Role of Electric Field and Surface Protonics on Low-Temperature

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Role of Electric Field and Surface Protonics on LowTemperature Catalytic Dry Reforming of Methane Tomohiro Yabe, Kensei Yamada, Kota Murakami, Kenta Toko, Kazuharu Ito, Takuma Higo, Shuhei Ogo, and Yasushi Sekine ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04727 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Sustainable Chemistry & Engineering

Role of Electric Field and Surface Protonics on LowTemperature Catalytic Dry Reforming of Methane AUTHOR NAMES Tomohiro Yabe*, †, Kensei Yamada†, Kota Murakami†, Kenta Toko†, Kazuharu Ito†, Takuma Higo†, Shuhei Ogo†, and Yasushi Sekine† AUTHOR ADDRESS †

Department of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo, 169-

8555 Japan Corresponding author T. Yabe, +81-3-5286-3114, [email protected] KEYWORDS: Carbon dioxide utilization; Dry reforming of methane; Ni catalyst; Surface protonics

ABSTRACT: Role of the electric field and surface protonics on low temperature catalytic dry reforming of methane was investigated over 1wt%Ni/10 mol%La-ZrO2 catalyst, which shows very high catalytic activity even at temperatures as low as 473 K. We investigated kinetic analyses using isotope and in-situ DRIFTS, and kinetic analyses revealed synergetic effects between the catalytic reaction and the electric field with less than one-fifth the apparent

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activation energy at low reaction temperatures. Results of kinetic investigations using isotopes such as CD4 and 18O2, in-situ DRIFTS in the electric field, and DFT calculation indicate that methane dry reforming proceeds well by virtue of surface protonics. CH4 and CO2 were activated by proton collision at the Ni–La-ZrO2 interface based on the “inverse” kinetic isotope effect.


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INTRODUCTION The utilization of CH4 and CO2 as raw materials offers a means of mitigating the increasing amounts of greenhouse gases (GHGs) and of producing value-added biogas.1–4 Dry reforming of CH4 (DRM) is the following chemical reaction between CO2 and CH4, a major component of natural gas, to produce the synthesis gas: a mixed gas of hydrogen and carbon monoxide.5–9 CH4 + CO2 → 2H2 + 2CO

H2980 = 247.2 kJ mol−1

(1)

The potential to convert two GHGs (CH4 and CO2) into useful chemical feedstocks makes DRM an attractive selection as a promising technology for the chemical fixation of GHGs. However, to obtain commercially utilizable CH4 conversion, the DRM reaction requires high reaction temperatures above 1073 K because of the stable structures of both CH4 and CO2 and because it is a strongly endothermic reaction. In these intense reaction conditions, catalyst stability, coke deposition, and the difficult choice for reactors remain as major issues to be resolved.10 To implement DRM reaction as promising process at mild reaction conditions, i.e., lower reaction conditions, electrically promoted DRM, a strongly endothermic reaction, using renewable power is useful as a power-to-gas technology.11,12 Thereby renewable and surplus electrical power can be stored chemically without large heat losses. Low-temperature dry reforming of the natural gas using catalysts offers opportunities to use the existing natural gas grid to store electricity generated from renewable resources. Additionally, H2 generated by chemical reactions can be injected to a natural gas network13,14; alternatively, it can be used as feedstock, fuel, and energy carrier. This power-to-gas technology using DRM reaction can store energy by endothermic conversion of natural gas including CO2 to H2 (and CO). Additionally, we can use gas from wells directly, including large amounts of CO2 available around Asia.12


