Microkinetic Analysis of the Methane Steam Reforming on a Ru

Article pubs.acs.org/IECR

Microkinetic Analysis of the Methane Steam Reforming on a RuSupported Catalytic Wall Reactor José Mauro Vásquez Castillo,† Takafumi Sato,† and Naotsugu Itoh*,† †

Department of Material and Environmental Chemistry, Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan S Supporting Information *

ABSTRACT: Green, efficient, and economic energy conversion from methane steam reforming requires understanding the reaction mechanism and intensification of reactor design to better convert energy at plant scale. On this research, a Ru catalytic wall reactor was prepared to conduct kinetic measurements of the methane steam reforming at low temperatures from 573 to 723 K, and then a microkinetic framework including a basic reforming mechanism and computational load saving methods was developed. The products were hydrogen and carbon dioxide, and the conversion of methane was promoted in high steam/carbon ratio (S/C) and low pressure region. Microkinetic simulation revealed that the methane decomposition and activated water decomposition path were the main reaction pathways, OH*, CO*, H*, and O* were the main intermediates, and small amount of intermediates on the catalyst surface during the reaction may promote methane reforming in the high S/C and low pressure region. It was found that the overall reforming was controlled equivalently by kinetic and thermodynamic mechanisms, according to the calculated contribution of steps and intermediates on reaction progress.

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INTRODUCTION Methane steam reforming from renewable sources such as biogas has the potential to fulfill social awareness, that is society demands for rapid, economic, and green chemical industrial process for the generation of sustainable hydrogen fuel.1,2 Hydrogen has been regarded as the best possible alternative fuel for the 21st century and the future because it can be used safely and it can be converted efficiently to thermal, electrical, and the others forms of energy necessary for society and industry.3 The methane steam reforming process is a catalytic system of mainly three chemical reactions, methane steam reforming reaction R1, water gas shift reaction R2, and reverse methanation reaction R3 as follows:4 CH4 + H 2 ⇄ CO + 3H 2 (R1) CO4 + H 2O ⇄ CO2 + H 2

(R2)

CH4 + 2H 2O ⇄ CO2 + 4H 2

(R3)

more, gas/pellet (catalyst washcoat) mass transfer limitation is reduced and intrinsic kinetic observation is easier than that of conventional reactors.6,7 Microkinetic reaction model is a detailed multiscale reaction model5,8 that uses molecular scale information to set up reaction mechanism and elementary reaction rate constants; reaction rates are calculated at catalyst (meso) scale and then coupled with a reactor model at process scale.5 Due to multiple scales descriptions, effective computations are important.9 Reduced methods for microkinetic simulation of methane steam reforming have been applied for Ni, Rh, and Pt catalyzed systems, principally. The computational load saving techniques encompass calculation of rate constants by phenomenological approximations,10,11 algebraic elementary reaction rate representation,8,9 reduced reactor models for microkinetic evaluation,12,13 and reduction of the microkinetic model to construct a closed form expression for the reaction rate.9,12,13 Microkinetic evaluation of steam reforming of methane with Ru catalyzed systems, important due to its cost and high activity at low temperature, can be developed by considering the above methods and by recurring to acknowledged Ru surface science data. Surface science investigations on Ru have identified and set chemisorptions energetic,14 adsorption sticking coeffi-

Development of efficient, economic, and green energy conversion process by steam reforming of methane requires detailed chemical models at the catalyst scale and intensification of reactor design for heat transfer, due to endothermic reactions, at the reactor scale.2,5 Intensification of reactor design have proposed catalytically coating only the reactor walls that results in an effectively direct heat transfer from the wall to the catalyst layer, material and reactor size reduction6,7 compared to the typical packed bed configuration. Further© 2017 American Chemical Society

Received: Revised: Accepted: Published: 8815

April 21, 2017 July 10, 2017 July 11, 2017 July 11, 2017 DOI: 10.1021/acs.iecr.7b01687 Ind. Eng. Chem. Res. 2017, 56, 8815−8822

