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Journal of Science: Advanced Materials and Devices (2016) 337e342 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Production of cobalt-copper from partial reduction of La(Co,Cu)O3 perovskites for CO hydrogenation Nguyen Tien Thao*, Le Thanh Son Faculty of Chemistry, Vietnam National University Hanoi, 19 Le Thanh Tong ST, Hanoi, 10999, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received 15 June 2016 Accepted 28 July 2016 Available online 18 August 2016 La(CoCuO3) nanoperovskites have been prepared by the mechano-synthesis method and treated with hydrogen to yield a high dispersion of bimetallic CoeCu sites The reduced LaCo1-xCuxO3 samples were characterized by XRD, H2-TPR, CO and H2 chemisorption and tested for CO dissociation and for alcohol synthesis from syngas The experimental results indicated that the activities in CO dissociation and hydrogenation on copper-cobalt metals extracted from perovskite lattice crystals are significantly different from those in the extra-perovskite lattice The overall catalytic activity in syngas conversion is correlated with the CoeCu metal surface, but the alcohol productivity e productivity of alcohols decreases in the order of LaCo0.7Cu0.3O3 > LaCo0.4Cu0.6O3 > Cu2O/LaCoO3 > LaCo0.9Cu0.1O3 > LaCoO3 The highest catalytic activity and alcohol productivity was obtained over sample of the reduced LaCo0.7Cu0.3O3 perovskite catalyst © 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: CO hydrogenation Metal dispersion CueCo Perovskite Introduction Perovskites, mixed oxides of the general formula ABO3, have extensively been applied in many fields due to their particular compositional structure [1] In principle, the ideal perovskite structure is cubic with the space-group Pm3m-Oh in which the A cation occupies at the body center, the cation (B) is at the cube corner, and the oxygen stays at the midpoint of the cube edges [1,2] By this way, the perovskite derivatives may be synthesized by the replacement of another element in A and/or B position [1,3,4] In the present work, we have partially substituted Co3ỵ in lanthanumcobaltate by Cu2ỵ to obtain La(Co,Cu)O3 perovskite catalyst precursors The partial reduction of La(Co,Cu)O3 perovskites may further produce metallic copper-cobalt metals those originate intentionally from the perovskite lattice As a result, a finely dispersed metal catalyst from perovskite precursors would be expected to use for several applications [2,4] In experimental, Crespin and Hall [5] had produced Co0/La2O3 from the reduction of LaCoO3 under hydrogen atmosphere Fierro et al [6] received the Ni/La2O3 after the complete reduction of LaNiO3 at 705 K Bedel et al [4] only carried out the partial reduction of La(Co,Fe)O3 orthorhombic perovskites at 723 K, producing a small amount of metallic cobalt while the perovskite lattice still preserved Thus, the perovskite product has exhibited a high catalytic activity in many applications such as CO oxidation [7] hydrogenation of ethylene [8], reforming of CO2 [9], and conversion of syngas (H2/CO) into many useful chemicals and liquid fuels [10,11] The latter conversion is a very important process since a mixture of alcohols is a crucial gasoline additive or green vehicle fuel today [12e14] In our previous work, we have reported some novel characteristics of lanthanum-cobaltate nanoperovskites prepared by mechano-synthesis method [10,11,14] The reduction of such materials leads to the formation of a well-homogenized supported bimetallic alloy Furthermore, the co-existence of two transition metal ions in the solid lattice results in the formation of dual sites which are active for many oxidation-reduction applications [4,15e19] This article is to present a way for the preparation of metals supported catalysts for the conversion of carbon monoxide into oxygenated compounds Experimental 2.1 Catalyst preparation * Corresponding author Fax: ỵ84 (04) 3824 1140 E-mail address: ntthao@vnu.edu.vn (N Tien Thao) Peer review under responsibility of Vietnam National