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Roles of desorbed radicals and reaction products during the oxidation of methane using a nickel mesh catalyst
Applied Catalysis A: General 258 (2004) 63–71
Roles of desorbed radicals and reaction products during the oxidation of methane using a nickel mesh catalyst Eugene B.H. Quah, Chun-Zhu Li∗ Department of Chemical Engineering, Monash University, P.O. Box 36, Vic. 3800, Melbourne, Australia Received 16 April 2003; received in revised form 22 July 2003; accepted 11 August 2003
Abstract Partial oxidation of methane with air was studied in a quartz tube reactor and a wire-mesh reactor at 850–950 ◦ C using pure nickel mesh as a catalyst. In the wire-mesh reactor, only the mesh catalyst itself was heated, giving more direct information on the reactions on the catalyst surface. In the quartz tube reactor, reactions took place both on the catalyst surface and in the gas phase. The use of a non-porous mesh as the catalyst at short contact times allowed for the elimination of mass transfer limitations for radicals, which would be considered as “irreducible” with porous catalysts at high temperature. Our experimental results demonstrated that the desorption of radicals from the catalyst surface at short contact times competed well with other reactions (including oxidation reactions) involving the radicals on the catalyst surface or within the gas film surrounding the wires. The desorbed radicals played an important role by participating in the subsequent reactions in the gas phase and/or on the catalyst surface. The use of two pieces of meshes (5 mm apart) in the quartz tube reactor at high gas flow rates indicated that the desorbed radicals could decrease the apparent catalyst activity due to the re-formation of CH4 from the radicals desorbed from the catalyst surface. The reactions in the quartz tube reactor at least partly represent the reactions inside a porous catalyst. Our study has thus given further insights into the fundamental reactions taking place inside the pores in a porous catalyst. © 2003 Elsevier B.V. All rights reserved. Keywords: Methane oxidation; Radicals; Ni mesh/gauze catalyst; Short contact time; Irreducible mass-transport limitation
1. Introduction The vast reserves of natural gas (mainly methane) have generated enormous scientific interests to convert natural gas directly to other more valuable forms of fuels and chemicals. In particular, the direct oxidative coupling of methane to higher hydrocarbons [1–11] and the direct partial oxidation of methane to syngas [12–18] have been the major focus of attention. In many of these studies, porous catalysts of large surface area have been widely used, mainly because they have been well developed. While porous catalysts are especially suitable for slow reactions requiring large surface areas, they may not be suitable for the catalytic oxidation of methane at high temperature, where the fast reaction rates often result in mass and heat transfer limitations and only the external surface is effective.
∗ Corresponding author. Tel.: +61-3-9905-9623; fax: +61-3-9905-5686. E-mail address: [email protected] (C.-Z. Li).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.08.023
It is often believed that the radicals (reaction intermediates) normally remain on the catalyst surface to undergo further reactions to form the final products that then desorb from the catalyst surface. However, there is now a growing body of evidence suggesting that the radicals formed on the catalyst surface can desorb and play an important role in the formation of products in the gas phase [19–25]. The reactive nature of these radicals, particularly at high temperature, means that the transport limitations of these radicals may be “irreducible”: the pellet size at which the internal concentration gradients would be negligible is much smaller than the minimal pellet size imposed by pressure drop considerations [25,26]. This is in contrast to the classical consideration of the transport limitation of the reactant and product molecules. Thus, for a porous catalyst used at high temperature, the desorbed radicals (and molecular species) may react in the “gas phase” inside the pores, which can in turn affect the reactions on the catalyst surface. Therefore, the understanding of the fundamental reaction mechanisms during the catalytic oxidation of methane at high temperature is difficult with the porous catalysts.
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The purpose of this study is to experimentally investigate the desorption of radicals from the catalyst surface and their subsequent reactions in the gas phase during the catalytic oxidation of methane with air. A non-porous nickel mesh was used as the catalyst to eliminate the so-called “irreducible” mass transport effects for the radicals. We have also used two layers of mesh catalyst in order to gain insights into the reactions taking place inside the micro-pores in a porous catalyst.
2. Experimental Pure Ni mesh (Goodfellow, UK) was used as the catalyst in this study. The Ni wire diameter was 0.25 mm. The mesh contained 40 wires per inch with an open area (transparency) of 37%. Prior to use, the catalyst mesh was washed with a mixture of methanol and chloroform to remove greases. Chemical Purity grade (>99.99%, Matheson) methane, Ultra High Purity grade argon (BOC) and Instrument grade air (BOC) were used without further treatment. Two reactors were used in this study. The first one was a quartz tube reactor made from a quartz tube with an inner diameter of 2.0 cm and a length of about 40 cm. A piece of mesh catalyst with 2.0 cm diameter was placed in the middle (isothermal zone) of the quartz tube reactor heated with an external furnace. Expansion of the mesh catalyst on heating ensured that the mesh catalyst would remain at its intended position. The temperature of the mesh catalyst was monitored with a K-type thermocouple attached to the outside wall of the reactor. Trial experiments confirmed that the temperature measured directly on the mesh catalyst differed from that measured outside the reactor by no more than 2 ◦ C. A second piece of mesh catalyst could also be loaded into the reactor with the distance (5 mm in the experiments reported here) between the two pieces of meshes remaining unchanged during an experiment. In the quartz tube reactor, both the mesh catalysts and the reactant gas mixture were heated to the same temperature in the isothermal zone of the furnace. The total gas flow rates (controlled by mass flow controllers) quoted here were measured at ambient temperature and pressure. The second reactor was a wire-mesh reactor. The detailed features of this reactor have been described previously [25,27–29]. Briefly, a piece of mesh catalyst was stretched between two copper electrodes and heated directly with an alternating current. Cooling water was circulated inside the copper electrodes and other parts of the reactor so that the mesh catalyst was virtually the only part heated to the required temperature, while the rest of the reactor remained at temperatures close to ambient temperature. Therefore, the gas reactants remained at room temperature before meeting with the hot catalyst. In passing through the catalyst, only a small portion of the gas molecules was heated up when coming into close contact with the mesh. The unheated portion of gas stream, passing through the apertures of the mesh,
would then rapidly quench the reaction mixture desorbed from the catalyst. The mesh catalyst temperature was measured by a pair of K-type thermocouples formed by contacting the bare thin K-type thermocouple wires directly with the mesh. Therefore, the temperature measured was that of the mesh itself where reactions took place, eliminating the heat transfer limitation. Reaction products were quantified with a HP 5890 gas chromatograph (GC) equipped with a HeyeSep DB column (15 ft × 1/8 in.), a flame ionisation detector and a thermal conductivity detector. All experiments achieved steady state operation soon (<10 min) after the mesh was heated to the required temperature. The same amount of mesh catalyst was used in each experiment in both reactors. All experiments were run for longer than 180 min with the catalyst activities remaining unchanged (within experimental error). The formation and consumption rates reported here represent an average of two or three rates measured after the reaction rates had reached steady state values.
