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Reducing Reaction Temperature, Steam Requirements, and Coke Formation During Methane Steam Reforming Using Electric Fields: A Microkinetic Modeling and Experimental Study Fanglin Che, Jake T. Gray, Su Ha, and Jean-Sabin McEwen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01587 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017
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ACS Catalysis
Reducing Reaction Temperature, Steam Requirements, and Coke Formation During Methane Steam Reforming Using Electric Fields: A Microkinetic Modeling and Experimental Study Fanglin Che,a Jake T. Gray,a Su Ha,a Jean-Sabin McEwen*abcd a
The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman WA, 99164
b c
Department of Physics and Astronomy, Washington State University, Pullman WA, 99164
Department of Chemistry, Washington State University, Pullman WA, 99164
d
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland WA, 99352
ABSTRACT: In this study, we approach several common problems with the Ni-catalyzed methane steam reformation reaction (MSR) using a two-pronged approach combining density functional theory (DFT) calculations with experimental work. Specifically, we look at the deactivation of a Ni catalyst due to coke formation, its high operating temperature requirements, and the high steam-to-methane (H2O/CH4) ratio needed for proper MSR operation. A DFT-based microkinetic model was developed in the presence and absence of electric fields and the results were compared with experimental results. The microkinetic model shows that under various electric fields, the most favorable MSR mechanism changed slightly. It also shows that the presence of a positive electric field decreases the surface coverage of carbon, increases the water coverage, accelerates the rate-limiting step of the C-H bond cleavage in methane, and increases the desorption rates of the syngas product (CO + H2) during MSR. Consequently, for a given methane conversion, a positive electric field allows for significantly lower H2O/CH4 ratio and operating temperatures as compared to systems without an electric field. These findings correspond well with experimental tests under a variety of operating conditions. In addition, improvement in the catalytic activity due to the presence of a positive electric field remained significant even at industrially relevant applied pressures - improving the hydrogen yield greatly. Overall, we find that an applied electric field can play a significant role in improving the catalytic activity of heterogeneous reactions. This information can guide the design of heterogeneous reactions in the presence of an electric field. By utilizing the electric field generated by various renewable energy sources, electric field assisted heterogeneous reactions can open up a paradigm in future energy research. KEYWORDS: Electric Field Assisted Catalysis; Low Temperature Methane Activation; Methane Steam Reforming; Low Steam-to-Carbon Ratio; Catalyst Stability; First Principles-Based Microkinetic Model.
the steam.4 These factors together greatly reduce the overall efficiency of the Ni catalyzed MSR reaction.
1. Introduction Hydrogen gas is most commonly produced from fossil fuels – steam reforming of methane over Ni-based catalysts in particular accounts for a staggering 95% of the total hydrogen gas produced on an industrial scale in the US.1 This is partly due to the abundance of natural gas from shale deposits and because Ni is an inexpensive, earth-abundant metal. Currently, methane steam reforming (MSR) over Ni faces three major issues: (i) the prolific formation of elemental carbon deposits over the catalyst during the course of the reaction (coking);2 (ii) high operating temperatures of 900 K or more required to overcome the highly endothermic nature of the reaction;3,4 and (iii) high steam-to-methane (H2O/CH4) ratios (greater than 3:1) used to limit coke formation, which requires a significant investment in energy to vaporize and handle
One approach to enhance the Ni catalytic performance during methane steam reforming is to modify the Ni catalyst itself by, for example, adding metal oxide supports or doping with other transition/noble metals. By using different catalyst supports such as silica, alumina, and zirconia, Matsumura and Nakamori were able to lower the temperature requirements of the MSR reaction from 1073 K to 773 K.5 They found that Ni supported on zirconia proved to be the most effective catalyst during MSR at a lower temperature of 773 K due to its superior affinity for water and ability to “store” many more OH groups than other catalyst/support systems. The ''stored'' OH groups on the catalysts greatly improve methane conversion by facilitating the cleavage of C-H bonds of methane and directly enhance the conversion of methane at lower temperatures. To address the coke formation issues, The1
