Insights into the Redox and Structural Properties of CoOx and MnOx: Fundamental Factors Affecting the Catalytic Performance in the Oxidation Process of VOCs

Volatile organic compound (VOC) abatement has become imperative nowadays due to their harmful effect on human health and on the environment. Catalytic oxidation has appeared as an innovative and promising approach, as the pollutants can be totally oxidized at moderate operating temperatures under 500 °C. The most active single oxides in the total oxidation of hydrocarbons have been shown to be manganese and cobalt oxides. The main factors affecting the catalytic performances of several metal-oxide catalysts, including CoOx and MnOx, in relation to the total oxidation of hydrocarbons have been reviewed. The influence of these factors is directly related to the Mars–van Krevelen mechanism, which is known to be applied in the case of the oxidation of VOCs in general and hydrocarbons in particular, using transitional metal oxides as catalysts. The catalytic behaviors of the studied oxides could be closely related to their redox properties, their nonstoichiometric, defective structure, and their lattice oxygen mobility. The control of the structural and textural properties of the studied metal oxides, such as specific surface area and specific morphology, plays an important role in catalytic applications. A fundamental challenge in the development of efficient and low-cost catalysts is to choose the criteria for selecting them. Therefore, this research could be useful for tailoring advanced and high-performance catalysts for the total oxidation of VOCs.

Flower-like Co3O4 Catalysts for Efficient Catalytic Oxidation of Multi-Pollutants from Diesel Exhaust

Nowadays, the oxidation activity at the low-temperature regime for Co3O4 catalysts needs to be improved to meet the stringent regulation of multi-pollutant diesel exhaust. Herein, nanoflower-like Co3O4 diesel oxide catalysts (DOCs) were fabricated with the addition of a low-content Pt to trigger better catalytic activities for oxidizing multi-pollutants (CO, C3H6, and NO) emissions by taking advantage of the strong-metal supporting interaction. Compared to the conventional DOCs based on Pt/Al2O3, the as-synthesized Pt/Co3O4 catalysts not only exhibited better multi-pollutants oxidation activities at the low temperature but also obtained better resistance toward NO inhibition. Moreover, Pt/Co3O4 catalysts showed exceptional hydrothermal durability throughout long-term tests in the presence of water vapor. According to the XPS and H2-TPR results, Pt promoted low-temperature catalytic activity by increasing the active surface oxygen species and reducibility due to the robust synergistic interaction between metallic Pt and supporting Co3O4. Meanwhile, TGA curves confirmed the Pt atoms that facilitated the desorption of surface-active oxygen and hydroxyl radicals in a low-temperature regime. Furthermore, instead of probing the intermediates during CO and C3H6 oxidation for Pt/Co3O4 catalysts, which included carbonates, formate, and acetate species, in situ DRIFTs experiments also revealed C3H6 oxidation mainly took place over metallic Pt sites.

Oxidative Dehydrogenation of 1-Butene to 1,3-Butadiene Over Metal Ferrite Catalysts

The oxidative dehydrogenation (ODH) of 1-butene to 1,3-butadiene was studied over a series of AFe2O4 catalysts, where A = Zn, Mn, Ni, Cu, Mg and Fe. The catalysts were characterised by XPS, EPR spectroscopy, BET surface area analysis, Raman spectroscopy and XRD. All the ferrites were active for ODH and gave an order of activity after 80 h on-stream of ZnFe2O4 > NiFe2O4 > MnFe2O4 > MgFe2O4 > CuFe2O4 > FeFe2O4. All catalysts lost significant surface area (up to ~ 80%) under reaction conditions of 0.75:1:15 oxygen:1-butene:steam with an overall GHSV of 10,050 h−1 at 693 K. Fe3O4 was unstable under reaction conditions and was converted to Fe2O3, which showed very low activity. Nickel ferrite was the only material that gave carbon dioxide as a significant product, all others were selective to 1,3-butadiene. Zinc ferrite gave a steady-state yield of 1,3-butadiene of ~ 80%. Inversion parameters were determined for the ferrites from XPS and a correlation was obtained between 1,3-butadiene yield and inversion parameter, indicating that Fe3+ in an octahedral hole is a key species in the mechanism of oxidative dehydrogenation. Butene isomerisation and ODH were shown to occur on different sites.

