Oxidation of Dry Methane on the Surface of Oxygen Ion-Conducting Membranes

A mix-valenced cobalt oxide, CoO_x, was prepared from cobalt nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by hydrogen peroxide. Further, other pure cobalt oxide species were refined from the CoO_x by temperature-programmed reduction (TPR) at 170, 230 and 300 °C (labeled as R-170, R-230 and R-300, respectively). They were characterized by X-ray (XRD), infrared (IR), thermogravimetry (TG) and TPR. The major composition of CoO_x is CoO(OH), with a small amount of Co⁴⁺ species; R-170 is CoO(OH) with a hexagonal structure; R-230 is Co₃O₄ with a spinel structure and R-300 is CoO with a cubic structure. Their catalytic activities toward the CO oxidation were further studied in a continuous flow microreactor. The results indicated that the relative activity decreased significantly with the oxidation state of cobalt, i.e., CoO(+2)Co₃O₄(+8/3)CoO(OH)( +3)CoO_x(>+3).     

Characterization and Catalytic Oxidation of Carbon Monoxide Over Supported Cobalt Catalysts

Three types of supported cobalt catalysts (CoO_x/SiO₂, CoO_x/TiO₂ and CoO_x/Al₂O₃) were prepared by incipient wetness impregnation with aqueous Co(NO₃)₂·6H₂O solution. The phase composition and the interactions of cobalt with supports under different calcined temperatures were investigated using thermogravimetry (TG), N₂-adsorption at −196 °C, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and diffuse reflectance spectroscopy (DRS). Their catalytic activities towards the CO oxidation were further studied in a continuous flow micro-reactor. The results showed that the interaction of cobalt oxide with supports was much stronger in the kinds of Al₂O₃ and TiO₂, while no conclusive evidence of any interaction was found for SiO₂. Besides the crystalline Co₃O₄ which was formed in three supported catalysts, both high-temperature phases CoAl₂O₄ and CoTiO₃ spinel were also detected under XRD, DRS and TPR analysis. The degree of interaction between cobalt oxide and the support not only affected the surface area and reduction behavior of the catalysts, the catalytic activity toward the CO oxidation also affected simultaneously. As the CoAl₂O₄ and CoTiO₃ spinel formed, both the surface area and catalytic activity decreased significantly.     

Perovskite-type oxide ACo0.8Bi0.2O2.87 (A=La0.8Ba0.2): a catalyst for low-temperature CO oxidation

Perovskite-type oxide ACo_0.8Bi_0.2O_2.87 (A=La_0.8Ba_0.2) has been investigated as a catalyst for the oxidation of carbon monoxide. X-ray diffraction results revealed that the catalyst is single-phase and cubic in structure. The results of chemical analysis indicated that in ACo_0.8Bi_0.2O_2.87, bismuth is pentavalent whereas cobalt is trivalent as well as bivalent; in La_0.8Ba_0.2CoO_2.94, cobalt ions exist as Co³⁺ and Co⁴⁺. The substitution of Bi for Co enhanced the catalytic activity of the perovskite-type oxide significantly. Over the Bi-incorporated catalyst, at equal space velocities and with the rise in CO/O₂ molar ratio, the temperature for 100% CO conversion shifted to a higher range; at a typical space velocity of 30000 h^–1 and a CO/O₂ molar ratio of 0.67/1.00, 100% CO conversion was observed at 250°C. Over ACo_0.8Bi_0.2O_2.87, at equal CO/O₂ molar ratio, the temperature for 100% CO conversion decreased with a drop in space velocity; the lowest being 190°C at a space velocity of 5000 h^–1. The result of O₂-TPD study illustrated that the presence of Bi ions caused the lattice oxygen of La_0.8Ba_0.2CoO_3– to desorb at a lower temperature. The results of TPR, ¹⁸O/¹⁶O isotopic exchange, and CO-pulsing investigations demonstrated that the lattice oxygen of the Bi-doped catalyst is highly mobile.     

