Research Article pubs.acs.org/acscatalysis
Dry Reforming of Methane on Rh-Doped Pyrochlore Catalysts: A Steady-State Isotopic Transient Kinetic Study Felipe Polo-Garzon,† Devendra Pakhare,‡ James J. Spivey,§ and David A. Bruce*,† †
Department of Chemical and Biomolecular Engineering, Clemson University, 127 Earle Hall, Clemson, South Carolina 29634, United States ‡ Pyrochem Catalyst Company, 11361 Decimal Drive, Jeffersontown, Kentucky 40299, United States § Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States S Supporting Information *
ABSTRACT: Increases in worldwide methane production from biological and fossil sources have led to an increased level of interest in the dry reforming of methane (DRM) to produce syngas. Experimental efforts have shown that select pyrochlore materials, such as the Rh-substituted lanthanum zirconate pyrochlore (LRhZ), are catalytically active for DRM, exhibit long-term thermal stability, and resist deactivation. This work seeks to allow further catalyst improvements by elucidating surface reaction kinetics via steady-state isotopic transient kinetic analysis (SSITKA) of dry reforming on the LRhZ pyrochlore. Isotopically labeled CH4 and CO2 were used in multiple SSITKA experiments to elucidate the migration of carbon atoms to product species. Short surface residence times at 650 and 800 °C (650 °C) are very short. However, DRM is not thermodynamically favorable at low ° ≫ 0), as shown in Figure 2; instead, RWGS temperatures (ΔGRxn reaction becomes important, as evidenced by the low H2/CO ratios. In kinetic terms, increases in surface residence times at lower temperatures provide additional time for H2, produced from the dry reforming of methane, to react with the CO2 fed to the reactor, which leads to water formation through the reverse water gas shift reaction (CO2 + H2 ⇌ CO + H2O). Non-normalized signals of the isotopic switch presented in panels c and d of Figure 6 can be found in Figure S6 of the Supporting Information. In Figure 6a−d, one can see that the decay of the 12CO2 signal is shifted a consistent amount from the Ar signal and this shift is roughly independent of temperature, which can be attributed to re-adsorption of CO2 in the catalytic bed or the reactor itself. In addition, it is known that CO2 re-adsorption is enhanced by the presence of basic sites, such as those arising from La species on the catalyst surface.29−31 On these basic sites, CO2 forms both active and inactive adsorbed species,28 which further delays its replacement after the isotopic switch. The shift of the CO signal with respect to the Ar signal increases when the reaction temperature is decreased. Finally, the residence time of the reactant 12 CO2 (area between the 12CO2 and the Ar signal as seen in Figure 6a) did not affect the average surface residence time of the reaction, expressed by the area between the signal of the product 12 CO and the tracer Ar. From Figure 6, it is seen that the 13CO signal closely matches the 13 CO2 signal at moderate and high temperatures (650 and 800 °C, respectively). This is due to quick oxygenation of CO to CO2, which desorbs and re-adsorbs, as explained in 13C Atom Migration between Reactants. At low temperatures, 500 and 450 °C, the signal for 13CO presents a delay with respect to the signal for
Figure 7. TOFChem and TOFITK as a function of temperature for DRM over LRhZ catalyst at 2 psig and a GHSV of 65333 cm3 gcat−1 h−1. 13
CO2, because reaction rates are slower and the transition between CO2 and CO is not as fast. Looking at the decay of the signals for the unlabeled reactant/ product (12CO2 and 12CO) and the increase in the magnitudes of the signals for the labeled reactant/product (13CO2 and 13CO), particularly at 450 °C (Figure 6d), one can see how few seconds after the switch (from approximately 2 to 5 s in Figure 6d) the 13 CO2 and the 13CO signals practically overlap, whereas a gap between the 12CO2 and CO signals is noticeable, with the 12CO2 dropping quicker than the CO signal. This supports the idea that the production of 13CO using 13CO2 is enhanced when compared to the production of 12CO using 12CO2, which was explained in Kinetic Isotope Effect (KIE). 2.2.3.2. Concentration of Surface Intermediates. The concentration of surface intermediates (NCO) is another parameter that can be calculated from SSITKA. Details are provided in section S2 of the the Supporting Information (Parameters calculated from SSITKA). 