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Spectroscopic study of NEMCA promoted alkene isomerizations at PEM fuel cell Pd–Nafion cathodes
Solid State Ionics 136–137 (2000) 713–720 www.elsevier.com / locate / ssi
Spectroscopic study of NEMCA promoted alkene isomerizations at PEM fuel cell Pd–Nafion cathodes Lloyd Ploense, Maria Salazar, Bogdan Gurau, E.S. Smotkin* Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 10 W. 33 rd Street, Chicago, IL 60616, USA
Abstract We recently reported the electrochemical promotion of the heterogeneously catalyzed isomerization of 1-butene to cis- and trans-butene with r values of 38 and 46, respectively, at the remarkably low temperature of 708C using Nafion as the electrolyte in a fuel cell configuration. This was the first reported case of a NEMCA promoted unimolecular and non-redox reaction concomitant with the reduction of butene to butane. We now present isotopic mass spectral and FTIR data confirming the mechanism involves abstraction of a proton from the catalytic surface for Markovnikov addition to the C-1 carbon concomitant with removal of a proton from C-3 to yield the isomer. This striking example of an acid catalyzed reaction at a metal surface is facilitated by the use of the super acidic Nafion electrolyte. 2000 Elsevier Science B.V. All rights reserved. Keywords: NEMCA; Gas chromatography; Mass spectroscopy; Infrared spectroscopy; Alkene isomerization; Fuel cell Materials: 1-butene; cis-2-butene; trans-2-butene; Nafion; Pd; Deuterium; D 2 O
1. Introduction Interest in the electrochemical activation of catalysts or Non-Faradaic Electrochemical Activation of Catalytic Activity (NEMCA), is steadily growing [1,2]. Low-temperature NEMCA processes are less frequently studied with the first case, using Nafion, reported by Tsiplakides [3]. We reported the first case of NEMCA for non-redox (unimolecular) catalytic reactions, specifically the isomerization of alkenes on high surface area Pd–C cathodes incorporated into polymer electrolyte fuel cell membrane electrode assemblies (MEAs) [4]. Rate enhancement *Corresponding author. Fax: 11-312-567-8874. E-mail address: [email protected] (E.S. Smotkin).
ratios for cis- and trans-butene of 38 and 46, respectively, at the remarkably low temperature of 708C in a galvanic fuel cell were obtained. A concerted mechanism can be stepwise described as the acid catalyzed isomerization of 1-butene by the Markovnikov addition of a spillover proton to an adsorbed alkene, followed by abstraction of a proton from the C3 carbon to yield 2-butene as shown in Scheme 1. We measured up to 28 butene isomerizations per electrogenerated proton prior to consumption of the proton by electrochemical production of butane. We now report isotopic studies using D 2 –D 2 O–Nafion– Pt anodes and (D 2 O–1-butene)–Nafion–Pd–C cathodes incorporated into fuel cell MEAs as shown in Scheme 2.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00567-1
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L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Scheme 1.
Scheme 2.
Advantage was taken of the isotope effect through the use of mass spectroscopy (MS) and Fourier transform infrared spectroscopy (FTIR) to confirm the proposed mechanism. Additionally, kinetic studies revealed substantially reduced isomerization rates in a deuterated environment in comparison to the previously published hydrogen environment data, substantiating the abstraction of adsorbed hydrogen from the Pd surface as rate-determining.
2. Experimental
2.1. Electrocatalytic reactor The MEAs were prepared from a 30 wt.% Pd–C (Engelhard Industries) catalyst ink and Nafion 117 by the method of Wilson [5]. For isotopic studies, the H 2 O was removed by submerging the MEA in D 2 O (Acros Organics, 99.8 atom% D) for 1 week prior to incorporation into the fuel cell assembly (FCA). The MEA was installed in a FCA consisting of a graphite 5 cm 2 serpentine anode flow field (ElectroChem) and an interdigitated cathode flow field, machined in-house. A nitrogen purge through the anode and cathode was maintained (30 min) at open-circuit voltage (Voc ) to remove air and atmospheric humidity from the assembly and the cathodic product analysis subsystems.
