Cobalt(III) Acetylacetonate Chemisorbed on Aluminum-Nitride-Modified Silica: Characteristics and Hydroformylation Activity

Co/AlN/SiO₂ catalysts were prepared by the saturative chemisorption of cobalt(III) acetylacetonate (Co(acac)₃). The support was bare silica or silica that had been modified with aluminum nitride (AlN) by repeated separate, saturated chemisorptions of trimethylaluminum and ammonia two or six times. Chemisorption of Co(acac)₃ occurred on all the supports up to a saturation ligand density of 2.7 acac nm^-2; the amount of bonded cobalt decreased from 2.1 to 1.5 at_Co nm^-2 with increasing extent of AlN modification of the support. Ligand exchange reaction, releasing Hacac, occurred less on AlN-modified silica than on bare silica. This induced difference in the reduction behavior of the catalysts, and catalytic activity in gas-phase hydroformylation of ethene, was lower with Co/AlN/SiO₂ than with Co/SiO₂ catalyst.     

The catalytic removal of CO and NO over Co-Pt(Pd, Rh)/γ-Al2O3 catalysts and their structural characterizations

A series of Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C₂H₄ over the samples calcined at 500°C and reduced at 450°C were determined. The activities of the samples calcined at 750°C and reduced at 450°C for NO selective catalytic reduction (SCR) by C₂H₄ were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/γ-Al₂O₃ for CO or C₂H₄ oxidation. For NO decomposition, the H₂-reduced Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C₂H₄, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/γ-Al₂O₃ has been reduced to zero valence during reduction by H₂ at 450°C, but in Co/γ-Al₂O₃ only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl₂O₄-like spinel which is difficult to reduce. For the samples calcined at 750°C, the cobalt exists as CoAl₂O₄ which cannot be reduced by H₂ at 450°C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/γ- Al₂O₃ has lower coordination symmetry than that in Co-Pt(Pd, Rh)/γ-Al₂O₃. This difference mainly results from the distorting tetrahedrally- coordinated Co²⁺ ions which have lower coordination symmetry than Co⁰ in the catalysts. The coordination number for the Co-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts. The TPR results indicate that the addition of noble metals to Co/γ- Al₂O₃ makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co⁰ is the main active phase. For NO decomposition and selective reduction, Co⁰ is also catalytically active, but it can be oxidized into Co₃O₄ by oxygen at high reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of cobalt precursors on NO oxidation over supported cobalt oxide catalysts

The effect of cobalt precursors such as cobalt acetate and cobalt nitrate on NO oxidation was examined over cobalt oxides supported on various supports such as SiO₂, ZrO₂, and CeO₂. The N₂ physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction with H₂ (H₂-TPR), NO chemisorptions, and temperature-programmed oxidation (TPO) with mass spectroscopy were conducted to characterize catalysts. The NO uptake as well as the catalytic activity for NO oxidation was dependent on the kinds of cobalt precursors and supports for supported cobalt oxides catalysts. Among tested catalysts, Co₃O₄/CeO₂ prepared from cobalt acetate showed the highest catalytic activity. The catalytic activity generally increased with the amount of chemisorbed NO. Reversible deactivation was observed over Co₃O₄/CeO₂ in the presence of H₂O. On the other hand, irreversible deactivation occurred over the same catalyst even in the presence of 5 ppm SO₂ in a feed. The strongly adsorbed SO₂ can prohibit NO from adsorbing on the active sites and also can prevent formed NO₂ from desorbing off the catalyst surface. The formation of SO₃ cannot be observed from the chemisorbed SO₂ on Co₃O₄/CeO₂ during TPO.     

Alumina and Alumina–Baria Supported Cobalt Catalysts for DeNO x : Influence of the Support and Cobalt Content on the Catalytic Performance

Co/Al₂O₃ and Co/Al₂O₃–BaO catalysts with low cobalt loading (0.1, 0.3 and 1 wt%) for the selective catalytic reduction (SCR) of NO _x by C₃H₆ were prepared. The distribution of cobalt species was investigated by UV–vis diffuse reflectance spectroscopy and by H₂-TPR in order to identify the active cobalt species in hydrocarbons (HC)-selective catalytic reduction (SCR). It was found that the nature of cobalt species strongly depends on the cobalt loading as well as on the properties of the support. The barium addition to the alumina slows down solid state diffusion processes, improving the thermal stability of the support and preventing diffusion of cobalt into the bulk. Highly dispersed surface Co²⁺ species over alumina were identified as active sites in the NO-SCR process. Accordingly, a high concentration of surface Co²⁺ sites in Co 1 wt%/Al₂O₃ improves the catalytic performance in NO-SCR, the long term stability as well as the water tolerance. On the contrary, the formation of Co₃O₄ particles in Co 1 wt%/Al₂O₃–BaO promotes the propylene oxidation by oxygen, decreasing the activity and selectivity of the catalyst in NO reduction.     

