The Effect of CO Partial Pressure on Important Kinetic Parameters of Methanation Reaction on Co-Based FTS Catalyst Studied by SSITKA-MS and Operando DRIFTS-MS Techniques

A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface.


Introduction
Low-temperature Fischer-Tropsch Synthesis (FTS) is a non-reversible, highly exothermic, and complex reaction (Equation (1)), which has been industrially applied for many decades for using syngas (CO and H 2 ), which is mainly derived from natural gas, biogas, and coal, towards the formation of chemicals and fuels [1][2][3].
(i) The amount (N CO , µmol g −1 ) and surface coverage (θ CO ) of reversibly adsorbed CO-s; (ii) the amount (N CHx , µmol g −1 ) and surface coverage (θ CHx ) of active reaction CH x -s (C α ) intermediates; (iii) the mean residence time of CO-s (τ CO , s) and active CH x -s (τ CHx , s) intermediates; (iv) the turnover frequency leading to CH 4 , TOF chem , or TOF ITK (s −1 ), respectively, estimated based on all Co surface metal atoms or on the active reaction intermediates (µmol g −1 ); (v) the amount (µmol g −1 ) of inactive carbonaceous species (C β ) formed during FTS (230 • C) readily hydrogenated at 230 • C in 50% H 2 /Ar; (vi) the amount (µmol g −1 ) of the refractory carbonaceous species (C γ ) formed during FTS but hydrogenated at higher temperatures (230-550 • C); Catalysts 2020, 10, 583 3 of 22 (vii) the relative reactivity (k eff ) of the various forms of linear-type adsorbed CO-s species formed over the Co surface during methanation after using operando transient DRIFTS-MS coupled with kinetic modelling.

Co/γ-Al 2 O 3 Catalyst Characterization
The powder X-ray diffraction (XRD) pattern of the 20 wt% Co-0.05 wt% Pt supported on Puralox SCCa γ-alumina carrier after calcination revealed the existence of a Co 3 O 4 crystalline phase (35.5-38.5 • 2θ range, JCPDS file No.  as reported in our previous publication [19]. The mean Co particle size (d Co , nm) was found to be 10.1 nm (based on Scherrer equation for Co 3 O 4 and the derived relationship between d Co and d Co3O4 particle size), resulting in a Co dispersion with D Co (%) of 9.5% and a Co metal surface area of 10.8 m 2 g −1 . This result was in good agreement with the D Co (%) estimated from H 2 chemisorption/Temperature-programmed desorption (TPD) measurements [19]. TEM studies on the present catalyst [19] showed the formation of agglomerates of individual cobalt particles, which is in agreement with the literature [44]. The textural properties of the present Co/γ-Al 2 O 3 catalyst were previously reported [19], where the specific surface area (SSA), pore volume (V p ), and average pore size (d p ) were found to be 88 m 2 g −1 , 0.21 cm 3 g −1 , and 8.8 nm, respectively. Figure 1A shows transient normalized concentration (Z) response curves of Kr (tracer gas), 13 CO, and 13 CH 4 obtained during the SSITKA step-gas switch 3 vol% 12 CO/25 vol% H 2 /Ar (5 h) → 3 vol% 13 CO/25% H 2 /1% Kr/Ar (t) performed at 230 • C over the Co/γ-Al 2 O 3 catalyst. It is seen that the Z( 13 CO) transient response curve lags behind that of Kr due to a measurable concentration (see Equation (5), Section 4.2)) of molecularly and reversibly chemisorbed CO formed after 5 h in FTS. The Z( 13 CH 4 ) response of gas phase 13 CH 4 clearly lags behind that of Z( 13 CO), and this is attributed to the 13 C-containing active reaction intermediates formed (following the 13 CO-s pool), which are sequentially hydrogenated to 13 CH 4 (g), and the concentration of which is estimated via Equation (6) in Section 4.2. Figure 1A,B, and Table 1 show that, by increasing the CO feed gas concentration from 3 to 7 vol% (36 to 84 mbar), the concentration (N CO , µmol g −1 ) and surface coverage (θ CO ) of reversibly adsorbed CO-s are increased (see Equations (5) and (7), Section 4.2), which are in agreement with the SSITKA work of Chen et al. [18]. Figure 1C presents the effect of CO partial pressure (P CO = 36-84 mbar) and, thus, of the H 2 /CO ratio (3.6-8.3, x vol% 12 CO/25 vol% H 2 /Ar) on the transient dimensionless concentration of 13 CH 4 formed during the SSITKA step-gas switch. A clear increasing delay in the appearance of 13 CH 4 with respect to the Z(Kr) of the tracer is observed. This behavior relates to both the reactivity of CH X -s (named C α ) active intermediates, which lead to methane and their surface coverage. The amount and surface coverage of the active CH x -s intermediates, N CHx (µmol g −1 ) and θ CHx , respectively, along with other important kinetic parameters are reported in Table 1. It is seen that N CHx and θ CHx are increased to a significant extent with increasing P CO . An increase by a factor of 2.5 is observed after increasing P CO from 36 to 84 mbar. Figure 2 presents transient rates (µmol g −1 s −1 ) of exchange, R ex CO , of adsorbed 12 CO-s with 13 CO(g) ( Figure 2A) and 12 CH x -s with 13 CH x -s (R ex CHx , Figure 2B) estimated from the SSITKA transient response curves recorded after 5 h in FTS at 230 • C (Figure 1, x vol% 12 CO/25 vol% H 2 /Ar → x vol% 13 CO/25 vol% H 2 /Kr/Ar, x = 3, 5, and 7). Estimation of these transient exchange rates was made after using Equations (3) and (4) (Section 4.2). The R ex CO appears very fast, where, within the first~8 s after the switch from the non-isotopic 12 CO/H 2 to the equivalent isotopic 13 CO/H 2 gas mixture, all reversibly adsorbed CO-s was exchanged. It is also illustrated that this R ex CO rate of exchange increases with partial pressure of CO since it reflects the rate of adsorption of 13 CO on the surface after a Co-s surface site is freed by the desorption of 12 CO-s. The rate of the latter process is proportional to the surface Catalysts 2020, 10, 583 4 of 22 coverage of 12 CO-s. If the 12 CO-s at steady-state is in equilibrium with gas-phase 12 CO, then the rate of desorption of 12 CO-s must be equal to the rate of adsorption or exchange for 13 CO(g) during the SSITKA step-gas switch. Considering the shape profile of R ex CO (Figure 2A), similar kinetics of 12 CO-s/ 13 CO(g) exchange occurs independently of P CO (36-84 mbar). The surface coverage of CO-s (θ CO ) in equilibrium with P CO increases with partial pressure of CO (area under the R CO vs. time response curve), as previously mentioned. and average pore size (dp) were found to be 88 m 2 g −1 , 0.21 cm 3 g −1 , and 8.8 nm, respectively. Figure 1A shows transient normalized concentration (Z) response curves of Kr (tracer gas), 13 CO, and 13 CH4 obtained during the SSITKA step-gas switch 3 vol% 12 CO/25 vol% H2/Ar (5 h) → 3 vol% 13 CO/25% H2/1% Kr/Ar (t) performed at 230 °C over the Co/γ-Al2O3 catalyst. It is seen that the Z( 13 CO) transient response curve lags behind that of Kr due to a measurable concentration (see Equation (5), Section 4.2)) of molecularly and reversibly chemisorbed CO formed after 5 h in FTS. The Z( 13 CH4) response of gas phase 13 CH4 clearly lags behind that of Z( 13 CO), and this is attributed to the 13 Ccontaining active reaction intermediates formed (following the 13 CO-s pool), which are sequentially hydrogenated to 13 CH4 (g), and the concentration of which is estimated via Equation (6) in Section 4.2.  13 CO, and 13 CH4 obtained after the Steady State Isotopic Transient Kinetic Analysis (SSITKA) step-gas switch 3 vol% CO/25 vol% H2/Ar (5 h) → 3 vol% 13 CO/25 vol% H2/1 vol% Kr/Ar (t), (B) Kr and 13 CO obtained after the SSITKA switch with 7 vol% CO in the feed gas composition, and (C) Kr and 13 CH4 obtained after the SSITKA  13 CO, and 13 CH 4 obtained after the Steady State Isotopic Transient Kinetic Analysis (SSITKA) step-gas switch 3 vol% CO/25 vol% H 2 /Ar (5 h) → 3 vol% 13 CO/25 vol% H 2 /1 vol% Kr/Ar (t), (B) Kr and 13 CO obtained after the SSITKA switch with 7 vol% CO in the feed gas composition, and (C) Kr and 13 CH 4 obtained after the SSITKA switch to x vol% 13 CO/25 vol% H 2 /1 vol% Kr/Ar (t) at 230 • C over the 20 wt% Co/γ-Al 2 O 3 catalyst, x = 3 (a), 5 (b), and 7 (c). The R CHx (µmol g −1 s −1 ) transient rate response curve depicted in Figure 2B show a similar shape with increasing P CO , where the rate maximum is shifted to higher reaction times and presents a longer tail until completion of CH x -s exchange. By increasing the CO partial pressure (x = 3, 5, and 7 vol% CO) in the feed gas stream, the initial R CHx rate (during the very first few seconds of the transient) increases from 0.95 µmol g −1 s −1 to 1.23 and 1.35 µmol g −1 s −1 , respectively, with the time required for complete exchange to occur at 50, 120 and 170 s, respectively. These features support the view of two kinds of CH x -s species formed in the CO/H 2 reaction (230 • C, 5 h). One is hydrogenated to CH 4 with a Catalysts 2020, 10, 583 5 of 22 faster rate than a second one of a lower rate, where the latter is responsible for the appearance of a tail ( Figure 2B). These features will be further discussed below.