Article

Industrial & Engineering Chemistry Research

40 cm3/min by the mass flow controller device. The water was supplied with a microfeeder into the preheater where it was evaporated. The molar flow ratio of steam to carbon (S/C) was from 2 to 5. The temperature of the reactor was controlled by the air oven thermostat from 573 to 723 K. The reaction pressure was controlled with a back pressure valve up to 0.2 MPa. After the water was removed by a cold trap at 193 K, the produced gas was analyzed by a gas chromatograph through the sampling loop attached to it and the gas flow rate was measured with a soap film flow meter. Microkinetic Modeling. Multiscale Approach for Microkinetic Modeling. Figure 2 shows the multiscale approach for

cients,15−17 and elementary steps mechanism.18−21 In these studies, Wei and Iglesia18 proposed a simple reforming mechanism on Ru catalyst derived from experimental evidence with simple intermediates and thus it can be regarded as one of the basic reforming mechanisms.9,22 Although the common practice on microkinetic is to consider detailed reaction mechanism in order to include all possible reaction pathways,23 a direct numerical evaluation of the Wei and Iglesia mechanism becomes possible. In this study, we first conducted methane steam reforming experiments with Ru catalytic wall reactor at 573 to 723 K, 2 to 5 of steam/carbon ratio (S/C), and at 0.1 and 0.2 MPa. After that, we constructed a microkinetic framework based on Wei and Iglesias’ mechanism,18 considering computational load saving methods and optimized with experimental results. We elucidated the effect of S/C, pressure and temperature on reaction kinetics by reaction path analysis (RPA) and sensitivity analysis (SA) to resolve important reaction path,24 rate controlling steps, and intermediates.25

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EXPERIMENTAL SECTION Catalytic Wall Reactor Preparation. The catalytic Ru/ Al2O3 wall reactor was prepared by preprocessing alumina tube and depositing Ru catalyst on the alumina tube. At first, an alumina tube was annealed at 773 K for 4 h. The inner tube was polished at 353 K by 8 min in a concentrated acid solution, rinsed with distilled water, and an ultrasonic rinse in acetone. The tube was then anodized with a Pt electrode set at 60 V in a 4 wt % oxalic acid electrolytic solution. The tube was submerged in water at 353 K. Then, a small amount of RuCl 3 solution was introduced to the inner side of preprocessed alumina tube and evaporated to dryness with an industrial blower (Ishizaki Electric, PJ-208A). Finally, the prepared catalytic wall reactor was allowed to dry in a thermostatic bath one night and then heated for 1 h under N2 and then H2 in an electric furnace at 823 K. The catalytic wall reactor prepared as above had a length of 100 mm, 6 mm of internal diameter, 0.1 mm of a catalytic layer thickness, and was composed of 2 wt % Ru/Al2O3 with 0.008 g of Ru. A metal surface area of 11.47 m2/g Ru and active site density of 2.72 × 10−5 mol active site/m2 Ru was determined by the H2 adsorption method (BELCAT-B, Microtrac BEL Co.). This value was similar to the literature one of 2.49 × 10−5 mol/m2 Ru.9 Catalytic Wall Reactor Apparatus. Figure 1 shows a schematic diagram of the reforming apparatus used in this research. The methane feed flow rate was varied as 5, 15, and

Figure 2. Multiscale approach for microkinetic model construction, evaluation, and reduction.

