Reoxidation Of Cobalt Research Articles

Bioethanol Steam Reforming over Cobalt-Containing USY and ZSM-5 Commercial Zeolite Catalysts

The ethanol steam reforming process (ESR) over cobalt-based zeolitic catalysts, differing significantly in the structure, was comprehensively examined. The cobalt spinel phase (10wt%) was deposited on the surface of USY and ZSM-5 zeolites (Si/Al ratio of 31). The catalysts were characterized in terms of their chemical (ICP) and phase composition (XRD), textural properties (low-temperature N2 adsorption), morphology (STEM/EDX), and reducibility (H2-TPR). The aforementioned characteristics were supplemented by the catalysts’ acidity and redox properties investigations (quantitative FT-IR studies of pyridine and carbon monoxide adsorption). Catalysts' activity was evaluated in the ESR process at 500C for various ethanol/water mixtures. Both catalysts exhibited 100% ethanol conversion, whereas their selectivity towards H2, CO2, and C2H4 strongly depended on the applied ethanol to water molar ratio. Comparable selectivities observed for the 1:4 ratio were improved for the 1:9 ratio for both catalysts, as expected. For the ratio of 1:12, the significant difference in the reaction paths (the ethanol dehydration for CoUSY and the ethanol steam reforming for CoZSM-5) was explained by the cobalt reoxidation process facilitated by water molecules for the CoUSY. The superior overall performance of the CoZSM-5 catalyst in the ESR process, in comparison to CoUSY, also results from its almost three times enhanced accessibility of the cobalt species, as confirmed by the quantitative FT-IR studies of CO sorption. The microscopic studies also indicated a better dispersion of the cobalt phase supported on the ZSM-5 support. Thus, the structure of ZSM-5 zeolite assures higher cobalt active phase dispersion being more beneficial for the ESR process.

Fischer-Tropsch synthesis over an alumina-supported cobalt catalyst in a fixed bed reactor – Effect of process parameters

Fischer-Tropsch synthesis has been investigated over a commercial type cobalt-based catalyst (20 %Co/0.5 %Re/γ-Al2O3) by varying the H2/CO ratio (2.55–1.12), CO conversion (15–75 %), reaction temperature (210 °C, 230 °C), and by adding water to the syngas. The experiments were conducted in a fixed bed reactor with the main purpose of obtaining experimental data to be used in the development and fitting of a mechanistic model. A positive effect of water on the catalyst activity was found for experiments with a H2/CO ratio higher than 1.7. Water was found to always increase the C5+ selectivity regardless of the H2/CO ratio. Increasing conversion led to increased C5+ selectivity. The selectivity to CO2 was significantly enhanced at higher conversions (high water partial pressure), particularly with the lowest H2/CO = 1.12, interpreted as the emergence of water-gas shift activity. Re-oxidation of cobalt, probably limited to small cobalt particles, is proposed as the main deactivation mechanism caused by water while a steeper deactivation curve was found for higher temperature, indicating that sintering also may play a role.

Partial oxidation of methane to syngas using Co/Mg and Co/Mg-Al oxide supported catalysts

Partial oxidation of methane (POM) was studied over a series of Co/Mg-Al catalysts. Various commercial hydroxides with different Mg/Al ratios were used as precursors of the oxides employed as catalytic supports, as well as MgO prepared in the laboratory. The effects of Co loading, Mg content, and calcination temperature (400–750 °C) were studied on the POM reaction. The catalytic performance was evaluated at 800 °C in a fixed-bed tubular quartz reactor under a high space velocity of 300 L N CH4/(gcat h) and a O2/CH4 molar ratio of 0.5. A very high activity was obtained during 4 h on-stream with a 20 wt. % Co catalyst prepared with a Mg-Al mixed oxide support having a MgO content of 63 wt. %. This catalyst gave CH4 conversions (91.3%) very close to the maximum corresponding to the thermodynamic equilibrium. Such notable performance could be attained with the catalyst precursor subjected to calcination at 500 °C for 6 h and subsequent in situ reduction under H2 flow at 800 °C for 2 h. Two main types of deactivation mechanisms were generally observed in most of the samples that suffered either a very rapid cobalt re-oxidation upon their exposure to the POM reaction atmosphere or significant deposition of carbonaceous deposits of different kinds. The characterization of the spent samples by transmission electron microscopy (TEM) revealed the presence of carbon deposits of different nature in the Mg/Al samples having a high MgO content, including filamentous carbon (whiskers), onion-shell like sheets and/or amorphous encapsulating layers. Conversely, no carbon deposits were observed in the spent Co/MgO catalyst, which underwent oxidation of the relatively less dispersed surface Co sites to form clearly defined crystals of Co oxides with particle diameters typically ranging between 25 and 100 nm.