University, Hanoi A series of CoeCu bases perovskites were synthesized by mechano-synthesis method The stoichiometric proportions of commercial lanthanum, copper, and cobalt oxides (99%, Aldrich) http://dx.doi.org/10.1016/j.jsamd.2016.07.011 2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) N Tien Thao, L.T Son / Journal of Science: Advanced Materials and Devices (2016) 337e342 were mixed together with three hardened steel balls (diameter ¼ 11 mm) in a hardened steel crucible (50 mL) A SPEX high energy ball mill working at 1000 rpm was used for mechanosynthesis Milling was carried out for h prior to a second milling step with an alkali additive Then, the resulting powder was mixed to 50% sodium chloride (99.9%) and further milled under the same conditions for 12 h before washing the additives with distilled water A sample was added into a beaker containing 1200 mL water and stirred by magnetic stirring for 90 prior to being sedimented for 3e5 h After the clean water is removed, the slurry was dried in oven at 60e80 C before calcination at 250 C for 150 Total adsorption Chemisorption Physisorption 1.40 H2 - Chemisorobed uptake (ml/g) 338 1.20 1.00 0.80 0.60 0.40 0.20 0.00 2.2 Characterization 10 20 30 40 50 Pressure (torr) The elemental chemical analysis of copper and cobalt in the perovskites was performed by atomic absorption spectroscopy using a PerkineElmer 1100B spectrometer Phase analysis and crystal domain size determination were performed by powder Xray diffraction (XRD) using a SIEMENS D5000 diffractometer with CuKa radiation (l ¼ 1.54059 Å) Bragg's angles between 15 and 75 were collected at a rate of 1 /min To measure the real surface area of the reduced perovskites, two other BET experiments were performed using a flow system (RXM100, Advanced Scientific Designs, Inc., ASDI) First, 70e100 mg of catalyst was calcined at 773 K (ramp of K/min) under 20 mL/min of O2/He (20 vol %) for 90 and then evacuated at 723 K for 90 (P 8.5  10À8 mmHg) Nitrogen adsorption was carried out at 77 K Each point of the adsorption isotherm was established by introducing a given amount of nitrogen from the reaction manifold into the reactor Temperature Programmed Reduction (TPR) experiments were carried out after evacuating N2 adsorption (BET measurement) TPR of the catalysts was then carried out by ramping under 4.65 vol.% of H2/Ar (20 mL/min) from room temperature to 773 K (5 K/min) for 90 The second BET measurement of the sample after reduction was also done in situ Chemisorption performance with H2 at 373 K and CO at room temperature was carried out after the second BET measurement The H2-chemisorption performance was similar to steps for BET measurement of nitrogen After the first isotherm that contains both physical adsorption and chemisorption was collected, the sample was evacuated at adsorption temperature for 5e10 in order to remove all physically adsorbed species prior to the second adsorption The difference between the first and the second isotherm gives the chemisorption isotherm H2-and CO-uptake were determined by extrapolating the straight-line portion of the adsorption isotherms to zero pressure as represented in Fig CO dissociation tests on the reduced samples were carried out using a RXM-100 system Prior to pretreatment, 40e50 mg of catalyst was ramped at 10 K/min up to the calcination temperature (773 K) under 20 vol.% O2/He (20 mL/min) for 90 and then cooled down to room temperature under a flow of 20 mL/min He for 60 in order to remove the physically adsorbed gas The pretreatment of the catalysts was then carried out from room temperature up to 798 K (5 K/min) for 90 under 4.65 vol % of H2/Ar (20 mL/min) and then cooled down to reaction temperature under a flowing 20 mL/min of He A number of CO/He (0.586 vol %) pulses (0.25 mL) were then injected and passed through the reactor prior to on line analysis using mass spectrometer (UTI-100) The m/ z signals 2, 18, 28, 44 were collected 2.3 Catalytic activity The catalytic tests were carried out in a stainless-steel continuous fixed-bed flow micro-reactor (BTRSeJr PC, Autoclave Engineers) The reaction pressure was controlled using a back-pressure Fig H2 e Chemisorption at 373 K over LaCo0.4Cu0.6O3 reduced at 773 K regulator The syngas mixture (H2/CO ¼ 2/1) was diluted in helium (20 vol %) A mixture of reactants and inert gas was supplied from a pressurized manifold via individual mass flow controllers The catalyst pellet size was 40 mesh Catalysts were pretreated in situ under flowing vol.