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From the perspective of catalysts for DRM reactions, active metal components are divided into two types: precious-metal-supported catalysts such as Ru, Rh, Pd, Ir, Pt, and base-metalsupported catalysts such as Fe, Co, Ni, and Cu. Many reports have described that precious-metalsupported catalysts have good DRM activity without carbon deposition. Nevertheless, because of the scarcity of precious metal resources and because of their high and fluctuating prices, studies have increasingly emphasized base-metal-supported catalysts. Among base-metal-supported catalysts, Ni-based supported catalysts are comparable to precious-metal-supported catalysts.9 However, Ni-based supported catalysts tend to deposit carbon because of the good diffusibility of carbon into the Ni lattice. Moreover, their high reaction temperatures lead to formation of filamentous carbon backwards on the Ni surface and lead to a loss of DRM performance. Our earlier report described that catalyst of 1wt%Ni/10 mol%La-ZrO2 showed high DRM activity even at 423 K with extremely low carbon deposition in an electric field.15 In this report, La-ZrO2 support catalyst indicated higher CH4 conversion than other rare-earth-oxide supports because of higher applied voltage of La-ZrO2. So, the electric conductivity on a catalyst support is an important factor for DRM activity in the electric field. The catalyst temperature was measured by a thermocouple on the catalyst and increased only by approximately 100 K by applying the electric field. So, the promotion for DRM activity was not due to increasing the temperature. Besides, many other reactions were also promoted by application of an electric field.16–21 Surface protonics, with surface proton hopping via adsorbed water and hydroxyl groups, the so-called Grotthuss mechanism22–25, on the CeO2 surfaces promotes catalytic performance.26,27 The ratedetermining step of conventional SRM (without the electric field) is known to be CH4 dissociative adsorption step over Ni and precious-metal-supported catalysts.28,29 Proton collision to CH4 in SRM with the electric field enables the formation of a transitional state and configures


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the three-atom (CH3-H-H)+ transition state.27 Additionally, the energy level of produced (CH3+ + H2) was much lower than CH4 physisorption and the transition state. Therefore, we investigated the role of electric field on low-temperature dry reforming of methane using several kinetic experiments including isotope tests and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements to elucidate reasons for enhanced catalytic activity of DRM by electric field application.

RESULTS AND DISCUSSION Temperature Dependence of the Catalytic Activity with/without the Electric Field and Electrical Conductivity with the Electric Field In our earlier study, Ni/La-ZrO2 catalyst was found to present the best performance among various catalysts in an electric field, even at 473 K.15 For this work, we prepared various catalysts having different loading amounts of Ni. Table 1 shows physical properties and nomenclature for xwt%Ni/10 mol%La-ZrO2 catalysts (x = 0.88, 1.3, 1.5, 2.7), as measured using XRF, N2 physisorption analyses, and TEM. BET surface area was almost identical for catalysts with various loading amounts of Ni on La-ZrO2 (denoted as NLZ). Then, catalytic activity tests were conducted in DRM with the electric field over the catalyst at temperatures of 404−856 K, as shown in Figure S1. Apparent activation energy Ea from both CH4 and CO2 consumption rate over the catalyst was elucidated at temperatures of 505−880 K with EF and 764−897 K without EF, to make the methane conversion almost the same range. The calculated apparent activation energy was much lower when the electric field was applied (CH4, 8.2 kJ mol-1; CO2, 12.1 kJ mol1

) compared to the value without the electric field (CH4, 66.1 kJ mol-1; CO2, 62.3 kJ mol-1) as


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shown in Table 2, suggesting that DRM proceeds via a different mechanism from that of the electric field from conventional catalytic reaction. To investigate the reduction behavior for various loading amounts of Ni on La-ZrO2, H2-TPR (Temperature Programmed Reduction) was conducted, as shown in Figure S2. As a result, NiO was reduced at 600−750 K and the part of La-ZrO2 was reduced at 900−1000 K. The difference for the reduction temperature among these catalysts was not observed. Reportedly, proton conduction via adsorbed water on the catalysts surface proceeds more readily at a range of lower temperatures such as 423 K. The surface protonics on the catalyst surface promote several reactions.16–21 Therefore, to investigate proton conduction in DRM with the electric field, the logarithmic apparent electrical conductivity was calculated as a function of inverse temperatures with the electric field at the same temperatures of Table 2, as depicted in Figure 1. Consequently, the electrical conductivity in DRM reaction increased in the range of lower temperatures such as 423 K. For that lower temperature range, it is known that increasing the amount of surface H2O species and hydroxyl groups increases the proton conductivity.30,31 Therefore, proton conduction occurred from surface proton species derived from CH4 via a reverse water gas shift reaction (RWGS: H2 + CO2 → CO + H2O) and surface hydroxyl groups on La-ZrO2. Additionally, the electrical conductivity in the (CD4 + CO2) reaction in lower temperature range as 423 K was lower than that in (CH4 + CO2) reaction because D species derived from CD4 and surface OD and D2O species were heavier than those in the H species. Otherwise, the electrical conductivity in (CO2 + Ar) flow, not containing CH4, did not increase, even at low temperatures, because proton conduction did not occur as a result of a lack of H species. These results suggest that proton conduction on Ni-supported La-ZrO2 catalyst occurs even in the DRM reaction with the electric field via surface hydrogen-containing adsorbed species and hydroxyl groups.