3. Results and discussion 3.1. Desorption of radicals from the Ni mesh catalyst Fig. 1 shows the formation rate of C2 H6 as a function of total gas flow rate (measured at ambient conditions) from the catalytic pyrolysis (CH4 /Ar = 0.3) and oxidation of CH4 in the wire-mesh reactor at 850 ◦ C. As was mentioned above, the catalyst activity remained unchanged throughout the experiments (>180 min), even for the pyrolysis experiments in the absence of any oxygen. After the oxidation
Fig. 1. The formation rate of C2 H6 as a function of total gas flow rate (measured at ambient conditions) from the catalytic pyrolysis and oxidation of CH4 in the wire-mesh reactor at 850 ◦ C. CH4 partial pressure at CH4 /Ar = 0.3 and at CH4 /air = 0.3 is 0.23 atm, and at CH4 /air = 4 is 0.8 atm.
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experiments, careful examination of the mesh catalyst with a scanning electron microscope revealed the absence of coke formation on the smooth catalyst surface. Our observation with the nickel mesh catalyst is in sharp contrast with many past observations [30–33] of significant coke formation and associated catalyst deactivation using porous catalysts. After pyrolysis experiments (CH4 /Ar = 0.3), some carbon, being a function of total gas flow rate, was found on the catalyst surface at low coverage (<20 wt.% when gas flow rate >10 l min−1 ), but no filamentous carbon growth was observed [34]. Clearly, the coverage of the catalyst surface by carbonaceous materials reached steady states without further increases, so that the catalytic activity was maintained. Increases in the total gas flow rate were observed to have not only increased the reaction rates but also changed the product selectivities. During the oxidation of CH4 with air at a low flow rate (e.g. 1.5 l min−1 in Fig. 1), the main products were CO and CO2 with a negligible amount of ethane (C2 H6 ) (Fig. 1). However, as is shown in Fig. 1, C2 H6 became a very important product at high gas flow rates (>8 l min−1 ). During pyrolysis, while no C2 H6 was observed at low flow rates, high C2 H6 formation rates with >95% selectivity were observed at increased gas flow rates (Fig. 1). Similar trends were also observed during the pyrolysis of pure CH4 and LPG [34] with the same nickel mesh catalyst and during the oxidation of CH4 in air with a Monel mesh catalyst [25]. As is shown in Fig. 1, drastic changes in reaction rates (and product selectivities) took place as the flow rate was increased over a narrow range between about 7.5–8.5 l min−1 . This somewhat “abrupt” effect of gas flow rate over a narrow flow rate range cannot be explained by the enhanced diffusion of molecular species to and from the catalyst surface. As was discussed in our previous study [25], this drastic effect of gas flow rate indicated the changes in the nature of the reactions involving radicals generated on the catalyst surface. It is widely accepted that the first steps in the catalytic pyrolysis and oxidation of methane are the initial activation of CH4 (and O2 for oxidation) due to their dissociative chemisorption, along with possible hydrogen abstraction from adsorbed methane by adsorbed oxygen species to form methyl radicals on the catalyst surface [35–37]. These radicals could desorb from the catalyst surface [22–25,38]. It should be noted that the mesh wires were always surrounded by a gas film whose thickness would be affected by the flow dynamics of the gas passing through the mesh. At low flow rates and in the absence of oxygen, the methyl radicals, even being desorbed from the catalyst surface, would spend a relatively long time in diffusing across the thick gas film from the catalyst surface into the bulk gas phase. During this diffusion process, the radicals could either be re-adsorbed onto the catalyst surface or recombine with the desorbed H radicals to re-form CH4 within the gas film. In the presence of oxygen, in addition to the recombination between CH3 and H, the CH3 and H radicals could also be consumed through their reactions with O2 or O-containing radicals within the hot gas film to form CO and CO2 . Only
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a small amount of CH3 would be able to desorb into the gas to form C2 H6 . The net result is the formation of CO, CO2 and H2 O with little formation of H2 and C2 H6 (see Fig. 1), with carbon selectivities of about 51, 35 and 14% for CO, CO2 and C2 H6 respectively at 1.5 l min−1 (CH4 /air = 4). With increasing gas flow rate, the thickness of the gas film would reduce and the supply of CH4 and air to the catalyst surface would increase, leading to increased reaction rates. More importantly, the reduced thickness of the gas film surrounding the mesh catalyst wires means that the CH3 radicals would only need to spend a relatively short time inside the gas film before they could get into the bulk gas phase. This represents a change in the mass transfer of the radicals. In other words, the escape of the radicals into the bulk gas phase could now compete favourably with the consumption of the radicals inside the hot gas film. As the hot portion of the gas stream is quenched by the cold/unheated portion of the gas stream after the catalyst, the methyl radicals would combine to form C2 H6 . As the gas flow rate was increased from 1.5 to 12 l min−1 (CH4 /air = 4, Fig. 1), the carbon selectivity to C2 H6 increased from 14 to 53%, while the selectivities to CO and CO2 decreased from 51 and 35% to 20 and 27% respectively. The formation of higher hydrocarbons from the coupling of radicals such as CH3 has also been recognised in some past studies [22,24–26,38–41]. In fact, it is surprising to note that the oxidation of CH4 (CH4 /air = 0.3) gave comparable C2 H6 formation rates to the pyrolysis of CH4 at the same CH4 partial pressure (CH4 /Ar = 0.3). Clearly, at high gas flow rates, the escape of CH3 radicals must have competed well with their oxidation on the catalyst surface and/or within the thin gas film. Even in the absence of oxygen (pyrolysis), the rapid escape of CH3 radicals has prevented its further dehydrogenation on the catalyst surface CH3 (s) = CH2 (s) + H(s) = CH(s) + 2H(s) = C(s) + 3H(s) (1) from becoming an important process and has therefore inhibited the continuous formation of coke [34]. The presence of sufficient H on the catalyst surface (e.g. during pyrolysis) also shifted Reaction 1 to the left. The facilitation of radical desorption with the use of non-porous mesh catalysts is the main reason for us to have sustained catalyst activity at high gas flow rates even for pure CH4 and LPG [34]. The lack of (high) bulk gas flow inside the porous catalyst particles is at least partially responsible for the coke formation and associated catalyst deactivation observed in many past studies using porous catalysts. 3.2. Roles of desorbed radicals and molecular species in the further reactions on the catalyst surface and in the gas phase at 850 ◦ C Fig. 2 shows the consumption/formation rates of reactants and main products during the oxidation of CH4 in the quartz
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Fig. 2. Consumption/formation rates of reactants and main products from the oxidation of CH4 with air (CH4 /air = 4.0) in the quartz tube reactor at 850 ◦ C. The sum of the formation rates of all products (C2+ , CO and CO2 ) can be taken as the CH4 consumption rate.
