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Partial Oxidation of Methane on Ni-MgO Catalysts Supported on Metal Foams Abolghasem Shamsi* National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 880, 3610 Collins Ferry Road, Morgantown, West Virginia 26507-0880
James J. Spivey Gordon A. and Marry Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
The catalytic partial oxidation of methane on Ni-MgO solid solutions supported on metal foams has been studied at temperatures from 600 to 800 °C, 1 atm, 3000-19 000 mL/g-h, CH4/O2 ) 2/1. The catalysts were prepared in two ways: (a) oxidized foams were dipped directly into a citrate/ethylene glycol solution containing nitrate precursors and (b) oxidized foams were primed with a bohemite primer and washcoated with Ni-MgO powders, derived from the above solution, in an acidic solution. Results show that the foam-supported catalysts reach near-equilibrium conversions of methane and H2/CO selectivities at the indicated space velocities. Rates of carbon deposition differ greatly among the catalysts, varying from 0.24 mg C/g cat h for the dipped foams to 7.0 mg C/g cat h for the powder-coated foams. Both of these rates are much lower than for the calcined, unsupported powder (57 mg C/g cat h). The results suggest that exposed Cr on all of the foam samples interact with the Ni-MgO catalyst to kinetically limit carbon formation. Introduction
Table 1. Thermodynamics of the Partial Oxidation of Methane
Partial Oxidation of Methane. The partial oxidation of methane (POX) has been widely investigated as a way to produce synthesis gas.1-10 The reaction produces synthesis gas with H2/CO ratio of 2/1 that is required in methanol and Fischer Tropsch synthesis, among other processes:
CH4 + 1/2O2 f CO + 2H2
(1)
The reaction is mildly exothermic and thermodynamically favorable at all conditions of interest (Table 1). Despite the advantages of this reaction, it is not practiced industrially because of the difficulty in preventing the highly exothermic complete oxidation reaction, which leads to unselective products.1 For this reason, POX is often studied in conjunction with the endothermic dry and/or steam reforming reactions in a mixed feed containing CH4/O2 with CO2 and/or H2O.11-13 This allows control of both the heat release and the H2/ CO ratio. Ni-MgO Catalysts. Most catalysts that have been investigated for the POX reaction have been based either on noble metals9,10 or Ni.2,12-15 As a general rule, noble metal-based catalysts tend to have higher activity and less tendency to deactivate by coke deposition.7,14,16-18 However, the high cost of materials based on noble metals limits their ultimate use in industrial operations.14 Ni-MgO has been used as a catalyst for the partial oxidation, steam, and CO2 reforming of methane in a number of studies.3,12,14,19-21 It is of particular interest because the formation of a Ni-MgO solid solution over * To whom correspondence should be addressed. E-mail:
[email protected].
∆Hr, kJ/mol
∆Gr, kJ/mol
reaction
25 °C
700 °C
25 °C
700 °C
CH4 + 1/2O2 f CO + 2H2
-35.7
-22.8
-86.4
-214
a wide range of compositions leads to a catalyst in which the Ni atoms can be reduced by hydrogen to active metallic Ni°.2 Unlike other conventional Ni-based catalysts on oxide supports, reduction in hydrogen forms small, strongly held metallic Ni particles because of the strong interaction of NiO with MgO in the solid solution.2,20 These particles, which are responsible for the catalytic POX activity, are also resistant to sintering. It has been shown that it is the larger Ni ensembles that are responsible for carbon formation.22 The smaller Ni particles formed in the Ni-MgO solid solution are active for the POX reaction but do not form sufficiently large particles to promote carbon formation. Deactivation. Deactivation of Ni-based POX catalysts typically results from carbon deposition, which is thought to be caused by two primary reactions: methane decomposition and/or the Bouduard reaction (CO disproportionation):
CH4 T C(s) + 2H2 ∆Hor ) +90.5 kJ/mol
(2)
2CO T C(s) + CO2 ∆Hor ) -170 kJ/mol
(3)
Figure 1 shows the equilibrium carbon formation and corresponding gas-phase composition of the POX reaction, assuming a starting composition corresponding to the stoichiometric ratio of 2 mol of CH4 and 1 mol of O2, 1 atm. This analysis shows that the net carbon deposition becomes less favorable at higher temperatures and higher O2/CH4 ratios. However, even at
10.1021/ie050114p CCC: $30.25 © 2005 American Chemical Society Published on Web 08/10/2005
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Figure 1. Equilibrium concentrations of syngas components and elemental carbon versus temperature; initial gas composition 2 mol of CH4 and 1 mol of O2, 1 atm (calculations carried out using HSC 4 software; Outokumpu Oy; Pori, Finland). Table 2. Equilibrium Carbon Depositiona
a
temp, °C
equilibrium carbon, mmol C/mol CH4
700 800 900
190 43.6 5.6
CH4/O2 ) 2/1, 1 atm.
