Exhaust Gas Reforming of Methane in a Catalytic Microchannel

Oxidative steam reforming (OSR) of methane is investigated under exhaust gas reforming conditions in a wall-coated catalytic microchannel reactor...
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Exhaust Gas Reforming of Methane in a Catalytic Microchannel Reactor Irem Sen and Ahmet K. Avci* Department of Chemical Engineering, Bogazici University, Bebek 34342, Istanbul, Turkey ABSTRACT: Oxidative steam reforming (OSR) of methane is investigated under exhaust gas reforming conditions in a wallcoated catalytic microchannel reactor. The process is run over different combinations of Pt- and Rh-based catalysts, namely 0.2% (by weight) Pt/Al2O3−2%Rh/Al2O3, 2%Pt/Al2O3−2%Rh/Al2O3, and 2%Rh/Al2O3−2%Rh/Al2O3. In each combination, coated catalysts are locked on the opposite walls of a rectangular microchannel to face each other. Parametric study is conducted in order to observe the effects of feed compositions (molar steam-to-carbon (H2O/C) and oxygen-to-carbon (O2/C) ratios) and temperature on methane conversion and on product distribution. The results show that methane conversion is enhanced with the increase in temperature and in the amounts of O2 and H2O in the feed stream. When the temperature is raised from 600 to 700 °C, methane conversion is found to improve by ca. 25% in all catalyst configurations. Increasing H2O/C and O2/C ratios improved methane conversion at most by 7% and by 23%, respectively. H2 production also increased with temperature, with the highest increase of 5% is observed over the 2%Rh/Al2O3−2%Rh/Al2O3 combination. Higher O2/C ratios improved the extent of methane total oxidation but decreased H2 production. The catalyst combination involving 2% Rh/Al2O3 coated oppositely onto the inner walls of the microchannel is found to exhibit the best performance in terms of methane conversion and H2 and CO amounts in the product stream. A 10% increase in methane conversion is observed either by changing the Pt content from 0.2% to 2% or by replacing 2%Pt with 2%Rh. The results show that Rh is superior to Pt in terms of oxidation and steam reforming activities.

1. INTRODUCTION Exhaust gas reforming (EGR) is a technology proposed to improve the efficiency and reduce the emissions of conventional internal combustion engines (ICE).1 In this system, a compact, catalytic reformer is integrated into the exhaust gas recirculation loop to convert an externally injected fuel into a hydrogen-rich mixture, which is fed back to the ICE to improve combustion efficiency and reduce undesired emissions.1 Heat needed to drive EGR is supplied by the sensible heat of the exhaust gas stream generated in the ICE and by the catalytic oxidation of part of the fuel injected into the reformer.2 Fast heat transfer along the reformer has a direct impact on the success of EGR, which is dictated by the catalyst type and reactor configuration. Pt- and Rh-based catalysts are known as the promising options for efficient hydrogen production via EGR.3,4 It is known that total oxidation (TOX), steam reforming (SR), and water gas shift (WGS) reactions occur consecutively over Pt and Rh catalyzed EGR,5,6 so that determining a proper reactor configuration is very important to ensure the effective transfer of the heat supplied to the reformer and generated by TOX:7

The exhaust stream also involves CO2 which can be reformed into H2 and CO by CO2 reforming (Reaction 4) which, however, is reported to be slow compared to SR when steam is present in the feed:6 CO2 + CH4 ⇔ 2H 2 + 2CO

SR: CH4 + H 2O ⇔ CO + 3H 2

(1)

ΔH ° = 206.2 kJ mol−1

Special Issue: Recent Advances in Natural Gas Conversion

(2)

WGS: CO + H 2O ⇔ CO2 + H 2

(4)

Current studies on EGR consider the use of either packed bed or monolith reactors.7,8 When a typical temperature distribution of EGR is observed in a packed-bed type reformer, a notable hot spot formation can be seen, showing that heat cannot be distributed evenly along the bed.7,8 This situation, which is caused by the inherently weak heat transfer properties of packed beds, can be enhanced by using a wall-coated monolith reactor in which the difference between maximum and reactor exit temperatures is much less than observed in the packed bed configuration. In monolith reactors containing precious metal catalysts, it is reported that in the hot zone catalyst activity is very high and is close to equilibrium, provided that the mass transfer limitations are overcome. In the cooler zone, however, WGS is thermodynamically favored which contributes to H2 production together with SR. The extent of these contributions is determined by the position and size of the temperature peak in the catalyst bed which again proves the importance of reactor configuration.8