2.2. Reactor preparation Standardized cell-conditioning procedures are used prior to evaluation of the MEAs in the FCAs. For isotopic studies D 2 was used in place of the H 2 for the anode feed. Oxygen is fed to the cathode during the conditioning period. The D 2 (CP grade, Matheson) was humidified with D 2 O at 758C (15 psig) and the O 2 (zero grade, Mittler) was saturated with D 2 O at 708C (0 psig) and supplied to the anode and cathode at 60 and 75 sccm, respectively. Following establishment of the reactor inlet flow conditions, the FCA was heated to 708C at Voc . After temperature stabilization, two-electrode cyclic voltammograms (CVs) are performed on the FCA until steady-state performance is attained, using a computer-controlled Hewlett Packard 6060B System DC Electronic load with an external power supply (Lambda LM E5). Two MEA assemblies were used for this study, one for the isotope studies and one for the normal hydrogenated system. Following H 2 / / O 2 CV conditioning, the cell was returned to open circuit and the HP loader was disconnected. A Pine AFRDE5 Bi-Potentiostat was then connected to the FCA at Voc for increased accuracy in the measurement of the small NEMCA currents from the 5 cm 2 cell employed. The cathode was purged (N 2 , 100 sccm) at Voc prior to introduction of 1-butene. The decrease in the open-circuit voltage was monitored during the N 2
L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
purge as the O 2 was purged. After stabilization of the Voc (about 30 min) the cell was switched to short circuit and cell current monitored. A rapid decay in cell current to a steady-state value within 2 min indicated that the remaining adsorbed oxygen at the cathode was purged. The cell was returned to Voc and the potential monitored for a minimum of 5 min. A steady-state value of 100620 mV indicated that the cathode plenum was purged of oxygen. The steadystate cell voltage results from the differential in the hydrogen partial pressure on opposite faces of the MEA.
2.3. 1 -Butene NEMCA isomerization After the O 2 purge 1-butene (Scott Specialty Gasses) was introduced at Voc to the cathode (25 sccm). After 15 min the cathode inlet, plenum and product analysis lines were sufficiently purged as indicated by a constant Voc . The flow-rate was reduced (3 sccm), the cell was stabilized at Voc (5–10 min) and at least three open-circuit samples (Voc (0.46 V) were obtained before commencing the potential dependent study.
2.4. GC and MS analysis The cathode effluent fed into the GC–MS through heated transfer lines. A Perkin Elmer AutoSystem GC with an automatic gas sampling valve (Arnel, 56-ml gas loop), in-series TCD–FID detectors, and a 10 in. 3 1 / 8 in. Carbograph 2 AP packed column was used. The effluent of the GC gas sample loop was passed to a UTI QMA III closed source quadrupole MS. Gas phase calibration of the GC established that the molar FID response factors for 1butene, cis-2-butene, trans-2-butene and butane were all within 62% of each other. Since the four C 4
715
peaks specified constituted greater than 99.9% of the FID detectable components in the chromatograms, peak area percentages were equivalent to cathode effluent mole percentages. Mass resolution and locations for the low mass range were calibrated with nitrogen and argon. Linearity of mass locations was established through high-range calibration with krypton and xenon. Samples of pure 1-butene, trans2-butene and butane were analyzed by MS for the purposes of spectral comparison. At least two spectra were taken at each potential. Since the FCA test stand–MS inlet interface required most of the cathode effluent it was not possible to perform simultaneous MS and FTIR analysis. Thus, the isomerization reaction was repeated after reconfiguration of the fuel cell output to the GC–FTIR system.
2.5. GC and FTIR analysis Cathode products were directed from the GC loop outlet to a Mattson Research Series II FTIR equipped with a DTGS detector and transmission cell. Samples of pure 1-butene, trans-2-butene and butane were also analyzed by FTIR for the purposes of spectral comparison and calibration. Two spectra were taken at each potential.
3. Results and discussion Replacement of D by H on the surface of the cathode catalyst is anticipated according to Scheme 3, if the isomerization mechanism does not involve simply an intramolecular hydride shift. Such a shift would not account for the observation of deuterium replacement of hydrogen on the alkene as revealed by MS and FTIR.
Scheme 3.
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L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Fig. 1. Potential dependent cathode product distribution calculated from GC–MS data.
Fig. 2. Potential dependent cathode product distribution calculated from GC–FTIR data.
