Chemical Transient Kinetics Applied to CO Hydrogenation over a Pure

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J. Phys. Chem. C 2009, 113, 10731–10739

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Chemical Transient Kinetics Applied to CO Hydrogenation over a Pure Nickel Catalyst Adam Bundhoo, Julien Schweicher, Alfred Frennet, and Norbert Kruse* Chemical Physics of Materials (Catalysis-Tribology), UniVersite´ Libre de Bruxelles (U.L.B.)-Faculte´ des Sciences, De´partement de Chimie, Campus Plaine CP 243, B-1050 Bruxelles, Belgium ReceiVed: March 24, 2009; ReVised Manuscript ReceiVed: April 28, 2009

CO hydrogenation over unsupported Ni model catalysts has been studied by chemical transient kinetics (CTK) to provide insight into the time-dependent surface processes leading to hydrocarbon formation at atmospheric pressures. Buildup and backward transients were triggered by stepwise changes of the CO flow into the reactor. CTK data have been evaluated, for the first time, to allow counting of the number of surface carbon, oxygen, and hydrogen atoms from the onset of catalytic reaction conditions to steady state. In this manner, it is shown that the total amount of these atoms may considerably exceed the monolayer limit on Ni metal. Back transients from CO/H2 to pure H2 show that the intermediates react in two steps, of which the second occurs with a common first-order decay time for all hydrocarbons (C1 to C4, in this case). This is in agreement with a chain growth mechanism, in which the C1 most abundant surface intermediate (masi) is always of the same type. Indications have been obtained that a CO insertion mechanism is in operation to form this masi. 1. Introduction 1

In 1902, Sabatier observed methane formation from the reaction of carbon monoxide and hydrogen on nickel surfaces. More than 100 years later, the same reaction still sparks considerable interest in academic research. With regard to the microscopic mechanism, it seems that alkyl species, CHx, are widely accepted as reaction intermediates. Accordingly, following CO dissociation on a metallic Ni surface, “carbidic” carbon would be stepwise hydrogenated to predominantly methane, while smaller amounts of higher hydrocarbons would be formed by insertion of a “methylene” monomer into a metal-carbon bond.2-4 Such chain lengthening propensity ranks Ni among those metals that are active in the Fischer-Tropsch (FT) synthesis. Its high activity also makes it an interesting candidate for fundamental research, despite the relatively low chain lengthening probabilities compared with those of Fe, Co, or Ru. Much of the current opinion on methane formation via CHx species is based on experiments in the late 1970s with oriented Ni single crystals5 and Ni films.6 Using electron spectroscopy, subsequent to kinetic studies, the former experiments clearly demonstrated the occurrence of various forms of surface carbon (carbidic versus graphitic). Araki and Ponec,6 however, reported that a partially 13C-precovered surface of Ni film mainly produces 13CH4 when switching from a reaction mixture of 13 CO/H2 to one with 12CO/H2. Following these early studies, truly microkinetic data were provided over the following two decades by means of transient kinetic response techniques.7-11 Basically, two different types of transient experiments can be envisaged: (a) isotopic transient studies under steady-state reaction conditions (SSITKA), i.e. without affecting the rate constants of the individual reaction steps leading to product formation and (b) abrupt partial pressure changes of reactants providing access to buildup and back transients, during which the chemical surface composition and rate constants of the constituting or, respectively, decaying reaction network may * Corresponding author. E-mail: [email protected]. Telephone: +32.2. 650.57.14. Fax: +32.2.650.57.08.

vary quite drastically. Such chemical transients can be quantitatively evaluated, provided reactor and gas pipes are properly designed and a calibration of reactant flows is possible. With respect to Fischer-Tropsch-related studies, largely conflicting results for the surface coverages were obtained using chemical transients and isotopic transients.7,12-14 Yang et al.9 advocated the view of low coverages but left open the question whether this applies to the whole catalyst surface or reflects structural effects (steps versus terraces) and site blocking by unreactive spectators (e.g., graphitic carbon). It seems at present that no reliable data is available that shows how the surface coverages develop with time from the onset of reactive chemisorption until the occurrence of the steady-state reaction and vice versa. The present paper is aimed at filling this gap. A “surface atom counting” technique is being developed to evaluate the timedependent quantities of carbon, oxygen, and hydrogen surface atoms during the ongoing reaction on a Ni catalyst. It will be shown that the overall amount of these atoms exceeds the monolayer limit under steady-state conditions. Quite generally, the knowledge of the surface composition under operating catalytic conditions must be considered as an essential prerequisite for the development of a sound understanding of CO hydrogenation. The time-dependent variation of reactants and products in the gas phase during buildup and back transients may provide further clues for deriving the mechanism. This has been demonstrated in our previous CTK studies over Co- and CoCu-based catalysts,15-17 which led us to suggest the FT reaction involves oxygen-containing intermediates, different from the view of other researchers employing SSITKA techniques.7,8,11,18,19 It should be noted that in none of these SSITKA studies were the amounts of carbon, oxygen, and hydrogen atoms measured under operating reaction conditions. In this work, we present such data for a pure Ni catalyst under transient and steady-state conditions of the FT reaction at atmospheric pressures. Eventually, the data will serve to either favor or discard certain mechanistic views described in the literature by, for example, Hindermann et al.20 For more recent reviews see Dry4 and more specifically, Gaube et al.21 With respect to methane formation, the question of the relevance of

10.1021/jp902647z CCC: $40.75  2009 American Chemical Society Published on Web 05/27/2009

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Figure 2. Principle of chemical transient kinetics applied to the CO + H2 reaction.

