γ-Al2O3 and Co0

Here, the deactivation of Co0.4Mo0.6Cx has been compared to that of 1 wt % Pt/γ-Al2O3 in a fixed-bed catalytic reactor, using mixtures of n-tetradeca...
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Ind. Eng. Chem. Res. 2008, 47, 7663–7671

7663

Partial Oxidation of n-Tetradecane over 1 wt % Pt/γ-Al2O3 and Co0.4Mo0.6Cx Carbide Catalysts: A Comparative Study Daniel J. Haynes,*,†,‡,§ David A. Berry,† Dushyant Shekhawat,† Tian-Cun Xiao,| Malcolm L. H. Green,| and James J. Spivey§ U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, West Virginia 26507, Department of Chemical Engineering, Louisiana State UniVersity, 110 South Stadium DriVe, Baton Rouge, Louisiana 70803, and Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom

Catalytic partial oxidation (CPOX) of liquid fuels is being widely studied as an option for producing a hydrogenrich gas stream for fuel cells. However, deactivation of catalysts by carbon deposition and sulfur poisoning in this process is a key technical challenge. Here, the deactivation of Co0.4Mo0.6Cx has been compared to that of 1 wt % Pt/γ-Al2O3 in a fixed-bed catalytic reactor, using mixtures of n-tetradecane and either 1-methylnaphthalene (1-MN) or dibenzothiophene (DBT) to simulate diesel fuel. The results show that Co0.4Mo0.6Cx is stable and active for the CPOX of n-tetradecane at 850 °C, 50000 scc/(gcat h), and an O/C ratio of 1.2. This catalyst produces slightly lower H2 and CO yields than Pt/γ-Al2O3, but still close to equilibrium values for 5 h. A low concentration of sulfur (50 ppmw as DBT) has little effect on either activity or selectivity for the carbide or Pt/γ-Al2O3 catalyst. However, the presence of 1-MN or a high sulfur concentration (1000 ppmw as DBT) deactivates both catalysts, resulting in reaction products that are typical of gas-phase reactions in a blank reactor. The addition of 1-MN or 1000 ppmw DBT to n-tetradecane produces qualitatively similar results on both catalysts: H2 production decreases continuously in the presence of either 1-MN or DBT, and CO drops to a stationary level. This drop in synthesis gas yields corresponds to an increase in steam, CO2, and olefin yields, suggesting that the contaminants deactivate sites that are active for steam and dry reforming reactions downstream of the reactor inlet, where rapid oxidation takes place. Once the contaminants are removed, initial activity returns more quickly for the carbide than for Pt/γ-Al2O3. 1. Introduction As fuel cell technology becomes more economically practical, an adequate supply of hydrogen must be provided to meet the largescale demand. For this reason, the catalytic reforming of logistic fuels has been attracting attention.1 Their wide availability and potential to produce a hydrogen-rich synthesis gas make them convenient energy sources. Although these fuels can be reformed to produce hydrogen-rich gas, they also contain large concentrations of sulfur and aromatic compounds. These species can inhibit the reforming reactions and deactivate supported metal catalysts.2,3 Deactivation by sulfur poisoning and carbon deposition is a key technical challenge, and the reforming of logistic fuels will require a catalyst that can tolerate these compounds. The following three reactions are widely used for the reforming of liquid fuels into synthesis gas4 catalytic partial oxidation (CPOX): CmHn + O2 f H2 + CO (∆H < 0) steam reforming (SR): CmHn + H2O f H2 + CO (∆H > 0) autothermal reforming (ATR): CmHn + O2 + H2O f H2 + CO (∆H ≈ 0) A fuel processor based on CPOX is considered to be the most practical system for hydrogen generation because no external * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (304) 285-1355. Fax: (304) 2850903. † National Energy Technology Laboratory. ‡ Currently with Parsons, P.O. Box 618, South Park, Pennsylvania 15129. § Louisiana State University. | University of Oxford.

heat or added steam is necessary.5,6 This means that the fuel processor can operate in remote, mobile, or military applications where water is scarce. Traditionally, group VIII noble metals, such as Pt and Rh, have been used as CPOX reforming catalysts.4,7-12 They exhibit considerable activity and selectivity toward hydrogen formation. Unfortunately, high metal prices have significantly limited their use for commercial reforming applications. Thus, it is necessary to develop economically viable catalysts that are as stable and active as noble metals. It was discovered in the early 1970s that transition metal carbides have catalytic properties similar to group VIII noble metals.13,14 While the transition metals by themselves are catalytically inactive for reforming reactions, certain carbides of these metals have catalytic properties that are characteristic of Pt, Ru, and other group VIII noble metals. This behavior was reported by Sinfelt and Yates,13 who found that molybdenum metal became active over time in the hydrogenolysis of ethane as a result of the formation of molybdenum carbide. Levy and Boudart14 reported similar behavior when they discovered that an unsupported tungsten carbide was able to isomerize neopentane into xylene, a behavior that was previously exhibited by Pt.15 It is speculated that this modified performance is a result of electron sharing between interstitial carbon atoms and the d orbitals of the transition metals.16,17 These electrons enhance the electronic properties of the metal such that their reactivities resemble those of group VIII elements.16,17 Typical applications for metal carbide catalysts have been hydrogenation,13,18 isomerization,14,19 and hydrotreating20-23 reactions. Recently, however, bulk molybdenum carbide (βMo2C) catalysts are receiving attention for use in synthesis gas production because of their high resistance to deactivation by

