Electrocatalytic reduction of nitrate on tin-modified palladium electrodes

Article abstract of DOI:10.1016/j.electacta.2014.06.011

The electrocatalytic reduction of nitrate was investigated in acidic media on Sn-modified polycrystalline palladium and Sn-modified palladium film electrodes (deposited on gold) by means of voltammetry in combination with on-line electrochemical mass spectrometry and on-line ion chromatography. The catalytic activity for this reaction is strongly enhanced by modification with Sn as pure Pd is not found to have a measurable activity for the nitrate reduction. Dominant volatile products are found to be N₂O and in a smaller amount N₂, while NH₂OH is found to be the dominant non-volatile product. Furthermore, the influence of hydrogen absorption in Pd is investigated by using a multilayer Pd film and a submonolayer Pd film on a polycrystalline Au electrode. By comparing the product distribution of these electrodes with the bulk Pd electrode the effect of hydrogen absorption is found to be of negligible importance for the nitrate reduction as the product distribution and formation as a function of potential are similar compared to the bulk Pd electrode.

Full text of DOI:10.1016/j.electacta.2014.06.011

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Electrochimica Acta
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Electrocatalytic Reduction of Nitrate on Tin-modified Palladium
Electrodes
Y.Y. Birdja, J. Yang, M.T.M. Koper∗,1
Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrocatalytic reduction of nitrate was investigated in acidic media on Sn-modified polycrystalline
palladium and Sn-modified palladium film electrodes (deposited on gold) by means of voltammetry
in combination with on-line electrochemical mass spectrometry and on-line ion chromatography. The
catalytic activity for this reaction is strongly enhanced by modification with Sn as pure Pd is not found to
have a measurable activity for the nitrate reduction. Dominant volatile products are found to be N2O and
in a smaller amount N2, while NH2OH is found to be the dominant non-volatile product. Furthermore, the
influence of hydrogen absorption in Pd is investigated by using a multilayer Pd film and a submonolayer
Pd film on a polycrystalline Au electrode. By comparing the product distribution of these electrodes with
the bulk Pd electrode the effect of hydrogen absorption is found to be of negligible importance for the
nitrate reduction as the product distribution and formation as a function of potential are similar compared
to the bulk Pd electrode.
Received 15 January 2014
Received in revised form 2 June 2014
Accepted 2 June 2014
Available online xxx
Keywords:
Nitrate reduction
Sn-modification
Pd film
On-line electrochemical mass spectrometry
Ion chromatography
© 2014 Published by Elsevier Ltd.
1. Introduction
the enzymes are then replaced by redox catalysts. It is very impor-
tant to understand this electrocatalytic process from a fundamental
Living organisms need nitrogen compounds in order to produce
more complex organic molecules such as proteins, nucleic acids and
amino acids. Air is composed of approximately 79% of dinitrogen
gas (N2), which is the simplest and most stable form of nitrogen.
However, N2 cannot be used in this form by most organisms. Plants
use nitrogen which is incorporated in compounds such as nitrate
ions (NO3−), ammonium ions (NH4+) and urea ((NH2)2CO) while
animals consume plants or herbivors for their nitrogen supply. The
inorganic compounds of nitrogen are controlled by the so-called
nitrogen cycle. Human activities such as combustion and agricul-
ture affect the nitrogen cycle, which leads to an increase in the
amount of nitrate in groundwater. According to the Council of
the European Union for the water quality for human consump-
tion, nitrate concentrations should be kept below 50 ppm [1]. A
higher concentration can pose health threats such as blue baby syn-
drome, cancer and eutrophication of water reservoirs. From this
point of view the removal of nitrate is an important process. In
nature this is taken care of by the bacterial denitrification process.
