J. Phys. Chem. C 2011, 115, 1255–1260
1255
Catalytic CO2 Hydrogenation on Nickel: Novel Insight by Chemical Transient Kinetics† E. Vesselli,‡,§ J. Schweicher,| A. Bundhoo,| A. Frennet,| and N. Kruse*,| Dipartimento di Fisica and CENMAT, UniVersita` degli Studi di Trieste, Via Valerio 2, I-34144, Trieste, Italy, IOM-CNR, Laboratorio TASC, Area Science Park, S.S. 14, km 163.5, I-34149, BasoVizza (Trieste), Italy, and Chimie Physique des Mate´riaux (Catalyse-Tribologie), UniVersite´ Libre de Bruxelles, Campus Plaine, CP 243, B-1050, Bruxelles, Belgium ReceiVed: July 15, 2010; ReVised Manuscript ReceiVed: September 10, 2010
Carbon dioxide hydrogenation on support-free nickel model catalysts was investigated by means of a time-resolved quantitative analysis of chemical transients triggered by abrupt changes in the reactant partial pressures. It was found that carbon dioxide adsorption is strongly affected by the presence of hydrogen and by coadsorption effects and thus influences the reaction rate in the buildup and back transients. The observed transients suggest that two reaction mechanisms operate in parallel, which is consistent with previous results obtained using a Ni singlecrystal termination. The initial reaction rate involves fast direct hydrogenation of CO2, whereas the rate under steady-state conditions is lower due to a change in the mechanism involving an oxygen-containing intermediate. Introduction Decreasing crude oil reserves mean that the catalytic hydrogenation of carbon monoxide (CO) and carbon dioxide (CO2) becomes a key process in chemical feedstock and synthesis of alternative fuels. Additionally, the chemistry of these reactions has attracted increased technological interest in the environmental issues related to the sequestration and reutilization of carbon oxides.1,2 The heterogeneous catalytic hydrogenation of carbon dioxide is involved in a number of processes, such as methanol synthesis (CO2 + 3H2 T CH3OH + 2H2O), the reverse water gas shift reaction (CO2 + H2 T CO + H2O), the Bosch reaction (CO2 + 2H2 T C + 2H2O), the Sabatier or methanation reaction (CO2 + 4H2 T CH4 + 2H2O), and dimethyl ether and formic acid synthesis.3,4 To provide a sound understanding of a catalytic reaction, a steady-state kinetic analysis is in most cases insufficient. It has become apparent during recent years that the information gleaned from spectroscopic studies during the ongoing reaction may be particularly revealing with regard to the identification of possible intermediates. Such “operando” studies can also be designed to provide kinetic insight far from the steady state or by isotope exchange (“SSITKA”) while maintaining the steadystate (for the original work, see ref 5). Another challenging issue faced by today’s research is to gain predictive power about the behavior of a catalytic system in order to develop new, cheaper, and more efficient catalysts. In this context, carbon dioxide hydrogenation is quite widely employed even if the underlying reaction mechanism is still largely unknown. CO2 methanation on nickel catalysts has been explored under standard catalytic reaction conditions using either the pure or the supported metal, in either the presence or the absence of promoters.6–11 One of the proposed reaction mechanisms originating from these studies starts with the CO2 decomposition into CO and O. At this point, the reaction proceeds via CO hydrogenation, and CHx species were envisaged as intermediates.9,10 This view is similar to that of some authors12–14 but has been challenged more recently by †
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Universita` degli Studi di Trieste. § IOM-CNR, Laboratorio TASC. | Universite´ Libre de Bruxelles.