TOX: CH4 + 2O2 → CO2 + 2H 2O ΔH ° = −802.3 kJ mol−1

ΔH ° = 247 kJ mol−1

Received: July 26, 2013 Revised: October 21, 2013 Accepted: October 24, 2013

ΔH ° = −41 kJ mol−1 (3)

© XXXX American Chemical Society

A

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operation of microchannel reactors using cocurrent and countercurrent flow was compared, and the latter one was shown to be advantageous in terms of methane conversion and CO selectivity. Karakaya et al.30 tested methane OSR in a microchannel reactor composed of Pt- and Rh-based catalysts that were coated on the opposite walls of the channel and compared the performance of this configuration with a bimetallic Pt−Rh coated microchannel. The results showed that the latter improved methane conversion up to 20% and CO selectivity up to 33%. Simsek et al.31 investigated the reaction in a microchannel involving coated and particulate forms of bimetallic 0.2%Pt-2%Rh/δ-Al2O3 catalyst. The coated configuration was described by a microchannel involving layers of the bimetallic catalyst coated onto the inner channel walls, whereas placing the catalyst in particulate form into the empty microchannel described the packed microchannel reactor. The results showed that there was no significant difference between the microchannel configurations in terms of methane conversion; however, the coated one gave significantly higher CO selectivity than the packed one in the whole parameter range. This work involves EGR of methane, i.e. methane OSR with the range of inlet conditions dictated by those of an exhaust-gas stream of a gasoline fueled ICE in a catalytic microchannel reactor, and aims to explore the impact of several parameters including reaction temperature, catalyst type, and feed conditions (molar steam-to-carbon (H2O/C) and oxygen-tocarbon (O2/C) ratios in the feed) on methane conversion and on product distribution. Details of the catalyst synthesis, microchannel reactor, experimental setup, and reaction conditions are provided in the Experimental section. Results and Discussion outlines the major outcomes of this work.

The use of catalytic microchannel reactors offers a potential solution for enabling heat transfer much faster than involved in packed-bed or in monolith type reformers. Microchannel reactors are known as the units having parallel, identical, catalyst coated channels with channel dimensions varying between ∼10−6−10−3 m.9 They have surface area-to-volume ratios ca. 100−500 times higher than those of conventional catalytic units and operate under laminar flow conditions which make them advantageous with enhanced heat transfer characteristics.10 As a result, heat transfer coefficients become one order of magnitude higher than those involved in the traditional units.10−15 The compact and metallic nature of the microchannel units also favor better heat transfer along the catalyst layer which is utilized efficiently during the reactions without the formation of any hot or cold spots.9,16−20 These properties make microchannel reactors promising for use in EGR in which heat transfer is critical as described above: energy supplied by the sensible heat of the ICE exhaust stream and by the exothermic heat of oxidation of the fuel injected into the reformer should be effectively transferred to the endothermic fuel reforming to favor the synthesis of a H2-rich product.21 The combination of Reactions 1−3 is also known as oxidative steam reforming (OSR). Methane OSR was studied by several research groups using conventional packed-bed reactor configuration. Tomishige et al.22 investigated methane OSR over Al2O3 supported Ni- and Pt-based catalysts. They observed that the Pt/Al2O3 catalyst gave higher methane conversions and allowed effective heat transfer from oxidation to steam reforming due to the reducibility of Pt that made it to be maintained in metallic state within the oxidation zone. Mukainakano and Li23 showed that, compared with the monometallic ones, bimetallic Pd−Ni and Rh−Ni catalysts gave better performances in terms of heat transfer and dampened hot spot formation. Li et al.24 studied the temperature profiles of alumina-supported noble metal (Rh, Pt, and Pd) catalysts in methane OSR. The results showed that the overlap of the TOX and SR zones, which was the most significant over the Rh-based catalyst, led to higher methane conversion, inhibited hot spot formation, and improved heat transfer efficiency. In addition to the studies that emphasize the dependence of heat transfer on catalyst type in packed-bed reactors, methane OSR was also studied in structured catalytic reactors. Fichtner et al.25 used microstructured honeycomb catalysts made of Rh in methane OSR. It was observed that high thermal conductivity of Rh led to good heat distribution in the flow direction so that conversions and CO and H2 selectivities were improved. Mei et al.26 tested methane SRcombustion coupling in a metal monolith catalytic reactor. It is demonstrated that proper reactor configuration involving high number or large size of the channels in the reforming side and nonuniform catalyst distribution in the combustion side improved heat transfer properties and increased methane conversion. When Ni-based monolith was tested, it was also seen that high methane conversions and selectivities to both H2 and CO were obtained.27 Methane OSR was also studied in a microchannel reactor manufactured from two different materials, Fecralloy and Nicrofer, both on which Rh was deposited.28 It was shown that Fecralloy established a stable alumina coating after high temperature calcination which increased the surface area of the reactor and provided sites available for the Rh particles. Makarshin et al.29 studied the effect of flow regime on the process efficiency of microchannel methane OSR. The

