Mechanistic study on the electrocatalytic reduction of nitric oxide on transition-metal electrodes

Article abstract of DOI:10.1006/jcat.2001.3275

The reduction of NO determines the performance of wastewater treatment catalysts for nitrate, nitrite, and NO removal, and the scrubbing of NO from gas streams. The mechanism of the electrochemical reduction of NO on a series of metals (Pd, Rh, Ru, Ir, and Au) was studied for the reduction of adsorbed NO and for continuous NO reduction. All transition metals showed a high selectivity toward N₂O at high potentials. With the exception of gold, a high selectivity toward NH₃ at low potentials was achieved and produced N₂, though with varying selectivity at intermediate potentials. Despite the varying Tafel slopes for the different metals, the similarities in the onset of the N₂O formation suggested a common mechanism for the N₂O formation on all metals. Metals that actively reduced N₂O produced more N₂ in their NO reduction. No N₂ and N₂O were formed in the NO adsorbate reduction in the absence of NO in solution. Palladium showed to be the best electro catalyst for the selective reduction of NO to N₂.

Full text of DOI:10.1006/jcat.2001.3275

Journal of Catalysis 202, 387–394 (2001)
doi:10.1006/jcat.2001.3275, available online at http://www.idealibrary.com on
Mechanistic Study on the Electrocatalytic Reduction of Nitric Oxide
on Transition-Metal Electrodes
A. C. A. de Vooys, M. T. M. Koper,1 R. A. van Santen, and J. A. R. van Veen
Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology,
5600 MB Eindhoven, The Netherlands
Received March 14, 2001; revised May 7, 2001; accepted May 7, 2001; published online August 17, 2001
choice. Other metals form, depending on potential, unde-
sirable products such as N2O and/or NH3.
The mechanism of the electrochemical reduction of nitric ox-
ide (NO) on a series of metals (Pd, Rh, Ru, Ir, and Au) has been
studied, both for the reduction of adsorbed NO and for the con-
tinuous NO reduction. All metals show a high selectivity to N2O
at high potentials and a high selectivity to NH3 at low potentials,
whereas N2 is formed at intermediate potentials (although gold
forms mainly N2O, and very little NH3). The behavior of the tran-
sition metals is very similar to that of platinum, suggesting that the
reaction schemes are essentially the same (especially the potential
windows in which the products are formed are similar). The mech-
anism that leads to N2O is believed to involve the formation of a
weakly adsorbed NO dimer intermediate, similar to recent sugges-
tions made for the gas-phase reduction of NO. The reduction of
adsorbed NO leads only to formation of NH3 and not to N2O or
N2. The electrochemical measurements suggest that NH3 forma-
tion involves a combined electron–proton transfer in equilibrium,
followed bya nonelectrochemicalrate-determiningstep. Theforma-
tion ofN2, produced at potentialsbetween theformation ofN2Oand
NH3, most likely takes place by the reduction of previously formed
There is as yet little or no mechanistic insight into why
palladium is the best catalyst in the selective reduction
of NO to N2. The only metal for which reasonably de-
tailed mechanistic information is presently available is plat-
inum (5).
Summarizing the results of our previous publication (5),
we found that there are two major reaction paths for NO
reduction on platinum, one at high potentials (0.3–0.7 V
vs RHE) which leads to nitrous oxide (N2O), and one at
low potentials (0–0.3 V) which leads mainly to ammonia
(NH3). The formation of N2O was observed to take place
only in the presence of NO in the solution. From the Tafel
slope, the pH dependence, and the kinetic order in NO
solution concentration, the following reaction scheme was
suggested:
+ NO(aq) → NOads
fast
c
2001 Academic Press
N2O.
NOads + NO(aq) + H+ + e → HN2O2,ads
HN2O2,ads + H+ + e → N2O(aq) + H2O + 2
r.d.s.
fast,
[1]
Key Words: NO reduction; electrocatalysis; transition metals;
differential electrochemical mass spectroscopy.
where denotes a free site at the surface. The most remark-
able feature of this scheme is the rate-determining step, in
1. INTRODUCTION
The reduction of nitric oxide (NO) is an important re- which we proposed a surface-bonded NO to combine with a
action in environmental catalysis, since it determines the solution-phase NO. The latter may also be interpreted as a
performance of wastewater treatment catalysts for nitrate, weaklybonded NO, aslongasitsconcentration isfirst-order
nitrite, and NO removal (1), and the scrubbing of NO from in solution NO to explain the experimentally observed first-
gas streams (2). Nitric oxide reduction has also been inves- order kinetics in NO solution concentration.
tigated as the cathodic reaction in fuel cells (3), because of
its high reduction potential.