others.15 However, CO and CO2 methanation on Ni also show several differences. For example, the selectivity9,10 may change from one case to the other, which, however, is not surprising in view of the fact that CO can be hydrogenated to long-chain hydrocarbons and oxygenates, whereas CO2 cannot. Thus, the reaction mechanism is still far from being deeply understood. In parallel, experimental and theoretical surface science studies yield a lot of fundamental, quantitative information about heterogeneous model catalytic systems. Despite some interesting work in relation to metal surfaces,16 major difficulties are encountered in handling the weak CO2 adsorption at surfaces under ultra-high-vacuum conditions. Recently, a series of combined experimental and theoretical papers have appeared, presenting a detailed study of carbon dioxide adsorption17,18 and reaction with hydrogen19,20 on the Ni(110) single-crystal surface at low temperature. At variance with other transition-metal (TM) surfaces and with other Ni single-crystal terminations where only linear physisorption occurs,21 it is remarkable that the CO2 molecule under UHV conditions is activated at the open Ni(110) surface through a 0.9 e- charge transfer to the adsorbed molecule. The latter bends and binds to the surface via the carbon atom in a “V” shape. The previously postulated dicoordinated adsorption via the two oxygen atoms is energetically less favored.16–18 Hydrogenation occurs through two parallel reaction channels, yielding hydrocarboxyl and formate intermediates through Eley-Rideal and Langmuir-Hinshelwood mechanisms, respectively. The former pathway is energetically favored (and, therefore, associated with a higher turnover rate) with an activation barrier of only 0.12 eV.19,20 Formate is generally observed as a stable species on Ni as well as on other TM catalysts, and its role in the reaction pathway is not yet understood because it may be either a spectator or a fundamental reactive intermediate.16,22–28 Further insight is evidently needed. In addition, other studies have recently tackled the role of Ni in the methanol synthesis reaction with respect to conventional Cu-based catalysts.29–32 Again, CO2 hydrogenation is the key step. It is, therefore, evident that a deep comprehension of the mechanisms underlying this process on nickel is of great scientific and technological interest. In the present paper, we report on the results obtained by studying CO2 methanation on unsupported pure Ni model
10.1021/jp106551r 2011 American Chemical Society Published on Web 09/27/2010
1256
J. Phys. Chem. C, Vol. 115, No. 4, 2011
Vesselli et al.
catalysts by means of chemical transient kinetics (CTK)5,33 at 1 bar. This approach has shown itself to be particularly useful in order to gain insight into the reaction mechanism of CO hydrogenation over Ni.15 On the basis of CTK studies, a novel mechanism has recently been proposed in order to describe the chain-lengthening process that occurs in the catalytic CO hydrogenation reaction on Co.33–35 Here, we adopt a similar approach to investigate the CO2 + H2 reaction on Ni.
is calculated using the instantaneous value of the outlet volumetric flow Dout. The variations in the surface coverage of the species involved in the reaction (in this case, carbon, oxygen, and hydrogen) can be calculated at any instant of the transient periods by integration of the corresponding surface flows. As an example, for carbon, this yields
∆ΘC(t) )
Methods Experimental. Chemical transient kinetics were studied by abruptly changing the partial pressure of one of the reactants via a four-way valve, thanks to a peculiar design of the reactor and of the gas-handling systems. The chosen flow rates are associated with the geometry of the system (reactor and connecting pipes). To allow the data to be quantitatively evaluated in terms of atomic surface amounts (“surface atom counting”), the reactor should operate like a CSTR (continuous stirred-tank reactor) to provide gradientless conditions. We have previously shown that our reactor design approaches these conditions reasonably well.15 Furthermore, the connecting pipes should provide plug-flow behavior. Clearly, these ideal conditions can never be reached in a real system. In our case, deviations from the plug-flow behavior cause the time resolution during transient measurements to be limited to about 1 s. Surface atom counting in CTK is based on the evaluation of the in-out mass balance.15 The variation of the partial