Competing mechanisms in CO hydrogenation over Co-MnOx catalysts

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Competing mechanisms in CO hydrogenation over Co-MnOx catalysts Motahare Athariboroujeny, Andrew Raub, Viacheslav Iablokov, Sergey Chenakin, Libor kovarik, and Norbert Kruse ACS Catal., Just Accepted Manuscript • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Catalysis

Competing mechanisms in CO hydrogenation over Co-MnOx catalysts Motahare Athariboroujeny1, Andrew Raub1, Viacheslav Iablokov1, Sergey Chenakin2, Libor Kovarik3 and Norbert Kruse*,1, 3

1

Voiland School of Chemical Engineering and Bioengineering, Washington State University, Wegner Hall 155, PO Box 646515, Pullman, WA 99164-6515

2

G.V. Kurdyumov Institute for Metal Physics NASU, Akad. Vernadsky Blvd. 36, 03142 Kyiv, Ukraine

3

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99332

Abstract

ACS Paragon Plus Environment

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We study the hydrogenation of CO at ambient pressure conditions over a Co-MnOx model catalyst using Chemical Transient Kinetics (CTK) under calibrated molecular flow conditions. Alkanes and alkenes are shown to form with markedly differing kinetics. Quantitation of the data allows accumulating carbon and oxygen coverages to be determined at any instant of the “build-up” transients. Anderson-Schulz-Flory (ASF) chain lengthening probabilities are evaluated while approaching the steady-state of the reaction. A linear dependence of these probabilities on the transient CO gas pressure provides evidence for a CO insertion mechanism being in operation under high-coverage conditions. A detailed kinetic analysis of reactant/product formation and scavenging is in agreement with this conclusion. However, for coverages below the monolayer limit, fast CO dissociation, probably hydrogen-assisted and promoted by Mn2+, also enables significant CHx-CHy coupling to occur. Evidence was obtained from High Resolution Transmission Electron Microscopy (HRTEM) that a phase transition from Co to Co2C was triggered at atmospheric pressure conditions for the Co-MnOx catalyst.


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Keywords: Fischer–Tropsch • CO hydrogenation • Chemical Transient Kinetics (CTK) • CO insertion • Cobalt carbide – manganese oxide

1. Introduction The catalytic hydrogenation of CO (Fischer-Tropsch synthesis) to form chainlengthened hydrocarbons and their derivatives was discovered about one hundred years ago 1-3. Fe- and Co-based catalysts have been employed in large-scale Fischer-Tropsch applications ever since. While supported Co catalysts tend to be more active and provide higher chain lengthening probabilities, their selectivity toward olefins and oxygenates is lower than that reported for supported Fe catalysts. With this background, efforts have been made to improve the selectivity performance of Co-based catalysts through “alloying” with other metals such as Cu 4-10 or by using metal oxide promoters. Regarding the latter, Co-MnOx catalysts, either undiluted

11-17

or dispersed on a carrier

18-28,

have

received considerable attention. The reason for the large interest in Co-MnOx systems is at least threefold. First, Anderson-Schulz-Flory (ASF) product distributions tend to show significant deviations for short-chain hydrocarbons over undiluted Co-MnOx. In particular,


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C3 olefin formation has been observed with relatively high yields. Second, there is farreaching consensus

18-28

on supported Co-MnOx catalysts being able to promote C5+

hydrocarbon and olefin formation at the expense of methane. These shifts in selectivity, which go along with moderate rate enhancements, indicate less hydrogen availability as compared to non-promoted catalysts. Only small concentrations of MnOx promoter have been shown to be effective, while larger ones lead to a deterioration of the catalyst’s performance. Johnson et al. 26 carefully studied the dependence of activity and selectivity of Co/SiO2 catalysts on Mn loading and found an optimum promoter action for a Co/Mn=10 atomic ratio. Third, recent high pressure studies reported the production of either lower olefins in the C2-C4 range

15

or long-chain oxygenates (alcohols/aldehydes)

with high selectivity 16. In the latter case, Co-Mn5O8 catalysts with varying Co/Mn atomic ratios were used in the presence of small amounts of potassium to optimize n-aldehyde formation at low pH2/pCO ratios. Interestingly, a Co - Co2C phase transition has been observed in these and similar studies 15, 16, 28, 29. The large variety of products accessible via catalytic CO hydrogenation raises the question for the underlying mechanisms spanning this complex reaction network.


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Hindermann et al.

30

reviewed the literature on the various pathways with a particular

emphasis on oxygenate formation. More recently, van Santen et al. 31 revisited this topic from a theoretical point of view. The general understanding of chain growth initiation and propagation yet continues to be a matter of debate and can be boiled down to either C-C coupling of CHx or CO insertion occurring 32-49. Regarding Co-MnOx catalysts, Morales et al. 19 adopted the view according to which MnO may act as a co-catalyst thereby creating active sites for the formation of CxHyOz intermediates via a CO insertion mechanism Hutchings et al.

14

50.

suggested α-hydroxylated alkyl groups to be formed involving the

coupling of electrophilic and nucleophilic C1 intermediates. A decoration model, in which small aggregates of MnOx populate the surface of Co nanoparticles, was developed by Johnson et al.

