Electrochemical promotion of NO reduction by hydrogen on a platinum/polybenzimidazole catalyst

Article abstract of DOI:10.1149/1.1566413

The electrochemical promotion of catalytic NO reduction by hydrogen was studied using a (NO, H₂, Ar), Pt polybenzimidazole (PBI)-H₃PO₄|Pt, (H₂, Ar) fuel cell at 135°C. A mixture of NO/H₂/Ar was used as the working mixture at one electrode and a mixture of H₂/Ar was used as reference and counter gas at the other electrode. Products of NO reduction (N₂ and H₂O) were analyzed by an on-line mass spectrometer. At high NO + H₂ + Ar flow rate (17 mL/min; 17 and 354 mL/min, respectively, at atmospheric pressure) the maximum rate enhancement ratio was 4.65. At low NO + H₂ + Ar flow rate (17 mL/min; 17 and 140 mL/min, respectively), NO reduction increased 20 times even without polarization compared to the high gas flow rate. The electrochemical promotion effect occurs at positive polarization with a maximum increase at approximately 0.08 V and with 1.5 times the zero polarization value. The promotion at the negative polarization can be attributed to the electrochemical production of the promoters. At low gas flow rates, a charge-induced change of the strength of chemisorptive bonds can take place.

Full text of DOI:10.1149/1.1566413

Journal of The Electrochemical Society, 150 ͑5͒ D87-D90 ͑2003͒
D87
0013-4651/2003/150͑5͒/D87/4/$7.00 © The Electrochemical Society, Inc.
Electrochemical Promotion of NO Reduction by Hydrogen on a
PlatinumÕPolybenzimidazole Catalyst
,z
*
*
I. M. Petrushina, V. A. Bandur, F. Cappeln, N. J. Bjerrum, R. Z. Sørensen,
*
R. H. Refshauge, and Qingfeng Li
Materials Science Group, Department of Chemistry, Technical University of Denmark, DK-2800 Lyngby,
Denmark
The electrochemical promotion of catalytic NO reduction by hydrogen was studied using a ͑NO, H2 , Ar͒, Pt polybenzimidazole
(PBI)-H3PO4͉Pt, (H2 , Ar͒ fuel cell at 135°C. A mixture of NO/H2 /Ar was used as the working mixture at one electrode and a
mixture of H2 /Ar was used as reference and counter gas at the other electrode. Products of NO reduction (N2 and H2O) were
analyzed by an on-line mass spectrometer. At high NO ϩ H2 ϩ Ar flow rate ͑17 mL/min; 17 and 354 mL/min, respectively, at
atmospheric pressure͒ the maximum rate enhancement ratio was 4.65. At low NO ϩ H2 ϩ Ar flow rate ͑17 mL/min; 17 and 140
mL/min, respectively͒, NO reduction increased 20 times even without polarization compared to the high gas flow rate. The
electrochemical promotion effect occurs at positive polarization with a maximum increase at approximately 0.08 V and with 1.5
times the zero polarization value. The promotion at the negative polarization can be attributed to the electrochemical production
of the promoters. At low gas flow rates, a charge-induced change of the strength of chemisorptive bonds can take place.
© 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1566413͔ All rights reserved.
Manuscript submitted August 27, 2002; revised manuscript received December 5, 2002. Available electronically March 28, 2003.
The present work is a continuation of our research in the fields of
electrochemical promotion1 and polymer electrolyte fuel cells.2,3
That a catalyst is often a system which consists of an electronic and
an ionic conductor makes it possible to use electrochemical tech-
niques to study the nature of the promotion effect in catalysts. Ap-
plication of galvanic cells with solid electrolytes for the study of
catalytic phenomena was suggested by Wagner.4 Later, Vayenas
et al.5 found that rate and selectivity of a catalytic reaction can be
remarkably and reversibly changed by electrochemical polarization
of the electronic conductive catalyst on an ionic conductive support.