% of H2/Ar (20 mL/min) prior to each reaction test The temperature was kept at 523 K (3 h), and 773 K (2h30) with a ramp of K/min Then, the reactor was cooled down to the reaction temperature while pressure was increased to 69 bars by feeding a reaction mixture of gases The products were analyzed using a gas chromatograph equipped with two capillary columns and an automated online gas sampling valve maintained at 443 K The temperature of transfer line between the reactor and the valves was kept at 493 K in order to avoid any product condensation Carbon monoxide and carbon dioxide were separated using a capillary column (Carboxen™ 1006 PLOT, 30 m  0.53 mm) connected to the TCD Quantitative analysis of all organic products was carried out using the second capillary column (Wcot fused silica, 60 m  0.53 mm, Coating Cp-Sil 5CB, DF ¼ 5.00 mm) connected to a FID detector (Varian CP e 3800) and mass spectrometer (Varian Saturn 2200 GC/MS/MS) Results and discussion 3.1 Catalyst characteristics The physical properties of all fresh catalyst samples are measured and shown in Table X-ray diffraction spectra of all samples were collected (but not shown) and the crystal phase is presented in Table [10] Although Table shows each sample contains at least two components, but it is noted that the main phase is perovskite a long with a very small amount of starting metal oxide material(s) as impurities [10] Both LaCoO3 and La(Co,CuO)O3 perovskites are presented as the well-structured Table Properties of the synthesized perovskites Nominal Sample XRD analysisa BET surface area (m2/g)b Chemical composition Co Cu LaCoO3 LaCo0.9Cu0.1O3 LaCo0.7Cu0.3O3 LaCo0.4Cu0.6O3 Cu2O/LaCoO3 P, P, P, P, P, 60 20 22 21 16.8 21.1 19.3 18.6 9.8 20.0 e 1.9 5.8 11.6 3.3 Co3O4 Co3O4 Co3O4, CuO Co3O4, CuO Cu2O, CuO a XRD spectra were compared to JCPDS files: P: Perovskite (JCPDS No 48e0123); Co3O4 (JCPDS No 42e1467); CuO(JCPDS No 45e0937) b BET surface area before reduction N Tien Thao, L.T Son / Journal of Science: Advanced Materials and Devices (2016) 337e342 1800 1500 L in (C p s) 1200 900 Cu O/LaCoO LaCo Cu O 600 LaCo Cu O 300 LaCo Cu O LaCoO 20 30 40 2θ (degree) 50 60 Fig XRD patterns of all catalyst precursor samples 25 20 TCD Signal (a.u) rhombohedral perovskite as shown in Fig [3e5,10] The crystal domain, determined from the FWHM of the (102) diffraction peak using Scherrer's equation after Warren's correction of instrumental broadening, is in the range from 7.9 to 10.5 nm The third column in Table indicates that all ground perovskites samples have medium surface area, ranging from 20 to 60 m2/g.Fig The reducibility of the ground perovskites is interpreted through the H2-TPR analysis as represented in Fig It is clearly observed that the copper efree perovskite sample shows two distinct visible peaks at 670 and 960 K The low temperature peak is firmly ascribed to the reduction of Co3ỵ to Co2ỵ and the other broad peak is attributed to the complete reaction of cobalt divalent to metallic phase, in good harmony with the results reported by several groups [1,4e6,9,10] A similar H2-TPR feature is also observed for LaCo0.9Cu0.1O3, the profile slightly shifts to the lower temperature (Fig 3) Thus, the first shaped-peak is observed at 650 K while the second is at 840 K It is worthily noted that the H2TPR baseline was completely recovered at 950 K showing that the reduction is essentially terminated at much lower temperature as compared with the case of LaCoO3 [10,12] The calculation of H2 consumed amount balance indicates that the reduction of Co3ỵand Cu2ỵ to Co2ỵ and Cu0 below 671 K, whereas that of Co2ỵ to Co0 at 840 