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The role of CO in the water-induced formation of cobalt oxide in a high conversion Fischer-Tropsch environment Moritz Wolf, Bridget Mutuma, Neil John Coville, Nico Fischer, and Michael Claeys ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04177 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018
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ACS Catalysis
The role of CO in the water-induced formation of cobalt oxide in a high conversion Fischer-Tropsch environment Moritz Wolf,†‡ Bridget K. Mutuma,§ Neil J. Coville,‡§ Nico Fischer,†‡ and Michael Claeys*,†‡ †
Catalysis Institute, Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa. DST-NRF Centre of Excellence in Catalysis c*change, Rondebosch 7701, South Africa. § DST-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Wits 2050, South Africa. Keywords: cobalt catalyst, Fischer-Tropsch synthesis, deactivation, oxidation, magnetometer, water, carbon monoxide ‡
ABSTRACT: A Co/C model catalyst was exposed to increasing partial pressures of water simulating high Fischer-Tropsch conversions at varying concentrations of synthesis gas. The stability of the metallic cobalt phase against oxidation and sintering under such conditions was monitored in an in situ magnetometer. Direct oxidation of cobalt to cobalt oxide with water-derived oxygen through a water splitting mechanism was shown to be kinetically hindered, even at high partial pressures of water. Combined threshold partial pressures of carbon monoxide and water were identified for rapid oxidation of cobalt, above which the removal of adsorbed oxygen species, originating from the dissociation of carbon monoxide on the metallic cobalt surface, by surface produced water was hindered resulting in water-induced oxidation.
In the Fischer-Tropsch (FT) synthesis, supported Co crystallites in the nanometer range have been reported to show very different deactivation resistance to oxidation when compared to bulk Co.1–3 This lower stability has been attributed to a higher relative contribution of the surface energy of the small particles to the overall energy.4 Thermodynamic predictions on the stability of metallic Co crystallites have shown a direct dependency of the oxidation of Co to CoO on the partial pressure ratio of H2O to H2 (pH2O/pH2) and the crystallite size,4 while the oxide formation out of the bulk Co phase is not feasible at FT conditions.4,5 Oxidation of bulk Co by H2O to Co3O4 or Co(OH)2 is even less likely (Fig. S1).5,6 Oxidizing H2O and reducing H2 are the major product and reactant of the FT synthesis, respectively. Two mechanisms may play a role in the oxidation of nano-sized Co to FT inactive CoO. Firstly, H2O may lead to a direct oxidation of Co via a H2O splitting mechanism, i.e. CoO is H2O-derived. Secondly, the generation of adsorbed highly electronegative oxygen species (O*) is intrinsically embedded in the FT mechanism via dissociation of CO on the Co surface, which can result in the formation of CoO.7 This is especially true if removal of O* by H2 via formation of H2O is hindered, e.g. at high FT conversions where high concentrations of H2O are produced.4,7 Recently, in situ magnetic measurements identified this indirect oxidation pathway as the major mechanism for the formation of CoO, while the direct oxidation of Co by H 2O to form CoO appeared to be kinetically hindered.2 This is hypothesized to be caused by the high stability of hydroxyl species on the surface of Co.8–10 Direct oxidation of Co by H2O to metalsupport compounds can also occur when utilizing metal oxide carriers.1–3,5 This solid-state reaction of Co with the support has been found to be pronounced for smaller Co crystallite sizes on SiO2, where cobalt silicate type species were formed.2 This observation is consistent with a previously reported size dependency of the oxidation of Co crystallites on Al2O3 under realistic