The electrochemical removal of nitrate is similar to this process as
point of view in order to identify possible limitations of the cata-
lysts and other factors that complicate the implementation of this
process on a large scale. Nitrogen compounds formed by electrocat-
alytic reduction of nitrate can also be of economic value. Examples
are: N2O as anesthesia, NH3 as fertilizer and NH2OH as precur-
sor for the nylon production. The formation of N2 from nitrate is
important as N2 is harmless for the environment. Most research
has been done on noble metal catalysts, often modified by another
adatom. Platinum has been widely studied for the nitrate reduction
together with the influence of various promoters. The promoters
influence the activity and selectivity of the nitrate reduction sig-
nificantly [2–5]. It has been found that the rate determining step
is the reduction of (adsorbed) nitrate to nitrite, which is the step
that is enhanced by the promoters, for which p-block metals are
often used [6]. Unfortunately, the exact role of these promoters is
not well understood.
Palladium itself is a poor catalyst for the nitrate reduction but
more active compared to platinum [7–9]. Research has been done
on the modification of Pd for promoting the nitrate reduction.
De Vooys et al. investigated Pd/Cu catalysts, on which nitrate is
reduced to N2 with a selectivity that was found to be inversely
proportional to the Cu coverage [10]. Here Cu enhances the rate
determining step of the nitrate reduction, whereas Pd is active
for NO and N2O reduction [11]. Also Ge has been investigated as
∗
Corresponding author. Tel.: +31 71 5274250; fax: +31 71 5274451.
E-mail address: m.koper@chem.leidenuniv.nl (M.T.M. Koper).
1
ISE member.
http://dx.doi.org/10.1016/j.electacta.2014.06.011
0013-4686/© 2014 Published by Elsevier Ltd.
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promoter for the nitrate reduction on Pd. N2O was found to be
the main product from a certain minimum Ge coverage [12], with
the selectivity towards hydroxylamine increasing for higher Ge
coverages [13]. Shimazu et al. have studied tin modified Pd [14].
They have investigated the electrocatalytic activity of Pd thin films
of varying thickness in HClO4, and found that the electrocatalytic
activity of PdSn electrodes is strongly dependent on the thickness of
the Pd layers and on the Sn coverage. However, the product distri-
bution, with the dominant product N2O, was not affected by these
parameters.
Palladium is well known for its unique ability to absorb hydro-
gen [15]. The hydrogen evolution on a metal usually takes place
following a Volmer-Heyrovsky-Tafel process [16–18]. In case of
Pd, there exists a so-called penetration reaction where adsorbed
H atoms penetrate the surface layer and move into a bulk site [17].
This occurs at similar potentials as the hydrogen UPD and HER
[15,19,20]. As a result, large currents are observed, which compli-
cate the study of surface processes such as anion adsorption and the
oxidation/reduction of adsorbed species. The use of Pd thin layers
deposited on a substrate circumvents this problem as the amount
of hydrogen that can be absorbed in the bulk is limited and there-
fore it is possible to differentiate between surface processes. In the
literature Pt and Au are often used as substrate for the deposition
of Pd films.
In this work Sn, the best promoter for the nitrate reduction on
Pt, is investigated for the enhancement of the reduction of nitrate
on Pd. A comparison will be made of the activity and the selec-
tivity towards volatile and non-volatile products between bulk Pd
electrodes and Pd multi- and submonolayer films in the absence
and presence of strongly adsorbing anions. Our study will employ
a combination of cyclic voltammetry with online electrochemical
mass spectrometry and online ion chromatography, as this allows
a direct and detailed comparison between activity, selectivity and
electrolyte compositions.
the surface oxide of a stable voltammogram obtained at 50 mV/s
was used to calculate the active surface area of this electrode.
For the bulk Pd working electrode the geometric area is used as
active surface area. A palladium wire and a reversible hydrogen
electrode (RHE), mounted via a Luggin capillary in the same elec-
trolyte, were used as counter and reference electrode, respectively,
unless mentioned otherwise (e.g. for special procedures as the Pd
deposition on Au or Cu UPD on Au). Electrochemical cleaning and
blank voltammetric experiments were performed in a cell sepa-
rate from the cell in which experiments on nitrate reduction were
performed.