This replacement of D ads by H ads precludes complete deuteration of the isomerization products. We propose that the H ads / D ads ratio attains steady state at each current as the H ads are replaced by D ads from the oxidation of D 2 at the anode and transported through the membrane to the cathode catalytic surface. The potential dependent GC–MS results are reported as mole percentages (Fig. 1). Similarly, GC– FTIR, used to determine the hybridization of the deuterium bearing carbons, also yields product mole percentages (Fig. 2).
3.1. Structural characterization by MS Complete isomerization is not attainable, thus isotope studies were conducted at the potential of the highest isomerization rate. Fig. 1 shows that the maximum conversion to cis- and trans-2-butene (55%) was at 0.15 V where butane represents 2% of the cathode effluent. The fragmentation patterns of 1-butene, cis- and trans-2-butene (Scheme 4) all yield primary peaks at m /z 56, 41 and 15. Since deuterium randomizes upon further frag-
Scheme 4.
L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Scheme 5.
mentation, only relationships between the m /z541 and 42 and m /z515 and 16 peaks are used for structural determination as clarified for trans-2butene-2-d and trans-2-butene-1-d (Schemes 5 and 6, respectively). Our proposed mechanism can be confirmed by examination of the ratios of the enhanced peaks (e.g. 42 enh / 41 enh and 16 enh / 15 enh ) where an enhanced
717
peak is the difference between the intensity of the peak at open circuit and that at the potential of interest. The enhancement spectra are obtained by taking a difference spectrum by subtracting the concentration-corrected open-circuit spectrum from that collected at 0.15 V. The difference spectrum are normalized against the base peak at m /z541 then superimposed upon the open-circuit spectrum (.97% 1-butene) and the spectrum of neat trans-2butene in Figs. 3 and 4, respectively. The quotient of the peak intensities was used to determine the location of the deuterium in the 2-butene products (Table 1). As shown in Schemes 5 and 6, the fragment at m /z516 can only arise from cis- and trans-2-butene-1-d. The approximately equal enhancement of the m /z516 and 42 peaks points strongly to the conclusion that cis- and trans-2butene-1-d are the primary deuterated species.
3.2. FTIR analysis Fig. 3 and Table 2, show that the maximum signal for cis- and trans-2-butene, 65.5%, was in short circuit during the FTIR experiment. At that potential butane comprised 25.3% of the cathode effluent as
Scheme 6.
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L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Fig. 3. MS difference spectrum normalized against the base peak at m /z541 and superimposed 1-butene open-circuit spectrum.
Fig. 4. MS difference spectrum normalized against the base peak at m /z541 and superimposed t-2-butene open-circuit spectrum.
L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
719
Table 1 Ratios calculated from difference mass spectrum superimposed upon mass spectrum of 1-butene at open circuit Mass ratio 1-Butene at open circuit Neat t-2-butene Difference spectrum at 0.15 V
16 / 15 0.164 0.016 0.627
42 / 41 0.039 0.033 0.506
compared to 2% at 0.15 V. FTIR spectra were determined at each potential, and the corresponding GC analyses are presented in Table 2. The potential dependent C–D stretching region of the FTIR is shown in Fig. 5. The calculated wavenumber ranges for the sp 3 and sp 2 hybridized C–D stretching are 2055–2201 and 2201–2275, respectively. The vertical line in Fig. 5 displays the boundary between the two calculated regions. The intensity observed to the left of the vertical line is not due to sp 2 hybridized C–D stretching (the spectra of octane-d 16 has two broad C–D peaks between 2050 and 2250 (Aldrich Spectral Reference Library) due to the harmonic and anharmonic stretching modes of the sp 3 hybridized C–D bond, respectively). Thus, the focus is on the peak to the right of the vertical line. A potential of 20.30 V was chosen in this experiment to produce butane-d 2 as the major product. Comparison of the spectra at 0.20 V and lower against the spectrum of 94.5% butane-d 2 shows that as the isomerization rate is increased, the sp 3 hybridized C–D peak becomes more prominent. This provides strong evidence that the isotopic shift of stretching frequencies of 1butene isomerization products corresponds to the location of deuterium on C 1 . In the region of the maximum total isomerization rates, 0.15–0.10 V, the rate for the D 2 anode cell is 25% less than that of the H 2 anode cell. The striking differences between the K values for the H 2 and D 2
Fig. 5. Potential dependent FTIR spectra of C–D stretching region.
anode cells displays further evidence that surface proton abstraction is the rate-limiting step. Furthermore, Figs. 6 and 7 show that the maximum for the EP promoted isomerization reactions occurs at more negative potentials and larger currents when H 2 is replaced by D 2 .