microkinetic modeling developed on this research. It employed recognized surface science data, phenomenological methods, and reduction techniques for primary relief of the computational load.13,18,26 The approach used microscale information (i.e., molecular scale data) to set the reaction mechanism with elementary reactions and their rate constants.13,18 For the mesoscale, the evaluation of each elementary reaction rate was carried out on the frame of the algebraic mean field (MF) microkinetic.13 At the reactor scale, coupling of microkinetic with a low-order reactor model was performed and the model was optimized with experimental results to execute analysis.13 With this information, a Langmuir−Hinshelwood macrokinetic rate law can be derived and then applied at a reactor scale to perform higher-dimensional modeling and simulation.12,13 Each step of the above method until optimization and analysis of the reduced reactor model will be described next. Elementary Reaction Mechanism of Methane Steam Reforming. We employed the methane steam reforming mechanism proposed by Wei and Iglesia.18 Table 1 shows the reaction mechanism. They proposed that steam reforming mechanism on Ru followed a heat crack mechanism20 with simple reaction intermediates and with gas species (e1 to e5), methane decomposition (e6 to e8), steam decomposition (e9, e10), hydrogen formation (e11), water gas shift, and carbon deposition (e2, e12) reaction paths. They developed this mechanism from experimental kinetic and isotopic studies in the high temperature region from 823 to 1023 K and in a wide pressure range from 0.1 to 0.5 MPa. Rate of Elementary Reaction. The reaction rate of each elementary step belonging to the above reaction mechanism was determined from the so-called MF approximation, which converts the chemical stoichiometry to a stoichiometry matrix and then implements the law of mass actions.5,8 Considering

Figure 1. Experimental apparatus. 8816

DOI: 10.1021/acs.iecr.7b01687 Ind. Eng. Chem. Res. 2017, 56, 8815−8822

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mechanism, compiled surface thermodynamic tables20,21 based on the UBI-QEP26 method for Ru (0001) metal surface were used. These tables supply the adsorption heat at 298 K, and the values were corrected at each experimental temperature by the statistical thermodynamics approximation of the adsorption process heat capacity proposed by Mhadeshwar et al.27 The enthalpy of each elementary reaction was computed by an abstract thermodynamic cycle consisting of the elementary reaction itself, and its corresponding reaction as if it had taken place on the gas phase.26,27 See the Supporting Information for more details. Intermediate species adsorption heats, Qi, and elementary reaction standard enthalpy, ΔHj°, were used as entries for the algebraic UBI-QEP calculation of the activation energy of each elementary step.26 Thermodynamic Consistency Conditions. The abovedescribed procedures determined Af,j, Bj, Ef,j, and consequently, kf,j was estimated from eq 4; meanwhile the backward reaction constants kb,j were calculated from the equilibrium constant of each reaction and from the corresponding intermediates desorption entropy ΔdSi° (to calculate step entropy according to the abstract thermodynamic cycle), to ensure local and global thermodynamic consistency.5,27,29 See details on the Supporting Information. Reduced Reactor Model. MF microkinetic was evaluated at the reactor scale by a reduced reforming reactor model according to the works of refs 12 and 13. We assumed isobaric and isothermal condition by considering previous work,30 uniform concentration gradient in gas phase, and dynamic change of species surface coverage on the catalyst surface. In addition, we considered constant total gas and active sites molar concentration.28,31 Gas phase:

Table 1. Methane Steam Reforming Elementary Steps Mechanisma CH4 + 2* ⇄ CH3* + H* CO2 + 2* ⇄ CO* + O* H2 + * ⇄ H2* CO + * ⇄ CO* H2O + * ⇄ H2O* CH3* + * ⇄ CH2* + H* CH2* + * ⇄ CH* + H* CH* + * ⇄ C* + H* H2O* + * ⇄ OH* + H* OH* + * ⇄ O* + H* H* + H* ⇄ H2* + * C* + O* ⇄ CO* + *

e1 e2 e3 e4 e5 e6 e7 e8 e9 e10 e11 e12 a

Gas phase reactions have been written as adsorption reactions for calculation in the microkinetic model. *Active site on Ru catalyst.

only Langmuir−Hinshelwood elementary reactions, see Table 1, the elementary reaction rates became: adsorption reactions: TOFk ⎛ p ⎞−βk′,k −α ′ α″ = k f, k ⎜ k ⎟ θ1 k ,1 − k b, k ∏ θi k ,i P ⎝ o⎠ i −αι′, i

surface reactions: TOFι = k f , ι ∏ θi i

(1) −αι″, i

− kb , ι ∏ θi i

(2)