SSITKA-MS after 5 h in FTS
The RCHx (μmol g −1 s −1 ) transient rate response curve depicted in Figure 2B show a similar shape with increasing PCO, where the rate maximum is shifted to higher reaction times and presents a longer tail until completion of CHx-s exchange. By increasing the CO partial pressure (x = 3, 5, and 7 vol% CO) in the feed gas stream, the initial RCHx rate (during the very first few seconds of the transient) increases from 0.95 μmol g −1 s −1 to 1.23 and 1.35 μmol g −1 s −1 , respectively, with the time required for complete exchange to occur at 50, 120 and 170 s, respectively. These features support the view of two kinds of CHx-s species formed in the CO/H2 reaction (230 °C, 5 h). One is hydrogenated to CH4 with a faster rate than a second one of a lower rate, where the latter is responsible for the appearance of a tail ( Figure 2B). These features will be further discussed below. The mean lifetimes τ CO and τ CHx (s) of the adsorbed CO-s and active CH x -s intermediates, respectively, were estimated based on the experimental results shown in Figure 1 after using Equations (8) and (9) (Section 4.2). The obtained results are given in Table 1 and Figure 3A as a function of the partial pressure of CO. The mean residence time τ CO (s) is practically independent of P CO (0.35-0.5 s), while that of τ CHx (s) largely increases with increasing P CO ( Figure 3A and Table 1). An increase bỹ 3.3 times is seen after increasing the P CO from 36 to 84 mbar (4.8 vs. 16 s).
Catalysts 2020, 10, x FOR PEER REVIEW 6 of 23 with increasing PCO to 60 mbar (5 vol%) and 84 mbar (7 vol%), respectively. These values agree very well with the measured experimental values given in Table 1, considering the slightly different reaction T (230 vs. 220 o C) and Co mean particle size used in these studies. intrinsic kinetic rates of methane formation in terms of TOFITK and TOFchem (s −1 ) as a function of CO partial pressure (bar) obtained after the SSITKA step-gas switch following 5 h in the CO/H2 reaction over the 20 wt% Co/γ-Al2O3 catalyst. Mean residence time of CO-s (τ CO , s) and active CH x (τ CHx , s), (B) surface coverages of CO-s (θ CO ) and active CH x (θ CHx ), (C) effective rate constant (k eff , s −1 ) of methane formation, and (D) intrinsic kinetic rates of methane formation in terms of TOF ITK and TOF chem (s −1 ) as a function of CO partial pressure (bar) obtained after the SSITKA step-gas switch following 5 h in the CO/H 2 reaction over the 20 wt% Co/γ-Al 2 O 3 catalyst.
Catalysts 2020, 10, 583 6 of 22 The concentration (µmol g −1 ) and surface coverage of adsorbed CO, θ CO , were found to increase by 10% to 65% with increasing CO partial pressure, ca. θ CO = 0.029, 0.032, and 0.048 for 36, 60, and 84 mbar P CO ( Figure 3B and Table 1). A significantly larger increase is obtained for the concentration (µmol g −1 ) and surface coverage of CH x -s ( Figure 3B), where θ CHx increases from 0.047 to 0.085 and 0.117 for 36 to 60 and 84 mbar CO partial pressure, respectively. An increase by a factor of~2.5 is obtained after increasing the P CO from 36 to 84 mbar. The latter values are in the range of those previously published over supported Co catalysts [8,39]. Carvalho et al. [45] reported that, for the methanation reaction at 250 • C over 20 wt% Co/SiO 2 and 20 wt% Co-0.1 wt% Pt/SiO 2 catalysts, the mean lifetime of CO-s and CH x -s active intermediates decreased with an increasing H 2 /CO gas ratio due to the greater surface coverage of hydrogen, θ H , and the lower surface coverage of CO-s, θ CO . They reported that these changes with H 2 /CO gas ratio were responsible for the increased hydrogenation rate of CH x -s. The decrease of θ CO with an increasing H 2 /CO gas ratio was reported [46] to have a positive effect on the ratio of θ H /θ *H , where θ *H represents the surface coverage of empty sites on the Co surface. This θ H /θ *H parameter plays a crucial role in the FTS reaction discussed below.
The k eff (s −1 ) value of the methanation reaction was estimated based on Equation (12) (Section 4.2). This is plotted against the partial pressure of CO as depicted in Figure 3C. It is noted that k eff = k CHx θ H . A three-fold decrease in k eff (s −1 ) is observed with increasing CO partial pressure, namely, k eff = 20.8, 8.31, and 6.26 × 10 −2 s −1 , for 36, 60, and 84 mbar CO, respectively ( Figure 3C).
Based on the surface coverages of θ CO and θ CHx , the τ CO and τ CHx values, the TOF chem and TOF ITK (s −1 ) of CH 4 formation were estimated (see Equations (10) and (11), Section 4.2) and results are shown in Figure 3D. The TOF chem (s −1 ) (estimated based on the total Co surface metal atoms) was found to slightly decrease with increasing P CO , namely: 9.74, 7.40, and 7.34 × 10 −3 s −1 , respectively, for 36, 60, and 84 mbar CO. However, in the case of TOF ITK (s −1 ), which is estimated based on the total concentration of active CH x -s and reversibly adsorbed CO-s, this was found to decrease by 2.1 and 2.9 times, respectively, in the case of 60 and 84 mbar CO compared to the case of 36 mbar CO in the feed gas stream (TOF ITK (s −1 ) = 127.7, 61.5, and 44.2 × 10 −3 s −1 , respectively, Figure 3D). These differences in TOF ITK (s −1 ) are attributed to the different concentration of active sites accommodating CO-s and CH x -s that truly participate in the carbon-path of methanation reaction, which were both influenced by the partial pressure of CO ( Figure 3B). The latter results are in good agreement with the work of Carvalho et al. [45] over the Co/SiO 2 and CoPt/SiO 2 catalysts, where, at higher hydrogen partial pressures (higher H 2 /CO), an increased θ H favoured the rate of CO hydrogenation to methane. The TOF ITK (s −1 ) was found to increase with an increasing H 2 /CO gas ratio. In fact, an increase by a factor of~1.6 was reported for the TOF ITK after an increase of H 2 /CO from 2 to 5 to be compared to the increase of~1.4 in the present work (CoPt/γ-Al 2 O 3 , after increasing the H 2 /CO gas ratio from~3.6 to 5.0 (Table 1, y f CO = 5 and 7 vol%). Kinetic methane selectivity values S CH4 (%) measured at 230 • C and for the applied partial pressures of H 2 and CO are also reported in Table 1. It is clearly seen that S CH4 (%) decreases substantially with increasing P CO . In particular, the increase of P CO from 36 mbar (3 vol%) to 60 (5 vol%) and 84 mbar (7 vol%) results in the decrease of CH 4 -selectivity by a factor of 1.55 and 1.71, respectively. Ma et al. [47] reported a very comprehensive kinetic study of methanation reaction over the 25 wt% Co/γ-Al 2 O 3 catalyst at 220 • C and for a wide range of partial pressures of H 2 and CO. The authors derived a CH 4 -selectivity relationship as a function of P H2 , P CO , and P H2O (see Reference [47], Equation (9)). We have applied the latter relationship to the present kinetic experimental data considering that P H2O can be given by: P H2O = y H2O P T = (F CO, f X CO /F T ) P T , where P T = 1.2 bar (total pressure), F CO, f = CO molar feed flow rate (mols CO/s), and F T = total feed molar flow rate (mols/s). It was found that S CH4 (%) at the lowest P CO (36 mbar, 3 vol% CO) decreases by a factor of 1.4 and 1.78 with increasing P CO to 60 mbar (5 vol%) and 84 mbar (7 vol%), respectively. These values agree very well with the measured experimental values given in Table 1, considering the slightly different reaction T (230 vs. 220 • C) and Co mean particle size used in these studies. Figure 4 presents the transient rates (µmol g −1 s −1 ) of 12 CH 4 and 13 CH 4 obtained during hydrogenation of carbonaceous species formed after 5 h in the CO/H 2 reaction at 230 • C, according to the following sequence of step-gas switches: x% 12 CO/25% H 2 /Ar (5 h, 230 • C) → x% 13 CO/25% H 2 /1% Kr/Ar (230 • C, 7 min, SSITKA) → Ar (3 min) → 50% H 2 /Ar (t), TIH (7 min, 230 • C) (see Section 4.3). The transient response curves of 12 CH 4 depicted in Figure 4A are due to the hydrogenation of inactive (spectator) 12 C-containing species (named C β ) not participating in the formation of CH 4 under steady-state FTS reaction conditions. It is seen that P CO in the feed gas stream influences both the shape of the transient rate and the amount of C β . The former reflects the kinetics of hydrogenation of C β , which is expected to depend on its reactivity (k) and the rate-determining step (RDS) in the reaction path of hydrogenation [48,49]. Of interest is the long tail out for more than 100 s in the hydrogen stream, irrespective of the CO partial pressure used in the feed stream. This result is similar to the TIH of carbon formed after the CO/He reaction at 250 • C on a Rh/MgO catalyst [49].