Silica-Supported Cobalt Catalysts for Fischer–Tropsch Synthesis: Effects of Calcination Temperature and Support Surface Area on Cobalt Silicate Formation

Cobalt silicate formation reduces the activity of the catalyst in Fischer–Tropsch synthesis (FTS). In this article, the effects of calcination temperature and support surface area on the formation of cobalt silicate are explored. FTS catalysts were prepared by incipient wetness impregnation of cobalt nitrate precursor into various silica supports. Deionized water was used as preparation medium. The properties of catalysts were characterized at different stages using FTIR, XRD and BET techniques. FTIR-ATR analysis of the synthesized catalyst samples before and after 48 h reaction identified cobalt species formed during the impregnation/calcination stage and after the reduction/reaction stage. It was found that in the reduction/reaction stage, metal-support interaction (MSI) added to the formation of irreducible cobalt silicate phase. Co/silica catalysts with lower surface area (300 m²/g) exhibited higher C₅₊ selectivity which can be attributed to less MSI and higher reducibility and dispersion. The prepared catalysts with different drying and calcination temperatures were also compared. Catalysts dried and calcined at lower temperatures exhibited higher activity and lower cobalt silicate formation. The catalyst sample calcined at 573 K showed the highest CO conversion and the lowest CH₄ selectivity.     

The influence of the initial acidity of HFER on the status of Co species and catalytic performance of CoFER and InCoFER in CH4-SCR-NO

The role of the initial acidity of ferrierite type zeolite on the status of cobalt and the catalytic activity of CoFER and InCoFER was investigated. Two FER zeolites were used: NH₄FER without any pretreatment (FER-1) and the same zeolite, dehydroxylated at 825 K (FER-2). Dehydroxylation removed most of the Si–OH–Al groups, therefore the resulting zeolite revealed practically no ion exchange capacity. The status of cobalt was followed by IR spectroscopy with probe molecules: CO (a probe for Co²⁺) and NO (a probe for Co³⁺). The introduction of cobalt by solid-state ion exchange produced divalent cobalt in exchange positions and in the form of oxide-like clusters, their respective concentration was determined by quantitative IR experiments of CO sorption. The amount of Co³⁺, present in CoFER-1 and InCoFER-1, was also determined. All these forms of cobalt were practically absent from CoFER-2 and InCoFER-2. The NO conversion and selectivity to N₂ of CoFER-2 in CH₄-SCR-NO was poor, indicating the essential role of the initial acidity of the ferrierite matrix on the formation of catalytically active Co species. The introduction of indium into CoFER only slightly increased the NO conversion and shifted the reaction path from NO₂ towards N₂ formation for FER-1, while greatly improved the catalytic performance of the FER-2 series.     

Spectroscopic and catalytic characterization of Co-exchanged mordenites subject to CO/O2 redox treatments

Co-mordenites were prepared by ion exchange and wet impregnation over Na-mordenite and H-mordenite. The prepared solids were calcined and aliquots of these samples were subject to redox treatments first with CO for 2 h at 773 K and then kept 2 h at the same temperature in flowing O₂. A thorough characterization of the solids was carried out by TPR, XPS, CO volumetric adsorption, and FTIR with CO, NO and pyridine adsorption as probe molecules. After the redox treatment, Na based samples showed an important increase in the CO adsorption. TPR and XPS results indicated reduction of the oxides present in the calcined samples and the migration of exchanged cobalt from hidden to more exposed sites was demonstrated by FTIR. The acid catalysts did not change their CO capacity of adsorption; exchanged cobalt ions were mainly in β-type sites and remained in this position after treatment. After the redox treatments, the activity for the selective reduction of NO_x with methane suffered a decrease on both types of mordenites, probably caused by the strong adsorption of NO and the reduction of β-type Co²⁺ to metallic cobalt that would diminish the active sites concentration and also block the channels, thus preventing the access of the reactants.     