The reaction scheme suggested for the formation of NH3
proceeds through the reduction of adsorbed NOads
:
The electrodes employed in the electrochemical reduc-
tion of NO are usually noble transition metals, because they
are the most active catalysts (4) and show the least for-
mation of metal oxides. Palladium has the highest activity
and selectivity to N2 (3), and therefore is the catalyst of
NO(aq) + → NOads
fast
NOads + H+ + e ↔ HNOads
fast
HNOads
+
→ unknown intermediate
r.d.s.
Unknown intermediate + 5H+ + 4e
→ NH+4 + H2O +
fast.
1 To whom correspondence should be addressed. Fax: ++31-40-2455054.
E-mail: m.t.m.koper@tue.nl.
[2]
387
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

388
DE VOOYS ET AL.
Presumably, the rate-determining step in this scheme in- for all RDE and adsorbate experiments. DEMS measure-
volves a breaking of the N–O bond. ments were performed employing a Balzers Prisma QMS
Some nitrogen (N2) is formed in the intermediate poten- 200 mass spectrometer. Details of the setup are described
tial region (0.2–0.4 V vs RHE), but we did not discuss the elsewhere (6). The DEMS signals of N2, N2O, and NO were
mechanism of its formation in our previous work. However, calibrated by oxidizing a monolayer of CO and measuring
given the fact that N2 is the most desirable product and that the amount of CO2. The signal was corrected for differences
its selective formation is apparently quite sensitive to the in sensitivity and fragmentation probability (7).
nature ofthe electrode surface, more detailed investigations
into the N2 formation pathway are clearly of interest.
A Hg/HgSO4 reference electrode was used for all
measurements in acidic solutions. In alkaline solutions a
In this paper, we study the metal dependence of the elec- Hg/HgO reference electrode was used. All potentials re-
trocatalytic reduction of NO. The metals investigated are ported in this paper, however, are converted to the RHE
Ru, Rh, Ir, Pd, Pt, and Au. We will show that the two sep- scale. All measurements were performed at room temper-
arate pathways for N2O and NH3 exist for all these elec- ature (20 C).
trodes, and we will argue that they take place via reaction
All glassware was cleaned in boiling H2SO4/HNO3 to re-
schemes similar to those on Pt. Most importantly, we will move organic contaminations. All solutions were prepared
present more detailed results on the formation of N2 on the with p.a. grade chemicals (Merck) and Millipore Gradient
different metal electrodes. From these results, we will sug- A20 water. All solutions were deaerated by purging with
gest that the key intermediate in the N2 formation is N2O argon. NO was bubbled through two 2 M KOH washing
rather than surface-bonded Nads
.
flasks in order to remove NO2 (8).
Adsorbate studies were performed by saturating the sur-
face in a solution of saturated NO or 2 mM NaNO2 under
potential control (usually 0.37 V) in a 0.1 M H2SO4 solution
in order to avoid oxidation of the surface by NO (9).
2. EXPERIMENTAL
Rotating disk electrodes (RDEs) were used in a home-
made setup. Platinum, palladium, and gold were pretreated
by repeated cycling in the hydrogen and oxygen evolution
region in 0.1 M H2SO4, after which the electrolyte was re-
placed with clean electrolyte. Rhodium, iridium, and ruthe-
nium were electrodeposited from the metal trichloride so-
lution at 0.1 V (vs RHE) onto disk electrodes made from the
same metal prior to a measurement in the electrochemical
cell, and traces of chloride were removed by thorough rins-
ing while the electrode was kept at 0.2 V. Iridium was de-
posited at ca. 80 C. The blank cyclic voltammogram of the
disk electrode was compared to that of a flag electrode, to
check for the absence of contaminations and surface oxides.
Differential electrochemical mass spectroscopy (DEMS)
electrodes consisted of platinum gauzes with the metal of
interest electrodeposited onto it, except for gold, in which
case a gold gauze was used. The electrodes were pretreated
in the same manner as the rotating disk electrodes.