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Reaction Orders for the CO Formation Rate from CH4 and CO2 Next, we investigated the effects of CH4 and CO2 partial pressures on the rate of CO formation derived from CH4 and CO2 per unit of electrical power in the electric field. Detailed calculation for CO formation derived from CH4 and CO2 is described in Supporting Information. These values are calculated based on DRM and RWGS. Results are presented in Table 3. For the effect of the contact time (W/F) for catalytic activities in the electric field over various xNLZ (x = 0.88, 1.3, 1.5), we confirmed that the reaction condition was in a kinetic region, as shown in Figures S3−. Results demonstrate that reaction orders of PCH4 for the formation rate of CO derived from CH4 and CO2 in DRM with the electric field were smaller than those in conventional DRM at temperatures of 723−773 K, which Bradford reported over Ni supported on oxides of several kinds such as MgO, TiO2, SiO2, and active carbon32 (reaction orders: 0.44−0.72). Therefore, applying the electric field is presumed to support CH4 activation. Actually, reaction orders of PCO2 for the formation rate of CO derived from CH4 and CO2 in DRM with the electric field were larger than those in conventional DRM, as Bradford has reported (0.27−0.64). Therefore, the reaction mechanism for DRM in the electric field differed from that for conventional DRM reactions. Additionally, increasing the loading amounts of Ni on La-ZrO2 caused increasing reaction orders of PCH4. Presumably, CH4 is activated by the electric field at the Ni-La-ZrO2 interface or on the Ni surface, although reaction orders of PCO2 were almost identical according to the loading amounts of Ni on La-ZrO2. Therefore, the support of Ni/La-ZrO2 possibly contributes to CO2 activation. Details of reaction mechanisms are discussed in the next section.


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Analysis of Reaction Mechanisms of DRM in an Electric Field To elucidate the reaction site for DRM in the electric field, we evaluated the turnover frequency with various amounts of Ni-supported catalyst (NLZ). Figure 2 presents the Ni specific surface area dependence or Ni perimeter dependence on activities for DRM in the electric field, as calculated from consumption rates for CH4 and CO2, and as analyzed using GC-TCD and GC-FID. Details of the experimental apparatus are described in Supplementary Information (SI). These analyses consider turnover frequencies (TOFs) of two kinds, as determined by the Ni-specific surface area (TOF-s) and Ni perimeter (TOF-p). Detailed procedures for calculations and STEMEDS mapping images are described in SI (Figures S6−S9). Results show that DRM activities (both CH4 and CO2 conversion) in the electric field exhibit strong dependence on the Ni perimeter, rather than on the Ni specific surface area, which indicates that activation for CH4 and CO2 dissociation in the electric field proceeds mainly at the Ni and La-ZrO2 interface. Next, to investigate the kinetic isotope effects in DRM in the electric field over 0.88NLZ catalyst, we conducted transient investigations using CH4/CD4 and CO2 with gas chromatography, as shown in Figure 3. In this experiment, to produce surface H2O/−OH (hydroxyl groups) and D2O/−OD on the catalyst, pre-reduction was conducted respectively using H2 and D2. In the figure, rH and rD denote CO formation rates using (CH4+CO2) and (CD4+CO2). As a result, rD/rH was calculated as 1.23±0.08 (CO from CH4) and 1.39±0.03 (CO from CO2). These results show good agreement with the “inverse” Kinetic Isotope Effect (KIE) theory on both CH4 and CO2 dissociation.33–39 That is, proton collision to CH4 in DRM with the electric field enables the formation of a transitional state and configures the three-atom (CH3-H-H)+ and transition state.27,