Fischer–Tropsch synthesis: An XAS/XRPD combined in situ study from catalyst activation to deactivation

A rhenium promoted (1wt.%Re/20wt.%Co/γ-Al2O3) and an un-promoted (20wt.%Co/γ-Al2O3) Fischer–Tropsch catalyst were studied in situ throughout the common steps of laboratory catalyst testing, that is, reduction, pressurization, the initial period before reaching pseudo-steady state, deactivation, and post-mortem analysis. High-resolution X-ray powder diffraction (HR-XRPD) was combined with X-ray absorption spectroscopy (XAS) in order to reveal the changes occurring during the experimental procedure. A mass spectrometer (MS) connected to the reactor outlet allow monitoring of the gas phase and accordingly the catalyst performance with respect to activity. Fischer–Tropsch synthesis was performed at 493K, 18bar, H2/CO=2.1, and >50% CO conversion. The reduction at 673K confirmed the transition from Co3O4 to CoO and eventually a mixture of face-centered cubic (fcc) metallic cobalt, hexagonal close packed (hcp) metallic cobalt and a fraction of unreduced cobalt. The promoted catalyst exhibits a transformation during the initial stages of the Fischer–Tropsch synthesis possibly associated with minor re-oxidation of cobalt to Co2+ tetrahedrally coordinated, part of a CoAl2O4 surface structure. Throughout Fischer–Tropsch synthesis, HR-XRPD and XAS signals are unaffected, suggesting the absence of bulk cobalt transformations occurring at the initial Fischer–Tropsch deactivation period. No significant gradients in the concentration of cobalt species or the size of the crystallites have been found as a function of the length of the catalytic bed.

Deactivation Behavior of Carbon Nanotubes Supported Cobalt Catalysts in Fischer-Tropsch Synthesis

The effects of electronic properties of inner and outer surfaces of Carbon Nano Tubes (CNTs) on the deactivation of cobalt Fischer-Tropsch (FT) catalysts were studied. The comparative characterization of the fresh and used catalysts by TEM, XRD, TPR, BET and H 2 chemisorption showed that cobalt re-oxidation, cobalt-support interactions and sintering are the main sources of catalyst deactivation. TEM showed that 480 h continuous FT synthesis increased the average particles size of the particles located inside the pores from 7 to 7.4 nm while the average particles size of the particles located outside of the tubes increased from 11.5 to 25 nm. XRD analysis of the used catalyst confirmed cobalt re-oxidation and interaction between cobalt and CNTs and creation of carbide phases. When the %Co conversion and H 2O partial pressure in the reactor are high, the deactivation rate is not dependent on the number of the catalyst active sites and is zero order to %CO conversion. In this case the main deactivation mechanisms are cobalt re-oxidation and metal support interactions. At lower amounts of the %Co conversion and H 2O partial pressure, deactivation can be simulated with power law expressions with power orders of 11.4 for the particles outside the tubes and 30.2 for the particles inside the tubes and the main deactivation mechanism is sintering. Due to the electron deficiency of the inner sides of the CNTs, the interaction between the cobalt oxides and the support is stronger leading to lower rates of sintering as compared with the particles located on the outer layers of the CNTs. Regeneration recovered the catalyst activity by 54.3% of the total activity loss.

Effects of Confinement in Carbon Nanotubes on the Activity, Selectivity, and Lifetime of Fischer−Tropsch Co/Carbon Nanotube Catalysts

The effects of electronic properties of the inner and outer surfaces of carbon nanotubes (CNTs) on the deactivation of cobalt Fischer−Tropsch (FT) catalysts were studied. The comparative characterization of the fresh and used 0.20 w (mass fraction) Co/CNT catalysts by transmission electron microscopy (TEM), X-ray diffraction (XRD), temperature-programmed reduction (TPR), Brunnauer−Emmett−Teller analysis (BET), and H2 chemisorption showed that cobalt reoxidation, cobalt-support interactions, and sintering are the main sources of catalyst deactivation. TEM showed that continuous FT synthesis for 480 h increased the average Co particle size located inside the pores from (7 to 8.5) nm, while the average Co particle size located outside of the tubes increased from (11.5 to 25) nm. XRD analysis of the used catalyst confirmed cobalt reoxidation, interaction between cobalt and CNTs, and the creation of carbide phases. At a high percent CO (%CO) conversion and H2O partial pressure, the deactivation rate is zero-order and independent of the number of active catalyst sites. In this case, the main deactivation mechanisms are cobalt reoxidation and metal support interactions. At lower %CO conversion and H2O partial pressure, the deactivation rate can be simulated with power law expressions of the order of 11.4 for the particles outside the tubes and 30.2 for the particles inside the tubes. In this case, the main deactivation mechanism is sintering. Because of the electron deficiency of the inner sides of the CNTs, the interaction between the cobalt oxides and the support is stronger, leading to lower rates of sintering as compared with the particles located on the outer layers of the CNTs. Regeneration recovered the catalyst activity with 9.1 % of the total activity loss.

