Si3N4

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Reaction kinetics and product selectivity in the oxidation of methane over Pd/Si3N 4 Dezheng Wang b, Franck Monner ~and Claude Mirodatos a "Institut de Recherches sur la Catalyse. CNRS. 2, Avenue Albert Einstein. Villeurbanne Cedex. France bState Key Lab tbr Catalysis. Dalian Institute of Chemical Physics, Chinese Academv of Science. P.O. Box 110, Dalian, 116023 China Kinetic measurements on the oxidation of methane over Pd/Si3N~ under fuel-rich conditions at high temperatures show that, unlike what is usually reported for fuel-lean conditions, methane oxidation is zero order in methane and positive order in oxygen. CO is a primary product at high temperature and high CH4/O 2 ratios, when the surface is covered by carbon rather than by oxygen adspecies. 1. INTRODUCTION The partial oxidation of methane is a potential reaction for the production of synthesis gas. The complete oxidation of methane is utilized in combustion turbines. Pd is well-known and well-studied as a complete combustion catalyst [1-3], but a study of its partial oxidation activity can lead to a better understanding of selectivity in methane oxidation. Most works on partial oxidation have used Ni. Pt and Rh catalysts but van Looij et al. [4] recently reported results on Pd. Most data suggest that these reactions, partial and complete oxidation, proceed on various group 8-10 metals via the same mechanism. This is a Langmuir-Hinshelwood mechanism with the dissociative adsorption of methane and oxygen. However, within the general model, some disagreement exists tbr Pd about the nature of the active surface. Earlier works suggested that the reaction proceeds on Pd oxide surface [1,2]. but more recently the active phase was assumed to be the metal surthce [3,4]. The discrepancy may come from the reaction conditions. High OJCH4 ratios (and relatively low temperatures) used in earlier works whose interest is complete oxidation do not permit the reduction of the oxide. while low O:/CH~ ratios (stoichiometric value and less) lead to a reducing reaction mixture and the active phase turns to be metallic, van Looij et al. [4] have further proposed that the catalyst bed under reaction conditions actually consists of three different surface phases" oxidic, metallic and carbidic. Both van Looij et al. [4] and Lyubovsky and Pfefferle [3] reported that the reaction is about first order in oxygen and zero order in methane and the initial products are CO: and H20 under oxygen-rich conditions. Here, we report results of kinetic measurements carried out under fuel-rich conditions to complement reported results under lean conditions. This approach aims at investigating more precisely the nature of oxygen and methane adsorption and the factor(s) determining the initial selectivity.