The voltammetric measurements for the On-Line Electrochem-
ical Mass Spectrometry (OLEMS) were carried out with an Ivium
A06075 potentiostat and the OLEMS measurements with an EvoLu-
tion mass spectrometer (European Spectrometry Systems Ltd.). The
system, a Quadrupole Mass Spectrometer Prisma QMS200 (Pfeif-
fer), is brought to vacuum with a TMH-071P turbo molecular pump
(60 l/s, Pfeiffer) and a Duo 2.5 rotary vane pump (2.5 m3/h, Pfeiffer).
During voltammetric measurements a hydrophobic tip placed close
underneath the working electrode (∼ 10 ꢁm) collected volatile
products from where they flow to the MS. The pressure inside
the MS was always kept below 10−6 mbar. During OLEMS mea-
surements the voltammetric scan rate was always 1 mV/s. The
configuration and pretreatment of the tip is described elsewhere
[22,23]. Before use the tip is cleaned in a solution of 0.2 M K2Cr2O7
in 2M H2SO4 and rinsed with ultrapure water. All mass fragments
decay during measurement, because it takes a long time for the
pressure to reach a steady state. We correct for this by subtracting
a double exponential function fitted to data points where there is
no activity observed. In this work all the results are background cor-
rected in this manner. Furthermore, fragmentation ratios were used
for NO and N2 as these mass fragments are partly the result of frag-
mentation of N2O. The fragmentation ratio between N2O (m/z = 44)
and NO (m/z = 30) was found to be 0.202 while between N2O and N2
(m/z = 28) it was 0.0685 [24]. A SEM voltage of 1500 V was used for
all species (m/z = 2, 14, 15, 28, 30, 31, 33, 44, 45, 46). Unfortunately it
is not straightforward to deduce quantitative information in terms
of faradaic efficiencies for the reaction products from OLEMS mea-
surements. Our setup is able to detect volatile products only, while
non-volatile products are formed as well. It is difficult to calibrate
the OLEMS together with e.g. ion chromatography simultaneously
as this is required to be able to derive the quantitative amounts of
all the reaction products. Furthermore a whole range of products
is formed depending on the applied potential. It would be better
to perform long term electrolysis at a single potential to calculate
faradaic efficiencies.
2. Experimental
The voltammetric experiments were all carried out at room
temperature in a typical three-electrode, one compartment elec-
trochemical cell. First the cell and all glassware were boiled in
a 1:1 concentrated sulfuric and nitric acid mixture. Ultrapure
water (Millpore MilliQ, resistivity > 18.2 Mꢀ· cm) was used to
boil, rinse and clean all glassware prior to each experiment and
to prepare the electrolytes with Suprapur (Merck) reagents. The
details of the cleaning procedure can be found elsewhere [21].
Before each measurement argon (purity grade 6.0) was purged
through the cell for deaeration. The Sn-modification of Pd was
done using SnCl2 (99.99+%, Sigma-Aldrich). Other used chemicals
were CuSO4 (99.995+%, Sigma-Aldrich) and HNO3 (Merck, supra-
pur, 65%).