pressure with time is measured for each species, while keeping the influx constant. The out-flux is also selectively followed as a function of the time so that the net adsorption rate (rdes - rads) is obtained. Because of variations in the catalyst’s activity while performing transients, variations of the volumetric flow rate have to be expected and are taken into account by using an external standard (Ne), which is added to the reactor outlet flow. The use of an internal standard (He, Ar) allows the definition of the effective beginning, and evolution, of the transients,15,33,34 thus untangling the significant information from the effects caused by the finite volume of the reactor. Analytically, for each component of the stream, the surface flow can be written as
Φsurf )
dngas (t) + Φin - Φout(t) dt
(1)
where
( )
dngas dp(t) d pV 1 )V (t) ) · dt dt kBT dt kBT
(2)
is calculated by the perfect gas law applied to the reactor partial pressures. The in-flux
Φin ) Dinpin ·
1 kBT
(3)
is obtained from the predetermined flow parameters at the entrance of the system, and
Φout ) Dout(t)pout(t) ·
1 kBT
(4)
t
t
t0
t0
∫ Φsurf(C)dt ) ∫ [Φsurf(CO2) + Φsurf(CO) + Φsurf(CH4)]dt (5)
CTK experiments were performed at a constant temperature and at a total pressure of 1 bar. The H2/CO2 ratio was varied from 0.25 up to 15 while maintaining the total volumetric flow constant (30.0 cc min-1). CO2 was introduced into the system in parallel with a reference standard (Ar) by using a 10% CO2/ Ar mixture in order to define the zero time of the transients and to compute the theoretical CO2 pressure. The latter allows the evaluation of CO2 conversion, which is defined in the present paper as
conv )
COtheor - COmeas 2 2 COtheor 2
(6)
Two classes of measurements were performed, which we define as H2-CO2 and CO2-H2 experiments. In the former case, by switching the four-way valve, a H2 + He stream was replaced by a H2 + CO2/Ar stream for the buildup transient and vice versa in the back transient. In the CO2-H2 experiment, a CO2/ Ar + He stream was instead replaced by a CO2/Ar + H2 flux. The catalyst temperature was controlled by means of a homemade oven. The catalyst was deposited on a Pyrex frit inside a home-designed Pyrex reactor with an inner diameter of 1.5 cm and a volume of ∼5 cm3. A portion of the outlet stream was introduced into a UHV analysis chamber through a 70 cm-long capillary yielding the proper pressure drop. The system was equipped with a Balzers QMG 420 residual gas analyzer. In the Results and Discussion section, the water transient is not reported because the time-resolved water signal could not be measured reliably due to water-pumping speed limitations in the detection apparatus. For the quantitative analysis at steady state (coverage determination), the mass balance was used to infer the water partial pressure in the outlet stream. Catalyst Preparation. The unsupported Ni catalyst was prepared by means of the oxalate precipitation route.15 The Ni powder was then treated by a redox cycle to promote Ni-oxalate decomposition directly in the reactor. Subsequently, a temperature-programmed oxidation (TPO) was performed under a 10% O2/Ar stream at 1 bar (30.0 cc min-1) with a temperature ramping rate of 3 °C min-1 from RT up to 400 °C. The following reduction to metallic Ni was obtained after treatment with a 10% H2/Ar flux (1 bar, 30.0 cc min-1) again up to 400 °C. The final catalyst mass was 0.535 g. The surface area was measured in situ, adopting the BET approach,36 and a low value (with a consequently large error bar) was obtained (1.7 ( 1.0 m2 g-1). This is due to the fact that a complete redox cycle was performed to decompose the Ni-oxalate instead of just a reduction in hydrogen. We have previously observed that a Ni model catalyst with a considerably larger surface area can be obtained by temperature-programmed decomposition of the
Catalytic CO2 Hydrogenation on Nickel
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1257
Figure 2. CO2 conversion at steady-state conditions as a function of the H2/CO2 ratio. Data are reported for the H2 + He f H2 + CO2/Ar and CO2/Ar + He f CO2/Ar + H2 experiments at T ) 360 °C and T ) 295 °C. Figure 1. Temperature-programmed reactivity test. The catalyst is annealed from RT to 400 °C under a 10% CO2/Ar + H2 stream at 1 bar (total volumetric flow was 28 cc min-1, H2/CO2 ratio ) 4:1). The temperature ramping rate was 2 °C min-1. The vertical dotted lines indicate the two temperatures selected for the CTK experiments.