10.1021/ie071295t CCC: $40.75  2008 American Chemical Society Published on Web 09/19/2008

7664 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

sulfur24,25 or by carbon formation.26,27 It has also been reported that the addition of cobalt to β-Mo2C leads to structural modifications that result in improved activity and stability in hydrotreating processes such as hydrodenitrogenation22,23 and hydrodesulfurization.20,21 This suggests that cobalt molybdenum carbide could be a durable partial oxidation catalyst capable of reforming liquid fuels with minimal carbon deposition and little deactivation by sulfur. There is a question of the stability of a transition metal carbide in an oxidizing environment, as the interstitial carbon that gives the metal its unique activity might be oxidized during the reaction. Although we are aware of no studies in the literature involving the CPOX of higher hydrocarbons (C2+) using transition metal carbides, several studies have shown that carbides are stable for the partial oxidation of methane (POM).24,26,28-31 According to some of these studies,26,31 the carbide is stable at an elevated pressure (g0.4 MPa), which seems to stabilize the surface lattice carbon. In a POM isotopic exchange study, Xiao et al.31 dosed a labeled β-Mo213C catalyst with a 12C/O2 mixture at 857 °C, 0.1 MPa, and O/C ) 1 and reported that the lattice carbon participates in the POM. However, because of the low pressure, they discovered that, after the first pulse, the selectivity toward H2 and CO began to drop rapidly and the selectivity to CO2 increased. At low pressures (0.1 MPa), both Xiao et al.31 and Claridge et al.26 noted that deactivation of the carbide occurred as a result of the rapid oxidation of surface lattice carbon. However, both studies found that this deactivation by oxidation could be avoided by running at higher pressures (0.4 MPa), which allows the recarburization and oxidation reactions to occur at similar rates. At these elevated operating pressures, Claridge et al.26 showed that β-Mo2C carbide catalyst can produce high synthesis gas yields in the CPOX of methane at 900 °C inlet temperature for over 10 h. In fact, according to their results, the reactivity of oxygen on the surface of the β-Mo2C resembles that of noble metals. The levels of oxidant in the feed also play a role in the stability of the carbide in a reforming environment. Cheekatamarla and Thomson32 discovered that running at high levels of excess oxidant during the SR (S/C > 1.5) and oxidative steam reforming (O/C > 0.4 with S/C ) 1.3) of trimethyl pentane at 900 °C resulted in a loss in H2 yield after 4 h of operation. Deactivation was shown to be a result of oxidation of the carbide. They found that a kinetic equilibrium could be established between carburizing agents and oxidizing agents by running at an optimal S/C ratio for SR and O/C ratio for oxidative steam reforming to produce stable synthesis gas yields. The study here examines Co0.4Mo0.6Cx as an alternative catalyst for the CPOX of logistic fuels. The CPOX activity and selectivity of the catalyst were measured using model fuel compounds chosen to represent diesel fuel. The experiments were designed to study the catalyst activity and resistance to deactivation by aromatics and organosulfur compounds that are commonly found in fuel mixtures. The results were compared to those obtained with a Pt/γ-Al2O3 catalyst, which serves as a basis of comparison because of its reforming properties and widely known inherent catalytic behavior in reforming hydrocarbons.2,10,12,33,34 Although Pt is not the optimal catalyst for the CPOX of hydrocarbons,11,12 it has been studied for the CPOX of diesel fuel constituents11,12 and is capable of producing high synthesis gas yields.35

Table 1. Experimental Conditions for CPOX Reactions parameter feed composition (mol%) N2 O2 TD O/C GHSV [scc/(gcat h)] total flow (sccm) temperature (°C) catalyst bed (mg) pressure (MPa) carbide Pt/γ-Al2O3