Preparation of Fischer–Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation

Homogeneous deposition-precipitation on either a silica or carbon nanofiber (CNF) support of cobalt from basic solution using ammonia evaporation was studied and compared with conventional deposition from an acidic solution using urea hydrolysis. In the low-pH experiment, the interaction between precipitate and silica was too high; cobalt hydrosilicates were formed requiring a reduction temperature of 600 °C, resulting in low cobalt dispersion. Lower interaction in experiments performed in a basic environment yielded a well-dispersed Co₃O₄ phase on silica, and after reduction at only 500 °C, a catalyst with 13-nm cobalt particles was obtained. On CNF from an acidic solution, cobalt hydroxy carbonate precipitated and displayed a low interaction with the support resulting after reduction at 350 °C in a catalyst with 25-nm particles. From basic solution we obtained high dispersion of cobalt on the CNF, probably related to the greater ion adsorption. After drying, Co₃O₄ crystallites were obtained that, after reduction at 350 °C, resulted in a catalyst with 8-nm Co particles. Samples prepared in the high-pH experiment had 2–4 times higher cobalt-specific activity in the Fischer–Tropsch reaction than their low-pH counterparts. CNF support materials combined with the high-pH deposition-precipitation technique hold considerable potential for cobalt-based Fischer–Tropsch catalysis.     

CO-NO/O2 redox reactions over Cu substituted cobalt oxide spinels

Catalytic conversion of NO and CO over Cu substituted cobalt oxide spinels show excellent activity for CO-O₂ and NO-CO reactions. Lower concentration of Cu in cobalt oxide spinel is having an enhancing effect on the catalytic conversion. Best activity among the tested catalyst was found for Co_2.9Cu_0.1O₄ and complete conversion (100%) is observed at 93 °C for CO oxidation by O₂ and 209 °C for NO reduction by CO. Prepared catalysts show promising activity compared to few of the precious metal based catalysts reported in the literature. The influence of moisture and oxygen on catalytic conversion has been studied.     

TiO2 promoted Co/SiO2 catalysts for Fischer–Tropsch synthesis

The addition of small amount of TiO₂ to silica-supported cobalt catalysts significantly increasing the dispersion of cobalt and Co metallic surface area resulting in the remarkable enhancement of the Fisher–Tropsch synthesis (FTS) activity in the slurry-phase reaction. The addition of TiO₂ adjusted the interaction between cobalt and silica support quite well to realize the favorite dispersion and reduction degree of supported cobalt, leading to high catalytic activity in FTS. The properties of various catalysts were characterized by in-situ DRIFT, XRD, TPR, N₂ physisorption and H₂ chemisorption.     

Effect of hierarchical meso-macroporous silica supports on Fischer-Tropsch synthesis using cobalt catalyst

Hierarchical meso-macroporous (HS-X) silica with different mesopore diameters synthesized by using rice husk ash as a silica source and chitosan as a natural template were applied for the first time as the cobalt support for Fischer-Tropsch synthesis. Unimodal mesoporous silica (MS-X) supports with equivalent mesopore diameters to HS-X supports have also been prepared for comparison. Effects of diffusion in MS-X and HS-X supports of different particle sizes on the catalytic activity and hydrocarbon selectivity were investigated. The cobalt crystallite sizes were increased with increasing mesopore diameters, whereas the highest amount of H₂ chemisorbed was found for the catalyst with the medium mesopore diameter. The HS-X supports revealed lower surface area and higher macroporosity which led to the formation of larger cobalt crystallite size and less chemisorbed H₂. However, the catalytic activity was much higher for cobalt supported on HS-X silica of both small and large catalyst particle sizes. Moreover, with the large catalyst particle size, the C₅₊ selectivity of cobalt supported on HS-X silica was much higher than that on MS-X silica, indicating the influence of mass transfer of reactants and products in macropores of HS-X supports.     

Catalytic Performance of Aged Rh/CeO2–ZrO2 for NO–C3H6–O2 Reaction Under a Stoichiometric Condition

The activity of Rh/CeO₂ for NO reduction by C₃H₆ was gradually deceased by mixing with ZrO₂ until 68 mol%. Rh supported on CeO₂–ZrO₂ with higher OSC was found to show lower catalytic activity. High OSC of CeO₂–ZrO₂ would probably stabilize the surface of Rh in oxidized state, resulting in low activity and low efficiency of C₃H₆ utilization for NO reduction. In situ FT-IR spectroscopy suggested that mononitrosyl species such as Rh(NO)^δ? and Rh(NO)^δ+ are reaction intermediates in the NO–C₃H₆–O₂ reaction over Rh/CeO₂–ZrO₂ catalysts.     