Surface chemical structures of CoO x /TiO2 catalysts for continuous wet trichloroethylene oxidation

An earlier sample of 5% CoO _x /TiO₂ used for the wet oxidation of TCE at 310 K forca. 6 h has been characterized with a fresh catalystvia XRD and XPS measurements. The binding energy for Co 2p_3/2 of the fresh sample appeared at 781.3 eV, which was very similar to the chemical states of CoTiO _x such as Co₂TiO₄ and CoTiO₃, whereas the spent catalyst indicated a 780.3-eV main peak for Co 2p_3/2 with a satellite structure at a higher energy region. This binding energy was almost equal to that of Co₃O₄ among reference Co compounds used. The phase structure of Co₃O₄ was revealed upon XRD measurements for all the catalyst samples. Based on these XPS and XRD results, a surface chemical structure of CoO _x species existing with the fresh catalyst can be proposed to be predominantly Co₃O₄ encapsulated completely by very thin filmlike CoTiO _x consisting of Co₂TiO₄ and/or CoTiO₃, with a tiny amount of Co₃O₄ particles covered partially by such cobalt titanates which may be responsible to the initial catalytic activity. Those CoTiO _x overlayers on Co₃O₄ particles may be readily removed into the wet media within 1 h at 310 K based on our earlier study, thereby giving rapid increase in the catalytic activity for that period.     

Novel aspects of the physical chemistry of Co/SiO2 Fischer–Tropsch catalyst preparations: Cobalt oxide-induced silica migration during calcination of cobalt nitrate-impregnated high surface area silica

Co/SiO₂ catalysts were prepared by aqueous cobalt nitrate impregnations of silicas with different surface areas to study the effect of the support surface area on the reactions occurring during impregnation and calcination and to define the stage and mode of metal–support interactions. TPR analyses of samples calcined in dry air showed the presence of various quantities of cobalt silicate species, while cobalt silicate formation was not discernible by other analytical techniques. Our conclusion, confirmed in our later studies, is that cobalt silicate does not form during impregnation or calcination, but is created during the reduction in the TPR instrument. Because of these and other ambiguities of the TPR analyses, in our continuing studies we preferred alternative analytical approaches.These studies on the calcination stage resulted in the following unusual findings: (1) X-ray photoelectron spectroscopy revealed drastic decreases in the surface cobalt concentration after calcination of high surface silicas impregnated with cobalt nitrate solutions. (2) Infrared spectroscopy indicated much less than expected Co₃O₄ formation upon calcination if high surface area silica was the support. (3) A method was devised to calculate the surface areas of individual components in mixtures. The calculations indicated about 20% surface area losses for the silica in calcined catalysts. (4) Scanning electron micrographs of a calcined catalyst on high surface area silica support showed smaller-sized decorations around the larger silica particles. Energy-dispersive X-ray analysis of the decorations showed Si as major, and Co as a minor component. Pure Co₃O₄ phases were not found by EDX analyses of these decorations. These four seemingly unrelated findings are attributed to a common cause: silica migration and weak bond formation between CoO and SiO₂. The extent of surface area losses (i.e. the extent of silica migration) is about an order of magnitude greater in CoO_x–SiO₂ catalysts than in analogously treated SiO₂. The migration of silica must have occurred in a relatively short time period during the thermal decomposition of cobalt nitrate, while simultaneous migration and oxidation of CoO to Co₃O₄ aggregates also occurred. The CoO species intercepted by SiO₂ were unable to oxidize, resulting in reduced quantity of Co₃O₄ formation. The extensive migration of silica is attributed to strong attraction between SiO₂ and CoO species, inducing the removal of silicic acid or silica molecules from the silica surface.     

K2NiF4型Pr2-xSrxCoO4±λ的合成及催化氧化活性

采用聚乙二醇凝胶法合成了Co系稀土复合氧化物Pr_2-xSr_xCoO_4±λ（0.2≤x≤1.0）,以CO和C₃H₈氧化为探针反应及XRD、TPD、TPR、XPS和化学分析等方法对催化剂的组成、结构进行了表征.结果表明：所有样品都具有K₂NiF₄型结构,x=0.2和x=1.0时,样品中有少量杂相,0.2＜x＜1.0时,样品为单一A₂BO₄物相;随x的增大,Pr_2-xSr_xCoO_4±λ催化剂的晶胞参数、晶胞体积和平均晶粒度递减,晶格畸变率、Co³⁺的含量、晶格氧的脱附量及氧空位浓度递增;x＝0.2和x=0.4时,Pr_2-xSr_xCoO_4±λ催化剂的O_1s谱只有结合能在529 eV附近的脱附峰,而x为0.6～1.0时,其O_1s谱呈双峰结构,且结合能在531 eV附近的脱附峰随x增加而增大;CO和C₃H₈氧化反应的催化活性与Co³⁺、晶格氧及氧空位有密切联系.     