oxalate precursor in argon. However, this catalyst turned out to be rather active in the Ni(CO)4 formation and subsequent decomposition of this compound at the walls of the pipes. This decomposition reaction is most troublesome for the correct operation of the capillary system preceding the mass spectrometric detection. The catalyst prepared via a complete redox cycle, therefore, provided a surface area of about 0.9 ( 0.5 m2, corresponding to (1.4 ( 0.8) × 1019 surface Ni atoms. This yields a systematic (and not a statistic) error bar of about 50% on the coverage values we obtain, meaning that the trends we discuss are consistently reliable, keeping, however, in mind that the uncertainty affects the absolute coverage value only. Results and Discussion To allow the characteristic times of adsorption and reaction steps to be measured by CTK, the selection of a suitable catalyst temperature is an essential prerequisite. Accordingly, temperature-programmed reaction experiments were performed to follow how the gas-phase composition changed due to reaction. The results are plotted in Figure 1 for an inlet H2/CO2 ratio equal to 4 (stoichiometric composition). In agreement with the previous literature,4,19 methane is the only detectable reaction product, apart from a small CO contribution in the 250-300 °C temperature range, which may be ascribed to low-temperature accumulation effects. CO2 conversion starts above 250 °C, and the maximum is reached at about 350 °C. On the basis of these results, two temperatures were selected for the CTK experiments: (i) 295 °C (corresponding to the inflection point of the reactants, CH4, and conversion profiles with respect to the temperature) and (i) 360 °C (maximum CO2 conversion). Subsequently, a full set of CTK experiments was performed in order to span the space of the reaction parameters. For each of the two selected temperatures, buildup and back transients were measured for H2/CO2 ratios from 0.25 up to 15. Buildup switching to reactive conditions was performed starting from
either hydrogen (H2 + He f H2 + CO2/Ar) or CO2 (CO2/Ar + He f CO2/Ar + H2) initial adsorption. The CO2 steady-state conversion is shown in Figure 2 as a function of the H2/CO2 ratio. The dashed lines are plotted to guide the eye. As can be seen, no influence of the switching order is found: whether the steady state is reached by switching from mere hydrogen or mere CO2 adsorption to reactive conditions, the conversion at the end of the buildup transient is essentially the same. As expected, the conversion at 360 °C is higher than at 295 °C whatever the H2/CO2 ratio, which is consistent with the conversion results plotted in Figure 1 as a function of temperature. Selected buildup and back transients are reported in Figure 3 for a temperature of 360 °C and a H2/CO2 ratio equal to 2. The behavior of H2, CO2, and CH4 is plotted as a function of time for both types of experiments, that is, for adsorption of either hydrogen or CO2 only, followed by reactive conditions caused by adding the respective reactant, and for returning from reactive to mere adsorption conditions (Figure 3a,b). The timedependent conversion for these experiments is shown in Figure 3c. For the top panel experiment (CO2/Ar + He f CO2/Ar + H2, Figure 3a), the CH4 signal is seen to grow after the H2 signal, indicating that hydrogen adsorbs before reaction occurs. This is not the case when switching inversely, that is, H2 + He f H2 + CO2/Ar + H2 (Figure 3b). Now there is a significant delay (of the order of 10 s) between CH4, which appears first, and CO2. Note also that the measured CO2 curve (blue) lags behind the theoretical one (gray), which is actually the argon reference for physical adsorption. The occurrence of this time lag is indicative of irreversible carbon dioxide adsorption at the surface in the presence of hydrogen. This is indeed compatible with a He f 10% CO2/Ar buildup transient not reported in Figure 3. In this case, almost no sticking of CO2 occurs, yielding a surface carbon coverage of 0.04 ML and an oxygen coverage of 0.08 ML37 (note that CTK “counts” surface atoms rather than molecules) at 360 °C and steady-state (saturation) conditions, in agreement with the C/O ) 1:2 stoichiometry in CO2. On the other hand, in the presence of hydrogen, the coverage of the two species is higher by an order of magnitude, as we will discuss later (data are reported in Figure 6). This indicates that
1258
J. Phys. Chem. C, Vol. 115, No. 4, 2011
Figure 3. CTK data for the reactants and main reaction products of CO2 hydrogenation on Ni. Buildup transients are shown in the left column, whereas back transients appear in the right one. Reaction conditions: T ) 360 °C, p ) 1 bar, H2/CO2 ratio ) 2, volumetric flow ) 30 cc min-1. (a) CO2/Ar + He f CO2/Ar + H2 experiment. (b) H2 + He f H2 + CO2/Ar experiment. (c) CO2 conversion. The t ) 0 s and t ) 300 s are the reactant pressure transient onset times as obtained from the reference Ar partial pressure for both the buildup and the back transients, respectively.