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Additionally, the energy level of produced (CH3+ + H2) was much lower than CH4 physisorption

and the transition state. Next, to confirm promotion of proton conduction on the catalyst surface in the condition of DRM with the electric field, we took in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements, as presented in Figure 4. Details of the experimental setup and reaction conditions are described in Supporting Information. The peak assignment is presented in Figure S10. The spectra at 573 K with the electric field (EF) provide more detailed information related to the change for adsorbed water. As shown in Figure 4(b), the peak for differential spectra in/after application of the electric field, which was assigned to rotation of the adsorbed water (850 cm−1), was observed only when an electric field was applied to 0.88NLZ catalysts in DRM condition at temperatures as low as 573 K. Additionally, eliminating the electric field caused the disappearance of the peak for rotation of adsorbed water. Reapplication of the electric field caused the appearance of the peak for rotation of adsorbed water again. This peak is regarded as having a strong relation with the Grotthuss mechanism, as derived from proton conduction. Moreover, from the viewpoint of investigating the rate-determining step in DRM with the electric field, surface reactions for the intermediates in DRM of carbonate (CO32−), bicarbonate (HCO3−) and formate (HCO2−), as shown in the following equations, were faster than those of CH4 and CO2 dissociation because the peaks for carbonate, bicarbonate, and formate at 1200−1800 cm-1 were not observed in/after the electric field. Detailed peak profiles of carbonate, bicarbonate, and formate were assigned using DRIFTS – temperature programmed desorption (TPD) with CO2, as shown in Figure S11. (I) Carbonate and bicarbonate formation CO2(ad) + O2−(support) → CO32−(ad)

(2)


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CO2(ad) + OH−(support) → HCO3−(ad)

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(3)

(II) Formation and decomposition of formate intermediates CO32−(ad) + 2H(ad) → HCO2−(ad) + OH−(ad)

(4)

HCO3−(ad) + 2H(ad) → HCO2−(ad) + H2O(ad)

(5)

HCO2−(ad) →CO(ad) + OH−(ad)

(6)

Therefore, after CO2 was dissociated and adsorbed onto the catalysts, H2 and CO were promptly produced via the intermediates above. Additionally, the transformation of surface carbonates and formates without the electric field using in-situ DRIFT measurement over Ni-La/SBA-15 catalysts was investigated in the previous report by Qian et al.40 They indicated that surface carbonates and formates was gradually degraded above 673 K. Moreover, because DRM proceeds above 700 K without the electric field, transformation (degradation) of surface carbonates and formates was not the rate determining steps for DRM. In the main text, Figure 4, noted as DR, because CO was not observed at 573 K without the electric field, DRM did not proceed at such low temperatures. Consequently, these results indicate that surface protonics, the mechanism by proton collision derived from its surface conduction, occur in DRM with the electric field over Ni/La-ZrO2 catalyst, resulting in activation of both CH4 and CO2 dissociation. Moreover, considering the results of reaction orders of PCH4 and PCO2 for the formation rate of CO derived from CH4 and CO2 together, the rate-determining step in DRM with the electric field is CO2 dissociation. As described above, CH4 and CO2 dissociated in a Langmuir–Hinshelwood-resembling mechanism via (I) and (II) routes, eqs. (2)−(6), above passes for H2 and CO formation. However, metal supported on La-ZrO2 catalysts is known to support the DRM reaction via a redox


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mechanism, the so-called Mars – Van Krevelen mechanism, including the consumption and regeneration of surface lattice oxygen in La-ZrO2.41−43 Here, to elucidate effects of the lattice oxygen in La-ZrO2 for the DRM reaction, the cyclic catalytic activity test (2 cycle) uses 18O2 to label the lattice oxygen in La-ZrO2, as shown in Figure 5. Results demonstrate that C18Ox derived from the lattice oxygen labeled by 18O formed at the initial stage after applying the electric field (“EF on” in the figure) in the DRM condition. Next, the oxygen defect produced in the former step was filled with C16O2, that is, the lattice oxygen was substituted to