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2. EXPERIMENTAL The partial oxidation of methane was studied in a conventional quartz tube reactor from 700 to 900~ and at atmospheric pressure. The tested catalyst was a 1 wt.%-Pd catalyst supported on Si3N4, a thermostable and neutral support used to avoid oxygen or hydroxyl adsorption and spillover [5]. The catalyst was prepared by impregnation and dried at 350i~,C. 26 mg of catalyst was used and the superficial volume of the reactor was 0.14 ml. The catalyst was reduced in hydrogen flow at 400~ for 50 min prior to reaction. There was a possibility of some activity loss at high CH4/02 ratios and kinetic measurements at the different CH4/O2 ratios were bracketed by measurements at the CH4/O2 -- 2 ratio and only data points where the catalyst activity was stable were used. The reaction mixture was made using mass flow controllers and was diluted ca. 1:9 in He to make a total flow of 100 ml/min to avoid reaction heat thermal diffusion problems. Flow rates of methane, oxygen and helium were changed to provide conditions of different partial pressures. The reactor effluent was analyzed by FID and TCD gas chromatography. At the temperatures used in this study, most conversions were not low enough to allow the reactor to be approximated as a differential reactor. For the regression of kinetic parameters, the integral reactor approach was used. Computed methane conversions were obtained by the integration of the plug flow reactor equation to the space time of the methane reactant and parameter estimation was performed using conversion as the regression variable [6]. As the gas mixture was diluted in He, the constant volume reactor equation was used. Power law kinetics was used for the rate equation. Least squares minimization was performed by a Nelder-Mead and simulated annealing algorithm due to Press et al. [7]. 3. R E S U L T S AND D I S C U S S I O N The production of CO+CO2 at different CHJO 2 ratios of fuel-rich mixtures, expressed as conversion of methane, is shown in Figure 1 and Tables 1-4. Water could not be reliably detected because it eluted from the GC as a very broad peak. For all the data shown, except tbr specific cases shown in Table 4, CO is less than 10% of CO+CO2, and if it is assumed that the main reaction is complete combustion, oxygen conversion can be computed with the stoichiometry of CH4 + 2 02. At higher conversions, 02 conversion can also be obtained from the partial pressure (flow rate) used minus the remainder oxygen detected and there were no inconsistencies between this with that computed from the methane conversion. Similar calculations of carbon-containing species showed there was no loss of carbon within experimental accuracy (ca. 5%). Figure 1 shows the results at 718~ where conversions are low enough for the reactor to be approximated as a differential reactor (conversion at the rightmost point may be too high 9 PcH4 = 0.14 atm" oxygen conversion = 20%). Figure 1 as In (rate) versus In (pressure) is an indication of the fit to a power law rate expression. Clearly, the fit is poor when data from the whole range are used with constant orders of reaction. The data are insufficient for detailed kinetic models but they can still be used in power laws to infer qualitative intbrmation. The data were taken three points at a time to fit two parameters (the rate constant and one reaction order), that is, the degree of freedom was one. The other reaction order ([3 in Table 1 and 2, ct in Table 3) was held constant at the indicated value under the tables because it was not a

3563 significant parameter as the initial partial pressure of corresponding reactant was held constant. The exact value at which this reaction order was held constant or allowing it to vary had only a small effect on the regression value of the fitted reaction order and the main effect of using a different value is to change the value of k by a constant factor. In Table 4. three data points were used to fit three parameters with no degree of freedom. Outside of these three data points, methane conversion was ca. 100% on the lean side and deactivation occurred on the fuel-rich side, e.g. the rightmost data point in Table 4 was not used because the catalyst showed deactivation, but the data is included to show the initial selectivity. The estimated reaction orders with respect to power law kinetics are shown in Tables 1-4, for data taken at different temperatures. In each row. xf~,are the computed conversions using the parameters of that row. Also shown are standard deviations, cy, obtained by Monte Carlo simulations of the experiment which assumed that temperature, flow rates and GC fluctuations are normally distributed with standard deviations of 0.5~ 1% of mass controller full scale and l xl 0 -4 mole fraction. The simulated distributions were not Gaussian and two or three deviations were quite large. However, the deviations were still distributed such that +cy are the 68% confidence limits and +_2orare the 95% confidence limits. -11 C"

E -12 (D 0

E r

-

6

-13

t~ c"

-14

I

-4

I

t

-3 In (methane partial pressure), In(atm)

I

-2

Fig. 1. Rate of methane oxidation versus partial pressure at 718 C.

Table 1. Variation of reaction with CH/O2 ratio at 718~ Pcm atm Xc,4 x~, x,~, x,~, x,~,

0.02 0.020 0.020

0.04 0.038 0.032

0.06 0.037 0.040

0.08 0.036 +--

0.038

0.05 7

0.036

0.039

0.031 0.034

Note:"rate = k PCH4a Po2~ mol/min/26 mg cat.,

0.10 0.025 +--

and parameter estimates. 0.14 0.017 +--

Parameter estimates +cy(standard deviation) k=0.085+0.13, ct=1.7+0.25 +-+-k=0.011+0.008. or= 1 . 0 + 0 . 1 8 0.026 <--k=0.0011+0.0011. ot=0.22+0.26 0.026 0.017 k=0.00033+0.0004. c,= -0.30+0.29 Po2 = 0.02 atm, assuming [3 = 1.0