methane formation does not simply proceed via the hydrogenation of a stable surface carbon species, as might be suggested if carbon dioxide is assumed to decompose on the Ni surface. It follows that hydrogen plays an active role in the CO2 reaction pathway. This is an indicator of a hydrogen-assisted CO2 activation/reaction mechanism, resembling the trend found under UHV conditions on a model system.19,20 This scenario does not exclude more complicated subsequent pathways on a chemically reconstructed surface containing oxygen, carbon, and hydrogen. Another remarkable feature can be observed in the conversion curves, plotted in Figure 3c. Following the procedure H2 + He f H2 + CO2/Ar + H2 (the red curve), the CO2 conversion goes through a relative maximum in the first 20 s and successively recovers to the same steady-state value as that obtained when starting from carbon dioxide adsorption (the cyan curve). This behavior once again points to the importance of hydrogen adsorption as a prerequisite for the reaction to occur. More precisely, a hydrogen precovered metallic surface turns out to
Vesselli et al. be more reactive than a surface constructed by coadsorption of CO2 and hydrogen. The high transient reactivity in the “overshoot regime” as compared with steady-state conditions may, therefore, be associated with a different mechanism of methane formation. In any case, preconditioning of the Ni catalyst surface is essential for the course of reaction events and is more timeconsuming than the actual CO2 turnover to CH4. Note in this respect that, for the CO2/Ar + He f CO2/Ar + H2 experiment, the reaction starts with a precoverage of 0.04 ML of carbon and 0.08 ML of oxygen (possibly 0.04 ML of molecular carbon dioxide) precluding (or at least inhibiting) the creation of a highly reactive hydrogen-covered Ni surface. In the same series of H2 + He f H2 + CO2/Ar + H2 measurements, the CH4 production (Figure 3b, the black curve) rises abruptly upon switching, but instead of running monotonously into the steady-state, local extrema are observed. As to the back transient, the methane signal does not follow a simple exponential decay. At least two different decay processes seem to be in operation. Note that the methane decay curve, showing a structured shape, extends well beyond both the Ar reference (“theoretical CO2”) and the experimental CO2 exponential profiles. This is consistent with the picture in which, during steady-state conditions, a C-containing surface species is formed that reacts only slowly when switching to pure hydrogen. The existence of a fast reaction route via hydrogen-assisted CO2 adsorption, as discussed on the basis of the buildup measurements, is also supported by the back transients. Panel b of Figure 3 shows a fast decay of the experimental CO2 curve, which is followed by a fast rise in the CO2 conversion (panel c, directly below). Thus, after switching to pure hydrogen, metallic sites are recovered, allowing the fast turnover of CO2. The occurrence of a maximum in the CO2 conversion then indicates the slow scavenging of a C-containing surface species beginning to take over. Clearly, such a reaction scenario cannot occur in CO2/Ar + He S CO2/Ar + H2 transient measurements and has, indeed, not been observed (Figure 3a). The presence of kinetic instabilities in the back transients of this reaction on Ni for low H2/CO2 ratios has already been reported.10 The kinetic features as described above were observed in all our buildup and back-transient measurements of the type H2 + He S H2 + CO2/Ar + H2. To demonstrate the time dependence of the different reaction processes as well as the relative extent to which they occur, we studied the influence of the temperature and H2/CO2 ratio, respectively. Buildup and back-transient profiles are plotted as normalized38 CH4 production in Figure 4 for the two selected temperatures, 295 °C (the gray lines) and 360 °C (the black lines), respectively. Obviously, lowering the H2/CO2 ratio causes the local extrema in the buildup stage to develop more clearly. They become clearly separated at a reaction temperature of 360 °C. Conversely, increasing the hydrogen partial pressure tends to merge the extrema. For H2/ CO2 ) 15 and independent of the temperature, the buildup curve seems to run quickly and monotonously into the steady state. It may be concluded that the (fast) hydrogen-assisted pathway on an essentially metallic surface dominates under these conditions. Although the buildup time for this pathway varies only a little with the hydrogen partial pressure, a substantial shift is observed for the pathway involving chemical construction of the active phase. Obviously, the lower the hydrogen partial pressure, the longer the construction takes. It is most likely that oxygen and, possibly, hydroxyl groups are involved in this process. As for the back transients, the general trend is that the overall methane decay time increases with decreasing hydrogen partial pressure. The decay times are longer for the lower reaction
Catalytic CO2 Hydrogenation on Nickel
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1259
Figure 5. Time delay dependence of the features in the methane production buildup and back transients indicated in Figure 4 as a function of the H2/CO2 ratio. Figure 4. Methane buildup (left) and back transients (right) as a function of the H2/CO2 ratio (T ) 295 °C, gray lines - T ) 360 °C, black lines). Dashed lines are depicted to guide the eye, thus following the position of the CH4 production rate peaks as a function of the reactant ratio. P1-P4 labels indicate the corresponding peaks in the transients. The t ) 0 s and t ) 300 s are the reactant pressure transient onset times as obtained from the reference Ar partial pressure for both the buildup and the back transients, respectively.
