Mechanistic Investigations of Heterogeneously Catalyzed Steam

Aug 25, 2014 - Pyrolytic coke is formed during the thermal cracking of fuels at .... new reaction mechanisms for the steam gasification of HC coke pre...
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Mechanistic Investigations of Heterogeneously Catalyzed Steam Gasification of Coke Precursors Susanne M. Opalka,* He Huang, and Xia Tang Physical Sciences Department, United Technologies Research Center, 411 Silver Lane, East Hartford, Connecticut 06108, United States S Supporting Information *

ABSTRACT: Atomic modeling was conducted to mechanistically investigate the CsOH steam gasification catalyst coating that has been successfully demonstrated to eliminate coke deposits during high temperature fuel pyrolysis. This effective coke mitigation was interpreted from the atomic modeling results to be due to the multiple functionalities of the CsOH coating for blocking the underlying metal surface from catalyzing coke formation, preventing deposition of coke-forming precursors and products, and catalyzing the oxidation of coke precursors in the presence of water. The discovery that the CsOH(010) surface was only predicted to strongly interact with hydrocarbon radicals that bombard surfaces during hydrocarbon pyrolysis led to the creation of novel reaction mechanisms for the effective steam gasification of radical coke precursors. The CsOH(010) surface was predicted to locally rearrange and form vacancies to facilitate the decomposition and oxidation of adsorbed hydrocarbon radicals. Two CsOH(010) heterogeneously catalyzed steam gasification reaction mechanisms were proposed involving hydrocarbon radical and H2O coreactants for the decomposition and oxidation of a methyl adsorbate. The first “H vacancy mechanism” oxidized a methyl radical through a bimolecular reaction with H2O. In the second “Cs insertion mechanism,” the adsorbed methyl radical was oxidized directly by reduction of the CsOH surface. The resulting OH vacancy was refilled by H2O dissociation, in order to restore the surface reaction site. This latter mechanism was more energetically downhill overall and had a modest rate-limiting energy barrier that could be easily overcome during fuel pyrolysis at high temperatures. These mechanisms are consistent with the experimentally observed stability of the CsOH coating, which functions as a true catalyst that is not consumed or dissolved over time. Observations of the CsOH coating behavior over a range of temperatures supported the hypothesis that effective coke mitigation functionality is the result of a dynamic balance between steam gasification of coke precursors arriving at the surface and the removal of already accumulated deposits.

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INTRODUCTION The formation of carbonaceous deposits, or coke, is a common, undesirable side-reaction that often compromises the performance and efficiency of hydrocarbon (HC) fuel refining and synthesis processes. Pyrolytic coke is formed during the thermal cracking of fuels at temperatures above 350 °C and is promoted with decreasing fuel hydrogen (H)/carbon (C) or oxygen (O)/ C ratios.1 Coke deposition progressively diminishes HC fuel process heat transfer rates, thermal efficiencies, fuel flow, and reactor product yields and, if unchecked, can ultimately lead to equipment shutdown. High temperature HC fuel processing technologies, such as steam reforming, ethylene cracking, heavy oil refining, partial oxidation, and substoichiometric combustion, are often plagued with pyrolytic coke catalyst deactivation and deposit build-up on reactor walls. High temperature pyrolitc coke buildup is also a principal operability issue for the thermal management of advanced aircraft, rocket, and missile engines that use kerosene-type fuels.2 As flight speeds of these vehicles increase to the high supersonic and hypersonic regimes, the high temperature ram air taken on board cannot cool the structure and, therefore, it is necessary to utilize the fuel as the primary coolant. The requirements for maximizing heat sink in hypersonic applications leads to operation at very high temperature (approximately 700 °C or higher) and pressure (greater than 40 bar) conditions where the HC fuel exists as a supercritical fluid phase. The densified supercritical © 2014 American Chemical Society

fuel has an increased potential for accelerated pyrolytic coking leading to system failure.3−5 The particular pyrolytic coke phase(s) formed, which can include filamentous, amorphous, and/or graphitic coke, depends on the availability of metal surfaces with catalytic functionality and the process time−temperature history.6 Both filamentous and graphitic carbon growth are heterogeneously catalyzed on metal surfaces. First HCs, especially radical fragments, dissociatively adsorb on metals, preferentially on surfaces with specific crystallographic orientations. This leads to C solubilization and saturation just below the surface.7 The nucleation of heterogeneous coke originates from the reemergence of the saturated C solute on the metal surface to nucleate metal carbide, filamentous, and/or graphitic precursor structures.8 In addition, coke precursors for amorphous deposits are homogeneously formed in the fuel fluid phase and then adsorb on solid surfaces. Even after adsorption, these species typically contain multiple sites that can participate in condensation and cyclization reactions with additional unsaturated and radical HC species that are arriving continuously at the surface.9 The conditions for formation of the different coke types overlap with each other, so that Received: April 22, 2014 Revised: August 22, 2014 Published: August 25, 2014 6188

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mechanisms on the down-selected CsOH catalyst. The most favorable adsorption intermediates for oxidative species and hydrocarbon fragments and their reaction products were identified. The results were used, with the aid of selected transition state determinations, to propose new reaction mechanisms for the steam gasification of HC coke precursors. These results were interpreted in conjunction with post-test reactor tube CsOH coating characterization findings to generate insights into the unique CsOH catalyst steam gasification functionalities that may be responsible for their simulated effective coke mitigation performance.

amorphous deposits often infiltrate high surface area heterogeneous coke nanostructures to form high strength composite materials.10 Coke mitigation technologies, in the form of additives, catalysts, or coatings, can be employed to prevent coke formation and/or buildup. A number of metallic and O-bearing metallic phases have been identified to be effective for steam gasification of petroleum coke,11 especially alkali and alkaline earth oxides and carbonates.12,13 In particular, cesium (Cs)containing very basic compounds, such as cesium carbonate (Cs2CO3) and cesium hydroxide (CsOH), provide much higher catalytic reactivity than the potassium (K)-containing basic compounds, such as K2CO3 or KOH, that have been widely used for coal gasification. The Cs phases have been shown to be highly cohesive with very stable, electron-rich surfaces that are resistant to dissolution and coke bonding.2 It is not economically feasible to use these strong basic Cs compounds to catalyze coal or petroleum coke steam gasification. However, they are ideal phases to formulate as wall coating catalysts for removing pyrolytic coke deposits via catalytic steam gasification in endothermic fuel systems for high temperature aeronautical applications. The coke mitigation catalyst can be applied as a passivation coating to metal wall surfaces to circumvent the metal-catalyzed nucleation and growth of coke deposits. Ideally, the coating should resist the adsorption and condensation of homogeneous coke species formed in the fuel fluid phase. It is also of benefit for low molecular weight free radical species (e.g., the methyl radical, H3C·) to be quenched at the surface, by adsorption, neutralization, or saturation, to prevent their involvement in fuel fluid phase radical reaction cascades that lead to coke precursor formation or that promote growth of coke surface deposits. However, the coating must resist the adsorption of electron-rich higher molecular weight unsaturated and/or polycyclic reaction products that are believed to nucleate amorphous coke formation. In cases where coke is still able to accumulate on the surface, coke mitigation coatings may also serve to catalyze the removal of coke deposits by steam gasification. Heterogeneous coke steam gasification involves the catalyzed water oxidative decomposition of coke precursors or already accumulated coke, analogous to the steam gasification of petroleum coke for H2 production.1 Coke deposits can be removed from surfaces by adding small amounts of H2O into the fuel stream, which can then endothermically react with adsorbed coke precursors to produce carbon monoxide (CO) and molecular hydrogen (H2) products.14 The net coke precursor (CxHy) steam gasification reaction is given as Cx Hy + x H 2O = (y/2 + x)H 2 + xCO