Here only bimolecular surface reactions were regarded, the reaction rate was expressed as a turn over frequency [1/s] and reversibility effects were taken into account by including the backward reaction term. Also note that the reaction rate constants kf and kb were normalized to the reference pressure po = 0.1 MPa, yielding the units of [1/s] for both constants.8 The reaction rate in molar units was readily calculated by the total concentration of active sites, Ct,s [mol/m2 Ru] for any reaction type: rj = Ct , sTOFj

Cg

A f, k = sk

=

Cg tres

(yk , o − yk ) +

∑ νj ,krjavcat (6)

j

Catalyst surface: Ct ,s

dθi = dt

∑ νj ,irj (7)

j

Initial conditions: t = 0 → yk = yk , o , θi = 0(i ≠ 1), θ1 = 1 (clean surface) (8)

(4)

The surface coverages were determined based on total constant active sites concentration as follows. Conservation of active sites:

The forward pre-exponential factor Af was calculated according to the reaction type. For adsorption reactions, Af was related to the sticking coefficient, s:9,12,28 KBT ⎛ Cg ⎞ ⎜⎜ ⎟⎟ 2πmk ⎝ Ct , s ⎠

dt

(3)

Reaction Rate Parameter Determination. The reaction rate constants, kf and kb, appearing on eqs 1 and 2 for each elementary reaction j were regarded equivalent to a modified Arrhenius type expression:9,27 ⎛ T ⎞ Bj k f, j = A f, j ⎜ ⎟ e−Ef,j(Q i , ΔH °j )/ RT ⎝ To ⎠

dyk

∑ θi = 1 i

(9)

The results of simulation were referred to steady state points, although the reactor model permits to investigate reaction system transient states, and this was out of the scope of the present research. Microkinetic Model Parameters Optimization. The adjustable parameters vector was given as Θ = [sk, Af,j, Bj, ΔdSi°, Qi]. All other parameters and constants on the microkinetic model were a function of this basic set. The optimization was constrained to experimentally recognized parameter ranges, values are given on the Supporting

(5)

We have here introduced the ratio of gas concentration and active sites concentration. For surface reactions, the forward pre-exponential values were estimated from transition state theory (TST).10,27 The range for temperature exponent, Bj, was determined from reported optimization data as a polynomial.9,27 For practical calculation of the adsorption heats of the intermediates present in the methane steam reforming 8817


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weight (note that this is different from the residence time defined on the reactor model) at S/C = 3, 0.1 MPa, and 673 K. The main produced gases were H2 and CO2 with a H2/CO2 ratio close to 4. CO was not detected in this study. This result indicates that the net reaction path was the inverse methanation reaction R3. Figure 4 shows the effect of S/C and pressure on methane conversion at 673 K. The increase in S/C enhanced methane

Information, and was carried on implementing directly the thermodynamic consistency conditions (see ref 27 and Supporting Information for discussion about direct and indirect optimization). Microkinetic Model Analysis. The computation procedures for the analysis of the reaction mechanism on Ru such as reaction paths, rate-controlling step, and surface intermediate are briefly presented. Type “1” integral RPA24 was performed to account for the degree of forward or backward progress for each reaction on the reaction path diagram: R f,1j =

∫t pos(rj) dt

(10)

R b,1 j =

∫t neg(rj) dt

(11)

The degree of rate control (DRC) and the degree of thermodynamic rate control (TRC) sensitivity coefficients defined by Campbell32 and Stegelmann et al.25 were applied to confirm the rate-determining step (RDS) and the thermodynamic mechanism control (i.e., controlling intermediates).

X jrc, A

Figure 4. Effect of S/C and pressure on methane conversion at 673 K (S/C = 2 (×), 3 (Δ), 4 (□), 5 (○) at 0.1 MPa, Gray marker triangle:0.2 MPa for S/C = 3) .