2. EXPERIMENTAL 2.1. Catalyst Preparation. The experiments carried out in this study involve testing of the combinations of three different catalysts − 0.2 wt %Pt/δ-Al2O3, 2 wt %Pt/δ-Al2O3, and 2 wt % Rh/δ-Al2O3 − in a microchannel reactor. The catalysts are prepared by the incipient-to-wetness impregnation method. Alumina (Al2O3) powder of 3 μm size (Merck, 120−190 m2/g, γ-phase) is used as the support material for synthesizing the catalysts. Thermally stable δ-phase of the support is obtained by drying γ-Al2O3 at 150 °C for 2 h and calcining it at 900 °C for 4 h.32 Calculated amounts of the metal precursors (Rh(NO3)3, [Pt(NH3)4](NO3)2, both supplied by Sigma-Aldrich) are dissolved in a certain amount of deionized water (ca. 1 mLsolution g−1support) to obtain an aqueous solution, which is then impregnated over the support by a peristaltic pump. The impregnation process is assisted by ultrasonic mixing and vacuum to ensure penetration of the dissolved metal salts to the inner pores of the support. The resulting slurries are then dried overnight in an oven at 120 °C and calcined in a muffle furnace at 500 °C for 3 h. Preparation of the catalytic microchannel reactor involves several mechanical, thermal, and chemical steps. First, a FeCrAlY sheet of 2 × 10−3 m (height) × 1 × 10−1 m (width) × 1 × 10−1 m (depth) (Goodfellow Cambridge Limited) is cut into 2 × 10−3 m (height) × 5 × 10−3 m (width) × 2 × 10−2 m (depth) plates by using wire electro discharge machining technique. The prepared plates are then calcined in a muffle furnace at 900 °C for 2 h to generate a native Al2O3 layer which is reported to enhance adhesion and improve the coating quality.30,31,33 In order to prepare the catalyst coatings, the B

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Figure 1. (a) Schematic representation of the steel housing and the details of the catalytic microchannel, (b) catalyst combinations used in the experiments, (c) the microchannel arrangement inside the quartz tube.

previously calcined 3-μm δ-Al2O3 supported catalyst powders are mixed with deionized water at a water-to-powder weight ratio of 5−8:1. The resulting slurries are blade-coated carefully onto the 5 × 10−3 m × 2 × 10−2 m plates and are dried at 120 °C overnight. This procedure is repeated until the weight per surface area becomes equal to ca. 0.02 gcat/cm2. Finally, the coated plates are calcined at 500 °C for 3 h in a muffle furnace. Once the catalyst coated plates are prepared, they are inserted into a 310-stainless steel cylindrical housing (outer diameter (OD) = 1.86 × 10−2 m, length (L) = 3 × 10−2 m) (Figure 1a). Interior of the housing is engineered by wire electro discharge machining to have a combined opening for the plates and the microchannel. Two catalyst coated plates, with combinations of 0.2%Pt-2%Rh (combination I), 2%Pt-2% Rh (combination II) and 2%Rh-2%Rh (combination III), are inserted with 0.5 × 10−3 m fitting to the grooves at each side so that one catalytic microchannel with the dimensions of H′ = 0.75 × 10−3 m × W′ = 4 × 10−3 m × D = 2 × 10−2 m is obtained (Figures 1a, 1b). During insertion, coatings over the first and last 0.5 × 10−3 m of the plate width are scraped due to contact with the grooves. The plates are further fixed at their positions by stuffing ceramic wool into the 1 × 10−2 m gap remaining between the end of the plates and the housing. 2.2. Reaction Tests. The housing containing the catalyst coated plates is placed into the center of a quartz tube with inner diameter of 2 × 10−2 m and length of 8 × 10−1 m. The housing is supported underneath by a hollow ring, which is an integral part of the quartz tube (Figures 1b, 1c). The ring is designed to provide sufficient circumferential overlap which keeps the housing stationary in the desired position and prevents bypass through the annular gap between the housing and the tube. During the experiments, bypass is also diminished due to the thermal expansion of the steel housing at high (>500 °C) temperatures. Temperature control of the catalytic zone is done by a Shimaden FP-23 programmable temperature controller and a K-type sheathed thermocouple that has immediate contact with the central point of the skin of the quartz tube in which the catalytic microchannel is located. The quartz tube is placed in the furnace such that the middle part of it, i.e. the catalytic zone, stays within the 1 × 10−1 m constanttemperature zone of the furnace. The nature of OSR may lead

