Dry Reforming of Methane over a Ruthenium/Carbon Nanotube Catalyst

Abstract

In this study, CH₄ dry reforming was demonstrated on a novel microwave-synthesized ruthenium (Ru)/carbon nanotube (CNT) catalyst. The catalyst was tested in an isothermal laboratory-packed bed reactor, with gas analysis by gas chromatography/thermal conductivity detection. The catalyst demonstrated excellent dry-reforming activity at modest temperatures (773–973 K) and pressure (3.03 × 10⁵ Pa). Higher reaction temperatures favored increased conversion of CH₄ and CO₂, and increased H₂/CO product ratios. Slight coke deposition, estimated by carbon balance, was observed at higher temperatures and higher feed CH₄/CO₂. A robust global kinetic model composed of three reversible reactions—dry reforming, reverse water gas shift, and CH₄ decomposition—simulates observed outlet species concentrations and reactant conversions using this Ru/CNT catalyst over the temperature range of this study. This engineering kinetic model for the Ru/CNT catalyst predicts a somewhat higher selectivity and yield for H₂, and less for CO, in comparison to previously published results for a similarly prepared Pt_Pd/CNT catalyst from our group.

1. Introduction

Because of enhanced petroleum production by hydraulic fracturing, vast quantities of co-produced natural gas are flared annually worldwide [1,2]. In Texas, for example, a lack of sufficient gas pipeline capacity has severely depressed local natural gas market prices, resulting in negative prices and more flaring [3]. In addition to flaring, there is also concern for fugitive CH₄ emissions associated with petroleum production [4]. Methane has a global warming potential 30 times higher than CO₂ [5].

Methane dry reforming (DR) (overall: CH₄ + CO₂ → 2CO + 2H₂, ∆H_298K = 2.473 × 10⁵ J) would remove two greenhouse gases while generating useful synthesis gas (H₂, CO) as a chemical feedstock [6,7,8], especially for important chemicals such as ammonia and methanol, and for Fischer–Tropsch liquids [9,10]. The energy needed for the reforming can be supplied by burning some of the CH₄. The CO₂ can be trucked in, especially from sequestration sources. While this scheme does not ultimately eliminate the greenhouse gases, it does provide some useful return in the form of fuels and/or chemicals. Alternatively, the DR could be solar powered [11].

Typical heterogeneous DR catalysts use inorganic oxide supports; e.g., Al₂O₃, SiO₂, TiO₂ [6,12,13,14]. The active sites fall into two groups. First are base-metals, including Fe, Co, and Ni [6,12,13]. The Ni is widely studied since it is catalytically active and cheap. However, the Ni catalyst sinters at common DR reaction temperatures (800–1100 K), and has a relatively weak coking resistance [6,13]. Cobalt offers an improved coking resistance compared to Ni [15]. In a recent development [16], a highly stable and coke-resistant catalyst composed of Mo-doped Ni nanoparticles on a MgO support was demonstrated for high DR activity.

The second active site group consists of noble metals, commonly Rh, Ru, Pt, Pd, and Ir [6,14,17]. Although expensive, noble metal catalysts resist coking, sinter less, and are more active for higher-temperature applications.

A potentially useful support for DR catalysts is carbon nanotubes (CNTs). The CNTs are one-dimensional cylindrical structures consisting of wrapped single or multi-layer graphene sheets [18]. Its special structure results in excellent thermal and electrical conductivities, high mechanical strength, large surface area, relatively high oxidation stability, surface chemical flexibility and porous structure [19].

The advantages of CNTs as catalyst supports for a number of heterogeneous catalysis processes have been identified. Dry reforming of CH₄ over Ni/CNT and Ni/SiO₂ catalysts produced approximately the same conversions, but Ni/CNT showed much better stability. This was attributed to a redirection of carbon deposits away from the Ni sites to growth along the CNT tips [20]. Dry reforming of CH₄ over Co/CNT and Co/MgO showed higher CH₄ conversions from Co/CNT, which also had lower carbon deposition rates and less catalyst deactivation [21].

