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Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics
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Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics Laurent Lemaitre, Adrien Berliet, Sylvie Maury, Mickael Rivallan ∗ IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP 3-69360 Solaize, France
a r t i c l e
i n f o
Article history: Received 29 October 2015 Received in revised form 31 January 2016 Accepted 12 February 2016 Available online xxx Keywords: DRIFT spectroscopy Fischer Tropsch Cobalt Chemometrics
a b s t r a c t Operando DRIFT analysis of the surface species observed on Fischer Tropsch catalysts at work has been performed under H2 /CO syngas at 503 K. Decomposition of the evolution in the IR operando spectra has been done by chemometric analysis in order to depict the modifications occurring on the surface of the nanoparticles as a function of time on stream. Results obtained on three catalysts with different reducibility determined from temperature programmed reduction experiments are confronted in order to correlate the spectral observations to the cobalt surface atoms speciation. Attribution of the IR components in the 2100–1750 cm−1 range is confirmed on the basis of the chemometric decomposition. The component falling in the 2068–2064 cm−1 range is assigned to CO sorbed onto Co◦ surface atoms of partially H2 -reduced nanoparticles. This component is found to progressively decrease as a function of catalyst time on stream which indicates a progressive reduction of the nanoparticles under syngas at 503 K. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Infrared spectroscopy is one of the most sensitive technique for the characterization of material surface, either by direct analysis of the hydroxyl groups in the 4000–3200 cm−1 region or indirectly by the means of probe molecule adsorption after specific thermal activation of the sample. In that latter case of in situ IR experiment the whole mid-IR spectrum is of interest. Probe molecules are generally chosen as a function of the surface sites wanted to be reflected in the IR spectra: acidic, basic, metallic. . . [1–3] In heterogeneous catalysis, carbon monoxide is one of the most conventional probe used since it allows at first a well-defined characterization of the acidic properties (when adsorption is performed at low temperature close to 100 K) with distinction of Lewis and Brønsted acid sites, of conventional industrial catalyst supports like alumina, silica, silico–alumina, zeolite. . .[1–3] In order to selectively enhance the contributions due to CO chemisorbed on supported noble metal atoms and not those of CO in interaction with acidic sites, CO contact is generally done at 298 K on the surface of reduced catalysts. The evolution in the IR spectra as a function of the progressive introduction of CO molecules up to saturation, and the profile of the spectrum in the carbonyl region (position of
∗ Corresponding author. E-mail address: [email protected] (M. Rivallan).
the maxima, intensity, shoulder) shed light onto many information relative to surface atom coordination, particle shape, dispersion, metal support interactions, metal location [4–6]. When considering analysis of transition metal surface atoms, one has to take into account the possible surface reactivity even at 298 K towards CO molecule, which may induce drastic change of the initial state of the surface reduced catalyst. [2] This is the specific case of CO contact on cobalt containing catalyst conventionally used in Fischer Tropsch synthesis (FTS). Spectral contributions due to CO on cobalt surface atoms fall in the 2210 down to 1900 cm−1 region depending on the oxidation state and nuclearity of the probed metal sites, which may exhibit polycarbonyl species in some specific cases. [2] Linear carbonyl sorbed on oxidized Co3+ (weak adsorption), Co2+ (weak adsorption) and Co+/␦+ absorb at 2195–2180, 2210–2120 and 2090–2065 cm−1 respectively, while mono-carbonyl species on reduced Co◦ absorb at 2075–1975 cm−1 . From the comparison of the spectral regions, it first incomes that the distinction between Co+/␦+ · · ·CO and Co◦ · · ·CO complexes is clearly not straightforward. Moreover it is also important to notice that the position of the peak observed in the 2075–1975 cm−1 region can be a lot affected by CO surface coverage [7]. Indeed, high CO surface coverage will decrease -backbonding from Co atoms to CO ligands (weaker Co◦ -CO bonding) and will shift to the left of the spectrum the contributions. Comparison of the position of the maxima has to be done at approximate similar CO surface coverage. Prior to CO contact, reduction of Fischer Tropsch
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Please cite this article in press as: L. Lemaitre, et al., Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.033