This phenomenon was named NEMCA effect ͑nonfaradaic electro-
chemical modification of catalytic activity͒ or electrochemical
promotion.4 The NEMCA effect has been demonstrated for more
then 50 heterogeneous2,5-22 and one homogeneous1 catalytic reac-
tions. According to Vayenas et al.,5-10 the NEMCA effect can be
described by the following parameters.
probe, may be only a part of the applied polarization or do not
change at all. Summarizing the results of Refs. 5-22, one can agree
with the assumption of Emery et al.20 and Metcalfe21,22 that
⌬⌽ ϭ ␰␩
͓4͔
where ␰ is a coefficient. Furthermore one can also assume 0 р ␰
р 1.
The electrochemical potential of the electron in a metal, ␮
¯
e
͑Fermi level͒, is a sum of the chemical potential of the electron in
the metal ␮ and e␾, where ␾ is the inner or Galvani potential. The
e
Galvani potential is a sum of an outer ͑or Volta͒ potential, ⌿, and a
surface potential, ␹. The work function of electron, e⌬⌽, is equal to
the electrochemical potential of the uncharged metal (⌿ ϭ 0),23
i.e.
⌬⌽ ϭ ⌬␮ ϩ ⌬␹
͓5͔
The rate enhancement ratio
␳ ϭ r/ro
͓1͔
and ⌬⌿ Þ 0 is the reason for the difference between ⌬⌽ and ␩
͑Eq. 4͒. The value of ␰ is defined by the value of the polarization
resistance of the electrochemical reaction at the catalyst support in-
terface: if this resistance is low, ␰ Ϸ 1; if this resistance is high,
␰ Ϸ 0 and by polarization we mainly charge the catalyst-support
electric double layer.20
and the enhancement factor
⌳ ϭ r Ϫ r ͒/ 1/2F͒
͓2͔
͑
͑
o
where r is the catalytic rate at current I, ro is the open-circuit cata-
lytic rate, and F is Faraday’s constant. In all studied cases of the
In our previous paper2 we have defined three types of electro-
chemical promotion depending on values of the faradaic current ͑I͒
and ⌳, however it is more precise to separate electrochemical pro-
motion and electrocatalysis.
electrochemical promotion
͉⌳͉ ӷ 1 and that was the reason for this
effect being called nonfaradaic.
Vayenas et al.5-10 explained the electrochemical promotion of a
catalyst as an effect of changes in the work function, e⌬⌽ ͑e is the
electron charge, ⌬⌽ is change of the electron extraction potential͒,
of the catalyst under polarization. The change of the electron extrac-
tion potential has been measured to be equal to the applied polar-
ization, i.e.
1. The electrochemical promotion effect is the charge-induced
change of the strength of chemisorptive bonds at the catalyst-
support-reactant boundary in heterogeneous catalysis or electrode-
catalyst interface in homogeneous catalysis ͑CI effect͒. The CI effect
is characterized by I ϭ 0, ␩ ϭ ⌬⌿, and ⌬⌽ ϭ 0. In this case the
catalyst ͑or an electrode in homogeneous catalysis͒ is charged to the
potential of the specific adsorption of one or more reactants.
2. The electrochemical promotion effect is the effect induced by
the electrochemical production of the catalyst promoters ͑EPP ef-
fect͒. I Þ 0; ⌳ ӷ 1; ⌬⌿ ϭ 0, and ⌬⌽ ϭ ␩. The electrocatalytic
effect is a separate case where the reactant ͑or reactants͒ of a cata-
lytic reaction is ͑are͒ oxidized or reduced electrochemically,
I Þ 0, ⌳ Ϸ 1.
␩ ϭ ⌬⌽
͓3͔
It was also suggested that this change in the work function was
caused by spillover of the products of the charge transfer from the
three-phase boundary to the catalyst-gas interface. The change in the
work function was measured using the Kelvin probe technique.
However, it has been found later18-22 that work function changes
of catalyst with different morphology, measured using a Kelvin
Most published cases of electrochemical promotion2-13 can be
ascribed to the EPP effect or mixed CI and EPP effects, depending
on the polarization resistance of the EPP reaction. The promotion of
the homogeneous catalytic reaction1 or alkene isomerizations16 are
examples of the CI effect.