Figure 1. Outlet flows of products during temperature-programmed decompostion of NiC2O4 (∼250 mg) under 10% H2 (bal. Ar). Ramp: 3 K min-1.

hydrogenation of surface carbon under atmospheric pressure conditions will be addressed. The stepwise hydrogen addition according to CHx + H T CHx+1 (x e 3) has recently been successfully modeled for a number of Ni model surfaces using density functional theory (DFT).22 2. Experimental Section 2.1. Catalyst. A metallic Ni powder catalyst, free of any support material, was prepared by (a) precipitating Ni oxalate from Ni(NO3)2 and oxalic acid in acetone23 and (b) decomposing the precipitate in the reactor using temperature-programmed decomposition (TPDec) in 10% H2. Figure 1 provides an example of the decomposition process. CO2 is formed with partial pressures peaking slightly below 600 K. Thus, the oxalate decomposes according to the simplified scheme NiC2O4 f Ni + 2 CO2 so as to produce a purely metallic catalyst. As shown in Figure 1, CO production is negligible, and small amounts of CH4 are most probably produced by hydrogenation of some carbon residues. The carbon mass balance shows that oxalate decomposition to carbon dioxide is complete. The specific surface area of the resulting Ni catalyst was measured in situ by Ar physisorption, following the BET method and using the same experimental setup as the kinetic reaction studies that are described below.24 For the Ni catalyst used in the present study, a value of 5 m2 g-1 was determined, corresponding to an average particle size of 135 nm. 2.2. Setup. A pyrex reactor with a volume of about 5 cm3 (ID, Ø ) 15 mm) and appropriate frit porosity was used for TPDec and CTK. Care was taken to thermally decompose Ni(CO)4, which is present as an impurity in CO high-pressure cylinders. Gas evolution during TPDec and CTK was continuously analyzed at the outlet of the reactor using a differentially pumped quadrupole mass spectrometer (Balzers QMG 422). All connections are composed of pyrex and stainless steel pipes with an inner diameter of 1.4 mm. The setup for chemical transient kinetics (CTK) as employed for CO hydrogenation studies has been described in details elsewhere.15,16 As shown in Figure 2, CTK allows buildup and

backward transients to be measured in response to stepwise changes of inlet flows. The first transient period, called buildup, is enforced by replacing a H2/He flux with H2/CO-Ar at constant temperature, usually above 500 K, and keeping a constant H2 inlet flow. Thus buildup transients provide kinetic information about the transition from a dynamic adsorptiondesorption equilibrium of hydrogen to steady-state H2/CO reaction conditions. The Ar internal reference enables us to calculate the instantaneous CO partial pressure in case no adsorption would have occurred (COtheoretical). The difference between COtheoretical and COimposed curves is due to the filling of the reactor dead volume. However, the difference between COtheoretical and COmeasured reveals the CO consumption by the catalyst during the experiments. Steady state is reached when consumption and product formation have reached a constant value maintained by the equality of their rates. The back transient period is enforced by switching the reactive flux to the initial H2/He flux. This time period thus provides information on how fast steady-state catalytic production fades away in pure hydrogen. Finally, the system is back in a dynamic adsorptiondesorption equilibrium of hydrogen. 2.3. Experimental Procedure. As mentioned above, our measurements consist of monitoring the time-dependent transition from a dynamic adsorption-desorption H2/Ni equilibrium to a CO/H2 steady-state reaction and vice versa. In this manner, buildup and back transients are constituted. Before any experiment, the individual volumetric flow rate of each gas is determined. This is done for H2 and He in a first circuit of the gas manifold and for H2 and CO-Ar (5% Ar, cylinder mixture) in a second one. These volumetric flow rates are verified at the very end of the experiment. The H2 flow rate is kept constant during each transient period, implying that the volumetric flow rate of He is chosen equal to that of CO-Ar. The quadrupole mass spectrometer (MS) is run in a multiplexed mode, i.e., 16 different m/z values are queried one after the other. Each mass is analyzed during 0.1 s with a dwell time of 50 ms, so that each cycle consumes a total time of about 2 s. Sensitivity coefficients (i.e., ratios between the MS ion current and partial pressure) are determined using reactor bypass lines. Experiments start by adjusting the mixtures of reactive H2/ CO-Ar flows. A reactor bypass line is used along with MS. During this time, H2/He flux passes through the reactor at reaction temperatures in order to ensure the dynamic H2 adsorption/desorption equilibrium with the catalyst to be established. Switching a four-way valve at this stage of the

Chemical Transient Kinetics and CO Hydrogenation experiment, i.e., replacing the flow of H2/He by H2/CO-Ar, enforces the buildup, which continues until steady state is reached. Switching back the four-way valve then leads to the back transient in which a H2/He flux passes through the reactor and scavenges the reactive state established during steady FT conditions. The last stage of the experiment consists of turning back to the H2/CO-Ar mixture and analyzing it under reactor bypass conditions in order to verify the values of the reactant inlet flows. In order to ensure differential reaction conditions, the working temperature was set to 523 K. In this way, the CO conversion was kept to values of ∼10%. 2.4. Method and Quantification of Data. Data are quantitatively evaluated by solving the time-dependent mass balance of a flow reactor. To do so, the gas flow into the reactor (inlet flow) is kept constant, while the flow leaving the reactor (outlet flow) is calculated using eq 3. Because adsorption of reactants and desorption of products change to entail a variation of the coverages during transients, the total volumetric flow rate has to change likewise. This is taken into account by using an inert internal standard (Ne, injected into the outflow after leaving the reactor with a constant flow rate), whose partial pressure is inversely proportional to the total volumetric flow rate.24 Thus, with the knowledge of gas flows into and out of the reactor, the net adsorption rate and, consequently, the accumulated amounts of surface species become accessible. This takes for granted that the catalyst surface reacts uniformly. Thus, either