In this work a polycrystalline palladium electrode and a
(ultra)thin palladium film deposited on a polycrystalline gold elec-
trode were used as working electrodes. The bulk Pd electrode was
pretreated by immersing in concentrated nitric acid for 5 min-
utes. Next the electrode was polished mechanically with alumina
pastes with particle sizes of 5 ꢁm and 0.3 ꢁm followed by son-
ication in ultrapure water. Before the experiment the electrode
was cycled in the blank electrolyte until a stable voltammogram
was obtained to verify a clean state of the surface. The gold elec-
trode for the Pd films was cleaned by first applying a potential of
+10 V in a 10% H2SO4 solution for 30 seconds followed by 15 sec-
onds rinsing in a 6 M HCl solution. After this the electrode was
flame-annealed and cooled down in air. In the blank electrolyte,
electrochemical cleaning consists of 200 cycles between 0 and 1.75
VRHE at a scan rate of 1 V/s. The charge needed for the reduction of
The IC experiments were performed on an ion chromatography
unit (Shimadzu, Prominence), which is equipped with a conductiv-
ity detector (CDD-10A vp, Shimadzu). The voltammograms related
to the IC measurements were recorded with an Autolab poten-
tiostat (Pgstat20). The samples analyzed with IC were collected
with an automatic fraction collector (FRC-10A, Shimadzu) during
voltammetry by a small teflon tip positioned close to the working
electrode in the cell [25]. The parameters for the fraction collector
(flow rate and sample volume) were set in such a way that during
voltammetry with a scan rate of 1 mV/s, each sample contains the
average reaction products over a 60 mV potential difference (for
the OLEMS measurements this potential range is approximately
5 mV at a scan rate of 1 mV/s). By employing an IC Y-521 cation
+
+
column (Shodex) the cations NH4 and HONH3 were detected. A
2.5 mM nitric acid solution (Merck, Suprapur, 65%) was used as elu-
ent. The column temperature was set to 30 ◦C and the flow rate of
the eluent at 0.8 ml/min. The cation concentration of the analyzed
samples could be calculated by the use of standard solutions of the
corresponding cation for characterization of the retention time and
calibration of the concentration.
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3. Results & Discussion
0
-5
3.1. Nitrate reduction on bulk palladium
Blank
-
NO
3
-10
3.1.1. Unmodified palladium
-
NO on PdSn
3
In Fig. 1 the blank voltammogram of the bulk Pd electrode is
shown together with the CV for nitrate reduction on this electrode.
The current is mainly due to hydrogen adsorption/absorption and
desorption. With nitrate in the solution (0.1 M HNO3) the current
remains unchanged which suggests that nitrate is not reduced on
a bare Pd electrode. This was verified with on-line electrochemical
mass spectrometry and ion chromatography. No significant amount
of reaction products was observed in this potential range. This is in
agreement with the results reported previously [26].
-15
2
N O
2
N
2
1
0
0.0
0.1
0.2
0.3
0.4
0.5
E vs RHE/V
3.1.2. Palladium modified with tin
Fig. 3. Mass signals for the nitrate reduction on PdSn in 0.1 M H2SO4. Scan rate:
1 mV/s
The modification of the Pd electrode with Sn adatoms (PdSn)
was carried out by spontaneous deposition with the Pd electrode
immersed in a 250 ꢁM SnCl2 solution. The time during which the
electrode was held in the solution determines the coverage of tin on
the surface. The CVs of the modified and unmodified Pd electrode
in blank solution at a relatively high scan rate of 1 V/s are shown in
Fig. 2. The reason that such a high scan rate was used is to discrim-
inate between the adsorption/absorption of hydrogen into the Pd
bulk with and without the presence of Sn adatoms on the electrode
surface.
absorbs hydrogen in the bulk, which occurs in the same potential
region. An alternative estimate (although also rather uncertain) can
be based on the current observed at the most negative potential,
which we assume to be mainly due to H adsorption/absorption and
ꢂ
ꢃ
j
PdSn
H2 evolution: ꢂSn = 1 −
. Furthermore, one can observe that
j
Pd
there is still Sn present on the surface after nitrate reduction, which
suggests that Sn on the electrode surface is stable. However, it was
observed that more scans lead to a slight decrease in the current
for the nitrate reduction. This is probably due to the Sn desorp-
tion and poisoning of the electrode by adsorbed products. In this
study we worked with a single Sn coverage of approximately 60-
80% as obtained by keeping the electrode for 60 seconds in the
Sn2+-containing solution.