16

O. Finally, the substituted

lattice oxygen (16O) was consumed to produce C16Ox. Results also show that C18Ox derived from the lattice oxygen labeled by 18O formed even in the second cycle. Therefore, DRM proceeded via a redox mechanism using the lattice oxygen in La-ZrO2. The formation rate for C16O was higher than that for C18O in both steps. Therefore, DRM also proceeded in the Langmuir–Hinshelwood mechanism. Otherwise, as a result of the catalytic activity test using CD4, DRIFTS, and tests in various PCH4 and PCO2 in the electric field as described above, surface protonics were presumed to promote CH4 and CO2 dissociation in redox mechanism. Additionally, to investigate CH4 and CO2 dissociation using the lattice oxygen in La-ZrO2 in the electric field, we alternately applied cyclic tests (2 cycle) using CH4 or CO2, respectively, as shown in Figure 6. Results indicate that CO formed in the first step of CH4 supply after application of the electric field, resulting in lattice oxygen defect (VO) formation. Then, CO also formed in the next step of CO2 supply after application of the electric field, resulting in filling of the oxygen defect by CO2. In the second cycle, CO formed in the CH4 supply in the electric field again. Therefore, CH4 and CO2 dissociation via a redox mechanism was promoted by electric field application. Next, considering that numerous surface protons are presumed to exist on La-ZrO2 in electric field via proton conduction, the CO2 adsorption energy for carbonate and bicarbonate species as


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intermediate in the DRM reaction with/without the lattice oxygen defect was compared using DFT calculations, as shown in Figure 7 to investigate promotion of CH4 and CO2 dissociation using lattice oxygen, thanks to numerous surface protons.44–48 For DFT calculations, the metal oxide surfaces were modeled by repeated slabs that were mutually separated by a 15 Å vacuum region. First, the (111) surface of cubic zirconia (t-ZrO2) was constructed by (1×1) unit cell containing three ZrO2 unit (total 36 atoms), as shown in Figure S12. There, one Zr atom of 12 atoms was replaced by one La atom. Results demonstrated that substitution of the first-layer zirconium was favorable and demonstrated that removal of the lattice oxygen was an endothermic reaction. Next, the CO2 adsorption over La-doped zirconium with and without oxygen defects was compared. The CO2 adsorption aside from −OH on La-ZrO2 was investigated with and without oxygen defects. The CO2 adsorption energy was calculated using the following equation (7). E(CO2 adsorption) = E(CO2/slab) – E(slab) – E(CO2)

(7)

The energy of CO2 was calculated by placing CO2 in a cubic box (10 Å × 10 Å × 10 Å).49−53 Results show that CO2 was adsorbed more easily onto La-ZrO2 with the oxygen defect than without the oxygen defect. Therefore, bicarbonate species formed more easily from surface H2O and hydroxyl groups (−OH) derived from proton conduction near the adsorbed CO2 on La-ZrO2, resulting in H2 and CO formation by bicarbonate species dissociation. In summary, CH4 and CO2 dissociation was promoted at the Ni interface onto the support by lattice oxygen defects in La-ZrO2 and surface protonics in DRM with the electric field at temperatures as low as 423 K. A schematic image of the mechanism for DRM with the electric field is presented in Figure 8. CONCLUDING REMARKS


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To elucidate the effects of an electric field on methane dry reforming, kinetic investigations were conducted using isotopes such as CD4 and

18

O2, and in-situ DRIFTS in the electric field.

Results show that the apparent activation energy decreased with the electric field, indicating that the reaction mechanisms with the electric field differ considerably. Additionally, proton conduction occurred from surface proton species derived from CH4, H2O produced sequentially from RWGS, and surface hydroxyl groups on La-ZrO2. Results of the TOF study demonstrate that DRM activities in the electric field exhibit strong dependence on the Ni perimeter, rather than on the Ni specific surface area, which indicates that activation for CH4 and CO2 dissociation in the electric field proceeds mainly at the Ni–La-ZrO2 interface. Results of in-situ DRIFTS and DFT calculations demonstrate that proton hopping on the catalyst occurs in DRM condition in the electric field and the rate-determining step of DRM in the electric field is CO2 dissociation, and that CH4 and CO2 dissociation was promoted by lattice oxygen defects in La-ZrO2 and surface protonics.