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Fig. 1. Evolutions in the operando DRIFT spectra (background subtracted) as a function of catalysts time on stream (measured after 1 (black spectrum), 15, 30, 60, 90 and 120 min (red spectrum) reaction at 503 K under H2 /CO = 7.5 at Patm). Results for FT0, FT1 and FT2 catalysts are reported on panels (a), (b) and (c) respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(FT) catalyst is generally performed under H2 gas in order to only probe Co◦ active sites. Reduction conditions (plateau temperature and time) are dependent on the preparation route of the catalyst such as cobalt content, particle size, dopants, nature of the support, calcination. Temperature programmed reduction (TPR) experiment is of importance in order to determine the temperature reduction required to avoid or limit the presence of partially reduced nanoparticles. Indeed, Co3 O4 oxide may require temperature up to 600 K to be correctly reduced into CoO [8,9] also depending on support effects and promoters. The presence of oxide species in interaction with CO may lead to oxidation mechanism even at 298 K with further formation of carbonates on the surface of the support (with relative basicity). To summarize about CO adsorption followed by in situ FTIR, it allows a full (but indirect) description of the accessible surface atoms on the active state of the catalyst and may address some information about the surface reactivity of the catalyst even at 298 K. One may imagine that the situation is even worst complex when IR spectroscopy is applied for the characterization of Fischer Tropsch catalysts under operando conditions, as close as possible to realistic process conditions, i.e. high temperature (close to 500 K), pressure (P > 1 bar), H2 /CO gas mixture with formation of reaction products. In that case of operando FTIR studies, CO is clearly seen as a reactant and not as a probe molecule anymore. Fischer Tropsch synthesis is one of the reaction of choice for IR operando monitoring of the catalyst evolution due to favorable experimental conditions: moderated temperature range, use of syngas mixture (‘light’ molecules CO and H2 ), as a consequence the literature is a lot furnished (see Refs. [10–14]). But even if the reaction is extensively studied by IR spectroscopy, the interpretation of the evolutions observed in the spectra as a function of the temperature, gas mixture (H2 presence or not), catalyst time on stream is not straightforward. In the present contribution, we will focus on the analysis of the surface species observed on Fischer Tropsch catalysts at work under H2 /CO syngas at 503 K. Decomposition of the evolution in the IR operando spectra will be done by chemometric analysis in order to depict the modifications occurring on the surface of the nanoparticles as a function of time on stream. Results obtained on three catalysts with different reducibility determined from temperature programmed reduction experiments will be confronted in order to correlate the spectral observations to the cobalt surface atoms speciation.
2. Experimental 2.1. Catalyst preparation Three FT catalysts (referred as FT0, FT1 and FT2 hereafter) have been prepared by incipient wetness impregnation of an aqueous cobalt nitrate solution on a silica-alumina support in order to obtain a catalyst containing 15 wt% Co. After drying for 2 h at 400 K, the solids were calcinated at 673 K for 2 h under air flow. The solids obtained were then reduced under pure hydrogen at 673 K during 16 h, and subsequently passivated in ambient air before operando analyses.
2.2. Characterization Temperature programmed reduction (TPR) experiments were performed in a fixed bed reactor. Typically, a 20–30 mg catalyst bed was mounted in the reactor and was pretreated in situ. A flow of 5% H2 in Ar, maintained at a flow rate of 20 mL min−1 , was used as the reducing gas. H2 consumption was monitored by measurement of the thermal conductivity (TCD) of the effluent gas on a GC. A DRIFT reactor cell (praying manthis, Harrick) has been used to follow catalyst surface evolution as a function of catalyst time on stream in the reaction of CO hydrogenation. Prior to IR operando measurements, the samples were pretreated at 673 K for 120 min and cooling down to 503 K in a 50% vol. H2 flow diluted in Ar (30 mL min−1 ). Afterwards, syngas mixture of CO and H2 (4 and 30 mL min−1 respectively, H2 /CO = 7.5) was introduced in the reactor. Gases introduced in the DRIFT reactor pass through the catalytic bed from top to bottom. The background subtracted IR spectra were recorded while the sample powder was kept at 503 K under H2 (GHSV ≈ 30 000 h−1 ). Chemometric calculations were performed by using the multivariate curve resolution-alternating least squares (MCR-ALS) method directly on the operando FTIR spectra. MCRALS is based on a linear model assuming the generalized law of Lambert–Beer where the individual response of each component are addable. The aim of this method is the decomposition of the original data matrix, which contains all the spectra recorded during CO hydrogenation reaction, into the product of two matrices, which contain the calculated concentration profiles and corresponding reference IR spectra respectively. Calculations were performed with a MCR-ALS toolbox running under Matlab.
Please cite this article in press as: L. Lemaitre, et al., Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.033
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Fig. 2. MCR decomposition of the operando DRIFT spectra into three reference spectra, results for FT0, FT1 and FT2 catalysts are reported on panels (a), (b) and (c) respectively.
Fig. 3. Concentration profiles (obtained from MCR decomposition), dimensionless and expressed as the relative contribution of the reference spectra in Fig. 2 to the spectra displayed in Fig. 1. Results for FT0, FT1 and FT2 catalysts are reported on panels a), b) and c) respectively. Numbers in parenthesis correspond to the percentage of variance expressed by each corresponding reference spectrum (A, B and C).
