Abstract: Co/TiO2 catalyst activation for Fischer-Tropsch (FT) reaction by CO in comparison to H2 has been performed. The catalyst, prepared by incipient wetness impregnation, has been characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses after separate reduction using CO and H2 respectively. Temperature programmed reduction (TPR) analyses were also conducted to study the reduction behaviour of the catalyst in presence of H2 and CO respectively. CO improved catalyst reduction and produced a more stable and active catalyst with higher selectivity and yield for C5+ hydrocarbons at extended time-on-stream

Jalama, Kalala

English

University of Johannesburg Institutional Repository

1  Fischer-Tropsch synthesis over Co/TiO2 catalyst: effect of catalyst activation by CO compared to H2 Kalala Jalama Department of Chemical Engineering, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, Johannesburg, South Africa, e-mail: kjalama@uj.ac.za  Abstract Co/TiO2 catalyst activation for Fischer-Tropsch (FT) reaction by CO in comparison to H2 has been performed. The catalyst, prepared by incipient wetness impregnation, has been characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses after separate reduction using CO and H2 respectively. Temperature programmed reduction (TPR) analyses were also conducted to study the reduction behaviour of the catalyst in presence of H2 and CO respectively.  CO improved catalyst reduction and produced a more stable and active catalyst with higher selectivity and yield for C5+ hydrocarbons at extended time-on-stream.  Keywords: Catalyst activation; CO; H2; Fischer-Tropsch  1. Introduction Fischer-Tropsch (FT) synthesis is a process that converts synthesis gas into liquid fuel over metal catalysts. Supported cobalt, fused iron and precipitated iron catalysts are the most used in commercial applications [1]. An activation process that consists of reducing the cobalt oxides particles in the catalyst to metallic cobalt is always required prior to the FT reaction. H2 is usually used to activate cobalt catalysts at 250-400oC [1]. This standard catalyst activation is usually limited by some interaction between the support and cobalt. Cobalt-support compounds formation have been reported to form in supported cobalt catalysts during activation using H2 [2-7].    Jongsomjit et al. [7] reported the formation of Co-titanate during standard reduction of a Co/TiO2 catalyst 2  using H2. The formed compound cannot be reduced below 800oC and limits the degree of reduction for the catalyst.  In some cases, the formation of these cobalt-support compounds is influenced by the presence of water produced during the reduction process [3]. Only a limited amount of literature has reported to date on cobalt catalyst activation process that does not involve water formation.  Li et al. [8] used CO to activate a ruthenium-promoted Co/TiO2 catalyst at 523 K and 1.68 MPa and compared its performance for FT reaction with an H2-activated catalyst. They measured lower activity and good stability on the CO-reduced catalyst. Pan and Bukur [9] reduced Co/ZnO catalyst with CO at 523 K and measured lower activity and olefin content, and more methane compared to the H2-reduced sample. These predominantly negative effects of cobalt catalyst activation by CO on the FT reaction performance could most likely be due to the low temperature and high CO partial pressure used for catalyst activation. Cobalt carbide and CoO were the major phases in the catalyst sample reduced by CO at 523 K [9].  To the best of our knowledge, there is no other report on the effect of activating cobalt-based catalyst with CO, under conditions that avoid cobalt carbide formation, on catalyst performance for FT reaction. Hence the aim of this study is to investigate the effect of Co/TiO2 catalyst activation by diluted CO (5%CO in He) at 350oC on FT reaction performance.  