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10733 the reactor is operated under differential conditions or it is wellmixed, which applies to our setup.15-17 In this case, the entire surface behaves as kinetically uniform, i.e., chromatographic elution effects are largely absent, whatever the conversion. To make sure that well-mixed conditions apply to the fixed bed reactor, we determined the decay characteristics for gas removal from the reactor. A rare gas, argon in this case, is used. An exponential decay is found (see Results), allowing us to determine the respective time constant and to verify that the derived volume corresponds to the geometrical value. A well-mixed reactor implies the absence of concentration gradients, and the general equation of mass conservation for each compound in an isothermal fixed bed reactor is

∂ngas dngas (t, x, y, z) ) (t) ) Φin - Φout(t) + Φsurf(t) ∂t dt

(1)

with

Φsurf(t) ) Φdes(t) - Φads(t) This equation considers the time-dependent variation of the number of molecules in the gas phase of the reactor (dngas)/ (dt)(t), increasing with the inlet flow Φin and decreasing with the outlet flow Φout. As far as the catalyst surface is concerned,

Figure 3. Outlet partial pressures, outlet global volumetric flow (left), and outlet molecular flows (right) during the buildup on pure nickel (T ) 523 K, Ptot ) Patm, Dtot ) 0.65 cm3 s-1, and H2:CO ) 2:1). To improve the perceptibility of delay times, we provided a zoom into the early buildup stage for the outlet molecular flows (bottom).

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the surface flow Φsurf(t) can be defined as the difference between the desorption flow Φdes(t) and the adsorption flow Φads(t) because these two terms cannot be determined separately one from another. At any time of the experiment, eq 1 allows us to calculate the surface flow Φsurf of each molecule, i.e., the net desorption rate for products and net adsorption rate (with opposite sign) for reactants. The term on the left-hand side of eq 1 is experimentally calculated from the variation of the partial pressures with time

dngas dpMS 1 (t) ) Vreac × (t) × dt dt kT

(2)

where Vreac is the volume of the reactor, and (dpMS)/(dt)(t) is the variation of the partial pressure of reactants or products as calculated from characteristic mass spectrometer signals. As mentioned above, the total volumetric flow rate during transients changes with time so that the time-dependent outlet is

Φout(t) ) pMS(t) ×

dV 1 (t) × dt kT

(3)

where pMS(t) is the partial pressure of the component and (dV)/ (dt)(t) is the total outlet volumetric flow, which is calculated at any time of the experiment from the partial pressure of the Ne reference injected at constant flow into the gas leaving the reactor. This provides access to the total outlet flow Dtot(t) ) (dV)/(dt)(t)

Dtot(t) )

PoutNe(0) × Dtot(0) PoutNe(t)

(4)

Figure 4. Surface flows of H2, CO, and CH4 and total carbon surface flow (C) as a function of time during the buildup on pure nickel (T ) 523 K, Ptot ) Patm, Dtot ) 0.65 cm3 s-1, and H2:CO ) 2:1).

Similarly, the hydrogen and oxygen surface flows are calculated from

Φsurf(H) ) 2Φsurf(H2) + 2Φsurf(H2O) + 4Φsurf(CH4) + 6Φsurf(C2H6) + 8Φsurf(C3H8) + 10Φsurf(C4H10)

(7)

Φsurf(O) ) Φsurf(CO) + 2Φsurf(CO2) + Φsurf(H2O)

(8) so that the outlet flows of all components at the reactor exit can be calculated from eq 3. As far as the inlet flow of reactants is concerned, its respective value for hydrogen Φin is independent of time during the course of the experiment (including the switch from adsorption to coadsorption-reactive conditions) and is determined by bypassing the reactor, while the inlet flow of CO is assumed to make a step from 0 to its bypass value, at the zero time of the buildup (determined by the Ar reference)

Φin(i) ) (Φout(i))by pass

with i representing H2 or CO, respectively (5)

Once the surface flows are calculated using the above equations, we first have access to the net desorption rate of products and net adsorption rate of reactants, in molecules s-1. Moreover, because surface flows of all components are calculated at any time of the buildup, instantaneous surface coverage can easily be obtained by integrating atomic surface flows, which are, for carbon, oxygen, and hydrogen, the sum of all molecular surface flows containing these atoms, weighted by the number of considered atoms in each molecule. The carbon surface flow is therefore determined by

Φsurf(C) ) Φsurf(CO) + Φsurf(CO2) + Φsurf(CH4) + 2Φsurf(C2H6) + 3Φsurf(C3H8) + 4Φsurf(C4H10)

(6)

The above description of data evaluation highlights one of the advantages of the method, which is the accessibility of coverages, i.e., atomic composition of the active catalyst surface under reaction conditions at any instant (transients and steady state) in a pressure range from 1 to 103 mbar. The application of the method to the CO + H2 reaction at global atmospheric pressure is developed and discussed in Results and Discussion. 3. Results Buildup and backward transients as measured on pure nickel are shown in Figures 3-8, using a H2:CO ratio of 2:1 at 523 K and global atmospheric pressure with an inlet volumetric flow rate of Dtot(0) ) 0.65 cm3 s-1. Figure 3 shows the buildup transients by switching from mere H2 adsorption to CO/H2 reactive conditions. The partial pressures of reactants and products as well as the total outlet flow are plotted as a function of time on the left-hand side of Figure 3 and are directly compared to the molecular outlet flows (calculated according to eq 3) on the right-hand side. The differences between the two graphs are striking and highlight the importance of measuring the global outlet flow at any instant of the experiment using the Ne reference gas as described in the Experimental Section. As shown in Figure 3 (left), switching to reactive conditions causes the total outlet flow to decrease substantially. Obviously, this is due to CO adsorption on the catalyst surface. In fact, the entire amount of CO is being adsorbed during the initial stages