ASSOCIATED CONTENT Supporting Information. Details of preparation procedures of the catalysts, catalytic activity test conditions, and characterizations of catalysts (H2-TPR, XRD, XRF, FE-TEM, CO pulse, in-situ DRIFTS). Temperature dependence for catalytic activities with an electric field over 0.88NLZ catalyst. Catalytic activities for the rate of CO formation from CH4 and CO2 on CH4 partial pressure and CO2 partial pressure over various catalysts. Effects of the contact time (W/F) for catalytic activities with an electric field over various catalysts. Detailed procedures of DFT calculation.


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Peak assignments for in-situ DRIFTS and TPD-DRIFTS using CO2. VESTA image of the optimized slab approximate model by DFT calculation. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions T. Y., K. Y., K. M., K. T., K. I., and T. H. conducted all experiments. T. Y., S. O. and Y. S. discussed all data. T. Y. and Y. S. composed the report. The manuscript was prepared through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors have no conflict of interest, financial or otherwise, related to this study. ACKNOWLEDGMENTS This work was supported by KAKENHI (grant number: 17H07194). S. Enomoto (Kagami Memorial Research Institute for Materials Science and Technology, Waseda University) is appreciated for lending great assistance for FE-TEM operations and EDS measurements. Present Address *E-mail: [email protected] REFERENCES


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Tables Table 1. Physical Properties for xwt%Ni/10 mol%La-ZrO2 Catalysts

Catalyst 0.88wt%Ni/10 mol%La-ZrO2 1.3wt%Ni/10 mol%La-ZrO2 1.5wt%Ni/10 mol%La-ZrO2 2.7wt%Ni/10 mol%La-ZrO2

BET surface

Particle size Nomenclature

area / m2 g-1 15.7 17.2 18.0 15.4

/ nm 20.3 24.2 25.1 29.7

0.88NLZ 1.3NLZ 1.5NLZ 2.7NLZ

Table 2. Apparent Activation Energy Ea, Calculated from CH4 and CO2 Consumption Rate, for Dry Reforming of CH4 with/without the Electric Field over 0.88NLZ Catalysta

DR (without EF) ER (with EF)

a

Ea

R2 value

/ kJ mol-1

/−

CH4

66.1

0.96

CO2 CH4 CO2

62.3 8.2 12.1

0.97 0.99 0.99

Reaction conditions: CH4:CO2:Ar = 1:1:2; 100 SCCM total flow rate; 3.0 mA input current; 100

mg catalyst weight; 505−880 K (with EF) and 764−897 K (without EF) catalyst bed temperature.


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Table 3. Reaction Order for the CO Formation Rate from CH4 and CO2 on CH4 Partial Pressure and CO2 Partial Pressure over Various xwt%Ni/LZO (XNLZ)a

r (CO from CH4) r (CO from CO2) a

0.88NLZ P CH4 P CO2 0.21 1.6 0.12 1.5

1.3NLZ P CH4 P CO2 0.35 1.3 0.26 1.4

1.5NLZ P CH4 P CO2 0.43 1.3 0.29 1.3

Reaction conditions: total flow rate, 100 SCCM diluted by Ar; 3.0 mA input current; 100 mg

catalyst weight; 423 K furnace temperature.


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Figures

CH4+CO2+Ar CD4+CO2+Ar

-3.1

CO2+Ar

log( / S cm-1) / -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3.2

-3.3

-3.4

576 K -3.5 1

1.5

2

2.5

1000 / T / K-1 Figure 1. Logarithmic apparent electrical conductivity as a function of inverse temperature with an electric field over 0.88NLZ catalyst, CH4 (or CD4) : CO2 : Ar = 1 : 1 : 2 or CO2 : Ar = 1 : 3; total flow rate, 100 SCCM; 3.0 mA input current; 100 mg catalyst weight; 404–856 K catalyst bed temperature.


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400

TOF-s(CH ) 4

350 300

TOF-p(CH4)

TOF-p

TOF-s(CO2)

250 5

TOF / s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

TOF-p(CO2)

4 3

TOF-s

2 1 0 18

20

22

24

26

28

30

32

Metal particle / nm Figure 2. Ni-specific surface area (TOF-s) dependency or Ni perimeter (TOF-p) dependency of activities for ERM (in the electric field) over Xwt%Ni/La-ZrO2 catalyst (X = 0.88−2.7), CH4 : CO2 : Ar = 1 : 1 : 2; total flow rate, 100 SCCM; 3.0 mA input current; 100 mg catalyst weight; 423 K furnace temperature.