Fig. 6. Comparison of isomerization rates in hydrogen and deuterium environments.
Table 2 Product distribution of cathode effluent in short circuit determined by FTIR Cell voltage (V)
1-Butene (%)
c-2-Butene (%)
t-2-Butene (%)
Butane (%)
0.46 0.30 0.20 0.15 0.10 0.00 20.30
97.4 95.1 91.8 72.4 31.5 9.1 0.5
0.7 1.4 3.6 13.1 27.9 23.1 1.5
1.4 2.6 3.9 13.4 34.2 42.4 3.4
0.3 0.8 0.6 1.6 6.3 25.3 94.5
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L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
concomitant with return of a proton from the C-3 carbon. This is the first mechanistic study of a NEMCA promoted unimolecular reaction.
Acknowledgements This work was supported by the Army Research Office at Research Triangle, Grant No. DAAHO-G94-00554. Thanks are due to Prof. Richard M. Lambert of Cambridge University for insightful discussions. Fig. 7. Comparison of enhancement factors in hydrogen and deuterium environments.
4. Conclusions FTIR and mass spectroscopy have been applied to the study of the mechanism of the NEMCA promoted butene isomerization to cis- and trans-2butene. The predominant pathway is an acid catalyzed reaction at the metal surface with the initial step involving abstraction of a proton from the catalytic surface for addition to the C-1 carbon
References [1] C.G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625–627. [2] International Society for Solid-state Ionics, Extended Abstracts of the 12th Intl. Conf. on Solid State Ionics, June 6–12, 1999, and references therein. [3] D. Tsiplakides, S.G. Neophytides, O. Enea, M.M. Jaksic, C.G.J. Vayenas, Electrochem. Soc. 144 (1997) 2072–2078. [4] L. Ploense, M. Salazar, B. Gurau, E.S. Smotkin, J. Am. Chem. Soc. 119 (1997) 11550–11551. [5] M.S. Wilson, S. Gottesfeld, J. Appl. Electrochem. 22 (1992) 1–7.
Spectroscopic study of NEMCA promoted alkene isomerizations at PEM fuel cell Pd–Nafion cathodes Lloyd Ploense, Maria Salazar, Bogdan Gurau, E.S. Smotkin* Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 10 W. 33 rd Street, Chicago, IL 60616, USA
Abstract We recently reported the electrochemical promotion of the heterogeneously catalyzed isomerization of 1-butene to cis- and trans-butene with r values of 38 and 46, respectively, at the remarkably low temperature of 708C using Nafion as the electrolyte in a fuel cell configuration. This was the first reported case of a NEMCA promoted unimolecular and non-redox reaction concomitant with the reduction of butene to butane. We now present isotopic mass spectral and FTIR data confirming the mechanism involves abstraction of a proton from the catalytic surface for Markovnikov addition to the C-1 carbon concomitant with removal of a proton from C-3 to yield the isomer. This striking example of an acid catalyzed reaction at a metal surface is facilitated by the use of the super acidic Nafion electrolyte. 2000 Elsevier Science B.V. All rights reserved. Keywords: NEMCA; Gas chromatography; Mass spectroscopy; Infrared spectroscopy; Alkene isomerization; Fuel cell Materials: 1-butene; cis-2-butene; trans-2-butene; Nafion; Pd; Deuterium; D 2 O
1. Introduction Interest in the electrochemical activation of catalysts or Non-Faradaic Electrochemical Activation of Catalytic Activity (NEMCA), is steadily growing [1,2]. Low-temperature NEMCA processes are less frequently studied with the first case, using Nafion, reported by Tsiplakides [3]. We reported the first case of NEMCA for non-redox (unimolecular) catalytic reactions, specifically the isomerization of alkenes on high surface area Pd–C cathodes incorporated into polymer electrolyte fuel cell membrane electrode assemblies (MEAs) [4]. Rate enhancement *Corresponding author. Fax: 11-312-567-8874. E-mail address: [email protected] (E.S. Smotkin).
ratios for cis- and trans-butene of 38 and 46, respectively, at the remarkably low temperature of 708C in a galvanic fuel cell were obtained. A concerted mechanism can be stepwise described as the acid catalyzed isomerization of 1-butene by the Markovnikov addition of a spillover proton to an adsorbed alkene, followed by abstraction of a proton from the C3 carbon to yield 2-butene as shown in Scheme 1. We measured up to 28 butene isomerizations per electrogenerated proton prior to consumption of the proton by electrochemical production of butane. We now report isotopic studies using D 2 –D 2 O–Nafion– Pt anodes and (D 2 O–1-butene)–Nafion–Pd–C cathodes incorporated into fuel cell MEAs as shown in Scheme 2.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00567-1
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L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Scheme 1.