Fig. 2 shows that the hydrogen adsorption/ desorption cur-
rent is diminished due to Sn adsorption. The ratio of the charge
accompanied by the hydrogen desorption on the modified (qPdSn
)
and unmodified Pd surface (qPd) can be used to estimate the
ꢀ
ꢁ
q
q
PdSn
Sn-coverage of the electrode by ꢂSn = 1 −
. The charge is
obtained by integrating the voltammogram inPtdhe hydrogen des-
orption region as seen by the shaded areas in Fig. 2. It should be
noted that this value for the coverage is a rough estimation as Pd
3.1.3. On-Line Electrochemical Mass Spectrometry
The formation of volatile reaction products during voltammetry
is determined by OLEMS. While an unmodified Pd electrode shows
no volatile products, a Sn-modified Pd electrode shows several
volatile reaction products. The CV together with the mass spec-
tra are shown in Fig. 3. N2O is the dominant product of the nitrate
reduction on Sn-modified Pd. Small amounts of other mass frag-
ments (m/z = 28, 14, 30, 45, 46) are also found. Mass signal 30 is
associated with NO, while mass signals 45 and 46 are assigned to
the nitrogen isotopes (N*) or oxygen isotopes (O*) of N2O as they
follow the same trend. The observation of a small amount of N2
(m/z = 28) suggests that further reduction of N2O is possible on a
PdSn surface.
1
0
-1
-2
Blank
0.1 M HNO
3
-3
0.0
0.1
0.2
0.3
0.4
0.5
E vs. RHE/V
3.1.4. Ion Chromatography
The Faraday current in Fig. 3 for the Sn-modified Pd electrode
differs from the profile of the mass signal vs potential as the mass
signal shows a peak at ∼ 0.12 V and ∼ 0.22 V for the negative-
and positive-going scan respectively, while the current does not.
It is likely that other non-volatile products are formed and these
were measured by ion chromatography. For the measurement
of non-volatile products, samples were collected during a linear
voltammetric scan in the same potential range and analyzed with
IC. The voltammogram is given in Fig. 4 in blue together with the
calculated concentration profile. It can be seen that hydroxylamine
is formed dominantly together with some ammonia. The formation
of these products starts at a potential just below 0.2 V. There is a
difference in the voltammogram with and without sample collec-
tion (shown later in Fig. 11) as a result of the tip which presumably
removes poisoning species from the electrode surface. The current
is therefore a little higher and shows fluctuations during sample
collection. In addition to nitrate reduction on a PdSn electrode,
Fig. 1. The cyclic voltammogram for the nitrate reduction on the unmodified Pd
electrode in 0.1 M H2SO4. Scan rate: 1 mV/s
6
4
2
qPdSn
qPd
jPdSn
0
-2
-4
-6
-8
jPd
Before Sn modification
After Sn modification
-
After NO reduction
3
0.0
0.2
0.4
0.6
0.8
E vs. RHE/V
Fig. 2. Effect of Sn-modification on the Pd electrode in 0.1 M H2SO4. Scan rate: 1 V/s
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0
-5
0
-5
Blank
Pd
-10
-15
-20
-25
NO3- on Pd (HClO4)
NO3- on PdSn (HClO4)
NO3- on Pd (H2SO4)
NO3- on PdSn (H2SO4)
-10
-15
-20
-25
PdSn
Pd
Pd + ~ 10
Pd + ~ 50
μ
μ
M Sn2+ in sol
M Sn2+ in sol
HONH+
HONH+3 (HClO4)
NH+4 (HClO4)
HONH+3 (H2SO4)
NH+4 (H2SO4)
0.5
0.4
0.3
0.2
0.1
0.0
3
NH+
0.4
0.2
4
PdSn
10μ
M Sn2+ in sol
M Sn2+ in sol
50μ
0.0
0.0
0.1
0.2
E vs RHE/V
0.3
0.4
0.0
0.1
0.2
E vs RHE/V
0.3
0.4
0.5
Fig. 4. Non-volatile reaction products for the nitrate reduction on PdSn in 0.1 M
Fig. 5. Anion effect for the nitrate reduction on PdSn. Scan rate: 1 mV/s
H2SO4 and Pd in a Sn2+ containing 0.1 M H2SO4 solution. Scan rate: 1 mV/s
widely used and more recently also Rh(111) [39], Ru(0001) [40]
and Ir, Re and PtRu [41].