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ΔHform = Hbulk phase − ΣHelement standard state phase

(3)

The most stable phases have the most negative ΔHform values. In order to identify the lowest energy surfaces of these bulk phases, they were cleaved along various high atomic density crystallographic planes to form 4 layer slabs. The slabs were then minimized with 12 Å vacuum separation in the z-direction, while fixing the atomic positions of the slab bottom two layers at the bulk lattice ground state minimum positions, so that the upper slab surface represented the slab−fuel interface. The slabs surface energies, γ (in J/m2), were determined from the difference in energies between the minimized slab structure composed of n phase formula units (nFU) and the corresponding stoichiometric equivalent, nFU, of the minimized bulk structure, divided by two times the slab unit interfacial surface area.

(1)

Analogous reactions are proposed for the disintegration of accumulated coke deposits.14 A high water concentration will promote the exothermic water gas shift reaction, leading to additional H2 production. CO + H 2O = CO2 + H 2

EXPERIMENTAL METHODS

DFT Calculations. The atomic structures, physical properties, and surface reactivity of Cs-based catalyst candidate phases were investigated with first-principles atomic modeling. The periodic atomic models were minimized using the plane wave basis Vienna ab initio simulation package (VASP) density functional theory code and projector augmented wave potentials with the Perdew, Burke, and Ernzerhof generalized gradient approximation for the exchangecorrelation functional (PAW GGA−PBE).15−19 Regular PAW GGA−PBE potentials were used for all atoms, except that the Cs_sv potential with the 5p and 6s semicore states treated as valence states was used to represent Cs. The Grimme DFT-D2 method20 for van der Waals corrections was not applied due to inconsistencies revealed in the testing of newly available DFT-D2 parameters for Cs21,22 (see results in Tables S1 and S3 of the Supporting Information). The parameters utilized for all models were converged in total energy to within 0.001 eV per atom. The parameters included: nonspin polarized electronic configuration, plane wave expansion cutoff of 400 eV, 0.2 /Å or finer space k-point mesh, Methfessel-Paxton smearing with an energy broadening of 0.2 eV, and an electronic self-consistent field convergence criterion of 10−5 eV. The ground state structures were minimized with the conjugate gradient algorithm until the Hellmann− Feynman forces were all below 0.02 eV/Å. The properties of a range of Cs and O bearing catalyst candidate phases were evaluated for their bulk and surface stabilities, basicities, and reactivities with representative adsorbate species relevant to steam gasification reactions. The selected solid-state Cs-based crystalline phases were based on the Inorganic Crystal Structure Database (ICSD) structure numbers23 with their corresponding literature references, including: CsOH ICSD.60972,24 Cs2CO3 ICSD.14156,25 Cs2O·SiO2 ICSD.411663,26 and Cs2O·2SiO2 ICSD.78564.27 The bulk periodic models were fully minimized with the VASP code to determine their ground state structures and electronic enthalpies, Hbulk phase. The bulk phase ground state electronic heats of formation, ΔHform (in kJ/mol), were determined by referencing the Hbulk phase with respect to the sum of the ground state energies of the stoichiometric equivalent of standard state elemental phases that comprise the bulk phase.

(2)

Atomic modeling was conducted to establish a fundamental understanding for the in situ continuous elimination of coke deposits observed experimentally by steam gasification with a CsOH catalyst.2 This study first surveyed the predicted properties and reactivity of Cs-based solid-state candidate catalyst phases and their most stable surfaces. The modeling then focused on the investigation of steam gasification

surface energy, γ = {[Hslab − (Hbulk × nFU )]/(2 × surface area)}

(4)

The fixed bottom surface of the more computationally feasible single slab configuration introduces an artifact in the surface energy 6189

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Table 1. Predicted Cs-Bearing Catalyst Phase Properties and Corresponding Inorganic Crystallographic Structure Database (ICSD)23 Crystal Structure Information catalyst candidates CsOH ICSD23 File No. space group a (Å) b (Å) c (Å) β (deg) heat of formation, ΔHform (kJ/mol) most stable surface plane surface energy, γ (J/m2) O Cs H

Cs2CO3

6097224 1415625 P212121 P121/c1 Lattice Parameters: Calculated [Measured − ICSD] 4.315 [4.319] 6.111 [6.120] 11.325 [11.538] 10.249 [10.270] 4.393 [4.469] 7.858 [8.140] 90 [90] 98.3 [95.8] −371.5 −1018.8 (010) (001) 0.07 0.45 Surface Atom Charges by Bader Method28 −1.85 −1.86 0.8 0.9 1.0

determination, compared to a double slab configuration with two free surfaces. However, this contribution was minimal for the very stable surfaces and did not interfere with the comparison of different crystallographic planes. The crystallographic plane and surface termination that yielded the lowest surface energy slab corresponded to the most stable surface for the bulk phase of interest (see Supporting Information, Table S2). The lowest energy surface slabs were used to investigate surface reactivity with the selected adsorbates, including: small hydrocarbons (butane, C4H10; propylene, C3H6; benzene, C6H6; and methyl radical, H3C·), H2O, O, H, CO, and carbon dioxide (CO2). The VASP calculations examined the properties and reaction mechanisms of adsorbate−slab complexes that are minimized to the ground state potential energy surface. These calculations do not incorporate the kinetic effects of elevated temperatures or sample intramolecular interactions that can occur with increased pressure. The electronic adsorption enthalpies were determined referencing the adsorbing molecules, including the O2 and H2 elemental standard states for the O and H, respectively, and the bare surface slabs. The most stable cesium hydroxide (CsOH) (010) hydroxylterminated surface was selected for an in depth investigation of the catalyzed steam gasification mechanisms. Reaction intermediates were investigated on a 3 × 3 in-plane expanded supercell of the four layer CsOH(010) surface slab, minimized with the bottom two layers fixed. A series of calculations were made to identify the most stable molecular adsorption configurations and reaction intermediates for proposed steam gasification pathways on the CsOH(010) surface. All of the reported adsorption enthalpies, ΔHadsorption, and reaction enthalpies, ΔHreaction, were determined from the difference of the ground state electronic enthalpies of the products and the reactants without applying zero point energy or finite temperature thermodynamic corrections