FT conditions.1 In both studies, this formation of metal-support compounds prohibited an exclusive and distinct characterization of the oxidation process of Co to CoO. In the present study, a 5 wt.% Co/C model catalyst was developed to study the effect of CO and H2O on the Co oxidation reaction in H2O-rich environments simulating high FT conversion levels. A similar model catalyst with SiO2 Stöber spheres as support material has been studied recently by Wolf et al.2 An exacerbating effect of the presence of CO on the stability of Co nanoparticles against oxidation to CoO by H2O was identified. However, the formation of cobalt silicates was underlying2 and may prevent distinct conclusions. A deactivation of Co-based catalysts via the formation of metal-support compounds is generally expected under high conversion FT environment for commonly utilized metal oxide carriers such as Al2O3 or SiO2.1–3 In contrast, solid carbon spheres (SCSs) as the support of choice in the present study may limit such a deactivation pathway. A Co/C model catalyst system has been previously utilized by Bezemer et al.11 In their study they did not observe oxidation of nano-sized (5.1 nm) Co/C during exposure to pH2O/pH2 ratios of 1-30 in the absence of CO and concluded that oxidation is not a deactivation mechanism in the commercial FT synthesis (pH2O/pH2 ≈ 1). Potential effects of CO were not tested. Herein, the stability of Co nanoparticles as a model catalyst for the oxidation to cobalt oxide was monitored in situ in a magnetometer (Fig. S2-4), which was developed by Claeys et al.1,2,12,13 in a collaboration between the University of Cape Town and Sasol. The reduced catalyst was exposed to H2O/H2/Ar atmospheres with stepwise increases in the partial pressure of H2O at 493 K to simulate CO conversions of 26-99% in the FT synthesis. After a subsequent exposure to ‘dry’ H2/Ar, CO was introduced and, once again, high conversions were simulated by co-feeding H2O to the reaction. Lastly, the partial pressure of synthesis gas was also increased.
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Monodisperse Co3O4 crystallites with a volume mean size of 3.8 nm were synthesized via heat treatment of Co acetate in benzyl alcohol in the presence of NH4OH(aq) (see Supporting Information for experimental details).2,14,15 Analysis of transmission electron microscopy (TEM) images of the Co 3O4 crystallites demonstrated a narrow size distribution and analysis by means of X-ray diffraction (XRD) exhibited the high crystallinity and purity of the oxide phase (Fig. 1). SCSs were prepared in a vertical chemical vapor deposition reactor at 1193 K with acetylene as carbon precursor.16 The purified onion-like SCSs have a surface mean diameter of 266 nm with a standard deviation of 137 nm (Fig. 1b). The BET surface area was found to be 10.8 m2/g while the carbon content was close to 100 wt.%. 16 The parent model catalyst was prepared by depositing the Co 3O4 crystallites onto the SCSs via ultrasonication in ethanol2 to give a Co loading of 4.4 wt.% according to elemental analysis data. Most crystallites were present as small agglomerates (Fig. 1c) indicating an insufficient amount of anchoring points on the relatively inert support.
Figure 1. TEM images of (a) the unsupported Co3O4 crystallites with volume-based size distribution, (b) the solid carbon spheres with number-based size distribution, and (c) the carbon supported Co3O4 crystallites representing the parent model catalyst. Normalized XRD patterns of (d) the model catalyst, (e) the SCSs, and (f) the unsupported Co3O4 nanoparticles with a reference pattern for Co3O4 are also shown.