the nitrate reduction was also investigated on a Pd electrode in
a solution containing Sn ions. This leads to reversible adsorption of
Sn during the voltammetric scan compared to irreversible adsorp-
tion of Sn which is the case for the Sn-modified Pd electrode. The
corresponding CV’s (in red) and the concentrations of protonated
In this work two methods have been used to prepare the thin
films. The first is commonly used in literature and is simply Pd elec-
trodeposition on the substrate which leads to films with a thickness
of several multilayers. We used polycrystalline Au as substrate and
the Pd electrodeposition was carried out at 0.27 VSCE for 60 sec-
onds in 0.1 M H2SO4 + 0.1 mM PdCl2. The Pd film was characterized
electrochemically by cycling in the blank electrolyte between 0 and
1.0 VRHE. The voltammogram given in Fig. 6 (blue trace) shows the
presence of the hydrogen adsorption/absorption and desorption as
for bulk Pd (black trace). As expected the current is lower compared
to the voltammogram of the bulk Pd electrode.
The second method for Pd film preparation is to deposit one
single monolayer of Pd on the substrate (Au in our case). The mono-
layer palladium film is obtained by a spontaneous irreversible redox
process in which a UPD Cu adlayer is replaced by Pd atoms. This
technique has been demonstrated to result in a uniform 2D deposit
+
hydroxylamine and ammonia, HONH3 and NH4+, as function of
potential are also shown in Fig. 4. The dashed lines correspond to
the pristine Pd electrode before adsorption of Sn adatoms. With
Sn2+ in the solution it is observed that the onset of nitrate reduc-
tion is delayed and the current is lower (smaller amount of reaction
products are formed) compared to the electrode with a Sn-modified
Pd electrode. Furthermore, a higher concentration of Sn species in
the solution has a negative influence on the catalytic activity for
nitrate reduction, which could be ascribed to a blocking effect by
the excess Sn ions.
3.1.5. Anion effect
From previous work on Pt it is known that the nitrate reduction
is sensitive to the presence of other adsorbing species as nitrate
first needs to adsorb on the surface for the reduction of NO−3 to
NO−2 . Therefore the nitrate reduction has been investigated in two
electrolytes: H2SO4 and HClO4. Fig. 5 shows that in the presence of
the non-adsorbing perchlorate anion the onset potential of nitrate
reduction on PdSn is around 0.37 V while in H2SO4 it is around
0.3 V. The nitrate reduction is suppressed by the strongly adsorbed
(bi)sulfate. This is similar to the results for nitrate reduction on
platinum [27]. Furthermore, also in HClO4 it is found that Pd is
inactive for the nitrate reduction without addition of Sn species.
The product distribution, however, is the same for both electrolytes
and shows the same trend as a function of potential.