Chemical Transient Kinetics and CO Hydrogenation

Figure 5. Surface coverages of carbon, oxygen, and hydrogen during the buildup on pure nickel (T ) 523 K, Ptot ) Patm, Dtot ) 0.65 cm3 s-1, and H2:CO ) 2:1).

of the buildup so that the CO partial pressure in the reactor outlet is very low; however, the H2 partial pressure increases proportionally. This could lead to the wrong conclusion that adsorbed hydrogen is being displaced into the gas phase by coadsorbing CO. However, the contrary is true as shown in Figure 3 (right). Most obviously, knowledge of the real outlet molecular flows (calculated by combining eqs 3 and 4) is needed to demonstrate that during the buildup the adsorption of CO is accompanied by consumption of gas phase hydrogen. It must be pointed out that as the CO “front” enters the reactor its chemisorption leads to an instantaneous decrease in the global volumetric flow as detected by a Ne positive peak and H2/He negative peaks in the mass spectrometer. This negative peak of H2 outlet flow is in fact the image of the CO consumption as seen at m/z ) 28, when the mass spectrometer “sees” the gas flowing out of the reactor (appearance of argon). The zero time of the transient measurements is taken as the time when Ar is being detected by the MS. Thus, the negative part of the H2 peak preceding the zero time must not be taken into account in the quantification of the surface mass balance. On the basis of the calculated outlet flows, we now turn to the quantitative evaluation of the time response during the buildup. As shown in Figure 3, during a certain period of time (about 5 s), the entire amount of CO entering the reactor is adsorbed onto the Ni catalyst surface. Yet no product formation occurs during this initial stage of the buildup. Note that before switching to reactive CO/H2 conditions the surface is in a dynamic adsorption/desorption equilibrium with hydrogen, of which the coverage is, according to the literature,10 near saturation (15 H per nm2) under our experimental conditions. Later, i.e., at times ranging from 4 to 9 s after its introduction into the reactor, all of the CO still adsorbs but small amounts of CH4 are progressively being desorbed as a product of the surface reaction. No hydrogen is being displaced from the surface. Only for times longer than 9 s does CO start to gradually appear in the outlet of the reactor. At the same time, methane production runs through a maximum. Additionally, some

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10735 amounts of H2O and CO2 are produced, and chain lengthening starts as evidenced by the appearance of C2H6 followed in sequence by C3H8 and C4H10, which are plotted in Figure 3 as C2+ (scaled up by a factor 5). The fact that chain lengthening does not occur without CO appearing in the gas phase suggests gaseous CO to be the monomer inducing C2+ formation. With the knowledge of outlet and inlet flows, we can also calculate instantaneous surface flows, Φsurf(t), for each component (eq 1). The results are shown in Figure 4 for the two reactants H2 and CO and for the main product CH4 during buildup. CO adsorption is characterized by an important consumption peak. The production of methane is accompanied by a consumption of hydrogen. Steady state is reached when the surface flows of reactants and products, in terms of carbon, oxygen, and hydrogen atoms, equal each other. Accordingly, after about 50 s, the surface flow of CH4, which is the main product of the reaction, is equal to half of the hydrogen surface flow and is close to the value of the CO surface flow (full carbon balance is achieved once C2+ surface flows are taken into account). Integration of the instantaneous surface flows (eqs 6-8), allows for following the buildup of the surface amounts of hydrogen, oxygen, and carbon. The respective data are shown in Figure 5. For hydrogen, the initial coverage corresponds to the equilibrium coverage of hydrogen on nickel at 523 K, which we consider to be about 15 H atoms per nm2 under our conditions.10 At the beginning of the buildup, carbon and oxygen start to cover the surface at an identical initial rate. Once CH4 forms (about 4s later), the increase in carbon coverage slows compared with the oxygen coverage. At steady state (reached after ∼50 s), carbon, oxygen, and hydrogen surface amounts are ∼15 C, ∼19 O, and ∼16 H atoms per nm2, respectively. These amounts are beyond the monolayer capacity, i.e., the surface is entirely covered under reaction conditions, most probably involving the atomic species associated or as a surface complex, rather than as individual atoms. Subsurface states may additionally be occupied. Moreover, during the ∼9 s delay time before C2+ formation, the carbon and oxygen amounts reach more than 6 C and 6 O atoms per nm2, respectively. All of these values lead us to conclude that the H2/CO reaction occurs on a chemically reconstructed surface, which is not metallic anymore. However, this chemical reconstruction does not involve major changes in the overall surface area of the catalysts. The BET surface areas as determined by in situ Ar physisorption24 before and after the reaction essentially lead to the same values. The steady-state CO conversion is about 10% under our experimental conditions. Thus, the approximation of differential reaction conditions may be applied. The product selectivity with formation of C1 and C2+ hydrocarbons suggests nickel behaves like a Fischer-Tropsch catalyst with a low ASF coefficient measured as R ) 0.25. Unfortunately, it seems that Ni is frequently considered as a pure methanation catalyst.6,25 The steady-state CO consumption rate is ∼0.20 molecules s-1 nm-2. We finally turn to the back transient measurements, shown in Figure 6. Note that outlet flows in the left-hand graph are normalized to their steady-state value in the right-hand graph. As can be seen, switching from H2/CO-Ar to H2/He causes the CO pressure at the outlet of the reactor to drop exponentially with a time dependence very similar to that of the inert standard (Ar). Thus, CO seems to behave like Ar in the back transient, which shows that it is irreversibly chemisorbed during the steady-state CO/H2 reaction. In order to provide a more quantitative assessment of the data, outlet flows are plotted

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Figure 6. Outlet flows (left) and normalization to their steady-state values (right) during the back transient on pure nickel (T ) 523 K, Ptot ) Patm, Dtot ) 0.65 cm3 s-1, and H2:CO ) 2:1).

