Activity For CO Oxidation Research Articles

Metal-Support Interaction at Palladium-Composite Manganese Oxide Interface and CO Oxidation Activity

Metal-Support Interaction at Palladium-Composite Manganese Oxide Interface and CO Oxidation Activity

Why nitrogen oxide inhibits CO oxidation over highly dispersed platinum ceria catalysts

The influence of nitrogen oxide on the lean CO oxidation activity of highly dispersed Pt/ceria and reference Pt/alumina catalysts has been studied by kinetic measurements and infrared spectroscopic characterization. Co-feeding of nitrogen oxide leads to the formation of nitrates on the supports that induce a highly oxidized character of the Pt sites and in the case of Pt/ceria, inhibit ceria lattice oxygens to react with CO adsorbed on Pt rim sites via a Mars-van Krevelen mechanism below the ignition temperature. The build-up of nitrates below the light-off temperatures is faster when CO is present in the feed. Above the light-off temperatures, carbonates replace the nitrates while the catalytic activity remains high.

Spectroscopic determination of metal redox and segregation effects during CO and CO/NO oxidation over silica-supported Pd and PdCu catalysts

We report the effects of alloying Cu into silica supported Pd nanoparticles on the catalytic activity, surface composition, and particle structure during CO oxidation Pd is highly active for CO oxidation but is inhibited by relevant competitive reagents (e.g. NO) at low temperatures (< 150 °C). By alloying Cu into Pd nanoparticles, NO inhibition is suppressed without any CO oxidation activity loss. Infrared spectroscopy illustrates the formation of surface nitrosyl (NO-Pd) is eliminated in the PdCu alloy catalyst, preventing CO oxidation inhibition by NO. X-ray absorption spectroscopy studies show that Pd in the monometallic catalyst forms a surface oxide after light-off. Conversely, in the PdCu catalyst, Pd remains metallic during reaction, independent of temperature, whereas the Cu oxidizes when the catalyst becomes active, after light-off. Furthermore, DRIFTS studies show that the Cu segregates to the surface as the catalyst becomes active for diesel oxidation, presumably becoming the O2 adsorption site.

Superior single-atom Pt/TiO2 mesoporous microspheres via microdrops-confined pyrolysis/deposition for low-temperature CO oxidation

This work demonstrate a polymer micelle-induced mesoporous single-atom Pt1/TiO2 microspheres with a mixed mother solution of triblock copolymer (Pluronic F127), H2PtCl6 and tetrabutyl titanate (TBT). The specific surface area and pore volume of mesoporous single-atom Pt1/TiO2 microspheres reach 179 m2/g and 0.39 cm3/g, respectively. The synthesis reaction is spatially confined in microdrops via aerosol spray. The obtained catalysts present superior activity for CO oxidation at low temperature. Typically, T100 (Temperature required to reach 100% conversion) of CO conversion (1.0% CO + 1.0% O2) is achieved at 120 °C with 0.5 at% Pt1/TiO2 catalysts, 50–60 °C lower than that with the same Pt loaded non-mesoporous catalysts from conventional photo-deposition and immersion-deposition of Pt on commercial TiO2 nanoparticles. Thus, this work paves a way to access efficient single-atom catalysts for CO oxidation.

Size-Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at LowTemperature and the Consequences for CO Oxidation Activity.

Understanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al2O3using in situ/operando spectroscopy and ex situ aberration-corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperatureand the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Size‐Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at Low Temperature and the Consequences for CO Oxidation Activity

AbstractUnderstanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al2O3 using in situ/operando spectroscopy and ex situ aberration‐corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperature and the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Investigating the impact of dynamic structural changes of Au/rutile catalysts on the catalytic activity of CO oxidation

AbstractThe surface properties of oxidic supports and their interaction with the supported metals play critical roles in governing the catalytic activities of oxide‐supported metal catalysts. When metals are supported on reducible oxides, dynamic surface reconstruction phenomena, including strong metal–support interaction (SMSI) and oxygen vacancy formation, complicate the determination of the structural–functional relationship at the active sites. Here, we performed a systematic investigation of the dynamic behavior of Au nanocatalysts supported on flame‐synthesized TiO2, which takes predominantly a rutile phase, using CO oxidation above room temperature as a probe reaction. Our analysis conclusively elucidated a negative correlation between the catalytic activity of Au/TiO2 and the oxygen vacancy at the Au/TiO2 interface. Although the reversible formation and retracting of SMSI overlayers have been ubiquitously observed on Au/TiO2 samples, the catalytic consequence of SMSI remains inconclusive. Density functional theory suggests that the electron transfer from TiO2 to Au is correlated to the presence of the interfacial oxygen vacancies, retarding the catalytic activation of CO oxidation.