0.2
0.0
-0.2
-0.4
Multilayer Pd
Monolayer Pd
2
1
0.0
0.2
0.4
0.6
0.8
1.0
E vs RHE/V
0
3.2. Nitrate reduction on (ultra)thin palladium films
Bulk Pd
-1
-2
-3
3.2.1. Preparation of palladium films
Pd
Au
Pd Pd
Pd Pd
Pd
PAd u
Thin palladium overlayers have been extensively studied in the
last decades as they are very interesting for electrocatalysis. Various
electrochemical reactions have been studied on these systems such
as adsorption and absorption of hydrogen [28], oxidation of formic
acid [29,30], formaldehyde [31], CO adlayers [32] and the reduc-
tion of oxygen [33]. Different substrates have been investigated for
the Pd deposition. Au(111) [28,34,35] and Pt(111) [29,36–38] are
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
E vs RHE/V
Fig. 6. Comparison of blank CV’s for bulk Pd, multilayer and submonolayer AuPd
electrodes. Scan rate: 100 mV/s
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0.1
0.0
0.00
-0.05
-0.1
Blank
Blank
NO3- on AuPd
NO3- on AuPd
-0.10
-0.2
H2
N2O
0.015
0.010
0.005
0.000
-0.005
H2
0.010
0.005
0.000
-0.005
0.0
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
E vs RHE/V
E vs RHE/V
Fig. 7. Nitrate reduction on the unmodified multilayer AuPd including the formation
Fig. 9. Nitrate reduction on the unmodified submonolayer AuPd electrode including
of volatile products. Scan rate: 1 mV/s
the formation of volatile products. Scan rate: 1 mV/s
3.2.2. On-Line Electrochemical Mass Spectrometry
covering the substrate [42–44]. First we deposited 1 monolayer of
Cu on Au by UPD at 0.05 VAg/AgCl for 3 minutes in 1 mM CuSO4 + 0.1
M H2SO4. Next the electrode was immersed in 0.1 mM PdCl2 + 0.1
M H2SO4 for 10 minutes. After rinsing the electrode was brought
into the electrolye where the potential was cycled between 0 and
0.5 VRHE until a stable voltammogram was obtained. The upper limit
of the potential is kept relatively low to prevent the formation of
oxides. This voltammogram is also shown in Fig. 6 (red trace). In
comparison with the bulk Pd and the multilayer AuPd electrodes,
the current is again much lower, and in this case it must be due to
hydrogen adsorption only. In our experiments we have not inves-
tigated the coverage of Pd on Au with other techniques. As the
charge for the hydrogen adsorption/desorption region for this Pd
film is variable and lower compared to the charge of a fully cov-
ered Pd monolayer (in Fig. 6 the coverage from the H adsorption
is approximately 0.5 ML), we will refer to it as a submonolayer Pd
film throughout this work.
In Figs. 7 and 8 the CVs and the mass signals as a function of
potential are shown for the nitrate reduction on an unmodified-
and Sn-modified multilayer Pd film on Au respectively. Without
Sn on the surface H2 (m/z =2) is the dominant product at poten-
tials < 0.05 VRHE together with trace amounts of N2O (m/z = 44)
only in the negative-going scan. When the electrode is modified by
Sn a large current enhancement is observed similar to the bulk Pd
electrode. The hydrogen formation is now strongly suppressed the
signal is weaker compared to the lowest ion current of the prod-
ucts shown in the figures and large amounts of N2O are produced
together with a smaller amount of N2 (m/z = 28). Furthermore a
small amount of NO (m/z = 30, as mentioned this signal has been
corrected for fragmentation of N2O) is observed together with the
mass fragments m/z = 14, 45 and 46. The latter three follow the
same trend as N2O and are therefore considered to be a fragment
of N2O after ionization, whereas the m/z = 14 (N) signal is also a
fragment of N2.
0.0
0
-1
-2
-0.5
-1.0
Blank
NO3- on Sn modified AuPd
Blank
-3
-1.5
NO3- on Sn modified AuPd
0.20
-4
N2O
N2
NO
N2O
N2
NO
0.06
1.5
0.15
0.10
0.05
0.00
0.003
0.002
0.001
0.000
-0.001
0.04
0.02
1.0
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.0 0.1 0.2 0.3 0.4 0.5
0.5
0.0
0.0
0.1
0.2
E vs RHE/V
0.3
0.4
0.5
0.0
0.1
0.2
E vs RHE/V
0.3
0.4
0.5
Fig. 8. Nitrate reduction on the Sn-modified multilayer AuPd electrode including
the formation of volatile products (inset: zoom in on the NO mass signal). Scan rate:
1 mV/s
Fig. 10. Nitrate reduction on the Sn-modified submonolayer AuPd electrode includ-
ing the formation of volatile products. Scan rate: 1 mV/s
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Y.Y. Birdja et al. / Electrochimica Acta xxx (2014) xxx–xxx
multilayer AuPd produces about twice as much hydroxylamine
0
-1
-2
-3
-4
compared to the Sn-modified submonolayer AuPd and is also more
selective towards hydroxylamine than the Sn-modified submono-
layer AuPd. The absolute current and the current profile are similar.