temperature, 295 °C. Figure 5 has been constructed in an attempt to evaluate the times for the appearance of the relative extrema as a function of the hydrogen partial pressure: see the dashed lines in Figure 4 drawn to guide the eye. If we number the extrema in order of appearance for both the buildup and the back transients (as indicated in Figure 4, peaks P1-P4), we obtain an easily reproducible trend. The decay of the delay times is obviously exponential and does not depend on the reaction temperature (apart from an offset). As the data are plotted after subtraction of t0, which is defined as the time delay at an infinite H2/CO2 ratio and obtained by a fitting procedure, the relationship between the hydrogen partial pressure and the delay times is directly evidenced. Remarkably, the time constants are the same in buildup and back transients for corresponding peaks (P1-P3 and P2-P4). We obtain decay constants of τP1-P3 ) 5.2 ( 0.8 and τP2-P4 ) 3.6 ( 0.3 for P1-P3 and P2-P4, respectively, where we fitted the experimental data with a function of the form A*exp(r/τ), r being the H2/CO2 ratio. This indicates that there is a different dependence of the reaction rate on the chemical potential of the reactants, depending on the reaction rate-limiting step. On the basis of the knowledge about the single molecule reaction mechanism on a model Ni surface,19,20 we suggest the following interpretation. At the beginning of the buildup (the onset of the CO2 transient), the conversion rate increases due to the hydrogen-assisted activation of impinging CO2 molecules, leading to direct conversion into methane. The reaction subsequently slows down because this mechanism causes oxygen to accumulate at the catalyst surface. Possibly an oxygen-containing C1 surface species is formed (a “formate-
derived” species could be a candidate), which may be subsequently hydrogenated with a lower rate. This picture is also compatible with the shape of the back transients. Hence, the observed behavior of the reaction transients is compatible with the coexistence of two parallel CO2 hydrogenation mechanisms, thus resembling the scheme that has been recently proposed on the basis of experiments performed on a single-crystal termination combined with ab initio calculations.20 In the latter case, two competitive hydrogenation mechanisms were identified, yielding a poorly reactive formate species and an active hydrocarboxyl intermediate, which yields CO, respectively. Finally, steady-state values for the amounts of carbon, oxygen, and hydrogen are reported in Figure 6 following time integration of the CTK data. First, it has to be stated that the hydrogen coverage in steady-state conditions is not affected by the starting configuration (i.e., hydrogen or carbon dioxide adsorption). According to Figure 6, the amounts of hydrogen in terms of monolayer (ML) equivalents increase with the conversion, that is, with the hydrogen partial pressure. We conclude that this trend is due to the formation of hydrogen-containing surface intermediates because the C and O coverages are almost constant and of the order of only 1 ML. A picture in which several MLs of H are adsorbed on the metal surface is, therefore, difficult to support. Even if the occurrence of hydrogen-rich surface intermediates is assumed, this result can only be understood by giving way to subsurface diffusion and occupation of bulk Ni sites. The existence of a subsurface hydrogen reservoir may also explain