In this work, Ru supported on a CNT catalyst was synthesized by a versatile microwave technique, and then tested for methane DR activity. There is no current literature on the use of Ru (as the sole metal) on CNT used as DR catalyst for CH₄. Observed data are compared against equilibrium. An engineering model, useful for calculations such as reactor design, and based on three global reactions, adequately represents the DR data.

2. Materials and Methods

2.1. Carbon Nanotubes

The multi-walled carbon nanotubes (Cheap Tubes Inc., Cambridgeport, VT, USA) used were 10–30 × 10⁻⁶ m length by 20-30 × 10⁻⁹ m outer diameter. Other compounds used in the synthesis (Sigma-Aldrich) were 95%+ purity. The synthesis strategy begins with CNT functionalization (becoming F-CNTs) with carboxylic acid groups, followed by the addition of Ru.

The Ru/CNT synthesis is described in detail elsewhere [22]. The synthesis begins with CNT carboxylation to enhance dispersibility for the subsequent Ru addition. A known mass of CNTs are dispersed into concentrated H₂SO₄ + HNO₃ (aqueous). The suspension is radiated with microwaves to 413 K for 1200 s, and then cooled (room temperature). Vacuum filtering (10⁻⁵ m pore size) follows, with Milli Q water washing to pH ≈ 7. These F-CNTs are then vacuum-dried at 343 K.

The dried F-CNTs are then dispersed in diethylene glycol by sonication. A RuCl₃.nH₂O, sodium acetate, and Milli Q water mixture is added, followed by more sonication. The new dispersion is processed in the microwave reactor at 473 K for 600 s. It is estimated that a roughly 20% conversion of the Ru starting material occurs. Upon cooling, the mass is vacuum filtered and ethanol-washed. The Ru/CNT are finally vacuum-dried at 343 K to a constant mass.

2.2. Catalyst Testing System

The apparatus used for the Ru/CNT catalysis testing is described elsewhere [23,24]. The CH₄, CO₂, and diluent He flow rates were set by calibrated mass flow controllers. Catalyst was uploaded into the fritted metal cup of a 6.35 × 10⁻³ m stainless steel Swagelok (Nupro) in-line filter. The stainless steel tube assembly was placed in a 3-zone electric furnace. Results presented elsewhere [23] show that the steel reactor vessel was inert during these DR experiments.

The on-line gas analysis was performed with a model 5890 Hewlett-Packard gas chromatograph/thermal conductivity detector (GC/TCD) fed through a gas sample loop and valve. The pressure in the reactor flow system was monitored and manually controlled. The operator alternately directed feed (bypass) or reactor effluent to the on-line GC/TCD.

In this DR research, the inlet CH₄ and CO₂ were diluted by 85% He, resulting in approximately constant total molar flow rate. The experimental CH₄, CO, CO₂ mole fractions allowed atomic carbon balances, and estimates of any carbon deposits. These data also facilitated H and O balances for H₂ and H₂O concentrations. Where feasible, H₂ levels determined this way were verified by the GC/TCD data for H₂.

System pressure was kept constant (3.03 × 10⁵ Pa, abs.). Temperatures studied were 773–973 K, while the molar CH₄/CO₂ feed ratio was 0.5–2.0. Most runs were at constant total inlet rate (1.12 × 10⁻⁶ std. m³/s). Total catalyst mass was 0.002 kg (0.0004 kg Ru/CNTs, 0.0016 kg Y-zeolite). The resulting gas space velocity (GSV) was 5.56 × 10⁻⁴ m³/s-kg. Variable total feed rate experiments were also done at an equimolar feed CH₄ and CO₂.

3. Results

3.1. Catalyst Characterization

The Ru/CNT was characterized using a scanning electron microscope (SEM, JEM 2800) and a transmission electron microscope (TEM, F2000). Figure 1 (left) shows SEM images of the synthesized Ru/CNT. After Ru deposition, the CNTs are intact with no apparent damage. Close inspection of the TEM image in Figure 1 (right) reveals Ru nanoparticles. Their approximate particle size range is 1–5 × 10⁻⁹ m. Subsequent SEM testing of the catalyst after all the DR runs showed little if any sintering of the Ru.

