SiO2

SiO2

NATURAL GAS CONVERSIONV Studies in Surface Science and Catalysis, Vol. 119 A. Parmalianaet al. (Editors) o 1998 Elsevier Science B.V. All rights reser...

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NATURAL GAS CONVERSIONV Studies in Surface Science and Catalysis, Vol. 119 A. Parmalianaet al. (Editors) o 1998 Elsevier Science B.V. All rights reserved.

155

M e c h a n i s t i c insights in the C O h y d r o g e n a t i o n reaction over N i / S i Q C. Marquez-Alvarez, G.A. Martin, and C. Mirodatos Institut de Recherches sur la Catalyse, CNRS. 2 Av. Albert Einstein, F-69626 Villeurbanne Cedex, France.

Steady-state isotopic transient kinetic analysis (SSITKA) is used to determine the amount of adsorbed CO, intermediates CHx and x value during the course of the methanation reaction catalysed by Ni/SiO2. The results support a mechanism in which CO dissociative adsorption is followed by a stepwise hydrogenation of carbon, the hydrogenation of CH into CH2 being the rate limiting step.

1. I N T R O D U C T I O N In two recent publications [1,2], our group presented a thorough characterisation of a Ni/SiO2 catalyst at different stages of the methanation reaction under a CO+2H2 mixture, as well as a detailed kinetic study, performed by means of m situ transient techniques: SSITKA (steady-state isotopic transient kinetic analysis) and DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). Characterisation by hydrogen chemisorption, magnetic measurements, electron microscopy, temperature programmed hydrogenation and DRIFTS browsed direct evidence that, in contact with the reacting mixture, after an initial period of carbon deposition, fast sintering and particle smoothing via nickel carbonyl transfer, leading to-a preferential growth of Ni(111) planes, a surface nickel carbide monolayer was developed, with a stoichiometry Ni2.sC. This carbide monolayer was shown to constitute a reservoir of reacting intermediates. A kinetic ensemble model [3] well fitted the kinetic data obtained under steady-state conditions at temperatures between 230 and 350~ following the isotopic switch 13CO+H2/12CO+H2. According to this model, and from DRIFTS and H2/D2 exchange results, it was concluded that the rate of hydrogenation is controlled by the probability for a hydrogen molecule to collide with an active site formed of one to two adjacent Ni atoms free from adsorbed CO. Several CO adspecies were identified by DRIFTS under reaction conditions, largely coveting the metal surface, with an average stoichiometry of CO/2Nis, the concentration of active sites thus being statistically determined by the CO coverage. Carbon atoms belonging to the carbidic layer associated to the active site are hydrogenated by hydrogen activated on the free Ni atoms. The regeneration of the carbidic layer is in turn ensured by CO dissociation after methane desorption. That study allowed us to experimentally determine important parameters which validate the proposed mechanism, such as the coverage of reaction intermediates, CHx, as a function of temperature and the stoichiometry of adsorbed species, CO and CHx. However, several aspects were not fully clarified concerning the exact rate limiting step and nature of the so called CHx

156 species. In the present paper, the SSITKA study is extended and the H2/D2 kinetic isotope effect analysed in order to get a more advanced description of the reaction mechanism.

2. EXPERIMENTAL The Ni/SiO2 catalyst used in the present study was described in previous papers [ 1-3]. The precursor (20 wt.% Ni) was obtained by contacting silica (Aerosil Degussa, 200 m2g~) with a solution of nickel nitrate hexamine. The solid was dried, then crushed to powder and reduced at 650~ for 15 h at 2~ min~ heating rate in a hydrogen flow (5 1 h~). The average Ni particle size was measured by hydrogen chemisorption, magnetic methods and electron microscopy and was found to be 4.2 run after reduction with a good agreement between the three techniques [1]. The isotopic transient experiments under steady state conditions were carried out by changing rapidly the composition of the feeding mixture from 12CO+2H2to ~2CO+2D2 or from 13CO+2D2 to 12CO+2D2 and vice versa at the inlet of the microreactor. Before the series of switches was introduced, a pseudo steady-state was attained by contacting the catalyst with a CO+2H2 mixture at 230~ for 4h. The conversion level was maintained below 10% at any temperature to ensure differential and isothermal conditions. The abrupt switches were obtained with an automatic four-way valve located just before the reactor. The signal distortion induced by the non-catalytic system (tubes, reactor...) was followed by the transient signal of a helium trace introduced in one of the feeds. The origin of the time scale was chosen as the time at which the inert tracer response starts. The gas composition at the reactor exit was continuously analysed with a VG-quadrupole mass spectrometer, and occasionally by gas chromatography using TCD and FID detectors. Details on the technique are reported in [4].