Effect of bimodal porous silica on particle size and reducibility of cobalt oxide

In this study, the effect of bimodal porous silica (BPS) on particle size and reducibility of cobalt oxide has been investigated. Unimodal porous silica (UPS) was used for comparison purposes. Both silica supports were impregnated with an aqueous solution of cobalt nitrate to obtain cobalt loadings of about 10 wt%. Pore structure, specific surface area, morphology and cobalt oxide crystallite size of the cobalt oxide loaded on porous silicas were systematically characterized by means of N₂-sorption, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The reduction behavior profiles and the activation energy for the reducibility of the cobalt oxide were studied by dynamic thermal gravimetric under flow of H₂. The average particle size of cobalt oxide loaded on the BPS sample was revealed to be slightly larger than that loaded on the UPS sample, likely because cobalt oxide particles were distributed both on mesopores and macropores. The reduction temperatures of the cobalt oxide loaded on the BPS sample were found to be evidentially lower than those of the cobalt oxide loaded on the UPS sample.     

Cobalt-exchanged mordenites: preparation,characterization and catalytic activity for the abatement of NO with CH<Subscript>4</Subscript> in the presence of excess O<Subscript>2</Subscript>

The abatement of NO with methane in the presence of oxygen was studied on various commercial MOR in the Na-form (Na-MOR) and H-form (H-MOR), or exchanged to various extents with cobalt (Co-MOR). The sodium and cobalt contents were determined by atomic absorption. Samples were characterized by FTIR and volumetric measurements of CO adsorption. Chemical analysis indicated that one cobalt species replaced two Brønsted acid sites in H-MOR and two Na⁺ ions in Na-MOR. The IR analysis of the OH stretching region, evidencing an unexpected presence of Brønsted acid sites (band at 3610 cm^?1) in Co-MOR, indicated that the exchange process had a more complex stoichiometry. The adsorption of CO at RT on Co-MOR, in addition to the bands of the corresponding H-MOR and Na-MOR matrices, yielded two types of Co^II-carbonyls, the first type occupied the?mordenite main channels, and the second one the mordenite smaller channels. Brønsted acid sites in mordenites were active for the selective catalytic reduction of NO with CH₄. Co-MOR samples were far more active than Na-MOR and H-MOR samples, showing that acid protons play a negligible role when Co is present. Co-MOR catalysts showing the highest activity had the largest amount of Co^II-carbonyls in the main channels. This result strongly suggests that Co^II in the main channels of MOR are the active sites for the CH₄ + NO + O₂ reaction.     

Synthesis and structure of cobalt layer silicate

Two cobalt layer silicates were synthesized by hydrothermal treatment at 275‡C and 54 atm from corresponding stoichiometric coprecipitate of cobalt hydroxide and silica. From IR absorption spectra and X-ray diffraction patterns, one sample was proved to be cobalt montmorillonite [Co[₃Si[₂O[_{5 2}(OH)₂, while the structure of the other was cobalt antigorite [Co₃Si₂O₅(OH)₄] of one silica layer combined with an octahedral layer of the type Co₃ ²⁺ (OH)₆.     

The Nature of Active Sites of Co/Al2O3 for the Selective Catalytic Reduction of NO with C2H4

The effects of Co loading and calcination temperatures on the catalytic activity of Co/Al₂O₃ for selective catalytic reduction (SCR) of NO with ethylene in excess oxygen were investigated. Co/Al₂O₃ showed high and low activities when calcined at high (800 °C) and low (350 °C) temperatures, respectively. The formation and dispersion of cobalt species for catalysts calcined at 350 and 800 °C as well as for Al₂O₃ were studied by XRD, UV–vis and FTIR spectra. Combined with DRIFTS results of ad-species and reaction experiments, it allowed us to correlate the catalytic activity with active sites of Co/Al₂O₃, and the catalytic functions of active cobalt species and support were clarified. Co₃O₄ species contributed to the oxidation of NO to various nitrates and of C₂H₄ to reactive formate species, even in the absence of O₂, whereas the side reaction of ethylene combustion occurred simultaneously when excess oxygen was present. Tetrahedral Co²⁺ ions in CoAl₂O₄, which acted as the active sites, were responsible for the reaction between formate and nitrate species to form organic nitro compound.     