Cs2O·2SiO2 7856427 P121/c1

6.835 [6.847] 13.709 [13.757] 17.041 [17.036] 108.4 [108.2] −1325.0 (001) 0.20

10.043 [10.061] 8.570 [8.609] 18.407 [18.414] 122.8 [122.8] −2197.8 (001) 0.09

−1.87 0.8

−1.84 0.9

method, where the atomic charge density was integrated within the atomic volume bounded by zero electronic flux interatomic regions of minimum charge density.28 Catalyst Coated Reactor Tube Coking Tests. The testing of reactor tubes with internally applied CsOH steam gasification catalyst coatings for high temperature fuel heat exchangers was previously reported in detail2 and is summarized briefly here along with an update on the CsOH coating characterization results for the interpretation of the atomic modeling. In these tests, a 10 wt % CsOH catalyst aqueous solution was used to coat on the inner wall of 1/8 in. OD/0.055 in. ID and 24 in. length Inconel 625 nickel alloy reactor tubes.2,29 A steam injection subsystem was installed on a single-tube reactor simulator to evaluate the effectiveness of the coating in situ coke deposit removal by catalytic steam gasification.2 Tests were carried out to compare the run duration (coking limit) of JP-7 HC fuel at 41.8 atm in an uncoated reactor tube with that in a CsOH catalyst coated reactor tube with 2 wt % steam injection. In the coking tests, the JP-7 fuel was preheated to 427 °C just prior to entering the reactor tube inlet. The fuel was further heated and underwent endothermic reactions as it passed through the reactor tube. The coking tests were conducted so that the fuel temperature was ∼750 °C at the reactor tube outlet. The pressure drop, defined as the pressure difference between the reactor inlet and exit, was proportional to the tube restrictions from coke deposition. The maximum run duration was determined to be when the pressure drop increased dramatically, indicating a significant fuel flow restriction. Following testing, the inlet, middle, and outlet portions of the reactor tubes were sampled for analysis by SEM (scanning electron microscopy), EDS (energy dispersive spectroscopy), and EPMA (electron probe microanalysis).

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RESULTS AND DISCUSSION Properties of Cesium-Bearing Phase. The Cs-bearing candidate catalyst phases, CsOH, Cs2CO3, Cs2O·SiO2, and Cs2O·2SiO2, were predicted by atomic modeling to be fairly stable and to form relatively unreactive, lowest energy surfaces. The calculated properties of these phases, including, lattice parameters, heat of formation, ΔHform, the most stable surface crystallographic plane and surface energy, γ, and the surface atom Bader charges, are given in Table 1. All minimized bulk phase dimensions were well within 2% of the experimental measured values. The four layer slabs formed from cleaving the candidate bulk structures along their most stable surface are shown in Figure 1. The CsOH bulk phase formed a very stable hydroxylated surface. Although the Cs2CO3 and Cs2O·SiO2 bulk phases had more stable ΔHform, their most stable minimized surfaces were highly corrugated with a variety of

ΔHadsorption (reaction) = (Hslab * adsorbate complex + ΣHdesorbed products) − (Hslab surface + ΣHadsorbates(reactants))

Cs2O·SiO2 41166326 P121/c1

(5)

where the * symbol denotes a surface-complexed adsorbate species. The convention followed was that a negative reaction enthalpy corresponded to an exothermic, favorable reaction and a positive reaction enthalpy to an endothermic, unfavorable reaction. Selected transition state configurations were optimized using the VASP nudged elastic band method with four intermediate images formed by linearly interpolating between the end point reaction intermediate steps. The activation barrier was determined as the enthalpy difference between the transition state and the preceding intermediate end point state. The charges on selected surface atoms were determined using an implementation of the Bader Quantum Theory of Atoms in Molecules 6190

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Table 2. Predicted Ground State Electronic Enthalpies for the Adsorption (ΔHadsorption) of Hydrocarbon Species, Steam Gasification Reactants and Products, and Formation of Surface Vacancies on Selected Cs-Bearing Catalyst Candidate Phases catalyst candidates CsOH Hydrocarbon Adsorbates C3H6 C6H6 C4H10 H3C· Steam Gasification Adsorbates H2O O (physisorbed) H CO CO2 (physisorbed) Surface Vacancy Formation Cs O H

−6.5 −6.3 −4.3 −78.3 −57.7 276.3 111.5 −15.7 −126.7

Cs2CO3

Cs2O·2SiO2

ΔHadsorption (kJ/mol) −35.9 −14.1 −13.2 10.6 −13.5 −2.8 −320.8 −22.9 ΔHadsorption (kJ/mol) −68.2 −65.5 258.8 340.4 −122.8 141.5 19.0 −20.4 −21.4 0.3 ΔHvacancy (kJ/mol) 306.4 401.2 268.7 487.7

305.4

Figure 1. Side views of minimized slabs with the most stable surface structures of the Cs-bearing phase (surface crystallographic plane) layers: (a) CsOH (010), (b) Cs2CO3 (001), (c) Cs2O·SiO2 (001), and (d) Cs2O·2SiO2 (001). The atom colors are blue for Cs, red for O, black for C, white for H, and yellow for Si.

different surface sites. These higher energy surfaces are more reactive. The very stable Cs2O·2SiO2 phase had a noncorrugated surface and a very low surface energy. Compounds containing both Cs and O are very strong Brønsted/Lewis bases, since Cs is the most electropositive element and will readily release an electron to the surrounding O atoms. The relative basicity of the candidate phases was interpreted to increase with the strength of the negative charge on the surface O atoms determined by the Bader method.28 From the O charges shown in Table 1, the order of decreasing basicity of the candidate phases was assigned as Cs2O·SiO2 > Cs2CO3 > CsOH > Cs2O·2SiO2. Alkali catalysts could promote hydrocarbon activation via O transfer or hydroxyl radical, HO·, attack. The O atoms may act as Lewis base electron-pair donors and abstract ·H from H2O to form HO·. The HO· is one of the most aggressive species known to oxidize and breakdown hydrocarbon fragments. An initial survey of adsorption reactions of representative HC species was conducted on three selected candidate phases: CsOH, Cs2CO3, and Cs2O·2SiO2. The reactant adsorption enthalpies are listed in Table 2. Selected adsorbate configurations on the CsOH(010) surface are shown in Figure 2. Complete HC molecules physisorbed weakly on the most stable surfaces, partly due to the inability of the VASP DFT methodology to describe van der Waals interaactions. Trial VASP minimizations of hydrocarbon adsorbates on CsOH(010) with DFT-D2 corrections using two different Cs parameters showed moderate physisorption for the C4H10 alkane and the C3H6 alkene molecules but increased repulsion of the aromatic C6H6 molecule (see Table S3 of the Supporting Information). This finding was interpreted to indicate that the