Transient Isothermal and Temperature-Programmed Hydrogenation (TIH/TPH)
( Figure 4A), and its maximum is also higher than the corresponding one of 12 CH4 ( Figure 4A). These features show that the 12 C-containing and 13 C-containing species are different in reactivity towards hydrogenation to methane. The very sharp transient response of 13 CH4 at t < 5 s is associated with the hydrogenation of the small amount of active CHx-s species (Cα) formed in CO/H2 (5 h) and exchanged (labelled 13 CHx-s) under the SSITKA switch. It can be clearly seen that there is a small decrease in the maximum rate of 13 CH4 formation by increasing the CO concentration in the feed gas stream from 3 to 5 vol% (4.4 vs. 4.0 μmol g −1 s −1 , respectively), while a further increase to 7 vol% leads to an increase in the rate maximum by ~25% (5.5 μmol g −1 s −1 ). However, this appears practically at the same time, ca. 4-6 s. Part of the 13 CH4 transient response ( Figure 4B) is due to the hydrogenation of CO-s formed in CO/H2 (5 h) and exchanged under the SSITKA switch ( Figures 1A, 1B, and 3B). The hydrogenation of 13 CO-s lasts for ~20 s in the case when 3 vol% CO was used in the feed. However, in the case of 5 and 7 vol% CO, the hydrogenation takes longer (~40 s). The amounts (μmol/g) and surface coverages (based on the exposed Co surface metal atoms) of the corresponding Cβ ( 12 CH4 transient) and those of Cα and CO-s ( 13 CH4 transient) are provided in Table 2. The θ( 13 CH4) was found to be 0.08, 0.12, and 0.17 for the 3, 5, and 7 vol% CO used in the feed gas stream, respectively. It is pointed out that the sum of the surface coverages of CO-s and Cα estimated during SSITKA is in very good agreement with the TIH results (θ( 13 CH4), Figure 4B). The amount (μmol g −1 ) of hydrogenated Cβ species given in Table 2 is found to be approximately the same irrespective of the partial pressure of CO in the feed gas stream, namely: 34.3 (θ = 0.10), 27.6 (θ = 0.08), and 30.4 (θ = 0.09) μmol g −1 for the 3, 5, and 7 vol% CO in the feed.  The transient rate of 13 CH 4 ( Figure 4B) reflects the hydrogenation of 13 C-containing reaction intermediates exchanged at 230 • C after the SSITKA switch. It decays much faster than that of 12 CH 4 ( Figure 4A), and its maximum is also higher than the corresponding one of 12 CH 4 ( Figure 4A).
These features show that the 12 C-containing and 13 C-containing species are different in reactivity towards hydrogenation to methane. The very sharp transient response of 13 CH 4 at t < 5 s is associated with the hydrogenation of the small amount of active CH x -s species (C α ) formed in CO/H 2 (5 h) and exchanged (labelled 13 CH x -s) under the SSITKA switch. It can be clearly seen that there is a small decrease in the maximum rate of 13 CH 4 formation by increasing the CO concentration in the feed gas stream from 3 to 5 vol% (4.4 vs. 4.0 µmol g −1 s −1 , respectively), while a further increase to 7 vol% leads to an increase in the rate maximum by~25% (5.5 µmol g −1 s −1 ). However, this appears practically at the same time, ca. 4-6 s. Part of the 13 CH 4 transient response ( Figure 4B) is due to the hydrogenation of CO-s formed in CO/H 2 (5 h) and exchanged under the SSITKA switch ( Figure 1A,B, and Figure 3B). The hydrogenation of 13 CO-s lasts for~20 s in the case when 3 vol% CO was used in the feed. However, in the case of 5 and 7 vol% CO, the hydrogenation takes longer (~40 s). The amounts (µmol/g) and surface coverages (based on the exposed Co surface metal atoms) of the corresponding C β ( 12 CH 4 transient) and those of C α and CO-s ( 13 CH 4 transient) are provided in Table 2. The θ( 13 CH 4 ) was found to be 0.08, 0.12, and 0.17 for the 3, 5, and 7 vol% CO used in the feed gas stream, respectively. It is pointed out that the sum of the surface coverages of CO-s and C α estimated during SSITKA is in very good agreement with the TIH results (θ( 13 CH 4 ), Figure 4B). The amount (µmol g −1 ) of hydrogenated C β species given in Table 2 is found to be approximately the same irrespective of the partial pressure of CO in the feed gas stream, namely: 34.3 (θ = 0.10), 27.6 (θ = 0.08), and 30.4 (θ = 0.09) µmol g −1 for the 3, 5, and 7 vol% CO in the feed. The 12 CH 4 transient response curves obtained during temperature-programmed hydrogenation (TPH), following 5 h in CO/H 2 reaction and 7 min TIH at 230 • C, are reported in Figure 5 for the three CO concentrations used in the feed gas stream (3, 5, and 7 vol% CO). The several hydrogenation peaks and shoulders that appeared in these TPH traces are attributed to inactive carbonaceous species (named C γ ) formed during the FTS reaction, and which could not be hydrogenated at 230 • C, as opposed to the C β hydrogenated to methane at 230 • C (TIH, Figure 4A). As shown in Figure 5A, the amount of C γ ( Table 2) is influenced by the CO feed gas concentration, as opposed to the shape and T max ( • C) of the 12 CH 4 traces. Furthermore, the fraction of the various types of C γ carbonaceous species was found not to be influenced by the CO concentration in the feed, according to the deconvolution of the TPH traces performed ( Figure 5B), and these results are reported in Table S1 (ESI). By increasing the CO concentration in the feed gas stream from 3 to 5 and 7 vol%, the amount of CH 4 formed was found to increase, ca. 128, 228, and 252 µmol g −1 , respectively. Additionally, the main peak maximum is ca. 290, 420, and 520 ppm, respectively ( Figure 5A).
Catalysts 2020, 10, x FOR PEER REVIEW 9 of 23 The 12 CH4 transient response curves obtained during temperature-programmed hydrogenation (TPH), following 5 h in CO/H2 reaction and 7 min TIH at 230 °C, are reported in Figure 5 for the three CO concentrations used in the feed gas stream (3, 5, and 7 vol% CO). The several hydrogenation peaks and shoulders that appeared in these TPH traces are attributed to inactive carbonaceous species (named Cγ) formed during the FTS reaction, and which could not be hydrogenated at 230 °C, as opposed to the Cβ hydrogenated to methane at 230 °C (TIH, Figure 4A). As shown in Figure 5A, the amount of Cγ (Table 2) is influenced by the CO feed gas concentration, as opposed to the shape and Tmax (°C) of the 12 CH4 traces. Furthermore, the fraction of the various types of Cγ carbonaceous species was found not to be influenced by the CO concentration in the feed, according to the deconvolution of the TPH traces performed ( Figure 5B), and these results are reported in Table S1 (ESI). By increasing the CO concentration in the feed gas stream from 3 to 5 and 7 vol%, the amount of CH4 formed was found to increase, ca. 128, 228, and 252 μmol g −1 , respectively. Additionally, the main peak maximum is ca. 290, 420, and 520 ppm, respectively ( Figure 5A).