the fact that the CH4 tail in the back transients of the H2 + He f H2 + CO2/Ar experiment is always longer than the H2 decay, indicating that the hydrogenation mechanism is still active in the absence of gas-phase hydrogen. These arguments are also supported by the relatively short H surface residence time at the temperatures adjusted in the experiments. As to the overall carbon and oxygen coverage, we observed that the former is almost constant over the spanned reaction parameters (0.3 ML), whereas the latter is between 0.5 and 1.0 ML. In agreement with previous suggestions,11 we can, therefore,
1260
J. Phys. Chem. C, Vol. 115, No. 4, 2011
Vesselli et al. of poorly reactive, oxygen-containing species accumulating at the surface with time. With regard to future investigations, we emphasize that, in order to obtain further insight into the reaction mechanism and the nature of the stable oxygenated intermediate, spectroscopic techniques under operating reaction conditions should be employed. Acknowledgment. Dedicated to Prof. Alfons Baiker on the occasion of his 65th birthday. E.V. acknowledges financial support from MIUR through PRIN2008 and from Commissariato del Governo di Trieste through Fondo Trieste. We gratefully acknowledge support by Shell Global Solutions. J.S. and A.B. are thankful for support by F.R.I.A. of the FNRS in Belgium. References and Notes
Figure 6. Average surface coverage for carbon, hydrogen, and oxygen under steady-state conditions at the end of the buildup transient as a function of the CO2 conversion efficiency for the four different experiments (H2 + He f H2 + CO2/Ar and CO2/Ar + He f CO2/Ar + H2, T ) 295 °C and T ) 360 °C). The diameter of the open and solid symbols is proportional to the H2/CO2 ratio, which varies from 0.25 up to 15.
conclude that the Ni surface is essentially “metallic” under reaction conditions rather than “oxidized”. Because the CO2 conversion rate shows a linear dependence on the H coverage in Figure 6, whereas the O/C coverage ratio varies from 2 to 3 at an otherwise constant C coverage, we have also obtained evidence that the oxygen removal (via the formation of hydroxyl groups and water) may be the limiting process at very high conversion. Conclusions Carbon dioxide hydrogenation on metallic Ni at 1 bar has been studied by means of an analysis of the chemical transients following abrupt changes in the reactant composition. Carbon dioxide interaction with Ni is strongly influenced by hydrogen coadsorption and coverage effects. The reaction does not simply proceed via the hydrogenation of a stable C species: rather, hydrogen influences the CO2-Ni interaction via a hydrogenassisted CO2 activation mechanism, yielding reactive intermediates with high conversion. Instead, a parallel, slower reaction mechanism seems to involve a “stable” intermediate. In the former case, a complex obtained by direct CO2 hydrogenation is suddenly dissociated and further reacts to produce gaseous methane. In the latter situation, instead, a surface intermediate is obtained (a “formate-derived” species) that, due to its slow hydrogenation, accumulates at the catalyst’s surface. The interplay of these two mechanisms generates a peculiar profile of the buildup and back transients, providing evidence for a rate-determining step that is different from the fast initial CO2 activation and that may be ascribed to the rate-limiting removal