Unveiling the electrochemical CO oxidation activity on support-free porous PdM (M = Fe, Co, Ni) foam-like nanocrystals over a wide pH range

Binary PdM-based nanocrystals are efficient electrocatalysts for a wide range of renewable and green fuel cell applications; however, their poisoning by CO is a great barrier for commercialization, so it is pivotal to solve this issue. Herein, we fabricated support-free PdM alloys (M = Fe, Co, and Ni) by a prompt one-step aqueous-solution co-reduction with sodium borohydride driven by the coalescence growth mechanism. This forms support-free PdM porous foam-like nanostructures with well-defined compositions from the ICP-OES analysis and clean surface without any hazardous chemicals or multiple reaction steps. The as-prepared PdM foam-like nanostructures were demonstrated for carbon monoxide oxidation (COOxid) electrocatalysis at varied electrolyte pH compared with commercial Pd/C catalyst. The foam-like PdFe nanocrystals achieved an excellent electrochemical COOxid activity that was at least 2.18-times of PdCo, 4.35-times of PdNi, and 1.56-times of Pd/C in both alkaline (KOH) and acidic (HClO4) electrolytes, but PdCo was the best in neutral (NaHCO3) medium. The superior activity of PdM is due to the strain and alloying effect, which promoted the superb CO oxidation durability for 1000 cycles than Pd/C catalyst. This study demonstrated the superiority of support-free bimetallic PdM alloys than Pd/C catalyst in all electrolytes, which may open new gates for understanding CO-poisoning in alcohol-based fuel cells.

A comparison of bifunctional MnOx catalysts prepared via different precipitants for simultaneous removal NO and CO

In this work, ammonium carbonate, sodium hydroxide, and ammonium hydroxide were selected as the precipitants to prepare bifunctional MnOx catalysts via the PEG-assisted co-precipitation method. SEM analysis revealed that MnOx catalysts prepared with ammonium carbonate (AC) displayed a spherical morphology with relatively large size, whereas the catalysts synthesized with sodium hydroxide (SH) and ammonium hydroxide (AH) exhibited polyhedron shapes with smaller size. The results demonstrated that Mn-AC catalyst presented superior catalytic activity for both NO reduction and CO oxidation, achieving 89% NO conversion at 100 ºC and over 80% CO conversion at 200 ºC, outperforming Mn-SH and Mn-AH catalysts. Mn-AC catalyst possessed more abundant Mn4+ species and surface chemisorbed oxygen than Mn-AH and Mn-SH catalysts, which served as a significant motivation for the difference in catalytic activity. Additionally, Mn-AC catalyst exhibited stronger surface acidity, which facilitated the adsorption and activation of ammonia species in the NH3-SCR reaction. Furthermore, in situ DRIFTS experiment suggested that the pre-desorption of CO promoted the adsorption and activation of NO. On this basis, the possible mechanism model of three precipitants on bifunctional MnOx catalysts was proposed.

A-site cations tailoring the activity of LnMnO3 perovskites for CO and propane oxidation

To understand the role of A-site cations of perovskites on the reactivity, the LnMnO3 perovskites (Ln = La, Nd, Sm) with varied A-site cations were prepared by a facile glycine combustion method and evaluated for CO and propane oxidation. The activity for both reactions followed the trend: NdMnO3 > SmMnO3 > LaMnO3, demonstrating that lanthanide cations at A sites significantly affect the catalytic performance. Combined with Fourier transmission infrared spectra, temperature programmed reduction and desorption techniques, it was revealed that both the strength of Mn–O bonds and the adsorption of reactants played crucial roles on the activity for CO oxidation and C3H8 combustion on the LnMnO3. The adsorption of the reactants was responsible for the higher activity on the NdMnO3 than on the SmMnO3, while the strength of Mn–O bonds for the higher activity on the SmMnO3 than on the LaMnO3. The NdMnO3 possessed both weak Mn–O bonds and favorable adsorption of the reactants achieved the highest activity for CO oxidation and C3H8 combustion. The best NdMnO3 catalyst achieved CO and propane complete conversion at 160 and 360 °C, 90 and 60 °C lower than the LaMnO3, respectively.