3. RESULTS AND DISCUSSION
3.1. Selectivity of the NO Reduction
We will first discuss the various products of the NO reduc-
tion and the potential dependence of their transformation.
The selectivity of the NO reduction in the presence of NO
in the solution has been measured both in the DEMS setup
by measuring the amount of N2O and N2 formed, and in
the RDE setup by determining the number of electrons
transferred per NO molecule from the Levich equation:
1
1
1
=
+
.
I
Ikin
0.62nFC D2/3 1/6ω1/2
In this equation, Ikin is the kinetic limited current den-
sity, n is the number of electrons per NO molecule, C is
Adsorbate studies were performed on a flag electrode of the bulk concentration of NO, D is the diffusion constant
the pure metal. All flag electrodes were flame annealed of NO in water, and is the kinematic viscosity. The follow-
prior to each measurement, except for gold, which was ing values were used: C = 1.40 10 6 mol cm 3, D = 2.5
5
3
1
cleaned by repeated cycling between the hydrogen and oxy- 10 cm2 s 1, and = 8.5 10 cm2 s (10). The kinet-
gen evolution regions. The electrodes were quenched in ically limited current cannot be determined exactly at po-
clean water under an argon atmosphere, except for ruthe- tentials where several reactions occur simultaneously, and
nium, which was quenched in an argon/hydrogen atmo- was therefore only determined in potential windows where
sphere to reduce residual surface oxides (ruthenium is more the selectivity to N2O was 100% .
susceptible to the formation of irreversible surface oxides
On all metals the potential window in which NO re-
than the other metals). The cyclic voltammogram in a clean duction takes place can be divided into three regions: at
solution was taken to check for the absence of surface ox- high potentials (0.4–0.7 V) N2O is the main product, N2 is
ides and contaminations.
formed with varying selectivity at intermediate potentials
The counter electrode in all cases consisted of a plat- (0.2–0.4 V), whereas NH3 is the main product at low poten-
inum flag. An AUTOLAB Pgstat 20 potentiostat was used tials (0–0.3 V). It is known that hydroxylamine (H2NOH)

MECHANISM OF THE NO ELECTROREDUCTION ON TRANSITION METALS
389
TABLE 1
Tafel Slope of the Adsorbed NO Reduction, in mV/dec
Pt
Rh
Ir
Ru
0.1 M H2SO4
0.1 M KOH
54 3(5)
58 3(5)
70
106
3
3
76
88
5
5
66 10
63
1
will be discussed in three separate sections, according to
the three potential regions.
3.2. Reduction of NO to NH3
The reduction of NO to NH3 can be studied by contin-
uous NO reduction in the potential region 0–0.3 V, or by
the reduction of an NO adsorbate layer in a clean NO-free
electrolyte. In the latter case, information about the rate-
determining step in the overall reaction scheme can be ob-
tained relatively easily by measuring a so-called Tafel plot,
which gives the potential dependence of the overall reac-
tion rate. Under the assumption that the reduction rate is
first- (12) or second-order (13) in the adsorbate coverage,
it can be shown that a plot of the peak potential (i.e., the
potential at which a maximum current is measured during
the reductive stripping voltammetry) vs the logarithm of
the scan rate is equivalent to a Tafel plot. Lateral interac-
tions between NO molecules are neglected, as the coverage
at the peak potentials is relatively low (ca. 0.2, i.e., in ap-
proximately half of the maximum coverage).
Table 1 summarizes the slopes of the Tafel plots obtained
for the NO adsorbate reduction on Pt, Rh, Ir, and Ru in
acidic and alkaline solution. The maximum coverage of NO
is in all cases similar (0.4–0.5 monolayer), and similar to val-
ues reported for single-crystal surfaces (14–16). Pd and Au
are not included, as in the case of palladium the peak poten-
tial overlaps with the hydrogen evolution reaction, and in
the case of gold because no NO adsorbate layer is formed.
As an example a typical Tafel plot for the NO reduction
of Ru in acidic solution is shown in Fig. 2. A value of ca.