value 80.1 17.8 2.1 1.2 50000 400 850 450 0.46 0.23

2. Experimental Section 2.1. Catalyst Preparation. The preparation of the bimetallic carbide catalyst (Co0.4Mo0.6Cx) has been described previously.22 To summarize, the carbide catalyst was prepared by the carburization of CoMo bimetallic oxide powder using temperature-programmed reduction in a C2H6/H2 mixture.22 The Pt/ γ-Al2O3 catalyst was synthesized by NexTech. A 1 wt % Pt loading was deposited onto the γ-Al2O3 support (W.R. Grace) using an incipient wetness technique. After metal incorporation, the catalyst was calcined at 500 °C for 1 h under O2. 2.2. Catalyst Activity Measurements. n-Tetradecane (TD) was used as a model logistic fuel compound to screen catalysts for activity and selectivity. A mixture of 5 wt % 1-methylnaphthalene (1-MN) in TD was used to simulate the presence of aromatic compounds. The sulfur tolerance of the catalysts was assessed through the CPOX of TD containing 50 or 1000 ppmw sulfur as dibenzothiophene (DBT). A detailed description of the experimental system used for the CPOX experiments is documented elsewhere.35 Briefly, catalyst tests were carried out in a fixed-bed continuous-flow reactor. The catalyst was diluted with quartz sand of the same particle size (5/1 by weight) to avoid channeling and minimize temperature gradients. The bed was placed in a tubular reactor (8 mm i.d.) with an axially centered thermocouple. Experimental conditions for the CPOX experiments are detailed in Table 1. It should be noted that, during several of the experiments, shortlived pump failures occurred. When this happened, the air flow was stopped immediately to minimize oxidation of the interstitial carbon in the carbide and also any carbon that formed on the surface. These flow interruptions were very short (about 1 min) and did not affect the experimental results. In addition to the Co0.4Mo0.6Cx and Pt/γ-Al2O3 catalysts, CPOX of TD was also examined over Co0.4Mo0.6Ox at 0.46 MPa to compare the activity of the oxide to that of the carbide. To obtain the oxide phase, the carbide catalyst was oxidized under a 5% mixture of O2 and N2 as the temperature was increased from ambient to 850 °C by 5 °C/min. Oxidation of the material was continued overnight at 850 °C to ensure that O2 uptake into the structure was complete. Phase identification of the oxide and carbide was performed in a PanAnalytical X’pert Pro X-Ray diffraction system (model number PW 3040 Pro). The CPOX of TD over a blank reactor (filled with quartz sand) was also conducted to observe the selectivity of gas-phase reactions and to determine the amount of carbon deposited on the quartz diluent used in the catalytic studies. Temperature-programmed oxidation (TPO) of the used catalyst was performed after each experiment to determine the amount of carbon formed during reforming reactions. A 5% mixture of O2 and N2 was passed over the catalyst as the temperature was ramped from 200 to 800 °C at 1 °C/min. Once

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7665 Table 2. Catalyst Properties property composition loading (wt %) surface area (m2/g) particle sizec (µm) metal dispersion (%) b

carbide22 Co0.4Mo0.6Cx 2.56a 40 170-310 N/A

Pt/γ-Al2O3 a

Pt/γ-Al2O3 1.0b 198 170-310 30

a Carbon content, i.e., % by weight of carbon (as carbide); see ref 22. As prepared. c Corresponds to 60-100 mesh size.

at 800 °C, the oxidation ran isothermally overnight. An online mass spectrometer was used to monitor the CO2 emitted during the TPO. The spent carbide underwent a recarburization procedure after each TPO to replenish any carbon that might have been oxidized. After each TPO, the CoMo sample was reduced under a 20% CH4/H2 mixture at 850 °C and 0.46 MPa for 1 h before the flow rates were changed for normal startup. As a result of this procedure, the same carbide sample was used for each experiment. 2.3. Product Analysis. The gases (H2, O2, N2, CO, CO2, and CH4) were analyzed continuously using a Thermo Onix mass spectrometer (model no. Prima δb, a 200-amu scanning magnetic sector). Gaseous hydrocarbon products were analyzed using an HP5890 gas chromatograph equipped with a flame ionization detector. Water concentration was not analyzed directly; however, it was calculated from mass balances of oxygen- and hydrogen- containing species in the product stream. The yield of each product was determined by yield of A (%) ) (moles of A produced × 100)/ (N × moles of hydrocarbon fed to the reactor) where N is the number of moles of H2 per mole of hydrocarbon for H2 yields and the number of moles of carbon in the hydrocarbon fuel for yields of CO and CO2. Hydrocarbon yield is defined as yield of hydrocarbon (%) ) (moles of hydrocarbon produced × 100 × M)/ (N × moles of hydrocarbon fed to the reactor) where M is the number of moles of carbon per mole of hydrocarbon product and N is the number of moles of carbon in the hydrocarbon feed. The conversion of hydrocarbons is defined as 6

carbon balance (%) ) [(CO + CO2 +

∑ iC H ) × 100]/ i

r

i)1

(N × moles of hydrocarbon fed to the reactor) where i is the number of carbon atoms and r is the number of hydrogen atoms contained in the hydrocarbon product. N is the number of moles of carbon in the hydrocarbon feed. 3. Results and Discussion 3.1. Catalyst Properties and Characterization. The physical properties of the two catalysts used in this study are given in Table 2. Results of the TPO of the fresh carbide are shown in Figure 1. The slight shoulder seen at 400 °C is probably due to the oxidation of excess carbon at the surface from the recarburization process. This carbon is more reactive than the interstitial carbon because it is not bound within the structure. As can be seen, the carbon is completely removed from the structure once the reaction temperature (850 °C) is reached. XRD profiles shown in Figure 2 confirm that bimetallic oxide and carbide phases have formed and that they are consistent with those reported by Xao et al.22

Figure 1. TPO of fresh Co0.4Mo0.6Cx catalyst.