to changes in the temperature of the reactor. However, the reaction system is arranged to investigate product distribution at isothermal conditions. The generated heat, which is low due to dilution of the feed with N2 and low methane quantity in the feed, is transferred from the catalytic microchannel to the outside of the quartz tube via conduction through FeCrAl plate and the steel housing which are in close contact with each other, so that the reactor is not adiabatic. It can also be said that the temperature difference between the microchannel and the skin of the quartz tube is minimal since the wall thickness of the quartz tube is quite small (2 × 10−3 m), the annular gap between the steel housing and the quartz tube is nearly closed at the reaction temperatures, and the microchannel is placed within a metallic block. The experiments are conducted in the following parameter ranges: (i) reaction temperature between 600 and 700 °C, (ii) steam-to-carbon ratio (H2O/C) between 1 and 1.5, (iii) oxygen-to-carbon ratio (O2/C) between 0.5 and 1. Here, the H2O/C ratio is defined as the number of moles of steam divided by that of methane at the inlet, and the O2/C ratio is defined as the number of moles of oxygen divided by that of methane at the inlet. The experimental program is summarized in Table 1. Catalysts used in the experiments are reduced in situ Table 1. List of Operating Conditions Involved in the Experiments parameter

value

reaction temperature (°C) H2O/C (mol/mol) O2/C (mol/mol)

600, 650, 700 1, 1.25, 1.5 0.5, 0.75, 1

under 40 NmL min−1 H2 flow at 800 °C for 2 h before the reaction tests. Total flow rate of the reactant gases is kept constant at 90 NmL min−1 during each experiment. Mole fraction of methane is kept constant at 0.167 in all runs. H2O/C and O2/C ratios (Table 1) are adjusted by varying the H2O, O2, and N2 flow rates to keep the total flow rate and methane partial pressure constant. Bronkhorst F-201CV series digital mass flow controllers are used to measure and control the flow rates of the gases (CH4, O2, CO2, H2, N2). Deionized water is C

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Figure 2. Schematic representation of the catalyst testing system (1. Gas regulators, 2. Mass flow controllers, 3. On−off valves, 4. Three-way valves, 5. Thermocouple locations, 6. Mixing zone, 7. Quartz tube).