3. RESULTS

3.1. Inverse H/D isotope effect Under the experimental conditions used in this study, i.e., a hydrogen to carbon monoxide ratio of 2 and temperature between 230 and 280~ an inverse isotope effect is observed for the methanation reaction. The rate of methane formation with H2 as a reactant (rn) is smaller than that observed using D2 (rD) by a factor 0.6. This isotope effect is nearly constant in the temperature range 230-280~ (Figure 1). However, as expected, the effect is less pronounced at higher temperatures, the ratio rn/rD being c.a. 0.8 at 600~ [2]. Similar effects have already been reported for CO hydrogenation on different metals [5].

3.2. Steady-state isotopic transients a3CO+2D2/~2CO+2D2

The transient responses of 13CO, 13CD4 and He, following the stepwise switch from 13CO+2D2(He) to ~2CO+2D2 have been obtained in the temperature range 250-280~ and compared to the 13CO+2H2/~2CO+2H2 transient results reported elsewhere [2]. In both cases, small delays are observed in the CO decay with respect to the inert tracer (He), indicating the presence of a reservoir of adsorbed CO in equilibrium with CO in the gas phase during the course of the reaction [2,6]. The accumulation of active intermediates giving CD4 (referred to as CDx) is also revealed by significant delays in the 13CD4decay response. The experimental delays (Zco, zc~) and the corresponding steady-state concentrations (Nco, NcDx) are shown in Table 1 for several temperatures.

157 Table 1 Time delay (,), concentration (N) and coverage (0) of CO and CDx intermediates determined from SSITKA experiments followin~ the switch 13CO+2D2/12CO+2D2. T (~

Zco (s)

Nco (mmol g-l)

ZCDx (s)

NCDx (mmol g~)

0co (ML)

0CDx (ML)

250 270 280

1.03 0.88 0.85

0.182 0.154 0.146

38.4 22.6 15.1

0.166 0.233 0.244

0.70 0.59 0.56

0.29 0.40 0.42

From Nco and N c ~ values, coverages are calculated by estimating the average stoichiometries of the adsorbed species. For this purpose, we make use of the rate equation (1) written according to the proposed ensemble model [2]: (1)

r = Ko exp (-Eo/RT) PH2 (1-0co) w

This rate equation contains the term (1-0co) w that represents the concentration of active sites, i.e., the fraction of the Ni surface occupied by reacting intermediates and, therefore, it equals 0CDx. Then, if A and W are the stoichiometries NiJCO and NiJCDx, respectively, it can be written: NCD~= (NiaNV) ( 1-A Nco/Nis)w

(2)

where, Nis is close to 3.6 10 20 surface Ni atoms per gram of catalyst [ 1]. Data in Table 1 well fit eq. (2), giving A=2.3 and W=I.0, in good agreement with stoichiometries calculated from the 13CO+2H2/12CO+2H2 transients (NiJCO=2.1 and NiJCHx=I.2, respectively [2]). The calculated coverages, in monolayer (ML), are shown in Table 1 and plotted in Figure 2 together with those of CO and CHx (from CO+H2 transients in ref. [2]). 1.0

1.0

0.8 r

0.6 ~

0.4-

0 0

0.2 0.0

[] .... Q ..........................................................

..-O ...........................

0.8

rd~ ~ ..............

0

.....

o

o

.....

..............

o~/o~ 230 240 250 2~;0'2"}0'280 Temperature (~

Figure 1. H/D isotope effect on reaction rate (r) and intermediate species coverage (0).

~,

0

0.6

......... rt ..... o'"

0.4 o

......... A...../k

..........2Ox

0.2 0.0

1.6

1.7

1.8

1.9

2.0

1000/T (K-1) Figure 2. CO, CHx and CDx steady-state coverages under CO+2H2 (full symbols) and CO+2D2 (open symbols) reactions.