60 mV/dec indicates the existence of an electron-transfer
FIG. 1. Selectivity of the reduction of NO in the solution. Solution
saturated with NO, (a–f) 0.1 M H2SO4, (g–l) 0.1 M KOH, (a, g) Pt,
(b, h) Ru, (c, i) Ir, (d, j) Rh, (e, k) Pd, (f, l) Au. Solid lines and filled
circles, selectivity to N2O; dotted lines and rectangles, selectivity to N2.
The error bars given in (a) and (g) also apply to the other figures.
can be formed as a minor product at low potentials (0 V)
on platinum (11); however, as the number of electrons per
NO molecule at low potentials is close to 5, indicating that
NH3 is the major product, and a quantitative determination
is quite complicated, we will exclude H2NOH formation
from our discussion. Figure 1 shows the selectivity to N2O
and N2 of the six metals studied in this paper, as obtained
from the DEMS measurements. At high potentials both the
electrical current and the DEMS signal become very small,
resulting in a large experimental uncertainty, reflected in
some of the selectivities reported being higher than unity.
The amount of N2 produced varies with the metals used,
as do the boundaries of the three potential regions. The se-
lectivity to N2 on palladium, for instance, is very high and
covers a larger potential window than on the other met-
als. As will be further discussed below, we believe that the
similarities in selectivity are an indication that the reaction
FIG. 2. Dependency of the peak position of the NO reduction with
the scan rate on ruthenium in 0.1 M H2SO4; surface at maximum NO
mechanism is similar on all transition metals. The results coverage.

390
DE VOOYS ET AL.
TABLE 2
3.3. Reduction of NO to N2O
Peak Potential of the NO Adsorbate Reduction at 5 mV/s, Com-
pared to the Potential at Which the Selectivity of the Reduction of
Solution NO Changes According to the RDE
The Tafel slopes of the NO reduction to N2O on the var-
ious metals, as derived from the kinetic limiting current of
the RDE measurements, are given in Table 3. The Tafel
slope was found to be close to 120 mV/dec in three cases:
platinum and ruthenium in acidic solutions, and iridium in
alkaline solutions. This implies that the first electron trans-
fer is rate-determining in these three cases. In all other cases
the Tafel slope was found to be significantly higher than
120 mV/dec, indicating that the rate-determining step is a
chemical step prior to the electrochemical steps. The values
given in Table 3 are significantly different from those given
by Colucci et al. (8) (78, 116 and 408 mV/dec on respectively
Pd, Rh, and Ru in acidic solutions), as determined from
Pt
Pd
Rh
Ir
Ru
0.1 M H2SO4
0.1 M KOH
Epeak
RDE
Epeak
RDE
0.21
0.25
0.18
0.2
0.01a
0.11
0.2
0.14
0.15
0.17
0.25
0.10
0.15
0.102
b
b
0.03a
0.101
0.2
b
a Measured on a gold electrode with a thin (2–3 ML) palladium over-
layer, and at 1 mV/s.
b There is no change in the selectivity.
step in equilibrium, followed by a rate-determining chem- voltammetry at stationary electrodes. Since reactions with
ical (potential-independent) step. The value reported for mass-transport limitations should preferably be studied at
platinum (5), together with the observed pH dependence, rotating electrodes, and because we took extreme care to
has led us to suggest that the NO reduction on platinum avoid surface oxidation of the less noble transition metals
follows scheme [2] discussed in the Introduction. The very (17, 18), we believe our values reflect more accurately the
similar Tafel slopes observed for the other metals, and the “real” Tafel slopes.