* Electrochemical Society Active Member.
z E-mail: njb@kemi.dtu.dk
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Journal of The Electrochemical Society, 150 ͑5͒ D87-D90 ͑2003͒
Figure 1. Setup for study of electrochemical promotion of catalytic reduc-
tion.
Figure 2. Dependence of NO conversion on polarization obtained in ͑NO,
H2 , Ar͒, Pt/polybenzimidazole (PBI)-H3PO4 /Pt, (H2 , Ar͒ fuel cell at high
NO ϩ H2 ϩ Ar flow rate ͑17 mL/min; 17 and 354 mL/min, respectively, at
atmospheric pressure͒ and at 135°C.
In most previous studies of electrochemical promotion of the
catalytic NO reduction, Na was pumped to the surface of the catalyst
using electrochemical reduction of Naϩ ions from Na ␤ -alumina
Љ
support.11-14 This supply of Na greatly enhanced the reduction of
NO on Pt, Pd, Rh, and several other catalytic materials with a reac-
tion rate enhancement as high as two orders of magnitude. Simulta-
neously the selectivity of production of N2 against N2O increased.
Lambert et al.11-14 showed that the effect of electrochemical promo-
tion is equivalent to the effect seen from depositing Na chemically
on the catalyst surface. It means that the effect had a pure EPP
nature.
The reason for NO reduction being promoted so remarkably is
that the supply of Na to the catalytic surface changes the electronic
properties of the surface. This happens in a way that strengthens the
N-catalyst bond at the expense of the N-O bond, thereby facilitating
the dissociation of NO, which is the limiting step of the whole
reaction.24 As N2O is produced by NO ϩ N͑ads͒ → N2O, the fast
dissociation of NO diminishes the production of N2O.24
The electrochemical promotion of NO reduction is increased
with the loading of Na until a certain point, where the supply of any
more Na leads to a poisoning of the system.12 This poisoning hap-
pens because when too much Na is present, it and Oads from the NO
dissociation begin to cover most of the active centers blocking out
new NO molecules.
developed in our group as well as by others.3 Graphite plates with
gas channels were used as holders and current collectors. Two alu-
minum end plates with attached heaters were used to clamp the
graphite plates. The temperature was controlled by a home-made
controller. Mass flowmeters ͑5850 S Brookssmart͒ and 0154 Brooks
Instrument controllers were used to control the inlet gas composi-
tion. The outlet gas composition was measured by an on-line quad-
rupole mass spectrometer ͑QMS 421, Pfeiffer͒ with a secondary
electron multiplier detector and a cross-beam ion source with a rhe-
nium filament. The ionization voltage was 70 V. The outlet gases
were admitted to the mass spectrometer from the fuel cell through
an 0.8 mm stainless steel capillary.
The mixture of NO and hydrogen, diluted by Ar (NO/H2 /Ar
ϭ 17 mL/min; 17 and 354 mL/min; or 17 mL/min; 17 and 140
mL/min, respectively, at atmospheric pressure͒ was used as a work-
ing mixture at one electrode and hydrogen/argon mixture (H2 /Ar
was 17 and 371 mL/min or 17 and 140 mL/min, respectively͒ was
used as a reference and a counter gas at the other electrode. The
temperature was 135-137°C.
Commercial gases NO ͑99.9%͒, H2 ͑99.9%, р10 ppm of O2 ,
р15 ppm of H2O), and Ar (Ͻ40 ppm O2 ϩ H2O) were used.
An EG&G Instrument ͑Princeton Applied Research͒ 283
potentiostat/galvanostat controlled by 352 SoftCorr™ III Software
was used for electrode polarization and steady-state voltammetric
measurements. The initial ͑at zero polarization͒ catalytic activity
was measured after the yield of gas products had stabilized, i.e.,
under steady-state conditions. For each value of polarization the
measurements were performed after stabilization of the yield of gas
products ͑1-1.5 h͒. NO conversion vs. polarization curves were re-
produced for each studied product and temperature. The open-circuit
potential of the working electrode was approximately 0.14 V.