Scheme 2.
Advantage was taken of the isotope effect through the use of mass spectroscopy (MS) and Fourier transform infrared spectroscopy (FTIR) to confirm the proposed mechanism. Additionally, kinetic studies revealed substantially reduced isomerization rates in a deuterated environment in comparison to the previously published hydrogen environment data, substantiating the abstraction of adsorbed hydrogen from the Pd surface as rate-determining.
2. Experimental
2.1. Electrocatalytic reactor The MEAs were prepared from a 30 wt.% Pd–C (Engelhard Industries) catalyst ink and Nafion 117 by the method of Wilson [5]. For isotopic studies, the H 2 O was removed by submerging the MEA in D 2 O (Acros Organics, 99.8 atom% D) for 1 week prior to incorporation into the fuel cell assembly (FCA). The MEA was installed in a FCA consisting of a graphite 5 cm 2 serpentine anode flow field (ElectroChem) and an interdigitated cathode flow field, machined in-house. A nitrogen purge through the anode and cathode was maintained (30 min) at open-circuit voltage (Voc ) to remove air and atmospheric humidity from the assembly and the cathodic product analysis subsystems.
2.2. Reactor preparation Standardized cell-conditioning procedures are used prior to evaluation of the MEAs in the FCAs. For isotopic studies D 2 was used in place of the H 2 for the anode feed. Oxygen is fed to the cathode during the conditioning period. The D 2 (CP grade, Matheson) was humidified with D 2 O at 758C (15 psig) and the O 2 (zero grade, Mittler) was saturated with D 2 O at 708C (0 psig) and supplied to the anode and cathode at 60 and 75 sccm, respectively. Following establishment of the reactor inlet flow conditions, the FCA was heated to 708C at Voc . After temperature stabilization, two-electrode cyclic voltammograms (CVs) are performed on the FCA until steady-state performance is attained, using a computer-controlled Hewlett Packard 6060B System DC Electronic load with an external power supply (Lambda LM E5). Two MEA assemblies were used for this study, one for the isotope studies and one for the normal hydrogenated system. Following H 2 / / O 2 CV conditioning, the cell was returned to open circuit and the HP loader was disconnected. A Pine AFRDE5 Bi-Potentiostat was then connected to the FCA at Voc for increased accuracy in the measurement of the small NEMCA currents from the 5 cm 2 cell employed. The cathode was purged (N 2 , 100 sccm) at Voc prior to introduction of 1-butene. The decrease in the open-circuit voltage was monitored during the N 2
L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
purge as the O 2 was purged. After stabilization of the Voc (about 30 min) the cell was switched to short circuit and cell current monitored. A rapid decay in cell current to a steady-state value within 2 min indicated that the remaining adsorbed oxygen at the cathode was purged. The cell was returned to Voc and the potential monitored for a minimum of 5 min. A steady-state value of 100620 mV indicated that the cathode plenum was purged of oxygen. The steadystate cell voltage results from the differential in the hydrogen partial pressure on opposite faces of the MEA.
2.3. 1 -Butene NEMCA isomerization After the O 2 purge 1-butene (Scott Specialty Gasses) was introduced at Voc to the cathode (25 sccm). After 15 min the cathode inlet, plenum and product analysis lines were sufficiently purged as indicated by a constant Voc . The flow-rate was reduced (3 sccm), the cell was stabilized at Voc (5–10 min) and at least three open-circuit samples (Voc (0.46 V) were obtained before commencing the potential dependent study.