26, 51

and was able to explain the existence of an optimum Mn loading

through maximization of the Co-MnOx interface. The same authors proposed Lewis acidbase interactions between Mn2+ and adsorbed CO to weaken the C-O bond and thereby promote CO dissociation. As a result, the ratio of adsorbed H/CO would decrease and the increase in C5+ selectivity and olefinic products, at the expense of methane, could be explained.


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The chemical nature of the most abundant surface intermediate (masi

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52)

leading to

hydrocarbon chain lengthening over Co-MnOx type catalysts remains unclear up until now. The present study aims to show that a CO insertion mechanism is indeed in operation for undiluted Co-MnOx model catalysts. However, due to the high rate of CO dissociation in the initial stages of the reaction on an essentially metallic Co particle surface, significant amounts of CHx species are likewise formed and enable paraffins to form also via C-C coupling. Chemical Transient Kinetics (CTK) under calibrated flow conditions will be used to monitor, as a function of time, CO consumption as well as paraffin/olefin production while the catalytically active phase is being constructed. Timedependent CO response and hydrocarbon ASF distributions merge into a single graph demonstrating that the chain lengthening probabilities of hydrocarbons vary linearly both with transient CO partial pressures under high coverage conditions and with CHx coverages for sub-monolayer coverages, respectively.


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2. Experimental Section Catalyst preparation Co-MnOx catalyst precursors (10/1 atomic ratio of metal) are prepared via the oxalate route of fast co-precipitation into a mixed-metal organic framework structure with oxalate playing the role of the metal chelating ligand 53. Using THF as the solvent, a mixed solution of Co (NO3)2·6H2O and Mn (NO3)2·4H2O is prepared and added to an excess of oxalic acid, H2C2O4·2H2O, under vigorous stirring. Relative metal atomic amounts in oxalate coprecipitation can easily be varied to obtain catalysts with well-defined compositions. After removal of the supernatant THF, the precipitate is centrifuged, dried overnight at room temperature and finally crushed and sieved to obtain a mixed-metal oxalate precursor with a size fraction between 125 and 250 µm for kinetic investigations and characterization studies. Mixed-metal oxalate precursors turn into active catalysts by thermally stripping oﬀ the oxalate ligand through hydrogen-assisted Temperature Programmed Decomposition (H2TPDec)). The decomposition is accomplished by heating the catalyst in a 30 ml.min-1 flow of 10% H2 in He, using a ramp of 3 °C.min-1 from ambient temperature to 390 °C, followed


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by a 1 hr holding step. The formation of gaseous products during oxalate decomposition is followed by calibrated quadrupole mass spectrometry (MS). H2-TPDec spectra show that major peaks for water, CO and CO2 form during decomposition of the oxalate framework structure. For details see the SI, Figure S1. Dynamic H2-D2 exchange is used to determine metallic surface areas of the catalysts formed via oxalate decomposition. The method was described in detail by Schweicher et al. 54 and consists of swiftly replacing H2 by D2, or D2 by H2, under dynamic flow conditions (30 ml.min-1) at 130 °C

and measuring the HD mass spectrometric response.

Contributions from gas phase exchange are taken into account through measurements at different partial pressures of H2 (D2) and extrapolating to zero H2 (D2) pressure conditions. For details see the SI, Figure S2. For a Co-MnOx (10/1) catalyst as used in the present study, we obtain a value of 128.6 μmol HD g -1Cat which translates into a specific metal surface area of 5.5 m 2. g -1Cat if an adsorption stoichiometry of H/Co=1 is assumed 55.

1

The total surface area of the Co-MnOx (10/1) catalyst is determined to be 48 m 2. g

Cat by

-

the BET method (Brunauer-Emmett-Teller) using N2 as the probe gas at 77 K. Note


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that TPDec and H2-D2 exchange are performed within the same apparatus without displacing the sample.

Chemical Transient Kinetics (CTK) Chemical Transient Kinetics (CTK) goes back to early work by Wagner and Hauffe in relation to research with electrochemical solid-state systems 56. CTK was first applied by Tamaru

57

to characterize the adsorbed state in heterogeneous catalysis. The approach

is different from Steady-State Isotopic Transient Kinetic Analysis (SSITKA) originally developed by Happel

58

and Bennet

59

and extensively reviewed by Shannon and

Goodwin 60 as well as Ledesma et al. 45. While the forcing disturbance in SSITKA is based upon isotopic labeling of one of the reactants while maintaining quasi-steady state conditions of the reaction, the leading principal in CTK is to trigger sudden changes in the

chemical composition of the reactant feed and to follow either the construction of the catalytically active phase or its scavenging as a function of time. The CTK methodological approach along with the data evaluation procedures developed in our laboratory were described in several preceding papers

39, 61-64.