Figure 2. Side views of unique adsorbate configurations predicted on the CsOH(010) 3 × 3 × 4 slab surface: (a) inverted *H2O, (b) O insertion to form hydroperoxyl, *OOH, and (c) CO2 adsorption and rearrangement to form carbonate, *CO3H, where * denotes a surface adsorbed species.

dissociative adsorption or chemisorption of saturated, unsaturated, and aromatic hydrocarbon species is unlikely to contribute to coke precursor formation or deposition on the Cs-bearing candidate surfaces. Another model showed that the CsOH(010) surface was predicted to only weakly adsorb a partially hydrogenated pseudoaromatic coke nanoparticle (ΔHadsorption of −7.0 kJ/mol for adsorption at a single site), indicating an intrinsic resistance of the electron-rich CsOH(010) surface to binding coke deposits. The surfaces, 6191

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Table 3. Comparison of Potential Steam Gasification Reactions and Their Predicted Ground State Reaction Enthalpies, ΔHreaction, Initiated with H2O with Those Initiated with H3C·, Where Both Scenarios Involve the Formation and Reaction of Surface H Vacancies (□H) on CsOH(010) ΔHreaction (kJ/mol)

reactions with H2O Csx(OH)x → Csx(OH)(x−1)O□H + 1/2H2 Csx(OH)(x−1)O□H + H2O → Csx(OH)x + HO· Csx(OH)(x−1)O□H *CH3 + HO· → Csx(OH)(x−1)O□H *CH2 + H2O

H Vacancy Formation 304.5 Csx(OH)x + H3C· → Csx(OH)(x−1)O□H + CH4 Adsorption on H Vacancy −126.4 Csx(OH)(x−1)O□H + H3C· → Csx(OH)(x−1)O□H *CH3 Decomposition of H Vacancy Adsorbed Methyl −240.4 Csx(OH)(x−1)O□H *CH3 + H3C·→ Csx(OH)(x−1)O□H *CH2 + CH4

CsxOHx + H 2O → Csx(OH)(x − 1)*OOH + H 2

10.2 −477.3 −137.4

which has a very favorable ΔHreaction of −217.9 kJ/mol (see Figure 2c). However, this is likely to be an activated reaction since carbonate formation was not observed to form spontaneously in any of the other CO2 absorption simulations. Steam Gasification Reactions on CsOH(010). The predicted attributes of the CsOH(010) surface, strong stability and flexibility for accommodating unstable adsorbates, motivated the decision to down-select CsOH(010) for in depth investigations of heterogeneously catalyzed steam gasification reaction mechanisms. Since the preliminary adsorbate survey did not show that direct dissociative H2O adsorption was favorable on the pristine hydroxylated CsOH(010) surface, other mechanisms needed to be developed to explain the role of O-bearing reactants, such as H2O, HO·, or O·, in facilitating the steam gasification of coke precursor or accumulated species. Preliminary examination revealed that the involvement of the CsOH(010) surface in an O exchange mechanism, where H2O indirectly contributes to C oxidation by refilling an O or OH vacancy, was not straightforward. This is because the hydroxylated CsOH(010) surface that is terminated with an outer H layer and the O atoms are not directly accessible. The atomic modeling showed that the formation of OH or H vacancies on this surface requires moderately large endotherms, 402 and 305 kJ/mol, respectively, to generate stoichiometric equivalents of O2 and/ or H2 products. Another possible HO· source could be from the homolytic dissociation of *OOH complexes to HO· and a surface H vacancy (□H), by the reaction

especially the less stable Cs2CO3 surface, were more reactive to the methyl radical fragment, H3C·. The most favorable H3C· adsorption configuration is different for each of the phases’ most stable surfaces. The H3C· radicals adsorb moderately on CsOH over Cs and very strongly on Cs2CO3 over O, forming a surface ether-like linkage on the latter surface. The H3C· radical adsorbed weakly over O on the stable Cs2O·2SiO2(001) surface. The adsorption survey of representative steam gasification reactants and products showed little evidence of surface reactivity (see Table 2). The major steam gasification reactant, H2O, had moderate adsorption enthalpies. The H2O adsorbed in an inverted configuration on all surfaces to facilitate their Hbonding interactions with the electron-rich, basic surface O atoms. This H2O bonding configuration on CsOH is shown in Figure 2a. No evidence was seen for facile H2O dissociation on any of the pristine Cs-bearing compound surfaces. Input models with H2O dissociated on the CsOH(010) surface reformed intact H2O adsorbates during VASP minimization. Atomic O physisorption from O2 dissociation was not favorable on any of the surfaces. However, the initial placement of an O atom near a Cs on the CsOH surface resulted in the O inserting within a surface hydroxyl (OH) to form a hydroperoxyl group (OOH) spontaneously during the CsOH model minimization (see Figure 2b), meaning that the formation of this intermediate is not an activated reaction step following O2 dissociation. (This is because the ionic movements during VASP minimizations generally only converge to a local minimum energy well and cannot overcome energy barriers to converge on a more global minimum structure.) The *OOH formation from O2 dissociation is a modestly endothermic reaction (ΔHreaction of 50.9 kJ/mol) on CsOH(010). The formation of this *OOH group from H2O dissociation during steam gasification, by the reaction

Csx(OH)(x − 1)*OOH → Csx(OH)(x − 1)O□H + HO·

(8)

However, this is unlikely. Even if was feasible to form the *OOH complexes, the dissociation reaction has a very large endotherm (ΔHreaction of 366.4 kJ/mol). Since the preliminary survey did not show significant CsOH(010) reactivity with intact HC molecules, the more strongly interacting H3C· reactant was selected for the investigations of steam gasification mechanisms. This reactant is a prevalent radical species during HC pyrolysis and inherently has a high number of wall collisions due to its small size. This is relevant because steam gasification catalysts are commonly coated on reactor walls and are bomdarded with HC fragments during HC pyrolysis. The H3C· reactants played two roles in this atomic modeling study, where the reactions with the relatively simple H3C· were used to symbolically represent similar reactions that could occur with any HC radical arriving at the CsOH surface. First, the H3C· was used as an analog of an adsorbing HC radical coke precursor to be steam gasified. Second, the H3C· was used as a coreactant for facilitating dehydrogenation reactions of other HC adsorbates.

