11550
J. Am. Chem. Soc. 1997, 119, 11550-11551
Scheme 1
Proton Spillover Promoted Isomerization of
n-Butylenes on Pd-Black Cathodes/Nafion 117
Lloyd Ploense, Maria Salazar, Bogdan Gurau, and
E. S. Smotkin*
used to study current-voltage and conversion-voltage relation-
ships in the isomerization of 1-butene to cis- and trans-2-butene,
shown in Scheme 1.
Department of Chemical and EnVironmental Engineering
Illinois Institute of Technology
The catalytic-electrocatalytic reactor consists of a membrane
electrode assembly (MEA), such as Pt-black/Nafion/Pd/C
sandwiched between sheets of porous carbon cloth, housed in
a FCA. The MEAs were prepared by the method of Wilson.15
A Pd/C catalyst ink was prepared by dispersing 0.15 g of 30
wt % Pd/C (Engelhard, Industries, Inc.) into 1 g of solubilized
Nafion (5% solution, Aldrich) and stirring (24 h). A Pt-black
ink was prepared similarly (0.1 g of Pt-black, Alfa Aesar Fuel
Cell Grade, and 0.35 g of Nafion). Catalyst decals were
prepared by repetitively brush painting an ink onto a 5 cm2 sheet
of Teflon, annealing (120 °C, 20 min), and weighing, until the
desired catalyst loading (28 mg of Pd/C and 30 mg of Pt-black)
was attained. The Pt and Pd/C decals were hot-pressed onto
opposite faces of a Nafion 117 sheet (130 °C, 100 psig, 30 s).
The temperature and pressure were increased (140 °C, 1500
psig), and pressing was continued for an additional 40 s. After
cooling to room temperature, the Teflon sheets were peeled off,
leaving catalytic films on both faces of the Nafion. The catalytic
layers were overlaid with a 20 wt % Teflonized porous carbon
cloth and inserted into the FCA. Current-voltage (I-V) data
was obtained with a Pine potentiostat, used with the FCA anode
shorted to the reference electrode input; the maximum current
was <25 mA/cm2 on high roughness factor (>250) electrodes.
Thus, the H2 anode is essentially a nonpolarizable reference
electrode. The flow rates were 60 SSCM H2 (15 psig) to the
anode and 2.2-7.2 SCCM (0 psig) 1-butene to the cathode.
The inlet feeds were saturated with H2O at 75 and 70 °C,
respectively, with the FCA at 70 °C. Cathode products were
quantified by GC-TCD/FID.
10 West 33rd Street, Chicago, Illinois 60616
ReceiVed August 18, 1997
ReVised Manuscript ReceiVed September 24, 1997
The electrochemical activation of heterogeneous catalysis,
described by Vayenas as non-Faradaic electrochemical modi-
fication of catalytic activity (NEMCA) or electrochemical
promotion,1 is currently attracting considerable interest.
NEMCA, an effect ascribed to the electrochemically induced
and controlled spillover of ions from a solid electrolyte onto a
catalyst surface, takes place at the three-phase boundary between
the electrode catalyst, electrolyte, and reactant vapor phase. Until
now, NEMCA has been demonstrated only for non-Faradaic
catalytic reactions which are identical to the net electrocatalytic
process (e.g. H2, O2/Pt-black/Nafion/Pt-black/O2, where catalytic
oxidation of hydrogen by co-fed oxygen is enhanced by the
electro-oxidation of hydrogen coupled to an oxygen cathode).
The rate enhancement ratio F, the enhancement factor Λ, and
an approximate relationship between the absolute value of Λ
and the exchange current Io are given in eq 1.
(r - ro)
(I/2F)
r
ro
F )
;
Λ )
;
|Λ| ≈ 2Fr0/I0 (1)
where r is the catalytic rate at a current I and ro is the open-
circuit catalytic rate. A key parameter for the quantitative
description of NEMCA is the exchange current,2 Io, which
expresses the rates of the anodic and cathodic processes of the
electrocatalytic reaction at open circuit.3
Figure 1 shows the I-V dependence of the steady state
cathode exhaust component fluxes (ν) of cis- and trans-2-butene,
and butane.
