React Kinet Catal Lett (2009) 98:19–26 DOI 10.1007/s11144-009-0062-9
Partial oxidation of methane into synthesis gas over a Pt-supported complex fluorite-like oxide: one-channel studies in realistic feeds Natalya N. Sazonova Æ Vladislav A. Sadykov Æ Aleksey S. Bobin Æ Svetlana A. Pokrovskaya Æ Elena L. Gubanova Æ Claude Mirodatos
Received: 16 December 2008 / Accepted: 20 May 2009 / Published online: 11 September 2009 Ó Akade´miai Kiado´, Budapest, Hungary 2009
Abstract Partial oxidation of methane (POM) into synthesis gas over a single channel of monolithic catalyst 1.4 wt% Pt/Gd0.3Ce0.35Zr0.35Ox/a-Al2O3 was investigated. The effect of the catalyst pretreatment and of the process parameters (temperature, contact time, feed composition) on the catalytic activity and synthesis gas (syngas) selectivity was studied at conditions close to realistic ones with a minimum impact of the mass and heat transfer phenomena. Experimental data are in favor of the direct route of the syngas formation via the methane pyrolysis— selective oxidation sequence. Keywords Methane Syngas Flow system Platinum Oxygen storage material Single channel
Introduction Catalytic partial oxidation of methane (POM) into syngas at short contact times is now considered as a viable alternative to the traditional steam reforming process [1, 2]. This process is attractive for the design of compact syngas generators equipped with structured catalysts characterized by a high surface to volume ratio and a low pressure drop. Hence, a lot of attention of different scientific groups has been paid to N. N. Sazonova (&) V. A. Sadykov A. S. Bobin S. A. Pokrovskaya E. L. Gubanova Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia e-mail:
[email protected] V. A. Sadykov A. S. Bobin S. A. Pokrovskaya Novosibirsk State University, Novosibirsk 630090, Russia C. Mirodatos Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, Villeurbanne Cedex, France
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the development of monolithic catalysts with the required efficiency and stability [3–6]. Fluorite-like oxides (doped ceria or ceria–zirconia) promoted with Pt group metals were shown to be promising active components for these catalysts due to their high oxygen storage capacity, thermal stability and strong metal-support interaction stabilizing the metal dispersion and preventing the carbon build-up [3, 7–12]. In our previous publications [7–9, 11, 12], Pt-supported powdered CeO2– ZrO2 mixed oxides doped with La, Pr or Gd have been studied by combining diffraction and spectroscopic techniques, and the effects of their structural and surface features on the surface/lattice oxygen mobility and reactivity that determine their activity in POM in the diluted feeds have been discussed in detail. However, for realistic feeds with a high content of CH4 and O2, the main features of POM and stability of these active components have not yet been verified due to problems caused by the temperature gradient usually developed within the packed layers of granulated catalysts or monolithic pieces [1–6]. The effects of the mass and heat transfer in POM studies with realistic feeds were shown to be minimized by supporting a thin layer of active component either on alumina tubular supports (the so-called annular reactor [13]) or on a single channel of a corundum monolithic substrate [14]. In this work, results of the application of this experimental approach are reported for the first time for the case of Pt-supported complex fluorite-like oxides exemplified here by the 1.4 wt% Pt/Gd0.3Ce0.35Zr0.35Ox active component. Apart from checking the applicability of this experimental approach in realistic feeds for such catalysts, this paper aims to verify the validity of conclusions based on results of high-vacuum TAP studies on the practical realization of the direct pyrolysis—selective oxidation mechanism of POM [15].
Experimental Catalyst preparation A piece of a hollow thin-walled triangular prismatic corundum substrate (wall thickness 0.2 mm, inner triangle side 2.33 mm, length 10 mm) used in this work was cut from an a-Al2O3 honeycomb monolith annealed at 1,300 °C (specific surface area 3 m2/g). A layer of a Gd0.3Ce0.35Zr0.35Ox complex oxide prepared via the Pechini route [8] was supported on this substrate by washcoating with the water suspension made by ultrasonically dispersing the oxide powder with the addition of peptizers and surfactants. Several consecutive impregnations (typically 4) were required to attain the active component content of 7–10 wt% (the porous layer thickness ca. 10 microns). After each impregnation, the samples were dried and calcined at 900 °C in air. Pt (1.4 wt%) was supported by the incipient wetness impregnation with an H2PtC6 solution. Catalyst testing The POM tests were carried out in a plug-flow quartz reactor with the inner diameter of 6 mm. The space between the reactor walls and the catalyst channel was sealed up
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with a-Al2O3 fibers to provide the gas flow only through the channel. The catalyst was pretreated for 1 h in the flow of either O2 at 700 °C (oxidizing pretreatment) or 30% H2 in He at 900 °C (reducing pretreatment). The experiments were carried out at atmospheric pressure. The reaction mixture (7–20 vol% CH4, CH4:O2 = 2, N2 to balance) was fed with the flow rate in the range of 5.6–18.0 L/h corresponding to the contact times 4.7–15.0 ms as estimated from the flow rate and the volume of channel walls. Before the experiments, the fresh catalyst was kept in the feed at 700 °C for 3 h to attain a steady state with reproducible catalytic properties. The inlet and outlet temperatures of the catalyst channel were monitored during the tests. Blank experiments with both the empty reactor and the reactor loaded with corundum substrate without the active component verified that homogeneous reactions did not occur under studied conditions. The reagents and reaction products were analyzed with a gas chromatograph (Tsvet-500) as well as with the on-line IR absorbance, electrochemical and polarographic gas sensors for different components. In all the experiments, the carbon balance was close to 100 ± 5%.