tube reactor at 850 ◦ C in the absence (blank experiments) and presence of the Ni mesh catalyst. The formation rates of various products are shown as “accumulated rates” in Fig. 2 (and other figures below). C2+ in the figure (and other figures) represents the sum of C2 H6 , C2 H4 , C3 H8 and C3 H6 , the latter always being present in trace amounts. The total formation rates of higher hydrocarbons (C2 H6 , C2 H4 , C3 H8 and C3 H6 ), CO and CO2 can be taken as a good approximation of the CH4 consumption rates. During the blank experiments (Fig. 2A), the product formation rates were seen to decrease rapidly with increasing gas flow rate. It appears that the homogeneous gas-phase reaction rates were directly related to the concentrations of radicals [10,42–44]. At a high gas flow rate in the absence of a catalyst (blank experiments, Fig. 2A and D), the short residence time did not generate radicals at concentrations high enough for the reactions to proceed at high rates. The short residence time also did not give enough time for the radicals to react (except, for example, recombination reactions). The data in Fig. 2 confirm that the use of a single piece of nickel mesh catalyst (Fig. 2B and E) did increase the consumption rates of CH4 (as the sum of C-containing products) and O2 compared with the blank experiments (Fig. 2A and D). Even at high gas flow rates where non-catalytic gas-phase reactions (blank experiments) took place to very
limited extents, the consumption rates of CH4 and O2 in the presence of the Ni mesh catalyst were still very significant and tended to reach stable values with increasing gas flow rate (>8 l min−1 ). However, when two pieces of mesh catalyst with a separation of 5 mm were loaded into the reactor (Fig. 2C and F), to our surprise, the consumption rate of CH4 and the formation rates of C2+ decreased significantly compared with the use of a single piece of mesh catalyst (Fig. 2B and E). To understand the reactions taking place in the quartz tube reactor, the net reaction rates due to the catalyst were calculated as the differences between the reaction rates in the presence of mesh catalyst and those in the absence of the catalyst (blank). These net consumption/formation rates in the quartz tube reactor are compared with those in the wire-mesh reactor at 850 ◦ C in Fig. 3 for gas flow rates higher than 7 l min−1 , where the radical desorption became significant (Fig. 1). The amount of mesh used in the wire-mesh reactor was the same as any single piece of mesh used in the quartz tube reactor (within experimental error). At 850 ◦ C, the CH4 consumption rate decreased in the sequence of wire-mesh reactor (WMR, one piece of mesh) > quartz tube reactor (QTR, single piece of mesh) > quartz tube reactor (QTR, two pieces of mesh) for the CH4 /air ratio of 4.0. In a confirmation of the observation shown in Fig. 2, it was surprising
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Fig. 3. Net consumption/formation rates of reactants and main products from the oxidation of CH4 with air in the quartz tube reactor (QTR) at 850 ◦ C compared those in the wire-mesh reactor (WMR).
to note that the use of two pieces of mesh catalyst (Fig. 3D) was less effective to convert CH4 than the use of one piece of mesh catalyst (Fig. 3C) in the quartz tube reactor. In other words, some additional CH4 might have formed as the reaction mixture, leaving the first piece of mesh catalyst, passed through the second piece of mesh catalyst in the quartz tube reactor. Similarly, the use of two pieces of mesh catalyst (Fig. 3H) in the quartz tube reactor did not at all double the consumption rate of O2 compared with that for the use of one piece of mesh catalyst (Fig. 3G). While the oxygen concentration was reduced as the gas stream passed from the first to the second piece of the mesh catalyst, this did not seem to be the only or the main reason for the little oxygen consumption caused by the second mesh. This is because the data from the wire-mesh reactor indicated that the oxygen consumption rate was not drastically affected by the O2 concentration over the CH4 /air range of 0.3–4: at gas flow rates >10 l min−1 , the oxygen consumption rate in fact remained unchanged at around 3.1 mmol min−1 (Fig. 3E and F). The apparent low activities of the second piece of mesh catalyst in comparison with the first piece, whose activity is in turn lower than that in the wire-mesh reactor at the same CH4 /air ratio of 4.0, can be explained by considering the roles of oxidation products (molecular species and radicals)
in the reactions on the catalyst surface and in the gas phase. The presence of molecular oxidation products in the gas phase would apparently “decrease” the catalyst activity. For example, CO, CO2 and H2 O formed from the oxidation of CH4 can all be adsorbed onto the catalyst, occupying (multi) active sites on the catalyst surface and leaving less active sites for the direct reactions of CH4 and O2 themselves on the catalyst surface. In fact, nickel is a good catalyst for the steam and dry reforming (with CO2 ) of methane [45–48]. The effects of reaction products on the apparent catalyst activity have been confirmed experimentally in our previous study [25] by operating the quartz tube reactor and the wire-mesh reactor in series, using one piece of Monel mesh catalyst in each reactor at 950 ◦ C. When the product stream from the quartz tube reactor was cooled to room temperature (to quench the radicals) and then fed into the wire-mesh reactor, the CH4 and O2 consumption rates due to the catalyst in the wire-mesh reactor were much lower than those when fresh CH4 /air mixture was fed directly into the wire-mesh reactor. Therefore, the presence of some oxidation products (molecular species) would influence the catalyst activity. However, the effects of molecular products from oxidation would not explain the reduction in CH4 consumption rate going from the use of one piece of mesh catalyst (Fig. 3C) to
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the use of two pieces of mesh catalyst (Fig. 3D). The presence of radicals, either formed from the non-catalytic homogeneous gas-phase reactions or, more importantly, desorbed from the catalyst surface into the gas phase, would also affect the “apparent” activity of the catalyst. For example, the H radicals desorbed from the first piece of mesh catalyst may combine with the CH3 radicals desorbed from the second piece of mesh catalyst to form CH4 , or vice versa. The re-formation of CH4 may also take place on the catalyst surface as a result of the re-adsorption of H or CH3 radicals. This appears to be a plausible explanation for the reduction of CH4 consumption rate due to the use of the second piece of mesh catalyst (Fig. 3C and D). The data in Fig. 3C and D also indicate that the use of the second piece of mesh catalyst in the quartz tube reactor increased the formation rates of CO and CO2 and decreased the corresponding formation rates of C2 H6 and other hydrocarbons. Clearly, the desorption of other O-containing radicals (e.g. OH radicals) from the first and second mesh catalyst or the formation of these O-containing radicals in the immediate vicinity of the meshes (especially initiated by the radicals desorbed from the catalyst) has resulted in the enhanced oxidation of CH3 radicals to form CO and CO2 . In fact, hydroxyl-type radicals were found to be largely responsible for the disappearance of methyl radicals via
formation of carbon oxides and re-formation of methane [40,41,49]. Both the re-formation of CH4 from H and CH3 (or other radicals) and the oxidation of CH3 would consume the CH3 radicals—precursors of C2 H6 . Furthermore, both H radicals and O-containing radicals (e.g. OH radicals) may also abstract H from C2 H6 to form C2 H5 and C2 H4 . C2 H5 is the precursor of C3 H8 . The data in Fig. 3C indeed showed the formation of C2 H4 and C3 H8 as major reaction products when one piece of mesh was used in the quartz tube reactor. The data in Fig. 3D also showed that the use of two pieces of mesh catalyst greatly reduced the formation of all higher hydrocarbons. Clearly, the further oxidation of radicals was the dominant fate of the radicals when two pieces of mesh catalyst were used, possibly due to the greatly increased concentration of O-containing radicals by two pieces of closely spaced mesh catalyst. In the case of the wire-mesh reactor, the rapid quenching of the gas stream meant that the dominant reactions would be the combination of radicals, giving high formation rates of C2 H6 and H2 (Fig. 3A and B). Decreasing CH4 partial pressure (decreasing CH4 /air ratio from 4 to 0.3, Fig. 3A and B) would reduce the generation and subsequent desorption of CH3 radicals by the catalyst, reducing the C2 H6 formation rate from about 2.0–1.0 mmol C min−1 at high gas flow rates.