Highly Active Co3O4-Based Catalysts for Total Oxidation of Light C1-C3 Alkanes Prepared by a Simple Soft Chemistry Method: Effect of the Heat-Treatment Temperature and Mixture of Alkanes

In the present work, a simple soft chemistry method was employed to prepare cobalt mixed oxide (Co₃O₄) materials, which have shown remarkably high activity in the heterogeneously catalyzed total oxidation of low reactive VOCs such as the light alkanes propane, ethane, and methane. The optimal heat-treatment temperature of the catalysts was shown to depend on the reactivity of the alkane studied. The catalytic activity of the Co₃O₄ catalysts was found to be as high as that of the most effective catalysts based on noble metals. The physicochemical properties, from either the bulk (using XRD, TPR, TPD-O₂, and TEM) or the surface (using XPS), of the catalysts were investigated to correlate the properties with the catalytic performance in the total oxidation of VOCs. The presence of S1 low-coordinated oxygen species at the near surface of the Co₃O₄-based catalysts appeared to be linked with the higher reducibility of the catalysts and, consequently, with the higher catalytic activity, not only per mass of catalyst but also per surface area (enhanced areal rate). The co-presence of propane and methane in the feed at low reaction temperatures did not negatively affect the propane reactivity. However, the co-presence of propane and methane in the feed at higher reaction temperatures negatively affected the methane reactivity.

Dynamics of Reactive Oxygen Species on Cobalt-Containing Spinel Oxides in Cyclic CO Oxidation

Reactive oxygen species (ROS) are considered to be responsible for the high catalytic activity of transition metal oxides like Co3-xFexO4 in oxidation reactions, but the detailed influences of catalyst composition and morphology on the formation of these reactive oxygen species are not fully understood. In the presented study, Co3O4 spinels of different mesostructures, i.e., particle size, crystallinity, and specific surface area, are characterized by powder X-ray diffraction, scanning electron microscopy, and physisorption. The materials were tested in CO oxidation performed in consecutive runs and compared to a Co3-xFexO4 composition series with a similar mesostructure to study the effects of catalyst morphology and composition on ROS formation. In the first run, the CO conversion was observed to be dominated by the exposed surface area for the pure Co-spinels, while a negative effect of Fe content in the spinels was seen. In the following oxidation run, a U-shaped conversion curve was observed for materials with high surface area, which indicated the in situ formation of ROS on those materials that were responsible for the new activity at low temperature. This activation was not stable at the higher reaction temperature but was confirmed after temperature-programmed oxidation (TPO). However, no activation after the first run was observed for low-surface-area and highly crystalline materials, and the lowest surface-area material was not even activated after TPO. Among the catalyst series studied here, a correlation of small particle size and large surface area with the ability for ROS formation is presented, and the benefit of a nanoscaled catalyst is discussed. Despite the generally negative effect of Fe, the highest relative activation was observed at intermediate Fe contents suggesting that Fe may be involved in ROS formation.