Electrochemical promotion of NO reduction also took place
when the catalyst support was an O2Ϫ conducter. For this kind of
system, an increase in NO reduction rate ͑up to ⌳ ϭ 700) was
obtained when O2Ϫ was removed from ͑or in some cases added to͒
the catalytic surface.9,10,15,17
In the present research the electrochemical promotion of catalytic
NO reduction by hydrogen was studied using a
NO,H ,Ar͒,Pt͉polybenzimidazole PBI͒-H PO ͉Pt, H ,Ar͒
͑ ͑
͘
3 4 2
͑
͗
2
fuel cell.25 Our purpose was to study the possibility of promoting
NO reduction with the adsorbed hydrogen atoms produced electro-
chemically.
Results and Discussion
The results of the investigation of the electrochemical promotion
of the catalytic NO reduction are given in Fig. 2–5. The presented
data has been obtained at high and low gas flow rates to create the
conditions where there are, respectively, an underproduction and an
overproduction of the Pt-Hads and Pt-Hϩads sites at the catalyst-gas
interface through the chemical reaction.
Experimental
The setup for study of electrochemical promotion of the catalytic
NO reduction is given in Fig. 1. The carbon-supported Pt catalyst
with Pt load around 0.5 mg/cm2 was used in this investigation.3 The
electrolyte was PBI doped with H3PO4 . The techniques of preparing
the PBI(H3PO4) membrane electrolyte and the Pt, C/PBI
(H3PO4)/Pt, C assembly ͑the working electrode area is 5 cm2͒ were
Data in Fig. 2 and 3 were obtained at high gas flow rate after the
catalyst was first polarized 0.1 V positively and then negatively to
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Journal of The Electrochemical Society, 150 ͑5͒ D87-D90 ͑2003͒
D89
Figure 5. Steady-state voltammetric curve obtained at Pt catalyst at low
NO ϩ H2 ϩ Ar flow rate ͑17 mL/min; 17 and 140 mL/min, respectively, at
atmospheric pressure͒ and at 135°C.
There are obvious anodic and cathodic faradaic reactions inside
this potential region ͑presence of the voltammetric waves, Fig. 3͒.
The anodic current at the potential of maximum effect was 0.2
mA/cm2. The value of ⌳ calculated for the maximum promotion
effect was 1.26 ϫ 103, i.e., ⌳ ӷ 1. This means that this effect has
an EPP nature.2
The closest published example of an electrochemically promoted
catalytic reaction is the catalytic oxidation of CO by O2 at the Pt
catalyst on YSZ support.19 According to Belyaev et al., this catalytic
reaction is promoted by ZOϪ2 oxygen species which are the products
of the interaction between electrochemically produced ZOϪ species
and chemically adsorbed oxygen species ZoO ͑where Z are the cata-
lyst active sites at the catalyst-support-gas interface and Zo are the
catalyst active sites at the catalyst-gas interface͒. We can assume
here that both electrochemically produced ZH and chemically pro-
duced ZoH species, and also the possible product of their interaction
with Hϩ, ZHϩ2 , can promote the NO catalytic reduction. The reac-
tion
Figure 3. Steady-state voltammetric curve obtained at Pt catalyst at high
NO ϩ H2 ϩ Ar flow rate ͑17 mL/min; 17 and 354 mL/min, respectively, at
atmospheric pressure͒ and at 135°C.
Ϫ0.3 V. Nitrogen and water are the NO reduction products. Without
polarization the NO conversion was 2% ͑Fig. 2͒. The reaction is
assumed to be
2NO ϩ 2H2 → N2 ϩ 2H2O
͓6͔
Dependence of the NO conversion on polarization is given in
Fig. 2. It can be seen that Reaction 6 can be electrochemically pro-
moted at negative polarization and exhibits a clear ‘‘volcano’’-type
promotion behavior.6 This means that there is a maximum promo-
tion effect ͑9.3% NO conversion͒ at a polarization of approximately
Ϫ0.15 V, or Ϫ0.01 V catalyst potential vs. the reversible hydrogen
electrode, RHE. The catalytic rate enhancement ratio at this maxi-
mum is 4.65. Figure 2 shows no NO conversion at a polarization
р0.1 and рϪ0.3 V.