Fig. 4. Consumption/formation rates of reactants and main products from the oxidation of CH4 with air (CH4 /air = 4.0) in the quartz tube reactor at 900 ◦ C. The sum of the formation rates of all products (C2+ , CO and CO2 ) can be taken as the CH4 consumption rate.
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The great importance of reaction products (molecular and radical species) to the observed catalyst activity can also be seen clearly by comparing the net reaction rates in the quartz tube reactor with those in the wire-mesh reactor shown in Fig. 3. At flow rates higher than about 7 l min−1 , the homogeneous gas-phase reactions (blank experiments in Fig. 2A) only led to minimal CH4 oxidation (<0.5 mmol min−1 for the whole length of the reactor or <0.25 mmol min−1 for the reactor length before the gas met the mesh catalyst). Even this low level of CH4 consumption in the gas-phase homogeneous reactions had generated enough products, particularly radicals, to greatly reduce the apparent catalyst activity, as can be seen by comparing the net reaction rates in the quartz tube reactor (one piece of mesh catalyst, Fig. 3C) with those in the wire-mesh reactor (Fig. 3B). The above discussion on the roles of desorbed radicals and reaction products desorbed from catalyst is also supported by the data from the experiments carried out at higher temperatures. Figs. 4 and 5 show the consumption/formation rates of reactants and main products from the oxidation of CH4 in the quartz tube reactor at 900 and 950 ◦ C respectively. As was true in the case of oxidation at 850 ◦ C, the observed decreases in the reaction rates (Figs. 4B, C and 5B, C) with increasing gas flow rate were mainly due to the reduced reaction rates of the gas-phase homogeneous reactions (Figs. 4A, D and 5A, D). Combining with Fig. 2 for
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the oxidation of CH4 at 850 ◦ C, one can make the following observations: 1. Although increasing temperature caused the overall CH4 consumption rate to increase, the use of the second piece of mesh catalyst never led to any significant increase in the consumption rates of CH4 at any flow rate investigated. Clearly, the re-formation of CH4 from the radicals (particularly those desorbed from the catalyst) took place over the whole studied temperature range of 850–950 ◦ C. 2. However, the use of the second piece of mesh catalyst always increased the O2 consumption rate, particularly at higher temperatures; this was accompanied by the decreased formation rates of C2 H6 and other higher hydrocarbons. It is believed that increasing temperature has led to increased generation of O-containing radicals on the catalyst, which could desorb into the gas phase. The desorbed radicals then reacted with CH3 and other C-containing radicals (desorbed from the catalyst) for the formation of CO2 . 3.3. Implications to the reactions in a porous catalyst Assuming that the gas flow inside the quartz tube reactor is of a plug flow pattern, for a total gas flow rate of 12 l min−1 (measured at room temperature), the residence time of gas
Fig. 5. Consumption/formation rates of reactants and main products from the oxidation of CH4 with air (CH4 /air = 4.0) in the quartz tube reactor at 950 ◦ C. The sum of the formation rates of all products (C2+ , CO and CO2 ) can be taken as the CH4 consumption rate.
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spent between the two pieces of mesh catalyst (5 mm apart) would be about 2 ms over the investigated temperature range of 850–950 ◦ C. This time is similar in magnitude to the time scale inside a pore in a porous catalyst. Therefore, our data would give some insight into the fundamental reactions taking place inside a porous catalyst. The data in Figs. 2–5 indicate that the use of two pieces of mesh catalyst at all temperatures studied in the quartz tube reactor tended to reduce the formation of higher hydrocarbons in favour of the formation of CO2 and H2 . CO2 /CO and H2 were the main products observed in the past studies [18,50,51] from the partial oxidation of CH4 using porous nickel catalysts under otherwise similar conditions. It is likely that the reactions taking place in our quartz tube reactor using the nickel mesh catalyst discussed above at least partly represent those taking place in a porous catalyst, although it is recognised that the pore structure in a porous catalyst particle may be more complicated than the space between the two pieces of mesh catalyst in our reactor. However, it is clear that the desorption and re-adsorption of radicals as well their reactions inside the pores within a porous catalyst would greatly reduce the effectiveness of the catalyst. The physical structure of the catalyst must be an important consideration of catalyst design for the catalytic pyrolysis and oxidation of methane and other hydrocarbons at high temperature.
4. Conclusions The oxidation of CH4 with air has been studied using pure nickel mesh as the catalyst in a wire-mesh reactor and in a quartz tube reactor: 1. Our experimental results demonstrated that the desorption of radicals from the catalyst surface at short contact times is an important process and competes well with other reactions (including oxidation reactions) involving the radicals on the catalyst surface or within the gas film surrounding the mesh wires. 2. The desorbed radicals would participate in the subsequent reactions in the gas phase and/or on the catalyst surface. The reactions involving radicals are largely responsible for the formation of higher hydrocarbons from the oxidation and pyrolysis of CH4 . The desorbed radicals can also recombine to result in the re-formation of CH4 , apparently reducing the catalyst activity. 3. The reactions in the quartz tube reactor at least partly represent the reactions inside a porous catalyst. Our study has thus given further insights into the fundamental reactions taking place inside the pores in a porous catalyst. The desorption and re-adsorption of radicals and the reactions involving radicals in the pores must also be considered for the better understanding of reactions taking place in a porous catalyst during the oxidation of CH4 .