Figure 8. Surface flows of carbon reactants and products as a function of carbon coverage during the buildup on pure nickel (T ) 523 K, Ptot ) Patm, Dtot ) 0.65 cm3 s-1, H2:CO ) 2:1).

Figure 7. Logarithmic plots of outlet flows during the back transient on pure nickel (T ) 523 K, Ptot ) Patm, Dtot ) 0.65 cm3 s-1, and H2:CO ) 2:1).

logarithmically in Figure 7. In this way, characteristic decay times can be determined from the initial slopes after switching the gas composition. τAr ∼ 6.5 s and τCO ∼ 4.7 s are found for Ar and CO, respectively. τAr ∼ 6.5 s provides the value of the dead volume in the reactor as the volume filled by argon during this time, with a volumetric rate of 0.65 cm3 s-1, is 4.23 cm3. This value is compared to the geometric volume, which is about 5 cm3. The difference between both values includes the volume of the catalyst. The finding that τAr > τCO is not surprising because argon is removed from the reactor only through total volumetric flow, while CO remainders may additionally continue to be irrevers-

ibly chemisorbed. To provide a quantitative account of the difference in the time constants, we compared the rate of CO chemisorption, which is ∼0.20 molecules s-1 nm-2 at steady state (6.6 × 1017 molecules s-1 for the whole catalyst), with the rate of the gas phase removal, i.e., 0.65 cm3 s-1, which is equal to 3 × 1018 molecules s-1 under our conditions. We thus find a ratio of 0.22 between chemisorption and gas phase removal, which is in reasonable agreement with the ratio (τAr - τCO)/τAr ) 0.28) determined from the measured time constants. Strictly speaking, characteristic time constants should be evaluated from the time dependence of surface flows Φsurf(t). Note that because Φsurf Ar ) 0, no comparison between the behavior of Ar and CO would be possible. However, any time constant τout calculated from the outlet flow of a molecule is not a simple combination of the surface time constant τsurf and the gas phase time constant τgas. These time constants characterize two phenomena occurring in sequence, i.e., desorption from the surface and removal from the catalyst by gas phase transport,

Chemical Transient Kinetics and CO Hydrogenation for which the total time constant τout must be weighted by the concentration of species adsorbed onto the catalyst surface and those present in the gas phase, respectively. Because the concentration difference can be estimated to amount to more than 2 orders of magnitude, τout differs very slightly from τsurf, and we can reasonably use Φout(t) for the calculations. In turning to the behavior of the carbon-containing products in Figure 7, we see that the slopes of the decay (dotted lines) for CH4 (right-hand side of the peak) and C2H6 are very similar. Indeed, both species are removed with the same time constant of τout ∼ τsurf ) 18 s. As hydrocarbon chain lengthening is described by the ASF distribution (at low conversion, which is the case in our experiments), we assume that the same desorption probability applies to all hydrocarbons. This leads us to conclude that the time constants for C3H8 and C4H10 must be the same as those calculated for CH4 and C2H6. Unfortunately, because the chain lengthening probability of our Ni catalyst is low, the intensities of C3H8 and C4H10 are close to the detection limit of the mass spectrometer. Yet the dotted lines in Figure 7 fit rather well with the corresponding experimental curves. Anyway, because the exponential decay of CH4 is characterized by a single value of the time constant, which is identical to that of longer chain hydrocarbons, the probability of hydrocarbon desorption (last step in the reaction with hydrogen) is the same for all surface precursors. The overall behavior of the normalized methane outlet flow deserves closer inspection because it is distinctly different from the other species. As shown in Figure 6, it first increases strongly (up to a of factor 4), reaches a maximum, and finally decreases. Note that the CH4 release is accompanied by a corresponding consumption of gaseous H2 (∼10 molecules nm-2 of CH4 are produced, while ∼20 molecules nm-2 of H2 are consumed). We propose to interpret the large transient production of CH4 as being due to the generation of metallic sites while emptying the surface. This emptying is a two-step process and involves hydrogenation of a masi (most abundant surface intermediate), which contains one carbon atom and is likely oxygenated. During the first step, the hydrogenation rate of the masi increases because liberated metallic surface sites enhance the atomic hydrogen supply. In the second step, the reactive transient species react to produce gaseous hydrocarbons with a common time constant of 18 s. We finally mention that the total amount of carbon desorbed from the surface during the back transient has been calculated by integrating the outlet flows of all carbon-containing molecules. A value of ∼11.5 C nm-2 is found, which is similar to the carbon coverage obtained at the end of the buildup period (15 C nm-2). Such similarity suggests largely reversible processes to be in operation, i.e., carbon amounts produced on the catalyst during buildup reactive conditions are being extensively removed by hydrogen during back transients. This is also in accordance with the BET surface areas, which show essentially the same values before and after transient studies (∼5 m2 g-1). These considerations do not exclude that Ni carbides form in certain amounts during FT operating conditions. 4. Discussion We have shown in the preceding section that CTK data can be quantitatively evaluated to provide information on the surface amounts of O, C, and H atoms, while constructing the catalytically active phase of the steady-state CO hydrogenation at atmospheric pressure. However, the method needs careful calibration, and a simple partial pressure analysis could easily lead to the wrong conclusions. As has been seen, molecular