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ofanidis et al.6 investigated a series of bimetallic Fe/Nicatalysts with various Fe/Ni ratios during the dry reforming of methane where Fe partially segregates from the alloy and forms FeOx species. The lattice oxygen of FeOx then reacts with surface carbon producing CO and directly assists in removing CO from the Ni surface at 923 K – 1073 K. A myriad of other dopants including Au,7 Cu,8 Rh,9 Ru,10 Pt,11 Ag,12 Pd,13 and Sn2,14 can also be used as promoters to reduce carbon deposits over a pure Ni surface during the MSR reaction. By virtue of their inherent coke resistance, these bimetallic catalysts also allow MSR operation under lower H2O/CH4 ratios. Another route, and the one we investigate here, to achieving coke resistant, lower-temperature catalysts while decreasing the H2O/CH4 ratio is through the application of external electric fields to the catalyst. Electric fields can significantly alter the catalytic activity and selectivity by changing the underlying potential energy surface of the catalytic reaction, which significantly alters the surface kinetics.15-17 Multiple studies have examined heterogeneous reactions in the presence of an electric field and there are several reports that examine how an electric field can facilitate a catalytic reaction including plasmaaided catalytic reactions,18,19 non-Faradaic electrochemical modification of catalytic activity (NEMCA),20 reverse fuel cells (i.e. solid oxide electrolysis cell (SOEC)),21-23 scanning tunneling microscope (STM) probe,24-26 and field ion microscope (FIM) investigations.27 Choi et al. showed that the concentration of hydrogen products in the MSR reaction is increased more than 70 vol.% by using steam microwave plasma.28 Manabe et al.29 applied a working current of 5 mA to the MSR reactor consisted of a Pt/CeO2 catalyst bed between two probes to decrease its operating temperature by 200 K. They showed experimentally that such a high working current can assist the adsorption of water on the catalytic surface and activate water to decompose into protons. The first C-H bond cleavage in methane is the rate-limiting step of the MSR reaction, which can be activated by the proton collision at the PdCeO2 interface. However, operating the catalytic reactions under a plasma, which is formed by a high density electric field, has a low energy efficiency due to the high consumption of electric power.4 On the other hand, the catalytic activity and selectivity of metal catalysts over a metal oxide can be dramatically changed via the NEMCA effect due to an electrochemical ion spillover process wherein oxide anions migrate from the metal oxide to the metal catalytic surface.30 Unfortunately, such ion spillover requires higher operating temperatures.4 SOEC provides highly efficient hydrogen (or syngas) production via the electrolysis of water (or the co-electrolysis of steam and carbon dioxide). Still, the operating temperature requirement for a SOEC is also high, requiring temperatures from 600-1000 oC.31 A high electric field can also be generated on the tip of a STM32. Aragonès et al. investigated the Diels-Alder reaction in an STM-tip “nanoreactor” in which a diene was chemically attached to a gold STM tip and brought into contact with a dieneophile bonded to a flat gold surface.33 Their results show that the electric field can significantly alter the reactivity of the two mole-
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cules. However, such STM-tip “nanoreactors” are unsuitable for scale-up or for implementing in practical applications. Recently, electro-reforming3,4,18 has emerged as a novel and promising approach to apply an electric field to the catalytic reaction, requiring less overall energy input than the three above-mentioned methods. Gorin et al. 34,35 experimentally applied a voltage between two Si electrodes to induce a uniform electric field and such electric fields significantly changed the selectivity of a Rh porphyrinTiO2 catalyst for the carbine reaction to favor the cyclopropanation product rather than the insertion product. Comparisons between experimental efforts and theoretical research in this area is lacking, however. The present work focuses on the development of a microkinetic model for MSR under the influence of an electric field (Sections 3.1 and 3.2) with corresponding experimental results that confirm our findings (Section 3.3). In particular, Section 3.1 outlines how the field-dependent microkinetic model was constructed from DFT calculations. We introduce our novel MSR-on-Ni design with better coke-resistance, catalytic performance, lower operating temperatures, and decreased H2O/CH4 ratio at industrially relevant reaction pressures in the presence of an external positive (+1 V/Å) and negative (-1 V/Å) electric field in Section 3.2. It is worth mentioning that only large fields can distort electronic orbitals to a certain degree that the chemical properties of an atom or a molecule can become altered (e.g. by establishing new bonding orbitals) and consequently new chemical reaction pathways can be established.36 Thus, the reason of choosing such high values of the electric field (±1 V/Å) in theory is to capture the electric field effects in the underlying heterogeneous catalytic reaction.