3. Results and discussion CO hydrogenation has been followed on three cobalt supported catalysts (FT0, FT1 and FT2) at 503 K, at ambient pressure with a H2 /CO ratio of 7.5, which favors methane formation but reduces the formation of long chain alkanes. On an industrial point of view, methane is not the first desired product but it allows understanding the surface modifications occurring on the catalyst under FischerTropsch synthesis. The nature and concentrations of the gaseous products measured in the reactor outlet will not be discussed in the present contribution. A special focus is done on the surface state of the Co surface atoms species. Fig. 1 reports the evolution in the IR operando spectra (2300–1300 cm−1 ) as a function of catalysts time on stream (FT0, FT1 and FT2). After syngas introduction at the inlet of the reactor (t = 0 s), information about cobalt active surface sites evolution can be indirectly obtained from the analysis of the carbonyl in the CO region (2090–1730 cm−1 ), since CO reactant is chemisorbed on the surface. When looking at the results obtained for FT0 catalyst (Fig. 2a), limited evolution in the IR spectra is observed in the carbonyl region which indicates small modifications of the initial cobalt nanoparticles. On the other hand, for FT1 and FT2 catalysts, the spectrum profile rapidly evolves following a shift to lower wavenumbers as a function of catalyst time on stream. FT1 and FT2 catalysts display a shift of the maximum of ca. = –29 and −26 cm−1 respectively after 120 min reaction (Fig. 2b and c). This shift is clearly related to nanoparticles surface modifications but it is difficult to
decorrelate the different effects that may result in such spectral observations: reduction level of cobalt nanoparticles, carburization, sintering, redispersion and/or defects creation (corner/edge/step sites), hydrocarbonyl formation [10,11,13,14]. It is also interesting to note first that for the three catalysts, the total carbonyl area increases indicating an enhanced number of cobalt surface atoms accessible (slightly for FT0), and second that the right part of the contribution exhibits a more and more marked shoulder at 1950 cm−1 , which may be due to the formation of bridged species. For FT0 catalyst, the concentration of surface carbonyl increase is mainly due to the creation of bridged surface sites. In order to better understand the evolution observed as a function of catalyst time on stream, temperature programmed reduction (TPR) experiments have been carried out on the reduced passivated catalysts (Fig. 4) to depict the initial state (at t = 0 s) of the cobalt nanoparticles after subsequent reductive treatment at 673 K under H2 . From H2 consumption measured as a function of the temperature increase, it first appears that the reduction done at 673 K before catalytic testing is not sufficient to totally reduce the cobalt nanoparticles. This is clearly evidenced for FT2 catalyst which reports a broaden H2 consumption extended up to 700 K. The maxima of the consumption are observed at 560, 570 and 600 K for FT0, FT1 and FT2 catalysts respectively. TPR experiments shed light onto i) the easier reducibility of cobalt nanoparticles for FT0 catalyst and ii) the advanced reduction state after H2 treatment at 673 K of FT0 compared to FT1 and FT2 catalysts.
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the IR beam), could be lower than the temperature ‘seen’ in the bottom of the reactor bed due to heat loss [15]. Hence, the reduction level indirectly measured by operando DRIFT (from reference spectrum C) is expected to be lower than what is observed in TPR experiment at a similar temperature. The component falling at 2068–2064 cm−1 is related to partially H2 -reduced cobalt nanoparticles at 673 K. Since FT1 reports an enhanced reducibility compared to FT2 (as displayed in TPR, Fig. 4), the initial concentration of reference spectrum C is consequently lower for FT1, and the position of the maxima (2064 cm−1 ) is also found at lower wavenumber than FT2 (2068 cm−1 ). After 2 h time on stream, reference spectrum C concentration observed for FT1 and FT2 decreases of about 36 and 42% respectively (Fig. 3B and C), which indicates that syngas mixture slowly completes the reduction of the nanoparticles (in agreement with references [9,16]) at a lower temperature range than under H2 flow (diluted in inert gas). 