Operando DRIFTS-Transient Isothermal Hydrogenation (TIH) of CO
Operando DRIFTS-Mass spectrometry experiments were conducted in the DRIFTS cell (PFR performance) in which the outlet was connected to a mass spectrometer [19]. After 2 h of methanation

Operando DRIFTS-Transient Isothermal Hydrogenation (TIH) of CO
Operando DRIFTS-Mass spectrometry experiments were conducted in the DRIFTS cell (PFR performance) in which the outlet was connected to a mass spectrometer [19]. After 2 h of methanation reaction at 200 • C (5 vol% CO/25 vol% H 2 /Ar), the infra-red (IR) cell was purged with Ar for 3 min, which was followed by a step-gas switch to 50 vol% H 2 /Ar in order to study the transient kinetics of hydrogenation of each kind of adsorbed CO-s. Figure 6A shows the DRIFTS spectrum in the CO region (1800-2250 cm −1 ) and its deconvolution into five IR bands, following the deconvolution methodology previously described by us [19]. The IR bands marked (1) and (2) are due to the R and Q branches of gas-phase CO. The IR bands marked as (3) and (4) centered at 2015 and 1980 cm −1 , respectively, are due to the presence of two linear-type reversibly chemisorbed CO-s. The third IR band centered at 1905 cm −1 is due to a bridged-type reversibly adsorbed CO-s [50][51][52][53][54]. At least one of these CO-s should be considered a precursor for methanation and higher HCs formation on the Co surface. The DRIFTS spectrum of Figure 6A is in agreement with our previous results (5 vol% CO, H 2 /CO = 2) and others reported for different H 2 /CO gas ratios [19,[50][51][52][53][54].
The DRIFTS spectrum recorded after the 3-min Ar purge is shown in Figure 6B (Ar, t = 0 s) and its deconvolution into two kinds of linear CO-s (L1, L2) and one bridged-type CO-s (B) is depicted in Figure 6C. Upon the step-gas switch Ar → 50 vol% H2/Ar (t), the surface coverage of adsorbed CO-s (sum of L1, L2, and B) progressively decreases with time in hydrogen stream, and the maximum of the IR band shifts slightly to lower wavenumbers ( Figure 6B). The latter is attributed to the effect of surface coverage of CO-s on its binding strength with the cobalt surface (lateral repulsive interactions between adjacent adsorbed CO-s). Thus, the bond energy between C and O within the adsorbed COs decreases, and, thereby, lowers the vibrational frequency as observed. DRIFTS-TIH deconvoluted spectra with time of hydrogenation of the two linear-type (L1, L2) adsorbed CO-s species were further analysed by applying the microkinetic modelling described in detail [19]. Due to the small integral band area of the third bridged-type (B) adsorbed CO-s, no further analysis was attempted. Figures 7A and 7B show DRIFTS spectra of the evolution of the IR bands of L1 and L2 linear-type adsorbed CO-s with time in hydrogen gas stream, after following the same deconvolution procedure shown in Figure 6C (deconvolution in Ar gas stream). Both L1 and L2 adsorbed CO-s formed after 2 h in the CO/H2 reaction at 200 °C were hydrogenated towards methane within the first 3 min in 50 vol% H2/Ar. Figure 7C plots the integral band intensity (Abs cm −1 ) of the L1 and L2 as a function of time in 50%H2/Ar gas treatment. Assuming the same extinction coefficient for the two linear-type CO-s species, it is seen that the relative population of the two CO-s is L1:L2 ∼65:35%. The integral bands of L1 and L2 follow a very similar decay with time in hydrogenation, which suggests similar kinetics of hydrogenation. This is confirmed by the following kinetic analysis.
The transient kinetics of hydrogenation of CO-s was modelled via the H-assisted CO dissociation mechanism [55][56][57], where the formation of hydroxymethylene species (HCOH-s) from the sequential hydrogenation of CO-s was considered as the rate-determined step (RDS) [19]. Based on this kinetic The DRIFTS spectrum recorded after the 3-min Ar purge is shown in Figure 6B (Ar, t = 0 s) and its deconvolution into two kinds of linear CO-s (L 1 , L 2 ) and one bridged-type CO-s (B) is depicted in Figure 6C. Upon the step-gas switch Ar → 50 vol% H 2 /Ar (t), the surface coverage of adsorbed CO-s (sum of L 1 , L 2 , and B) progressively decreases with time in hydrogen stream, and the maximum of the IR band shifts slightly to lower wavenumbers ( Figure 6B). The latter is attributed to the effect of surface coverage of CO-s on its binding strength with the cobalt surface (lateral repulsive interactions between adjacent adsorbed CO-s). Thus, the bond energy between C and O within the adsorbed CO-s decreases, and, thereby, lowers the vibrational frequency as observed.
DRIFTS-TIH deconvoluted spectra with time of hydrogenation of the two linear-type (L 1 , L 2 ) adsorbed CO-s species were further analysed by applying the microkinetic modelling described in detail [19]. Due to the small integral band area of the third bridged-type (B) adsorbed CO-s, no further analysis was attempted. Figure 7A,B show DRIFTS spectra of the evolution of the IR bands of L 1 and L 2 linear-type adsorbed CO-s with time in hydrogen gas stream, after following the same deconvolution procedure shown in Figure 6C (deconvolution in Ar gas stream). Both L 1 and L 2 adsorbed CO-s formed after 2 h in the CO/H 2 reaction at 200 • C were hydrogenated towards methane within the first 3 min in 50 vol% H 2 /Ar. Figure 7C plots the integral band intensity (Abs cm −1 ) of the L 1 and L 2 as a function of time in 50%H 2 /Ar gas treatment. Assuming the same extinction coefficient for the two linear-type CO-s species, it is seen that the relative population of the two CO-s is L 1 :L 2~6 5:35%. The integral bands of L 1 and L 2 follow a very similar decay with time in hydrogenation, which suggests similar kinetics of hydrogenation. This is confirmed by the following kinetic analysis.
analysis, an apparent rate constant (keff, s −1 ) associated with this RDS was derived using Equation (2), where keff is the product of the intrinsic rate constant k times θH (keff = k θH).
In Equation (2), α is equal to the ratio of the integral IR band intensity for each individual lineartype adsorbed CO-s at a given time, t, in H2/Ar to the integral IR band intensity recorded at t = 0 (after the 3-min Ar purge before the H2/Ar step-gas switch was made).   (2), the slope of the linear ln[α] vs. time plot provides the apparent rate constant (keff) of each of the two linear adsorbed CO-s. It is shown that the L1 and L2 adsorbed CO-s exhibit very similar reactivities (keff_L1/keff_L2 ~1, keff_L1 = 0.016 s −1 ) toward hydrogenation to CH4. Since keff = k θH, the L1 and L2 linear-type CO-s have very similar intrinsic reactivities (kL1 ~kL2). The keff value estimated is smaller than the one reported by us on the same Co/γ-Al2O3 catalyst at 230 °C and for H2/CO = 2 (5% CO, 10% H2). More precisely, keff_L1 = 0.021 and keff_L2 = 0.027 s −1 with kL1/kL2 to be 0.78 (230 °C). Assuming that the pre-exponential factors are the same for the two rate constants, then the kL1/kL2 ratio takes the value of 0.76 at 200 °C. Furthermore, the difference in the activation energies (E1-E2, kcal/mol) of hydrogenation of these two linear-type CO species was estimated to be ~0.3 kcal mol −1 . The increase in the hydrogen partial pressure from 0.12 to 0.3 bar (10 to 25 vol% H2, P=1.2 bar) has a very small influence on the intrinsic rate constant of CO-s hydrogenation via the Hassisted CO dissociation mechanism (RDS: HCO-s + H-s → HCOH-s + s).
The transient evolution of CH4 formation during the DRIFTS-TIH experiment was recorded by online mass spectroscopy (operando methodology). This is depicted in Figure 8. At the same time, the gas-phase CO response was measured and compared to that of the Kr tracer, as shown in Figure  8. Based on the Kr and CO response curves, it is illustrated that desorption of CO during the The transient kinetics of hydrogenation of CO-s was modelled via the H-assisted CO dissociation mechanism [55][56][57], where the formation of hydroxymethylene species (HCOH-s) from the sequential hydrogenation of CO-s was considered as the rate-determined step (RDS) [19]. Based on this kinetic analysis, an apparent rate constant (k eff , s −1 ) associated with this RDS was derived using Equation (2), where k eff is the product of the intrinsic rate constant k times θ H (k eff = k θ H ).
In Equation (2), α is equal to the ratio of the integral IR band intensity for each individual linear-type adsorbed CO-s at a given time, t, in H 2 /Ar to the integral IR band intensity recorded at t = 0 (after the 3-min Ar purge before the H 2 /Ar step-gas switch was made). Figure 7D presents linear plots of ln[α] versus time in H 2 /Ar for the two (L 1 , L 2 ) linear adsorbed CO-s species. Based on Equation (2), the slope of the linear ln[α] vs. time plot provides the apparent rate constant (k eff ) of each of the two linear adsorbed CO-s. It is shown that the L 1 and L 2 adsorbed CO-s exhibit very similar reactivities (k eff_L1 /k eff_L2~1 , k eff_L1 = 0.016 s −1 ) toward hydrogenation to CH 4 . Since k eff = k θ H , the L 1 and L 2 linear-type CO-s have very similar intrinsic reactivities (k L1~kL2 ). The k eff value estimated is smaller than the one reported by us on the same Co/γ-Al 2 O 3 catalyst at 230 • C and for H 2 /CO = 2 (5% CO, 10% H 2 ). More precisely, k eff_L1 = 0.021 and k eff_L2 = 0.027 s −1 with k L1 /k L2 to be 0.78 (230 • C). Assuming that the pre-exponential factors are the same for the two rate constants, then the k L1 /k L2 ratio takes the value of 0.76 at 200 • C. Furthermore, the difference in the activation energies (E 1 -E 2 , kcal/mol) of hydrogenation of these two linear-type CO species was estimated to be~0.3 kcal mol −1 . The increase in the hydrogen partial pressure from 0.12 to 0.3 bar (10 to 25 vol% H 2 , P=1.2 bar) has a very small influence on the intrinsic rate constant of CO-s hydrogenation via the H-assisted CO dissociation mechanism (RDS: HCO-s + H-s → HCOH-s + s).
The transient evolution of CH 4 formation during the DRIFTS-TIH experiment was recorded by online mass spectroscopy (operando methodology). This is depicted in Figure 8. At the same time, the gas-phase CO response was measured and compared to that of the Kr tracer, as shown in Figure 8. Based on the Kr and CO response curves, it is illustrated that desorption of CO during the 50%H 2 /Ar gas treatment of the catalyst sample was negligible. The CH 4 transient response (Figure 8) presents two rate peak maxima. The first one, which appeared at~8 s, is largely due to the CO-s and active CH x -s formed after 2 h of CO/H 2 reaction at 200 • C, whereas the second peak centered at~40 s is largely due to the C β species presented and discussed above (Section 2.3, Figure 4A). Integration of this transient response of CH 4 provides the total amount (µmol/g or θ) of CO-s, and active CH x -s (C α ) and C β species formed after 2 h in CO/H 2 reaction. This amount was found to be 31.1 µmol/g (θ = 0.09), as opposed to the value of θ = 0.20 obtained at 230 • C after 5 h in CO/H 2 ( Table 2). It is reasonable to suggest that, since θ CO decreases with increasing rection T, and θ CHx decreases with time-on-stream [11,21], then inactive C β appears to grow with an increasing reaction temperature from 200 to 230 • C.
Catalysts 2020, 10, x FOR PEER REVIEW 12 of 23 50%H2/Ar gas treatment of the catalyst sample was negligible. The CH4 transient response (Figure 8) presents two rate peak maxima. The first one, which appeared at ~8 s, is largely due to the CO-s and active CHx-s formed after 2 h of CO/H2 reaction at 200 °C, whereas the second peak centered at ~40 s is largely due to the Cβ species presented and discussed above (Section 2.3, Figure 4A). Integration of this transient response of CH4 provides the total amount (μmol/g or θ) of CO-s, and active CHx-s (Cα) and Cβ species formed after 2 h in CO/H2 reaction. This amount was found to be 31.1 μmol/g (θ = 0.09), as opposed to the value of θ = 0.20 obtained at 230 °C after 5 h in CO/H2 ( Table 2). It is reasonable to suggest that, since θCO decreases with increasing rection T, and θCHx decreases with time-on-stream [11,21], then inactive Cβ appears to grow with an increasing reaction temperature from 200 to 230 °C.

