRD, XPS, SEM, TEM, and STEM measurements were used to confirm the phase composition, surface morphology, and structure of synthesized samples. After calcination treatment, Pt nanoparticles were semi-inlayed on the surface of the CeO2 nanorod, and TiO2 was highly dispersed into the catalyst system, resulting in the formation of (TiO2-Pt)/CeO2 with high specific surface area and large pore volume. The unique structure can provide more reaction path and active sites for catalytic CO oxidation, thus contributing to the generation of catalysts with high catalytic activity. The outstanding catalytic performance is ascribed to the stable structure and proper TiO2 doping as well as the combined effect of Pt, TiO2, and CeO2. The research results are of importance for further development of high catalytic performance nanoporous catalytic materials.

Fabricating Robust Pt Clusters on Sn-Doped CeO2 for CO Oxidation: A Deep Insight into Support Engineering and Surface Structural Evolution.

The size effect on nanoparticles, which affects the catalysis performance in a significant way, is crucial. The tuning of oxygen vacancies on metal-oxide support can help reduce the size of the particles in active clusters of Pt, thus improving catalysis performance of the supported catalyst. Herein, Ce-Sn solid solutions (CSO) with abundant oxygen vacancies have been synthesized. Activated by simple CO reduction after loading Pt species, the catalytic CO oxidation performance of Pt/CSO was significantly better than that of Pt/CeO2 . The reasons for the elevated activity were further explored regarding ionic Pt single sites being transformed into active Pt clusters after CO reduction. Due to more exposed oxygen vacancies, much smaller Pt clusters were created on CSO (ca. 1.2 nm) than on CeO2 (ca. 1.8 nm). Consequently, more exposed active Pt clusters significantly improved the ability to activate oxygen and directly translated to the higher catalytic oxidation performance of activated Pt/CSO catalysts in vehicle emission control applications.

In situ Observation of Porosity Formation in Porous Single-crystalline TiO2 Monolith for Enhanced and Stable Catalytic CO Oxidation.

Introducing pores in single crystals creates a new type of porous materials that incorporate porosity and structural coherence. Herein, we use in situ transmission electron microscopy to disclose the porosity formation by converting KTiOPO4 (KTP) single crystals into porous single-crystalline (PSC) TiO2 monoliths in a solid-solid transformation. The isolated crystalline nuclei of TiO2 clusters with identical lattice orientation on KTP surface moves TiO2 /KTP interface toward mother phase for growing PSC TiO2 monoliths. The relative density in PSC TiO2 monoliths dominates porosity while the macroscopic dimensions remain unchanged in the transformation. The single-crystalline nature of porous architecture stabilizes oxygen vacancy to activate lattice oxygen while the three-dimensional percolation enhances species diffusion. PSC TiO2 monoliths with deposited Pt clusters show enhanced and stable catalytic CO oxidation in air at ∼75 °C for 200 hours of operation.

In situ Observation of Porosity Formation in Porous Single‐crystalline TiO2 Monolith for Enhanced and Stable Catalytic CO Oxidation

AbstractIntroducing pores in single crystals creates a new type of porous materials that incorporate porosity and structural coherence. Herein, we use in situ transmission electron microscopy to disclose the porosity formation by converting KTiOPO4 (KTP) single crystals into porous single‐crystalline (PSC) TiO2 monoliths in a solid‐solid transformation. The isolated crystalline nuclei of TiO2 clusters with identical lattice orientation on KTP surface moves TiO2/KTP interface toward mother phase for growing PSC TiO2 monoliths. The relative density in PSC TiO2 monoliths dominates porosity while the macroscopic dimensions remain unchanged in the transformation. The single‐crystalline nature of porous architecture stabilizes oxygen vacancy to activate lattice oxygen while the three‐dimensional percolation enhances species diffusion. PSC TiO2 monoliths with deposited Pt clusters show enhanced and stable catalytic CO oxidation in air at ∼75 °C for 200 hours of operation.

Effective modeling of coupled reaction and transport inside the catalytic filter wall

A new extension of 1D mathematical model describing diffusion limitation in a catalytic filter wall is proposed. Transport limitations in the flowing gas inside free pores and in the coated catalyst are considered together with a Langmuir–Hinshelwood (LH) reaction kinetics including inhibition effects. Catalytic CO oxidation is investigated as a test reaction both at low and high reactant concentrations. The computed 1D concentration profiles are compared to detailed 3D pore-scale simulations involving the structure of a real filter wall obtained from X-ray tomography (XRT) scans. The best agreement is achieved when both gas-in-pores and intra-catalyst diffusion with LH effectiveness factor are considered in the 1D model. The impact of mass transfer limitations on CO light-off curves is then simulated using a 1D+1D model of the entire monolith filter. The extended model including diffusion limitation in free pores predicts the presence of undesired reactant slip at high flow rates as observed in experiments, which was not possible with the previously published models.