Co–Ce Oxides Supported on SBA-15 for VOCs Oxidation

Reported here are new data on the structural and catalytic properties of a series of mono-component cobalt and bi-component Co–Ce catalysts supported on SBA-15 (Santa Barbara Amorphous-15)). The catalysts performance has been evaluated by tests on combustion of methane, propane, and n-hexane. It was established that the preparation of the Co–Ce catalysts by the ‘two-solvent’ technique does not significantly change the mesoporous structure, however, its pores are clogging with the Co and Ce guest species. Cobalt and cerium are uniformly distributed and preferentially fill up the channels of SBA-15, but oxide agglomerates located on the surface are observed as well. The highest activity of the mono-component cobalt sample is explained by its higher reducibility as a result of lower interaction of the cobalt oxide with the SBA-15. The fine dispersion of cobalt and cerium oxide and their strong interaction in the channels of the SBA-15 molecular sieve, leads to the formation of difficult-to-reduce oxide phases and, consequently, to lower catalytic activity compared to monocomponent cobalt oxide catalyst. The synthesised mesoporous structure can prevent the agglomeration of the oxide particles, thus leading to the successful development of a new and stable catalyst for decreasing greenhouse gas emissions.

Photocatalytic Oxidation of Propane Using Hydrothermally Prepared Anatase-Brookite-Rutile TiO2 Samples. An In Situ DRIFTS Study

Photocatalytic oxidation of propane using hydrothermally synthesized TiO₂ samples with similar primary crystal size containing different ratios of anatase, brookite and rutile phases has been studied by measuring light-induced propane conversion and in situ DRIFTS (diffuse reflectance Fourier transform infrared spectroscopy). Propane was found to adsorb on the photocatalysts, both in the absence and presence of light. The extent of adsorption depends on the phase composition of synthesized titania powders and, in general, it decreases with increasing rutile and brookite content. Still, the intrinsic activity for photocatalytic decomposition of propane is higher for photocatalysts with lower ability for propane adsorption, suggesting this is not the rate-limiting step. In situ DRIFTS analysis shows that bands related to adsorbed acetone, formate and bicarbonate species appear on the surface of the photocatalysts during illumination. Correlation of propane conversion and infrared (IR) data shows that the presence of formate and bicarbonate species, in excess with respect to acetone, is composition dependent, and results in relatively low activity of the respective TiO₂. This study highlights the need for precise control of the phase composition to optimize rates in the photocatalytic oxidation of propane and a high rutile content seems to be favorable.

Operando Reactor-Cell with Simultaneous Transmission FTIR and Raman Characterization (IRRaman) for the Study of Gas-Phase Reactions with Solid Catalysts

Raman and transmission FTIR spectroscopic techniques have been coupled in a new homemade reactor-cell designed in a joint CSIC-LCS collaboration. The setup is easily adapted to any FTIR and fiber-coupled Raman spectrometers and gas analysis techniques. It allows for simultaneous operando FTIR and Raman spectroscopic measurement, which provide complementary characterization of adsorbed species, reaction intermediates, and structural properties of the catalyst. This system was validated with the study of vanadium-based catalysts during propane oxydehydrogenation (ODH). The combined use of both spectroscopies with gas analysis techniques to measure the activity contributes to the understanding of propane ODH and the identification of the role of different oxygen species bound to vanadium sites. For example, the simultaneous characterization of the catalyst under the same conditions by IR and Raman confirms that the V═O mode has the same frequency in both spectroscopies and that bridging oxygen sites (V-O-V, V-O-Zr) present higher activity than terminal V═O bonds. These results demonstrate the high potential of the new simultaneous transmission IR-Raman operando rig to correlate the activity and the structure of catalysts, thus assisting the rational design of catalytic processes.

Effect of Ce and La dopants in Co3O4 nanorods on the catalytic activity of CO and C3H6 oxidation

The catalytic activity is enhanced by Ce but inhibited by La dopant. The catalysts have been characterized in light of structural properties, reducibility, mobility of adsorbed oxygen and lattice oxygen, and surface reaction intermediates.