Influence of CO Partial Pressure on Intrinsic Kinetic Parameters of Methanation Reaction
The increase of CO partial pressure from 36 to 84 mbar (3-7 vol% CO, P = 1.2 bar), while keeping the H2 partial pressure constant at 300 mbar (25 vol%), led to significant variations in several intrinsic kinetic parameters of the methanation reaction conducted at 230 °C and after 5 h over the 20 wt% Co-0.05 wt% Pt/γ-Al2O3 commercially relevant catalyst. The θCO was found to be small, ca. 0.029, 0.032, and 0.049 for 36, 60, and 84 mbar, respectively, but with an increasing trend as the CO partial pressure increases and the H2/CO gas ratio decreases. An increase of θCO by a factor of ~1.68 was obtained as the CO partial pressure increases from 36 to 84 mbar. The latter result agrees with the characteristic features of Langmuir isotherm and the fast 12 CO-s/ 13 CO(g) exchange, which was revealed under SSITKA (see Figures 1A, 1B, and 2A). The increase of θCO with PCO finds agreement with the SSITKA work conducted by Chen et al. [18] over the 17.1 wt% Co-0.04 wt% Pt/SiO2 catalyst. However, the reaction conditions of T = 260 °C and PH2 = 0.45 bar (PCO in the 15-90 mbar range) are different than in the present work (230 °C, PH2 = 0.3 bar). The authors reported that θCO increased from ~0.18 to ~0.30 in the examined PCO range. The differences in θCO between that work [18] and the present one must be seen as the result of the influence of several reaction parameters on θCO, which were not the same. For example, the reaction T, the time-on-stream (5 h in the present work vs. 16 h in Reference [18]), the H2 partial pressure, and the Co mean particle size (10 nm in the present work vs. 15 nm in Reference [18]).
The effect of La-promotion of Co/γ-Al2O3 catalysts toward methanation was investigated by Vada et al. [58] using the SSITKA technique at similar FTS reaction conditions (PCO = 0.036 bar, H2/CO = 10, T = 220 °C) to those used in this work. They found similar surface coverages of θCO = 0.11 and