CuGaO2 delafossite as a high-surface area model catalyst for Cu+-activated reactions

Copper is one of the goldilocks metals in catalysis. Deciphering the local atomic environment and oxidation state of active centers in supported copper catalysts, as well as the design of materials to control their stability under reaction conditions remains a great challenge today. We show here that a mixed-oxide of copper delafossite with gallium (CuGaO2, Cu1+ and Ga3+) in the form of porous nanoplates, mainly exposing (110) facets with Cu1+ and Ga3+ cations in well-defined surface positions is a promising material as a model catalyst to explore the activity and stability of Cu1+-activated reactions. It is shown that the delafossite structure is preserved after calcination or reduction pretreatments (in pure O2 or H2, respectively). Extensive reduction of the mixed-oxide leads to a stable catalyst with higher activity for CO oxidation, as a result of the formation of geminal dicarbonyl species on Cuδ+, with 0 < δ < 1, under reaction conditions.

Insights into the underpinning effect of graphene in Cu1Mn10 on enhancing the low-temperature catalytic activity for CO oxidation

CO emission is a critical issue of industrial processes such as steel-smelting, cement manufacturing, and waste incineration. Catalytic oxidation based on Cu–Mn binary catalysts shows great potential for efficient removal of CO, whereas their practical applicability is limited by the inferior low-temperature catalytic activity and the high catalyst cost owing to a substantial quantity of Cu. In this study, doping graphene is designed to adjust the electron transfer capability to improve the low-temperature catalytic activity as well as reduce the amount of Cu, and thereby Cu1Mn10 catalysts doped with slight amounts of graphene (x%G-Cu1Mn10, x is 1∼5) were fabricated. It was found that the introduction of graphene could form effective electron transport channels to enhance the intermetallic interaction and oxygen vacancy generation, thus improving the low-temperature catalytic performance of the Cu1Mn10 catalyst. Among all the catalysts, 4%G-Cu1Mn10 exhibited the highest activity, achieving CO conversion of 92% at 110 °C at a weight hourly space velocity of 120,000 mL/(g∙h). The introduction of graphene also enabled the catalyst with excellent catalytic activity and stability at a relative humidity of 70%. Attractively, 4%G-Cu1Mn10 can be further loaded into the polyester fabric, presenting great application potentials in the effective elimination of CO during the dust removal process since the flue gas temperature in the dust collector is just around the T90% and the catalyst that is inside of fabric fiber rather than on the fabric surface can be rarely influenced by the dust. In general, doping graphene provides a facile method to enhance the low-temperature activities of the Cu–Mn binary catalysts and cut down the use of valuable Cu, showing great application potential.

CO oxidation under lean and stoichiometric conditions over ceria-zirconia with very low metal contents (Cu, Co, Ag and Pt)

In this work, several ceria-zirconia based catalysts with very low (and equimolar) metal contents were prepared, characterised and tested for the CO oxidation reaction (under lean and stoichiometric conditions), trying to emulate those conditions found in a diesel oxidation catalyst (DOC) system from a diesel engine and those encountered under gasoline exhaust (λ = 1). The metals chosen are Cu, Co, Ag and Pt (as a reliable benchmark). The results reveal enormous differences among reducibility and catalytic activity despite quite similar structural and textural properties of the catalysts, showing differences among dispersion (Ag-catalyst seems to present a low level of dispersion). This catalyst seems to be characterised, as well, by a strong electronic interaction between Ce and Ag centres which is suggested to yield an improved reducibility under H2-TPR conditions. Nevertheless, the order in catalytic activity (Cu > Ag > Co ≈ Pt >> support) seems not to follow the order found in reducibility and the Cu-catalyst seems to be the most active independently on the reaction conditions, yielding nearly overlapped CO oxidation catalytic curves. Interestingly, a strong correlation between the catalytic activity under the two conditions tested and the OSC values of the Ce0.8Zr0.2O2-supported metal catalysts is found. Therefore, OSC parameter measured at 150 °C can be used as a relevant descriptor to evaluate the CO oxidation activity at low and medium conversions for the investigated catalysts, much better than the H2-TPR measurements.