Once carefully calibrated, the CTK


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technique allows the calculation of rates (in mol.s-1 or molecules.s-1) of reactant consumption and product formation at any time during the transient periods. The time-dependent response behaviour of reactants and products in switching experiments is followed by quadrupole MS and GC-MS (Agilent 7890A GC/5975 MS). To initiate the construction of the catalytically active phase (“build-up” transient), a flow of hydrogen gas in dynamic adsorption-desorption equilibrium with the catalyst surface is abruptly replaced by a flow of CO/H2, while keeping both the total flow and the H2 flow constant. Measured flows are recalculated to obtain true molecular outlet flows. This becomes possible by injecting Ne at a known (and constant) flow rate into the reactor exit (see below for details). We also note that the procedure of initiating the build-up with a catalyst in dynamic H2 equilibrium guarantees the Co surface to be in a quasi-metallic state. After establishing steady-state conditions of the CO+H2 reaction, a back-transient is triggered by switching to the original H2 adsorption-desorption conditions. This way, the scavenging of the catalytically active phase is monitored as a function of time. The CTK quantitative evaluation in terms of “surface atom counting” was previously des ads described elsewhere 39, 62, 64. In brief, the surface flows defined by φsurf i (t) = φi (t) ― φi


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des (t) have to be calculated for each chemical component (φads i (t) and φi (t) are the

instantaneous flow rates of adsorption and desorption, respectively). This requires the time-dependent mass balance ∂ngas i ∂t

(t,x,y,z) =

dngas i dt

in des ads (t) = φ ― φout i (t) + φi (t) ― φi (t) i

to be solved for gradient-free conditions (which changes the partial differential equation into an ordinary one).

dnigas dt

(t) is calculated straightforwardly from the ideal gas law. φin i =

0 for products and φout i (t) =

pi(t)Dtot(t) RT

, with Dtot(t) =

pNe(0)Dtot(0) pNe(t)

determined from the Ne

partial pressures and volumetric flow rate at time t=0 (before product formation) and time t, respectively. Note that φout Ne is constant since Ne is introduced at a constant flow rate after the reactor. The calculation of Dtot from the time-dependent Ne bypass is key to any quantitative evaluation of atomic surface amounts from atmospheric CTK measurements (in the absence of oxygenate formation which only occurs at elevated total pressures): surf surf surf surf φsurf C (t) = φCO (t) + φCO2 (t) +yφCyH2y + 2(t) +zφCzH2z(t) surf surf surf φsurf O (t) = φCO (t) + 2φCO2 (t) + φH2O(t) 𝑡

θC(𝑡) = ∫0φsurf C (t) 𝑡

θO(𝑡) = ∫0φsurf O (t)


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A similar procedure can be applied to determine θH(t) (as done by Schweicher et al. 39), however, since the build-up of the catalytically active phase starts from a hydrogen equilibrated surface, assumptions are necessary for the Had equilibrium coverage preceding the build-up. Possible errors in determining θC(t) and θO(t) include gas holdup and “chromatographic” effects due to molecule-specific transport limitations in the catalyst bed and the capillary leading to the ultra-high vacuum (UHV) chamber of the quadrupole mass spectrometer (MS). As shown in the SI (Figure S4), gas hold-up has no measurable influence on the response time of Ar and CO when triggering the respective flows. Inertia effects due to the filling of the catalyst-loaded reactor yet occur but are unlikely to falsify the quantitation of the data since the response behaviors of Ar and CO for the SiC-filled reactor are identical. The water response time is 4 to 5 s slower as compared to that of inert He. This delay is independent of the presence of a catalyst bed and is determined by the slow diffusional transport through the capillary line of the MS.

High Resolution Transmission Electron Microscopy (HRTEM)


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Transmission electron microscopy (TEM) studies are performed with an aberration corrected FEI Titan 80-300 operated at 300 kV. The instrument is equipped with a CEOS GmbH double-hexapole aberration corrector as the probe forming lens, which allows for sub-angstrom resolution in scanning imaging modes. The present STEM observations are performed with a HAADF detector. The probe convergence angle is 18 mrad and the inner collection angle is 52 mrad. Compositional analysis is performed with Electron Energy Loss Spectroscopy (EELS) using JEOL ARM 200 aberration corrected STEM and Gatan’s Quantum 965 imaging filter. The analysis of the obtained spectra is performed with the Gatan Digital Micrograph software package. Samples for TEM observations are prepared by dispersing dry powder on lacey-carbon coated Cu TEM grids.

3. Results Chemical Transient Kinetics (CTK)

Figure 1 shows the build-up of the catalytically active phase triggered by quickly changing the gas phase composition from H2 (with He as carrier gas) to a pH2/pCO=1 mixture (diluted by Ar). As can be seen, the calibrated outlet flow rates of all species are


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time-delayed as compared to the scaled Ar reference (corresponding to the theoretically expected CO response, COtheor, in the case where no chemisorption occurs). Strikingly, it takes ~ 20 s until gaseous CO appears in the mass spectra (note that during this time the number of collisions between reactant molecules and catalyst surface is ~ 60 nm-2). Thus, the entire amount of CO entering the reactor undergoes irreversible chemisorption on the catalyst surface during this time. In other words, the catalytically active phase is being constructed in the initial stages of the experiment. Note that the mean residence time of adsorbed CO at 220 °C is shorter than the delay time associated with its 𝐸𝑑𝑒𝑠

appearance in the gas phase (using Frenkel’s equation, 𝜏 = 𝜏0 ∗ exp ( 𝑅𝑇 ) , with Edes= 140 kJ/mole for fcc Co(110) 65 and τ0= 10-14 s the CO residence time at T=493 K is about τ = 6 s; smaller pre-exponential factors, as frequently observed for CO adsorption on metal single crystal surfaces, would cause significantly shorter lifetimes).