2.4. GC and MS analysis The cathode effluent fed into the GC–MS through heated transfer lines. A Perkin Elmer AutoSystem GC with an automatic gas sampling valve (Arnel, 56-ml gas loop), in-series TCD–FID detectors, and a 10 in. 3 1 / 8 in. Carbograph 2 AP packed column was used. The effluent of the GC gas sample loop was passed to a UTI QMA III closed source quadrupole MS. Gas phase calibration of the GC established that the molar FID response factors for 1butene, cis-2-butene, trans-2-butene and butane were all within 62% of each other. Since the four C 4
715
peaks specified constituted greater than 99.9% of the FID detectable components in the chromatograms, peak area percentages were equivalent to cathode effluent mole percentages. Mass resolution and locations for the low mass range were calibrated with nitrogen and argon. Linearity of mass locations was established through high-range calibration with krypton and xenon. Samples of pure 1-butene, trans2-butene and butane were analyzed by MS for the purposes of spectral comparison. At least two spectra were taken at each potential. Since the FCA test stand–MS inlet interface required most of the cathode effluent it was not possible to perform simultaneous MS and FTIR analysis. Thus, the isomerization reaction was repeated after reconfiguration of the fuel cell output to the GC–FTIR system.
2.5. GC and FTIR analysis Cathode products were directed from the GC loop outlet to a Mattson Research Series II FTIR equipped with a DTGS detector and transmission cell. Samples of pure 1-butene, trans-2-butene and butane were also analyzed by FTIR for the purposes of spectral comparison and calibration. Two spectra were taken at each potential.
3. Results and discussion Replacement of D by H on the surface of the cathode catalyst is anticipated according to Scheme 3, if the isomerization mechanism does not involve simply an intramolecular hydride shift. Such a shift would not account for the observation of deuterium replacement of hydrogen on the alkene as revealed by MS and FTIR.
Scheme 3.
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L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Fig. 1. Potential dependent cathode product distribution calculated from GC–MS data.
Fig. 2. Potential dependent cathode product distribution calculated from GC–FTIR data.
This replacement of D ads by H ads precludes complete deuteration of the isomerization products. We propose that the H ads / D ads ratio attains steady state at each current as the H ads are replaced by D ads from the oxidation of D 2 at the anode and transported through the membrane to the cathode catalytic surface. The potential dependent GC–MS results are reported as mole percentages (Fig. 1). Similarly, GC– FTIR, used to determine the hybridization of the deuterium bearing carbons, also yields product mole percentages (Fig. 2).
3.1. Structural characterization by MS Complete isomerization is not attainable, thus isotope studies were conducted at the potential of the highest isomerization rate. Fig. 1 shows that the maximum conversion to cis- and trans-2-butene (55%) was at 0.15 V where butane represents 2% of the cathode effluent. The fragmentation patterns of 1-butene, cis- and trans-2-butene (Scheme 4) all yield primary peaks at m /z 56, 41 and 15. Since deuterium randomizes upon further frag-
Scheme 4.
L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Scheme 5.
mentation, only relationships between the m /z541 and 42 and m /z515 and 16 peaks are used for structural determination as clarified for trans-2butene-2-d and trans-2-butene-1-d (Schemes 5 and 6, respectively). Our proposed mechanism can be confirmed by examination of the ratios of the enhanced peaks (e.g. 42 enh / 41 enh and 16 enh / 15 enh ) where an enhanced
717
peak is the difference between the intensity of the peak at open circuit and that at the potential of interest. The enhancement spectra are obtained by taking a difference spectrum by subtracting the concentration-corrected open-circuit spectrum from that collected at 0.15 V. The difference spectrum are normalized against the base peak at m /z541 then superimposed upon the open-circuit spectrum (.97% 1-butene) and the spectrum of neat trans-2butene in Figs. 3 and 4, respectively. The quotient of the peak intensities was used to determine the location of the deuterium in the 2-butene products (Table 1). As shown in Schemes 5 and 6, the fragment at m /z516 can only arise from cis- and trans-2-butene-1-d. The approximately equal enhancement of the m /z516 and 42 peaks points strongly to the conclusion that cis- and trans-2butene-1-d are the primary deuterated species.
3.2. FTIR analysis Fig. 3 and Table 2, show that the maximum signal for cis- and trans-2-butene, 65.5%, was in short circuit during the FTIR experiment. At that potential butane comprised 25.3% of the cathode effluent as
Scheme 6.