3. RESULTS AND DISCUSSION Methane OSR experiments are conducted with the operating conditions given in Section 2.2 by changing one parameter in each run while keeping all others constant. Equilibrium methane conversion for each experiment is calculated using HSC Chemistry software34 via Gibbs free energy minimization method. The experimental conditions given in the previous section guaranteed that conversions below equilibrium values are obtained for all different cases tested. 3.1. Methane Conversion. The effect of operating conditions summarized in Table 1 on methane conversion is given in Figure 3. Regardless of the catalyst combination used in the experiments, methane conversion increased with temperature (Figure 3a). In all combinations methane conversion increased by ca. 25% when the temperature is raised from 600 to 700 °C. The results also show that the activity decreases in the order of 2%Rh-2%Rh (III) > 2%Pt-2% Rh (II) > 0.2%Pt-2%Rh (I). When combinations I and II are compared, even though Pt loading is increased ten-fold in the latter, such an increase is not observed in conversion, and the difference is found to be less than ca. 15%. This can possibly be explained by the degree of Pt dispersion, which is known to be better at lower metal loadings.12 Comparison of combinations II and III indicates that replacing Pt with Rh can improve methane conversion. This finding is in accordance with the literature, stating the superior hydrocarbon SR and TOX activities of Rh over Pt.6,33 The effects of feed compositions, i.e. H2O/C and O2/C ratios at the reactor inlet on methane conversion, are given in Figures 3b and 3c, respectively. For all catalyst combinations, increase in both ratios led to improved methane conversions, which is found to change more significantly with the amount of O2 in the feed; a maximum activity increase of 7% is observed upon changing H2O/C from 1 to 1.5. However, a 23% activity increase is observed in the cases where the O2/C ratio is increased from 0.5 to 1. Increasing the partial pressure of O2 in the feed favors TOX and increases the amount of steam produced, which contributes to the SR conversion. The

fed to the system by using Shimadzu LC-20AD HPLC pump with constant and pulse-free flow. Liquid water is vaporized before mixing with the other gases. All piping is heat traced by a heating tape/temperature controller system and insulated by ceramic wool to prevent condensation. The flow diagram of the complete experimental setup is shown in Figure 2. The impact of one parameter is observed by taking the others constant at their default values, which are 650 °C, 1.5, and 1 for reaction temperature and H2O/C and O2/C ratios, respectively. Analysis of reactant and product gases are performed by using a Shimadzu GC-2014 gas chromatograph equipped with a Carboxen-1000 packed column and a thermal conductivity detector. The column temperature is programmed to hold at 40 °C for 13 min, then to ramp to 150 °C in 2 min, and then to hold for another 8.5 min. The detector temperature is set to 175 °C. Argon is used as a carrier gas with a constant flow rate of 30 NmL min−1. Two salt-ice cold traps held in Dewar flasks are placed before the GC unit to remove water vapor in the product mixture. Reactant and product streams are sampled and injected online using a six-way valve. Research grade gases of high purity (CH4, N2, H2, O2, CO2, Ar > 99.99%, all supplied by Linde) are used in reaction tests and GC analysis. Methane conversion (XCH4) is obtained by calculating the percent difference of molar flow rate of CH4 in the inlet (FinCH4) and the outlet (Fout CH4) streams: XCH4 =

in out − FCH FCH 4 4 in FCH 4

× 100 (5)

The molar compositions of H2, CO, and CO2 produced (Ci) are determined by the ratio of the number of moles of produced component i (Fout i ) to the total number of moles of the product stream (Fout total): Ci(i = H 2 , CO, CO2 ) =