ZH ꢀ Z ϩ Hϩ ϩ e
͓7͔
takes place in the potential range between 0 and 0.4 V vs. RHE,25
i.e., at the potentials of the electrochemical promotion of NO reduc-
tion ͑Fig. 2͒. The nature of promotion of NO reduction by the ad-
sorbed hydrogen species is probably the same as the nature of the
promotion of the same reaction by the adsorbed Na atoms.11-14 It has
been shown that the rate-determining step of the catalytic NO reduc-
tion at a Pt catalyst is dissociative chemisorption of NO because Pt
is relatively ineffective at this step.24 Adsorbed hydrogen species
can act to increase the adsorption strength of electronegative adsor-
bates ͑NO͒ and weaken the N-O bond in the adsorbed molecule and
therefore promote NO dissociation.13
Data in Fig. 4 and 5 were obtained at low gas flow rate after the
catalyst was first polarized Ϫ0.3 V negatively and then positively to
0.2 V. It can be seen from comparison of Fig. 2 and 4 that NO
reduction is increased 20 times even without polarization. Moreover,
under these conditions negative polarization decreased the rate of
NO reduction ͑i.e., an opposite effect to what was found at high gas
flow rates͒. It can also be seen that the electrochemical promotion
effect did occur at a positive polarization with maximum increase at
approximately 0.08 V polarization and with 1.5 times the zero po-
larization value.
Figure 4. Dependence of NO conversion on polarization obtained in ͑NO,
H2 , Ar͒, Pt/polybenzimidazole (PBI)-H3PO4 /Pt, (H2 , Ar͒ fuel cell at low
NO ϩ H2 ϩ Ar flow rate ͑17 mL/min; 17 and 140 mL/min, respectively, at
atmospheric pressure͒ and at 135°C.
The steady-state voltammetric behavior of the catalyst at a low
gas flow rate is shown in Fig. 5. It is obvious from Fig. 5 that in the
potential range of the promotion effect faradaic current is absent.
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Journal of The Electrochemical Society, 150 ͑5͒ D87-D90 ͑2003͒
This means that the promotion effect has a CI nature.2 Increase of
the NO conversion under open-circuit conditions and changes in the
nature of the promotion effect can be explained by a high concen-
tration of Pt-H sites at the low gas flow rates. The increased number
of the adsorbed hydrogen species ͑as in the NO promotion with
Na12,24͒ can complicate the NO ͑electronegative adsorbate͒ chemi-
sorption, especially at negative polarization. At positive polarization,
however, the charge-induced change of the strength of chemisorp-
tive bonds can take place ͑CI effect͒.
Technical University of Denmark assisted in meeting the publication
costs of this article.
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Conclusions
The possibility of the electrochemical promotion of the catalytic
NO reduction by hydrogen at the Pt-PBI(H3PO4)-gas boundary has
been demonstrated. It has also been shown that the nature of this
promotion effect can vary depending on the flow rate of the
NO/H2 /Ar gas mixture. At high NO ϩ H2 ϩ Ar flow rate ͑17 mL/
min; 17 and 354 mL/min, respectively, at atmospheric pressure͒, it
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means that this effect has an EPP nature, the catalytic reaction was
promoted by the electrochemically produced adsorbed hydrogen
species.
At low NO ϩ H2 ϩ Ar flow rate ͑17 mL/min; 17 and 140 mL/
min, respectively, at atmospheric pressure͒, NO reduction increased
20 times even without polarization. Moreover, under these condi-
tions negative polarization decreased the rate of NO reduction ͑i.e.,
an opposite effect to what was found at high gas flow rates͒. How-
ever, the electrochemical promotion effect did occur at positive po-
larization with maximum increase at approximately 0.08 V and with
1.5 times the zero polarization value. In the potential range of the
promotion effect faradaic current is absent. It means that the promo-
tion effect has a CI nature. This means that the effect was caused by
the charge-induced change of the strength of chemisorptive bonds.
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Acknowledgments
This investigation has been supported by the PSO-F&U contract
no. 3089 and by the Danish Research Councils ICAT project no.
9702636.
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