Investigation of Reaction Mechanism of NO-C3H6-CO-O2 Reaction over NiFe2O4 Catalyst

To elucidate the reaction mechanism of NO-C₃H₆-CO-O₂ over NiFe₂O₄, we investigated the dynamics of the adsorbed and gaseous species during the reaction using operando Fourier transform infrared (FTIR). The NO reduction activity dependent on the C₃H₆ and CO concentrations suggested that NO is reduced by C₃H₆ under three-way catalytic conditions. From FTIR measurements and kinetic analysis, it was clarified that the acetate species reacted with NO-O₂ to form N₂ via NCO, and that the rate-limiting step of NO reduction was the reaction between CH₃COO^- and NO-O₂. The NO reduction mechanism of the three-way catalyst on NiFe₂O₄ is different to that on platinum-group metal catalysts, on which NO reduction proceeds through N-O cleavage.

Insights into the solar light driven thermocatalytic oxidation of VOCs over tunnel structured manganese oxides

Tunnel structured manganese oxides with high photothermal performance were prepared, and their light driven thermocatalytic activity was carefully investigated.

Oxidation of chlorinated alkanes over Co3O4/SBA-15 catalysts. Structural characterization and reaction mechanism

This work presents novel results on the application of Co₃O₄/SBA-15 catalysts for the oxidation of chlorinated hydrocarbons.

Scalable Synthesis of Efficient Water Oxidation Catalysts: Insights into the Activity of Flame-Made Manganese Oxide Nanocrystals

Chemical energy storage by water splitting is a promising solution for the utilization of renewable energy in numerous currently impracticable needs, such as transportation and high temperature processing. Here, the synthesis of efficient ultra-fine Mn3O4 water oxidation catalysts with tunable specific surface area is demonstrated by a scalable one-step flame-synthesis process. The water oxidation performance of these flame-made structures is compared with pure Mn2O3 and Mn5O8, obtained by post-calcination of as-prepared Mn3O4 (115 m(2)  g(-1)), and commercial iso-structural polymorphs, probing the effect of the manganese oxidation state and synthetic route. The structural properties of the manganese oxide nanoparticles were investigated by XRD, FTIR, high-resolution TEM, and XPS. It is found that these flame-made nanostructures have substantially higher activity, reaching up to 350 % higher surface-specific turnover frequency (0.07 μmolO2  m(-2)  s(-1)) than commercial nanocrystals (0.02 μmolO2  m(-2)  s(-1)), and production of up to 0.33 mmolO2  molMn (-1)  s(-1). Electrochemical characterization confirmed the high water oxidation activity of these catalysts with an initial current density of 10 mA cm(-2) achieved with overpotentials between 0.35 and 0.50 V in 1 m NaOH electrolyte.

Thermal evolution of cobalt deposits on Co3O4(111): atomically dispersed cobalt, two-dimensional CoO islands, and metallic Co nanoparticles

Cobalt deposition onto Co₃O₄(111) leads to formation of atomically dispersed cobalt species, which form ordered two-dimensional oxide islands upon annealing.

Co3O4Catalysts on CeO2-ZrO2Supports and Co3O4-CeO2Catalysts on Al2O3/SiO2Supports for the Oxidation of Propylene

Different compositions of Co3O4catalysts on CeO2-ZrO2solid solution (Ce0.9Zr0.1O2and Ce0.8Zr0.2O2) have been studied for the oxidation of propylene. The optional amount of Co3O4active phase on CeO2-ZrO2support of 30 wt% was found. The mixed Co3O4-CeO2-ZrO2with the same composition of the optimal supported ones showed approximately the same activity, which was not higher than the activity of the mixed Co3O4-CeO2catalyst. Catalytic activities of mixed Co3O4-CeO2with different loading contents supported on high surface area supports (Al2O3, SiO2) were then measured. The optimal composition of active phase was still 30 wt% but the minimum temperature of the highest activity increased to above 300°C due to the inert nature influence of the support.

TAP study of toluene total oxidation over a Co3O4/La-CeO2 catalyst with an application as a washcoat of cordierite honeycomb monoliths

The total oxidation of toluene was studied over a Co₃O₄/La-CeO₂ catalyst in a Temporal Analysis of Products (TAP) set-up in the temperature range 713 K to 873 K in the presence and absence of dioxygen.