(6)

is unlikely because it is an even more endothermic reaction (ΔHreaction of 353.9 kJ/mol). The insertion of adsorbed O was not observed on the minimized Cs2CO3 and Cs2O·2SiO2 models. The CO and CO2 steam gasification products were only weakly adsorbed on the candidate surfaces, except for the stronger CO2 adsorption tilted over the surface H on the CsOH surface. Weakly bound products are easily volatilized and will drive the steam gasification reactions to completion to free up surface sites for additional reactions. It was possible to form a carbonate complex by staging an insertion of CO2 within a surface OH on CsOH, by the reaction CsxOHx + CO2 → Csx(OH)(x − 1)*CO3H

ΔHreaction (kJ/mol)

reactions with H3C·

(7) 6192

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Here, incoming H3C· fragments can dehydrogenate HC adsorbates to favorably form methane, CH4, and activate adsorbate decomposition reactions. The number of dehydrogenation steps required for steam gasification of a given HC adsorbate is dependent on its C to H ratio. The poor reactivity of the pristine CsOH(010) surface prompted the investigation of whether H vacancies are required to activate the surface for the steam gasification HC dehydrogenation and oxidation reactions. Hydrogen vacancy sites could spontaneously dissociate H2O to form highly reactive HO· to promote these reactions. However, the formation of each HO· would require two steps: first, the generation of a surface H vacancy and, second, a H2O dissociation reaction to fill the H vacancy and form an HO·. The possible preliminary reactions that could lead to the formation and reaction of H vacancies are compared in Table 3 for the H2O and H3C· reactants. In the absence of HC radicals, spontaneous H dissociation from CsOH to form H2 is a significantly endothermic reaction. A more plausible route to vacancy formation is the in situ reaction of the CsOH surface with HC radicals formed during fuel pyrolysis. Atomic simulations showed that the H3C· H abstraction to form methane involves an almost thermo-neutral reaction. The H3C· first adsorbs over a surface OH on the CsOH(010) surface with a weak ΔHadsorption of −5 kJ/mol. Hydrogen abstraction was shown by transition state calculations to have an activation enthalpy, ΔHactivation, of only 18.3 kJ/mol and to require an overall ΔHreaction input of 10.2 kJ/mol. The activation barrier transition state structure and enthalpy pathway for H3C· abstraction of H from CsOH(010) is shown in Figure 3. This mode for vacancy formation is much more facile than spontaneous H desorption. The modeling showed that H surface vacancies could promote heterogeneously catalyzed steam gasification mechanisms involving HC radical fragments. Hydrocarbon radicals were found to more favorably form and react with H surface vacancies than H2O. These results are summarized in Table 3. The H3C· adsorption strength is increased almost 6 times over a H vacancy on the CsOH(010) surface [Csx(OH)(x−1)O□H*CH3], compared to over the strongest binding Cs site on the pristine surface [Csx(OH)x*CH3] (see Table 2). If HO· radicals are able to be formed from the reaction of H2O with any remaining H vacancies, then the HO· are more reactive than H3C· with surface HC adsorbates, such as Csx(OH)(x−1)O□H*CH3. However, the conjecture was made that, in concentrated HC atmospheres, HC species will saturate H vacancy sites, making the extra reaction steps for HO· radical formation less likely from a mechanistic standpoint. Further examination of possible elementary reaction steps on the CsOH surface revealed two plausible steam gasification pathways that could take place directly with H2O as a reactant, without requiring H2O dissociation to form HO· reactants. These mechanisms are illustrated for the representative HC radical species, H3C·, adsorbed on CsOH, for the overall steam gasification reaction

Figure 3. Reaction pathway for H vacancy (□H) formation on CsOH(010) surface by H3C· abstraction mechanism.

vacancy on the CsOH(010) surface, as discussed previously and illustrated in Figure 3. A second H3C· adsorbs strongly at the vacancy, setting the stage for H2O complexation and oxidative attack. Water then dissociatively inserts into the *CH3 surface complex to form a hydroxymethyl (*CH2OH) intermediate and release H2. The *CH2OH intermediate further dehydrogenates to form a formate (*CHO) complex. Decomposition of the *CHO complex releases CO and results in the filling of the CsOH H vacancy. This completes the turnover of the reaction site. The H vacancy mechanism may not be competitive because the final *CHO decomposition step is a very endothermic reaction. This formate complex is comparable to that formed in the water gas shift associative reaction pathway of CO and H2O on oxide catalysts. Previous investigations of the latter pathway on ceria−zirconia catalysts also revealed that the formate dissociation was also very endothermic and limited the turnover of active catalyst sites.30 A major unresolved question for this mechanism was the identification of a feasible pathway for H2O reaction with the *CH3 complex. The favorability of the H2O insertion reaction increases if the *CH 3 complex is dehydrogenated to *CH2 or *CH by reactions with incoming HC. However, the simulations of numerous reaction intermediates showed that all of these adsorbates repelled

Csx(OH)x *CH3 + H 2O → Csx(OH)x + CO + 5/2H 2 (9)

The first pathway, referred to as the “H vacancy mechanism”, involves the coupled reaction of H3C· and H2O. The minimized reaction intermediate structures and reaction enthalpies for this mechanism are shown in Table 4 and Figure 4, respectively. One H3C· radical first abstracts a ·H to form CH4 and a H 6193

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Table 4. Ground State Electronic Reaction Enthalpies, ΔHreaction, for the H Vacancy Mechanism for H3C· Steam Gasification on the CsOH(010) Surface, Where the H Vacancy Is Denoted as □H elementary reaction steps for overall reaction: 2 H3C· + H2O → CH4↑ + 2H2↑ + CO↑

sequential ΔHreaction (kJ/mol)

cumulative ΔHreaction (kJ/mol)

Csx(OH)x + H3C· → Csx(OH)(x−1)O□H + CH4↑ Csx(OH)(x−1)O□H + H3C· → Csx(OH)(x−1)O□H*CH3 Csx(OH)(x−1)O□H*CH3 + H2O → Csx(OH)(x−1)O□H*CH2OH + H2↑ Csx(OH)(x−1)O□H*CH2OH → Csx(OH)(x−1)O□H*CHO + H2↑ Csx(OH)(x−1)O□H*CHO → Csx(OH)x + CO↑

10.2 −477.3 41.4 −30.4 172.6

10.2 −467.1 −425.7 −456.1 −283.5

Figure 4. Elementary steps for the H vacancy mechanism for H3C· steam gasification on the CsOH(010) surface, where the H vacancy is denoted as □H.