2. Experimental procedure 2.1 Catalyst preparation and characterization The details on catalyst preparation are reported in earlier studies [10, 11].   Temperature programmed reduction (TPR) was performed using 100 mg of fresh Co/TiO2 catalyst sample. Bulk Co3O4 samples (10 mg) were also used to facilitate 3  reduction peak identification. The sample was first degassed at 150 oC in a flow of Ar (30 Nml/min) for 45 minutes and then cooled to 60 oC before replacing the Ar flow with the analysis gas (5% H2 in Ar or 5%CO in He, 30 Nml/min) to start the TPR analysis. The temperature was increased at a rate of 10oC/min.    X-ray diffraction (XRD) analysis was carried out using Rigaku Ultima IV X-ray powder diffractometer with PDXL analysis software.  The following analysis parameters were used: Cu-Kα (λ = 1.54 Ǻ) radiation source, current and voltage set at 30 mA and 40 KV and step width of 2θ = 0.01o. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos Axis Ultra DLD system. The following settings were used: monochromatic Al Ka X-ray source operating at 120 W; pass energies of 160 eV for survey spectra; 40 eV for high resolution scans; hybrid operation mode using a combination of magnetic immersion and electrostatic lenses; acquisition area of approximately 300 x 700 µm2; take off angle of 90° and a base pressure of ca. 1x10-9 Torr.  2.2 Catalyst evaluation 500 mg of fresh catalyst (particles with sizes between 0.5 and 1 mm) were loaded in a fixed-bed stainless steel reactor (internal diameter: 16 mm, length: 260 mm) and dried at 150 oC in a flow of N2 (30 Nml/min) for 60 min before exposure to the reducing gas mixture. Catalyst reduction was done at 350oC (heating rate: 10oC/min) for 14 hours using 5%H2 in Ar or 5%CO in He (30 Nml/min) at atmospheric pressure.  FT runs were performed at 220oC, 20 bar using a premixed gas with H2:CO ratio of ca. 2 and a feed flowrate of ca. 10 Nml/min. The outlet gas stream was analysed by online gas chromatography using a Supel-Q Plot fused silica capillary column 30 m x 0.32 mm and a stainless steel general 60/80 Carboxen 1000.   4   3. Results and discussion 3.1 Catalyst characterization The data for TPR analysis using H2 or CO are summarized in fig. 1.  Fig. 1. TPR data for bulk Co3O4 using a) CO; b) H2, and for Co/TiO2 sample using c) CO and d) H2.  The first reduction peak for bulk Co3O4 in presence of CO (fig. 1a) started from ca. 237oC and reached its maximum value at ca. 342oC. It can be attributed to the first reduction step of Co3O4 to CoO. A second peak starting at ca. 370oC showed a rapid CO consumption rate up to a temperature of 390oC after which an almost steady CO consumption rate was observed up to ca. 407oC followed by a maximum value at ca. 437oC. This peak can be attributed to the reduction of CoO to Co0. The peak starting at ca. 480oC is attributed to significant carbon deposition. In presence of H2, the bulk 10.4510.6510.8511.0511.2511.4511.6511.85012345678910200 300 400 500 600 700 800 900 1000Temperature [oC]TCD signal [a.u.]342oC437oC350oC424oC351oC 413oC390oC378oC 534oCadcb5  Co3O4 sample (fig. 1b) showed two peaks with maximum values at ca. 350 and 424oC attributed to the two-step reduction of Co3O4 to Co and Co0.  The data show that bulk Co3O4 can be reduced by CO or H2 in a similar temperature range. The data for the supported catalyst in presence of CO (fig. 1c) also showed two peaks with their maxima at ca. 351 and 413oC respectively. These peaks, also attributed to the two-step reduction of Co3O4 to CoO and Co0, are well in the same temperature range as that required for the reduction of bulk Co3O4. However, a delayed peak for carbon deposition was only observed from ca. 560oC compared to 480oC in the case of bulk Co3O4. The role of the TiO2 support in delaying carbon deposition is unclear and will require future investigation.  