Results and discussion In all the experiments with a broad variation of contact time, temperature and feed composition, the oxygen conversion was complete. A relatively small (20–30 °C) temperature gradient along the channel length was observed due to efficient heat dissipation by radiation and by thermal equilibration with the corundum substrate [13]. The front edge temperature was higher than the rear one, which is a typical feature of POM due to exothermal oxygen consumption within a narrow inlet part of the catalytic layer/channel [16]. Hence, the outlet temperatures were used to present the temperature dependence of the methane conversion (Fig. 1). As follows from this 20
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Toutlet ,°C Fig. 1 Methane conversion (1, 2) and product sum CO ? H2 (3, 4) versus the outlet feed temperature (Toutlet) for the oxidized (1, 3) and reduced (2, 4) Pt/GdCeZrO catalyst. Feed composition: 7% CH4 ? 3.5% O2 in N2. Contact time: 15 ms. Equilibrium values of CH4 conversion are shown by dashdot line
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figure, the catalytic activity depends strongly on the catalyst pretreatment: the oxidized catalyst is more active and selective than the reduced one, the difference being more pronounced at higher temperatures. The observed difference can be explained by the variation of the catalyst state due to the pretreatment. The hydrogen pretreatment reduces cationic Pt2? species stabilized due to strong interaction with the support [8, 17]. As the result, the size of the Pt particles increases, thus decreasing the Pt-support interface. Moreover, binding the carbonates and water with coordinatively unsaturated surface cations is expected to be stronger for the reduced surface than for the oxidized one, which hampers the surface/near-surface mobility of oxygen and oxygen-containing species such as hydroxyls and hydroxocarbonates. Since the catalytic activity of Pt-supported doped ceria–zirconia oxides correlates with both the surface density of cationic Pt species (responsible for the activation of CH4 molecules) and surface/near-surface oxygen mobility (responsible for the supply of the oxygen-containing species to the Pt-support interface to gasify the adsorbed CHx species) [15], the observed effect of the pretreatment agrees with the general bifunctional route of POM on these systems. Hence, all the experiments presented below were carried out with the oxidized catalyst. Within the whole temperature range studied, both the methane conversion and the product sum CO ? H2 were significantly lower than the equilibrium values and increased with the temperature (Fig. 1). At the contact time of 15 ms and temperatures below 700 °C, the H2/CO ratio is about 4. With the temperature increase, this ratio approaches the value of 2 corresponding to the stoichiometry of the POM reaction. First of all, this suggests that at low temperatures, the direct/ reverse water gas shift reactions are too slow to provide equilibration between products of deep and partial oxidation. Second, such a high ratio H2/CO of ca. 4 could not be achieved even if the primary process of methane combustion is accompanied by the steam reforming reaction, which yields the H2/CO ratio of 3. Hence, this result seems to reflect some specific features of the POM mechanism, namely, the fast pyrolysis of methane on the Pt sites with the formation of hydrogen and adsorbed CHx species. Due to the rather high residence time at low temperatures, CHx species are preferentially oxidized in the presence of gas phase oxygen into CO2 and H2O, thus increasing the H2/CO ratio in the products. With the temperature increase, the acceleration of endothermic reactions of steam and dry reforming approaches the H2/CO ratio to the equilibrium values. Figs. 2, 3, and 4 present the dependences of the methane conversion and H2/CO selectivities on the contact time for different feed compositions. For the most diluted feed, the methane conversion increases with the contact time (Fig. 2). This dependence is satisfactorily described by the integral first-order rate equation for the plug-flow reactor (not shown for brevity), which agrees well with the same conclusions for powdered active components [9]. The apparent first-order rate constants of the methane consumption were estimated for the experimental points with the similar concentrations of other components in the most diluted reaction mixture. Their temperature dependence gives the effective activation energy of ca. 16 kcal mol-1. This value is rather high, suggesting small (if any) effects of heat and mass transfer on the performance of the catalyst channel in studied conditions, which agrees with the similar conclusions for the annular reactor [13].