Acknowledgements The financial support of this study by Monash University is gratefully acknowledged.
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Roles of desorbed radicals and reaction products during the oxidation of methane using a nickel mesh catalyst Eugene B.H. Quah, Chun-Zhu Li∗ Department of Chemical Engineering, Monash University, P.O. Box 36, Vic. 3800, Melbourne, Australia Received 16 April 2003; received in revised form 22 July 2003; accepted 11 August 2003
Abstract Partial oxidation of methane with air was studied in a quartz tube reactor and a wire-mesh reactor at 850–950 ◦ C using pure nickel mesh as a catalyst. In the wire-mesh reactor, only the mesh catalyst itself was heated, giving more direct information on the reactions on the catalyst surface. In the quartz tube reactor, reactions took place both on the catalyst surface and in the gas phase. The use of a non-porous mesh as the catalyst at short contact times allowed for the elimination of mass transfer limitations for radicals, which would be considered as “irreducible” with porous catalysts at high temperature. Our experimental results demonstrated that the desorption of radicals from the catalyst surface at short contact times competed well with other reactions (including oxidation reactions) involving the radicals on the catalyst surface or within the gas film surrounding the wires. The desorbed radicals played an important role by participating in the subsequent reactions in the gas phase and/or on the catalyst surface. The use of two pieces of meshes (5 mm apart) in the quartz tube reactor at high gas flow rates indicated that the desorbed radicals could decrease the apparent catalyst activity due to the re-formation of CH4 from the radicals desorbed from the catalyst surface. The reactions in the quartz tube reactor at least partly represent the reactions inside a porous catalyst. Our study has thus given further insights into the fundamental reactions taking place inside the pores in a porous catalyst. © 2003 Elsevier B.V. All rights reserved. Keywords: Methane oxidation; Radicals; Ni mesh/gauze catalyst; Short contact time; Irreducible mass-transport limitation
1. Introduction The vast reserves of natural gas (mainly methane) have generated enormous scientific interests to convert natural gas directly to other more valuable forms of fuels and chemicals. In particular, the direct oxidative coupling of methane to higher hydrocarbons [1–11] and the direct partial oxidation of methane to syngas [12–18] have been the major focus of attention. In many of these studies, porous catalysts of large surface area have been widely used, mainly because they have been well developed. While porous catalysts are especially suitable for slow reactions requiring large surface areas, they may not be suitable for the catalytic oxidation of methane at high temperature, where the fast reaction rates often result in mass and heat transfer limitations and only the external surface is effective.
∗ Corresponding author. Tel.: +61-3-9905-9623; fax: +61-3-9905-5686. E-mail address: [email protected] (C.-Z. Li).
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.08.023
It is often believed that the radicals (reaction intermediates) normally remain on the catalyst surface to undergo further reactions to form the final products that then desorb from the catalyst surface. However, there is now a growing body of evidence suggesting that the radicals formed on the catalyst surface can desorb and play an important role in the formation of products in the gas phase [19–25]. The reactive nature of these radicals, particularly at high temperature, means that the transport limitations of these radicals may be “irreducible”: the pellet size at which the internal concentration gradients would be negligible is much smaller than the minimal pellet size imposed by pressure drop considerations [25,26]. This is in contrast to the classical consideration of the transport limitation of the reactant and product molecules. Thus, for a porous catalyst used at high temperature, the desorbed radicals (and molecular species) may react in the “gas phase” inside the pores, which can in turn affect the reactions on the catalyst surface. Therefore, the understanding of the fundamental reaction mechanisms during the catalytic oxidation of methane at high temperature is difficult with the porous catalysts.
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The purpose of this study is to experimentally investigate the desorption of radicals from the catalyst surface and their subsequent reactions in the gas phase during the catalytic oxidation of methane with air. A non-porous nickel mesh was used as the catalyst to eliminate the so-called “irreducible” mass transport effects for the radicals. We have also used two layers of mesh catalyst in order to gain insights into the reactions taking place inside the micro-pores in a porous catalyst.
2. Experimental Pure Ni mesh (Goodfellow, UK) was used as the catalyst in this study. The Ni wire diameter was 0.25 mm. The mesh contained 40 wires per inch with an open area (transparency) of 37%. Prior to use, the catalyst mesh was washed with a mixture of methanol and chloroform to remove greases. Chemical Purity grade (>99.99%, Matheson) methane, Ultra High Purity grade argon (BOC) and Instrument grade air (BOC) were used without further treatment. Two reactors were used in this study. The first one was a quartz tube reactor made from a quartz tube with an inner diameter of 2.0 cm and a length of about 40 cm. A piece of mesh catalyst with 2.0 cm diameter was placed in the middle (isothermal zone) of the quartz tube reactor heated with an external furnace. Expansion of the mesh catalyst on heating ensured that the mesh catalyst would remain at its intended position. The temperature of the mesh catalyst was monitored with a K-type thermocouple attached to the outside wall of the reactor. Trial experiments confirmed that the temperature measured directly on the mesh catalyst differed from that measured outside the reactor by no more than 2 ◦ C. A second piece of mesh catalyst could also be loaded into the reactor with the distance (5 mm in the experiments reported here) between the two pieces of meshes remaining unchanged during an experiment. In the quartz tube reactor, both the mesh catalysts and the reactant gas mixture were heated to the same temperature in the isothermal zone of the furnace. The total gas flow rates (controlled by mass flow controllers) quoted here were measured at ambient temperature and pressure. The second reactor was a wire-mesh reactor. The detailed features of this reactor have been described previously [25,27–29]. Briefly, a piece of mesh catalyst was stretched between two copper electrodes and heated directly with an alternating current. Cooling water was circulated inside the copper electrodes and other parts of the reactor so that the mesh catalyst was virtually the only part heated to the required temperature, while the rest of the reactor remained at temperatures close to ambient temperature. Therefore, the gas reactants remained at room temperature before meeting with the hot catalyst. In passing through the catalyst, only a small portion of the gas molecules was heated up when coming into close contact with the mesh. The unheated portion of gas stream, passing through the apertures of the mesh,
would then rapidly quench the reaction mixture desorbed from the catalyst. The mesh catalyst temperature was measured by a pair of K-type thermocouples formed by contacting the bare thin K-type thermocouple wires directly with the mesh. Therefore, the temperature measured was that of the mesh itself where reactions took place, eliminating the heat transfer limitation. Reaction products were quantified with a HP 5890 gas chromatograph (GC) equipped with a HeyeSep DB column (15 ft × 1/8 in.), a flame ionisation detector and a thermal conductivity detector. All experiments achieved steady state operation soon (<10 min) after the mesh was heated to the required temperature. The same amount of mesh catalyst was used in each experiment in both reactors. All experiments were run for longer than 180 min with the catalyst activities remaining unchanged (within experimental error). The formation and consumption rates reported here represent an average of two or three rates measured after the reaction rates had reached steady state values.