Influence of CO Partial Pressure on Intrinsic Kinetic Parameters of Methanation Reaction
The increase of CO partial pressure from 36 to 84 mbar (3-7 vol% CO, P = 1.2 bar), while keeping the H 2 partial pressure constant at 300 mbar (25 vol%), led to significant variations in several intrinsic kinetic parameters of the methanation reaction conducted at 230 • C and after 5 h over the 20 wt% Co-0.05 wt% Pt/γ-Al 2 O 3 commercially relevant catalyst. The θ CO was found to be small, ca. 0.029, 0.032, and 0.049 for 36, 60, and 84 mbar, respectively, but with an increasing trend as the CO partial pressure increases and the H 2 /CO gas ratio decreases. An increase of θ CO by a factor of~1.68 was obtained as the CO partial pressure increases from 36 to 84 mbar. The latter result agrees with the characteristic features of Langmuir isotherm and the fast 12 CO-s/ 13 CO(g) exchange, which was revealed under SSITKA (see Figure 1A,B, and Figure 2A). The increase of θ CO with P CO finds agreement with the SSITKA work conducted by Chen et al. [18] over the 17.1 wt% Co-0.04 wt% Pt/SiO 2 catalyst. However, the reaction conditions of T = 260 • C and P H2 = 0.45 bar (P CO in the 15-90 mbar range) are different than in the present work (230 • C, P H2 = 0.3 bar). The authors reported that θ CO increased from~0.18 to~0.30 in the examined P CO range. The differences in θ CO between that work [18] and the present one must be seen as the result of the influence of several reaction parameters on θ CO , which were not the same. For example, the reaction T, the time-on-stream (5 h in the present work vs. 16 h in Reference [18]), the H 2 partial pressure, and the Co mean particle size (10 nm in the present work vs. 15 nm in Reference [18]).
The effect of La-promotion of Co/γ-Al 2 O 3 catalysts toward methanation was investigated by Vada et al. [58] using the SSITKA technique at similar FTS reaction conditions (P CO = 0.036 bar, H 2 /CO = 10, T = 220 • C) to those used in this work. They found similar surface coverages of θ CO = 0.11 and θ CHx = 0.05 as in the present work ( Figure 3B). Yang et al. [59] in their SSITKA work on Co/γ-Al 2 O 3 using a CO partial pressure of P CO = 55 mbar, H 2 /CO = 10, T = 210 • C and P T = 1.85 bar, and, after 5 h on FTS reaction stream, they reported a similar θ CHx value but larger θ CO~0 .3. Thus, a larger P H2 (0.55 vs. 0.3 bar in the present work) and lower T (210 vs. 230 • C in the present work) led to larger θ CO values. In the case of TOF chem , a value of~12.5 × 10 −3 (s −1 ) was reported [59] and compared to the present value of 7.4 × 10 −3 (s −1 ) for very similar P CO (55-60 mbar) but larger P H2 (0.55 vs. 0.3 bar in the present work). A k eff (CH x ) value of~0.1 s −1 was reported [59] compared to the value of 0.08 s −1 ( Table 1) for the previously mentioned comparative experimental conditions.
Carvalho et al. [11,45] in their SSITKA works conducted at 250 • C over Co-based catalysts reported that θ CO decreases and θ CHx increases with an increasing H 2 /CO gas ratio. The authors claimed that higher θ CHx could be linked to larger hydrogenation rates for higher hydrocarbons, which is in agreement with the decrease of S CH4 and increase of S C5+ [45]. These results are similar to those reported by Pena et al. [60] and those in the present work (Table 1), where, after increasing the CO partial pressure, an increase of θ CHx and a concomitant decrease in S CH4 are obtained. In addition, Keyvanloo et al. [33] reported that, by increasing the CO partial pressure, while keeping the H 2 partial pressure constant, the S CH4 decreases. This result is similar to our findings, where the S CH4 (%) behaviour with P CO and P H2 was presented and discussed in Section 2.2 in relation to kinetic modelling studies [47] conducted on the Co/γ-Al 2 O 3 . In the present work, it was found that, after increasing the H 2 /CO gas ratio (decrease of CO partial pressure), the ratio of θ CO /θ CHx grows toward unity. This result agrees with that found by Carvalho et al. [45], where, for H 2 /CO = 2, θ CO was higher than θ CHx , whereas, for H 2 /CO = 5, θ CHx was higher than θ CO . This trend of θ CO /θ CHx with the H 2 /CO gas ratio is attributed to the competitive H 2 and CO chemisorption where, after increasing the hydrogen partial pressure in a given range, θ H increases against θ CO , which results in increasing rates for -CH x coupling and further hydrogenation to higher hydrocarbons (lowering the S CH4 ).
The mean residence time of active -CH x species (τ CHx , s) leading to CH 4 , was found to increasẽ 2.5 times by increasing the CO feed gas concentration from 3 to 5 vol%, whereas a further increase to 7 vol% causes only a slight increase (~30%). This result agrees very well with previous works. More specifically, τ CHx is in the range of 5-15 s for the present catalytic system and the FTS reaction conditions applied, which is similar to other previous SSITKA works over Co-based catalysts supported on SiO 2 or Al 2 O 3 , where τ CHx ranged from 8 to 25 s [8,11,15,16,59]. Vada et al. [58] for Co supported on alumina carrier found that, by decreasing the H 2 /CO gas ratio, the τ CHx increases (7, 13, and 29 s for H 2 /CO = 15, 10, and 5, respectively, CO = 2 vol%). This result is in harmony with those of the present work. However, it should be noted that, in that work [58], the effect of H 2 /CO gas ratio was investigated by varying the P H2 as opposed to the present work where the P CO varied and P H2 was kept constant ( Figure 3A).
According to the results shown in Table 1, the TOF chem (s −1 ) for CH 4 formation at 230 • C decreases by~25% when the P CO increases from 36 to 84 mbar. A similar behaviour was reported for the methanation reaction at 260 • C on Co/SiO 2 (P H2 = 450 mbar) [18]. The drop in TOF of CH 4 formation is attributed to the deterioration of site activity (k eff ) of the -CH x species and not of a possible decrease of the surface coverage of -CH x , as illustrated in Table 2. Given the fact that k eff = k θ H (see Section 4.2), it is reasonable to suggest that, as θ H decreases with increasing P CO , then deterioration of intrinsic activity (k) of -CH x should be considered less than 25% in that range of P CO . Weststrate and Niemantsverdriet [46] pointed out the importance of θ CHx and θ V (coverage of empty catalytic sites) for hydrogen adsorption related to the hydrogenation of CO toward methane formation. The surface coverage of CO-s defines the amount of hydrogen able to chemisorb next to adsorbed CO-s for a given P CO , and, as a consequence, the τ CHx strongly depends on the H 2 /CO gas ratio [46,58].
The TOF ITK (s −1 ) of CH 4 formation estimated based on the active CO-s and CH x -s intermediates measured by SSITKA shows large differences when compared to TOF chem for a given P CO , according to the results reported in Table 1. The influence of P CO on TOF ITK is largely different than in the case of TOF chem . By increasing the CO partial pressure from 36 to 84 mbar, the TOF ITK decreases by a factor of 2.9 (127.7 vs. 44.2 × 10 −3 s −1 ) to be compared to the value of 1.3 for the TOF chem , which indicates the more sensitive nature of TOF ITK on P CO . It is interesting to mention the variation of TOF ITK /TOF chem with P CO , where a decrease from 13 to 6 is obtained by increasing the P CO from 36 to 84 mbar.
The exchange rates of CO-s and CH x -s in the SSITKA experiment were estimated (see Section 4.2, Equations (3) and (4)) and reported (Figure 2) for the first time to the best of our knowledge. It is clearly illustrated the significantly larger adsorption/desorption rate of CO, R ex CO , compared to that of hydrogenation of -CH x (R ex CHx ) under working FTS reaction conditions (5-7 vol% CO in the feed), but the similar rates for the lowest feed concentration of 3 vol% CO. Furthermore, the shape profile of R ex CHx suggests the influence of P CO on the formation of different kinds of CH x -s in the methanation reaction path with different reactivity toward hydrogenation to methane. Bell and co-workers [61] were the first to describe an advanced mathematical analysis of the 13 CH 4 -SSITKA response curve for the methanation reaction, which reveals the distribution of reactivity of -CH x -s (f(k)-reactivity distribution function). Thus, the heterogeneity of sites exists for this particular active reaction intermediate. Weststrate and Niemantsverdriet [46] pointed out that the long CH x -s residence time (e.g., 10 s) observed on the Co(0001) surface, which is similar to the τ CHx values reported in Table 1, cannot be due to the existence of low values of θ H or θ H /θ *H , where θ *H is the surface coverage of empty sites for hydrogen chemisorption. The authors proposed, based on DFT calculations and the θ CHx values observed (0.1-0.2 ML), that CH x are formed on step edge sites, which then migrate to the terrace sites. The latter was considered the rate-determining step (RDS) that gives rise to residence times of~10 s. Given the fact that more than one kind of step edge sites can be formed on the Co surface with a distribution that depends on the Co particle size, hydrogenation of such CH x -s species is more sensitive to the P CO , which is shown in the present work ( Figure 2B). An increase of CO feed gas concentration from 3 to 7 vol% causes the increase of τ CHx by~3.5 times (Table 1).

The Influence of H 2 Partial Pressure on the Reactivity of CO-s in the Methanation Reaction
Transient Isothermal Hydrogenation (TIH) of adsorbed CO-s coupled with DRIFTS performed at 200 • C after 2 h in 5 vol% CO/25 vol% H 2 /Ar along with microkinetic analysis, which was based on a hydrogen-assisted CO hydrogenation mechanism, revealed practically the same reactivity (k) for each of the two L 1 and L 2 linear-type adsorbed CO-s species populated on the present Co/γ-Al 2 O 3 catalyst ( Figure 7C,D). According to the results depicted in Figures 6C and 7C, the initial surface coverage of L 1 and L 2 adsorbed CO-s is largely different. The θ CO apparently had no influence on the hydrogenation activity of each of these two linear-type CO-s, thus, on the RDS (HCO-s + H-s → HCOH-s + s), in agreement with the recent work of Zijlstra et al. [34]. The authors based on Density Functional Theory (DFT) calculations reported that the C-O bond dissociation energy on step-edge sites is hardly affected by CO co-adsorbates. The comparison between the CO-s hydrogenation rate constant (k) found in the present work (Figure 7, Section 2.4) for a H 2 /CO gas ratio of 5 and that from previous results [19] for a H 2 /CO gas ratio of 2 (same 5% CO concentration was used in the FTS reaction) over the same catalyst, suggest that the H 2 /CO gas ratio hardly affects the k of RDS (H-assisted CO hydrogenation mechanism).
The results of the operando DRIFTS-Mass spectrometry methodology applied in the isothermal hydrogenation of adsorbed CO-s and in the transient mode, which are depicted in Figures 6-8, combined with the SSITKA-MS results depicted in Figure 3B, allowed to learn about the nature of the various kinds of adsorbed CO-s, their distribution (individual θ CO (L i ), i = 1, 2), and their reactivity (k i , s −1 ) toward hydrogenation to CH 4 . On the other hand, the SSITKA-MS alone and the TIH-Mass spectrometry alone ( Figure 4B) allowed to estimate only the total surface coverage of all types of CO-s and an effective reactivity, k eff (s −1 ) for methanation associated with the active CH x -s formed from all these related adsorbed CO-s precursor species. Thus, the powerful combination of SSITKA with an operando methodology becomes clear from the results of the present example of methanation reaction on Co/γ-Al 2 O 3 , as demonstrated also in the WGS reaction and others reported therein [24].