Figure 2. XRD profiles of Co0.4Mo0.6Ox and Co0.4Mo0.6Cx catalysts: (() monclinic CoMoO4 phase, (2) bimetallic carbide phase.

3.2. CPOX of n-Tetradecane. The CPOX activity of the catalysts was assessed by reforming TD for 5 h. Yields of H2, CO, CO2, and olefins; conversions of TD; and amounts of carbon formed for the three catalysts and blank reactor are reported in Table 3. The H2, CO, CO2 and CH4 yields produced over the Co0.4Mo0.6Cx are presented in Figure 3 to illustrate the activity over this time period. The initial drop in activity for the carbide is likely due to reaction of excess carbon (reactive carbon seen during the TPO shown in Figure 1) on the surface left over from the recarburization process. In the absence of a catalyst (i.e., over quartz sand), the carbon balance is very high, ∼90%. However, compared to the results obtained with the catalysts, the selectivities to H2 and CO are much lower than those predicted by thermodynamic equilibrium because of the high yields of olefins and CO2 products resulting from the gas-phase reactions occurring in the system. Results from the activity screening indicate that the oxide phase is less active than the carbide, as expected. Comparison of the reforming data in Table 3 shows that the carbon balance is considerably lower, as are the selectivities to H2 and CO, over the Co0.4Mo0.6Ox compared to the carbide. The significant difference in activity between the carbide and oxide phases demonstrates the stability of the lattice carbon in the carbide under CPOX reforming conditions and the need for the carbon to remain in the bulk structure to consistently produce high synthesis gas yields. Both Co0.4Mo0.6Cx and Pt/γ-Al2O3 show very stable behavior during the CPOX of TD for 5 h. The TD conversion to gaseous products is always more than 98%, and the H2/CO ratio is 1:1

7666 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 3. Yields of H2, CO, CO2, and Olefins; TD Conversion; and Amount of Carbon Formed from Equilibrium, Blank Reactor, Co0.4Mo0.6Ox, Co0.4Mo0.6Cx, and Pt/γ-Al2O3 after 5 h on Stream at 850°C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx and Co0.4Mo0.6Ox) or 0.23 MPa (Pt/γ-Al2O3 and Quartz Sand), and 50000 scc/(gcat h)

H2 yield (%) CO yield (%) CO2 yield (%) olefin yield (%) carbon balance (%) amount of carbon formed (g/gcat)

equilibriuma

quartz sand (blank)

Co0.4Mo0.6Ox

Co0.4Mo0.6Cx

Pt/γ-Al2O3

85 86 9 0 100 0

6 35 20 21 90 0.80

48 51 26 1.2 85 0.07

76 75 20 ndb 98 0.61

81 81 17 1.9 >99 0.85

a Equilibrium conversion values were calculated using HSC Chemistry thermodynamic software.36 The calculation was made assuming the complete CPOX of TD at 850 °C, O/C ) 1.2, and 0.23 MPa. b nd ) not detected.

Figure 3. Yields of H2, CO, CO2, and CH4 during the CPOX of TD over Co0.4Mo0.6Cx at 850 °C, O/C ) 1.2, 0.46 MPa, and 50000 scc/(gcat h): (9) H2, (() CO, (2) CO2, (b) CH4.

Figure 4. TPO of carbon deposited on (() Pt/γ-Al2O3, (9) Co0.4Mo0.6Cx, (×) Co0.4Mo0.6Ox, and (2) quartz sand. Conditions for each experiment: 850 °C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx and Co0.4Mo0.6Ox) or 0.23 MPa (Pt/γ-Al2O3 and quartz sand), and 50000 scc/(gcat h).

for both Co0.4Mo0.6Cx and Pt/γ-Al2O3. Although the TD conversion is nearly complete for both catalysts, their selectivities differ from each other and from equilibrium values (Table 3). Co0.4Mo0.6Cx shows slightly lower H2 and CO yields than Pt/γ-Al2O3. Both catalysts show significantly higher CO2 selectivities than the equilibrium value. An additional difference between the Co0.4Mo0.6Cx and Pt/ γ-Al2O3 catalysts is the formation of olefins, which are known coke precursors.37 No olefins are detected from the CPOX of TD over the carbide catalyst, whereas significant amounts of olefins are formed over the Pt/γ-Al2O3 catalyst. A final difference is carbon formation. The results of the TPO of carbon deposited on the three catalysts and quartz sand (blank reactor) are shown in Figure 4. [Although the carbon in the carbide structure could be oxidized during the TPO, based on the carbon content of the carbide (Table 2) and catalyst loading (480 mg), the total amount of carbon in the carbide could account for no more than 26 mgcarbon/gcat, or 4% of the carbon in Table 3.] Two peaks are observed in the TPO for the Pt/γ-Al2O3 catalyst. The low-temperature peak (∼550 °C) can be assigned to carbon deposits on the metal sites,38,39 which are oxidized easily at these temperatures because the metal catalyzes the carbon-oxygen reaction. The high-temperature peak (750 °C) can be attributed to the carbon deposited on the alumina support.38,39 The TPO profile for the Co0.4Mo0.6Ox has a small, single low-temperature peak at ∼290 °C. Bouchy et al.40 and Boskovic et al.41 reported similar TPO curves when using MoO3 for the isomerization of n-alkanes and found broad peaks around 380 and 350 °C, respectively. They attributed the CO2 to be a result of the combustion of an oxycarbide with the stoichiometry MoO2.34C0.10H1.38. Comparison of these TPO results with Figure 4 suggests that the introduction of cobalt into the molybdenum