⎛ ⎞ ⎜ ⎟ ⎛ ∂ ln Y ⎞ ∂ ln YA ⎟ A ⎟ = ⎜⎜ =⎜ ⎟ TST ⎝ ∂ ln k f , j ⎠ kf ,j′≠j ⎜⎜ ∂⎜⎛ −G°j ⎟⎞ ⎟⎟ RT ⎠ ⎠ G°j′≠TSTj K eq, j ⎝ ⎝ Gi°

XiTRC ,A

⎛ ⎜ ∂ ln YA =⎜ ⎜ ∂ −Gi° ⎝ RT

( )

conversion at the same residence time, indicating that the reaction pathways and intermediates related to water probably assisted the reforming of methane. On the other hand, the increase in pressure decreased the methane conversion. Taking into account that the overall gas stoichiometric shown in Figure 3 was similar to R3, that is, the molecular number increasing reaction from reactants to products, the negative effect of increasing pressure on methane conversion was reasonable in the view of Le Chatelier’s principle. Figure 5 shows the temperature effect on methane conversion at S/C = 3 and 0.1 MPa and the conversion at

(12)

⎞ ⎟ ⎟ ⎟ G°TST ⎠ j Gi°′≠ i

(13)

The correspondent steps and intermediates with the greatest magnitude of DRC and TRC are designated as the ratedetermining step and controlling intermediates, respectively. Moreover, a negative DRC or TRC indicates that the reaction can be promoted by TST or intermediate stabilization because decreasing Gibbs free energy means stabilization. The composition was considered in this study, whereas Campbell’s32 original definition takes as a metric the overall reaction rate itself.

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RESULTS AND DISCUSSION Experiment of Methane Steam Reforming with Catalytic Wall Reactor. Figure 3 shows the produced gas molar composition of the methane steam reforming reaction against the inlet residence time W/FCH4 related to catalyst

Figure 5. Temperature effect on methane conversion at S/C = 3 and 0.1 MPa (markers are residence times, W/FCH4 [kg Ru s/mol] = 0.3 (×), 0.72 (Δ), 2.1 (□), dotted line are calculated conversion at equilibrium).

equilibrium thermodynamically calculated. At each residence time, experimental methane conversion increased with increasing temperature the same as calculated conversion at equilibrium. These trends were in accordance to the endothermic methane steam reforming system, R1−R3. Moreover, the conversion at the longest residence time was most close to the calculated conversion at equilibrium in each temperature. The longer contact time between gas and catalyst surface made the reaction proceed to equilibrium. Reaction Kinetics of Methane Steam Reforming with Catalytic Wall Reactor. In this section, reaction kinetics and reaction control mechanisms are discussed by using the simulation results. Note that results on this section are referred

Figure 3. Molar composition for methane steam reforming gas at S/C = 3, 0.1 MPa, and 673 K. 8818


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Industrial & Engineering Chemistry Research to steady state values and at the shortest residence time (far from equilibrium) 0.3 kg Ru s/mol, if no other mention is made. Macrokinetic Effects. After optimization, the microkinetic simulation results had a squared residual of 9.2%, agreeing well with the experimental results. The optimized parameter values are presented on the Supporting Information. The RPA was conducted at S/C = 3, 673 K, 0.1 MPa, and 0.3 kg Ru s/mol. Figure 6 shows the major reaction pathways for

Figure 7. Estimated surface intermediate coverage for steady state at residence time of 0.3 kg Ru s/mol and 673 K [S/C = 2 (×), 3 (Δ), 4 (□), 5 (○) at 0.1 MPa, gray marker triangle is 0.2 MPa for S/C = 3].

Table 2 shows free active sites coverage at the same conditions of Figure 7. The surface coverage of H* and O* Table 2. Free Active Sites Surface Coverage Determined from Microkinetic Model under Steady State at Residence Time 0.3 kg Ru s/mol and 673 K

Figure 6. Major reaction pathway for methane steam reforming at S/C = 3, 673 K, 0.1 MPa, and 0.3 kg Ru s/mol (arrow thickness indicates the integral degree of reaction progress, R1).