Clearly, the

dissociative chemisorption of CO, likely hydrogen-assisted to meet the challenge of high C-O activation barriers 36, 38, 40, 49, 66, 67 , is the most obvious process occurring during the first 20 s of the CTK experiment and there is no doubt to us that this is associated with


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the accumulation of carbon and oxygen atoms (or any type of species containing these atoms, possibly including hydrogen) on the catalyst surface. Such species are then subject to reaction with hydrogen to yield methane and, respectively, water, as will be further discussed later on. The first product seen to desorb from the catalyst surface is methane. It appears with a short delay of 2 s relative to the scaled Ar signal and runs through a maximum before decreasing again. A similar behavior was also observed in more recent CTK measurements of other groups using different types of supported Cobased catalysts

48, 68, 69.

According to Figure 1, the steady-state methane level is only

obtained after relatively long reaction times. The time-dependence of the methane production is compatible with the view that (at least) two different mechanistic processes are in operation, whereby early methane most probably results from the hydrogenation of surface carbon. H2O and CO2 yet appear largely delayed relative to early methane production. It was previously argued that water production is kinetically inhibited

39, 69-72.

Interestingly, both CO and CO2 appear with identical delay times; their onsets coincide with the maximum of methane production in Figure 1. A steady-state turnover frequency of 0.025 molecules s−1 site−1 can be calculated by comparing the measured CO outlet


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flow (5.2*1018 molecules.s-1 after 500 s time-on-stream, not shown in Figure 1) with the scaled Ar response (8.1*1018 molecules.s-1) which corresponds to a reaction rate of 3.3 μmol CO g-1catalyst (SI, Figure S2).

We note without showing that the steady-state

selectivity is highest for propene (42 %, ex CO2) among all hydrocarbons. The production of chain-lengthened C2+ hydrocarbons needs closer inspection, see inset of Figure 1. Yields normalized to the steady-state demonstrate that C2+ paraffins appear in sequence of their carbon number, which is in agreement with a consecutive reaction involving a C1 species for chain lengthening. Propene, as well as higher olefin homologues (SI, Figure S5), yet appear with a delay time longer than that of paraffins. Therefore, olefins are unlikely to be precursors of alkanes (especially at short reaction times where the formation kinetics dominate). Strikingly, all alkanes start forming before the appearance of gaseous CO. They run through an early maximum before levelling off to reach the steady-state later on. By contrast, propene starts forming time-correlated with gaseous CO and runs smoothly into the steady-state without “overshoot” production.


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Figure 1. Outlet flows (molecules.s-1, detected by MS) during a complete CTK build-up experiment for CO hydrogenation over a Co-MnOx (10/1) catalyst (T=220 °C, Ptot=Patm, total volumetric flow rate 54 ml/min, pH2/pCO = 1). The insert shows hydrocarbon outlet flows normalized to the steady state value along with the CO response.

Turning to back-transient measurements, once the reactant CO is removed from the pH2/pCO mixture under constant flow conditions, both saturated and unsaturated hydrocarbon production is immediately affected. Figure 2 shows the relative outlet flows (by normalizing the absolute outlet flows to their steady-state values) of chain-lengthened


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paraffins and olefins. The respective C2+ species of each type of hydrocarbon show a rather different time dependence. All paraffins first increase and pass through a maximum before decreasing with an identical decay time of 23 ± 2 s in a pseudo-first order kinetic process. The time-dependent CH4 response is different in that the exponential decay after passing its production maximum seems to be delayed. We associate this behavior with the removal of significant amounts of (sub-) surface carbon (for a discussion of the reaction-induced accumulation of carbon amounts, see below). Quite generally, the occurrence of maxima in the back-transients has to be associated with alterations in the hydrogenation rate of the respective surface precursor species. Obviously, as the scavenging of the catalytically active phase in the absence of co-adsorbing CO proceeds, metallic sites are created for H2 dissociation. While the availability of atomic Had increases, the surface concentration of precursor species decreases with time; the rates of supply and consumption balance while running through the maximum of paraffin production in Figure 2. The behavior observed for olefins is completely different from that of paraffins. According to Figure 2, these species fade away right from the beginning of the back-


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transients. A common exponential decay behavior is involved, as shown in the insert of Figure 2. The first-order time constant for propene is 8 ± 1 s while that for longer olefins is commonly ~ 10 s. As already mentioned for the build-up transients, ethene is not observed at all. It is interesting to note that high pressure CO hydrogenation studies with the same nominal catalyst composition also showed little ethene formation

16.