K [4,11,15] An increased amount of intra-perovskite lattice copper leads to a significant affect on the perovskite reducibility [15] For LaCo0.7Cu0.3O3 sample, the lower peak is visible, but the other is very broadening from 643 to 943 K When a larger amount of intra-lattice cobalt is replaced by copper ions, the two distinct peaks in H2-TRP trace of the perovskite sample seems to coalesce into a single peak at 687 K while that of the physical mixture of Cu2O/LaCoO3 still preserves two visible peaks at 670 and 1018 K This is interpreted by the assumption of mutual interaction between cobalt and copper ions in the reduced form; the reduced copper metal is to promote the reducibility of cobalt ions in the framework when copper was essentially extracted from the perovskite lattice at lower reduction temperature as demonstrated by H2-TPR results [1,10,15,19] Moreover, hydrogen is well known to be easily dissociated to hydrogen atoms on metallic copper sites Consequently, the reduction of cobalt ions (Co3ỵ and Co2ỵ) by atomic hydrogen is presumably taken place at lower temperatures [1,10,15] Thus, the reduction of La(Co,Cu)O3 is to provide a finely dispersed CoeCu atoms on the catalyst surface and the formation of bimetallic alloy is not ruled out [19] To determine the dispersion of metals formed, we have measured both H2 and CO chemisorptions for the reduced perovskite forms Unfortunately, the determination of each individual component dispersion level is a very difficult task due to a synergic interaction between two metals after the partial reduction of 339 15 Cu2O/LaCoO3 10 LaCo0.4Cu0.6O3 LaCo0.7Cu0.3O3 LaCo0.9Cu0.1O3 LaCoO3 300 450 600 750 900 1050 Temperature (K) Fig H2-TPR profiles for the catalyst samples from room temperature to 1050 K in 20 mL/min of 4.65 vol.% of H2/Ar flowrate perovskites [10,11,15,19] In this case, we have recorded total H2 and CO chemisorbed volume (mL/g) of each sample (Fig 1) The volume of H2 and CO uptake of all reduced samples is in turn presented in Table The reduced samples were investigated the ability of CO dissociation to C* intermediate which further hydrogenates to carbon skeleton through performing the dissociation of CO versus temperature programmed from room temperature to 798 K The relationship between CO dissociation conversion and temperature is displayed in Fig It seems that the CO decomposition level is related with the chemical composition and dissociation temperatures [15,17,20] The presence of copper component gives rise to slight decreased CO dissociation conversion in the temperature range of 500e798 K The CO dissociation conversion decreases with the order of LaCoO3 > CuO/ LaCoO3 > LaCo0.7Cu0.3O3 > LaCo0.4Cu0.6O3 ! LaCo0.9Cu0.1O3 (Fig 4) It is well known that cobalt metal has shown a very good activity in the dissociation of CO while copper is inactive for the CO splitting [17,21,22] In this case, copper plays an important role in the synthesis of alcohols through the protection of the OH functional groups during the hydrogenation conditions [11,20,22] Thus the higher CO conversions over the cobalt-rich samples are certainly comprehensive [4,21] Based on the ability of the reduced samples to dissociate CO molecule, we are firmly expected that the reduced copper-free perovskite is active for the synthesis of hydrocarbons while the perovskite containing copper may act as promising catalysts for the hydrogenation of CO to linear primary alcohols 3.2 Catalytic activity in hydrogenation of carbon monoxide All the prepared samples are pretreated under hydrogen flowrate at 798 K prior to test for the hydrogenation of carbon monoxide at 548 K, 68.9 bars and space velocity VVH ¼ 5000 hÀ1 (H2/CO/ He ¼ 8/4/3) The products contain a mixture of linear primary alcohols and n-alkane in addition to small amounts of secondary alcohols and isoparafins and the formation of products is believed to be associated with the catalyst metal surface [22] Thus, we presented the correlation between CO conversion and product selectivity versus the CO-chemisorbed volume uptake With the