Fischer–Tropsch synthesis using Co/SiO2 catalysts prepared from mixed precursors and addition effect of noble metals

Fischer–Tropsch synthesis in Co/SiO₂ catalysts, which were prepared by mixed impregnation of cobalt (II) nitrate and cobalt (II) acetate, was studied under mild reaction conditions (Total pressure=1 MPa, H₂/CO=2, T=513 K). X-ray diffraction indicated that highly dispersed cobalt metal was the main active sites on the catalyst prepared by the same method. It was considered that the metallic crystallines, which were readily reduced from cobalt nitrate, promoted the reduction of Co²⁺ to metallic a state in cobalt acetate by H₂ spillover mechanism during the catalyst reduction process. The reduced cobalt, from cobalt acetate, was highly dispersed one and remarkably enhanced the catalytic activity. The addition of a small amount of Ru to this type of catalyst remarkably increased the catalytic activity and the reduction degree. Its turn over frequency (TOF) increased but the selectivity of CH₄ was unchanged. However, when Pt or Pd were added into catalysts, they exhibited a higher selectivity of CH₄. Although Pt and Pd hardly exerted an effect on cobalt reduction degree, they promoted cobalt dispersion and decreased the value of TOF. Characterization of these bimetallic catalysts suggested that a different contact between Co and Ru, Pt or Pd existed. Ru was enriched on the metallic cobalt surface but, Pt or Pd dispersed well in the form of Pt–Co or Pd–Co alloy.     

Methane activation by NO2 on Co loaded SBA-15 catalysts: The effect of mesopores (length, diameter) on the catalytic activity

In a general model of “three-function deNO_x” catalyst, the partial oxidation of methane by NO₂ is an important step (CH₄ + NO₂ → C_xH_yO_z + NO). To study the effect of the length and diameter, in the mesopores of SBA-15, we have synthesized catalysts with 3 wt.% cobalt supported on SBA-15, with differences in length and diameter of channels. Three different cobalt species were detected on all catalysts. We demonstrated by TPSR experiments that the activity of cobalt/SBA-15 catalysts is affected by the length, the diameter and connections between mesopores of the SBA-15 supports. We show that by changing textural properties of silica support the temperature of 100% conversion of NO₂ into NO can decrease by more than 100 °C.     

Designing a structured catalyst for selective reduction of O<Subscript>2</Subscript> by hydrogen in the presence of NO

The influence of the promoter (Pd) modifying additives of oxides of rare-earth (La₂O₃, CeO₂) and transition (NiO, CuO) metal oxides on the catalytic activity of Co₃O₄/cordierite in reactions of O₂ and NO reduction by hydrogen was studied. Introducing Pd and rare-earth metal oxides into the composition of cobalt oxide catalyst results in an increase in its activity in H₂ + 1/2O₂ → H₂O, H₂ + NO → 1/2N₂ + H₂O reactions and an increase in selectivity upon oxygen reduction by hydrogen in the presence of nitric oxide, due possibly to a decrease in the strength of oxygen bounds with the surface and the formation of low-temperature forms of oxygen, which is not typical of unpromoted cobalt oxide catalyst. A structured Pd-Co₃O₄-La₂O₃/cordierite catalyst was developed that surpasses the commercial granulated silver-manganese catalyst used in industry to purify the technological gases used in the production of hydroxylamine sulfate of oxygen impurities with reference to activity and selectivity (in the process of oxygen reduction in the presence of nitric oxide), and to thermal stability.     

Nature and Reactivity of the Active Species Formed After NO Adsorption and NO + O2 Coadsorption on an Fe-Containing Zeolite

Reactivity of the NO adspecies on Fe-ZSM-11 was studied by FTIR in situ. The effect of Fe content and the oxidation state of Fe in the samples were correlated with the catalytic activity. The relation between the adsorbed species, the Brønsted sites and catalytic activity in the SCR of NO_x to N₂ was also investigated. Moreover, FTIR allowed us to identify the active sites and the adsorption complexes present in FeMFI. Samples prepared by the sol–gel method with different Fe content displaying vastly different activity and selectivity in the reduction of NO to N₂ with isobutane in excess of O₂. Thus, in contact with pure nitric oxide, NO ions, mononitrosyl groups, nitro groups and nitrate ions have been identified. Feⁿ⁺ active sites are the most probable centers for NO oxidation to NO₂ and its further conversion to adsorbed nitro groups and nitrate ions, steps that are crucial for NO reduction. The concerted action of Feⁿ⁺ and H⁺ sites of the catalysts over the NO conversion to N₂ and isobutane conversion was analyzed.