(1) Lackner, K. S. Science 2003, 300, 1377. (2) Song, C. Catal. Today 2006, 115, 2. (3) (a) Ko¨ppel, R. A.; Sto¨cker, C.; Baiker, A. J. Catal. 1998, 179, 515. (b) Wambach, J.; Baiker, A.; Wokaun, A. Phys. Chem. Chem. Phys. 1999, 1, 5071. (c) Baiker, A. Appl. Organomet. Chem. 2000, 14, 751. (4) Krylov, O. V.; Mamedov, A. Kh. Russ. Chem. ReV. 1995, 64, 877. (5) Tamaru, K. AdV. Catal. 1964, 15, 65. (6) Ravindranathan Thampi, K.; Kiwi, J.; Gra¨tzel, M. Nature 1987, 327, 506. (7) Gu¨r, T. M.; Huggins, R. A. Science 1983, 219, 967. (8) Peebles, D. E.; Goodman, D. W.; White, J. M. J. Phys. Chem. 1983, 87, 4378. (9) Fujita, S.-I.; Takezawa, N. Chem. Eng. J. 1997, 68, 63. (10) Fujita, S.-I.; Terenuma, H.; Nakamura, M.; Takezawa, N. Ind. Eng. Chem. Res. 1991, 30, 1146. (11) Men, Y.; Kolb, G.; Zapf, R.; Hessel, V.; Lo¨we, H. Catal. Today 2007, 125, 81. (12) Goodman, D. W.; Kely, R. D.; Madey, T. E.; White, J. M. J. Catal. 1980, 64, 479. (13) Araki, M.; Ponec, V. J. Catal. 1976, 44, 439. (14) Watwe, R. M.; Bengaard, H. S.; Rostrup-Nielsen, J. R.; Dumesic, J. A.; Norskov, J. K. J. Catal. 2000, 189, 16. (15) Bundhoo, A.; Schweicher, J.; Frennet, A.; Kruse, N. J. Phys. Chem. C 2009, 113, 10731. (16) Freund, H.-J.; Roberts, M. Surf. Sci. Rep. 1996, 25, 225. (17) Ding, X.; De Rogatis, L.; Vesselli, E.; Baraldi, A.; Comelli, G.; Rosei, R.; Savio, L.; Vattuone, L.; Rocca, M.; Fornasiero, P.; Ancilotto, F.; Baldereschi, A.; Peressi, M. Phys. ReV. B 2007, 76, 195425. (18) Ding, X.; Pagan, V.; Peressi, M.; Ancilotto, F. Mater. Sci. Eng., C 2007, 27, 1355. (19) Vesselli, E.; De Rogatis, L.; Ding, X.; Baraldi, A.; Savio, L.; Vattuone, L.; Rocca, M.; Fornasiero, P.; Peressi, M.; Baldereschi, A.; Rosei, R.; Comelli, G. J. Am. Chem. Soc. 2008, 130, 11417. (20) Vesselli, E.; Rizzi, M.; De Rogatis, L.; Xunlei, D.; Baraldi, A.; Comelli, G.; Savio, L.; Vattuone, L.; Rocca, M.; Fornasiero, P.; Baldereschi, A.; Peressi, M. J. Phys. Chem. Lett. 2010, 1, 402. (21) Gokhale, A. A.; Dumesic, J. A.; Mavrikakis, M. J. Am. Chem. Soc. 2008, 130, 1402. (22) Hu, Z. M.; Takahashi, K.; Nakatsuji, H. Surf. Sci. 1999, 442, 90. (23) Collins, S. E.; Baltana`s, M. A.; Bonivardi, A. L. J. Catal. 2004, 226, 410. (24) Fujitani, T.; Nakamura, I.; Uchijima, T.; Nakamura, J. Surf. Sci. 1997, 383, 285. (25) Jung, K. D.; Bell, A. T. J. Catal. 2000, 193, 207. (26) Hu, Z. M.; Nakatsuji, H. Chem. Phys. Lett. 1999, 313, 14. (27) Wambach, J.; Illing, G.; Freund, H.-J. Chem. Phys. Lett. 1991, 184, 239. (28) Le Peltier, F.; Chaumette, P.; Saussey, J.; Bettahar, M. M.; Lavalley, J. C. J. Mol. Catal. A 1998, 132, 91. (29) Rasmussen, P. B.; Kazuta, M.; Chorkendorff, I. Surf. Sci. 1994, 318, 267. (30) Nerlov, J.; Chorkendorff, I. Catal. Lett. 1998, 54, 171. (31) Nerlov, J.; Chorkendorff, I. J. Catal. 1999, 181, 271. (32) Nerlov, J.; Sckerl, S.; Wambach, J.; Chorkendorff, I. Appl. Catal., A 2000, 191, 97. (33) Frennet, A.; de Bocarme´, T. V.; Bastin, J.-M.; Kruse, N. J. Phys. Chem. B 2005, 109, 2350. (34) Kruse, N.; Schweicher, J.; Bundhoo, A.; Frennet, A.; de Bocarme´, T. V. Top. Catal. 2008, 11244, 9045. (35) Schweicher, J.; Bundhoo, A.; Frennet, A.; Kruse, N.; Daly, H.; Meunier, F. C. J. Phys. Chem. C 2010, 114, 2248. (36) Frennet, A.; Chitry, V.; Kruse, N. Appl. Catal., A 2002, 229, 273. (37) A monolayer (ML) is defined as the coverage corresponding to one adsorbed atom/molecule per available surface metal (Ni) atom. (38) The CH4 signal has been normalized to the steady-state value to allow direct comparison between different reaction conditions.
JP106551R