exception of mixture Cu2O/LaCoO3 sample, Fig shows an increased CO conversion with the CO chemisorbed-volume in order of LaCo0.7Cu0.3O3 > Cu2O/ LaCoO3 > LaCoO3 > LaCo0.4Cu0.6O3 > LaCo0.9Cu0.1O3 The selectivity to alcohols obtained over these catalyst samples are presented in Fig Although product selectivity seems to be well correlated with the CO-chemisorbed volume, it should be less meaningful as compared the product selectivities at different conversion values (Fig 6) Thus, a comparison between the productivities may give more insight into the catalytic behavior [10,22] Undoubtedly, Fig 340 N Tien Thao, L.T Son / Journal of Science: Advanced Materials and Devices (2016) 337e342 Table Effect of hydrogen pretreatment temperature on alcohol productivity over sample LaCo0.7Cu0.3O3 in CO hydrogenation at 548 K (VVH ¼ 5000 hÀ1, 69 bar, H2/CO/He ¼ 8/4/3) H2evolume (mL/gcat) CO conversion (%) Alcohol selectivity (%) Alcohol productivity (mg/gcat/h) 623 723 773 823 0.78 0.93 0.84 0.56 17.5 16.1 25.1 8.7 27.2 41.9 42.9 43.5 43.0 49.4 70.1 33.4 100 90 80 70 LaCo0.7Cu0.3O3 LaCoO LaCoO3 CuO CuO/LaCoO3 2/LaCoO3 LaCo LaCo0.7Cu0.3O3 0.7Cu0.3O3 60 50 40 30 20 LaCo LaCo0.9Cu0.1O3 0.9Cu0.1O3 LaCo0.4Cu0.6O3 450 500 550 600 650 700 Temperature (K) 750 800 CO -Chemisorbed uptake (mL/g cat) CO disociation (%) Reduction temperature (K) 0.028 Cu2O/LaCoO3 0.018 LaCoO3 0.016 LaCo0.4Cu0.6O3 0.014 0.012 Fig CO dissociation ability at different temperatures on the reduced samples after pre-treatment at 798 K in H2/Ar (0.586 vol % CO/He pulses (0.25 mL) were then injected through the catalyst) LaCo0.7Cu0.3O3 0.028 Cu2O/LaCoO3 0.018 LaCoO3 0.016 Conversion Alcohols LaCo0.4Cu0.6O3 0.014 LaCo0.9Cu0.1 0.012 15 25 35 45 55 65 75 CO Conversion (%) and Alcohol productivity (mg/gcat/h) Fig Correlation between the volume of CO chemisorbed uptake and CO hydrogenation activity at 548 K (VVH ¼ 5000 hÀ1, 69 bar, H2/CO/He ¼ 8/4/3) 10 20 30 40 50 60 70 80 90 100 Alcohol and hydrocarbon selectivity (wt.%) Fig Correlation between the volume of CO chemisorbed uptake and CO hydrogenation activity at 548 K (VVH ¼ 5000 hÀ1, 69 bar, H2/CO/He ¼ 8/4/3) alcohol productivity gradually increases and reaches a maximal value at 773 K and then sharply decreased at higher reduction temperatures This observable trend is explained by the fact that the surface composition is strongly associated with pretreatment temperature because of the reduction of CoeCu based perovskites happening in a multiple-step process at different temperatures [1,2,4,8,10] The surface concentration of cobalt and copper metals is very sensitive to the reduction temperatures [4,5,9,11,15,21,25] As an increased in H2-reduction temperature, the (Cu0eCo0)surface/ (CueCo)total molar surface ratio is varied and probably approached a highest value around 773 K as elucidated by hydrogen chemisorption data (Table 2) [23] A higher reduction temperature gives rise to a sintering of atomic copper metals and as consequence the active sites for the formation of OH alcohol functional group gradually decreases Indeed, it was widely reported that the reduction of perovskites can be described either by the contrasting-sphere model or by the Volume of H2 -uptake / SBET (mL.g/m2) CO -Chemisorbed uptake (mL/g cat h) shows the productivity of alcohols decreases monotonically with CO uptake in the order of LaCo0.7Cu0.3O3 > LaCo0.4Cu0.6O3 > Cu2O/ LaCoO3 > LaCo0.9Cu0.1O3 > LaCoO3 This observation is not with respect to the order of CO chemisorbed volume, but in good agreement with the ratio of H2 volume uptake/BET surface area of the sample after reduction (Fig 7) This phenomenon is explained by the composition of the catalyst surface and the available abundance of bimetallic cobalt-copper sites on the catalyst surface after reduction [11,16,22e24] Certainly, the presence of intra-lattice copper (LaCo0.7Cu0.3O3) has a promotional effect on the formation of alcohols as compared with the extra-lattice copper (Cu2O/LaCoO3) or the copper-free perovskite sample [11,23] The formation of La(Co,Cu)O3 perovskite precursors would