S/C

pressure [MPa]

θ1 [%]

2 3 4 5 3

0.1 0.1 0.1 0.1 0.2

55.42 65.65 72.73 77.16 56.66

increased with increasing S/C and decreasing pressure. The high reactivity of methane was probably derived from a large amount of free active sites. This trend is an agreement with the general results reported by other reforming kinetic works.22,35,36 On the other hand, negative effects of high S/C and high pressure have also been reported.33,34 These inconsistencies were probably due to the difference of the experimental conditions. The experimental endothermic behavior of the methane steam reforming reaction was interpreted as a profit in the rate of all elementary reactions influenced by temperature boost. Figure 8 shows the integral degree of rate progress for all

methane steam reforming at the above conditions. About the methane/carbon path, CH4 → C* → CO/CO2, the reaction moved completely forward, that is Rf1 was way greater than Rb1. The CO desorption reaction was the only one with nearly zero R1 value, for both forward and backward reactions, which means that the carbon path progressed to CO2 desorption rather than CO desorption. About the water path, H2O → O*, the overall reaction also moved completely in the forward direction to decompose water to O* and complete hydrogen production, and the degree of progress for water path was larger than the methane/carbon path, thicker arrows, proving the importance of water as noted on the experimental results of Figure 4. The determined overall gas stoichiometry as R3 was on the lines of what has been established since the first kinetic studies of Xu and Froment4 on Ni and on more recent and specific studies for Ru catalyst.22,33 In addition, the preference of CO2 generation path over CO was consistent as recognized on refs 34 and 35. Figure 7 shows the estimated surface intermediate coverage (θi) for steady state at residence time 0.3 kg Ru s/mol and 673 K. CO*, CH3*, H*, and O* were main species covering the catalyst surface. The coverage of CO* and CH3* decreased with increasing S/C and decreasing pressure. In the experiments, the methane steam reforming reaction was enhanced in the high S/C and low pressure region. These findings mean that surface intermediates such as CO* and CH3* clogged the catalyst surface, and therefore the overall reforming reaction was halted at some extent. In the case of high methane conversion, a cleaner catalyst surface was generated by reduction of intermediates coverage, which probably increased the number of catalytic cycles and promoted the overall reaction.

Figure 8. R1 results at S/C = 3, 0.1 MPa, residence time of 0.3 kg Ru s/mol [the number of reaction corresponds to Table 1, 573 K (×), 623 K (Δ), 673 K (□), 723 K (○)].

elementary steps at all temperatures. The number of reaction corresponds to that in Table 1. For all reactions, the magnitude of rate progress increased with increasing temperature. The degree of rate progress for reactions e2, e3, and e4 are negative, and these reactions correspond to the adsorption of CO2, H2, and CO. Other kinetic studies on Ru catalyst of methane steam reforming22,35,36 have also investigated the global endothermic 8819


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Figure 10 shows TRC of all surface intermediates on the methane molar fraction at 673 K, 0.1 MPa, and S/C = 3 and 5. The most influential intermediates were CO*, H*, and in a second scale O*, OH*. The CO*, H*, and O* TRC were in agreement with the surface coverage results as shown in Figure 7, since these intermediates occupied most of the active sites on the catalyst surface. CH3* which also occupied most of the active sites showed just a very low sensitivity, nonetheless needs to be included as a potential important intermediate. This inconsistency is left to be clarified yet, since it has been argued that a direct correlation between coverage and TRC holds.25 The magnitude of TRC for CO*, H* was greater than any DRC magnitude, implying that the control of the overall reaction depended not only on each elementary steps but also mostly on the intermediate stabilization. The positive and negative sign of CO* and H* intermediates TRC probably means that modifying the catalyst structure for further detachment (destabilization) of CO* and attachment (stabilization) of H* would boost the methane steam reforming reaction progress. This potential promotion of the reforming rate is markedly at higher magnitude of TRC in low S/C region because direct modification of CO* and H* intermediates attachment principally can liberate more active sites on the catalyst surface, thus the reforming reaction would keep progressing. Unlike the DRC case where the energy of the intermediates is preserved, meaning the surface coverage state remains unchanged, the TRC results show a wider improvement on methane steam reforming reaction by virtue of changing the catalyst surface coverage. The identification of kinetic and thermodynamic control parts was consistent with other reported findings, and other paths and intermediates can be activated as well in the different conditions. CH4 adsorption step was regarded by Wei and Iglesia18 as the only RDS and also has been identified on refs 19 and 36. In this study, steps on the water path have been evaluated as controlling too, as suggested by refs 10, 28, and 33. As for the controlling intermediates, identification of CO*, H* has been quite acknowledged but the importance of OH* has been neglected due to low coverage.9,28,36 Other researchers have reported that the Ru surface can attach strongly OH* along O*,19 and Aparicio10 claimed that a large amount of OH* promoted the O* generation. From our results and these findings, we can conclude that CO*, H*, OH*, and O* were key intermediates for methane steam reforming on Ru catalytic wall reactor.