The thermogravimetric analysis (TGA, in air) of our Ru/CNTs is shown in Figure 2. Weight loss in the 403–873 K range is due to Ru/CNT decomposition in air. The 10% remaining mass is likely RuO₂. Elemental analysis from energy-dispersive X-ray spectroscopy (EDX) (

Table 1. Elemental analysis of samples by energy-dispersive X-ray spectroscopy (EDX).

	C	O	Fe	Ni	Ru
Raw CNTs	96.50	1.38	0.68	1.44	-
Ru/CNTs	83.97	7.28	-	-	8.75

) is consistent with this hypothesis.

To extend the physical mass of the solids for ease of handling, the Ru/CNT were mixed with Y-zeolite of similar particle size. The Y-zeolite was the inert Na form, used as delivered (Alfa Aesar, Haverhill, MA, USA) with a 5.5:1 molar ratio SiO₂:Al₂O₃. The solids mixture was 20 wt.% Ru/CNT. Results presented elsewhere [23] show that the added zeolite was effectively inert during our DR experiments. The particles (~3 × 10⁻⁵ m diameter) were small enough to avoid significant external and internal mass transfer resistances (Mears and Weisz-Prater criteria, respectively, as discussed in [23]). Based on CO adsorption and BET surface area tests of the Ru/CNT-zeolite catalyst, the Ru site density was ~9 × 10⁻⁸ moles/m² [23].

3.2. Modeling

Modeling begins with reaction equilibrium calculations. Then we propose a three reaction global engineering kinetic model calibrated against experimental data.

3.2.1. Equilibrium Calculations

In this study, all experimental runs were tested with the Chemkin-Pro^® equilibrium application [25]. The calculation is based on the element-potential method used in Stanjan [26]. The equilibrium composition minimizes total Gibbs Free Energy.

The equilibrium calculation is run with specified temperature (constant), pressure (constant), and feed composition. The allowed equilibrium species are H₂, H₂O, CO, CO₂, CH₄, He, and solid carbon (Cs, when allowed—assumed graphite). The Chemkin-Pro database provides the thermodynamic properties of the species.

Table 2. Impact of solid carbon (Cs) on equilibrium; T = 973 K, P = 3.03 × 10⁵ Pa, initial CH₄/CO₂ = 1.

Species	Feed (Mole Fractions)	Equil (w/o Cs) (Mole Fracs)	Equil (w/Cs) (Mole Fractions)
CH4	0.0725	0.0227	0.0010
CO2	0.0725	0.0160	0.0284
CO	--	0.0930	0.0253
H2	--	0.0796	0.0802
H2O	--	0.0067	0.0440
He	0.8550	0.7820	0.7497
Cs	n/a	n/a	0.0714
SUM	1.0000	1.0000	1.0000
H2/CO	n/a	0.856	3.17

compares the equilibrium simulation results with and without the solid carbon Cs for an experimental case. For a feed where CH₄/CO₂ = 1, the amount of H₂ is roughly about the same in the two cases. However, allowing Cs results in much less equilibrium CH₄ and CO. Indeed, Cs is the second largest quantity species after H₂, and the equilibrium H₂/CO ≥ 1. Without Cs, equilibrium H₂/CO < 1. A similarly large impact of Cs in equilibrium was reported elsewhere [27].

3.2.2. Global Kinetic Model

An engineering global kinetic model has the advantage of offering relatively easy calculations for an otherwise complex reaction system. It is not meant to substitute for a detailed elementary reaction mechanism that requires a complex software package such as Chemkin-Pro to evaluate. The global model is a screening tool, and is limited to the region over which it is calibrated.

Analysis began with an experimental carbon balance. Measured feed CH₄ + CO₂ mole fractions were compared to measured outlet CH₄ + CO₂ + CO. Heavy He dilution minimized the impact of any changes in total molar rate by reactions. Figure 3 presents a parity plot of the experimental carbon balance. Points below the diagonal suggest carbon deposition occurred during those runs.