158 As shown in Figure 2, at any temperature the intermediate species coverage obtained in the presence of D2 is higher and, correspondingly, the CO coverage smaller, than those obtained with HE. This isotope effect on the steady-state concentration of intermediates slightly diminishes (0Cnx/0CDx ratio increases) at increasing temperatures (Figure 1). For the CO+2D2 reaction, the whole carbide monolayer participates in the reaction at temperatures higher than 270~ as at these temperatures 0czx stabilises at c.a. 0.4 ML, which corresponds to the measured C/Nis ratio under the reaction conditions [ 1]. For the CO+2H2 reaction, that state is not reached but at temperatures higher than 350~ 3.3. Steady-state isotopic transients CO+2H,JCO+2D2 The transient response of hydrogen and methane molecules has been followed when switching from H2 to D2 in the reaction mixture at 250~ As it has been pointed out, a H/D isotope effect exists under this conditions, that provides different reaction rates (rH=2.8, rD=4.3 gmol s1 g-l) and coverages of intermediate species when changing between both gases. Therefore, this switch does not leave the steady-state strictly unchanged. For this reason, a sufficiently low temperature (250~ was chosen in order to minimize the changes in the steady-state, while keeping the conversion level high enough for the products composition to be accurately analysed. The normalized transient responses of methane molecules (CH4.,D,, with n=0,1,2,3,4), corresponding to fragments of m/e=16-20, are shown in Figure 3. The curves have been corrected for the contribution of H20, HDO and D20 fragments to the signal intensity. The fragmentation patterns of all the methane molecules were also used to correct every main fragment signal intensity for heavier methane fragments contributions. The fragmentation patterns of CI-I4 and CD4 where obtained experimentally and those of CH3D, CH2D2 and CHD3 estimated by interpolation. It can be seen in Figure 3 that a sequential transient from CH4 to CD4 is produced step by step, following the switch from H2 to D2, except for CH3D and CD4 species, that start to appear at the same time. As for the hydrogen molecules, a transient production of HD is observed that reveals the fast dissociative adsorption/desorption equilibrium established between the gaseous hydrogen and the metallic surface [2].

4. DISCUSSION The set of isotopic transients 13CO+2H2/12CO+2H2 and 13CO+2D2/12CO+2D2have allowed us to determine the steady-state concentration of intermediates CHx (or CDx) during the methanation reaction on Ni/SiO2. This study has verified experimentally that the concentration of those intermediates increases in the presence of D2, as shown in Figure 2. This is in agreement with a higher D2 adsorption coefficient on Ni with respect to H2 [7], as the hydrogenation steps will be favoured in the presence of D2 and the concentration of the corresponding reaction intermediate increased [8]. This increased concentration of reaction intermediates can explain the observed inverse isotope effect, according to Ozaki [9] who proposed that isotope effects could be due both to a kinetic effect on the rate-determining step and to a thermodynamic effect on the concentration of an intermediate. Our results confirm the calculations made by van Nisselrooij et al. [10] who estimated by statistical thermodynamics the rate and equilibrium constants isotope effect for a mechanism in which the dissociative adsorption of carbon monoxide and

159 hydrogen is followed by the hydrogenation of adsorbed oxygen and carbon, and predicted a 0CH/0Ct~ ratio smaller than 1. They successfully reproduced their data on the kinetic isotope effect and proposed the hydrogenation of CH species as the rate limiting step. The isotopic transient CO+2HJCO+2D2 results have shown that, under methanation reaction conditions, a fast equilibrium of hydrogen dissociative adsorption takes place, as deduced from the transient formation of HD. Also, all the methane molecules, from CH4 to CD4, are produced when replacing H2 by D2. Both results support the mechanism of CO dissociation followed by a stepwise hydrogenation of surface carbon. We have calculated the binomial distribution of CHz.,D, molecules by taking the probability p for a H atom to be present in a certain methane molecule equal to the H/H+D ratio in the ensemble of H2.iDi molecules at any time, and the probability for D atoms as 1-p. Such a distribution would be expected in the case that the hydrogenation steps (as well as the hydrogen dissociative adsorption equilibrium) were fast and the CO dissociation, the rate limiting step. The change of this H/H+D ratio with time (Figure 4, dotted line) has been calculated from the H2, HD, and D2 distribution obtained after H2 was substituted by D2 in the reaction mixture at 250~ (see reference [2]). The calculated CH4.nD, binomial distribution (not shown for brevity) reveals a sequential deuteration from CH4 to CD4, all the CH4-nDn transients being regularly spaced in time. With respect to these transients, the experimental data reported in Figure 3 show a delay in time and a quite lower CHD3 intensity due to its simultaneous appearance with CD4. This disagreement between the model and the experimental data rules out that CO hydrogenation is the rate determining step, and therefore reinforces the previous conclusion that the rate is controlled by the hydrogenation of carbon adspecies. 1.0 0.8

0.8

8 .~ 0.6

0.6

i

0.4 0.2 0

3 0

.