Fiout out × 100 Ftotal

(6) D

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contrast with the notable effect of O2-rich feed configurations, the impact of steam addition to the feed gave limited changes in conversion (Figure 3b). Steam addition favors SR (Reaction 2) thermodynamically but is known to have a nonmonotonic effect on the reaction kinetics.35 Considering the fact that the measured conversions are below the calculated thermodynamic limits, it can be stated that the reaction kinetics shape up the product distribution, and steam had only a slightly positive effect on the SR rates obtained over the three catalyst combinations. The reason of having measured conversions less than the thermodynamic ones in the whole parameter range can be explained by the short contact time involved (ca. 30 ms based on total microchannel volume including the catalyst layers), which is not enough for the contact of the reactants with the catalysts to give higher methane conversions. 3.2. H2 Production. The effect of reaction temperature, feed ratios (H2O/C, O2/C), and the catalyst combination on the compositions of H2, CO, and CO2 in the product stream is presented in Table 2. H2 production is found to increase with temperature, and the highest increase of 5% is observed over combination I. This trend is obvious for the cases where 2%Pt2%Rh and 2%Rh-2%Rh catalyst combinations are used, but no general trend is observed for the 0.2%Pt-2%Rh case. For the cases where O2/C ratios are changed, H2 produced increased with temperature with the exception of the case having the O2/ C ratio of 1 and the H2O/C ratio of 1.5. In this situation, TOX is favored with the increase of O2 partial pressure in the feed stream, so that higher amount of steam is produced, which is then utilized in SR to produce more H2. However the highest amount of H2 is produced at the lowest temperature (600 °C). At this point WGS (Reaction 3) gains importance since it is shifted in a forward direction to produce more H2. As temperature increases, the reverse WGS is favored and H2 production is reduced. Increasing the steam quantity in the feed through the H2O/C ratio improved H2 yield with the exception of the experiments conducted at 700 °C over 2%Rh-2%Rh catalyst. When the H2O/C ratio is increased, the increase in H2 production is also expected, since the amount of steam in the feed promotes SR equilibrium, and hence the amount of H2 produced. This claim is supported by experiments in which the H2O/C ratio is increased from 1 to 1.5 over 0.2%Pt 2%Rh and 2%Pt-2%Rh for all temperatures. This phenomenon is reversed at 700 °C for the 2%Rh-2%Rh case, and H2 production decreased with the increase in the H2O/C ratio, which can be explained by the WGS effect. The temperature (700 °C) is high for the WGS, so that it is reversed to produce more CO by using the produced CO2 and H2. Since the reverse WGS becomes dominant at 700 °C, its negative effects on H2 production cannot be compensated by SR. As more H2 is produced with more steam addition, WGS consumes more hydrogen to produce more CO, which decreases the partial pressure of H2 in the product stream. The fluctuations seen in the H2 production with steam addition can also be explained by the nonmonotonic effect of steam on SR kinetics.35 The results presented in Table 2 show that there is no direct relationship between the O2/C ratio and H2 production over the three catalyst combinations investigated. When the O2 amount in the feed is increased, it is observed that more methane is converted via TOX. As a result, more steam is produced which should increase SR conversion and accordingly H2 production. However, the effect of WGS should also be considered; the amount of H2 produced is likely to be affected

Figure 3. a. Effect of temperature on methane conversion over 0.2 wt %Pt-2 wt %Rh, 2%Pt-2%Rh, and 2%Rh-2%Rh catalyst combinations (H2O/C = 1, O2/C = 1). b. Effect of the H2O/C ratio on methane conversion over 0.2 wt %Pt-2 wt %Rh, 2%Pt-2%Rh, and 2%Rh-2%Rh catalyst combinations (T = 650 °C, O2/C = 1). c. Effect of the O2/C ratio on methane conversion over 0.2 wt %Pt-2 wt %Rh, 2%Pt-2%Rh, and 2%Rh-2%Rh catalyst combinations (T = 650 °C, H2O/C = 1.5).

resulting increase in the extents of both reactions lead to methane conversions up to 27% over 0.2%Pt-2%Rh, 40% over 2%Pt-2%Rh, and 53% over 2%Rh-2%Rh (Figure 3c). Considering the fact that TOX is faster than and occurs before SR, highest conversion values and highest rates of increase in conversion with the O2/C ratio obtained over 2%Rh-2%Rh indicate that Rh is a better oxidation catalyst than Pt. In E

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Table 2. Effects of Reaction Temperature, Feed Ratios, and the Catalyst Combinations on the Compositions of H2, CO, and CO2 (CH2,CCO,CCO2) in the Product Stream H2

CO

H2O/C T (°C)

1

0.2%Pt-2%Rh 600 1.67 650 0.67 700 1.51 2%Pt-2%Rh 600 0.43 650 0.47 700 1.64 2%Rh-2%Rh 600 0.65 650 1.81 700 9.29