similar pH dependence (Table 2), may be interpreted ac-
cording to a similar scheme, where a higher Tafel slope NO reduction to N2O on platinum, palladium, rhodium,
could point toward a slower first electron-transfer step. and ruthenium includes only species which are adsorbed
Colucci et al. (8) suggested that the mechanism of the
We believe that the continuous reduction of NO to NH3 at the surface. Gootzen et al. (19) suggested for platinum
in the potential range of 0–0.3 V takes place through the electrodes that the key step in the N2O formation is the
same mechanism as the adsorbate reduction, as the on- dissociation of NO, which also implies that only adsorbed
set in NH3 production in the continuous reduction occurs species are involved in the formation of N2O. However,
at roughly the same potentials as the adsorbate reduction we find that NO from the solution must be involved in the
(Table 2). It is difficult to obtain kinetic information reaction scheme for two reasons. The first reason is that
however, as the selectivity to NH3 at 0.2–0.3 V is not 100% . the potential window in which the reduction of adsorbed
Two metals behave differently from the other metals and NO takes place (0–0.3 V) is much lower than the poten-
have to be discussed separately, namely palladium and gold. tial window considered here (0.4–0.7 V). Second, N2O is
On palladium the reduction of adsorbed NO takes place at not produced during the reduction of adsorbed NO. Since
significantly lower potentials than on the other noble met- the only difference between the reduction of adsorbed NO
als (ca. 0 V). Also, the potential at which NH3 is formed and the reduction of NO at high potentials is the presence
in the continuous reduction is low compared to the other of dissolved NO, solution NO must be directly involved in
metals (ca. 0.1 V). On gold the selectivity to N2O does not the reaction sequence to produce N2O. Since these obser-
drop to zero, as on the other noble metals, since the adsorp- vations hold for all metals studied here, we suggest that the
tion of NO on gold is weak and little formation of NH3 is formation of an (NO)2 dimer as the rate-determining step,
observed.
as has been proposed for platinum (5), is also valid for the
The mechanism of the formation of NH3 from NO must other metals. The higher Tafel slopes observed for some of
include the breaking of the N–O bond. The nature of this the systems could be explained by assuming that the first
step, and its relationship to the rate-determining step, is electron transfer shifts to a stage after the rate-determining
an important mechanistic issue. In scheme [2], the dis- step, and the surface dimerization reaction becomes the
sociation of the N–O bond is assumed to take place af- rate-determining chemical step.
ter the protonation. However, NO is known to dissoci-
ate at room temperature on metals like rhodium, iridium,
and ruthenium, and hence we must consider this possi-
bility in the overall reaction scheme. Since the possibility
of NO dissociation is also an important mechanistic issue
in the formation of N2O and N2, we postpone its discus-
sion to Section 3.5, where we will discuss a number of
general mechanistic features in relation to the gas-phase
reduction.
TABLE 3
Tafel Slopes of the NO Reduction between 0.4–0.7 V in a Solution
Saturated with NO, in mV/dec
Pt
Pd
Rh
Ir
Ru
Au
0.1 M H2SO4 117
4
160 18 436 32 201 39 133 13 356 16
0.1 M KOH 322 23 246 26 201 39 113 34 242 21 207 18

MECHANISM OF THE NO ELECTROREDUCTION ON TRANSITION METALS
391
It is unclear whether dissolved NO reacts directly from alkaline solutions. The activity found in alkaline solu-
the solution, or from a weakly adsorbed state at the sur- tions increases in the order Au < Ru Ir < Rh Pt
face. Either possibility will lead to the same N2O formation Pd, with the onset of the reaction being 0, 0.1, 0.4, 0.45,
kinetics and the same adsorbate reduction kinetics, since 0.45, and 0.6 V, respectively (Fig. 3). This ordering is com-
this weakly adsorbed NO will desorb when the solution is parable to the production of N2 in alkaline solutions shown
replaced with clean electrolyte.
in Fig. 1, the potentials of the onset showing a reasonable
Finally, we note that reaction scheme [1] predicts the rate agreement with the onset of the N2O reduction (Au
of the reaction to be first-order in the concentration of solu- Ru Pt < Rh < Ir Pd, the onset of the reaction being 0,
tion NO, because the surface is always covered with NOads 0, 0.45, 0.45, 0.55, and 0.6 V, respectively). Iridium is more
and hence only the amount of NO(aq) changes with the active in the N2 production than suggested by its N2O re-
NO concentration. A kinetic order of unity has indeed been duction activity, which makes it an exception.
observed on platinum in acidic solutions. Due to the large
To compare the activity and selectivity in acidic solutions
error bars in the determination of the kinetic limited cur- is more problematic, since the N2O reduction is known to be
rent (Table 3), we have not pursued these measurements very sensitive to the competitive adsorption of SO24 anions
for the other systems, but both DEMS measurements and (21, 22). The measurements should therefore be carried
an analysis of the RDE data for palladium, using the pro- out in an HClO4 solution instead of H2SO4. Unfortunately,
cedure suggested by Markovic et al. (20), suggest a kinetic the activity of rhodium and iridium cannot be obtained in
order close to one.