A significant shift of reduction peaks to higher temperatures was observed for the TPR analysis of the TiO2-supported cobalt catalyst sample using H2 (fig. 1d). The first reduction peak, attributed to the first reduction step of Co3O4 to CoO reached its maximum at ca. 378oC compared to 351oC in the presence of CO (fig. 1c). In addition, a broad reduction peak with maximum at ca. 534oC and extending up to ca. 620oC suggests the second reduction step of cobalt species with various levels of interaction with the TiO2 support to Co0.  These data suggest that reduction using H2 leads to strong cobalt-titania interactions in the catalyst compared to CO. It is possible that the latter prevented or limited these interactions in the catalyst during the reduction process to improve catalyst reducibility. Previous studies [5, 11] have also reported improved cobalt catalyst reduction due to the presence of CO in the reducing gas. XRD analyses were performed to confirm the cobalt phases that were present in the catalyst after reduction with CO or H2 at 350oC for 14 hours as described in section 2.2. The samples were cooled to room temperature under the flow of the reducing gas and passivated using 5% O2 in He for 30 min. Some portions of the passivated 6  samples were also sent for XPS analysis. Diffraction patterns for H2-reduced (fig. 2a) and CO-reduced (fig. 2b) samples were similar and both showed CoO and Co0 as major cobalt phases.    Fig. 2.  XRD data for Co/TiO2 catalyst after reduction with a) H2; b) CO   Different from other studies [8, 9, 12], no cobalt carbide was detected in CO-reduced samples as shown by XRD data.  A separate analysis was performed on the TPR apparatus by heating a CO-reduced catalyst sample to 350oC in a flow of H2 (5%H2 in Ar) in order to convert any cobalt carbide to Co metal and methane or light hydrocarbons respectively. The outlet of the TPR reactor was connected to an FID for possible methane/hydrocarbon detection. No peak was detected. XPS analyses were performed to supplement TPR and XRD data, and determine the surface composition of H2- and CO-reduced catalyst samples.  C 1s data (fig. 3a) for both H2- and CO-reduced catalyst samples were similar with a major peak at 284.8 eV due to adventitious carbon. 10 20 30 40 50 60 70 80 90 100 110 120AnataseRutileCoOCo2θ [o]Counts [a.u.](a)(b)7   Fig. 3. XPS data for a) C 1s and b) Co 2p Carbidic carbon peak has been reported to occur at ca. 282.5 eV [13]. This peak was not observed in the data reported in fig. 3 and confirms, in agreement with XRD data and FID analysis, that catalyst reduction with diluted CO did not lead to cobalt carbide formation. Most studies have performed cobalt carburization using CO at higher pressures [8, 12, 14, 15] and lower temperatures compared to the conditions selected in this study.   Li et al. [8] and Yang et al. [12] used a pressure of 1.68 MPa and a temperature of 523 275280285290295300305Intensity [cps]Binding Energy [eV]H2-reducedCO-reduced770780790800810Binding Energy [eV]H2-reducedCO-reducedIntensity [cps] Co 2p1/2Co 2p3/2a)b)8  K to carburize a Ru-promoted Co/TiO2 catalyst in CO. Karaca  et al. [14] and  Kwak  et al. [15] carburized a reduced CoPt/Al2O3 catalyst at 2.0 MPa and 220oC in a flow of pure CO. The overall Co3O4 carburization with CO can be written as  2Co3O4 + 14CO → 3Co2C + 11CO2             (1) The reaction proceeds with gas contraction and is thermodynamically favoured at higher pressures. Very low pressure for CO (5% CO in He at atmospheric pressure) was selected in this study in order to avoid or limit cobalt carbide formation. These observations are in agreement with Liotta et al. [16] who also reported reduction of Co3O4 to CoO and subsequently to Co metal during TPR analysis of a Co/Al2O3-BaO catalyst using a low CO pressure (1%CO in He).  