3564 Table 2. Variation PcH4, atm 0.02 Xcm 0.21 xt~, 0.22 x,~,

of reaction with CH4/O,. ratio at 813~ and 0.04 0.06 0.08 0.10 0.14 0.11 0.071 0.050 0.045 0.031 0.11 0.071 ~ ~-~ 0.11

xf~,

0.070

0.050

~--

0.067

0.053

0.044

~

0.052

0.042

0.031

x,~,

parameter estimates. Parameter estimates +o( standard deviation) k=0.0012+0.0012, a = 0.01 +0.25 k=0.00070+0.0011, e~= -0.19+0.32 k=0.0021 +0.0032, c~=0.21 +0.28 k=0.0017+0.0015, a=0.13+0.26

~--

Note: rate = k Pc.fl Po213mol/min/26 mg cat., Po2 = 0.02 atm. assuming 13 = 1.0 Table 3. Variation of reaction with C H / O 2 ratio at 764~ and parameter estimates. Po2- atm 0.02 0.04 0.08 O. 12 Parameter estimates Xcm 0.035 0.091 0.25 0.68 +o(standard deviation) xt~, 0.035 0.092 0.24 +-k=0.0066+0.0073, ,,,,

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

~=1.5+0.21 x,~,

0.091

0.26

0.46

k=0.021 +0.021, 13=1.9+0.25

Note: rate = k PcHfl Po2 0 ml/min/26 mg cat., Pc,4 = 0.08 atm, assuming e~ = 0.0 Table 4. Variation of reaction with CH/O2 Pc,4, atm 0.06 0.08 0.10 Po2- atm 0.06 0.04 0.02 XCH4 0.35 0.14 0.04 Xl~, 0.35 0.14 0.04

Sco 6% 7% 15% Note: rate = k Pc./' Po2~ml/min/26 mg cat.

ratio at 870~ and parameter estimate. 0.11 Parameter estimate 0.01 icy(standard deviation) 0.014 ~--k=0.0033+3.0, = -0.59+0.55, 13=1.6+0.7 30%

Table 1 and Figure 1 show that at 718~ the reaction order in methane, a, is changing from positive to approximately zero, while Table 2 shows that at 813~ ot is close to zero in the range of CHJO2 ratios used. The standard deviations shown in Table 2 are conservative because the data suggest a constant or. When they were analyzed together, they gave a-0.05+0.13. In Tables 3 and 4, the reaction order in oxygen, [3, is consistent with a value somewhat larger than one. This is in contradistinction with the reaction under high O2/CH 4 ratios previously used in the literature, where the reaction order with respect to oxygen is zero and the order with respect to methane is unity [3,4]. One can further deduce general qualitative features from the above results by considering that zero or negative orders suggest competitive adsorption and may provide a qualitative evaluation of the relative rates of adsorption. Thus, since the reaction orders with respect to either oxygen or methane can be zero or unity depending on the partial pressures, oxygen and