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Ar purge

CO from CH4

0.15

CO formation rate / mmol min -1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO from CO2

0.12

0.09

0.06

H2 reduction CH4+CO2

D2 reduction CD4+CO2

0 0

10

20

30

10

20

30

Time on stream / min Figure 3. CO formation rate with the electric field over 0.88NLZ catalyst, CH4 (or CD4) : CO2 : Ar = 1 : 1 : 2; total flow rate, 100 SCCM; 3.0 mA input current; 100 mg catalyst weight; 423 K furnace temperature.


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(A) 0.1 He purge after ER2 ER2 after ER1 ER1

Abs. (Kubelka-Munk) / Arb. unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abs. (Kubelka-Munk) / Arb. unit


Page 28 of 33

850 0.005

(B)

(ER2 − after ER2) spectra

850

(ER1 − after ER1) spectra

DR 4000

3500 3000

2500 2000

1500 1000

1200

Wavelength / cm-1

1100

1000

900

800

700

Wavelength / cm-1

Figure 4. In-situ DRIFTS spectra with/without the electric field (EF). (A) Comparison of before/in/after applying EF (without EF: DR; with EF: ER). (B) Differential spectra in/after applying EF in H2O rotation region: 0.88NLZ catalyst; CH4 : CO2 : He = 3:3:34; 40 SCCM total flow rate; 9 mA current; 573 K preset temperature.


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Cycle 1

EF on

Cycle 2

EF on

1

1

H2

H2

CH4 0.8

CH4 0.8

18

C O2 C16O2

0.6

C18O 16

C O 0.4

0.2

Flow / mmol min -1

Flow / mmol min -1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C18O2 C16O2

0.6

C18O C16O

0.4

0.2

0 0

1

2

3

4

5

6

7

8

0 0

1

Time / min

2

3

4

5

6

7

8

Time / min

Figure 5. Formation rates of various gas production including isotopic C18O and C18O2 in cyclic transient isotopic experiments using 18O2 for ER (with EF) over 0.88NLZ catalyst, CH4 : CO2 : Ar = 1 : 1 : 2; total flow rate, 100 SCCM; 3.0 mA, input current; 100 mg catalyst weight; 423 K furnace temperature.


29


Ar purge

Ar purge

CH4 only

CO2 only

EF on

EF on

Ar purge

CH4 only EF on

CO2 only EF on H2

1

Flow / mmol min -1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

CH4 CO2

0.8

CO He

0.6

0.4

0.2

0

1 min

Time / min

Figure 6. Formation rates of various gas production in cyclic ER (with EF) experiments over 0.88NLZ catalyst, CH4 : Ar = 1 : 3 or CO2 : Ar = 1 : 3; total flow rate, 100 SCCM; 3.0 mA input current; 200 mg catalyst weight; 423 K furnace temperature.


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: La : Zr : O (in lattice) : O (in CO2)

Ead = -124 kJ mol-1

Ead = -46 kJ mol-1

(a) Carbonate species on ZrO2 (111) with lattice oxygen defect

(b) Bicarbonate species on ZrO2 (111) with lattice oxygen defect

Ead = -21 kJ mol-1

Ead = -68 kJ mol-1 (c) Carbonate species on ZrO2 (111) without lattice oxygen defect

(c) Bicarbonate species on ZrO2 (111) without lattice oxygen defect

Figure 7. Calculated carbonate and bicarbonate species on ZrO2 (111) with/without lattice oxygen defect. Color coding is the following: La atoms, dark green; Zr atoms, light green; O (in lattice), red; O (in CO2), blue.


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Figure 8. Schematic illustrating the mechanism of dry reforming of CH4 over Ni/La-ZrO2 catalyst in an electric field.


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TABLE OF CONTENTS/ABSTRACT GRAPHIC:

SYNOPSIS: This paper specifically elucidates effects of an electric field on methane dry reforming proceeding at extremely low temperatures.


33

Role of Electric Field and Surface Protonics on Low-Temperature

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