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10737 flows have to be calculated by taking into account the variation of the total flow rate leaving the reactor during the buildup phase. Once this is done, carbon, oxygen, and hydrogen surface amounts are obtained by integration of net surface flows at any instant of the buildup phase, and this procedure has enabled us to demonstrate that the total amounts of O, C, and H atoms are beyond the monolayer limit under steady-state conditions of the reaction, i.e., ∼15 C, ∼19 O, and ∼16 H atoms per nm2 (according to the buildup integration). Particularly striking in this analysis is the fact that the amounts of hydrogen remain little affected (or increase somewhat at the most, Figure 5) compared with those representing the dynamic adsorptiondesorption equilibrium. Thus, CO adsorption at 523 K is not associated with a H2 displacement as it is frequently believed (ref 26 and references therein). It is likely that surface hydroxyl as well as CHx2,3 species are formed in this manner. In this context, note that water formation during buildup transients is associated with a rather long delay time (it appears with tdelay > 20 s, which is considerably later than that for methane), i.e., more than 10 O atoms per nm2 have built up before the onset of water formation. The same arguments apply to surface carbon, of which the amounts seem to develop somewhat slower than those of oxygen. It must be pointed out once again that the atomic surface amounts of oxygen, carbon, and hydrogen are much higher in our studies than previously reported by others in similar transient studies. For example, Biloen et al.,8,27 in their 12CO/13CO isotopic transients over Ni-based catalysts, provide data that suggest a strong chromatographic, frontal elution effect to be in operation. As compared with the argon reference, the decay of 12CO and its nearly time symmetric replacement by 13CO take place with a considerable delay, which is on the order of ∼30 s. The reason for the discrepancy is most likely provided by the reactor design. Accordingly, Yang et al.9 use a copper tubular reactor of 4.5 mm ID, although the catalyst bed is about 7 cm in height, which describes this reactor as the plug-flow type. In our case, the pyrex reactor has gradientless characteristics close to CSTR, so that chromatographic effects are negligible. Furthermore, as pointed out in the Experimental Section, changes in the total flow rate must be taken into account in order to allow quantitative evaluation of the data in terms of amounts of surface atoms. Surface coverages in the work of Yang et al.9 are calculated by evaluating the chromatographic effect and referencing to Θ ) 1 of a fresh catalyst exposed to CO gas at 373 K. One of the interesting features of the CTK method as used here is the possibility to determine, as a function of time, the net rates of adsorption and desorption of reactants and products (Figure 4) as well as the surface amounts of atomic species (Figure 5). Thus, combining these two types of information allows us to analyze how these rates vary as a function of the coverage (Figure 8). During the first 8 s (Figure 3), all of the CO entering the reactor sticks to the surface. Such irreversible chemisorption can be explained by CO dissociation. The corresponding carbon coverage building up during this time amounts to about 4-6 C nm-2. Within the same period of time, small amounts of methane, corresponding to about 10% of the integrated amount of irreversibly chemisorbing CO, start to desorb. No other product species is formed at these short reaction times. CO starts to appear in the gas phase after about 5-6 C nm-2 have been accumulated on the surface (after ∼8 s of reaction). Only from that time on, C2+ and CO2 appear in the gas phase.

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Hydrocarbons are liberated not all at once but follow a simple sequence with times of appearance increasing from C2H6 to CnH2n+2 (maximum value of n observable in this case is n ) 4), corresponding to very different values of carbon coverage and CO partial pressure. The formation of CH4 begins even when no gaseous CO is observed. It goes through a maximum with a rate of formation more than twice that of the steadystate value. Chain lengthening and CO2 formation, starting with the appearance of gaseous CO, present clues for considering gaseous CO as the monomer inducing chain lengthening. The maximum rate of methane production occurs at times when the rate increase in C2H6 and CO2 products is strongest (between 8 and 10 C nm-2). When the amount of surface carbon has reached ∼15 C nm-2, the distribution of the various hydrocarbons formed corresponds to the ASF distribution with an R value of 0.25. Shortly after passing the maximum of CH4 production, the CnH2n+2 and CO2 products reach their steady-state values monotonously. The occurrence of the CH4 production peak implies that the first stage of irreversible CO chemisorption leads to an intermediate species that only desorbs as methane, until the CO partial pressure is high enough to allow gaseous CO to induce chain lengthening. Basically, two mechanisms of CH4 formation are in line with this behavior: (i) stepwise carbon hydrogenation, CHx + H T CHx+1 (x e 3) and (ii) hydrogenation of oxygen-containing intermediates. A straightforward distinction between both mechanisms is not possible here because spectator formation and phase compositional changes (Ni carbide formation, for example) do not allow the stoichiometry of the masi (most abundant surface intermediate) complex to be reconstructed from the counted number of adsorbed atoms. However, there are a number of indications that mechanism (ii) dominates. First, as we have seen from the above discussion, despite being the first product and surface carbon being accumulated quickly, it takes several seconds for methane to appear in the gas phase. Second, the surface amounts of C, O, and H atoms exceed the monolayer limit for all reaction times from the onset of methane formation up to its steadystate production. Thus the surface is crowded, and metallic fractions are scarce or even unavailable to allow for a simple sequence of Langmuir-Hinshelwood type hydrogenation steps of surface carbon. Third, a surface complex seems to be formed whose generic composition allows chain lengthening to occur. It is in the spirit of an Anderson-Schulz-Flory type chain lengthening mechanism to consider all hydrocarbons to be formed via this masi complex. This third point brings us finally to a discussion of the hydrocarbon decay times as measured in our back transient studies when switching from CO/H2 synthesis conditions to hydrogen reactive desorption (Figures 6 and 7). Clearly, because the Anderson-Schulz-Flory chain lengthening probability is low here (R ) 0.25), only the CH4 and C2H6 data can be evaluated with statistical significance. However, because the ASF distribution applies (measurements are performed at low CO conversion), the CH4/C2H6 decay should describe the C2+ behavior reasonably well, which turns out to be the case. Interestingly, a two-step hydrogenation reaction of the masi seems to be in operation. Only for the second step, a (common) decay time can be evaluated. Actually, such a two-step process is in agreement with the role that hydrogen plays in exchange reactions between hydrocarbons and D2, isomerization and hydrogenolysis.28,29 Accordingly, surface hydrogenation takes place first and depends on the availability of atomic hydrogen, whereas reactive desorption is second and encounters a hydrogen