3565 methane compete for adsorption sites. This statement implies that their relative rate constants of adsorption are about equal. When the CHJO2 ratio is high, methane adsorption is faster and the surface is largely covered by adsorbed CHx species. Although it is generally known that the catalytic surface can be covered by carbon (the ultimate state being methane cracking in the absence of oxygen), it was never inferred that methane adsorption can be faster than oxygen adsorption, which we do here on the basis of a zero reaction order. Table 4 also shows an interesting point about selectivity. Sco, which is the percentage of CO in the CO:+CO products, increases with increasing CHJO2 ratio at high temperature. Since at the same time oxygen conversions are almost unchanged and methane conversions decrease, most of this CO is a primary product (direct route) and not a secondary, product (indirect route) of a consecutive reaction of CO: or H_~O retbrming of methane or reverse water-gas shift. This statement apparently contradicts the tact that Pd is generally an excellent complete oxidation catalyst [1-3]. Moreover, van Looij et al. [4] have shown that under stoichiometric partial oxidation conditions, the primary products are CO2 and H20. A mechanism can be constructed based on recent investigations on noble metal gauzes (Pt, Ru) at very, short contact time in TAP reactors [8-11] in which CO had been seen as a primary, product, which would account tbr the apparent discrepancy with the indirect route proposed by van Looij et al. [4] and Buyevskaya et al. [11]. Most partial/complete oxidation results can be summarized as" i) under conditions of a limited conversion of oxygen and with a rather large contact time (low temperature and low CHJO2 ratio). Sco is low (complete combustion conditions), ii) under conditions of very short contact times. Sco is large (direct conversion to syngas), iii) under conditions of a complete conversion of oxygen and a large contact time (low temperature and stoichiometric CHJO: ratio). Sco will be large also but mostly due to the twosteps indirect route of methane combustion then H:0-CO_, reforming to syngas. In this study, under conditions like (i). that is. with a conventional laboratory quartz tube reactor. 100 cc/min flow and limited conversions, but with the difference that high CHJO: ratios were used. a moderately high Sco was obtained. At lower temperature, the CHJO2 ratio has to be very high before a highselectivity to CO is seen: roughly, Sco is >30% tbr CH4/O2 ratio > 50 at 700~ C H J O 2 ratio > 20 at 800~ and C H J O 2 ratio > 10 at 900~ (these conditions lead to deactivation of the catalyst). Previously. a high Sco had only been obtained under conditions (ii). Conditions (ii) and the experiments here seem superficially different, but it can be argued that they have common factors which are high temperature and an adsorbed phase with a high CH,(ads)/O(ads) ratio. (ii) seems genuinely different from (i) and (iii). If the difference between (i) and (ii) is different contact times, their different selectivities is most likely due to the consequent different temperatures. At short contact times (ca. milliseconds) under TAP conditions or at the very high flow rates of Hickman and Schmidt [12] and Heitnes Hofstadt et al. [13], reaction temperatures are high. Since methane adsorption and CO and oxygen desorption are activated whereas oxygen adsorption is not, the rates of methane adsorption, and CO and oxygen desorption are increased relative to oxygen adsorption and can lead to conditions where the direct route predominates. In this direct route, the partial oxidation scheme can be described as a competition between the desorption of CO and its further oxidation to CO2 which can be diffusion-controlled depending on relative carbon (CH0, CO, OH and oxygen concentrations [10,14]. This is in contrast to the case of large contact times (order of seconds) in most plug-flow reactors (conditions (i)), and (conditions

3566 (iii)) where the first part of the reactor catalyzes complete combustion and the second part, depleted in oxygen, is a dry and steam reformer, as described by Prettre et al. [15]. The conditions in this work that can produce CO by the direct route also lead to deactivation of the catalyst. This deactivation is not a very rapid process, occurring on the order of hours while at extremely high CH4/O2 ratios, the catalyst deactivates more rapidly (about 30,--50% in one hour). The activity was usually stable for CH4/O, ratios less than 7. The slow loss in activity for higher CH4/O 2 ratios could be reversed by oxidation at 600~ for 30 min. It is likely that islands of adsorbed carbide are formed which slowly convert to graphite (filaments and/or encapsulating carbon), as observed by van Looij et al. [4]. Thus, a condition for producing CO as a primary product appears to be a surface covered with carbon (prominent methane adsorption) at the expense of surface oxygen. 4. CONCLUSION Methane oxidation on Pd under fuel-rich conditions has reaction orders in methane and oxygen of 0 and > 1, respectively. Methane and oxygen compete for sites and their adsorption ' rate constants do not differ much at 700---900~ CO is a primary product at high temperature and CHjO, ratio, where the surface is mainly covered with carbonaceous species rather than with surface oxygen. Acknowledgement" D. Wang thanks CNRS for his stay in France, and the Chinese National Science Foundation (grant no. 29873043) for support. Part of the work was supported by the CNRS programme "Catalyse et Catalyseurs pour l'Industrie et l'Environnement". REFERENCES

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