effect. The microkinetic modeling in this study supports and further elucidates this trend. Microkinetic Analysis. Figure 9 shows steady state DRC results of each elementary reaction forward constant on

Figure 9. DRC results of each elementary reaction forward constant on methane molar fraction at steady state at 0.1 MPa, 0.3 kg-Ru s/mol, and 673 K [S/C= 3 (Δ), 5 (○), filled symbols are RDS at each S/C].

methane molar fraction at 0.1 MPa, 0.3 kg Ru s/mol, and 673 K. In the case of low S/C at S/C = 3, the DRC of steps 1, 2, 9, and 10 were similar and all other steps had almost zero response. The entire mechanism was probably controlled by these four steps. The step 2, carbon dioxide dissociative adsorption, and step 10, hydroxyl intermediate decomposition, were determined as the RDS because these steps were independent, meaning a steady state solution is possible by choosing them as RDS, and they showed consistently high DRC at other temperatures (not shown here). In the case of high S/C at S/C = 5, the step 1, methane dissociative adsorption, and step 2 dominated the entire reforming mechanism. Step 6 was competitive with step 2 but, not independent with the methane adsorption step, because steady state solution is not possible. Thus, in the low S/C region, the kinetic controlling steps of the entire reforming reaction mechanism was both methane/carbon and water path responsive steps; meanwhile in high S/C region, the controlling steps shifted to the methane/carbon path solely because by high water concentration a threshold of degree of reaction progress on the water path was exceeded. The negative sign of the controlling step’s DRC suggests that modifying the catalyst structure to get more stable TST (of the RDS) would enhance the methane steam reforming. This possible improvement would be greater in high S/C region due to the large amount of free active sites, meaning there is a wider room to decrease methane molar fraction, almost 0.07 for methane adsorption step, and continue the methane steam reforming reaction. Note that here the catalyst modification should not change other steps TST or the intermediates surface coverage. The latter is discussed in Figure 10.

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CONCLUSIONS Microkinetic analysis of the methane steam reforming reaction on Ru and experimental measurements at low temperatures from 573 to 723 K, on a catalytic wall reactor, determined a reforming system where the methane and a highly activated water reaction path moved forward to the inverse methanation reaction path (R3), the reaction was promoted due to a cleaner catalyst surface in high S/C and low pressure region, and the overall endothermic behavior arose from the profit of all elementary reactions degree of progress with high temperatures. Furthermore, the overall reforming was controlled equivalently by kinetic and thermodynamic mechanisms, according to the calculated contribution of steps and intermediates on reaction progress. The kinetic control in the low S/C region involved a water path, OH* decomposition and a methane path, CO2 dissociative adsorption, whereas that in the high S/C region shifted completely to the methane path,

Figure 10. TRC results of each surface intermediate Gibbs free energy on methane molar fraction at steady state at 0.1 MPa, residence time 0.3 kg Ru s/mol and 673 K [S/C = 3 (Δ), 5 (○), filled symbols are the most important intermediates at each S/C]. 8820