From Basic Research on Supported-Metal Catalysts to their Practical Applications for Reduction of Food Waste

Abstract Supported metal catalysts are one of the most important heterogeneous catalysts, and they are widely used in chemical processes such as naphtha reforming, purification of exhaust gas from automobiles and so on. However, rational design of supported metal catalysts is still a target in heterogeneous catalysis. The author had a hypothesis that metal nanoparticles in ordered pores of zeolite or mesoporous silica may exhibit novel catalytic properties. Based on this idea, we used micropores of zeolites to accommodate metal carbonyl clusters, and then mesoporous silica to prepare metal nanoparticles. First, we studied ship-in-bottle synthesis of Rh carbonyl clusters in zeolite and its reactivity. Rh6(CO)16 was synthesized in NaY zeolite accompanied with formation of a mononuclear Rh(CO)2, which acted as a carrier of 13CO in 12CO-13CO exchange reaction. Then we worked on synthesis of platinum nanoparticles in mesoporous silica, and found that high activity in preferential oxidation of CO in hydrogen and in low-temperature oxidation of ethylene. The high catalytic performance in ethylene oxidation has led to practical application of platinum/silica catalysts for preservation of fruits and vegetables, resulting in reduction of food waste. This article describes the development process of this research.

Spatial confinement: An effective strategy to improve H2O and SO2 resistance of the expandable graphite-modified TiO2-supported Pt nanocatalysts for CO oxidation

The expandable graphite (EG) modified TiO2 nanocomposites were prepared by the high shear method using the TiO2 nanoparticles (NPs) and EG as precursors, in which the amount of EG doped in TiO2 was 10 wt.%. Followed by the impregnation method, adjusting the pH of the solution to 10, and using the electrostatic adsorption to achieve spatial confinement, the Pt elements were mainly distributed on the exposed TiO2, thus generating the Pt/10EG-TiO2–10 catalyst. The best CO oxidation activity with the excellent resistance to H2O and SO2 was obtained over the Pt/10EG-TiO2–10 catalyst: CO conversion after 36 hr of the reaction was ca. 85% under the harsh condition of 10 vol.% H2O and 100 ppm SO2 at a high gaseous hourly space velocity (GHSV) of 400,000 hr−1. Physicochemical properties of the catalysts were characterized by various techniques. The results showed that the electrostatic adsorption, which riveted the Pt elements mainly on the exposed TiO2 of the support surface, reduced the dispersion of Pt NPs on EG and achieved the effective dispersion of Pt NPs, hence significantly improving CO oxidation activity over the Pt/10EG-TiO2–10 catalyst. The 10 wt.% EG doped in TiO2 caused the TiO2 support to form a more hydrophobic surface, which reduced the adsorption of H2O and SO2 on the catalyst, greatly inhibited deposition of the TiOSO4 and formation of the PtSO4 species as well as suppressed the oxidation of SO2, thus resulting in an improvement in the resistance to H2O and SO2 of the Pt/10EG-TiO2–10 catalyst.

Important factors of the A-site deficient Mn perovskites design affecting the CO oxidation activity

A-site deficient La0.4Sr0.4MnxTi1−xO3 (LSMT, x = 0, 0.4, 0.6, 0.8) perovskites were investigated for their structural change and effect on CO oxidation activity. The XRD, XPS, TPR and O2-TPD analysis represent the change in Mn concentration, known as an active species in oxidation, affecting the crystal structure, oxidation state of the B-site cation (Mn), number of oxygen vacancies, and the reducibility of the catalysts. The shift in the main perovskite diffraction peak in XRD spectra of the perovskites with a change in Mn content represents the formation of cationic vacancies. At the lower temperature (<200 oC), CO oxidation efficiency of LSMT perovskite catalysts was found to be almost identical. This can be attributed to their similar perovskite structure and surface area. Compared to the higher temperature (>200 oC) CO oxidation activity, the LSMT catalyst with lower Mn content of 0.4 shows relatively better performance than the higher Mn content samples, which signifies the active role of the Mn oxidation state and lattice oxygen species of the perovskite catalysts. Moreover, the LSMT4446 catalyst shows stable activity for more than 12 h with an average CO conversion of 80 %. From these results, we found that the factors of the Mn oxidation state and the type of oxygen species are crucial when designing an Mn-based perovskite catalyst for catalytic applications.