The model catalyst was carefully reduced in the fixed-bed reactor of the in situ magnetometer (Fig. S5) at a holding temperature of 573 K and a high gas hourly space velocity (GHSV) of 30000 mL (gcatalyst h)-1 in a 50% H2/Ar atmosphere to minimize the concentration of formed H2O. After cool-down to 493 K, the magnetization of the catalyst corresponded to a degree of reduction (DOR) of 89.9%. The GHSV was adjusted to 12500 mL
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(gcatalyst h)-1 and the catalyst was exposed to different concentrations of H2O by incrementally increasing the partial pressure of H2O (22-7200 mbar) while keeping the partial pressures of H2 (144 mbar) and Ar (856 mbar) constant. This allowed for the exposure to pH2O/pH2 ratios from 0.15 to 50 simulating the gas phase composition expected at CO conversion levels between 26-99% (Table S1). The magnetization of the model catalyst in the strong external magnetic field was monitored in situ and correlated to the amount of metallic, ferromagnetic Co present. Oxidation of metallic Co leads to a decrease of the magnetization, while reduction of oxidic Co phases results in an increase. The adsorption of species on the Co surface can affect the magnetization of surface Co atoms due to associated changes of the electronic properties. Species that can be expected for H2O dissociation, such as O* and OH*, are reported to increase the magnetization.2,13 Thus, an initial increase of the magnetization (Fig. 2a) at the H2O levels 1-3 (pH2O/pH2 = 0.15-5) can be attributed to such adsorbed species on the metallic Co surface. OH* species in particular show a high stability on the Co surface.8–10 At a pH2O/pH2 ratio of 10, a first decrease in magnetization was observed (1% of the magnetization after reduction), i.e. the surface of the metallic Co particles was partially oxidized and transformed into a (at 493 K) non-magnetic cobalt oxide, presumably CoO. After exposure to all H2O levels, the magnetization corresponded to 96% of the initial magnetization even though the atmosphere consisted of up to 50 times more H2O than H2. This confirmed the high stability of Co crystallites against direct oxidation to cobalt oxide by H2O as hypothesized in our previous study.2 The observed direct oxidation appears to be kinetically hindered as indicated by a slow continuous decrease of the magnetization with limited dependency on the pH2O/pH2 ratio. The partially oxidized catalyst re-reduced rapidly when H2O was removed from the feed stream showing thermodynamically controlled reaction characteristics. Hence, the direct oxidation of Co to cobalt oxide is completely reversible at low concentrations of H2 in Ar. Measurement of the remnant magnetization upon removal of the external field allows for conclusions to be made on the magnetic domain size. In the present study, the magnetic domain size can be expected to correspond to the crystallite size of Co as single-domains are energetically favored for spherical Co particles below a diameter of approx. 68 nm.17 Small fcc-Co crystallites below 15 nm are not capable of retaining a remnant magnetization as they are superparamagnetic.18,19 Although, the exact size is still under debate.11,20 The weight fraction of Co that displayed remnant magnetization upon removal of the external field (γ) increased when oxidation was first observed after exposure of the catalyst to pH2O/pH2 ratios of 10-20 (Fig. 2a). H2O is reported to have a significant effect on the sintering tendency of Co nanoparticles11,13,21–23 leading to higher γ values.
Figure 2. Magnetization at maximal field strength (20 kOe) relative to the magnetization after reduction (squares) and the weight fraction of Co displaying remnant magnetization (diamonds) as a function of time on stream at varying pH2O/pH2 ratios (solid) in the (a) absence of CO, (b) initial presence of 68 mbar CO, and (c) initial presence of 340 mbar CO. 2
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ACS Catalysis Highly mobile (partially) oxidized surface Co atoms are hypothesized to drive this hydrothermal sintering23 leading to a loss in specific surface area.7 The increase of the remnant magnetization (Fig. S6) at low H2O levels together with a decrease of the overall magnetization confirms hydrothermal sintering. An expected preferential (partial) oxidation of smaller Co crystallites4 increased γ after exposure to higher H2O levels (pH2O/pH2 =30-50). When H2O was removed from the feed stream, γ increased readily and full magnetization was recovered. Hence, previously observed oxidation could be accompanied by sintering. The increase of γ during the re-reduction could also be due to the recovery of surface oxidized nanoparticles with a Co core size just below the critical diameter for superparamagnetism. After re-reduction, the crystallites would then be above this critical size leading to an increase in γ. In the