Effect of CO Partial Pressure on the Formation of Inactive C β and C γ Carbonaceous Species
Transient isothermal (TIH) and temperature programmed hydrogenation (TPH) conducted following the SSITKA step-gas switch ( 12 CO/H 2 → 13 CO/H 2 ) over alumina-supported cobalt for the first time was described in detail in our previous publication [19]. In the present work, the influence of CO partial pressure in the 36-84 mbar range (constant H 2 partial pressure of 300 mbar) on the rate of formation of inactive carbonaceous species (based on the measured amount for a given time-on-stream, TOS) was investigated. The transient rate of 12 CH 4 formation estimated during TIH at 230 • C ( Figure 4A) corresponds to the hydrogenation of inactive C β , C x H y -s (y 0), the structure of which was revealed via in situ DRIFTS [19], which is in agreement with other studies on Co-based FTS catalysts at similar reaction temperatures [46,62].
The amount of C β ( Table 2) was found to be 1.2 and 1.1 times higher in the case where 3 vol% CO compared to 5 and 7 vol% CO, respectively, were used (34.3, 27.6, and 30.4 µmol C g −1 , respectively). The shape of 12 CH 4 response ( Figure 4A) revealed two kinds of inactive C β species (C β ,I-small peak at~5 s, and C β ,II-broad peak with maximum at~30 s) in the case where 3 vol% CO was used in the feed gas stream, as opposed to the case of 5 and 7 vol% CO, where only single and very similar in shape peaks were observed, which likely correspond to adsorbed carbonaceous species of the same chemical structure. Based on our previous DRIFTS work [19] and the results of Figure 4A (peak shapes), the C β ,I-small peak could be attributed to the presence of a small concentration of HCOO-s (formate species) formed on the Co-alumina interface able to be hydrogenated at 230 • C. This assignment finds support from the TIH experiments reported by Paredes et al. [23].
The rate of hydrogenation of CO-s and active CH x -s ( Figure 4B) is significantly larger than that of inactive C β ( 12 CH 4 response, Figure 4A), and this explains the accumulation of C β in CO/H 2 reaction conditions, where θ H is lower than that established in 50%H 2 /Ar gas treatment. In the former case, CO and H 2 competes for Co active sites, where the former presents larger adsorption energy. Thus, θ CO will depend largely on the partial pressure of CO only, as opposed to θ H , which will also depend on the concentration of Co surface vacant sites [18]. As mentioned previously (see Section 3.1), by reducing θ H , the concentration of active CH x -s formed will be reduced. As shown in Table 2 and Figure 4, with decreasing CO partial pressure in the feed gas stream (ca. 3 vol%), θ CO and θ CHx become smaller ( Figure 3B), which leads to less blockage of Co active sites for H-s formation. Thus, more C β can be formed. The latter finds support from the literature [17], where -CH x monomers polymerization and termination (C β formation) proceed with faster rates than the H-assisted CO dissociation step at FTS reaction conditions. Based on the amounts of C β (Table 2), the effect of P CO is not that large. Approximately, a 15% decrease occurs by increasing the P CO from 36 to 84 mbar at 230 • C on the present Co/γ-Al 2 O 3 commercially relevant catalyst.
The TPH results shown in Figure 5A,B, Table 2, and Table S1 (ESI) reveal the likely existence of four kinds of carbonaceous species of different distribution but of similar individual activity (note the similar T M values, Table S1), independent of CO partial pressure. The surface coverage of each of these C γ carbonaceous species largely depends on P CO (Table 2). By increasing the CO feed concentration from 3 to 7 vol%, the total amount of C γ was nearly increased by a factor of two ( Table 2). The chemical structure of C γ was reported [31,32,34] to include: (i) oligomeric strongly adsorbed hydrocarbon species (peaks 1, 2, and 3, T M = 320-390 • C) and (ii) polymeric carbon (Peak 4, T M = 400-450 • C). The latter species were not eliminated during DRIFTS-TIH at 230 • C as reported in our previous publication [19] (IR band centred at 2915 cm −1 ), but at higher temperatures (230 < T < 400 • C). It was reported [63] that larger amounts of polymeric carbon are formed with decreasing hydrogen partial pressure at constant P CO over Co-based FTS catalysts. The possibility that formates could be considered as part of C γ should be excluded, as these were hydrogenated under DRIFTS-TPH at T < 300 • C [19].
Pena et al. [60] in support of the present findings showed that, by increasing the H 2 /CO gas ratio, higher amounts of polymeric carbon reacted with hydrogen under TPH. Keyvanloo et al. [33] showed via TPH following FTS reaction under industrial conditions that, by increasing the H 2 /CO ratio at a constant hydrogen partial pressure, more hydrogenated polymeric carbon was formed, which is similar to the present findings, where C γ is larger in the case of 7 vol% CO with most of carbonaceous species to be hydrogenated at T > 350 • C but lower than 550 • C. Thus, this cannot be attributed to atomic (low Ts) or graphitic carbon (high Ts). It was reported [33,64] that, by increasing P CO at constant P H2 , the deactivation rate increases, which might be due to the increase of hydrogen-deficient polymeric inactive carbonaceous species. Additionally, Pena and co-workers [60,65] showed via TPH and Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy (STEM-EELS) that, under low hydrogen partial pressures in FTS reaction conditions, more stable carbon species are formed with polymeric carbon and strongly adsorbed hydrocarbon-like species to be responsible for the deactivation of the catalyst at a lower H 2 /CO gas ratio. This result agrees with the present work where, at a lower H 2 /CO gas ratio, larger amounts of C γ are formed.

Co/γ-Al 2 O 3 Catalyst-Synthesis and Characterization
The 20 wt% Co supported on Sasol Germany's Puralox SCCa γ-Al 2 O 3 catalyst was prepared by SASOL S.A., as described elsewhere [19,42]. More precisely, 20 wt% Co and 0.05 wt% Pt were deposited on the alumina support via a slurry-phase impregnation procedure with aqueous metal precursor solutions described in References [42,43]. The resulting slurry was dried under vacuum and calcined in static air at 250 • C. Sequential impregnation and calcination steps were applied in order to achieve 20 wt% Co nominal loading. Prior to any catalytic and transient kinetic measurements, the catalyst was in situ reduced in pure H 2 (1 bar, 50 NmL min −1 ) at 425 • C for 10 h (from room T to 425 • C, the temperature of the solid was increased at β = 1 • C min −1 ), which was followed by an He-purge and cooling to 230 • C for FTS. A 5 vol% O 2 /He (2 h) was applied at 30 • C, following catalytic measurements, for catalyst passivation. The latter sample was further used in DRIFTS studies (Section 4.4). The as-received commercially relevant catalyst in powder form was sieved to less than a 106-µm particle size for avoidance of internal mass transport resistances before catalytic and transient kinetic measurements. Textural (BET/BJH), structural (PXRD, HAADF-TEM), and H 2 -TPR/H 2 -TPD studies over the examined catalyst were described and presented in detail in our previous publication [19].

SSITKA-MS Following 5 h in FTS
Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments with the use of 13 CO were conducted in order to investigate the effects of the variation of CO partial pressure (constant H 2 partial pressure), or the H 2 /CO gas ratio, on the carbon pathway of methane formation at 230 • C. After 5 h in CO/H 2 (x vol% 12 CO/25 vol% H 2 /Ar, GHSV = 30,000 h −1 ), the SSITKA step-gas switch to x vol% 13 CO/25 vol% H 2 /Ar (t) was made. At this point, it is worth mentioning that internal and external mass transport resistances in the catalytic bed placed in the quartz micro-reactor used were absent, as previously reported [19,66]. During the SSITKA step-gas switch, the mass numbers (m/z) 15, 17, 18, 28, 29, and 84, which correspond to 12 CH 4 , 13 CH 4 , H 2 O, 12 CO, 13 CO, and Kr, respectively, were continuously monitored by MS. The contribution of H 2 O (m/z = 18) to the m/z = 17 was considered for the analysis of 13 CH 4 signal to concentration (ppm or mol%) in the case a different signal at m/z = 18 in the two 12 CO/H 2 and 13 CO/H 2 gas mixtures would be recorded. An ideally designed and conducted 12 CO/ 13 CO-SSITKA experiment for FTS should provide the same signals at m/z = 18 [19,21].
The transient rates (µmol g −1 s −1 ) of CO and active CH x -s intermediates which are exchanged ( 12 CO-s is replaced for 13 CO-s and 12 CH x -s for 13 CH x -s) during SSITKA were estimated based on Equations (3) and (4), respectively.
where F T is the total molar flow rate (mol s −1 ), y f CO is the mole fraction of CO in the feed, X s.s CO is the CO conversion (%) at a steady state, W is the mass of the catalyst (g), y 13CH4 is the mole fraction of 13 CH 4 at the effluent stream from the micro-reactor in the new steady-state obtained in 13 CO/H 2 , Z is the dimensionless concentration of a given gas-phase species, and A(t) = Z Kr (t) − Z13 CO (t) .
Equation (3) is based on the fact that the rate of 13 CO fed in the reactor (mols 13 CO/s) during the step-gas SSITKA switch (until the new steady-state is reached) must be equal to the sum of the rates of: (i) exchange of 13 CO with 12 CO-s (R ex CO ), (ii) conversion of 13 CO into 13 CH 4 , (iii) conversion of 13 CO into 13 C 2+ -hydrocarbons, and (iv) the rate of non-reacted 13 CO(g) leaving the reactor (rate of outlet flow of 13 CO(g) from the micro-reactor). The Z C2 , Z C3 , and those of higher hydrocarbons (Z Cn ) that appear in Equation (3) represent the fractional change in 13 C for the C 2 -, C 3 -, and C n -hydrocarbons, respectively, and can only be measured after using the SSITKA-GS-MS methodology [17,67]. In the present work, only the Z C1 (Z( 13 CH 4 )) was measured by mass spectrometry. However, according to the experimental work of Rebmann et al. [67] on a 13 wt% Co/γ-Al 2 O 3 catalyst, the Z C2 and Z C3 transient curves were very close (almost identical) to that of Z C1 . Given the fact that, in the present work, the total carbon selectivity to C 1 -C 3 is more than 85%, the Z Cn (n > 3) terms in Equation (3) were neglected, and it was considered that Z C1~ZC2~ZC3 . In Equations (3) and (4), the accumulation of 13 CO(g) in the CSTR micro-reactor used [66] was found to be very small when compared to the other terms. Thus, it was also neglected.
The concentration (µmol g −1 ) of the reversibly adsorbed CO-s and active CH x -s intermediates leading to CH 4 are given by the following material balance Equations (5) and (6), respectively [19].
The surface coverage, θ, and the mean residence time, τ (s), of the reversibly adsorbed CO-s and active CH x -s intermediates, can be estimated based on the following Equations (7)-(9).
where t s.s. is the time at which the new steady-state is obtained under the 13 CO/H 2 gas mixture.
The kinetic rate TOF (s −1 ) of CH 4 formation is estimated based on the Co dispersion, Equation (10) (TOF chem , s −1 ), or the concentration of active reaction intermediates (Equations (5) and (6)) found in the reaction path of CH 4 formation, TOF ITK (s −1 ), via Equation (11) [19]. In addition, the effective rate constant, k eff (s −1 ), for the methanation reaction is estimated based on Equation (12), which holds when one of the hydrogenation steps of -CH x /CH x O intermediates is the RDS of the methanation reaction (hydrogenation and not CO dissociation of CO-s is the RDS). ) N Co,surf is the total number of surface cobalt per gram of catalyst (µmol g −1 ) estimated on the basis of the dispersion and loading values of the Co/γ-Al 2 O 3 catalyst.