Similar

observations were made with different catalyst compositions both in our laboratory 73 and elsewhere 74, 75. We also note without showing (for details see SI, Figure S3) that the time constants for the decay of CO reactant and Ar reference in back-transients are very similar. Such behavior was previously observed for Co-MgO catalysts and places a strong argument in favor of a CO insertion mechanism being in operation under synthesis conditions 39, 62, 64.


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Figure 2. Normalized outlet flows (detected by GC-MS) during back-transients (same reaction conditions as in Figure 1). The insets provide a zoom into the different time dependences of fading olefins (bottom) and paraffins (top), respectively.

Build-up and back transients for hydrocarbons have also been performed for various pH2/pCO ratios. The general response behavior for chain-lengthened hydrocarbon formation and scavenging remains the same as that shown in Figures 1 and 2 (for differences regarding methane, see SI, Figure S6).


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Since the reaction starts on an essentially “metallic” Co particle surface in dynamic adsorption-desorption equilibrium with hydrogen, a quantitation in terms of accumulating amounts of carbon and oxygen atoms becomes possible during build-up by solving the time-dependent mass balance under the (validated) assumption of gradient-free reactor conditions (see Experimental and SI). Furthermore, ASF chain lengthening probabilities at any instant during build-up can be constructed and related to both the transient CO pressures and calculated carbon amounts. Figure 3 shows the results of these efforts for the cases of pH2/pCO=1 (equal reactant pressures) and pH2/pCO=2. Accumulated carbon and oxygen amounts (“coverages”) as a function of time are plotted in Figures 3c and d. Note that carbon amounts in excess of a monolayer are encountered when approaching steady-state reaction conditions. This applies most obviously to measurements with low H2 partial pressures and indicates subsurface carbon diffusion to take place. This conclusion is strongly supported by the occurrence of carbidic C1s states in X-ray Photoelectron Spectroscopy (SI, Figure S9, S10) and the observation of Co2C structures in High Resolution Transmission Electron Microscopy (Figure 4 and Figure S11-S13). We note that this is, to the best of our knowledge, the first time that the Co - Co2C phase


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transition is being reported for a Co-based catalyst in the absence of an alkali promoter at atmospheric pressure. The carbon accumulation behavior is reminiscent of that reported earlier for Co-MgO catalysts 39. We also note that for higher H2 partial pressures, pH2/pCO≥3, carbon coverages on Co-MnOx catalysts do not exceed the monolayer capacity any longer (SI, Figure S8). The oxygen amounts seem to converge toward a saturation level. They are lower than those of carbon but still above the monolayer limit if calculated relative to the available Co surface area after 220 s time-on-stream. This is a consequence of the kinetically slow water (and CO2) formation. We previously advanced the hypothesis that accumulating and interacting oxygen/hydrogen amounts, in the absence of water production, result in surface hydroxyl groups which react with CO to form formate-type species 39, 63, 76. Indeed, such species are observed by operando-type Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS, Figure S14), similar to experiments with Co-MgO catalysts

76.

The accumulation of oxygen amounts in Figure 3, due to the formation of

either type of surface species, is therefore understandable. However, different from carbon being most likely associated with Co metal, surface oxygen in the form of hydroxyl


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and formate may also be spatially co-located with the Co-MnOx interface or, to a lesser extent, with MnOx sites. Oxygen atom-counting in Figure 3 c) and d) has therefore been related to the overall number of sites of the Co-MnOx (10/1) catalyst, as derived from BET measurements. Turning to transient chain lengthening probabilities, Figure 3 a) and b) clearly demonstrates a linear dependence on transient CO gas pressures for both pH2/pCO ratios. This behavior provides compelling evidence for a CO insertion mechanism being in operation, specifically at overall high surface coverages. However, for accumulating carbon amounts below the monolayer limit, a linear dependence α=f(ϴC) is found as well. In the case of pH2/pCO=2, this linearity seems to extend over a larger range of ϴC values; changing slopes as in the case of pH2/pCO=1 are not observed. Since a linear dependence would also hold for hydrogenated carbon species, C-C coupling of CHx seems to be indicated here. The reason why a linear α=f(ϴC) holds well for pH2/pCO=2 is most probably related to a lower rate of carbon subsurface diffusion under conditions of higher H2 partial pressures. Note also that the slope of α=f(ϴC) at sub-monolayer coverages is steeper for pH2/pCO=1 than for pH2/pCO=2. This is understandable because, in the former case, CO


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hydrogenation toward chain-lengthened products is under-stoichiometric with regard to hydrogen. As a consequence, faster-increasing ϴC values also lead to higher chain lengthening probabilities in the early stages of the reaction. In the latter case, for pH2/pCO=2, it remains an open question whether CO insertion and CHx-CHx coupling occur independently or jointly. Linear relationships of both α vs. (ϴC) and (CO) transient would be compatible with a scenario in which CO is being inserted into a metal-C bond. Further work is necessary to unravel the possibility of such synergies in chain growth mechanisms.