A closer look of the marker distribution in the graphic reveals:

• Circle I shows the runs at 773, 823, and 873 K. These are very close or even at the parity line suggesting little carbon deposition at these lower temperatures.
• Circle II and III present the runs at 923 K and 973 K. Compared to circle I, higher temperature favors coke formation.
• Circle III shows two cases at 973 K. They are even further from the parity line than the other runs at 973K (Circle II, feed CH₄/CO₂ range 0.51–1.01) due to higher feed CH₄/CO₂ (1.53, 2.08). The same observation is obtained at the other temperatures—higher CH₄/CO₂ at comparable temperatures favors coke formation.
• For all runs, the further below the parity line, the higher H₂/CO is observed. It is implied that the formation of higher H₂ is coincident with coke formation.

In view of Figure 3, our global kinetic model uses the following three reactions:

Dry Reforming   CH₄ + CO₂ = 2CO + 2H₂

(1)

Reverse Water Gas Shift   CO₂ + H₂ = H₂O + CO

(2)

Methane Decomposition   CH₄ = Cs + 2H₂

(3)

Equation (1) presents the ideal DR reaction, and is chosen naturally. The reverse water gas shift (RWGS, Equation (2)) occurs during reforming [28]. Reactions 1 and 2 suggest H₂/CO <1, which is coincident with most of our cases. However, numerous runs showed H₂/CO >1 at higher temperatures and feed CH₄/CO₂. This might be explained by either the CH₄ decomposition (MD, Equation (3)) generating more H₂, or Boudouard reaction (2 CO → CO₂ + Cs) consuming more CO. However, higher temperatures favor MD equilibrium but discourage Boudouard. It has been claimed that DR occurs through a catalytic decomposition of CH₄ to adsorbed C and H atoms, thus facilitating carbon deposits [28]. Finally, Reactions 2 and 3 are known to occur during CH₄ DR [21].

The selected reversible reactions for the global model and their rate expressions are summarized in

Table 3. Global kinetic engineering model with primary dry reforming (DR) and secondary reverse water gas shift (RWGS), methane decomposition (MD) reactions.

Reaction	Rate Expression ri	Appr. to Equil. ηi	Kpi
Dry Reforming CH4 + CO2 = 2CO + 2H2	$r_1=k_1{P}_{CH4}{P}_{CO2}\left(1-{\eta }_1\right)$	${\eta }_1=\frac{P_{CO}^2{P}_{H2}^2}{P_{CH4}{P}_{CO2}{K}_{p1}}$	$exp\left(-\frac{31234}{T}+34.093\right)$
Reverse Water Gas Shift CO2 + H2 = CO + H2O	$r_2=k_2{P}_{CO2}\left(1-{\eta }_2\right)$	${\eta }_2=\frac{P_{CO}{P}_{H2O}}{P_{H2}{P}_{CO2}{K}_{p2}}$	$exp\left(-\frac{430300}{T^2}-\frac{3344.7}{T}+3.3995\right)$
Methane Decomposition CH4 = Cs + 2H2	$r_3=k_3{P}_{CH4}\left(1-{\eta }_3\right)$	${\eta }_3=\frac{P_{H2}^2}{P_{CH4}{K}_{p3}}$	$exp\left(-\frac{10534}{T}+12.851\right)$

. If the approach to equilibrium η_i > 1, the reaction goes left to right; if η_i < 1, the reaction goes in reverse; if η_i = 1, the reaction is at equilibrium. The equilibrium constants K_pi (Table 3), are regressed vs. temperature from an on-line calculator [29]. For Reactions 1 and 3, the first order CH₄ kinetics are suggested elsewhere [28]. The Reaction 2 first-order CO₂ kinetics are also suggested elsewhere [30]. A regression strategy was used to estimate the Arrhenius parameter pairs (A_i, E_i), Assuming a packed-bed reactor (PBR) model (

Table 4. Key packed-bed reactor (PBR) simulation equations using 3-reaction global model (Table 3).