O2/C

CO2

H2O/C

O2/C

H2O/C

O2/C

1.25

1.5

0.5

0.75

1

1

1.25

1.5

0.5

0.75

1

1

1.25

1.5

0.5

0.75

1

2.76 2.42 5.25

3.68 3.12 2.96

2.04 3.63 3.79

0.77 3.23 3.22

3.68 3.12 2.96

1.61 1.65 3.02

2.18 3.23 4.24

1.65 3.11 5.75

1.55 3.25 3.23

0.58 3.06 4.34

2.65 3.11 5.75

6.76 8.51 8.10

7.49 10.06 8.57

8.23 12.61 14.37

4.28 4.72 4.80

3.97 7.92 9.22

8.23 12.61 14.37

0.39 1.07 3.51

0.46 1.40 5.34

2.16 2.69 5.05

0.49 3.51 5.54

0.46 1.40 5.34

0.57 1.04 4.25

0.57 1.26 5.66

1.00 1.70 4.88

2.21 3.24 2.48

0.57 4.26 6.83

1.00 1.70 4.88

7.46 13.58 12.88

9.83 17.14 14.17

9.13 17.55 15.89

7.25 6.23 7.05

7.27 10.88 8.74

9.13 17.55 15.89

0.62 6.54 8.91

0.76 7.83 7.70

2.85 6.06 8.27

1.85 4.43 9.83

0.76 7.83 7.70

1.15 2.44 3.26

0.77 3.92 2.71

1.17 3.95 6.74

2.57 2.56 2.36

2.47 3.06 3.46

1.17 3.95 6.74

11.32 15.39 14.28

16.46 15.64 16.39

18.86 16.97 14.02

7.14 6.83 7.25

12.37 11.81 11.49

18.86 16.97 14.02

consumed by WGS as the H2O amount is increased. Therefore, fluctuations in the amount of CO produced are not unrealistic. The experiments that investigate the effect of adding O2 into the feed show that there is not a certain relationship between the O2/C ratio in the feed and CO production (Table 2). The increase in the O2/C ratio has a direct effect on TOX but has a slight effect on SR and on WGS via the CO2 produced by TOX. When the O2/C ratio is increased, more CO2 is produced by TOX and at 650 and 700 °C, the cases involving total O2 conversion, less methane stays reserved for SR reaction which decreases CO production in the presence of excess H2O. H2O produced by TOX may also be used in WGS which even decreases the CO amount in the system. The balance of these three reactions becomes very sensitive when changing the O2 amount in the feed stream, so a general trend cannot be observed. The outcomes can also be supported by the fact that the reaction pathways can possibly be altered by the increased O2 percentage in the feed.36 When different catalyst configurations are compared, it is seen that there is not much noticeable difference between their CO production performances. The amount of CO produced varies within the range of 5% when all the experimental results are investigated. When 0.2%Pt-2%Rh and 2%Pt-2%Rh combinations are compared, it is observed that very similar amounts of CO are produced over both configurations, with the former delivering somewhat higher CO production in some cases. The difference can be linked to the lower Pt loading, which leads to lower extent of TOX, and hence to higher CO production per unit methane conversion. Similar performances are generally observed for 2%Pt-2%Rh and 2%Rh-2%Rh catalyst combinations, but it can also be noted that, in certain cases, the latter one delivered higher CO production. 3.4. CO2 Production. The results provided in Table 2 show that CO2 production is mainly carried out by TOX (Reaction 1). CO2 is also produced by WGS (Reaction 3) when it runs in forward direction at low temperatures, but it may also be consumed by WGS at high temperatures, when the reaction runs in backward direction. In the experiments conducted at 650 and 700 °C, complete O2 conversion achieved indicates that increasing temperature only enhances SR for these cases, i.e. there is further CO2 production by TOX. Considering the fact that the reverse-WGS becomes important and starts to consume CO2 produced by TOX at elevated temperatures, a decrease in CO2 production should be expected. WGS

by WGS equilibrium, which is sensitive to CO2 produced by TOX. Comparison of the catalyst configurations shows that increasing Pt affects H2 production indirectly via TOX. In several runs, combination I (involving 0.2%Pt) delivered H2 production higher than obtained over combination II (involving 2%Pt). For example, in the experiments conducted at 600 and 650 °C with O2/C = 1, it is seen that more H2 is produced on combination I than on combination II. This observation shows that, by increasing the loading of Pt, which is known as an active oxidation catalyst, the amount of methane converted by TOX becomes higher. In other words, less H2 is produced due to reduced amounts of methane that is steam reformed. The increased TOX activity at 600 and 650 °C can be verified by the higher moles of CO2 produced (Table 2). When 2%Pt-2%Rh and 2%Rh-2%Rh are compared, it is seen that more H2 is produced when the latter is used for all cases. Rh is known to be superior to Pt in SR,12 so it is very reasonable to obtain higher H2 production in the cases where 2%Rh-2%Rh is used. This catalyst delivered the maximum H2 composition of 9.8% (by mole) at 700 °C, H2O/C ratio of 1.5, and O2/C ratio of 0.75. High activity of Rh is in agreement with the previous studies.33 3.3. CO Production. The results presented in Table 2 show that CO production increases with temperature in most of the experiments. For the cases in which the O2/C ratio is fixed to 0.5, CO production decreased upon temperature change from 650 to 700 °C over all catalyst combinations. This situation can be explained by the fact that there is not enough O2 in the feed stream for oxidation to provide enough H2O for the endothermic SR, which leads to a decrease in the amount of CO in the product stream. When the WGS effect is considered at these temperatures, it is expected for WGS to increase the CO amount at the highest temperature (700 °C) since it shifts backward to produce CO by using CO2 and H2. However, in the shortage of O2, CO2 production is very small which prevents WGS to convert it into CO. Apart from the O2/C = 0.5 case, the highest amount of CO is produced at 700 °C. The amount of CO produced tends to increase with the H2O/C ratio (Table 2). However, this trend is not valid for some cases. The nonmonotonic behavior can be explained by the balance between SR (Reaction 2) and WGS (Reaction 3). Addition of H2O increases CO production by SR, but CO is F