Deactivation behavior of ruthenium promoted Co/γ-Al2O3 catalysts in Fischer–Tropsch synthesis

Abstract Detailed activity study and the deactivation of Ru-Co/γ-Al2O3 catalyst for Fischer–Tropsch (FT) synthesis over 1000 h was investigated considering different deactivation mechanisms. Morphology changes of the catalyst during FT synthesis were studied using XRD, TPR, BET, ICP, carbon determination, H2 chemisorption and re-oxidation techniques. When the P H 2 O / ( P H 2 + P CO ) in the reactor is above 0.75 the deactivation rate is not dependent on the number of the catalyst active sites and is zero order to CO conversion. In this case the main deactivation mechanisms are: cobalt re-oxidation, metal support interactions and aluminates formation. The deactivation of Ru-Co/γ-Al2O3 is related to cobalt cluster size. At lower amounts of P H 2 O / ( P H 2 + P CO ) deactivation can be simulated with a power law expression with a power order of 39.7 and the main deactivation is due to sintering. Regeneration of catalyst at 275 °C recovered the catalyst activity by 69.9% of total activity loss due to the reduction of oxidized cobalts. Catalyst regeneration at 400 °C recovered the activity by 21.9% of total activity loss due to the reduction of refractory forms of oxidized cobalt generated by cobalt–alumina interactions. 7.2% of total activity loss is irreversible and can be assigned to aluminates formation, sintering and coke deposition.

Partial oxidation of methane to synthesis gas over Co/Ca/Al 2O 3 catalysts

A series of calcium-modified alumina-supported cobalt catalysts were prepared with a two-step impregnation method, and the effect of calcium on the catalytic performances of the catalysts for the partial oxidation of methane to syngas (CO and H 2) was investigated at 750 °C. Also, the catalysts were characterized by XRD, TEM, TPR and ( in situ) Raman. At 6 wt.% of cobalt loading, the unmodified alumina-supported cobalt catalyst showed a very low activity and a rapid deactivation, while the calcium-modified catalyst presented a good performance for this process with the CH 4 conversion of ∼88%, CO selectivity of ∼94% and undetectable carbon deposition during a long-time running. Characterization results showed that the calcium modification can effectively increase the dispersion and reducibility of Co 3O 4, decrease the Co metal particle size, and suppress the reoxidation of cobalt as well as the phase transformation to form CoAl 2O 4 spinel phases under the reaction conditions. These could be related to the excellent catalytic performances of Co/Ca/Al 2O 3 catalysts.

Catalytic reduction of hexachlorobenzene and pentachlorobenzene by cobalt(I) salen electrogenerated at vitreous carbon cathodes in dimethylformamide

Abstract Cyclic voltammetry and controlled-potential electrolysis have been used to investigate the catalytic reduction of hexachlorobenzene and pentachlorobenzene by cobalt(I) salen that is electrogenerated at a carbon cathode in dimethylformamide containing 0.050 M tetramethylammonium tetrafluoroborate. Cyclic voltammograms for the reduction of cobalt(II) salen in the presence of either hexachlorobenzene or pentachlorobenzene show an enhanced cathodic peak for the catalyst–substrate reaction as well as loss of the anodic peak for reoxidation of cobalt(I) salen, revealing the effectiveness of the catalyst. However, cobalt(I) salen shows no catalytic activity toward tetrachlorobenzene or any less-chlorinated species. Controlled-potential electrolyses of cobalt(II) salen–hexachlorobenzene mixtures give both pentachlorobenzene and three isomers of tetrachlorobenzene, whereas bulk catalytic reduction of pentachlorobenzene affords three different tetrachlorobenzenes; for both substrates, substantial starting material remains unreduced. To account for the formation of the various products, a mechanistic scheme is proposed and discussed.