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Fig. 2 Methane conversion versus contact time at 750 °C for various feed compositions: 7% CH4 (1), 12% CH4 (2), 20% CH4 (3). CH4/O2 = 2. N2 balance
For the highly concentrated (20 vol% CH4) feed, the dependence of the conversion on contact time is weak (Fig. 2). With due regard for a decline in the CO selectivity (Fig. 3) and the concomitant increase in the H2 selectivity (Fig. 4) with contact time, this phenomenon suggests a larger contribution of at least steam reforming at longer contact times. This can be explained by the consumption of oxygen within a narrower inlet part of the channel at longer contact times (hence, a bigger oxygen consumption for deep oxidation products leading to a decrease in the CH4 conversion via selective oxidation route). In the part of the channel where the gas-phase oxygen is absent, the active component is more reduced at the longer 100
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Fig. 3 CO selectivity versus contact time at 750 °C for various feed compositions: 7% CH4 (1), 12% CH4 (2), 20% CH4 (3). CH4/O2 = 2. N2 balance
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Fig. 4 H2 selectivity vs. contact time at 750 °C for various feed compositions: 7% CH4 (1), 12% CH4 (2), 20% CH4 (3). CH4/O2 = 2. N2 balance
contact times and at the higher CH4 concentrations in the feed, thus facilitating the activation of such mild oxidants as H2O, and, hence, providing the CH4 conversion via the steam reforming route. Since POM in general proceeds faster than methane steam reforming [18], in certain ranges of operation parameters such a variation of the state of the active component along the channel could provide the weak dependence of the CH4 conversion on the contact time. The CO selectivity decreases with the contact time at the higher concentrations of methane in the feed and does not change significantly for the most diluted feed (Fig. 3). The most pronounced difference in the CO selectivity for various feed compositions is observed at the shortest (4.7 ms) contact time. For consecutive reactions, a decline in the product selectivity with the contact time is usually explained by the formation of a product in the fast primary reaction with its subsequent consumption in slower secondary reactions. In our case, the highest CO selectivity at short contact times for the most concentrated feed agrees with the conclusion on the syngas formation in the primary reaction of the methane pyrolysis on partially oxidized Pt sites (vide supra). Respectively, the decline in the CO selectivity with the contact time can be explained by the CO consumption in secondary reactions, such as oxidation with molecular oxygen and water gas shift reaction (WGSR). This agrees well with approaching the WGSR product to the equilibrium value with an increase in the contact time (not shown for brevity). In contrast to CO, selectivity with respect to H2 increases with the contact time at 750 °C for all the feeds (Fig. 4). This implies that CO and H2 are formed via different routes. With due regard for a higher H2 selectivity at the lowest temperatures (vide supra), these results can be tentatively explained by the increase in the adsorbed oxygen reactivity on Pt and/or by the increase in the surface/near surface oxygen mobility in the support with the temperature increase. Therefore, at the highest temperatures used in our experiments, the primary route of CH4 transformation on partially oxidized Pt appears to include the formation of CO and
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hydroxyls, the latter being subsequently desorbed as H2O [19, 20]. Hence, the increase in the H2 selectivity with the contact time is explained by secondary reactions of water gas shift (agrees with the decline in the CO selectivity) and steam reforming of methane. Conclusions POM in conditions close to the real ones has been studied for a single channel of a monolithic catalyst with the supported 1.4 wt% Pt/Gd0.3Ce0.35Zr0.35Ox active component. The oxidized catalyst was shown to be more active and selective than the pre-reduced one, which demonstrates a positive impact of the Pt-support interaction and oxygen mobility in the support on the POM performance. The presented data agree in general with a mechanism that includes the direct route of the syngas formation via the methane pyrolysis—selective oxidation sequence revealed by high-vacuum TAP studies for this type of the active component [15]. An analysis of the effect of the process parameters (temperature, contact time, feed composition) on the methane conversion and product selectivities revealed that in the studied conditions, the primary process of the CH4 transformation on partially oxidized Pt proceeds with the somewhat different products distribution as dependent upon the temperature. Thus, lower temperatures favor a higher H2/CO ratio mainly due to combustion of CHx species at the Pt-support interface. At higher temperatures, the primary process of the CH4 transformation produces syngas with a low H2/CO ratio due to a higher reactivity of oxygen species adsorbed on Pt. At the longer contact times and higher temperatures, the syngas yield and product selectivity are apparently strongly affected by secondary reactions of the methane steam reforming and the direct/reverse water gas shift reactions. Acknowledgements This work was carried out in frames of Associated Russian–French laboratory on catalysis. Support by RFBR–CNRS Project 05-03-34761 is gratefully acknowledged.
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