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Figure 3. Chain lengthening probabilities (calculated from build-up transients) as a function of the transient CO partial pressure and carbon surface coverage for pH2/pCO=1 (a), pH2/pCO=2 (b). c) and d) shows the carbon and oxygen coverages for the respective pH2/pCO ratio. Carbon amounts refer to the specific cobalt surface area titrated via H2/D2 exchange. Oxygen amounts are related to the overall specific surface area measured by BET, as described in the SI.

High Resolution Transmission Electron Microscopy (HRTEM) The quantitative evaluation of the CTK results in terms of accumulating carbon concentrations beyond monolayer limits suggests carbon subsurface diffusion, or


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possibly a Co - Co2C phase transformation, takes place. To further develop this hypothesis, microscopic and spectroscopic studies are being performed. Using X-ray photoelectron spectroscopy (XPS), Co - C bonds are indeed identified as described in Figure S9. The occurrence of a “carbidic” C1s line in XPS translates into Co2C structures in HRTEM. This is demonstrated in Figure 4. In particular, HRTEM reveals nanolamellar regions within Co (fcc) where the stacking sequence is modified, consistent with Co2C/Co (hcp). Based on the detection of carbon in Electron Energy Loss Spectroscopy (EELS) (Figure S13), we assign these regions to Co2C. The nanolamellar Co2C structures are observed along with metallic twin boundaries, dislocations and stacking faults. While the dominant fraction is metallic Co fcc, the atmospheric FischerTropsch conditions obviously also provoke a Co - Co2C phase transition, at least partly. Quite generally, the stress associated with twin formation is fairly high in fcc metals. Deformation twinning is therefore likely influenced by interactions with dislocations, in particular, for nanosized systems such as ours where the formation of Co2C hcp may help decrease the deformation stress. Although our observations are subject to further in-


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depth research, it seems that the Co - Co2C phase transition can be initiated during the Fischer Tropsch reaction at atmospheric pressure and in the absence of alkali.

Figure 4. Combined HRTEM/EELS of spent Co-MnOx (10/1) catalyst. Overview images are provided in a) and (with chemical resolution) in b). A mixed-metal (Co,Mn)O phase (NaCl structure) is identified in c) along with a spinel-type (Co,Mn)3O4 phase which is most probably associated with the oxygen passivation procedure. d) Demonstrates the formation of Co2C due to atmospheric CTK experiments. Besides Co2C and twin Co fcc, stacking faults are also commonly observed. EELS data in e) and f) prove the occurrence of metal-oxide phases in c) and Co metallic phases in d).


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The occurrence of a (Co,Mn)O phase in Figure 4c is particularly intriguing. While previous HRTEM and 3D Atom Probe Tomography (APT) studies with Co4Mn1K0.1 after CO hydrogenation clearly demonstrated the occurrence of a Mn5O8 phase a Co@Co,Mn,K core-shell structure

77,

16

along with

the present HRTEM/EELS study with Co10Mn1

misses the Mn5O8 phase, but proves the occurrence of a (Co,Mn)O phase instead. In general, while Co concentrations in the (Co,Mn)O phase are higher than those of Mn, they may vary across this phase. Such variations in concentration are also seen in individual (Co,Mn)O particles where outer regions are highly enriched in Mn suggesting selective segregation of Co. This picture of disintegrating (Co,Mn)O particles is fully consistent with HRTEM/EELS and XPS analyses of the fresh samples (Figure S9- Figure S11) where the (Co,Mn)O phase is lean in Mn, as expected with regard to the nominal Co/Mn=10 bulk composition. The absence of Mn5O8 in activated Mn-lean Co/MnOx catalysts is likely due to a ”solubility” effect, i.e. the large dispersion of Mn in a Co/Mn-oxalate co-precipitated precursor (forming a metal-organic framework) during catalyst preparation. Further characterization studies with systematically varying Co/Mn ratios are necessary to arrive


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at more detailed conclusions. Figure 4b proves the close proximity of Co/Co2C and Mnrich (Co,Mn)O phases, including the occurrence of Co@(Co,Mn)O core@shell-type structures similar to observations with APT of Co4Mn1K0.1 single particles. Interestingly, according to Johnson et al. 26, the occurrence of Mn2+ states at the surface of Co particles for catalysts with low Mn loading (Mn/Co≤0.1) has to be considered crucial in assessing the Mn2+ promoter effect in Co/SiO2 catalysts. The same authors report larger amounts of Mn, i.e. Mn/Co>0.1, cause Mn-rich particles with diameters greater than several nanometers to block Co active sites and to obliterate the Mn promoter effect observed for Mn/Co≤0.1.