4. Conclusions Fig. 4. H2 -TPR profiles of FT0, FT1 and FT2 catalysts (black, red and blue curves respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Chemometric analysis has been carried out on the spectra series recorded under operando conditions (H2 /CO syngas) in the 2300–1300 cm−1 range. The decomposition looks similar for FT1 and FT2 catalysts (Figs. 2 and 3) and leads to three reference spectra (A–C), while the evolution in the spectra for FT0 catalyst could be explained at 99.9% with only two reference spectra (A and B): - Reference spectrum A displays a maximum at 2035 cm−1 . The spectrum reports an asymmetrical component centered at 2035 cm−1 due to CO linearly adsorbed on Co◦ surface atoms and a broad signal below 1950 cm−1 due to CO bridged species. This reference spectrum also takes into account CO gas phase contribution (2200–2100 cm−1 ). - Reference spectrum B is related to the storage of products species like CO2 adsorbed at the basic surface sites leading to the further formation of formate/carboxylate species (1600–1300 cm−1 ) [10]. - Reference spectrum C (for FT1 and FT2 catalysts) displays a maximum at ca. 2068–2064 cm−1 . The component is very thin and quite symmetrical which may imply the presence of well-defined metal surface atoms positions prompt to sorb CO. From the position of the maxima, the reference spectrum can be linked to Co+/␦+ · · ·CO and/or Co◦ · · ·CO and/or HCo◦ · · ·CO without any distinction. Reference spectrum C cannot be due to carbon insertion into Co◦ nanoparticles (CO· · ·Co ◦ C surface complex) [11], since at t = 0 s FT1 and FT2 catalysts have never been in contact with CO molecule. Moreover, the concentration profile obtained for Reference spectrum C (Fig. 3) partially decreased during 120 min time on stream at 503 K, which also discards possible hydrocarbonyl surface species assignment (HCo◦ · · ·CO) not stable at such temperature range [14]. The component centered at 2068–2064 cm−1 may be related to CO adsorbed on Co◦ atoms at the surface of oxygen-rich nanoparticles in agreement with TPR profiles measurements and the nonobservation of such component for FT0 catalyst (which exhibits enhanced reducible properties). It is worthy to note that the temperature reduction ‘seen’ by the catalyst particles located at the upper surface of the DRIFT reactor (i.e. 100 m depth analyzed by
Chemometric tools can be a powerful technique to decompose operando DRIFT spectra series. In the present case of Fischer Tropsch synthesis, chemometric analysis allows a description of the modifications occurring on the surface of the nanoparticles as a function of catalyst time on stream. The mathematical decomposition of the spectral evolutions done on three catalysts with different reducibility are confronted in order to correlate the spectral observations to the cobalt surface atoms speciation. The results shed light onto the attribution of the IR components in the 2100–1750 cm−1 range. The component falling in the 2068–2064 cm−1 range is assigned to CO sorbed onto Co◦ surface atoms of partially H2 -reduced nanoparticles. This component is found to progressively decrease as a function of catalyst time on stream which indicates a progressive reduction of the nanoparticles under syngas at 503 K. References [1] K. Hadjiivanov, Advances in Catalysis, in: Friederike C. Jentoft (Ed.), Academic Press, 2014, pp. 99–318. [2] K.I. Hadjiivanov, G.N. Vayssilov, Advances in Catalysis, Academic Press, 2002, pp. 307–511. [3] F. Thibault-Starzyk, F. Maugé, Characterization of Solid Materials and Heterogeneous Catalysts, in: M. Che, J.C. Védrine (Eds.), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2012, pp. 1–48. [4] M. Rivallan, E. Seguin, S. Thomas, M. Lepage, N. Takagi, H. Hirata, F. Thibault-Starzyk, Angew. Chem. 49 (2010) 785–789 (International ed. in English). [5] M. Rivallan, S. Thomas, M. Lepage, N. Takagi, H. Hirata, F. Thibault-Starzyk, ChemCatChem 2 (2010) 1599–1605. [6] S. Thomas, M. Rivallan, M. Lepage, N. Takagi, H. Hirata, F. Thibault-Starzyk, Microporous and Mesoporous Mater. 140 (2011) 103–107. [7] N. Kumar, K. Jothimurugesan, G.G. Stanley, V. Schwartz, J.J. Spivey, J. Phys. Chem. C 115 (2011) 990–998. [8] C.-W. Tang, C.-B. Wang, S.-H. Chien, Thermochim. Acta 473 (2008) 68–73. [9] N.E. Tsakoumis, M. Rønning, Ø. Borg, E. Rytter, A. Holmen, Eleventh International Symposium on Catalyst Deactivation Delft (The Netherlands,) October 25–28, 2009, 154 (2010) 162–182. [10] A. Paredes-Nunez, D. Lorito, N. Guilhaume, C. Mirodatos, Y. Schuurman, F.C. Meunier, Catal. Today 242 (2015) 178–183. [11] J. Couble, D. Bianchi, J. Phys. Chem. C 117 (2013) 14544–14557. [12] J. Couble, D. Bianchi, Appl. Cata. A: Gen. 445–446 (2012) 1–13. [13] M. Kollár, A. de Stefanis, H.E. Solt, M.R. Mihályi, J. Valyon, A.A. Tomlinson, J. Mol. Catal. A: Chem. 333 (2010) 37–45. [14] G. Kadinov, C. Bonev, S. Todorova, A. Palazov, J. Chem. Soc. Faraday Trans. 94 (1998) 3027–3031. [15] H. Li, M. Rivallan, F. Thibault-Starzyk, A. Travert, F.C. Meunier, Phys. Chem. Chem. Phys. PCCP 15 (2013) 7321–7327. [16] A.M. Saib, A. Borgna, J. van de Loosdrecht, P.J. van Berge, J.W. Niemantsverdriet, Appl. Cata. A: Gen. 312 (2006) 12–19.