PBR Balances Species j	Net Rates rj	Mole Fractions yj	Partial Pressures
$d{F}_j/dW=r_j$ At W = 0, Fjo = value	$r_{CH4}=-r_1-r_3$ $r_{CO2}=-r_1-r_2$ $r_{CO}=2r_1+r_2$ $r_{H2}=2r_1-r_2+2r_3$ $r_{H2O}=r_2$	$y_j=\frac{F_j}{{{\displaystyle \sum }}_j{F}_j}$ Total molar rate includes inert gas	$P_j=y_{j}P$ P = total pressure

), our DR experiments were simulated. The experimental CH₄ and CO₂ conversions are sufficiently high (>10%) that an integral PBR model is used rather than a simple differential reactor.

An original Matlab program was developed to simultaneously integrate all PBR species balances with the input of all experimental inlet mole fractions and flow rates at a given temperature and pressure, and then to compare the calculated outlet mole fractions with experimental values. The Matlab program repeats the regression process, resulting in optimized rate constants k_i at that temperature. The optimized rate constants k_i at each temperature were correlated (Figure 4) to obtain quality Arrhenius parameters of each reaction (

Table 5. Arrhenius parameters for Table 3 from Figure 4 analysis; $k_i=A_{i}exp\left[-E_i/\left(RT\right)\right]$.

Reaction i	Parameter Ai (mole, s, kg_cat, Pa)	Parameter Ei (J/mole)
1	8.575 × 10−8	82,446
2	0.109	110,646
3	0.206	128,064

).

A similar analysis method [31] was successfully applied to optimize a 3-global reaction set for methane dehydroaromatization (MDA), inspired elsewhere [32]. In this MDA study, a detailed kinetic mechanism for the surface elementary reactions [33] was evaluated in a PBR model to create a simulated experimental database.

Using the best-fit Arrhenius parameters (Table 5) with the 3-reaction global kinetic set, the experimental DR runs were again simulated, with predicted values compared to the observed. Figure 5 shows example simulated and experimental (outlet) mole fractions at 923 K, 3.03 × 10⁵ Pa, feed 1.12 × 10⁻⁶ m³/s, and feed CH₄/CO₂ = 0.5. Species H₂ and CO rise steadily, while CO₂ and CH₄ both drop. The experimental outlet mole fractions are well simulated by the global 3-reaction model.

Offering further insight, Figure 6 shows model-based η_i profiles for the Figure 5 case. The DR and MD are far from equilibrium everywhere, while RWGS moves quickly toward equilibrium [28].

4. Discussion

The results above illustrate how the 3-reaction global model above was built up based on the observed reactor outlet species concentrations with various experimental temperatures and feed molar CH₄/CO₂ ratios. In this section, these results are discussed by demonstrating the utility of the 3-reaction global model by comparison against useful quantities such as reactant conversions and product H₂/CO ratios. The equilibrium values are also presented in these cases for comparison.

4.1. Species Concentrations at Reactor Outlet

For all experimental runs, the CH₄, CO₂, CO mole fractions were directly measured by GC/TCD. Oxygen atom balances were used, based on measured inlet and effluent CO₂ and CO concentrations, to estimate H₂O. No trace of O₂ was detected in any case. Combined with H atom balance, H₂ was estimated using measured inlet and outlet CH₄.

To compare the 3-reaction model with experimental results, Figure 7 shows outlet species for all cases at 873 K at constant feed rate but different feed CH₄/CO₂. Figure 8 presents the compositions of all cases at CH₄/CO₂ = 1.0 with constant feed rate as a function of temperature. Acceptable agreements establish the precision of the 3-reaction model. The outlet concentrations of CO, H₂O, and H₂ are significantly impacted by reaction temperature, less so by the inlet CH₄/CO₂.

4.2. Methane and Carbon Dioxide Conversions

Estimated CH₄ and CO₂ conversions are based on inlet and outlet mole fractions due to the heavy He dilution. Figure 9 shows variations with temperature of experimental, equilibrium, and 3-reaction model predicted conversions at feed CH₄/CO₂ of 2.0 and 0.5. The 3-reaction model performed very well in predicting observed conversions.

The CH₄ equilibrium conversions are much greater than in the experiment, and are insensitive to temperature, suggesting a kinetic potential to improve conversions. Other experimental temperature runs showed similarly consistent results. Figure 10 shows similar results with variation of feed CH₄/CO₂. At 973 K, experimental CO₂ conversions are much closer to equilibrium, suggesting that 973 K is nearly the upper temperature limit for improving CO₂ conversions with this catalyst.