4. Discussion Our CTK results for the hydrogenation of CO over Co-MnOx model catalysts can be summarized as follows. Forward (build-up) transients show that a) CO chemisorption is essentially irreversible during the first 20 s, b) methane production for equal reactant pressures occurs first (long before the appearance of gaseous CO) and runs through a maximum, c) CO and CO2 appear time-correlated and coincide with the maximum of methane production, d) olefins are produced with slower kinetics than paraffins and are


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time-correlated with gaseous CO, e) C2+ formation follows the methane time-dependence and can precede the appearance of gaseous CO, f) water formation has the longest delay time of all products. Backward (scavenging) transients show that a) CO fades away with essentially the same time constant as argon (τ = 6 s) , b) alkanes produce a maximum and then desorb with the same time constant of 23 ± 2 s (except for methane, where τ ~ 83 s), c) olefins do not produce a maximum and they clear away with τ = 10 ± 1 s. These time-dependent features are essentially the same for pH2/pCO=1 and pH2/pCO=2, except that the methane production rises to higher levels with less of an “overshoot” feature during build-up for pH2/pCO=2 and higher. The CTK data are in agreement with a CO insertion mechanism being in operation. This conclusion is also strongly supported by evaluating transient chain lengthening probabilities, α, as a function of transient CO pressures. A linear relationship is obtained at overall high surface coverages, conditions which usually describe the Fischer-Tropsch regime. Early C2+ formation (before the appearance of transient CO pressures) translates into a linear relationship α=f (ϴC). We therefore conclude that C-C coupling may occur to produce paraffins below the monolayer limit. This behavior is different from earlier measurements with Co-MgO model catalysts


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where paraffins occur time-correlated with (and after) the appearance of transient CO pressures 39. Our characterization studies demonstrate a reaction-induced Co - Co2C phase transition to take place. Given the atmospheric pressure conditions and the relatively short time scales of our CTK measurements, we suspect this process (involving nucleation and growth once critical subsurface carbon concentrations are reached) is in its early stages. It would appear likely that the initial Co2C formation occurs at defects or Co-MnOx interfaces. The predominant phase under atmospheric reaction conditions remains metallic Co of (mainly) fcc crystal structure. (Co,Mn)O mixed oxides, also formed during the activation of the mixed CoMn-oxalate (10/1) precursor, are likewise subject to reconstruction. Postreaction HRTEM studies clearly indicate the occurrence of disintegrated particles with Mn-rich (Co,Mn)O composition, including Co@(Co,Mn)O core@shell-type structures. These results provide corroborating evidence that (Co,Mn)O-derived Mn2+ states must be considered in order to explain the selectivity performance of Co-MnOx catalysts 26.


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The occurrence of a CO insertion mechanism raises the question for which bonds are targeted to produce suitable “most abundant reactive intermediates” (mari52) enabling chain growth. This question may have different answers depending on the specific reaction conditions (nominal chemical composition of the catalyst, pH2/pCO ratios, surface coverages etc.) 38, 39, 41, 42, 44, 63, as is exemplified in the present contribution. For example, Zhuo et al., on the basis of density function theory (DFT) calculations, consider RCHx species as the target for an insertion of chemisorbed CO

41.

The same authors also

recognize the importance of coverage effects in theoretical calculations of barriers and adsorption enthalpies. CO insertion, as reported in the present study over Co-MnOx catalysts, applies to highcoverage conditions of the atmospheric Fischer Tropsch reaction. DRIFTS results indicate the formation of formate/carboxylate-type surface species. Since no such species occur on pure Mn-oxide (see Figure S14), we conclude that they are associated with the Co/Co2C phase or its interface with MnOx aggregates. It has to be emphasized, however, that the mere occurrence of symmetric and asymmetric stretching modes of adsorbed formate (see figure S14) cannot be regarded as experimental proof for such species to


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play the role of a mari 76. Instead, the time dependence of these modes has to be studied and compared with product formation rates in CTK before a distinction between masi and mari type species becomes possible. Such experimental studies, which are under way for the present Co/MnOx catalyst system, should also be corroborated by DFT calculations of frequency shifts for varying adsorption modes (monodentate vs. bidentate). Recent efforts by the Meunier group allowed “fast” and “slow” formate decomposition to be followed after atmospheric CO hydrogenation over Co/SiO2-Al2O3 catalysts78. Using the integrated band intensity of the symmetric O-C-O vibrational mode the authors were able to monitor characteristic decay times for both types of formate species. Neither of these was compatible with the kinetics of hydrocarbon formation, though.

On the other hand, the possibility of forming methanol via “fast formate”

hydrogenation (involving adsorbed methoxy as intermediate) was considered cogent with the measured decay characteristics. It is instructive in this regard to also look to transient reactive studies with Cu and Cu-SiO2 catalysts where bridging bidentate transforming into monodentate formate species caused an increase of both types of O-C-O stretching frequencies79. Moreover, only monodentate formate in an Oad coadsorbed environment


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produced significant methanol formation upon reaction with hydrogen while bidentate formate did not. While it is to be expected that the reactive properties of formate-type species are different for Co-MnOx catalysts, it is remarkable that in both cases (titration by hydrogen of preformed monodentate formate on the one hand and CTK of the CO hydrogenation on the other) large amounts of carbon dioxide are formed. We suggest taking the CO2 appearance in CTK (Figure 1) as a lead for the formation (and partial decomposition) of formate-type species. In later stages of the CTK build-up phase, water rejection from the adsorbed phase would then result in a readjustment of the surface formate concentrations (translating into decreasing gaseous CO2 amounts). One of the leading arguments in support of the above mechanistic considerations is the experimental finding of kinetically inhibited water formation despite accumulating oxygen amounts in the initial stages of the reaction. Assuming fast hydrogen-assisted CO activation to occur on an essentially metallic Co surface (likely involving HCOad 41, 66 or COHad

49

and HCOHad

38, 41, 49

species according to theoretical calculations), as present

during the early stages of our build-up transients, will produce CHad and OHad

40, 66.