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.     

Synthesis,Characterization, and Catalytic Activity of Mn‐doped Perovskite Oxides for Three‐Way Catalysis

Mn‐doped La_0.8Sr_0.2CoO₃ perovskite oxides (La_0.8Sr_0.2Co_1–xMn_xO₃; x = 0, 0.1, 0.3, 0.5) were synthesized by a modified sol‐gel method. The phase‐pure oxides were obtained. CoO and carbonates were formed on the surface of La_0.8Sr_0.2CoO₃. With increasing doping content, these impurities were reduced while the stability of the perovskite structure was improved. The valence state of B‐site ions and the amount of absorbed oxygen were influenced by Mn doping. The catalytic activity of the perovskite catalysts was investigated for CO oxidation and simultaneous removal of CO, C₃H₈, and NO. For CO and NO removal, La_0.8Sr_0.2Co_0.9Mn_0.1O₃ exhibited the best performance. For C₃H₈ removal, the reactivity was promoted linearly with the doping content. The structure‐activity relationship is also discussed.     

Study on a cobalt silica catalyst during reduction and Fischer–Tropsch reaction: In situ EXAFS compared to XPS and XRD

The reduction of a 25 wt% Co/SiO₂ catalyst for the hydrocarbon synthesis has been followed by several techniques: XRD, TPR, XPS and in situ EXAFS. Before reduction the cobalt is present as a Co₃O₄ spinel phase. A two-step reduction of Co₃O₄ to CoO and then to Co° is observed by EXAFS. This is consistent with XPS (surface) and TPR or XRD (bulk) studies. During CO/H₂ reaction, cobalt is always in the metallic state (EXAFS). The coordination number of cobalt has been determined at each reduction step and during CO hydrogenation reaction.     

Electrochemical behaviour of FexCo3?xO4 with (x?=?0, 1, 2 and 3) oxides thin film electrodes in alkaline medium

Spinel-type Fe _x Co_3−x O₄ thin films with x = 0, 1, 2 and 3 were prepared, on stainless steel supports, using the thermal decomposition method at 400 °C. The X-ray diffraction patterns show the presence of a spinel-type structure with a low crystallinity. The electrochemical behaviour was investigated in 1 M KOH, using open-circuit-potential measurements and cyclic voltammetry. The studies allowed to observe the redox reactions occurring at the Fe _x Co_3−x O₄ (x = 1 and 2) oxide surface, namely Fe₃O₄/Fe(OH)₂ or Fe₃O₄/Fe₂O₃, Co₃O₄/CoOOH, Co(OH)₂/CoOOH and CoO₂/CoOOH by comparing the experimental data with those obtained for the Co₃O₄ and Fe₃O₄ films as well as with those referred to in the literature. The results show that iron ions play the major role in the solid-state surface redox transitions in the negative potential range, whereas the cobalt ions are the key species in the positive potential range. However, the contribution of each component, although small, has to be considered in both potential regions.     

Effect of the surface area of cobaltic oxide on carbon monoxide oxidation

Various surface area (S_BET) of cobaltic oxide (Co₃O₄) are prepared by different methods, i.e., precipitation–oxidation, impregnation and hydrothermal. The effect of S_BET of Co₃O₄ on the catalytic property toward CO oxidation is investigated. The results indicate that the optimum S_BET of Co₃O₄ could increase the catalytic activity. Characterization of the cobaltic oxide using X-ray diffraction (XRD), N₂-adsorption at –196 °C, infrared (IR) and temperature-programmed reduction (TPR) reveals that the increase of S_BET on Co₃O₄ can weaken the bond strength of Co–O and promote more lattice oxygen desorption from Co₃O₄ to cause the reduction become easy. We conclude that the S_BET effect, caused by various prepared methods and refined conditions, are responsible for the activity enhancement of Co₃O₄. The T₅₀ (the conversion of CO reached 50%) is decreased significantly when the S_BET is increased, i.e., PO-R230 > PO-C400 > I-C550 > H-150 ~D-Strem.     