Figure 5. Elementary steps for the Cs insertion mechanism for H3C· steam gasification on the CsOH(010) surface, where the OH vacancy is denoted as □OH.

Table 5. Ground State Electronic Reaction Enthalpies, ΔHreaction, for the Cs Insertion Mechanism for H3C· Steam Gasification on the CsOH(010) Surface, Where the OH Vacancy Is Denoted as □OH elementary reaction steps for overall reaction: 3 H3C· + H2O → 2CH4↑ + 3/2H2↑ + CO↑

sequential ΔHreaction (kJ/mol)

cumulative ΔHreaction (kJ/mol)

Csx(OH)x + H3C· → Csx(OH)x*CH3 Csx(OH)x*CH3 + H3C· → Csx(OH)(x−1) □OH*CH2OH + CH4↑ Csx(OH)(x−1)□OH*CH2OH + H3C· → Csx(OH)(x−1)□OH*CHOH+ CH4↑ Csx(OH)(x−1)□OH*CHOH → Csx(OH)(x−1)□OH*CO + H2↑ Csx(OH)(x−1)□OH*CO → Csx(OH)(x−1) □OH + CO↑ Csx(OH)(x−1)□OH + H2O → Csx(OH)x + 1/2H2↑

−78.3 −200.0 −125.5 −58.0 40.3 −156.4

−78.3 −278.3 −403.8 −461.8 −421.5 −577.9

This mechanism does not require the initial H vacancy formation or the direct reaction of H2O or HO· with the HC adsorbate. Here, the mechanism starts with a H3C· adsorbing over Cs on the pristine CsOH(010) surface. This adsorbate is then successively dehydrogenated by a series of collisions with other HC radicals. In the first dehydrogenation reaction, the resulting methylene complex (*CH2) spontaneously inserts

H2O, so that it was difficult to envision how a H2O insertion reaction could take place. In the second steam gasification pathway, referred to as the “Cs insertion mechanism”, the CsOH surface directly participates in HC radical dehydrogenation and oxidation. The intermediate structures and reaction enthalpies for this mechanism are shown in Figure 5 and Table 5, respectively. 6194

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desorption being the only slightly endothermic reaction step. Several transition states of the Cs insertion mechanism were examined more closely to find the most rate-limiting step. Their selection was based upon the assumptions that the CH3 radical and H2O adsorption reaction steps were not rate-limiting. The transition states were determined for the least exothermic *CHOH dehydrogenation step and the endothermic *CO desorption step with the VASP nudged elastic band methodology. The activation barrier was higher (ΔHactivation = 82.4 kJ/ mol) for moderately exothermic *CHOH dehydrogenation (ΔHreaction = −58.0 kJ/mol) than for the endothermic *CO desorption reaction (ΔHactivation = 18 kJ/mol, ΔHreaction = 40.3 kJ/mol). The activation barrier transition state structure and cumulative reaction enthalpies for the *CHOH dehydrogenation reaction is shown in Figure 7. While this is a moderate

between the Cs site and an adjacent surface OH, forming a hydoxymethyl (*CH2OH) configuration. Subsequent dehydrogenation by a HC radical produces the hydroxymethylidyne (*CHOH) configuration. The *CHOH complex can readily dehydrogenate to volatilize CO and H2 products, leaving behind an OH vacancy in the CsOH surface. The OH vacancy is subsequently filled by H2O dissociation, generating 1/2 mol H2. The atomic modeling revealed that the versatile interactions of the CsOH(010) surface with HC radicals, such as H3C·, are likely to play a strong role in the effective CsOH heterogeneous catalysis of coke precursor steam gasification mechanisms that has been observed experimentally.2 On one hand, the almost thermoneutral reaction for H vacancy formation via a H3C· H extraction could easily create defect sites that can strongly bind and position intermediates for further interaction with incoming reactants. On the other hand, the CsOH(010) surface shows a surprising capacity to spontaneously rearrange in interactions with radical adsorbates. One example of this was the spontaneous insertion of the *CH2 intermediate into the Cs−O(H) surface bond, which set the stage for formation of the CO product upon subsequent dehydrogenation. The potential effectiveness for these two mechanisms heterogeneously catalyzing steam gasification is evident from the comparison of their cumulative reaction enthalpies with that predicted for the uncatalyzed homogeneous gas phase steam gasification pathway in Figure 6. Here, the homogeneous

Figure 6. Cumulative reaction enthalpies, ΔHreaction, comparing the homogeneous reaction pathway with the two heterogeneous pathways on the CsOH(010) surface for the steam gasification of a methyl radical, H3C·.

pathway involves the reaction of a single H3C· moiety with H2O in the gas phase. The heterogeneous pathways are driven even more downhill by the increased reactivity of the vacancy bound H3C· in the H vacancy pathway and the saturation of incoming H3C· to CH4 in both pathways (production of 1 CH4 in the H vacancy mechanism and at least 2 CH4 in the Cs insertion mechanism). The reaction energies in Figure 6 do not include the reaction energies for H3C· formation during pyrolysis. The Cs insertion mechanism appears to be more energetically viable compared to the H vacancy mechanism. The first reaction step of this mechanism of H3C· adsorption on Cs is more favorable than the formation of an H vacancy and is more likely to take place. Also, the Cs insertion reaction enthalpies were the most consistently downward overall, with the CO

Figure 7. Reaction pathway for dehydrogenation of the *CHOH intermediate in the Cs insertion mechanism.

reaction barrier, it certainly would be accessible under high temperature fuel pyrolysis conditions. The overall Cs insertion mechanism cumulative reaction enthalpy pathway updated with selected transition states is shown in Figure 8. Atomic modeling was used in this study to develop a plausible mechanism for the CsOH heterogeneously catalyzed 6195

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coke plugging in the reactor tube but rather due to quench coke deposits plugging fuel flow downstream of the reactor. In comparison, the uncoated reactor tube test with JP-7 ran for ∼12 min until the reactor was plugged by coke deposits. Thus, the experimental results showed that the in situ continuous catalytic steam gasification was able to reduce coke deposition rate by more than 10 times. The coke deposits in the CsOH catalyst coated reactor tube after the 122 min long test were determined by LECO carbon analyses to be less than 0.8 mg/ cm2 along the length of the tube, corresponding to a maximum coke restriction of only ∼2% of the tube cross-section.2 This is consistent with the observation that the reactor pressure drop did not increase over the run duration because of effective coke mitigation by the CsOH catalyst coating. The polished CsOH-coated reactor tube cross-section samples were characterized for elemental Cs concentration, coating morphology, and coke deposit by SEM, EDS, and EPMA. The post-test results clearly indicated that CsOH catalyst coating remained intact after the long duration test with JP-7 and 2 wt % steam injection. The SEM images of the ascoated (untested) reactor tube and the inlet, middle, and outlet sections of the tested coated reactor tube are shown in Figure 9.