10

0

~ 20

0.2..

30

Time (s) Figure 3. Normalized transient responses of methane molecules following the switch CO+2HjCO+2D2 at 250~

0.0

9

0

1'0

,

,

20

30

Time (s) Figure 4. H/H+D atomic ratio in hydrogen (dotted line) and methane (full line) molecules for same switch as in Figure 3.

Further arguments in favour of the proposed mechanism can be found by comparing the H/H+D ratio in the ensemble of CH4-nDn curves with that of H2-iDi molecules (Figure 4, full and dotted lines, respectively). Methane being produced by hydrogenation of a pool of CHx moieties, the delay observed in the H/H+D curve for methane species reveals the accumulation of H in that CH.~ pool. The mean time for the substitution of those H atoms by D, given by the area between both curves in Figure 4, is XH= 2.3 S. From this "in, the amount of H accumulated as CHx species at 250~ is calculated as NH = 1.6-2.4 1019 H atoms per gram of catalyst (the uncertainty arising from the slight change in the methanation rate when replacing H2 by D2). At

160 this temperature, by means of 13CO+2H2/12CO+2H2 transients, the amount of CHx species per gram of catalyst was determined [2] as Ncrt, = 3 1019. Therefore, the average value of x for the pool of CHx species lies between 0.5 and 0.8 hydrogen atoms per carbon atom. This result shows that the CH is the most abundant surface intermediate, which in turn indicates that the hydrogenation of CH species is the rate limiting step, in agreement with the model developed by van Nisselrooij et al. [ 10].

5. CONCLUSION The use of isotopic transient experiments has driven to a direct determination of the concentration of reaction intermediates and adsorbed (spectator) CO species under the steadystate methanation reaction catalysed by Ni/SiO2. Moreover, it has been concluded that CH species are the most abundant surface intermediates. The results support a mechanism of CO dissociative adsorption, followed by a stepwise hydrogenation of carbon, in which the hydrogenation of CH into CH2 is the rate limiting step. Modelling of the transient substitution of H by D in the produced methane is at present being carried out in order to determine the kinetic constants for the elemental steps and further validate the proposed mechanism.

ACKNOWLEDGEMENTS CMA acknowledges a postdoctoral fellowship granted by the Spanish Ministerio de Educaci6n y Ciencia.

REFERENCES 1. 2. 3. 4.

5.

6. 7. 8. 9. 10.

M. Agnelli, M. Kolb and C. Mirodatos, J. Catal., 148 (1994) 9. Agnelli, H.M. Swaan, C. M/lrquez-Alvarez, G.A. Martin and C. Mirodatos, J. Catal., in press. J.A. Dalmon and G.A. Martin, J. Catal., 84 (1983) 45. C. Mirodatos, J. Phys. Chem. 90 (1986) 481; Catal. Today, 9 (1991) 83; in Catalyst Characterization, B. Imelik and J.C. Vedrine (eds.), p. 651. Plenum Press, New York, 1994. L. Luytens and J.C. Jungers, Bull. Soc. Chim. Belg., 54 (1945) 303; M.M. Sakharov and E.S. Dokukina, Kinet. Catal., 2 (1961) 639; C.S. KeUner and A.T. Bell, J. Catal., 67 (1981) 175; Y. Kobori, S. Naito, T. Onishi and K. Tamaru, J. Chem. Soc. Chem. Commun. (1981) 92; T. Mori, H. Masuda, H. Imai, A. Miyamoto, S. Baba and Y. Murakami, J. Phys. Chem., 86 (1982) 2753. J.T. Gleaves, J.R. Ebner and T.C. Kuechler, Catal. Rev.-Sci. Eng., 30 (1988) 49. T.P. Wilson, J. Catal., 60 (1979) 167 G. Henrici-Oliv6 and S. Oliv6, The chemistry of the hydrogenation of carbon monoxide, Springer-Verlag, Berlin, 1984. A. Ozaki, Isotopic studies of heterogeneous catalysis, Academic Press, New York, 1977. P.F.M.T. van Nisselrooij, J.A.M. Luttikholt, R.Z.C. van Meerten, M.H.J.M. de Croon and J.W.E. Coenen, Appl. Catal., 6 (1983) 271.