4.3. Syngas Molar Ratio H₂/CO

As a high syngas H₂/CO ratio is often preferred for many processes, the product mole fraction ratio H₂/CO is an important index of reforming catalyst effectiveness [34]. Figure 11 shows that higher temperatures and feed CH₄/CO₂ favor higher H₂/CO. The 3-reaction global model simulates well the experimental values. At both 773 and 973 K, equilibrium ratios far exceed the observed values.

The stoichiometric H₂/CO for the ideal DR reaction is 1.0. At 773 K, the observed H₂/CO at feed CH₄/CO₂ = 1 is <1. This is attributed to the RWGS reaction, which is more thermodynamically favored at these relatively low temperatures [35]. At 973 K with CH₄/CO₂ = 1, the experimental H₂/CO is ≈1.0.

4.4. Comparison of Ru/CNT to Pt_Pd/CNT

A study similar to the one presented here was performed earlier with Pt_Pd/CNT catalyst, also admixed with inert zeolite [24]. Both the Ru/CNT and Pt_Pd/CNT catalysts were prepared in very similar ways. Based on CO adsorption and BET surface area tests of the metal/CNT-zeolite catalysts, the Pt_Pd and Ru catalyst site densities were estimated as 1.3 × 10⁻⁷ and 9.0 × 10⁻⁸ mole/m², respectively [23].

Table 6. Comparison of performances from Ru/CNT (model, this study) and Pt_Pd/CNT (model from [21]). T = 773K, P = 3.03 × 10⁵ Pa, 0.001 kg catalyst, total feed 5.6 × 10⁻⁵ mole/s, feed CH₄/CO₂ = 1.

	XCH4	XCO2	YCO	YH2	YCs	SCO	SH2	SCs	H2/CO
Ru/CNT	0.084	0.099	0.180	0.076	0.0021	2.148	0.901	0.0255	0.838
Pt_Pd/CNT	0.075	0.091	0.164	0.066	0.0016	2.201	0.879	0.0213	0.798

presents calculated results from the 3-reaction (DR, RWGS, MD) engineering kinetic models presented in this paper for Ru, and in [24] for Pt_Pd. Each simulation was run with a hypothetical packed-bed reactor with 1 gram of metal/CNT-zeolite catalyst held at a constant 773 K, 3.03 × 10⁵ Pa pressure, feeding CH₄ and CO₂ at 5.6 × 10⁻⁵ mole/s total rate (equimolar). While the differences are fairly small, there is a definite trend that the Ru/CNT catalyst, even with a smaller site density, produces a synthesis gas richer in H₂. The drawback is a slightly larger Cs.

5. Conclusions

An Ru catalyst supported on CNT, prepared by a novel microwave synthesis technique, was tested for activity on dry reforming (DR) of CH₄ to synthesis gas. The DR studies were undertaken in an isothermal packed bed reactor. The catalyst showed impressive activity and stability at modest temperatures. The outlet species concentrations and conversions were influenced by temperature and feed molar ratio of CH₄/CO₂. Based on the analysis of the experiment data, a small amount of carbon deposition was observed during DR, and it was enhanced by higher temperatures and feed CH₄/CO₂. Reactant conversions did not reach equilibrium values, suggesting potential to improve conversions and product yield. In order to account for both observed reactant conversions and product species concentrations, a global engineering model consisting of three reversible reactions was developed. The model reactions are dry reforming (ideal), reverse water gas shift, and methane decomposition. The three-reaction model adequately represents the observed species profiles as functions of temperature (773–973 K) and feed CH₄/CO₂ (0.5–2.0). Linear Arrhenius plots for the forward rate constants are observed for each reaction in the model over the calibration temperature range. The calibrated engineering model for the Ru/CNT catalyst predicts a higher selectivity and yield for H₂ in comparison to previously published results for a similarly prepared Pt_Pd/CNT catalyst from our group, although at the expense of slightly more carbon deposits.