structure creates a more reactive oxycarbide, corresponding to the ∼100 °C lower TPO peak. The high reactivity for this species might also be the reason for the considerably lower amount of carbon deposited on the surface compared to Co0.4Mo0.6Cx and Pt/γ-Al2O3. Interestingly, the TPO curve of the used carbide catalyst is similar to that of the fresh catalyst shown in Figure 1. This oxidizible carbon on the Co0.4Mo0.6Cx catalyst has a reactivity similar to that found on the blank reactor and on the γ-Al2O3 support of the Pt/γ-Al2O3. The small shoulder peak at ∼590 °C suggests that a small amount of more reactive carbon is deposited on the carbide. Lower total carbon formation on the carbide catalyst compared to the Pt/γ-Al2O3 catalyst (0.61 versus 0.85 gcarbon/gcat) can be attributed to the metal ensemble size and the stability of the adsorbed carbon atoms. It is known that carbon formation for a supported metal catalyst requires an ensemble of six to seven metal atoms,37 which is presumably not available in carbide catalyst. Also, the difference in TPO peak temperatures between the two catalysts suggests that the carbon formed during the CPOX reaction on the carbide is intermediate in stability between the carbon associated with the Pt and that associated with the alumina support. Thus, the carbon deposited on the carbide is sufficiently reactive to result in less net carbon than for Pt/γ-Al2O3 after 5 h on stream. 3.3. Effects of Polynuclear Aromatics. The effects of aromatics on catalyst activity were studied using a 5 wt % mixture of 1-MN in TD. These tests were carried out by first running with TD only for 1 h, switching to 5 wt % 1-MN in TD for 2 h, and then switching back to TD for 2 h to examine recovery. The H2, CO, CO2, and CH4 yields in Figure 5a,b stabilize after 30 min for both catalysts during the CPOX of TD only. However, after the introduction of 1-MN, the H2 and CO yields

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7667 Table 4. Comparison of C2, C3, and Benzene Yields for CPOX of TD Only and CPOX of TD after 2 h of 5 wt % 1-MN in the Feed at 850°C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/ γ-Al2O3), and 50000 scc/(gcat h) Co0.4Mo0.6Cx

Pt/γ-Al2O3

TD only 5 wt % 1-MN TD only 5 wt % 1-MN ethane yield (%) ethylene yield (%) propane yield (%) propylene yield (%) benzene yield (%) a

Figure 5. Step response plots for (a) Co0.4Mo0.6Cx and (b) Pt/γ-Al2O3 catalysts after 5 wt % 1-MN was introduced into the feed at 850 °C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/ (gcat h): (9) H2, (() CO, (2) CO2, (b) CH4.

Figure 6. Olefin yields for the (() Pt/γ-Al2O3 and (9) Co0.4Mo0.6Cx catalysts after 5 wt % 1-MN/TD was introduced into the feed at 850 °C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/(gcat h).

from both the carbide and Pt/γ-Al2O3 decrease. On both catalysts, H2 yield decreases continuously until 1-MN is removed, whereas the CO yield drops to a stationary level that is higher for Pt/γ-Al2O3 (∼43%) than for the carbide (∼30%). Coinciding with the decline in synthesis gas yields is a rise in yields of CO2, CH4 (Figure 5a,b), and olefins (Figure 6). The CPOX reaction is generally believed to take place in two regions of catalytic activity in series:42-44 a short combustion zone at the top of the bed followed by a longer endothermic reforming zone. In the first region, the fuel undergoes a complete

0.1 nda nd nd nd

1.0 11.7 0.1 3.5 2.5

0.4 1.2 nd 0.4 0.3

1.0 11.4 0.1 2.9 1.9

nd ) not detected.