second part of the experiment Ar was partially substituted by CO so as to expose the catalyst to dry synthesis gas (H2/CO = 2.1). These conditions initiated a rapid loss in magnetization (3.5% of the magnetization after reduction; Fig. 2b). This change can be ascribed to adsorbed CO* species, which decrease the magnetization of surface Co atoms by up to 20%.2,13 Significant oxidation of Co to cobalt oxide by H2O is unlikely due to the low CO conversion of 1.5%, i.e. low concentration of H2O. Carbidization of the surface is also unlikely at this low partial pressure of CO (68 mbar). After these initial changes, the magnetization of the catalyst was stable for 20 h under the dry synthesis gas conditions. The increase in the magnetization was less pronounced when H2O was co-fed at H2O levels 1-4 than in the absence of CO (0.8% vs. 1.8%). This could arise when adsorbed CO* species counteract the positive effect of OH* on the magnetization.13 The presence of OH* on the Co surface is further indicated by a significant decrease of the CO conversion (Fig. S8) due to competitive adsorption of reactants and H2O. The observed preferential adsorption of H2O-derived species such as OH* over CO results in deactivation due to blockage of active sites of the catalyst even before oxidation was observed and is in line with previous results.2 A decrease of the magnetization was observed at a pH2O/pH2 ratio of 20, and increasing the ratio further did not affect the slow oxidation of Co. As expected, the increased partial pressure of H2O exacerbated the activity of the catalyst due to a combination of oxidation of Co to FT inactive cobalt oxide and intensified competitive adsorption (Fig. S8). In the presence of CO, a higher H2O content was required to initiate the first decrease in magnetization than found in the absence of CO. This effect could also be due to the larger Co crystallite size.4 The overall decrease of the magnetization while co-feeding H2O was only 1.5% of the magnetization after reduction and this can be attributed to initial oxidation under dry synthesis gas or a stabilization effect in the presence of CO. Competitive adsorption of CO and H2O or carbidization may stabilize the Co phase. The catalyst was again exposed to dry synthesis gas after H2O was removed from the feed stream. Full recovery of the CO conversion level (Fig. S8) and the amount of metallic Co was obtained rapidly when compared to the initial exposure to dry synthesis gas (Fig. 2b). The recovery of Co was more quickly than after exposure to H2O in the absence of CO (< 5 min vs. ≈20 min). The decomposition of Co carbides 24 and the desorption of species from the Co surface can be expected to occur rapidly and can rationalize the findings. Oxidation/re-reduction of oxidized Co would then play a minor role. The weight fraction of Co showing remnant magnetization was constant throughout this second part of the experiment. The γ value only
increased after CO was removed from the feed stream, and this is due to a transition through the threshold size for superparamagnetism during recovery of partially oxidized Co surface. In the third part of the experiment, the partial pressure of synthesis gas was increased (1060 mbar in 406 mbar Ar) to provide a higher concentration of CO and the GHSV was adjusted to 15000 mL (gcatalyst h)-1. The adsorption of CO rapidly decreased the magnetization to 96.5% relative to the initial magnetization after reduction (Fig. 2c) at a final CO conversion of 1.0%. No increase of the magnetization was observed when H2O was cofed to the system (pH2O/pH2 ≤ 1.5) due to the increased partial pressure of CO. The higher concentration of adsorbed CO* species allows for a constant CO conversion at these H2O levels (Fig. S9) preventing competitive adsorption. Deactivation of the catalyst was observed at a pH2O/pH2 ratio of 5 and correlated with a decrease in magnetization, i.e. oxidation of surface Co atoms decreased the activity of the catalyst. The characteristics of the loss indicates a thermodynamically driven oxidation of Co that was induced by the increased concentration of H2O. The regeneration of O* by H2 via formation of H2O was seemingly hindered leading to oxidation of surface Co to cobalt oxide by CO-derived oxygen. Another decrease to 93.5%, at a pH2O/pH2 ratio of 10 further demonstrates the indirect effect of H2O on the oxidation mechanism, which is a function of pH2O/pH2. Once the initial oxidation was completed, a slow further decrease of the magnetization (0.5%) was observed, which can be attributed to the formation of cobalt carbides or slow oxidation of Co by oxygen species from H2O. Once again, the decrease in the magnetization was fully reversible when H2O was subsequently removed from the feed stream (Fig. 2c). The recovery rate in dry synthesis gas was slower than during the previous rejuvenation at a lower partial pressure of CO (>15 min vs.