NEMCA has been studied for over 40 catalytic reactions4 on
Pt,5 Pd,6 Rh,7 Ni, Au, Cu, and Ag8 surfaces interfaced to a
variety of solid electrolytes, such as yttria-stabilized-zirconia
The NEMCA for the isomerization of 1-butene is apparent
at low overpotentials and increases dramatically, even prior to
0.2 V where butane reduction becomes significant. Butane was
the only reduction product detected by GC analysis; the butene
reduction current efficiency was 100% from open circuit to short
circuit. The striking electrochemical promotion of the isomer-
ization of 1-butene to cis- and trans-butene is curtailed at 0.16
and 0.1 V, respectively, as the butene reduction current
increases. Nafion-coated Pd/C exhibited no open circuit
isomerization rate enhancement in comparison to bare Pd/C
catalysts.16 The F values for cis- and trans-butene at their
maximum fluxes are approximately 38 and 46, respectively. Of
the 40 reactions tabulated by Vayenas,1 only two reactions have
higher F values: the oxidation of ethylene by O2 on Pt/YSZ at
260-450 °C (F ) 55) and the oxidation of ethylene by O2 on
Rh/YSZ at 250-400 °C (F ) 90). However, this is the first
report of F values for nonredox reactions, and with the exception
of hydrogen-oxygen recombination, the isomerization of butene
at 70 °C is the lowest temperature reported for NEMCA.
(YSZ),9 â′′-Al2O3,10 CaF2, CsHSO4, CaZr0.9In0.1
O
3-R, SrCe0.95
-
11
Yb0.05O3-R
,
Nafion,12 TiO2, and aqueous alkaline solutions.13
We now report the first demonstration of NEMCA for
nonredox catalytic reactions, specifically the isomerization of
alkenes on high surface area Pd/C or unsupported Pd-Ru
cathodes interfaced to Nafion with a Pt-black/H2 counter
electrode. A Nafion electrolyte fuel cell assembly (FCA)14 was
(1) Vayenas, C. G.; Bebelis, S. I. Solid State Ionics 1997, 94, 267-277.
(2) Vayenas, C. G.; Jaksic, M. M.; Bebelis, S.; Neophytides S. Modern
Aspects of Electrochemistry; Bockris, O’M., Conway, B. E., White R. E.,
Eds.; Plenum Press: New York, 1996; Vol. 29; pp 57-202.
(3) Bard A. J.; Faulkner L. R. Electrochemical Methods; John Wiley &
Sons: New York, 1980; p 101.
(4) Vayenas, C. G.; Babelis, S.; Yentekakis, I. V.; Lintz, H. G. Catal.
Today 1992, 11, 303-441.
(5) Sobyanin, V. A.; Sobolev, V. I.; Belyaev, V. D.; Mar’ina, O. A.;
Demin, A. K; Lipilin, A. S. Catal. Lett. 1993, 18, 153-164.
(6) Vayenas, C. G.; Babelis, S.; Ladas, S. Nature 1990, 343, 625-627.
(7) Pliangos, C.; Yentekakis, I. V.; Verykios, X. E.; Vayenas, C. G. J.
Catal. 1995, 154, 124-136.
(8) Bebelis, S.; Vayenas, C. G. J. Catal. 1992, 138, 570-587.
(9) Cavalca, C. A.; Larsen, G.; Vayenas, C. G.; Haller, G. L. J. Phys.
Chem. 1993, 97, 6115-6119.
(14) Schematics and details concerning fuel cell assemblies can be found
in: Proceedings of the First International Symposium on Proton Conducting
Membrane Fuel Cells I; Gottesfeld, S., Halpert, G., Landgrebe, A., Eds.;
The Electrochemical Society, Inc.: Pennington, NJ, 1995; pp 34-182.
(15) Wilson, M. S.; Gottesfeld, S. J. Appl. Electrochem. 1992, 22, 1-7.
(16) A packed bed reactor was constructed with 316 stainless steel tubing
(reactor volume 3 cm3). The catalysts (Nafion-coated Pd/C or bare Pd/C)
were dispersed on glass wool prior to packing. The reactor was typically
charged with ca. 0.5 g of Pd/C. Humidified 1-butene was fed (4 mL/min)
at 70 °C. Analysis of the effluent by GC/FID showed comparable rates of
1-butene conversion (<2%) for Nafion-coated and bare Pd/C catalysts.
(10) Karavasilis, Ch.; Bebelis, S.; Vayenas, C. G. J. Catal. 1996, 160,
205-213.
(11) Chiang, P. H.; Eng, D.; Stoukides, M. J. Catal. 1993, 139, 683-
687.
(12) Tsiplakides, D.; Neophytides, S. G.; Enea, O.; Jaksic, M. M.;
Vayenas, C. G. J. Electrochem. Soc. 1997, 144, 2072-2078.
(13) Neophytides, S. G.; Tsiplakides, D.; Stonehart, P.; Jaksic, M. M.;
Vayenas, C. G. Nature 1994, 370, 45-47.
S0002-7863(97)02884-9 CCC: $14.00 © 1997 American Chemical Society