2.2.3.3. Turnover Frequency. In general terms, catalyst turnover frequency (TOF) represents the number of product molecules produced per catalyst site per unit time. Initially, this calculation appears to be straightforward until one recognizes that there are multiple ways to define a catalyst site. To date, the three most common definitions of TOF are the reaction rate per surface metal atom (TOFChem), the rate per active site (TOFtrue), and the rate per active intermediate (TOFITK) as determined by in situ and/or isotopic labeling techniques.32,33 The relationship among these three TOF numbers follows a predictable trend rate [surface metal atoms] rate ≤ [active sites] rate ≤ [active intermediates] (2) or TOFChem ≤ TOFtrue ≤ TOFITK
The turnover frequency from isotopic transient kinetics (TOFITK), assuming pseudo-first-order irreversible reaction with a single-intermediate pool, can be calculated from τavg as follows TOFITK = 1/τavg
The reaction rate per surface metal atom or TOFChem can be calculated using
TOFChem = 3831
rCONAvA m ABET DOI: 10.1021/acscatal.6b00666 ACS Catal. 2016, 6, 3826−3833
Research Article
ACS Catalysis Table 1. Surface Reaction Kinetic Parameters for DRM on LRhZa temp (°C) 800 650 500 450
labeled gas 13
13
CO2/ 13 CO2 13 CO2 13 CO2
CH4e
rateb (mmol of CO gcat−1 s−1)
τavgc (s) f
0.495 0.168 0.047 0.043
0.23 0.57f 1.19g 2.29g
TOFITK (s−1)
Nd (mmol/gcat)
θ
4.44 1.74 0.84 0.44
0.111 0.097 0.055 0.098
0.99 0.85 0.49 0.86
Reaction conducted at 2 psig and a GHSV of 65333 cm3 gcat−1 h−1. bSteady-state rate. The steady-state MS signal was averaged over at least 1 min. Surface residence time of intermediates. dN = rate × τavg. eReplicas were done using both 13CO2 and 13CH4. fExperimental errors are ±0.15 s. g Data from single experiments, as conditions are not relevant for DRM applications. a c
where rCO equals the rate of CO production per weight of catalyst, NAv is Avogadro’s number, Am is the surface area per metal atom, and ABET is the surface area per weight of catalyst, which is routinely measured by physisorption techniques (e.g., BET analysis of nitrogen physisorption). With oxide materials, there is a further complication to the calculation of TOF, namely, that it is presently impossible to determine the surface structure of bulk mixed oxides in powder form. Thus, quantification of the exact number of surface sites of a given type is an approximation at best, which means that any calculation of the TOF is likely in error because the exact number of accessible surface sites under reaction conditions, especially for reactions occurring at 800 °C, is unknown.34 In this work, calculations of TOFChem assumed that all metal atoms in the pyrochlore surface (Rh, Zr, and La) are active for catalysis, not only the Rh atoms. This assumption is validated by previous DFT calculations11 that showed single-atom Rh sites as well as mixed metal sites involving combinations of Rh, Zr, and La atoms were favored reaction sites in the main reaction pathway. TOFChem and TOFITK data for DRM activity using the LRhZ pyrochlore catalyst are shown in Figure 7. A comparison of these values (i.e., TOFChem ≤ TOFITK) provides good agreement with theory, as shown in eq 1.32 The slightly lower values for TOFChem when compared to TOFITK agree with the assumption that each surface metal atom can allocate up to one surface intermediate. The proximity between TOFChem and TOFITK indicates that roughly each surface metal site is active (surface metal atoms ≈ active intermediates), which is also supported by the fact that the surface coverage of intermediates per metal atom (θ) approaches unity (see Table 1). This suggests that although the inclusion of the Rh dopant is what makes the lanthanum zirconate pyrochlore active for DRM,6 once the Rh dopant is included, the other metal atoms, namely, Zr and La atoms, become active sites in the reaction mechanism of DRM over LRhZ. In our previous computational work,11 it was shown that the preferred adsorption sites for the species involved in the main reaction mechanism for DRM involve all surface metal (Rh, Zr, and La) sites. Table 1 summarizes the surface reaction kinetic parameters for DRM on LRhZ found in this study.