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respectively. H2 composition in the product stream increased with temperature but correlated negatively with the O2/C ratio. The catalyst combination involving 2% Rh/Al2O3 coated oppositely onto the inner walls of the microchannel is found to exhibit the best performance in terms of methane conversion and H2 and CO production. Increasing Pt content ten-fold and replacing the same amount of Pt with Rh give the same response (10% increase) in methane conversion. Rh is observed to be superior to Pt in methane oxidation and in methane steam reforming.

equilibrium is also affected by the CO2 present in the exhaustgas stream of ICE (i.e., the stream fed to EGR) at a composition of ca. 9%. This may be considered as another factor that favors the reverse-WGS due to the Le Chatelier’s principle. Addition of steam into the feed is found to favor CO2 production (Table 2) due to possible promotion of the WGS reaction. However in some cases, such as 2%Rh-2%Rh catalyst combination run at 700 °C, CO2 production is found to decrease. This observation can be explained by the onset of the reverse-WGS as a result of high temperatures and H2 concentrations resulting from the high SR conversions obtained over Rh (see Section 3.1). Increasing the O2/C ratio also favored CO2 production (Table 2). Partial pressure of O2 in the feed has a notable effect on the TOX, so the production rate of CO2 by TOX increased positively with the O2/C ratio, and it even surpassed the consumption rate of CO2 by WGS even at high temperatures. Comparison of the 0.2%Pt-2%Rh and 2%Pt-2%Rh catalysts show that the latter is more effective when CO2 production is considered (Table 2). This result stems from the fact that TOX conversion increase with the Pt loading, which eventually favors CO2 production. When 2%Pt-2%Rh and 2%Rh-2%Rh are compared, the latter is found to deliver higher amounts of CO2. The highest CO2 composition of 14.4% (by mole) is achieved at 700 °C over 0.2%Pt-2%Rh when H2O/C is equal to 1.5 and O2/C is equal to 1. Over 2%Pt-2%Rh, maximum CO2 composition of 17.5% is observed at 650 °C when H2O/C = 1.5 and O2/C = 1.0 and over 2%Rh-2%Rh maximum CO2 composition of 18.9% is observed at 600 °C when H2O/C = 1.5 and O2/C = 1.0. These results show that the feed compositions leading to maximum CO2 production over the three catalyst combinations are the same, but the last one (2% Rh-2%Rh) delivered the highest CO2 yield at the lowest temperature. This outcome indicates that, at lower temperatures, Rh is more active than Pt in methane oxidation under the operating conditions studied. The experiments run according to the conditions in Table 1 are conducted without carbon formation even at H2O/C ratios as low as 1.0. Carbon-free operation is verified by the fact that reactor effluent flow, which is periodically monitored, is measured to be free of fluctuations, indicating that the channel is not physically blocked by solid carbon deposition. Moreover, no signs of deposition are detected after the removal of the spent catalysts from the reactor. These findings are in alignment with the recently reported results on microchannel methane-to-syngas conversions carried out at similar operating conditions and catalyst configurations, which report the absence of carbon formation by detailed SEM-EDX studies.31,33

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AUTHOR INFORMATION

Corresponding Author

*Phone: +90-212-359-7785. Fax: +90-212-287-2460. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is provided by Bogazici University Research Fund through project BAP-6349. Ahmet K. Avci acknowledges the TUBA-GEBIP program.

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REFERENCES

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