Electrical Properties,Cation Distributions,and Thermal Expansion of Manganese Cobalt Chromite Spinel Oxides

As the oxidation and chromium volatilization of chromia‐forming alloy interconnects can cause Solid oxide fuel cells (SOFC) cathode poisoning and cell degradation, spinel coatings like Mn_1.5Co_1.5O₄ have been applied as a barrier to oxygen and chromium diffusion. To evaluate their long‐term stability, the properties of the reaction layer between the Mn_1.5Co_1.5O₄ coating and Cr₂O₃ scale formed on the alloy surface need to be characterized. Therefore, compositions of Mn_1.5?0.5xCo_1.5?0.5xCr_xO₄ (x = 0–2) were prepared to investigate their electrical properties, cation distributions, and thermal expansion behavior at high temperature. With increasing Cr content in manganese cobalt spinel oxides, the cubic crystal structure is stabilized and the electrical conductivity and coefficient of thermal expansion both decrease. The cation distributions determined from neutron diffraction show that Cr and Mn have stronger preference for octahedral sites in the spinel structure as compared with Co.     

Stability of Pt-Co bimetallic particles: effect of the support induced morphology on reactivity in CO hydrogenation

The structure and the stability of bimetallic particles are controlled by their interaction with surface oxide species on support or by support geometry. Location and size of the bimetallic particles estimated by XRD and XPS, depends on the environment and treatment. Thus, the Pt-Co bimetallic particles entrapped in NaY zeolite cages become unstable upon mild oxidation by O₂ and/or reaction with surface protons causing the Co²⁺ ions formed to migrate irreversibly from the supercages into the sodalite cages as determined by TPR and TPD. Differences in the selectivity pattern observed for the CO hydrogenation over alumina and NaY zeolite supported Pt-Co catalysts, can be interpreted by the interaction between bimetallic particles with surface cobalt oxide species and the zeolite cage wall, respectively.On leave from Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, PR China.     

Methanol oxidation over model cobalt catalysts: Influence of the cobalt oxidation state on the reactivity

X-ray photoelectron and absorption spectroscopies (XPS and XAS) combined with on-line mass spectrometry were applied under working catalytic conditions to investigate methanol oxidation on cobalt. Two cobalt oxidation states (Co₃O₄ and CoO) were prepared and investigated as regards their influence on the catalytic activity and selectivity. In addition adsorbed species were monitored in the transition of the catalyst from a non-active state, to an active one. It is shown that the surface oxidation state of cobalt is readily adapted to the oxygen chemical potential in the CH₃OH/O₂ reaction mixture. In particular, even in oxygen-rich mixtures the Co₃O₄ surface is partially reduced, with the extent of surface reduction following the methanol concentration. The reaction selectivity depends on the cobalt oxidation state, with the more reduced samples favouring the partial oxidation of methanol to formaldehyde. In the absence of oxygen, methanol effectively reduces cobalt to the metallic state, also promoting H₂ and CO production. Direct evidence of methoxy and formate species adsorbed on the surface upon reaction was found by analysing the O 1s and C 1s photoelectron spectra. However, the surface coverage of those species was not proportional to the catalytic activity, indicating that they might also act as reaction inhibitors.     

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.     