X‐ray Absorption Spectroscopic Study of LiCoO2 as the Negative Electrode of Lithium‐Ion Batteries

Lithium cobalt oxide (LiCoO(2)) particles are modified using rotor blade grinding and re-annealing and used as the active electrode material versus lithium in the 3-0 V potential interval, in which a maximum capacity of 903 mA h g(-1) is achieved. X-ray absorption near edge structure spectra reveal the complete reduction of Co(3+) to Co metal at 0 V. Cell recharge leads to an incomplete reoxidation of cobalt. A maximum reversible capacity of 812 mA h g(-1) is obtained, although a poor capacity retention upon prolonged cycling may limit its application.

Fischer–Tropsch synthesis: effect of water on Co/Al2O3 catalysts and XAFS characterization of reoxidation phenomena

Both low loaded 15% Co/Al2O3 and more highly loaded 25% Co/Al2O3 catalysts are studied, in order to explore the impact of cluster size on the stability of the cobalt cluster to support-influenced reoxidation processes at high H2O/CO ratios. XAFS and activity data suggest that there are two regions for the water effect: at lower H2O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H2O/CO ratios cobalt re-oxidation processes occur. The latter regime where water was added at and above 25% are examined. Synthesis conditions were maintained constant while argon balancing gas was replaced by added water. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under inert gas. The EXAFS results suggest that, unlike the smaller clusters on unpromoted and, especially noble-metal promoted, 15% Co/Al2O3 catalysts, the larger crystallites (>10nm by chemisorption and XRD) on 25% Co/Al2O3 undergo oxidation by H2O to CoO, most likely confined to the surface. The clusters are re-reduced when H2O was switched off, and the activity displayed an important recovery.

X-ray photoelectron spectroscopy analysis of platinum- and/or cobalt-loaded zeolites relevant for selective catalytic reduction of NOx

Bimetallic Pt–Co and the corresponding monometallic Pt and Co samples, exchanged with Na–mordenite, Na–ZSM5 and K–ferrierite zeolites, have been studied by photoelectron spectroscopy. The oxidation state of the elements and their abundance at the surface were studied by exposing the samples, in the pretreatment chamber of the spectrometer, to several different reaction environments that resemble the conditions encountered in the selective catalytic reduction (SCR) of exhaust gases. In calcined Co–zeolites, Co2+ ions occupy exchanged positions in the structure and also coordinate to lattice oxygen with a high dispersion. Incorporation of Pt2+ in Co-containing samples influences the electronic state of cobalt in both the calcined and reduced states. Upon reduction in hydrogen at 623 K, highly dispersed bimetallic Pt–Co clusters are generated, which coexist with Co2+ and Pt2+ ions in both exchanged positions and coordinated to lattice oxygen. Exposure to the SCR reaction mixture of H4 +NO +O2 +H2 leads to reoxidation of cobalt in the bimetallic Pt–Co clusters, forming Co2+ and Co(OH)2 on the zeolite surface, while Pt migrates towards the outer surface, yielding larger Pt particles. Copyright © 1999 John Wiley & Sons, Ltd.

Partial Oxidation of Methane to Synthesis Gas Using LnCoO3Perovskites as Catalyst Precursors

In this work a series of cobalt-containing perovskites LnCoO3(Ln=La, Pr, Nd, Sm, and Gd) has been studied as catalyst precursors for the partial oxidation of methane to synthesis gas. All the perovskite precursors were prereducedin situ, producing cobalt metal finely dispersed over the rare earth sesquioxide support described here as Ln–Co–O. Of the catalyst tested the system Gd–Co–O showed exceptionally better performance for CO and H2production (with methane conversion of 73% and selectivities of 79 and 81% for CO and H2, respectively, at 1009 K). The production of synthesis gas over the other catalysts decreased in the following order: Sm–Co–O>>Nd–Co–O>Pr–Co–O. The catalyst La–Co–O was active for methane combustion and only traces of CO and H2were observed under the reaction conditions. XRD and XPS analyses of the catalyst La–Co–O showed that under the reaction conditions the cobalt metal is completely reoxidized, regenerating the original LnCoO3perovskite structure. For the reaction over Nd–Co–O the cobalt is only partially reoxidized to NdCoO3. For Gd–Co–O and Sm–Co–O, the most stable and active catalysts for the partial oxidation of methane no reoxidation to LnCoO3was observed. TPR and XRD studies showed that the perovskite NdCoO3is reduced in two steps, first to NdCoO2.5and further to Co°/Nd2O3and in both stages it was demonstrated that the reoxidation with O2is capable of recovering the perovskite structure. TPO experiments with reduced La–Co–O, Nd–Co–O, Sm–Co–O, and Gd–Co–O catalysts indicated that reoxidation of cobalt also takes place in two steps: first by oxidation of the supported Co° to the spinel Co3O4(Co2+Co3+2O4) followed by a further oxidation of the Co2+to Co3+with a simultaneous solid state reaction with Ln2O3, regenerating the perovskite structure. It was observed that the temperature for the second oxidation step is strongly dependent on the nature of the lanthanide. Based on these results it is proposed that the deactivation of the catalysts Ln–Co–O by reoxidation of cobalt metal is related to the thermodynamic stability of the parent perovskite structure. We also present evidence that hydroxyl groups on the rare earth oxide, specially in the La–Co–O system, might make some contribution to the reoxidation of cobalt metal during the reaction via a reverse spillover process.