Figure 8. Detailed cumulative reaction enthalpy plot for the simulated heterogeneous CsOH-catalyzed reaction pathways with the Cs insertion mechanism for H3C· steam gasification.

steam gasification of coke precursors to explain the effective suppression of coke formation that has been observed experimentally.2 The CsOH-catalyzed steam gasification of coke precursors is closely related to other heterogeneously catalyzed reaction mechanisms: (a) the steam gasification of petroleum coke (C) for syngas and H2 production (eq 1 with y = 0) and (b) the water gas shift reaction for increasing the H2 yield from syngas (eq 2). The most favorable proposed Cs insertion mechanism is analogous to the O-transfer mechanism refined with experimental reactor studies for C steam gasification catalyzed by K2CO3.11 This two step mechanism involves the direct C oxidation by the catalyst as the first step and H2O dissociation to replenish the catalyst surface O depleted by the oxidation reactions as the second step. The analogous bifunctional, regenerative mechanism for the water gas shift reaction was also shown experimentally to involve two reaction steps: first, CO oxidation to CO2 at one catalytic site and, second, H2O dissociation at another catalytic site to generate O for refilling the O vacancy. In the regenerative mechanism, both the O and electrons must migrate to enable the O vacancy at the first site to be refilled and the reaction site to turnover.30 The difference between the Cs insertion mechanism and these other bifunctional mechanisms is that the refilling of the CsOH OH vacancy from water dissociation occurs at the same site that it was depleted (monofunctional). Oxygen migration is hindered in the very stable, highly ionic CsOH structure. The less favorable H vacancy mechanism developed for CsOH of coke precursors is analogous to the associative mechanism for the water gas shift reactions. In this mechanism, both CO and H2O react together at a single surface site without involving O exchange with the catalyst. This mechanism forms intermediates that strongly bind to block catalyst sites and can compete with the regenerative mechanism. Further experimental studies are needed to validate and refine the fundamental basis for the Cs insertion steam gasification mechanism on CsOH(010), especially to examine the mechanism for the gasification of already deposited coke. Mechanistic Insights into Effective Coke Mitigation. The maximum run duration for JP-7 with the CsOH catalyst coated reactor tube and 2 wt % steam injection test was more than 122 min at the 750 °C fuel exit temperature.2 Significantly higher temperatures would be required for homogeneous steam gasification reactions to be comparable in activity in the absence of a heterogeneous catalyst. This test was not terminated from

Figure 9. Scanning electron microscopy micrographs of untested and tested reactor tubes with CsOH catalyst coating: (a) untested reactor tube and (b) inlet, (c) middle, and (d) outlet sections of the tested reactor tube.

The CsOH catalyst coating was uniform and remained adhered to the tube after testing. Some cracks were formed at the middle and outlet sections of the tube that were filled with coke deposits. Despite the cracks in the tested reactor tube middle section, no coke deposit was observed on the coating surface. The catalyst coating in the outlet section formed more cracks than in the middle section. A very thin layer (∼1 μm in thickness) of coke deposited was measured on the outlet section coating layer. The Cs concentrations in all sections of the tested coated reactor tube were the same as the as-coated (untested) one, indicating no CsOH catalyst was lost during the test (see EDS results in Table 6). The catalyst coating in ascoated (untested) reactor tube contained 17 wt % C, introduced from the epoxy mounting material infiltrated into the catalyst layer for microscopic examination. The C contents in tested reactor tubes at the middle section (which 6196

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CsOH heterogeneously catalyzed enhancement of steam gasification, compared to the corresponding homogeneous steam gasification reactions that only become active in the HC fluid phase at significantly higher temperatures. Similar types of reaction mechanisms are likely to be involved in the steam gasification of already formed coke deposits. The coke deposits formed inside the CsOH coating cracks may be due to the limited mass transfer and local depletion of the H2O oxidant of the HC fuel entrained within the cracks. They also raise questions about the role of the CsOH coating in catalyzing the gasification of already accumulated coke. Under the intermediate temperatures in the tube middle section, the CsOH coating appeared to prevent coke buildup on the coating surface by effectively oxidizing any surfacing crack coke deposits, as well as coke precursors arriving at the surface. Here, the rate of coke or precursor oxidation was interpreted to be greater or equal to the rate of coke formation, resulting in the negligible coke deposits observed by SEM (see Figure 9c). The SEM results showed coke buildup on the CsOH catalyst coating in Figure 9d under the severe conditions at the outlet end of the coated reactor. This was attributed to the very high coking rate at the high outlet temperature. This limited coke buildup is hypothesized to be due to the steady-state net coke resulting from the dynamic balance between coke deposition and catalytic steam gasification under these severe conditions. The fact that this deposit was limited provides evidence that the CsOH coating also played an active role in gasifying coke deposits, thus preventing the increasing coke deposition to the point of restricting fuel flow that was observed in reactor tubes without the CsOH coating.2 Further experimentation is recommended to investigate the hypothesis that the CsOH coating can gasify already accumulated coke and also to identify the operating window for effective coke mitigation by the CsOH coating. Tube reactor runs could be made under the steady state conditions for endothermic fuel cracking,2 with an intermittent application of steam injection (i.e., alternating steam injection on for 1 min and off for 1 min). The duration of the runs would be determined by the plugging of the reactor tubes due to coke deposits or by fuel supply depletion in the absence of coke build-up. The tests could compare the steam gasification behavior of uncoated and CsOH-coated reactor tubes run for a given set of conditions. The coke buildup during the off-steam periods should be significantly less on the CsOH-coated tube than on the uncoated tube. Steam gasification of accumulated coke should be catalytically enhanced in the coated tubes. In addition to the run-time determinations, post-test LECO carbon and microscopic analyses of the CsOH-coated reactor tube could be used to verify whether the CsOH coating facilitated the removal of coke deposits. The reaction order and kinetic parameters of the rate-limiting step(s) for the steam gasification catalyzed by the CsOH coating could be determined from a series of isothermal runs carried out with varying steam/fuel ratios and temperatures. The post-test analyses could also be used to verify the existence of steadystate coke mitigation conditions that enable extended operation without coke plugging.