Cobalt and Copper Composite Oxides as Efficient Catalysts for Preferential Oxidation of CO in H2-Rich Stream

A series of Co–Cu composite oxides with different Co/Cu atomic ratios were prepared by a co-precipitation method. XRD, N₂ sorption, TEM, XPS, H₂-TPR, CO-TPR, CO-TPD and O₂-TPD were used to characterize the structure and redox properties of the composite oxides. Only spinel structure of Co₃O₄ phase was confirmed for the Co–Cu composite oxides with Co/Cu ratios of 4/1 and 2/1, but the particle sizes of these composite oxides decreased evidently compared with Co₃O₄. These composite oxides could be reduced at lower temperatures than Co₃O₄ by either H₂ or CO. CO and O₂ adsorption amounts over the composite oxides were significantly higher than those over Co₃O₄. These results indicated a strong interaction between cobalt and copper species in the composite samples, possibly suggesting the formation of Cu _x Co_3?x O₄ solid solution. For the preferential oxidation of CO in a H₂-rich stream, the Co–Cu composite oxides (Co/Cu = 4/1–1/1) showed distinctly higher catalytic activities than both Co₃O₄ and CuO, and the formation of Cu _x Co_3?x O₄ solid solution was proposed to contribute to the high catalytic activity of the composite catalysts. The Co–Cu composite oxide was found to exhibit higher catalytic activity than several other Co₃O₄-based binary oxides including Co–Ce, Co–Ni, Co–Fe and Co–Zn oxides.     

Effects of iron and cerium in La1−yCeyCo1−xFexO3 perovskites as catalysts for VOC oxidation

Perovskite-type mixed oxides La_1−yCe_yCo_1−xFe_xO₃ with high specific surface area were prepared by reactive grinding. These catalysts were characterized by N₂ adsorption, X-ray diffraction, oxygen storage capacity (OSC), H₂-temperature-programmed reduction (TPR-H₂), O₂-, and CH₃OH-temperature-programmed desorption (TPD). The catalytic performance of the samples for volatile organic compounds (VOC), CH₃OH, CO and CH₄ oxidation was evaluated. Cerium allows an enhancement of the reducibility of the B-site cations in perovskite structure during OSC and TPR-H₂ and an increase in the amount of β-O₂ desorbed during TPD-O₂. As opposed to cerium, the addition of iron in the perovskite structure causes a drop in B-site cations reducibility and a decrease of the oxygen mobility in the bulk. As a consequence, the catalytic activity in VOC oxidation is enhanced by introduction of cerium and weakened by iron in the lattice.     

Comparative Study on Catalytic Performances for Low-temperature CO Oxidation of Cu–Ce–O and Cu–Co–Ce–O Catalysts

Two series of Cu–Ce–O and Cu–Co–Ce–O catalysts were prepared by co-precipitation method. The prepared catalysts were characterized by XRD, IR, TPR, XPS, BET and ICP-AES. The catalytic activities of the catalysts for low-temperature CO oxidation were evaluated through a microreactor-GC system. TPR results indicate that the addition of cobalt to the Cu–Ce–O can increase the dispersion of copper oxide, and the interaction between cobalt and copper can enhance the reducibility of each other. XPS analysis show that Ce⁴⁺, Cu²⁺, along with Co₃O₄, are present on the surface of Cu_0.4Co_0.6Ce₄ catalyst. The Co/Cu atomic ratio and the calcination temperature have significant effect on the activities of the catalysts. Compared with Cu₁Ce₄ catalyst, the Cu_0.4Co_0.6Ce₄ catalyst has better activity and thermal stability.     

Study of Oxygen Reactivity in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>3−δ</Subscript> Perovskites for Total Oxidation of Toluene

Abstract  

The total oxidation of toluene is studied over catalytic systems based on perovskite with general formula AA′CoO_3-δ (A = La, A′ = Sr). The systematic and progressive substitution of La³⁺ by Sr²⁺ cations in the series (La_1−x Sr _x CoO_3−δ system) of the perovskites have been studied to determine their influence in the final properties of these mixed oxides and their corresponding reactivity performance for the total oxidation of toluene as a model volatile organic compound with detrimental effects for health and environment. The structure and morphology of the samples before and after reaction have been characterized by XRD, BET and FE-SEM techniques. Additional experiments of temperature programmed desorption of O₂ in vacuum and reduction in H₂ were also performed to identify the main surface oxygen species and the reducibility of the different perovskites. It is remarkable that the La_1−x Sr _x CoO_3−δ series presents better catalytic performance for the oxidation of toluene, with lower values for the T₅₀ (temperature of 50 % toluene conversion) than the previously studied LaNi_1−y Co _y O₃ 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.