temperatures typical of the POX reaction, 700-900 °C, carbon deposition at equilibrium is significant (Table 2; conditions are the same as for Figure 1). Thus, carbon deposition can be minimized by kinetically limiting its formation. This can be done by increasing the concentration of oxygen-containing species (e.g., O2 or CO2) on the surface of the catalyst. Experimental Challenges. The study of the POX reaction in a fixed bed reactor is complicated by the fact that several reactions in addition to the POX reaction (eq 1) take place. For example, even studies using a feed containing only CH4 and O2 must take into account the dry and steam reforming reactions, which can take place in a typical fixed bed reactor along the reactor bed on these Ni-MgO (solid solution) catalysts.13 At temperatures near 800 °C, the following sequence of reactions (combustion-reforming) can occur: very near the reactor inlet, oxygen is quickly consumed to produce primarily CO2 and H2O. Next, steam and dry reforming reactions (in addition to water gas shift) take place. This problem was recognized even in the very earliest work on the POX reaction, which was studied using Ni catalysts.23 The highly exothermic nature of the complete oxidation reaction
CH4 + 2O2 f CO2 + 2H2O ∆Hor ) -861 kJ/mol (4) also leads to hot spots at the reactor inlet, complicating not only the study of individual reactions but also the measurement of the true reaction temperature, as noted by several investigators.1,24 At slightly lower temperatures of 700-750 °C, Ruckenstein and Hu also observed that the reaction sequence
includes a pyrolysis mechanism, in which dissociatively adsorbed CH4 forms C and H atoms.13 These surface carbon atoms are oxidized by adsorbed oxygen to form CO, with hydrogen desorbing molecularly. More information on these two mechanisms of combustion-reforming and pyrolysis can be found in a recent literature review by York and co-workers.1 Regardless of the exact mechanism, it is clear that the study of the POX reaction is complicated by the fact that a number of reactions in addition to eq 1 take place. A more complex problem is that the nature of the catalyst itself may change with time and reactor position due to the redox properties of the gas and temperature in the reactor. As an example, Dissanayake et al. found that for a Ni/alumina catalyst, three distinct catalyst phases evolve from an initially uniform catalyst: near the entrance, the high temperature and oxidizing environment lead to the formation of NiAl2O4.15 In the second zone, which is somewhat more reducing, the stable phase is NiO/Al2O3, and in the last and most reducing zone, metallic Ni/Al2O3 is present. Severe temperature gradients may also be present. This same group reported that the reaction temperature in some portions of their Ni-based catalyst were as much as 300 °C higher than that measured in close proximity.25 Clearly, these differing temperature gradients, product gas compositions, and even catalyst compositions as a function of time-on-stream and position make the study of this reaction difficult. This may account for some of the differences in the literature for studies carried out at nominally similar conditions, and the search for alternate reactor designs that can minimize these effects, such as fluid beds.26,27 Metal Foams. Metal foams are one of a number of alternatives for high-temperature monolithic catalyst supports for reforming reactions. Other supports include straight-channel ceramic monoliths,28,29 straight-channel metallic monoliths,30 and ceramic foams.31-34 Typically, the metal foams have an open, reticulated structure with a void volume greater than ∼85%. They are
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often characterized by two measures: “density”, which is the complement of the void volume (5% “density” ) 95% void volume), and “ppi”, pores per inch, a gross measure of the mean pore size (40 ppi corresponds to a mean pore diameter of 1/40 in. or ∼635 µm per pore). Their high mechanical strength and thermal conductivity make these materials important for a number of industrial applications, including catalyst supports. In addition, their three-dimensional structure allows for radial heat and mass transfer that is not possible in conventional straight-channel monoliths. This property can help minimize temperature and concentration gradients compared to fixed beds, straight-channel monoliths, or even ceramic foams. There have been relatively few reports on the use of these materials as catalyst supports. Pestryakov et al. report the use of metal foams as supports for the oxidation of alcohols35 as well as for automotive exhaust, CO, and propane.36 For alcohol oxidation, they prepared foams apparently consisting of bulk, low surface area Ag and Ag-Cu (