HClO4, since these metals can reduce ClO4 to Cl (26),
which is known to adsorb strongly. In general, however, the
series for the N2 production from NO in acidic solutions
is similar to the series of the N2O reduction activity. Gold,
for instance, is inactive in both cases, followed by iridium,
rhodium, and ruthenium, whereas palladium is very active.
Platinum does not fit in the series, as the rate in N2O reduc-
tion in both HClO4 and H2SO4 is higher than on palladium,
whereas palladium is more active in the selective formation
of N2 from NO.
The qualitative agreement between the N2O reduction
activity and the selectivity in the NO reduction to N2 is a
strong argument in favor of the “N2O serial pathway.” Also
the fact that no N2 is produced during the NO adsorbate
reduction in the absence of NO in the solution disfavors the
“adsorbate parallel pathway.” In fact, the NO adsorbate is
generally not reactive at all at potentials for which N2 is
produced during the continuous NO reduction.
3.4. Reduction of NO to N2
As mentioned in the Introduction, there is only scant
mechanistic information available on the formation of N2
from the electrocatalytic NO reduction. However, it seems
that one may assume two alternative working hypotheses.
The first hypothesis is that N2 is formed by the reduction
of N2O, i.e., in series with the N2O formation. This is a
known reaction in electrocatalysis that has been studied
by a number of authors (21–25). The second hypothesis is
that N2 is formed by a reaction of surface species (such as
NOads, Nads, NOHads, or other yet unidentified species), i.e.,
in parallel with the N2O formation.
If the first hypothesis would be true, i.e., N2 would be
formed from the reduction of N2O, one would expect a cor-
relation between N2O reduction activity and the selectiv-
ity toward N2 in the reduction of NO. Figure 3 gives the
activity of the various metals in the N2O reduction in
3.5. Mechanistic Issues and Comparison to Gas-Phase
Reduction of NO
3.5.1. NO dissociation. As noted in Section 3.2, it is
known that NO may dissociate at room temperature on
metals such as Rh, Ir, and Ru, whereas it is generally ad-
sorbed molecularly on Pt and Pd. An overview of the NO
dissociation behavior on metal surfaces was recently given
by Brown and King (27). At higher coverages, the NO dis-
sociation process may be inhibited because of the lack of ad-
jacent empty sites or because strong lateral interactions be-
tween the dissociation products render the molecular state
more stable. The necessity of having empty surface sites for
triggering NO dissociation has been shown recently in de-
tailed UHV surface science studies of NO reactivity on Rh
(28, 29). In most UHV studies, the Nads species is consid-
ered as an intermediate in the N2O, N2, and NH3 formation.
Hence, we will have to consider the possibility of NO dis-
sociation on Rh, Ru, and Ir in the electrochemical context.
FIG. 3. Activity of the transition metals and gold in the N2O reduc-
tion: 0.1 M KOH saturated with N2O. Crosses, Pd; squares, Pt; circles, Rh;
thick solid line, Ir; plusses, Ru; triangles, Au.

392
DE VOOYS ET AL.
the NO adsorbate reduction on Pt, where no such positive
shifts in reduction potential with lower NO coverage are
observed.
If NO dissociation plays a role on Rh, Ir, and Ru, then
the observed Tafel slopes for the adsorbate reduction may
suggest that it is not the rate-determining step in the ammo-
nia formation. However, it should be realized that it is not
a priori clear to what extent the NO dissociation has to be
considered as potential or even pH dependent, as the over-
all NO dissociation process at electrochemical interfaces
involves protonation of the oxygen:
FIG. 4. Reaction of NO adsorbed under OCP (open circles) and at
0.45 V (filled triangles) on polycrystalline rhodium compared to the blank
(solid line); scan rate, 20 mV/s.
NO + 2H+ + 2e → Nads + H2O.
[3]
It is obvious that the above experiments are not decisive
in pinning down the possible role of NO dissociation on
the less-noble transition metals Ru, Ir, and Rh. More direct
spectroscopic information is clearly needed.
The results in Fig. 4 do suggest, however, that at high
NO coverages, which seems the relevant surface condition
for the continuous NO reduction experiments, NO dissoci-
ation is not likely to take place at potentials above 0.3 V.