718
L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
Fig. 3. MS difference spectrum normalized against the base peak at m /z541 and superimposed 1-butene open-circuit spectrum.
Fig. 4. MS difference spectrum normalized against the base peak at m /z541 and superimposed t-2-butene open-circuit spectrum.
L. Ploense et al. / Solid State Ionics 136 – 137 (2000) 713 – 720
719
Table 1 Ratios calculated from difference mass spectrum superimposed upon mass spectrum of 1-butene at open circuit Mass ratio 1-Butene at open circuit Neat t-2-butene Difference spectrum at 0.15 V
16 / 15 0.164 0.016 0.627
42 / 41 0.039 0.033 0.506
compared to 2% at 0.15 V. FTIR spectra were determined at each potential, and the corresponding GC analyses are presented in Table 2. The potential dependent C–D stretching region of the FTIR is shown in Fig. 5. The calculated wavenumber ranges for the sp 3 and sp 2 hybridized C–D stretching are 2055–2201 and 2201–2275, respectively. The vertical line in Fig. 5 displays the boundary between the two calculated regions. The intensity observed to the left of the vertical line is not due to sp 2 hybridized C–D stretching (the spectra of octane-d 16 has two broad C–D peaks between 2050 and 2250 (Aldrich Spectral Reference Library) due to the harmonic and anharmonic stretching modes of the sp 3 hybridized C–D bond, respectively). Thus, the focus is on the peak to the right of the vertical line. A potential of 20.30 V was chosen in this experiment to produce butane-d 2 as the major product. Comparison of the spectra at 0.20 V and lower against the spectrum of 94.5% butane-d 2 shows that as the isomerization rate is increased, the sp 3 hybridized C–D peak becomes more prominent. This provides strong evidence that the isotopic shift of stretching frequencies of 1butene isomerization products corresponds to the location of deuterium on C 1 . In the region of the maximum total isomerization rates, 0.15–0.10 V, the rate for the D 2 anode cell is 25% less than that of the H 2 anode cell. The striking differences between the K values for the H 2 and D 2
Fig. 5. Potential dependent FTIR spectra of C–D stretching region.
anode cells displays further evidence that surface proton abstraction is the rate-limiting step. Furthermore, Figs. 6 and 7 show that the maximum for the EP promoted isomerization reactions occurs at more negative potentials and larger currents when H 2 is replaced by D 2 .
Fig. 6. Comparison of isomerization rates in hydrogen and deuterium environments.
Table 2 Product distribution of cathode effluent in short circuit determined by FTIR Cell voltage (V)
1-Butene (%)
c-2-Butene (%)
t-2-Butene (%)
Butane (%)
0.46 0.30 0.20 0.15 0.10 0.00 20.30
97.4 95.1 91.8 72.4 31.5 9.1 0.5
0.7 1.4 3.6 13.1 27.9 23.1 1.5
1.4 2.6 3.9 13.4 34.2 42.4 3.4
0.3 0.8 0.6 1.6 6.3 25.3 94.5
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concomitant with return of a proton from the C-3 carbon. This is the first mechanistic study of a NEMCA promoted unimolecular reaction.
Acknowledgements This work was supported by the Army Research Office at Research Triangle, Grant No. DAAHO-G94-00554. Thanks are due to Prof. Richard M. Lambert of Cambridge University for insightful discussions. Fig. 7. Comparison of enhancement factors in hydrogen and deuterium environments.
4. Conclusions FTIR and mass spectroscopy have been applied to the study of the mechanism of the NEMCA promoted butene isomerization to cis- and trans-2butene. The predominant pathway is an acid catalyzed reaction at the metal surface with the initial step involving abstraction of a proton from the catalytic surface for addition to the C-1 carbon
References [1] C.G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625–627. [2] International Society for Solid-state Ionics, Extended Abstracts of the 12th Intl. Conf. on Solid State Ionics, June 6–12, 1999, and references therein. [3] D. Tsiplakides, S.G. Neophytides, O. Enea, M.M. Jaksic, C.G.J. Vayenas, Electrochem. Soc. 144 (1997) 2072–2078. [4] L. Ploense, M. Salazar, B. Gurau, E.S. Smotkin, J. Am. Chem. Soc. 119 (1997) 11550–11551. [5] M.S. Wilson, S. Gottesfeld, J. Appl. Electrochem. 22 (1992) 1–7.