H2- and CO-reduced catalyst samples displayed similar Co 2p spectra (fig. 3b). Peaks for Co 2p3/2 and Co 2p½ were detected at 780.6 and 796.5 eV respectively.  Satellite peaks for Co 2p3/2 and Co 2p½ were also observed at 786.6 and 802.8 eV respectively and suggested that cobalt was present on both H2- and CO-reduced catalyst surface as CoO [17].  Surface atomic compositions for the catalyst samples are reported in table 1.  Table 1 Catalyst surface composition determined by XPS Catalyst Surface atomic % Co/Ti atomic ratio   Co 2p O 1s Ti 2p   H2-reduced catalyst 5.73 68.07 26.20 0.22 CO-reduced catalyst 6.99 68.10 24.91 0.28  9  CO-reduced catalyst had 6.99% of surface cobalt compared to 5.75% for the H2-reduced sample. Also the Co/Ti ratio of 0.28 after reduction using CO was higher than 0.22 measured after reduction using H2. These data indeed suggest that catalyst reduction with CO leads to high cobalt dispersion on the catalyst surface. 3.2 Catalyst evaluation The performance of H2- and CO-reduced catalysts for FT reaction has been compared at early and extended time on stream (TOS). CO conversion and methane selectivity as function of TOS are reported in fig. 4. Averaged values are reported in table 2.   051015202516 18 20 22 24 26CO conv. and CH4select. [%]TOS [h]CO conv_H2 reduced CO conv_CO reducedCH4 sel_H2 reduced CH4 sel_CO reduced(a)051015202530110 112 114 116 118 120 122CO conv. and CH4select. [%]TOS [h]CO conv_H2 reduced CO conv_CO reducedCH4 sel_H2 reduced CH4 sel_CO reduced(b)10  Fig. 4. CO conversion and methane selectivity over Co/TiO2 catalyst reduced using H2 and CO respectively at a) early TOS and b) extended TOS 11  Table 2  Summary FT data   Run details CO conv. [%] -rCO [mol/gCat/h] rCH4 [mol/gCat/h] CH4 select. [%] C2-C4 select. [%] C5+ select. [%] rC5+ [mg/gCat/h] Alpha*3 4 5 6 7 8 Overall**Early TOS (< 30h)H2-reduced catalyst 19.9 ± 0.4 0.00333 0.00064 19.4 ± 0.4 26.8 53.8 25.1 0.73 0.80 0.49 0.57 0.41 0.26 0.29CO-reduced catalyst 20.9 ± 0.2 0.00350 0.00052 14.9 ± 0.5 14.8 70.2 34.4 0.91 0.99 0.52 0.58 0.39 0.31 0.21 0.28Extended TOS (>100h)H2-reduced catalyst 17.6 ± 0.3 0.00294 0.00040 13.5 ± 0.4 24.7 61.7 25.4 0.67 1.55 0.80 1.25 0.88 0.73 0.55 0.59CO-reduced catalyst 21.1 ± 0.1 0.00352 0.00051 14.6 ± 1.1 16.4 69.0 34.0 0.92 1.04 0.53 0.60 0.38 0.33 0.25 0.30*based on gas product**w eight basedOlefin to paraffin ratio*12  At early TOS, H2- and CO-reduced catalyst samples had comparable CO conversions of ca. 20 and 21% respectively. This was not expected since better catalyst reduction and higher cobalt dispersion (as indicated by Co/Ti ratio from XPS analysis) were obtained when the catalyst was activated by CO. This would lead to high active sites density in the catalyst resulting in significantly higher conversion. Thus, the observed data suggest that the specific FT activity at early TOS was higher for the H2-reduced sample, possibly due the strong metal support interaction (SMSI) in the catalyst [18] compared to the CO-reduced catalyst. However, higher methane selectivity of ca. 19% was measured on the H2-reduced catalyst compared to ca. 15% for the CO-reduced catalyst.  A decline in CO conversion rate from 0.00333 to 0.00294 mol/gCat/h (ca. 12% activity loss) and methane selectivity from 19.4 to 13.5% was observed for H2-reduced catalyst sample at extended TOS. The CO-reduced catalyst showed more stability as the CO conversion rate and methane selectivity at early and extended TOS were comparable. Fig. 5 shows the gas phase product distribution for both H2- and CO-reduced catalysts at early and extended TOS and their corresponding alpha values in table 2. Up to C8 hydrocarbons were detected in the gas product after a cold trap at room temperature. The product distribution for the CO-reduced catalyst was practically the same at both early and extended TOS with the corresponding alpha values of 0.91 and 0.92 respectively. The H2-reduced catalyst had alpha values of 0.73 and 0.67 at early and extended TOS respectively.    