Table 6. Composition of CsOH catalyst coating analyzed by energy dispersive spectroscopy element, wt % reactor tube sample

C

O

Al

Si

Cs

untested tube tested tube middle tested tube outlet

17.0 24.7 26.0

39.9 31.4 31.2

5.8 5.8 5.6

26.6 27.4 26.6

10.4 10.2 10.2

experienced intermediate temperatures) and outlet section (which experienced the highest temperatures) were approximately 7−9 wt % higher than the as-coated one. It is not possible to distinguish whether the C content increase was due to increased coke deposits or epoxy infiltration inside the CsOH catalyst layer. However, these SEM/EDS observations are consistent with the minimal coke deposits (less than 0.8 mg/cm2) that were previously measured by LECO carbon analyses along the length of the post-test CsOH-coated reactor tube.2 It is hypothesized from these combined observations that the very limited coke deposition was the net result of a dynamic balance, i.e., coke deposition versus coke and/or precursor removal via the catalytic steam gasification process. The EPMA analyses of the outlet section of the tested coated reactor tube indicated that the CsOH catalyst remained uniformly distributed in the coating and was not consumed or dissolved in the steam gasification process. Overall, the elimination of significant coke buildup under fuel pyrolysis conditions may be attributed to the multiple functionalities of the CsOH coating for passivating the underlying metal surface, preventing deposition of cokeforming precursors and products and catalyzing the oxidation of coke precursors in the presence of water. The atomic modeling of CsOH surface reactions provided insights into the fundamental basis for these observed functionalities. The very stable, relatively unreactive CsOH surface was predicted to act like a barrier, thus preventing the underlying nickel alloy tube surface from catalyzing coking reactions. This CsOH barrier was predicted to weakly interact with aromatic hydrocarbons and pseudoaromatic coke nanoparticles that could nucleate homogeneous coke deposits. The pristine CsOH surface was also shown to be relatively unreactive with possible O-bearing reactants that could play a role in the direct oxidation of HC molecules. The strong predicted reactions of H3C· radical fragments with the CsOH surface were an important discovery for the development of the unique steam gasification mechanisms that are presented in this paper. In these mechanisms, HC radicals arriving at the surface are adsorbed, effectively shutting down radical cascades that can lead to the formation of homogeneous coke nuclei in the fuel fluid phase or can promote coke deposition. The surface-complexed radicals are then further decomposed by reactions with additional radicals arriving at the surface. The intermediates are then converted to products through orchestrated CsOH surface rearrangements. Their oxidation occurs either by the direct involvement of the water coreactant (H vacancy mechanism) or through local reduction of the CsOH surface, the latter requiring indirect involvement of water for the reoxidation of the CsOH surface (Cs insertion mechanism). This modeling demonstration of facile reaction site turnover was an important validation of the stable CsOH catalytic functionality, which was not observed to be consumed or dissolved from the surface with time by EPMA. These reaction pathways also provided feasible explanations for the significant

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CONCLUSIONS Atomic modeling was conducted to mechanistically investigate the CsOH steam gasification catalyst coating that has been experimentally demonstrated to successfully eliminate coke deposits during high temperature fuel pyrolysis. The very stable 6197

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Energy & Fuels CsOH(010) candidate surface was modeled to weakly interact with aromatic molecules, as well as pseudoaromatic coke nanoparticles. These findings were interpreted to indicate that the CsOH(010) surface served as a barrier to the underlying metal surface and was also resistant to the formation or adhesion of coke deposits. The discovery that the CsOH(010) surface was only predicted to strongly interact with HC radicals that bombard surfaces during HC pyrolysis led to the creation of novel reaction mechanisms for the effective steam gasification of radical coke precursors. The strong binding of HC radicals serves to quench radical reaction cascades that can lead to fuel fluid phase coking reactions or promote growth of coke surface deposits. The CsOH(010) surface was predicted to have the unique capability to locally rearrange and form vacancies to facilitate the decomposition and oxidation of adsorbed HC radicals. These observations led to the development of two proposed CsOH(010) heterogeneously catalyzed steam gasification reaction mechanisms from the atomic modeling results. Since the direct CsOH(010) dissociation of H2O was not predicted to be feasible, these mechanisms established alternative roles for the H2O coreactant in HC adsorbate oxidation. The first H vacancy mechanism involved methyl radical adsorption at a newly created H-vacancy site, followed by bimolecular reactions with the H2O coreactant for adsorbate oxidation and reaction site restoration. In the second Cs insertion mechanism, the adsorbed methyl radical was dehydrogenated by reactions with incoming HC radicals and oxidized by rearrangement of the CsOH reaction site. This left an OH vacancy that was refilled by H2O dissociation to regenerate the reaction site. This latter mechanism was more energetically downhill overall and had a modest rate-limiting energy barrier that could be easily overcome at the high fuel pyrolysis temperatures. These mechanisms are consistent with the experimentally observed stability of the CsOH coating, which functions as a true catalyst that is not consumed or dissolved over time. Observations of the CsOH coating behavior over a range of temperatures supported the hypothesis that effective coke mitigation functionality must dynamically balance both steam gasification of coke precursors arriving at the surface and the removal of already accumulated deposits. Further experimental studies are needed to validate and refine the fundamental basis for the Cs insertion steam gasification mechanism on CsOH(010), especially to extend the mechanism to explain the gasification reactions of already deposited coke.

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ACKNOWLEDGMENTS

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REFERENCES

The authors gratefully acknowledge financial support and guidance for the atomic modeling research and for the CsOH coating characterization under the Air Force Research Laboratory Contract FA8650-09-C-2901, managed by J. Tim Edwards, Air Force Research Laboratory, at Wright-Patterson Air Force Base. The authors acknowledge the experimental support provided by David McHugh and Julie Wittenzellner. The authors also acknowledge guidance by Martin Haas (project leader) and Meredith Colket (senior fellow).

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ASSOCIATED CONTENT

S Supporting Information *

Influence of DFT-D2 van der Waals corrections and spin polarization on VASP fully minimized CsOH bulk lattice parameters (Table S1), selection of CsOH slab configuration and lowest energy surface (Table S2), impact of DFT-D2 van der Waals corrections on VASP ionic minimization of representative hydrocarbon adsorbates on CsOH (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6198

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(27) de Jong, B. H. W. S.; Slaats, P. G. G.; Super, H. T. J.; Veldman, N.; Spek, A. L. J. Non-Cryst. Solids 1994, 176, 164−171. (28) Henkelman, G.; Arnaldsson, A.; Jónsson, H. Comput. Mater. Sci. 2006, 36, 254−360. (29) Huang, H. In-situ continuous coke deposit removal by catalytic steam gasification. U.S. Patent 7,883,674, February 8, 2011. (30) Opalka, S. M.; Vanderspurt, T. H.; Radhakrishnan, R.; She, Y.; Willigan, R. R. J. Phys.: Condens. Matter 2008, 20, 064237-1−06423712.

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