3. Results and discussion 3.1. Desorption of radicals from the Ni mesh catalyst Fig. 1 shows the formation rate of C2 H6 as a function of total gas flow rate (measured at ambient conditions) from the catalytic pyrolysis (CH4 /Ar = 0.3) and oxidation of CH4 in the wire-mesh reactor at 850 ◦ C. As was mentioned above, the catalyst activity remained unchanged throughout the experiments (>180 min), even for the pyrolysis experiments in the absence of any oxygen. After the oxidation
Fig. 1. The formation rate of C2 H6 as a function of total gas flow rate (measured at ambient conditions) from the catalytic pyrolysis and oxidation of CH4 in the wire-mesh reactor at 850 ◦ C. CH4 partial pressure at CH4 /Ar = 0.3 and at CH4 /air = 0.3 is 0.23 atm, and at CH4 /air = 4 is 0.8 atm.
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experiments, careful examination of the mesh catalyst with a scanning electron microscope revealed the absence of coke formation on the smooth catalyst surface. Our observation with the nickel mesh catalyst is in sharp contrast with many past observations [30–33] of significant coke formation and associated catalyst deactivation using porous catalysts. After pyrolysis experiments (CH4 /Ar = 0.3), some carbon, being a function of total gas flow rate, was found on the catalyst surface at low coverage (<20 wt.% when gas flow rate >10 l min−1 ), but no filamentous carbon growth was observed [34]. Clearly, the coverage of the catalyst surface by carbonaceous materials reached steady states without further increases, so that the catalytic activity was maintained. Increases in the total gas flow rate were observed to have not only increased the reaction rates but also changed the product selectivities. During the oxidation of CH4 with air at a low flow rate (e.g. 1.5 l min−1 in Fig. 1), the main products were CO and CO2 with a negligible amount of ethane (C2 H6 ) (Fig. 1). However, as is shown in Fig. 1, C2 H6 became a very important product at high gas flow rates (>8 l min−1 ). During pyrolysis, while no C2 H6 was observed at low flow rates, high C2 H6 formation rates with >95% selectivity were observed at increased gas flow rates (Fig. 1). Similar trends were also observed during the pyrolysis of pure CH4 and LPG [34] with the same nickel mesh catalyst and during the oxidation of CH4 in air with a Monel mesh catalyst [25]. As is shown in Fig. 1, drastic changes in reaction rates (and product selectivities) took place as the flow rate was increased over a narrow range between about 7.5–8.5 l min−1 . This somewhat “abrupt” effect of gas flow rate over a narrow flow rate range cannot be explained by the enhanced diffusion of molecular species to and from the catalyst surface. As was discussed in our previous study [25], this drastic effect of gas flow rate indicated the changes in the nature of the reactions involving radicals generated on the catalyst surface. It is widely accepted that the first steps in the catalytic pyrolysis and oxidation of methane are the initial activation of CH4 (and O2 for oxidation) due to their dissociative chemisorption, along with possible hydrogen abstraction from adsorbed methane by adsorbed oxygen species to form methyl radicals on the catalyst surface [35–37]. These radicals could desorb from the catalyst surface [22–25,38]. It should be noted that the mesh wires were always surrounded by a gas film whose thickness would be affected by the flow dynamics of the gas passing through the mesh. At low flow rates and in the absence of oxygen, the methyl radicals, even being desorbed from the catalyst surface, would spend a relatively long time in diffusing across the thick gas film from the catalyst surface into the bulk gas phase. During this diffusion process, the radicals could either be re-adsorbed onto the catalyst surface or recombine with the desorbed H radicals to re-form CH4 within the gas film. In the presence of oxygen, in addition to the recombination between CH3 and H, the CH3 and H radicals could also be consumed through their reactions with O2 or O-containing radicals within the hot gas film to form CO and CO2 . Only
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a small amount of CH3 would be able to desorb into the gas to form C2 H6 . The net result is the formation of CO, CO2 and H2 O with little formation of H2 and C2 H6 (see Fig. 1), with carbon selectivities of about 51, 35 and 14% for CO, CO2 and C2 H6 respectively at 1.5 l min−1 (CH4 /air = 4). With increasing gas flow rate, the thickness of the gas film would reduce and the supply of CH4 and air to the catalyst surface would increase, leading to increased reaction rates. More importantly, the reduced thickness of the gas film surrounding the mesh catalyst wires means that the CH3 radicals would only need to spend a relatively short time inside the gas film before they could get into the bulk gas phase. This represents a change in the mass transfer of the radicals. In other words, the escape of the radicals into the bulk gas phase could now compete favourably with the consumption of the radicals inside the hot gas film. As the hot portion of the gas stream is quenched by the cold/unheated portion of the gas stream after the catalyst, the methyl radicals would combine to form C2 H6 . As the gas flow rate was increased from 1.5 to 12 l min−1 (CH4 /air = 4, Fig. 1), the carbon selectivity to C2 H6 increased from 14 to 53%, while the selectivities to CO and CO2 decreased from 51 and 35% to 20 and 27% respectively. The formation of higher hydrocarbons from the coupling of radicals such as CH3 has also been recognised in some past studies [22,24–26,38–41]. In fact, it is surprising to note that the oxidation of CH4 (CH4 /air = 0.3) gave comparable C2 H6 formation rates to the pyrolysis of CH4 at the same CH4 partial pressure (CH4 /Ar = 0.3). Clearly, at high gas flow rates, the escape of CH3 radicals must have competed well with their oxidation on the catalyst surface and/or within the thin gas film. Even in the absence of oxygen (pyrolysis), the rapid escape of CH3 radicals has prevented its further dehydrogenation on the catalyst surface CH3 (s) = CH2 (s) + H(s) = CH(s) + 2H(s) = C(s) + 3H(s) (1) from becoming an important process and has therefore inhibited the continuous formation of coke [34]. The presence of sufficient H on the catalyst surface (e.g. during pyrolysis) also shifted Reaction 1 to the left. The facilitation of radical desorption with the use of non-porous mesh catalysts is the main reason for us to have sustained catalyst activity at high gas flow rates even for pure CH4 and LPG [34]. The lack of (high) bulk gas flow inside the porous catalyst particles is at least partially responsible for the coke formation and associated catalyst deactivation observed in many past studies using porous catalysts. 3.2. Roles of desorbed radicals and molecular species in the further reactions on the catalyst surface and in the gas phase at 850 ◦ C Fig. 2 shows the consumption/formation rates of reactants and main products during the oxidation of CH4 in the quartz
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Fig. 2. Consumption/formation rates of reactants and main products from the oxidation of CH4 with air (CH4 /air = 4.0) in the quartz tube reactor at 850 ◦ C. The sum of the formation rates of all products (C2+ , CO and CO2 ) can be taken as the CH4 consumption rate.