Hence, it is suggested that NO dissociation does not play a
significant role during N2O and N2 formation. Evidence for
alternative mechanisms, in relation to gas-phase and UHV
experiments, is discussed in the next two sections.
It is difficult to obtain any direct evidence for NO dissoci-
ation from electrochemical experiments. Gootzen et al. (19)
suggested that the observation of an oxide reduction peak
after NO adsorption on Pt at open-circuit potential (OCP)
was due to the reduction of the adsorbed oxygen formed
from NO dissociation. However, we showed recently that
the peak is observed only on roughened electrodes (5), sug-
gesting quite a different mechanism, and in keeping with
the gas-phase situation where NO adsorbs molecularly on
Pt at room temperature. Interestingly, an oxide reduction
peak is observed, at ca. 0.5 V, on a smooth flame-annealed
Rh electrode after adsorption of NO at OCP (ca. 0.8 V),
3.5.2. NO dimerization. The reaction scheme [1] sug-
as illustrated in Fig. 4. When NO is adsorbed at 0.45 V, gested for the N2O formation on Pt involves the formation
no oxide reduction peak is observed, and a much higher of a (protonated) surface-bonded (NO)2 dimer. The very
NO reduction charge is measured (see Fig. 4), indicating a similar conditions under which N2O is formed on the other
higher coverage of NO under these conditions. However, metals is suggestive of a more general applicability of this
these observations still do not prove the occurrence of NO reaction scheme.
dissociation. On Rh, surface oxides are known to be more
In fact, there is a growing surface science literature on the
stable than on Pt, and therefore on Rh, certainly at a po- formation of NO dimers on silver, copper, molybdenum,
tential of 0.8 V, a more competitive co-adsorption of oxides tungsten, and palladium, and it has also been suggested to
and NO is expected than on Pt. The peak at 0.5 V can hence exist on platinum and rhodium (27). It has been shown that
be explained by the reduction of co-adsorbed oxides, which on silver the NO dimer is the key intermediate in the N2O
are not formed by NO dissociation, but similar to the for- formation. NO is adsorbed weakly on Ag (27). Apparently,
mation of surface oxides on roughened platinum.
strongly adsorbed NO is not required for the formation of
An interesting observation to be made from Fig. 4 that the NO dimer or the production of N2O. Theoretical DFT
could point toward the role of NO dissociation is the fact studies have also supported the existence of the NO dimer
that the apparently lower coverage of NO formed at 0.8 V on Ag (30).
starts to be reduced at a significantly less negative potential
Even though we are not aware of similar UHV-based ev-
than a saturated layer formed at 0.45 V. This could mean idence for the NO dimer on Au, the generic similarity be-
that the surface sites that become available after reduc- tween Ag and Au suggests that it is reasonable to assume a
tion of the surface oxides trigger the NO dissociation. The similar N2O formation mechanism on Au. This would cer-
saturated NO adlayer is not reactive or is much less reac- tainly explain the unique selectivity of Au in reducing NO
tive at these potentials, as is also evidenced by the obser- to N2O as observed in our electrochemical study. Silver in
vation of a stable NO stretching peak in FTIR experiments fact has a similarly high selectivity toward N2O in the elec-
of NO on single-crystal Rh (14). A similar—though less trochemical NO reduction (31). Also, this weakly adsorbed
pronounced—shift in the reduction peak is observed when state of NO involved in the dimer formation may explain
a nonsaturated NO adlayer is formed at 0.45 V by dosing the first-order kinetics in the NO solution concentration,
from a less concentrated NO solution. These observations as mentioned in the Introduction. Interestingly, there is no
are in stark contrast with the coverage effects observed for clear catalytic effect of any metal in reducing NO to N2O,

MECHANISM OF THE NO ELECTROREDUCTION ON TRANSITION METALS
393
TABLE 4
All transition metals show a high selectivity toward N2O
at high potentials and, with the exception of Au, a high
selectivity toward NH3 at low potentials. At intermediate
potentials, all metals, again with the exception of Au, pro-
duce N2, though with varying selectivity.