Please cite this article in press as: L. Lemaitre, et al., Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.033
ARTICLE IN PRESS
CATTOD-10064; No. of Pages 4
Catalysis Today xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics Laurent Lemaitre, Adrien Berliet, Sylvie Maury, Mickael Rivallan ∗ IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP 3-69360 Solaize, France
a r t i c l e
i n f o
Article history: Received 29 October 2015 Received in revised form 31 January 2016 Accepted 12 February 2016 Available online xxx Keywords: DRIFT spectroscopy Fischer Tropsch Cobalt Chemometrics
a b s t r a c t Operando DRIFT analysis of the surface species observed on Fischer Tropsch catalysts at work has been performed under H2 /CO syngas at 503 K. Decomposition of the evolution in the IR operando spectra has been done by chemometric analysis in order to depict the modifications occurring on the surface of the nanoparticles as a function of time on stream. Results obtained on three catalysts with different reducibility determined from temperature programmed reduction experiments are confronted in order to correlate the spectral observations to the cobalt surface atoms speciation. Attribution of the IR components in the 2100–1750 cm−1 range is confirmed on the basis of the chemometric decomposition. The component falling in the 2068–2064 cm−1 range is assigned to CO sorbed onto Co◦ surface atoms of partially H2 -reduced nanoparticles. This component is found to progressively decrease as a function of catalyst time on stream which indicates a progressive reduction of the nanoparticles under syngas at 503 K. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Infrared spectroscopy is one of the most sensitive technique for the characterization of material surface, either by direct analysis of the hydroxyl groups in the 4000–3200 cm−1 region or indirectly by the means of probe molecule adsorption after specific thermal activation of the sample. In that latter case of in situ IR experiment the whole mid-IR spectrum is of interest. Probe molecules are generally chosen as a function of the surface sites wanted to be reflected in the IR spectra: acidic, basic, metallic. . . [1–3] In heterogeneous catalysis, carbon monoxide is one of the most conventional probe used since it allows at first a well-defined characterization of the acidic properties (when adsorption is performed at low temperature close to 100 K) with distinction of Lewis and Brønsted acid sites, of conventional industrial catalyst supports like alumina, silica, silico–alumina, zeolite. . .[1–3] In order to selectively enhance the contributions due to CO chemisorbed on supported noble metal atoms and not those of CO in interaction with acidic sites, CO contact is generally done at 298 K on the surface of reduced catalysts. The evolution in the IR spectra as a function of the progressive introduction of CO molecules up to saturation, and the profile of the spectrum in the carbonyl region (position of
∗ Corresponding author. E-mail address: [email protected] (M. Rivallan).
the maxima, intensity, shoulder) shed light onto many information relative to surface atom coordination, particle shape, dispersion, metal support interactions, metal location [4–6]. When considering analysis of transition metal surface atoms, one has to take into account the possible surface reactivity even at 298 K towards CO molecule, which may induce drastic change of the initial state of the surface reduced catalyst. [2] This is the specific case of CO contact on cobalt containing catalyst conventionally used in Fischer Tropsch synthesis (FTS). Spectral contributions due to CO on cobalt surface atoms fall in the 2210 down to 1900 cm−1 region depending on the oxidation state and nuclearity of the probed metal sites, which may exhibit polycarbonyl species in some specific cases. [2] Linear carbonyl sorbed on oxidized Co3+ (weak adsorption), Co2+ (weak adsorption) and Co+/␦+ absorb at 2195–2180, 2210–2120 and 2090–2065 cm−1 respectively, while mono-carbonyl species on reduced Co◦ absorb at 2075–1975 cm−1 . From the comparison of the spectral regions, it first incomes that the distinction between Co+/␦+ · · ·CO and Co◦ · · ·CO complexes is clearly not straightforward. Moreover it is also important to notice that the position of the peak observed in the 2075–1975 cm−1 region can be a lot affected by CO surface coverage [7]. Indeed, high CO surface coverage will decrease -backbonding from Co atoms to CO ligands (weaker Co◦ -CO bonding) and will shift to the left of the spectrum the contributions. Comparison of the position of the maxima has to be done at approximate similar CO surface coverage. Prior to CO contact, reduction of Fischer Tropsch
http://dx.doi.org/10.1016/j.cattod.2016.02.033 0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Lemaitre, et al., Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.033