tube reactor at 850 ◦ C in the absence (blank experiments) and presence of the Ni mesh catalyst. The formation rates of various products are shown as “accumulated rates” in Fig. 2 (and other figures below). C2+ in the figure (and other figures) represents the sum of C2 H6 , C2 H4 , C3 H8 and C3 H6 , the latter always being present in trace amounts. The total formation rates of higher hydrocarbons (C2 H6 , C2 H4 , C3 H8 and C3 H6 ), CO and CO2 can be taken as a good approximation of the CH4 consumption rates. During the blank experiments (Fig. 2A), the product formation rates were seen to decrease rapidly with increasing gas flow rate. It appears that the homogeneous gas-phase reaction rates were directly related to the concentrations of radicals [10,42–44]. At a high gas flow rate in the absence of a catalyst (blank experiments, Fig. 2A and D), the short residence time did not generate radicals at concentrations high enough for the reactions to proceed at high rates. The short residence time also did not give enough time for the radicals to react (except, for example, recombination reactions). The data in Fig. 2 confirm that the use of a single piece of nickel mesh catalyst (Fig. 2B and E) did increase the consumption rates of CH4 (as the sum of C-containing products) and O2 compared with the blank experiments (Fig. 2A and D). Even at high gas flow rates where non-catalytic gas-phase reactions (blank experiments) took place to very
limited extents, the consumption rates of CH4 and O2 in the presence of the Ni mesh catalyst were still very significant and tended to reach stable values with increasing gas flow rate (>8 l min−1 ). However, when two pieces of mesh catalyst with a separation of 5 mm were loaded into the reactor (Fig. 2C and F), to our surprise, the consumption rate of CH4 and the formation rates of C2+ decreased significantly compared with the use of a single piece of mesh catalyst (Fig. 2B and E). To understand the reactions taking place in the quartz tube reactor, the net reaction rates due to the catalyst were calculated as the differences between the reaction rates in the presence of mesh catalyst and those in the absence of the catalyst (blank). These net consumption/formation rates in the quartz tube reactor are compared with those in the wire-mesh reactor at 850 ◦ C in Fig. 3 for gas flow rates higher than 7 l min−1 , where the radical desorption became significant (Fig. 1). The amount of mesh used in the wire-mesh reactor was the same as any single piece of mesh used in the quartz tube reactor (within experimental error). At 850 ◦ C, the CH4 consumption rate decreased in the sequence of wire-mesh reactor (WMR, one piece of mesh) > quartz tube reactor (QTR, single piece of mesh) > quartz tube reactor (QTR, two pieces of mesh) for the CH4 /air ratio of 4.0. In a confirmation of the observation shown in Fig. 2, it was surprising
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Fig. 3. Net consumption/formation rates of reactants and main products from the oxidation of CH4 with air in the quartz tube reactor (QTR) at 850 ◦ C compared those in the wire-mesh reactor (WMR).
to note that the use of two pieces of mesh catalyst (Fig. 3D) was less effective to convert CH4 than the use of one piece of mesh catalyst (Fig. 3C) in the quartz tube reactor. In other words, some additional CH4 might have formed as the reaction mixture, leaving the first piece of mesh catalyst, passed through the second piece of mesh catalyst in the quartz tube reactor. Similarly, the use of two pieces of mesh catalyst (Fig. 3H) in the quartz tube reactor did not at all double the consumption rate of O2 compared with that for the use of one piece of mesh catalyst (Fig. 3G). While the oxygen concentration was reduced as the gas stream passed from the first to the second piece of the mesh catalyst, this did not seem to be the only or the main reason for the little oxygen consumption caused by the second mesh. This is because the data from the wire-mesh reactor indicated that the oxygen consumption rate was not drastically affected by the O2 concentration over the CH4 /air range of 0.3–4: at gas flow rates >10 l min−1 , the oxygen consumption rate in fact remained unchanged at around 3.1 mmol min−1 (Fig. 3E and F). The apparent low activities of the second piece of mesh catalyst in comparison with the first piece, whose activity is in turn lower than that in the wire-mesh reactor at the same CH4 /air ratio of 4.0, can be explained by considering the roles of oxidation products (molecular species and radicals)
in the reactions on the catalyst surface and in the gas phase. The presence of molecular oxidation products in the gas phase would apparently “decrease” the catalyst activity. For example, CO, CO2 and H2 O formed from the oxidation of CH4 can all be adsorbed onto the catalyst, occupying (multi) active sites on the catalyst surface and leaving less active sites for the direct reactions of CH4 and O2 themselves on the catalyst surface. In fact, nickel is a good catalyst for the steam and dry reforming (with CO2 ) of methane [45–48]. The effects of reaction products on the apparent catalyst activity have been confirmed experimentally in our previous study [25] by operating the quartz tube reactor and the wire-mesh reactor in series, using one piece of Monel mesh catalyst in each reactor at 950 ◦ C. When the product stream from the quartz tube reactor was cooled to room temperature (to quench the radicals) and then fed into the wire-mesh reactor, the CH4 and O2 consumption rates due to the catalyst in the wire-mesh reactor were much lower than those when fresh CH4 /air mixture was fed directly into the wire-mesh reactor. Therefore, the presence of some oxidation products (molecular species) would influence the catalyst activity. However, the effects of molecular products from oxidation would not explain the reduction in CH4 consumption rate going from the use of one piece of mesh catalyst (Fig. 3C) to
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the use of two pieces of mesh catalyst (Fig. 3D). The presence of radicals, either formed from the non-catalytic homogeneous gas-phase reactions or, more importantly, desorbed from the catalyst surface into the gas phase, would also affect the “apparent” activity of the catalyst. For example, the H radicals desorbed from the first piece of mesh catalyst may combine with the CH3 radicals desorbed from the second piece of mesh catalyst to form CH4 , or vice versa. The re-formation of CH4 may also take place on the catalyst surface as a result of the re-adsorption of H or CH3 radicals. This appears to be a plausible explanation for the reduction of CH4 consumption rate due to the use of the second piece of mesh catalyst (Fig. 3C and D). The data in Fig. 3C and D also indicate that the use of the second piece of mesh catalyst in the quartz tube reactor increased the formation rates of CO and CO2 and decreased the corresponding formation rates of C2 H6 and other hydrocarbons. Clearly, the desorption of other O-containing radicals (e.g. OH radicals) from the first and second mesh catalyst or the formation of these O-containing radicals in the immediate vicinity of the meshes (especially initiated by the radicals desorbed from the catalyst) has resulted in the enhanced oxidation of CH3 radicals to form CO and CO2 . In fact, hydroxyl-type radicals were found to be largely responsible for the disappearance of methyl radicals via
formation of carbon oxides and re-formation of methane [40,41,49]. Both the re-formation of CH4 from H and CH3 (or other radicals) and the oxidation of CH3 would consume the CH3 radicals—precursors of C2 H6 . Furthermore, both H radicals and O-containing radicals (e.g. OH radicals) may also abstract H from C2 H6 to form C2 H5 and C2 H4 . C2 H5 is the precursor of C3 H8 . The data in Fig. 3C indeed showed the formation of C2 H4 and C3 H8 as major reaction products when one piece of mesh was used in the quartz tube reactor. The data in Fig. 3D also showed that the use of two pieces of mesh catalyst greatly reduced the formation of all higher hydrocarbons. Clearly, the further oxidation of radicals was the dominant fate of the radicals when two pieces of mesh catalyst were used, possibly due to the greatly increased concentration of O-containing radicals by two pieces of closely spaced mesh catalyst. In the case of the wire-mesh reactor, the rapid quenching of the gas stream meant that the dominant reactions would be the combination of radicals, giving high formation rates of C2 H6 and H2 (Fig. 3A and B). Decreasing CH4 partial pressure (decreasing CH4 /air ratio from 4 to 0.3, Fig. 3A and B) would reduce the generation and subsequent desorption of CH3 radicals by the catalyst, reducing the C2 H6 formation rate from about 2.0–1.0 mmol C min−1 at high gas flow rates.