combustion until all of the available oxygen has reacted, forming steam and CO2 and leaving some unreacted hydrocarbons. The large exotherms that accompany this oxidation promote thermal breakdown of the remaining fuel into smaller hydrocarbons and H2. Once all of the oxygen is consumed, the unconverted fuel fragments react over the remainder of the catalyst with steam and CO2 via the endothermic steam reforming (SR) and dry reforming reactions, respectively, to form H2 and CO. Given this proposed sequence, it is hypothesized that the results shown in Figure 5a,b are likely a result of the deactivation of the secondary reforming sites downstream of the oxidation zone that catalyze the SR and dry reforming reactions on both catalysts. Evidence for this can be seen as CO2 yields increase for both catalysts after the introduction of 1-MN, suggesting incomplete CO2 reforming reactions. In addition, the continuous decrease in H2 yield appears to be linked to the increased steam formation. Mass balance calculations accounting for H and O atoms show an increase in unaccounted for H and O with a H/O atomic ratio of 2.5 ( 0.9 for Pt/γ-Al2O3 and 2.6 ( 1.2 for the carbide during the 2 h when 1-MN was present in the feed. Although 1-MN could possibly be oxidized in the oxidation zone,39 the deactivation is likely due to the lower reactivity of 1-MN compared to TD over the Pt/γ-Al2O3 and carbide catalysts. In a study to determine the relative reactivities of different compounds in gasoline and diesel surrogate fuel mixtures for the CPOX reaction, Subramanian et al.45 reported that the overall reactivity of the fuel mixture is not an average of the reactivities of the fuels, but that the most reactive component consumes the O2 first and the remaining compounds then undergo pyrolysis. 1-MN is difficult to reform because its polynuclear unsaturated structure is chemically similar to that of coke,4,39 including electron-rich double bonds that adsorb strongly to the surface of the catalysts. The adsorption of 1-MN can block the normal gas-surface interactions of the sites that would otherwise have been involved in the SR and dry reforming reactions, leading to an increase in homogeneous reaction products in the system. Olefins and lower hydrocarbons, which are known gas-phase reaction products,42,46-48 increase in the product stream in the presence of 1-MN (Figure 6 and Table 4), whereas the selectivities toward H2 and CO decrease. After 2 h on stream with TD and 5 wt % 1-MN, both catalysts are completely deactivated such that their dry gas selectivities resemble that of a blank reactor (quartz sand), shown in Table 3. The effects of 1-MN appear to be partially reversible, at least over the time scale of these experiments. Each catalyst was able to recover some activity after 1-MN was removed from the feed, with the H2 and CO yields recovering more quickly for the carbide than for Pt/γ-Al2O3 (Figure 5a,b). The recovery of the catalysts suggests an inhibition by 1-MN rather than irreversible poisoning of the active sites. The short recovery time for the

7668 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

Figure 7. Step response plots for (a) H2 and (b) CO yields from (() Pt/γAl2O3 and (9) Co0.4Mo0.6Cx catalysts after 50 ppmw DBT was introduced into the feed at 850 °C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/(gcat h). Table 5. Comparison of C2, C3, and Benzene Yields for TD Only and for TD after 2 h of 50 ppmw Sulfur in the Feed at 850°C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/(gcat h) Co0.4Mo0.6Cx

ethane yield (%) ethylene yield (%) propane yield (%) propylene yield (%) benzene yield (%) a

Pt/γ-Al2O3

TD only

50 ppmw sulfur

TD only

50 ppmw sulfur

0.1 nda nd nd nd

1.0 0.6 0.1 0.2 nd

0.4 1.2 nd 0.4 0.3

1.0 7.3 0.1 1.9 1.3

nd ) not detected.

carbide also suggests that the aromatic species are less strongly adsorbed to its surface than to Pt/γ-Al2O3. 3.4. Effects of Sulfur-Containing Feed. Two different experiments were performed to study the effects of added sulfur: one at low sulfur concentration (50 ppmw) and another at high sulfur concentration (1000 ppmw). The experiments were carried out by first running TD for 1 h. Then, TD with 50 or 1000 ppmw sulfur as DBT was introduced. The sulfur-containing feed was then run over the catalyst for 2 h, and the feed was switched back to TD for 2 h to examine recovery. 3.4.1. 50 ppmw Sulfur. The data in Figure 7a,b indicate that the effects of 50 ppmw sulfur were relatively small on both catalysts. This low sulfur content alters the H2 and CO yields of both catalysts, leading to the formation of significantly more C2-C3 olefins as well as benzene on the Pt/γ-Al2O3 (Table 5). After removal of the sulfur from the feed, both catalysts recovered activity, suggesting that the low concentration of

Figure 8. Step response plots for the (a) Co0.4Mo0.6Cx and (b) Pt/γ-Al2O3 catalysts after 1000 ppmw DBT was introduced into the feed at 850 °C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/(gcat h): (9) H2, (() CO, (2) CO2, (b) CH4.