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Fig. 1. Evolutions in the operando DRIFT spectra (background subtracted) as a function of catalysts time on stream (measured after 1 (black spectrum), 15, 30, 60, 90 and 120 min (red spectrum) reaction at 503 K under H2 /CO = 7.5 at Patm). Results for FT0, FT1 and FT2 catalysts are reported on panels (a), (b) and (c) respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(FT) catalyst is generally performed under H2 gas in order to only probe Co◦ active sites. Reduction conditions (plateau temperature and time) are dependent on the preparation route of the catalyst such as cobalt content, particle size, dopants, nature of the support, calcination. Temperature programmed reduction (TPR) experiment is of importance in order to determine the temperature reduction required to avoid or limit the presence of partially reduced nanoparticles. Indeed, Co3 O4 oxide may require temperature up to 600 K to be correctly reduced into CoO [8,9] also depending on support effects and promoters. The presence of oxide species in interaction with CO may lead to oxidation mechanism even at 298 K with further formation of carbonates on the surface of the support (with relative basicity). To summarize about CO adsorption followed by in situ FTIR, it allows a full (but indirect) description of the accessible surface atoms on the active state of the catalyst and may address some information about the surface reactivity of the catalyst even at 298 K. One may imagine that the situation is even worst complex when IR spectroscopy is applied for the characterization of Fischer Tropsch catalysts under operando conditions, as close as possible to realistic process conditions, i.e. high temperature (close to 500 K), pressure (P > 1 bar), H2 /CO gas mixture with formation of reaction products. In that case of operando FTIR studies, CO is clearly seen as a reactant and not as a probe molecule anymore. Fischer Tropsch synthesis is one of the reaction of choice for IR operando monitoring of the catalyst evolution due to favorable experimental conditions: moderated temperature range, use of syngas mixture (‘light’ molecules CO and H2 ), as a consequence the literature is a lot furnished (see Refs. [10–14]). But even if the reaction is extensively studied by IR spectroscopy, the interpretation of the evolutions observed in the spectra as a function of the temperature, gas mixture (H2 presence or not), catalyst time on stream is not straightforward. In the present contribution, we will focus on the analysis of the surface species observed on Fischer Tropsch catalysts at work under H2 /CO syngas at 503 K. Decomposition of the evolution in the IR operando spectra will be done by chemometric analysis in order to depict the modifications occurring on the surface of the nanoparticles as a function of time on stream. Results obtained on three catalysts with different reducibility determined from temperature programmed reduction experiments will be confronted in order to correlate the spectral observations to the cobalt surface atoms speciation.
2. Experimental 2.1. Catalyst preparation Three FT catalysts (referred as FT0, FT1 and FT2 hereafter) have been prepared by incipient wetness impregnation of an aqueous cobalt nitrate solution on a silica-alumina support in order to obtain a catalyst containing 15 wt% Co. After drying for 2 h at 400 K, the solids were calcinated at 673 K for 2 h under air flow. The solids obtained were then reduced under pure hydrogen at 673 K during 16 h, and subsequently passivated in ambient air before operando analyses.
2.2. Characterization Temperature programmed reduction (TPR) experiments were performed in a fixed bed reactor. Typically, a 20–30 mg catalyst bed was mounted in the reactor and was pretreated in situ. A flow of 5% H2 in Ar, maintained at a flow rate of 20 mL min−1 , was used as the reducing gas. H2 consumption was monitored by measurement of the thermal conductivity (TCD) of the effluent gas on a GC. A DRIFT reactor cell (praying manthis, Harrick) has been used to follow catalyst surface evolution as a function of catalyst time on stream in the reaction of CO hydrogenation. Prior to IR operando measurements, the samples were pretreated at 673 K for 120 min and cooling down to 503 K in a 50% vol. H2 flow diluted in Ar (30 mL min−1 ). Afterwards, syngas mixture of CO and H2 (4 and 30 mL min−1 respectively, H2 /CO = 7.5) was introduced in the reactor. Gases introduced in the DRIFT reactor pass through the catalytic bed from top to bottom. The background subtracted IR spectra were recorded while the sample powder was kept at 503 K under H2 (GHSV ≈ 30 000 h−1 ). Chemometric calculations were performed by using the multivariate curve resolution-alternating least squares (MCR-ALS) method directly on the operando FTIR spectra. MCRALS is based on a linear model assuming the generalized law of Lambert–Beer where the individual response of each component are addable. The aim of this method is the decomposition of the original data matrix, which contains all the spectra recorded during CO hydrogenation reaction, into the product of two matrices, which contain the calculated concentration profiles and corresponding reference IR spectra respectively. Calculations were performed with a MCR-ALS toolbox running under Matlab.
Please cite this article in press as: L. Lemaitre, et al., Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.033
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Fig. 2. MCR decomposition of the operando DRIFT spectra into three reference spectra, results for FT0, FT1 and FT2 catalysts are reported on panels (a), (b) and (c) respectively.
Fig. 3. Concentration profiles (obtained from MCR decomposition), dimensionless and expressed as the relative contribution of the reference spectra in Fig. 2 to the spectra displayed in Fig. 1. Results for FT0, FT1 and FT2 catalysts are reported on panels a), b) and c) respectively. Numbers in parenthesis correspond to the percentage of variance expressed by each corresponding reference spectrum (A, B and C).