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B = Temperature exponent Cg = Total gas molar concentration, mol/m3 Ct,s = Concentration of active sites, mol/m2 Ru Cp,a = adsorption heat capacity approximation from statistical thermodynamics,25 J/mol s E = Elementary step activation energy, J/mol Keq = Elementary step equilibrium constant k = Elementary step rate constant, 1/s KB = Boltzmann constant, J/K m = Equals M/NA with M the molecular mass of the gas species and NA the Avogadro number neg = Operator, neg(·)=min(·, 0) p = Gas specie partial pressure, Pa pos = Operator, pos(·)=max(·, 0) Q = Intermediate specie adsorption heat, J/mol R1 = Type 1 integral reaction rate of each step, mol/m2 r = reaction rate of a elementary step, mol/m2 Ru s s = Gas specie sticking coefficient T = Gas temperature, K TOF = Turn over frequency, 1/s t = Simulation time, s tres = residence time, Vr/(uAr), Vr [m3] total reactor volume, u [m/s] gas superficial velocity, Ar [m2] total reactor transversal area Xrc = Degree of rate control XTRC = Thermodynamic degree of rate control Y = Gas specie molar fraction or intermediate coverage y = Gas molar fraction

CH4 and CO2 dissociative adsorption. The discussion of thermodynamic control pointed principally to CO*, H* intermediates as the most important ones. The developed microkinetic model of the methane steam reforming on Ru still poses challenges on both experimental and modeling grounds. Surface science and/or isotopic trace experiments are needed to corroborate the elementary steps, intermediates, and reaction rate constants parameters on methane steam reforming on Ru at low temperatures. As for the microkinetic model, it needs to be extended to include the coverage effect on the reaction rate constants, and also extension of the reactor model is needed to perform higher dimensional simulation, like CFD. Even with the above considerations, the developed microkinetic model of the methane steam reforming on Ru has advanced the elucidation of the reforming mechanism. Wei and Iglesias mechanism has been quantified and proved to generate consistent results with experimental data and other kinetic works, and we have suggested that further progress of the methane steam reforming reaction can be achieved by catalyst modification that induces stabilization/destabilization of the proper TST steps and intermediates. Practically, this has been achieved for bimetals development of electrocatalysts in the hydrogen evolution reaction,37 and for CO oxidation on Pt.5 In all, the present research is a first step to a detailed understanding of the catalytic conversion energy on Ru catalytic wall reactor at the micro level, in order to better convert energy at a process scale. This multiscale approach has been called molecular Process-Product-Process engineering, 3PE, by Charpentier,2 described the molecular level to control process scale, and applied to this research, it has the finality of produce energy with fewer amounts of resources and aligned with society environmental concerns, that is a sustainable energy process.

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Greek Letters

ΔH° ΔdS° α β θ ν

ASSOCIATED CONTENT

Subscripts

S Supporting Information *

‘ “ b f i j o ι κ

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01687. Details about the microkinetic model, chosen parameter optimization ranges, parameter optimization results, and model validation (simulation and experimental results) (PDF)

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Elementary step standard enthalpy, J/mol Intermediates desorption entropy, J/molK Intermediate specie stoichiometry coefficient Gas specie stoichiometry coefficient Intermediate specie surface coverage General stoichiometric coefficient of elementary step

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81-28-689-6178. E-mail: [email protected].

Reactive indicator on a given elementary step Product indicator on a given elementary step Backward reaction of elementary step Forward reaction of elementary step Intermediate specie indicator, 1 (free active site) to 11 General elementary step indicator Atmospheric or initial condition Surface reaction index, 6 to 12 (see Table 1,) Gas species index, CH4, CO2, H2, CO, and H2O

REFERENCES

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ORCID

Naotsugu Itoh: 0000-0003-4596-093X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS José Mauro Vásquez Castillo acknowledges the support from the Japanese Government (Monbukagakusho: MEXT) Scholarship.

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NOMENCLATURE A = Arrhenius pre-exponential factor, 1/s acatv = Superficial area of Ru by reactor volume, m2 Ru/m3 reactor 8821


Article


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Microkinetic Analysis of the Methane Steam Reforming on a Ru

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