Transient Isothermal (TIH) and Temperature-Programmed Hydrogenation (TPH) Experiments
Transient hydrogenation experiments (Isothermal and Temperature-Programmed) were performed following the SSITKA step-gas switch: x% 13 CO/25% H 2 /Kr/Ar (230 • C, 7 min) → Ar (3 min) → 50% H 2 /Ar, TIH (7 min, 230 • C) → TPH up to 600 • C (β = 10 • C min −1 ). During the TIH and TPH runs, the mass numbers (m/z) 15, 17, and 18 for 12 CH 4 , 13 CH 4 , and H 2 O were continuously monitored by online mass spectrometry. Under TIH at 230 • C, the 12 CH 4 response reflects the inactive carbon-containing species (named C β ) formed during the non-isotopic 12 CO/H 2 gas mixture (5 h) and which cannot be exchanged with 13 CO during the 13 CO/H 2 -SSITKA switch. On the other hand, the 13 CH 4 response reflects both the adsorbed 13 CO-s and active 13 CH x -s species that are exchanged during the SSITKA switch. Under TPH, the response of 12 CH 4 is due to adsorbed refractory carbonaceous inactive species (C x H y -s, named C γ ) that were not hydrogenated at 230 • C. The transient concentration response curves of 12 CH 4 (t) and 13 CH 4 (t) recorded during TIH/TPH were converted into transient rates (mol g −1 s −1 ) using Equation (13) (the accumulation term in the Continuous Stirred Tank Reactor (CSTR,~1.5 mL volume micro-reactor) was found negligible when compared to the outlet rate of CH 4 from the micro-reactor) and after calibration of the MS signal with standard 1.04 mol% 12 CH 4 /Ar and 2.1 mol% 13 CH 4 /Ar gas mixtures.

Operando DRIFTS-Mass Spectrometry-Transient Isothermal Hydrogenation (TIH) of CO
The reactivity toward hydrogen of the various types of chemisorbed CO-s formed after 2 h of CO/H 2 reaction at 200 • C was determined by operando DRIFTS kinetic experiments. The temperature of 200 • C instead of 230 • C was selected since this allowed the recording of more spectra (lower hydrogenation rate of CO) during this transient experiment. DRIFTS spectra were collected by using a Perkin-Elmer Frontier FT-IR spectrometer equipped with an MCT detector and a high-temperature/high pressure temperature controllable DRIFTS reactor cell (Harrick Scientific, Praying Mantis) with CaF 2 windows. An average spectrum was recorded after 256 scans (resolution of 4 cm −1 , scanning speed rate of 2 cm s −1 ). The sieved and passivated catalyst sample (as described in Section 4.1) was placed in the DRIFTS reactor cell (~90 mg) and its temperature was increased to 425 • C and kept for 2 h in 50 vol% H 2 /Ar (50 NmL min −1 ), which was followed by cooling in H 2 /Ar gas flow to the methanation reaction temperature, T = 200 • C, for background spectrum of solid catalyst acquisition. The DRIFTS cell was subsequently purged in Ar gas flow for 20 min and then exposed to the CO/H 2 feed gas stream (5 vol% CO/25 vol% H 2 /Ar) for 2 h. At the end of the reaction, an averaged DRIFTS spectrum was collected. The transient kinetics of hydrogenation of individual adsorbed CO-s species formed during the reaction step was investigated by transient isothermal hydrogenation (TIH) in 50 vol% H 2 /Ar gas flow. The recorded DRIFTS spectra in the CO region under CO/H 2 and TIH reaction conditions were deconvoluted following the procedure previously described in detail [19]. In the case of a TIH reaction run, where no gas phase IR CO bands were observed, the following criteria for deconvolution were used: (i) FWHM < 100 cm −1 , (ii) R 2 > 0.99, and (iii) IR band center of the CO-s species was allowed for a small shift (< 20 cm −1 ) to lower wavenumbers (surface coverage effect). By considering fulfillment of the above three criteria, three IR bands of adsorbed CO-s were obtained.
During TIH, the gas-phase responses of CO and CH 4 from the outlet of the DRIFTS reactor cell were continuously monitored by the connected mass spectrometer (MS), which were used for performing the SSITKA-MS and other transient kinetic experiments that followed (TIH/TPH).

Conclusions
The present study aimed at providing deeper fundamental understanding on how the CO partial pressure affects important kinetic parameters of the methanation reaction path at 230 • C and 1.2 bar total pressure over a commercially relevant 20 wt% Co-0.05 wt% Pt/γ-Al 2 O 3 catalyst. Towards this goal, advanced SSITKA combined with transient isothermal and temperature-programmed hydrogenation experiments as well as transient operando DRIFTS-MS CO hydrogenation to CH 4 coupled with a microkinetic modelling were performed.
For the first time, to the best of our knowledge, the exchange rates of CO-s and CH x -s related to the SSITKA experiment were estimated via strict application of 13 C-material balances. The adsorption/ desorption rate of CO-s, R ex CO , was found to be significantly larger in comparison to the hydrogenation of CH x -s (R ex CHx ), and its dependence on the partial pressure of CO was probed. The large influence of CO partial pressure on the formation of different kinds of active CH x -s during the methanation reaction was suggested by the shape of the transient rate profile of the CH x -s exchange.
By increasing the CO partial pressure, an increase of θ CO was observed and this agrees with the characteristic features of the Langmuir isotherm and the fast CO exchange observed. Additionally, by increasing the CO partial pressure, an increase of θ CHx and a concomitant decrease in S CH4 were observed, which was linked to the increase of θ CO /θ CHx by decreasing the CO partial pressure (competitive chemisorption of H 2 and CO reactants) and the growing rate of -CH x coupling (increase of θ CHx ).
The amount (µmol/g) and reactivity in hydrogen of the inactive carbonaceous species formed after 5 h of FTS at 230 • C over the Co/γ-Al 2 O 3 catalyst was estimated via TIH (C β ) and TPH (C γ ) following SSITKA. The amounts of C β and C γ were found to be influenced by the CO partial pressure (36-84 mbar, P H2 = 300 mbar). In particular, larger amounts of adsorbed active CH x -s and CO-s were found to react with hydrogen at 230 • C toward CH 4 by increasing the CO partial pressure, as opposed to the amount of inactive carbonaceous species (-C x H y , named C β ), which slightly decreased by increasing the CO concentration in the feed. The latter was related to the fact that θ CO and θ CHx become larger, which likely reduces the surface coverage of H-s, and, thus, the rate of formation of C β . The amount of inactive carbonaceous species C γ was found to significantly increase with increasing CO partial pressure. The C γ could be due to hydrogen-deficient polymeric carbon and strongly adsorbed hydrocarbon-like species, but not formate (HCOO), atomic, or graphitic carbon.
Based on microkinetic modelling performed on the operando DRIFTS Transient Isothermal Hydrogenation conducted at 200 • C following the methanation reaction (5 vol% CO/25 vol% H 2 /Ar, 2 h), the presence of two linear-type adsorbed CO-s species (L 1 and L 2 ) of a 65:35 (mol/mol) ratio but of very similar activity (k) was revealed. It was concluded that θ CO had no influence on the hydrogenation activity of linear-type CO-s and on the RDS (HCO-s + H-s → HCOH-s + s) of the considered H-assisted CO hydrogenation mechanism.