13   Fig. 5. Light hydrocarbons distribution (based on gas product) over H2- and CO-reduced catalyst at a) early reaction times and b) extended reaction times.   Overall olefin to paraffin ratios (table 2) at early TOS were comparable and around 0.28 and 0.29 for CO- and H2-reduced catalysts respectively but shifted to a higher value (doubled) for the H2-reduced catalyst at extended TOS. Catalyst evaluation data, as summarized in table 2 show that reduction with CO produces a more stable and active catalyst that reaches steady state faster with higher C5+ selectivity (69%) and -2.5-2-1.5-1-0.500 1 2 3 4 5 6 7 8 9log (Wn/n)Carbon numberH2-reduced catalystCO-reduced catalyst(a)-3-2.5-2-1.5-1-0.500 1 2 3 4 5 6 7 8 9log (Wn/n)Carbon numberH2 reduced catalystCO-reduced catalyst(b)14  production rate of  34 mg/gCat/h  at extended TOS compared to 61.7% and  25.4 mg/Cat/h for H2-reduced catalyst.  Li et al. [8] also observed that cobalt-based catalyst pre-treated with CO reaches a steady state faster and is stable.   Concomitantly with the decline in H2-reduced catalyst activity at extended TOS, the following observations can be summarized from the data in table 2: i) decline in methane formation rate from 0.00064 to 0.00040 mol/gCat/h (ca. 38% decline); ii) increase in overall olefin to paraffin ratio (double) and iii) steady rate for C5+ formation which remained unchanged around 25 mg/gCat/h. These observations suggest a decline in the density of catalytic sites which are more selective for hydrogenation and CH4 formation. It has been proposed that SMSI leads to a raft-like morphology of metal on a TiO2 support [19].  Plane sites have been visualized to possess specificity for hydrogenation and are significantly poisoned by strong CO adsorption during FT reaction [20].  4. Conclusion Different from previous studies which reported negative effect of cobalt catalyst activation by CO, this study used significantly lower partial pressure of CO and a higher temperature for the reduction process. Under these conditions, no cobalt carbide was detected. CO or H2 led to different types of interactions of cobalt species and the TiO2 support which led to differences in catalyst reducibility and performance for FT reaction. CO improved catalyst reduction and produced a more stable and active catalyst with higher selectivity and yield for C5+ hydrocarbons at extended TOS compared to the H2-reduced catalyst.   15  Acknowledgements Financial support from the National Research Foundation (NRF) and the University of Johannesburg is acknowledged.  Cardiff Institute of Catalysis is also acknowledged for assistance with XPS analyses.  References [1] A. Steynberg, M. Dry, Studies in Surface Science and Catalysis 152 (2004) 533. [2] A. Kogelbauer, J.C. Weber, J.G. Goodwin Jr., Catalysis Letters 34 (1995) 259.  [3] Y.Zhang, D. Wei, S. Hammache, J.G. Goodwin Jr., Journal of Catalysis 188 (1999) 281.  [4] B. Jongsomjit, J. Panpranot, J.G. Goodwin Jr., Journal of Catalysis 204 (2001) 98.  [5] B. 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Fischer-Tropsch synthesis over Co/Tio2 catalyst : effect of catalyst activation by CO compared to H2

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Fischer-Tropsch synthesis over Co/Tio2 catalyst : effect of catalyst activation by CO compared to H2

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