Bundhoo et al. molecule interacting with respective surface precursors. Following this scenario, metallic fractions liberated in the first step gradually increase the availability of atomic hydrogen so as to reduce the masi intermediate, while in a second hydrogenation step the resulting precursor is transformed into a hydrocarbon. A quantitative evaluation of the back transients in terms of precursor amounts becomes possible once their chemical composition is known. These amounts should follow the ASF distribution, with the same R value calculated via the desorption rates during steady state. Unfortunately, as mentioned previously, the exact stoichiometry of the precursors cannot be straightforwardly derived from our surface atom counting. However, assumptions can be made and tested for their validity. Following this conceptual approach, we assume hydrocarbon liberation into the gas phase to involve hydrogenation of ASFdistributed oxygenates. In line with previous suggestions, these oxygenates should be alkoxy-type species, (ORad),15-17 whose surface amounts can then be determined using the differential expression of the surface flow during back transient and steady state (with a common hydrocarbon decay time constant of τ ) 18 s found from Figure 7)

-

d(ΘOR)transient (ΘOR)steady state ) ) Φsurf(t) dt τ

(9)

The ORad distribution calculated in this way is ΘOCH3 ∼ 2.2 nm-2, ΘOC2H5 ∼ 0.5 nm-2, ΘOC3H7 ∼ 0.1 nm-2, and small amounts of ΘOC4H9. The difference between the overall ORad coverage and the global steady-state carbon amount (∼11.5 C nm-2) must then lead to the masi coverage, i.e., Θmasi ∼ 8.6 nm-2. From these back transient derived values, we can then calculate the total atomic surface amounts of hydrogen, carbon, and oxygen constituting the masi. In this way, we receive 12.1 C nm-2, 20.0 O nm-2, and 18.4 H nm-2, respectively. The values are in rather good agreement with those calculated during the buildup (15 C nm-2, 19 O nm-2, and 16 H nm-2, Figure 5). The slight deviation does not come as a surprise and is either due to side reactions (i.e., with surface carbon) during the back transients or to an overestimation of the hydrogen surface coverages preceding the buildup (15 H nm-2). It is also interesting to correlate ΘOCH3 ∼ 2.2 nm-2 and Θmasi ∼ 8.6 nm-2 with the CH4 peak integration during the back transient, which provides ∼10 CH4 nm-2. Once again, the accordance is remarkable, so we conclude that the CH4 back transient peak is indeed associated with a two-step process, i.e., hydrogenation of a C1 masi into O-CH3, followed by desorption from the surface through a reaction with H2. The same applies to the higher homologues, with the same desorption probability whatever the chain length, which is in complete agreement with the Anderson-Schulz-Flory distribution observed. Similarly, the quantitative evaluation of the back transient experiments has demonstrated the capability of the method to account for the difference between the time constants characterizing the emptying of the reactor of argon and CO where, at first glance, τCO is surprisingly smaller than the respective value for argon. The above considerations demonstrate that important information may be gathered from quantitative CTK experiments. Surface atomic amounts and kinetics can be evaluated at any time of the reaction, which is clearly different from steadystate analyses where only the product k Θ is accessible. This was already recognized by Tamaru.13 The same author proposed more than 40 years ago that detailed mechanistic insight can only be gleaned from investigations of the catalyst in its “catalytically working state”.13 According to our view and that

Chemical Transient Kinetics and CO Hydrogenation of many others in the catalysis community, this is key to most hydrogenation reactions since these provide prototype examples for the pressure gap problem encountered in transferring surface science results to heterogeneous catalysis high pressure conditions.16,29 There is also no doubt that a combination of the CTK method with time-resolved spectroscopic measurements would be very welcome and help develop a more complete picture of the FT reaction mechanism at atmospheric pressures. As far as the nature of the masi is concerned, in previous studies with Co- and CoCu-based catalysts, we proposed methane and C2+ to be formed by hydrogenation of formatecarboxylate-type intermediates.15-17 According to Davis,30 in the high-pressure synthesis a carbide mechanism should be in operation for cobalt-based catalysts, although oxygenate intermediates appear more likely on iron-based catalysts. In the present study, over pure Ni, at atmospheric pressures similar to those over Co-based catalysts, the exact stoichiometry of the intermediate clearly cannot be derived, although evidence is provided that the masi is likely to be an oxygenate. It is interesting to note that such oxygenates were detected in IR studies over Co- and Ni-supported catalysts.31 Formate and methoxy species in this study showed similar reaction behaviors as those formed over Cu-ZnO-based catalysts during methanol synthesis from CO and hydrogen. 5. Conclusions CTK studies of CO hydrogenation have been performed on pure nickel and yield, except for the lower chain lengthening probability, similar results as those obtained on Co-based catalysts.15-17 As with these latter metals, clear evidence is obtained that the surface under steady-state catalytic reaction conditions is covered with large amounts of carbon, oxygen, and hydrogen atoms a sum that exceeds the monolayer limit of the Ni surface considerably. The use of a well-mixed reactor and determination of the instantaneous outlet volumetric flow are combined with surface atom counting and lead to a new insight hitherto not accessible experimentally. The experimental evidence provided shows that hydrocarbon formation does not proceed on a metallic surface in atmospheric pressure conditions. Although the stoichiometry of the masi cannot be derived precisely in this study, surface atom counting and transient period calculations show that it should be an oxygenate-type species. Large back transient methane production has been observed and is most probably associated with the increasing availability of surface atomic hydrogen following H2 dissociation on regenerating metallic sites. A two-step process seems to be in operation and is in accordance with the reduction of the C1 surface masi into hydrocarbons via surface alkoxyde intermediates. A common time constant for hydrocarbon desorption has been evaluated and associated with this second step. The back transients finally seem to reproduce a largely metallic catalyst surface, demonstrating that the processes involved in CO hydrogenation over pure Ni are reversible. In order to provide further insight into the fairly complicated mechanism of the Fischer-Tropsch reaction, chemical transient kinetics as those reported here will have to be performed at higher than atmospheric pressures. Furthermore, the catalyst nanostructural properties will have to be considered in more detail because it