Current Density of the NO Reduction in Saturated NO
at 0.42 V, in mA/cm2
Pt
Pd
Rh
Ir
Ru
Au
The NO reduction to N2O is suggested to take place
via the formation of an NO dimer, similar to the mecha-
nism proposed for platinum (5). The varying Tafel slopes
found for the different metals may reflect a shift in the rate-
determining step with the metal or, more precisely, a shift
in the occurrence of the first electron-transfer step in this
mechanism. Despite the varying Tafel slopes, the similar-
ities in the onset of the N2O formation (i.e., at potentials
much more positive than the onset of the adsorbate re-
duction) suggest a common mechanism for the N2O for-
mation on all metals. The existence of an NO dimer has
been proved on various metal surfaces in UHV. Moreover,
its weak adsorption strength would explain why all metals
show a similar activity in the NO reduction to N2O.
We have also presented evidence that the mechanism of
formation ofN2 issimilar on allmetals(except for Au, which
does not produce N2), and involves N2O as the key interme-
diate. This deduction was based on the correlation between
the selectivity of a certain metal toward producing N2 from
NO and its activity in reducing N2O. Metals that actively
reduce N2O also produce more N2 in their NO reduction.
Moreover, no N2 and N2O are formed in the NO adsorbate
reduction in the absence of NO in solution, suggesting that
there is no parallel pathway for N2 formation.
The situation for NH3 formation is less clear. Certainly
for the reduction of the NO adsorbate on Rh (and presum-
ably also Ru and Ir) we cannot exclude the role of NO dis-
sociation, which was not considered for Pt. Tafel slope mea-
surements nevertheless suggest an electron-transfer step to
be involved in, or prior to, the rate-determining step, even
though this cannot be taken as proof for similar mecha-
nisms. At high NO coverages, NO reactivity on Rh is lower,
suggesting a suppression of the NO dissociation by the
blocking of free sites.
0.1 M H2SO4
0.1 M KOH
0.49
0.28
0.55
0.52
0.08
0.33
0.48
0.54
0.11
0.43
0.20
0.62
as is illustrated by the very similar activities observed for all
metals as collected in Table 4. This also strongly suggests the
involvement of weakly adsorbed intermediates in the key
step of the reaction scheme. As mentioned in the previous
section, under these conditions of high surface coverage,
NO dissociation does not seem very likely.
The idea that N2O formation takes place through the
formation of a weakly adsorbed NO dimer offers a con-
sistent explanation of our observations. It would certainly
explain why the reaction is so similar on all the different
metals. However, spectroscopic proof of the existence of
NO dimers under electrochemical conditions is certainly
necessary to validate our interpretation.
3.5.3. N2O as intermediate in N2 formation. The data
presented in Section 3.4 offered evidence for the formation
of N2 from N2O, i.e., in a serial pathway, rather than from
other NO-derived adsorbates in a parallel pathway. This
interpretation is based on the observation that there is a
correlation between the selectivity of a given metal in pro-
ducing N2 from NO and its activity in reducing N2O, and
that no N2, nor N2O, is formed during the NO adsorbate
reduction.
Interestingly, there is a very similar suggestion in the re-
cent gas-phase literature. The formation of N2O has not
been observed under UHV conditions. However, Zaera
and Gopinath (32) have suggested on the basis of isotope
exchange measurements that the formation of N2 under a
constant fluxofNO isnot due to the recombination of two N
adatoms, but rather via an N2O-type intermediate formed
by the combination of an NO adsorbate and N adatom. Ac-
cording to Zaera and Gopinath, this reaction would only
take place under conditions of a high gas-phase pressure of
NO.
Finally, the best electrocatalyst for the selective reduction
of NO to N2 is palladium. At high potentials this is related
to its high activity in reducing N2O to N2. At low potentials
this is related to its low activity in the reduction of adsorbed
NO to NH3.
4. CONCLUSIONS
ACKNOWLEDGMENTS
In this paper, we have studied the mechanism of the elec-
trocatalytic reduction of NO on Ru, Rh, Pd, Ir, Pt, and Au
electrodes by a combination of electrochemical measure-
ments and on-line mass spectrometry. Our objectives were
to assess to what extent the mechanistic schemes for N2O
and NH3 formation recently suggested for the NO reduc-
tion on platinum can also be applied to the other metals. In
addition, we have studied the mechanism of N2 production
from NO.
This research was supported financially by the Netherlands Foundation
for Scientific Research (NWO) and the Royal Netherlands Academy of
Arts and Sciences (KNAW).
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