The

importance of (acidic) surface hydroxyl for the activation of adsorbed CO (or RCO) via


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proton transfer has recently been pointed out by Gunasooriya et al. on the basis of DFT calculations 49. The authors essentially consider CO insertion into a metal-carbon surface bond followed by proton transfer to the oxygen atom of newly generated R-C-O species. Chain lengthened hydrocarbons are then formed by water rejection and surface hydroxyl reproduction. The CTK evidence provided in the present study shows that water rejection does not coincide with hydrocarbon formation during the early stages of the CTK buildup. Furthermore, CO insertion seems to occur at high coverages, in particular for pH2/pCO=1. The dearth of site vacancies under these conditions would appear to favor CO insertion into O-H rather than into metal-C bonds, for steric reasons. It may be speculated, however, that for higher hydrogen partial pressures, pH2/pCO=2, where a linear dependence of the chain lengthening probability with both ϴC and transient pCO is found below saturation coverages (Figure 3), CO insertion into metal-C bonds of adsorbed RCHx could indeed be occurring. While the configurational aspects allowing adsorbed formate/carboxylate to become a mari type species remain vague at this point in time, the possible role of alkoxy type species as imminent precursors of hydrocarbons calls for some additional considerations.


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Besides the observation of O-C stretching modes attributable to adsorbed alkoxy (see Figure S14), it is interesting to recall that added amounts of higher alcohols to H2/CO reactant mixtures over undiluted Co-MnOx catalysts caused an enhancement of CO conversion and C4+ product yields through reductive cleavage. Clearly, this observation enticed Hutchings et al. to suggest alcohols participate in the chain growth mechanism 14.

Furthermore, Westrate et al., in studies with single crystal Co (0001) samples, showed

O-C bond scission in higher alcohols to lead to hydrocarbons 80. The important role of ORad as intermediate species explaining terminal oxygenate and olefin formation would also appear quite obvious. While oxygenates in terms of alcohols and aldehydes only occur at high pressures for thermodynamic reasons

16,

olefins may be produced at

atmospheric conditions, as shown in the present study for Co-MnOx catalysts. Hutchings et al. 14 proposed adsorbed alkoxy species to reduce to alkyl and hydroxyl, followed by a water elimination step. Although the authors did not explicitly consider olefin formation in their mechanistic considerations, one could easily imagine a fast E2 elimination step to lead to olefins, thereby explaining the rejection with short time constants during scavenging as observed in our CTK studies. There is no doubt, however, that additional


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studies will be necessary to elucidate the details of this process. In particular, such studies should aim at providing an explanation for the suppression of ethene production. In summary, although a CO insertion mechanism seems to be commonly in operation, we believe that the mechanistic details of it remain complicated. This complexity arises since chemical bonds must break and others must form in a quasi-concerted manner under the high-coverage conditions of the Fischer-Tropsch reaction where empty metallic sites are scarce. Subsurface carbon diffusion preceding the bulk Co - Co2C phase transition adds another intricacy to unravelling and setting the mechanistic benchmarks of the relevant surface reactions. Moreover, our findings highlight the importance of coverage effects on the mechanistic pathways of the Fischer-Tropsch process. There also seems to be no doubt that the specific reaction conditions (in terms of temperatures and total as well as partial pressures) and the chemical nature of the catalyst support (which may be either inert or partaking) have a significant influence on the chemical composition of the reactive surface layers and, therefore, on the relative importance of competing CO insertion and C-C coupling scenarios.


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Associated Content Supporting Information Available

Catalyst preparation, activation and physical characterization, H2-D2 exchange, Argon (COtheor) and CO response, Gas hold-up and water response behavior, Paraffins and olefin information during build-up, Methane response behavior for varying H2/CO ratios, Surface atom counting, X- ray photoelectron spectroscopy (XPS), High Resolution Transmission Electron Microscopy (HRTEM) and Electron Energy Loss Spectroscopy (EELS), Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS).

Corresponding Author * Norbert Kruse ([email protected])

Author Contributions M. A. performed all CTK measurements and drafted the first version of the manuscript. V. I. and A. R. participated in the CTK measurements. M.A. and V.I. also provided DRIFTS data. S. C. and L. K. performed XPS and HRTEM/EELS measurements, respectively. N.


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K. coordinated the research project and drafted the paper. There are no conflicting interests of authors and their institutions.

Acknowledgements

We thank the National Science Foundation for financial support under contract no. CBET-1438227. A portion of the research was performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL. PNNL is a multiprogram national laboratory operated for the U.S. DOE by Battelle. Some characterization studies were performed at CPMCT of the Universite Libre de Bruxelles (Belgium).

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Competing mechanisms in CO hydrogenation over Co-MnOx catalysts

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