to form CO involves more elementary steps than the decomposition of CO2 to CO, as predicted by DFT simulations. A SSITKA allowed for the calculation of average surface residence times, surface species concentrations, and turnover frequencies at different temperatures. The residence time increased at lower temperatures, which allows time for the reaction of H2 produced from the dry reforming of methane with the CO2 fed to the reactor, promoting the competing reaction, the reverse water gas shift reaction: CO2 + H2 ⇌ CO + H2O. Extremely short residence times (650 °C) because activation barriers are more easily overcome and the diffusion of intermediates on the surface is favored. The observed short residence times have an associated error estimated from the replicas taken in this study. It is difficult to draw definitive conclusions about the exact value of the kinetic parameters because of the extremely fast rate of the reactions involved. Despite this fact, this work helps provide an understanding of the reaction kinetics from the comparison of kinetic parameters under several conditions so that all calculations appropriately compensated for systematic errors due to the arrangement of the experimental apparatus and the recording capabilities of the mass spectrometer. The calculated values of turnover frequencies corroborated that all surface metal atoms (Rh, Zr, and La) take part as active sites for at least some DRM reactions on the LRhZ pyrochlore material, as previously seen from DFT results. An inverse kinetic isotope effect (increase in the reaction rate) was observed when the 12CO2 ↔ 13CO2 switch was performed, which is attributed to reduced re-adsorption of the product CO and, therefore, a more rapid formation of vacant Rh catalyst sites. A normal kinetic isotope effect (reduction in the reaction rate) was observed when the 12CH4 ↔ 13CH4 switch was performed, due to reduction in the vibrational energy of the C−H bond and therefore more difficult dehydrogenation of the 13C atom in the 13 CHO intermediate, which constitutes a rate-determining step. The DFT data used in this work along with the estimated surface residence times can be employed to predict overall catalyst performance by means of a microkinetic model, which will be presented in one of our upcoming publications.
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3. CONCLUSIONS Isotopic labeling studies validated findings from DFT calculations about the main reaction pathway for the dry reforming of methane on the Rh-substituted lanthanum zirconate pyrochlore catalyst, in which the CH4 dehydrogenation/oxygenation to CO proceeds as follows: CH4(g) ⇌ CH4* ⇌ CH3* ⇌ CH2* ⇌ CH2O* ⇌ CHO* ⇌ CO* ⇌ CO(g). On the other hand, CO2 can dissociate directly and indirectly (through COOH formation) to CO.11 The observed migration of a labeled C atom from CH4 to CO2 but not from CO2 to CH4 confirms that CH4 dehydrogenation/oxidation
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00666.
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Experimental details (section S1), parameters calculated from SSITKA (section S2), and Figures S1−S6 (PDF)
AUTHOR INFORMATION
Corresponding Author
*Phone: +1 864 656 5425. E-mail:
[email protected]. 3832
DOI: 10.1021/acscatal.6b00666 ACS Catal. 2016, 6, 3826−3833
Research Article
ACS Catalysis Notes
(28) Pakhare, D.; Schwartz, V.; Abdelsayed, V.; Haynes, D.; Shekhawat, D.; Poston, J.; Spivey, J. J. Catal. 2014, 316, 78−92. (29) Zhang, Z.; Verykios, X. E.; MacDonald, S. M.; Affrossman, S. J. Phys. Chem. 1996, 100, 744−754. (30) Irusta, S.; Cornaglia, L. M.; Lombardo, E. A. Mater. Chem. Phys. 2004, 86, 440−447. (31) Ghelamallah, M.; Granger, P. Fuel 2012, 97, 269−276. (32) Goodwin, J. G., Jr.; Hammache, S.; Shannon, S. L.; Kim, S. Y. Encyclopedia of Surface and Colloid Science. Taylor and Francis, 2006; pp 2755. (33) Shannon, S. L.; Goodwin, J. G. Chem. Rev. 1995, 95, 677−695. (34) Wachs, I. E.; Routray, K. ACS Catal. 2012, 2, 1235−1246.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported as part of the Center for Atomic Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant DE-SC0001058. We thank Dr. Victor Abdelsayed at the National Energy Technology Laboratory (U.S. Department of Energy, Morgantown, WV) for providing the catalyst (LRhZ) used in this study. We thank Dr. John Kuhn at the University of South Florida for helpful discussions about the experimental setup and reactor design. We thank Yiran Ren in the Department of Environmental Engineering and Earth Sciences at Clemson University for conducting BET measurements on the catalyst material and acknowledge the helpful staff and resources affiliated with the Clemson University Electron Microscopy Laboratory.
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DOI: 10.1021/acscatal.6b00666 ACS Catal. 2016, 6, 3826−3833