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10739 may be expected that size and morphology of Ni (and Co) particles influence transient kinetics. Acknowledgment. This work was financially supported by the Fonds de la Recherche Scientifique, F.R.S.-F.N.R.S. (F.R.I.A. grants for A.B. and J.S.) which is gratefully acknowledged. We are also most thankful for support from Shell Global Solutions. References and Notes (1) Sabatier, P.; Senderens, J. B. C. R. Acad. Sci. Paris 1902, 134, 514. (2) Schulz, H. Top. Catal. 2003, 26, 73. (3) Schulz, H. Appl. Catal., A. 1999, 186, 3. (4) Dry, M. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: New York, 2008, Vol. 6, p 2965. (5) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; White, J. M. J. Catal. 1980, 64, 479. (6) Araki, M.; Ponec, V. J. Catal. 1976, 44, 439. (7) Happel, J.; Suzuki, I.; Kokayeff, P.; Fthenakis, V. J. Catal. 1980, 65, 59. (8) Biloen, P.; Helle, J. N.; van den Berg, F. G. A.; Sachtler, W. M. H. J. Catal. 1983, 81, 450. (9) Yang, C.-H.; Soong, Y.; Biloen, P. Abundancy and reactiVity of surface intermediates in methanation, determined with transient kinetic methods; Proceedings of the 8th International Congress on Catalysis, Berlin, July 2-6, 1984, Vol. II, pp 3-14. (10) Pannell, R. B.; Chung, K. S.; Bartholomew, C. H. J. Catal. 1977, 46, 340. (11) Agnelli, M.; Swaan, H. M.; Marquez-Alvarez, C.; Martin, G. A.; Mirodatos, C. J. Catal. 1998, 175, 117. (12) Vannice, M. A. J. Catal. 1975, 37, 462. (13) Tamaru, K. AdV. Catal. 1964, 15, 65. (14) Dautzenberg, F. M.; Helle, J. N.; van Santen, R. A.; Verbeek, H. J. Catal. 1977, 50, 8. (15) Frennet, A.; Hubert, C. J. Mol. Catal A: Chem. 2000, 163, 163. (16) Frennet, A.; Visart, T.; Bastin, J.-M.; Kruse, N. J. Phys. Chem. B 2005, 109, 2350. (17) Kruse, N.; Schweicher, J.; Bundhoo, A.; Frennet, A.; Visart de Bocarme´, T. Top. Catal. 2008, 48, 145. (18) van Dijk, H. A. J.; Hoebink, J. H. B. J.; Schouten, J. C. Chem. Eng. Sci. 2001, 56, 1211. (19) Radstake, P. B.; den Breejen, J. P.; Bezemer, G. L.; Bitter, J. H.; de Jong, K. P.; Frøseth, V.; Holmen, A. On the Origin of the Cobalt Particle Size Effect in the Fischer-Tropsch Synthesis. In Natural Gas Conversion VIII, Proceedings of the 8th Natural Gas Conversion Symposium, Natal, Brazil, May 27-31, 2007; Fa´bio, F. B., Schmal, M., Sousa-Aguiar, E. F., Eds., Elsevier: Oxford, U.K., 2007, Vol. 167, pp 85. (20) Hindermann, J. P.; Hutchings, G. J.; Kiennemann, A. Catal. ReV. Sci. Eng. 1993, 35, 1. (21) Gaube, J.; Klein, H.-F. J. Mol. Catal A: Chem. 2008, 283, 60. (22) Andersson, M. P.; Abild-Pedersen, F.; Remediakis, I. N.; Bligaard, T.; Jones, G.; Engbæk, J.; Lytken, O.; Horch, S.; Nielsen, J. H.; Sehested, J.; Rostrup-Nielsen, J. R.; Nørskov, J. K.; Chorkendorff, I. J. Catal. 2008, 255, 6. (23) Buess, P.; Caers, R. F. I.; Frennet, A.; Ghenne, E.; Hubert, C.; Kruse, N. Catalysts and Processes Using Them. U.S. Patent 2003/0036573, 2003. (24) Frennet, A.; Chitry, V.; Kruse, N. Appl. Catal., A 2002, 229, 273. (25) Coenen, J. W. E.; van Nisselrooy, P. F. M. T.; de Croon, M. H. J. M.; van Dooren, P. F. H. A.; van Meerten, R. Z. C. Appl. Catal. 1986, 25, 1. (26) Ponec, V. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: New York, 1997, Vol. 4, p 1876. (27) Yang, C.-H.; Soong, Y.; Biloen, P. J. Catal. 1985, 94, 306. (28) Garin, F. Top. Catal. 2006, 39, 11. (29) Frennet, A. In Hydrogen Effects in Catalysis: Paal, Z., Menon, P. G., Eds.; M. Dekker: New York, 1988. (30) Davis, B. H. Fuel Process. Technol. 2001, 71, 157. (31) Sanchez-Escribano, V.; Larrubia Vargas, M. A.; Finocchio, E.; Busca, G. Appl. Catal., A 2007, 316, 68.

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