Reoxidation and Deactivation of Supported Cobalt Fischer−Tropsch Catalysts

The role of water in the deactivation of Al2O3-supported cobalt Fischer−Tropsch catalysts has been studied at different water partial pressures. In addition, model studies at nonreacting conditions using H2O/H2 feeds were carried out in order to study the possible reoxidation of cobalt by water. High-pressure gravimetry, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) were used as characterization methods in the model studies. The kinetic studies showed significant deactivation when water was added to the feed in fixed-bed reactor experiments. In the model studies, a large extent of bulk cobalt reoxidation was only observed in the absence of H2. Only a small amount of reoxidation could be observed when H2 was present, even at high H2O/H2 ratios. However, XPS studies indicated significant surface oxidation of cobalt at a lower H2O/H2 ratio. Surface oxidation or oxidation of highly dispersed cobalt phases is therefore concluded to be responsible for the observed deactivati...

Influence of cyanide and sulphide ions on the reduction and reoxidation of cobalt(II) in thiocyanate solution at mercury electrodes

The process of reduction and reoxidation of cobalt(II) in thiocyanate solution at hanging mercury drop electrode has been investigated by cyclic voltammetric, chronoamperometric and anodic stripping methods. In 0.1 M NaSCN and 0.4 M NaClO 4 solution containing 1×10 −3 M cobalt(II), the voltammogram on the first cycle at 0.05 V s −1 gives a cathodic peak at −1.06 V with hysteresis on reversal, and an anodic wave with a peak potential of −0.28 V and with two shoulders near −0.38 and −0.45 V, respectively. Multicyclic voltammograms under the same conditions give a cathodic peak at −0.90 V and an anodic peak at −0.45 V. The reduction and reoxidation of cobalt(II) in thiocyanate solution is accelerated by the reduction products of thiocyanate ion, cyanide and sulphide ions, which are produced during the electroreduction of cobalt(II). A mechanism of reduction and reoxidation of cobalt(II) which involves a chemical reduction of thiocyanate ion by electroreduced metallic cobalt and takes into account cyanide and sulphide ions is proposed. The hysteresis on the cathodic wave is caused by the difference in reduction potentials of cobalt(II)-thiocyanate and-cyanide complexes. Cyclic voltammetric study of cobalt(II) in perchlorate solution containing trace amounts of cyanide and sulphide ions supports these conclusions.

Influence of cyanide and sulphide ions on the reduction and reoxidation of cobalt(II) in thiocyanate solution at mercury electrodes

The process of reduction and reoxidation of cobalt(II) in thiocyanate solution at hanging mercury drop electrode has been investigated by cyclic voltammetric, chronoamperometric and anodic stripping methods. In 0.1 M NaSCN and 0.4 M NaClO 4 solution containing 1×10 −3 M cobalt(II), the voltammogram on the first cycle at 0.05 V s −1 gives a cathodic peak at −1.06 V with hysteresis on reversal, and an anodic wave with a peak potential of −0.28 V and with two shoulders near −0.38 and −0.45 V, respectively. Multicyclic voltammograms under the same conditions give a cathodic peak at −0.90 V and an anodic peak at −0.45 V. The reduction and reoxidation of cobalt(II) in thiocyanate solution is accelerated by the reduction products of thiocyanate ion, cyanide and sulphide ions, which are produced during the electroreduction of cobalt(II). A mechanism of reduction and reoxidation of cobalt(II) which involves a chemical reduction of thiocyanate ion by electroreduced metallic cobalt and takes into account cyanide and sulphide ions is proposed. The hysteresis on the cathodic wave is caused by the difference in reduction potentials of cobalt(II)-thiocyanate and-cyanide complexes. Cyclic voltammetric study of cobalt(II) in perchlorate solution containing trace amounts of cyanide and sulphide ions supports these conclusions.