provide intimidate dual copper-cobalt sites which are prerequisite for the formation of alcohols from CO and H2 [17,19,25] This issue is further supported by the examination of the catalytic activity at different pretreatment conditions Table displays the alcohol selectivity/productivity versus the reduction temperatures obtained on LaCo0.7Cu0.3O3 It is noted that the Hydrocarbons Alcohols LaCo0.9Cu0.1O3 0.07 0.06 0.05 0.04 0.03 0.02 0.01 LaCoO LaCoO3 Cu2O/LaCoO3 LaCo0.9Cu0.1O3 LaCo0.7Cu0.3O3 LaCo0.4Cu0.6O3 Cu 2O/LaCoO3 Perovskite catalysts Fig The correlation between H2-volume uptake (mL/g)/SBET ratio and the samples reduced at 773 K prior to chemisorption of hydrogen at 373 K 20 15 10 Propanol Butanol Pentanol+ 25 Methanol Ethanol Propanol Butanol Pentanol+ 30 Propanol Butanol Pentanol+ Methanol Ethanol 40 35 Propanol Butanol Pentanol+ Alcohol Distribution (%) 45 Methanol Ethanol 50 Methanol Ethanol N Tien Thao, L.T Son / Journal of Science: Advanced Materials and Devices (2016) 337e342 623 723 773 Temperature (K) 823 Fig Effect of hydrogen pretreatment temperature on alcohol distribution over sample LaCo0.7Cu0.3O3 in CO hydrogenation at 548 K (VVH ¼ 5000 hÀ1, 69 bar, H2/CO/ He ¼ 8/4/3) Ln [(wt.%)/n] -1 -2 α3 = 0.43 α2 = 0.42 -3 α1 = 0.38 -4 Carbon number 10 11 Fig ASF distribution of products obtained at pretreatment temperature of 773 K over LaCo0.7Cu0.3O3 (548 K, VVH ¼ 5000 hÀ1, 69 bar, H2/CO/He ¼ 8/4/3) (a1 ¼ C1OHeC7OH, a2 ¼ C2OHeC7OH, a3 ¼ C1eC10 hydrocarbons) nucleation mechanism [1,2,5,8,9] Thus, the total metal surface area is strongly dependant on the pretreatment conditions [8,10,18,20] In the present study, the LaCo0.7Cu0.3O3 is reduced at 773 K gives the most effective catalyst for the formation of higher alcohols from CO hydrogenation reaction The alcohol product distribution is presented in Fig which contains a mixture of linear primary alcohols from methanol to heptanol [10,22] The product distribution is recalculated as Anderson-Schulz-Flory (ASF) rule and the plot between ln(wt.%/n) versus carbon number (n) is drawn in Fig [20,22] Since the carbon chain growth factor of alcohols (designated as a1) is not on par with that of hydrocarbons (a3), we have recalculated the second one (a2) excluding methanol because methanol may be independently produced by different pathways [11,19,26e30] In the case, the alcohol chain growth factor (a2) of C2OH e C7OH stays at middle value between a1 and a3 This indicates that the formation of skeletal carbons of primary alcohols occurs parallel to that of hydrocarbons on cobalt catalyst surface [20,22,24e27] A close distance between cobalt and copper sites on catalyst surface has steered the formation of hydrocarbons into primary alcohols by insertion of undissociated CO molecule absorbed on copper sites [11,19,20,22,28e30] Conclusions A set of La(Co,Cu)O3 perovskite samples prepared by grounding method was pretreated in H2 prior to test for the CO hydrogenation reaction The presence of copper ions in the perovskite lattice 341 results in a significant effect on the perovskite reducibility Under the same pretreatment conditions the LaeCoeCu based perovskites is easily reduced, yielding metallic cobalt and copper sites dispersed over a La2O3 matrix The CO dissociation ability of cobalt is remarkably affected by the presence of neighboring copper atoms The overall activity of the catalysts in syngas conversion strongly depends on pretreatment temperature and the metal surface area The intra-framework copper 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determined by extrapolating the straight-line portion of the. .. activity in the dissociation of CO while copper is inactive for the CO splitting [17,21,22] In this case, copper plays an important role in the synthesis of alcohols through the protection of the OH... to the reduction of Co3ỵ to Co2ỵ and the other broad peak is attributed to the complete reaction of cobalt divalent to metallic phase, in good harmony with the results reported by several groups
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