3. Results and discussion CO hydrogenation has been followed on three cobalt supported catalysts (FT0, FT1 and FT2) at 503 K, at ambient pressure with a H2 /CO ratio of 7.5, which favors methane formation but reduces the formation of long chain alkanes. On an industrial point of view, methane is not the first desired product but it allows understanding the surface modifications occurring on the catalyst under FischerTropsch synthesis. The nature and concentrations of the gaseous products measured in the reactor outlet will not be discussed in the present contribution. A special focus is done on the surface state of the Co surface atoms species. Fig. 1 reports the evolution in the IR operando spectra (2300–1300 cm−1 ) as a function of catalysts time on stream (FT0, FT1 and FT2). After syngas introduction at the inlet of the reactor (t = 0 s), information about cobalt active surface sites evolution can be indirectly obtained from the analysis of the carbonyl in the CO region (2090–1730 cm−1 ), since CO reactant is chemisorbed on the surface. When looking at the results obtained for FT0 catalyst (Fig. 2a), limited evolution in the IR spectra is observed in the carbonyl region which indicates small modifications of the initial cobalt nanoparticles. On the other hand, for FT1 and FT2 catalysts, the spectrum profile rapidly evolves following a shift to lower wavenumbers as a function of catalyst time on stream. FT1 and FT2 catalysts display a shift of the maximum of ca. = –29 and −26 cm−1 respectively after 120 min reaction (Fig. 2b and c). This shift is clearly related to nanoparticles surface modifications but it is difficult to
decorrelate the different effects that may result in such spectral observations: reduction level of cobalt nanoparticles, carburization, sintering, redispersion and/or defects creation (corner/edge/step sites), hydrocarbonyl formation [10,11,13,14]. It is also interesting to note first that for the three catalysts, the total carbonyl area increases indicating an enhanced number of cobalt surface atoms accessible (slightly for FT0), and second that the right part of the contribution exhibits a more and more marked shoulder at 1950 cm−1 , which may be due to the formation of bridged species. For FT0 catalyst, the concentration of surface carbonyl increase is mainly due to the creation of bridged surface sites. In order to better understand the evolution observed as a function of catalyst time on stream, temperature programmed reduction (TPR) experiments have been carried out on the reduced passivated catalysts (Fig. 4) to depict the initial state (at t = 0 s) of the cobalt nanoparticles after subsequent reductive treatment at 673 K under H2 . From H2 consumption measured as a function of the temperature increase, it first appears that the reduction done at 673 K before catalytic testing is not sufficient to totally reduce the cobalt nanoparticles. This is clearly evidenced for FT2 catalyst which reports a broaden H2 consumption extended up to 700 K. The maxima of the consumption are observed at 560, 570 and 600 K for FT0, FT1 and FT2 catalysts respectively. TPR experiments shed light onto i) the easier reducibility of cobalt nanoparticles for FT0 catalyst and ii) the advanced reduction state after H2 treatment at 673 K of FT0 compared to FT1 and FT2 catalysts.
Please cite this article in press as: L. Lemaitre, et al., Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.033
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the IR beam), could be lower than the temperature ‘seen’ in the bottom of the reactor bed due to heat loss [15]. Hence, the reduction level indirectly measured by operando DRIFT (from reference spectrum C) is expected to be lower than what is observed in TPR experiment at a similar temperature. The component falling at 2068–2064 cm−1 is related to partially H2 -reduced cobalt nanoparticles at 673 K. Since FT1 reports an enhanced reducibility compared to FT2 (as displayed in TPR, Fig. 4), the initial concentration of reference spectrum C is consequently lower for FT1, and the position of the maxima (2064 cm−1 ) is also found at lower wavenumber than FT2 (2068 cm−1 ). After 2 h time on stream, reference spectrum C concentration observed for FT1 and FT2 decreases of about 36 and 42% respectively (Fig. 3B and C), which indicates that syngas mixture slowly completes the reduction of the nanoparticles (in agreement with references [9,16]) at a lower temperature range than under H2 flow (diluted in inert gas). 