sulfur is insufficient to irreversibly poison the catalyst activity. TPO results (not shown) indicate that both catalysts had more carbon form on the surface than for the CPOX of TD only (0.91 gcarbon/gcat for Pt/γ-Al2O3 and 0.95 gcarbon/gcat for the carbide). Yet, with the larger amounts of carbon deposited, both catalysts were able to recover activities to near-precontaminant levels, suggesting that the carbon did not accumulate on the active sites. 3.4.2. 1000 ppmw Sulfur. Figure 8a,b demonstrates that 1000 ppmw of sulfur significantly affects both catalysts. The results are qualitatively similar to the effects of 1-MN addition. After the sulfur is introduced, the H2 yield drops continuously over time, whereas the CO yield decreases initially and then remains relatively constant for both catalysts. The decrease in synthesis gas yield is followed by an increase in yields of CO2, CH4 (Figure 8a,b), and olefins (Figure 9). A study by Liu et al.49 indicated that the interaction of sulfur on a molybdenum carbide catalyst during the HDS reaction formed a MoSxCy layer on the surface, which deactivated the catalyst. In the present study, it is likely that the high concentration of sulfur formed a sulfide layer on the carbide, and its formation can be directly linked to the drop in catalytic activity. A qualitatively similar effect of DBT was observed on the Pt/γ-Al2O3 catalyst. The behavior is likely a result of the adsorption of sulfur on the platinum. Palm et al.50 reported that the addition of 11 ppmw of sulfur decreased the conversion of Pt/γ-Al2O3 in the autothermal reforming of surrogate gasoline compounds for 20 h. They attributed the deactivation to the adsorption of sulfur to the surface of the catalyst. The high concentration of sulfur used in the present study would be expected to saturate the

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7669

Figure 9. Olefin yields for the (() Pt/γ-Al2O3 and (9) Co0.4Mo0.6Cx catalysts after 1000 ppmw DBT/TD was introduced into the feed at 850 °C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/ (gcat h). Table 6. Comparison of C2, C3, and Benzene Yields for TD Only and for TD after 2 h of 1000 ppmw Sulfur in the Feed at 850°C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/(gcat h) Co0.4Mo0.6Cx

ethane yield (%) ethylene yield (%) propane yield (%) propylene yield (%) benzene yield (%) a

Pt/γ-Al2O3

TD only

1000 ppmw sulfur

TD only

1000 ppmw sulfur

0.1 nda nd nd nd

1.2 12.8 0.1 3.9 2.4

0.4 1.2 nd 0.4 0.3

0.9 11.6 0.1 2.9 1.9

nd ) not detected.

platinum surface much more quickly. As a result, deactivation occurs more rapidly, and the conversion to synthesis gas is decreased. On both catalysts, the sulfur reduces the rate of the reforming reactions, similarly to the addition of 1-MN, suggesting that sulfur also deactivates the sites downstream of the oxidation zone that engage in the endothermic reforming reactions. As a result, there is an increase in gas-phase reactions in the system. Consequently, the catalyst selectivities, shown in Figures 8a,b and 9 as well as Table 6, qualitatively resemble the effects of 1-MN. As for experiments with 1-MN addition, a material balance suggests that the continuous decrease in H2 can be attributed to the formation of steam. The ratio of unconverted H and O atoms (H/O) was determined to be 2.9 ( 0.9 for Pt/ γ-Al2O3 and 3.0 ( 1.1 for the carbide. After 2 h time on stream, sulfur deactivated both catalysts over time such that their selectivities resembled that of a blank reactor (quartz sand) after 2 h on stream (Table 3). Similarly to the effects of 1-MN, the effects of DBT are potentially reversible over the time scale of these experiments. After the sulfur was removed from the feed, the original catalyst activity was nearly restored for the carbide, but not for the Pt/ γ-Al2O3 catalyst, which only partial recovered. DBT, like 1-MN, acts as a reversible inhibitor rather than a poison for the carbide catalyst, suggesting that, if a surface sulfide is formed, the original carbide (or an equally active form of the catalyst) is formed rapidly once DBT is removed. For the Pt/γ-Al2O3 catalyst, the sulfur appears to remain more strongly bound to the catalyst, leading to longer recovery times. 3.5. Carbon Formation. The presence of 1-MN and high DBT concentrations in the feed lead to 2-3 times more carbon

Figure 10. TPO of (() Pt/γ-Al2O3 and (9) Co0.4Mo0.6Cx catalysts after (a) 1-MN and (b) 1000 ppmw sulfur experiments. Conditions for each experiment: 850 °C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/(gcat h). Table 7. Carbon Deposited on Carbide and Pt/γ-Al2O3 Catalysts after 5 h on Stream at 850°C, O/C ) 1.2, 0.46 MPa (Co0.4Mo0.6Cx) or 0.23 MPa (Pt/γ-Al2O3), and 50000 scc/(gcat h) feed TD 5 wt % 1-MN/TD 1000 ppmw sulfur/TD