2. Methods 2.1 Computational Setup. Density functional theory (DFT) calculations were performed using the Vienna Ab Initio Simulation Package (VASP) code,37 which uses a plane wave basis set. Ion-electron interactions were modeled by the projector-augmented wave (PAW) method.38 The Generalized Gradient Approximation39 with the Perdew-Wang 91 functional was used to calculate the exchange-correlation energy.40 The plane-wave energy cutoff was set to 400 eV. The Ni lattice constant was calculated to be 3.521 Å. From the XRD spectrum (Figure S4), the Ni(111) facet shows a higher signal as compared to the signals from the (100) facet and the (110) facet. From our previous work, we concluded that the electric field effects on the methane decomposition and water/Ni interactions on Ni(111) and Ni(211) are similar.41,42 As a result, we only consider a flat Ni(111) surface for the said reaction in this paper and posit that any effects observed are good predictions of those we would find on a Ni(211) surface. We used a flat p(3×3) Ni(111) surface for simulating our heterogeneous catalyzed MSR system, which allows adsorbate coverages as low as 1/9 Monolayer (ML). We constructed a Ni(111) surface using a four-layer slab and set the vacuum layer to 11 Å, which is appropriate for avoiding interactions between 2
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ACS Catalysis
adsorbates and the periodic image of the slab while also suppressing ‘field-emission’ to the vacuum. A MonkhorstPack 4×4×1 k-point mesh43 was applied for the surface calculations. Spin-polarization was used for all calculations. Our previous work had shown that the adsorption of the MSR-involved species with and without electric fields had converged with respect to a higher energy cutoff (450 eV), a finer k-point mesh of (6×6×1) and a 5-layer slab for Ni(111).42,44 We also examined the van der Waals corrections by applying the optB8845,46 functional for our calculations of the methane reaction energy and the dehydrogenation of water and compared our results with the PW91 functional. We found that the reaction energies change by less than ~0.1 eV with different applied fields, and thus all the data used to construct the microkinetic model was performed using the PW91 functional. We applied the same approach proposed by Neugebauer and Scheffler47 to simulate a uniform electric field, introducing a dipole layer in the middle of the vacuum with opposite charges on either side. More details about the computational setup are given in our previous work.41,42,44,48 2.2 Experimental 2.2.1 Catalyst Preparation. A solid (conductive) but porous catalyst was prepared by sintering nickel particles (99%, Alfa Aesar, 95 um diameter) to a nickel foam support (99.9%, 1 mm thickness, GoodFellow). To accomplish this, the particles were suspended in an organic jelly (Vehicle V-006A, Heraeus) to make a spreadable paste. This paste was then spread evenly on both sides of several 1 cm diameter nickel foam disks, allowing time between sides for the paste to dry. The paste-covered disks were then placed in an oven around 100°C for several hours. After drying, the disks were stacked together and placed in a 1 cm diameter quartz tube reactor and sintered at 850°C for four hours under a H2/Ar atmosphere, total resistance was 0.5 Ω. Finally, lengths of silver wire (Alfa Aesar, 99.9%, 0.404 mm gauge) were attached to the top and bottom of the disk stack using silver paste (SPI Supplies) and dried in the oven for several hours. Figure 1 shows a schematic drawing of the catalyst and its incorporation into the reactor. The addition of spherical particles increases the magnitude of the surface field without requiring dangerously high applied voltages.
Figure 1. Simplified representation of the Ni-based methane steam reforming system in the presence of an electric field. Detailed reactor schematics can be found in the SI of our previous work.42 2.2.2 Reactor Construction. The reactor consists of a single quartz tube (Quartz Scientific, 1 cm inner diameter) divided into two distinct sections: a preheating section and a reactive section. Both sections are heated independently, but only the reactive section requires precise temperature control. The preheating section consists of a bed of loosely-packed silicon carbide particles (SiC, Pfaltz & Bauer, 99%) maintained at around 160°C using a variac (Staco Energy Products, 3PN1010). The reactive section is controlled precisely using a PID temperature controller (Cole-Parmer Digi-Sense) and a K-type thermocouple (Omega) that is in direct contact with the catalyst bed. Both sections are heated using ceramic furnaces from Watlow. 2.2.3 MSR Reaction. 18 MΩ H2O is introduced to the reactor via a syringe pump (Cole Parmer) and all gases (Ar, H2, and CH4) are controlled using a rotameter (Aalborg). The system is first purged under Ar for ten minutes before heating to the reaction temperature. H2 gas is then introduced to pretreat the catalyst and ensure a clean surface before each test. After five minutes of pretreatment the desired electric field is set via a DC power supply (VOLTEQ HY20010EX) and the syringe pump is turned on to introduce H2O to the system. CH4 flow is begun after two or three minutes of H2O/H2 co-feeding, at which point the H2 is shut off. The system is then allowed to reach steady state over the course of thirty minutes
Scheme 1. The proposed overall mechanism of the Ni-based methane steam reforming reaction.
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before sampling begins. Gas chromatography (SRI Instruments 8610C) is used to determine the product stream composition and the flow rate is measured using a digital flowmeter (Alltech).