Effect of dopants on soot oxidation over doped Ag/ZrO2 catalysts for catalyzed gasoline particulate filter

Soot oxidation was studied using silver-zirconia catalysts doped with Y, La, Ce and Pr under a low concentration of O2, typical for gasoline direct injection engine conditions, in view of practical gasoline particulate filter application. The Ag/La-ZrO2 catalyst revealed the best catalyst performance for soot oxidation exhibiting a Tmax of 405 °C for a fresh sample and 420 °C after aging at 700 °C for 6 h. Additionally, the doped catalysts exhibited improved activity for co-oxidation of CO and C3H6. The Ag/ZrO2 catalyst benefited from doping via the synergy between silver particles and doped zirconia.

Enhanced methane combustion and CO oxidation with Pd and Pt substitution LaSrCuO4 perovskites

The LaSrCuO4 as parent perovskite purely lattice was synthesized by new presented solid states technique and Pd and Pt substitution in LaSrCuO4 catalysts instead of Cu were prepared in this method at low temperature. All the perovskites were investigated by X-ray Diffraction, HRTEM, O2-TPD, H2-TPR, BET and XPS analyses. The CO oxidation and methane combustion catalytic activity of prepared perovskites were investigated which the LaSrCuO4 catalytic performance noticeably enhanced via Pd and Pt substitution instead of Cu. Both Langmuir and intra-facial mechanisms were intensified with increasing of surface area and mobility of lattice oxygen due to Pd and Pt doping in CO oxidation and methane combustion reactions. Pd 3d3/2 and 3d5/2 XPS spectra in LaSrCu0.9Pd0.1O4 confirmed Pd incorporation in perovskite structure due to presence of high oxidation state of Pd3+ or Pd4+. Pt 4 f5/2 and 4 f7/2 peaks approved Pt4+ oxidation state in LaSrCu1−xPtxO4 (x = 0.1,0.2) catalysts. The H2-TPR profiles of the Pt and Pd doped LSC perovskites revealed that the reduction peaks of the Cu were shifted to lower temperatures by increasing in Pd and Pt partial substitution due to H2-spillover effect of palladium and platinum.

Microwave-assisted CO oxidation over LaNiO3 and Ce-promoted LaNiO3

Microwave-assisted catalytic reactions have been investigated for energy-efficient chemical processes. However, catalysts with comparatively high heating properties, activities, and economics have been developed with limited success. In this study, we investigated the catalytic properties of Ce addition to LaNiO3 catalyst, which has high CO oxidation activity and high microwave heating properties. Ce-added catalysts exhibited high heating properties under microwave heating. Increasing the Ce content in the catalyst enhanced the dielectric loss factor, the ability to absorb microwaves and convert them to heat, and improved the microwave heating properties of the catalyst. The catalytic activity toward CO oxidation over Ce-added catalysts decreased in the order of La0.8Ce0.2NiO3 > La0.9Ce0.1NiO3 > LaNiO3 at all oxygen partial pressures when compared to the same microwave power. To clarify the role of Ce addition, temperature programmed reduction (TPR) by H2, CO, and XAFS measurements were performed. TPR results suggested that Ce promotes oxygen mobility and then affects both CO oxidation and microwave heating properties.

Catalytic behavior of Pt single-atoms supported on CeO2

In this study, we demonstrate that the CO oxidation activity of Pt/CeO2 single-atom catalysts (≤0.4 Pt/nm2) is significantly low despite the increase in reducibility, which is associated with the formation of oxygen vacancies that are critical for oxygen activation, with increasing Pt surface density. This result can be related to the negligible amount of CO adsorbed onto the Pt/CeO2 single-atom catalysts. As the Pt surface density increases to 0.8 Pt/nm2, the activity sharply increases; at this loading, Pt clusters are formed and the interactions with CO are enhanced. Notably, after the controlled reduction treatment using CO, the catalytic activity of 0.4 Pt/CeO2 increases to the level of 0.8 Pt/CeO2. The sudden increase in activity can be explained by the formation of a partially reduced Pt cluster and the enhanced CO interactions with the Pt atoms of the cluster. These results indicate that Pt cluster formation and its partial reduction are essential for low-temperature CO oxidation. Our study explains the cause of the significantly low CO oxidation activity of Pt/CeO2 single-atom catalysts and how controlled reduction treatment of these catalysts enhances their activity.