Fig. 4. Consumption/formation rates of reactants and main products from the oxidation of CH4 with air (CH4 /air = 4.0) in the quartz tube reactor at 900 ◦ C. The sum of the formation rates of all products (C2+ , CO and CO2 ) can be taken as the CH4 consumption rate.
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The great importance of reaction products (molecular and radical species) to the observed catalyst activity can also be seen clearly by comparing the net reaction rates in the quartz tube reactor with those in the wire-mesh reactor shown in Fig. 3. At flow rates higher than about 7 l min−1 , the homogeneous gas-phase reactions (blank experiments in Fig. 2A) only led to minimal CH4 oxidation (<0.5 mmol min−1 for the whole length of the reactor or <0.25 mmol min−1 for the reactor length before the gas met the mesh catalyst). Even this low level of CH4 consumption in the gas-phase homogeneous reactions had generated enough products, particularly radicals, to greatly reduce the apparent catalyst activity, as can be seen by comparing the net reaction rates in the quartz tube reactor (one piece of mesh catalyst, Fig. 3C) with those in the wire-mesh reactor (Fig. 3B). The above discussion on the roles of desorbed radicals and reaction products desorbed from catalyst is also supported by the data from the experiments carried out at higher temperatures. Figs. 4 and 5 show the consumption/formation rates of reactants and main products from the oxidation of CH4 in the quartz tube reactor at 900 and 950 ◦ C respectively. As was true in the case of oxidation at 850 ◦ C, the observed decreases in the reaction rates (Figs. 4B, C and 5B, C) with increasing gas flow rate were mainly due to the reduced reaction rates of the gas-phase homogeneous reactions (Figs. 4A, D and 5A, D). Combining with Fig. 2 for
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the oxidation of CH4 at 850 ◦ C, one can make the following observations: 1. Although increasing temperature caused the overall CH4 consumption rate to increase, the use of the second piece of mesh catalyst never led to any significant increase in the consumption rates of CH4 at any flow rate investigated. Clearly, the re-formation of CH4 from the radicals (particularly those desorbed from the catalyst) took place over the whole studied temperature range of 850–950 ◦ C. 2. However, the use of the second piece of mesh catalyst always increased the O2 consumption rate, particularly at higher temperatures; this was accompanied by the decreased formation rates of C2 H6 and other higher hydrocarbons. It is believed that increasing temperature has led to increased generation of O-containing radicals on the catalyst, which could desorb into the gas phase. The desorbed radicals then reacted with CH3 and other C-containing radicals (desorbed from the catalyst) for the formation of CO2 . 3.3. Implications to the reactions in a porous catalyst Assuming that the gas flow inside the quartz tube reactor is of a plug flow pattern, for a total gas flow rate of 12 l min−1 (measured at room temperature), the residence time of gas
Fig. 5. Consumption/formation rates of reactants and main products from the oxidation of CH4 with air (CH4 /air = 4.0) in the quartz tube reactor at 950 ◦ C. The sum of the formation rates of all products (C2+ , CO and CO2 ) can be taken as the CH4 consumption rate.
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spent between the two pieces of mesh catalyst (5 mm apart) would be about 2 ms over the investigated temperature range of 850–950 ◦ C. This time is similar in magnitude to the time scale inside a pore in a porous catalyst. Therefore, our data would give some insight into the fundamental reactions taking place inside a porous catalyst. The data in Figs. 2–5 indicate that the use of two pieces of mesh catalyst at all temperatures studied in the quartz tube reactor tended to reduce the formation of higher hydrocarbons in favour of the formation of CO2 and H2 . CO2 /CO and H2 were the main products observed in the past studies [18,50,51] from the partial oxidation of CH4 using porous nickel catalysts under otherwise similar conditions. It is likely that the reactions taking place in our quartz tube reactor using the nickel mesh catalyst discussed above at least partly represent those taking place in a porous catalyst, although it is recognised that the pore structure in a porous catalyst particle may be more complicated than the space between the two pieces of mesh catalyst in our reactor. However, it is clear that the desorption and re-adsorption of radicals as well their reactions inside the pores within a porous catalyst would greatly reduce the effectiveness of the catalyst. The physical structure of the catalyst must be an important consideration of catalyst design for the catalytic pyrolysis and oxidation of methane and other hydrocarbons at high temperature.
4. Conclusions The oxidation of CH4 with air has been studied using pure nickel mesh as the catalyst in a wire-mesh reactor and in a quartz tube reactor: 1. Our experimental results demonstrated that the desorption of radicals from the catalyst surface at short contact times is an important process and competes well with other reactions (including oxidation reactions) involving the radicals on the catalyst surface or within the gas film surrounding the mesh wires. 2. The desorbed radicals would participate in the subsequent reactions in the gas phase and/or on the catalyst surface. The reactions involving radicals are largely responsible for the formation of higher hydrocarbons from the oxidation and pyrolysis of CH4 . The desorbed radicals can also recombine to result in the re-formation of CH4 , apparently reducing the catalyst activity. 3. The reactions in the quartz tube reactor at least partly represent the reactions inside a porous catalyst. Our study has thus given further insights into the fundamental reactions taking place inside the pores in a porous catalyst. The desorption and re-adsorption of radicals and the reactions involving radicals in the pores must also be considered for the better understanding of reactions taking place in a porous catalyst during the oxidation of CH4 .
Acknowledgements The financial support of this study by Monash University is gratefully acknowledged.
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