3. Results and Discussion Scheme 1 shows the overall mechanism of the methane steam reforming (MSR) over the Ni catalyst in the presence and absence of electric fields examined in this study. In this proposed mechanism: (i) a water molecule adsorbed on the catalytic surface can form a surface OH group and H or further decompose to form O and 2 H atoms; (ii) a dissociated methane molecule can also decompose on the Ni(111) surface and form a CHx group (x = 0-3); (iii) the CHx group (x = 0-3) can then combine with the surface OH or an O species to from CHxOH and CHxO and dehydrogenate to syngas as the final products.42,44,48,49 Here, we mainly present our field-dependent microkinetic model of the overall MSR reaction and compare the results with our experimental findings. In particular, we look at how an applied electric field influences methane conversion of the MSR reaction with: (i) different operating temperatures; (ii) feeds with various H2O/CH4 ratios; and (iii) tunable, industrially relevant reaction pressures. 3.1 Microkinetic Model. In order to quantitatively determine the catalytic activity of Ni during the overall MSR reaction in the presence and the absence of an electric field, we constructed a microkinetic model of the aforementioned MSR reaction. All elementary reactions involved in Scheme 1 were considered both with and without electric fields. 3.1.1 Rate constants for adsorption and desorption. As shown in the proposed reaction mechanism, there are four adsorption steps: +∗⟶ ∗ , + 2 ∗ ⟶ ∗ + ∗ , 2 ∗ ⟶ + 2 ∗, and ∗ ⟶ + ∗, (where ‘*’ indicates a free Ni active site). For a fielddependent adsorption process ( +∗⟶ ∗ ), the rate constant (ka) in the absence of an electric field is calculated via collision theory50-52: =
(1)
where S0 is the initial sticking probability for adsorption in the absence of an electric field at zero coverage.53 From literature, the S0 values of H2O,54,55 CH4,56 CO,57,58 and H259,60 over a Ni(111) surface are 10-5, 10-7, 1, and 0.2, respectively. We assume electric field and temperature effects on the initial sticking coefficients are negligible. as is the surface area of an active site, and m denotes the molecular weight of the gas phase species of interest. When a simulated field is present, the rate constants of adsorption are influenced by changes to the activation barrier for adsorption ( − = 0, as shown in Eq. (2). =
e #$%&'(#%&'()*+/
(2)
Here, we are essentially neglecting the effect of lateral interactions in our model. Furthermore, a fielddependent desorption rate for ∗ ⟶ + ∗ is also incorporated in our model, where the desorption rate constant (kd) is obtained by51,52:
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1 -./0 ∗
1 235 46
7 #%'(⁄
(3)
= where 9:;< stands for the internal partition functions of the gas phase molecule A (rotational and vibrational par∗ tition functions) and > = includes frustrated translationalvibrational, rotational-vibrational, and vibrational partition functions of a molecule A* on the surface. The ther2 mal wavelength is given by ? = . is the
field-dependent desorption activation energy ∗ ⟶ + ∗ for CO and H2O. As for the associative desorption of H2 on the surface, the field dependent activation energy for the ( 2 ∗ ⟶ + 2 ∗ ) process, and is given by51,52,61: = BC*
=
@
5 -./0 @∗ 5 235 4 6
7 #%' (A
* = 2BC* − DC* * * − E ⁄FG + FG +
DC*
=
BC*
* 2 E ⁄
−
(4) (5) * E ⁄
* E 5 ⁄
(6) (7)
DC*
where and are the binding energy of ∗ ⟶ + ∗ and the electronic dissociation energy of molecular hydrogen with various electric fields and zero point energy corrections, respectively. Additionally, all examined vibrational frequencies for adsorbates and gas molecules were calculated in the presence of different simulated electric fields. 3.1.2 Rate Constants for the surface elementary reactions. Based on previous work,42,44,48,49 we applied the nudged elastic band (NEB) method to establish an initial guess for the transition state (TS) image and then from this initial guess used the climbing image nudged elastic band (CINEB)62,63 method to find the true TS of each elementary reaction involved in the MSR mechanism under applied fields. In this way, we obtain the field-dependent activation energies ( * ) for each possible reaction step. Using transition state theory64,65 combined with statistical mechanics and our DFT calculations, we identify the field-dependent forward ( H ) and backward (G ) rate constants as well as the equilibrium constant (I ) at certain experimental temperatures for the proposed elementary surface reaction over Ni(111). More details regarding the rate constant calculations via transition state theory can be found in our previous work.49 All H , G , and I are calculated in the temperature range spanning from 773 K to 1173 K in increments of 50 K. The dependence of these kinetic parameters in the presence and the absence of an electric field for all possible MSR elementary reactions was also taken into account and are given in Table S1-S9. 3.1.3 Establishing a field-dependent microkinetic model. As shown in Scheme 1, there are 16 surface species (17 if we count the free sites (θ) involved in the proposed MSR mechanism). We can get: J.