Co0.4Mo0.6Cx (gcarbon/gcat) Pt/γ-Al2O3 (gcarbon/gcat) 0.61 0.84 1.3

0.85 2.3 2.4

formation than without contaminants in the feed. Table 7 shows the carbon measured by TPO of both catalysts after 1-MN and 1000 ppmw sulfur experiments. Similarly to the CPOX of TD only, the carbide catalyst had lower amounts carbon deposited on its surface than Pt/γ-Al2O3. However, the TPO peaks in Figure 10a,b suggest that the carbon is qualitatively similar on the two catalysts, with the Pt/γ-Al2O3 simply having more carbon. Comparison of the TPO peaks of Figures 4 and 10 demonstrates that the TPO peak temperatures after the 1-MN and TD experiments are similar for the carbide, showing that the reactivities of the carbon are similar. However, more carbon is formed by the addition of 1-MN. The presence of DBT leads to the deposition of a more refractory carbon, as shown by the shift of the TPO peak to higher temperatures. For Pt/γ-Al2O3, the TPO results for 1-MN and DBT are qualitatively and quantitatively similar. The contaminants cause a much greater accumulation of carbon on the surface such that, compared to Figure 4, the carbon deposited on the metal or support cannot be distinguished. 3.6. Stability of Carbide. Reports in the literature have demonstrated that carbide catalysts can become deactivated

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through the oxidation of the carbide phase. For instance, Brungs et al. found that bulk β-Mo2C lost activity as it was converted to MoO2 during the dry reforming of methane.51 In another study, Cheekatamarla and Thomson observed that β-Mo2C was also deactivated as the surface carbide was oxidized during the SR and oxidative steam reforming of trimethyl pentane.52 These results raise concerns about the stability of the Co0.4Mo0.6Cx catalyst used for the CPOX of hydrocarbons in this study, because the presence of oxygen at CPOX conditions might oxidize the carbide to the bimetallic oxide phase. However, the results show that the Co0.4Mo0.6Cx catalyst studied here remains active during the CPOX of TD for 5 h with no loss of activity (Figure 3). Because the carbide was subjected to an extreme oxidizing environment during the TPO after each experiment, the catalyst was treated in a H2/CH4 reduction gas mixture to re-form the carbide phase if any oxidation had taken place, a process directly related to the original synthesis method for the carbide.22 At the start of each experiment, CPOX of TD was performed to verify that the activity was comparable to that measured in previous experiments. In each experiment, the carbide demonstrated quantitatively identical synthesis gas yields after startup, within experimental error, indicating that the active carbide phase was in fact present. 4. Conclusions A Co0.4Mo0.6Cx catalyst exhibited behavior similar to that of a conventional 1 wt % Pt/γ-Al2O3 catalyst in the CPOX of logistic fuel surrogates. The presence of 50 ppmw sulfur decreased H2 and CO yields on both catalysts. However, the presence of 1-MN and 1000 ppmw sulfur in the feed deactivated both catalysts and led to an increase in gas-phase reactions. For the carbide, activities were restored quickly to near-precontaminant levels after removal of the contaminants from the feed. This was not true for the Pt/γ-Al2O3 catalyst. This behavior indicates that, at these reaction conditions, 1-MN and sulfur at high concentrations act as reversible inhibitors of the reforming reactions on the carbide catalyst, but they appear to irreversibly poison the Pt/γ-Al2O3. The ability of the carbide to recover most of its initial activity quickly also indicates that the contaminants are weakly adsorbed to its surface. Acknowledgment We gratefully acknowledge Mr. Donald Floyd for his invaluable assistance with this investigation. This work was supported by National Energy Technology Laboratory Contract DE-AC26-04NT41817 Subtask 41817.610.01.01. Literature Cited (1) Jain, S.; Chen, H.-Y.; Schwank, J. Techno-economic analysis of fuel cell auxiliary power units as alternative to idling. J. Power Sources 2006, 160 (1), 474–484. (2) Cheekatamarla, P. K.; Lane, A. M. Catalytic autothermal reforming of diesel fuel for hydrogen generation in fuel cells: I. Activity tests and sulfur poisoning. J. Power Sources 2005, 152, 256–263. (3) Cheekatamarla, P. K.; Lane, A. M. Catalytic autothermal reforming of diesel fuel for hydrogen generation in fuel cells: II. Catalyst poisoning and characterization studies. J. Power Sources 2006, 154 (1), 223–231. (4) Shekhawat, D.; Berry, D. A.; Gardner, T. H.; Spivey, J. J., Catalytic Reforming of Liquid Hydrocarbon Fuels for Fuel Cell Applications. In Catalysis, Spivey, J. J., Dooley, K., M., Eds.; Royal Society of Chemistry: London, 2006; Vol. 19, pp 184-253. (5) Ahmed, S.; Kumar, R.; Krumpelt, M. Fuel processing for fuel cell power systems. Fuel Cells Bull. 1999, 2 (12), 4–7. (6) Ahmed, S.; Krumpelt, M. Hydrogen from hydrocarbon fuels for fuel cells. Int. J. Hydrogen Energy 2001, 26 (4), 291–301.

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ReceiVed for reView September 26, 2007 ReVised manuscript receiVed March 21, 2008 Accepted June 27, 2008 IE071295T