4. Conclusions Fig. 4. H2 -TPR profiles of FT0, FT1 and FT2 catalysts (black, red and blue curves respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Chemometric analysis has been carried out on the spectra series recorded under operando conditions (H2 /CO syngas) in the 2300–1300 cm−1 range. The decomposition looks similar for FT1 and FT2 catalysts (Figs. 2 and 3) and leads to three reference spectra (A–C), while the evolution in the spectra for FT0 catalyst could be explained at 99.9% with only two reference spectra (A and B): - Reference spectrum A displays a maximum at 2035 cm−1 . The spectrum reports an asymmetrical component centered at 2035 cm−1 due to CO linearly adsorbed on Co◦ surface atoms and a broad signal below 1950 cm−1 due to CO bridged species. This reference spectrum also takes into account CO gas phase contribution (2200–2100 cm−1 ). - Reference spectrum B is related to the storage of products species like CO2 adsorbed at the basic surface sites leading to the further formation of formate/carboxylate species (1600–1300 cm−1 ) [10]. - Reference spectrum C (for FT1 and FT2 catalysts) displays a maximum at ca. 2068–2064 cm−1 . The component is very thin and quite symmetrical which may imply the presence of well-defined metal surface atoms positions prompt to sorb CO. From the position of the maxima, the reference spectrum can be linked to Co+/␦+ · · ·CO and/or Co◦ · · ·CO and/or HCo◦ · · ·CO without any distinction. Reference spectrum C cannot be due to carbon insertion into Co◦ nanoparticles (CO· · ·Co ◦ C surface complex) [11], since at t = 0 s FT1 and FT2 catalysts have never been in contact with CO molecule. Moreover, the concentration profile obtained for Reference spectrum C (Fig. 3) partially decreased during 120 min time on stream at 503 K, which also discards possible hydrocarbonyl surface species assignment (HCo◦ · · ·CO) not stable at such temperature range [14]. The component centered at 2068–2064 cm−1 may be related to CO adsorbed on Co◦ atoms at the surface of oxygen-rich nanoparticles in agreement with TPR profiles measurements and the nonobservation of such component for FT0 catalyst (which exhibits enhanced reducible properties). It is worthy to note that the temperature reduction ‘seen’ by the catalyst particles located at the upper surface of the DRIFT reactor (i.e. 100 m depth analyzed by
Chemometric tools can be a powerful technique to decompose operando DRIFT spectra series. In the present case of Fischer Tropsch synthesis, chemometric analysis allows a description of the modifications occurring on the surface of the nanoparticles as a function of catalyst time on stream. The mathematical decomposition of the spectral evolutions done on three catalysts with different reducibility are confronted in order to correlate the spectral observations to the cobalt surface atoms speciation. The results shed light onto the attribution of the IR components in the 2100–1750 cm−1 range. The component falling in the 2068–2064 cm−1 range is assigned to CO sorbed onto Co◦ surface atoms of partially H2 -reduced nanoparticles. This component is found to progressively decrease as a function of catalyst time on stream which indicates a progressive reduction of the nanoparticles under syngas at 503 K. References [1] K. Hadjiivanov, Advances in Catalysis, in: Friederike C. Jentoft (Ed.), Academic Press, 2014, pp. 99–318. [2] K.I. Hadjiivanov, G.N. Vayssilov, Advances in Catalysis, Academic Press, 2002, pp. 307–511. [3] F. Thibault-Starzyk, F. Maugé, Characterization of Solid Materials and Heterogeneous Catalysts, in: M. Che, J.C. Védrine (Eds.), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2012, pp. 1–48. [4] M. Rivallan, E. Seguin, S. Thomas, M. Lepage, N. Takagi, H. Hirata, F. Thibault-Starzyk, Angew. Chem. 49 (2010) 785–789 (International ed. in English). [5] M. Rivallan, S. Thomas, M. Lepage, N. Takagi, H. Hirata, F. Thibault-Starzyk, ChemCatChem 2 (2010) 1599–1605. [6] S. Thomas, M. Rivallan, M. Lepage, N. Takagi, H. Hirata, F. Thibault-Starzyk, Microporous and Mesoporous Mater. 140 (2011) 103–107. [7] N. Kumar, K. Jothimurugesan, G.G. Stanley, V. Schwartz, J.J. Spivey, J. Phys. Chem. C 115 (2011) 990–998. [8] C.-W. Tang, C.-B. Wang, S.-H. Chien, Thermochim. Acta 473 (2008) 68–73. [9] N.E. Tsakoumis, M. Rønning, Ø. Borg, E. Rytter, A. Holmen, Eleventh International Symposium on Catalyst Deactivation Delft (The Netherlands,) October 25–28, 2009, 154 (2010) 162–182. [10] A. Paredes-Nunez, D. Lorito, N. Guilhaume, C. Mirodatos, Y. Schuurman, F.C. Meunier, Catal. Today 242 (2015) 178–183. [11] J. Couble, D. Bianchi, J. Phys. Chem. C 117 (2013) 14544–14557. [12] J. Couble, D. Bianchi, Appl. Cata. A: Gen. 445–446 (2012) 1–13. [13] M. Kollár, A. de Stefanis, H.E. Solt, M.R. Mihályi, J. Valyon, A.A. Tomlinson, J. Mol. Catal. A: Chem. 333 (2010) 37–45. [14] G. Kadinov, C. Bonev, S. Todorova, A. Palazov, J. Chem. Soc. Faraday Trans. 94 (1998) 3027–3031. [15] H. Li, M. Rivallan, F. Thibault-Starzyk, A. Travert, F.C. Meunier, Phys. Chem. Chem. Phys. PCCP 15 (2013) 7321–7327. [16] A.M. Saib, A. Borgna, J. van de Loosdrecht, P.J. van Berge, J.W. Niemantsverdriet, Appl. Cata. A: Gen. 312 (2006) 12–19.
Please cite this article in press as: L. Lemaitre, et al., Surface modifications of cobalt Fischer Tropsch catalyst followed by operando DRIFT and chemometrics, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.033