Comparison of Figs. 8 and 11 (products on multilayer AuPd) to
Figs. 10 and 12 (products on submonolayer AuPd) suggests that the
Sn-modified submonolayer AuPd produces more NO and N2O and
is less efficient in the further reduction of NO to hydroxylamine
and ammonia than the multilayer AuPd. An explanation for this
may be related to the amount of Pd that is needed for the further
reduction of NO to hydroxylamine or ammonia. The amount of Pd
is less on the submonolayer AuPd compared to the multilayer AuPd
which results in a lower amount of hydroxylamine and ammonia,
but a higher amount of NO and N2O.
Blank
NO3- on AuPd
NO3- on Sn-modified AuPd
Without sample collection
-5
0.8
HONH3+
NH4+
0.6
0.4
0.2
0.0
0.0
4. Conclusions
Tin adsorbed on a Pd electrode can effectively increase the activ-
ity for the nitrate reduction in acidic media. I−t is believed that Sn
0.1
0.2
0.3
E vs RHE/V
0.4
0.5
−
catalyzes the rate determining step from NO3 to NO2 as it does
on a platinum electrode. A pristine Pd electrode only shows hydro-
gen evolution and has no activity for the nitrate reduction, while
on PdSn N2O is found to be the dominant product together with a
smaller amount of N2. In addition to these volatile products hydrox-
ylamine is produced dominantly together with a small amount of
ammonia. Sn ions in the solution lead to a lower catalytic activity
compared to the Sn-modified electrode as a higher concentration of
Sn ions lowers the activity. It was also found that the nature of the
anions influences the onset of the nitrate reduction. The stronger
adsorbing sulfate anion delays the nitrate reduction compared to
the non-adsorbing perchlorate anion. However, the product distri-
bution is not affected.
Fig. 11. Non-volatile product formation for nitrate reduction on the Sn-modified
multilayer AuPd electrode. Scan rate: 1 mV/s
In Figs. 9 and 10 the formation of volatile products on a sub-
monolayer AuPd electrode is shown with and without modification
by Sn. Unmodified 1 ML AuPd results only in hydrogen evolution,
while Sn-modification leads to a significant increase in activity.
Again N2O is the dominant product followed by N2 and NO. This is
similar to the Sn-modified multilayer Pd film and bulk Pd electrode.
3.2.3. Ion Chromatography
In addition, this paper has shown that the ability of palladium
to absorb hydrogen plays a negligible role in the nitrate reduction.
The nitrate reduction on a multilayer and on a submonolayer Pd
film deposited on Au without Sn-modification leads only to hydro-
gen evolution and no nitrate reduction in agreement with the lack
of activity of bulk palladium. It should be noted that trace amounts
of N2O were observed in a small potential region for the multilayer
AuPd but not on the submonolayer AuPd. When both AuPd elec-
trodes are modified with Sn, on both electrodes N2O is formed as
the dominant volatile product and NH2OH as the dominant non-
volatile product. This is similar to the product distribution of the
nitrate reduction on the Sn-modified bulk Pd electrode.This product
spectrum is similar to the Sn modified platinum [6,24,25], whereas
a Sn-modified rhodium electrode produces more ammonia [45],
owing to the higher propensity of Rh to break the N-O bond.
The non-volatile product formation using IC for the unmod-
ified AuPd electrodes (multilayer and submonolayer Pd on Au)
is not shown here as there were no products observed. After
Sn-modification the non-volatile product formation as function
of potential is shown in Fig. 11 for the multilayer AuPd and in
Figure 12 for the submonolayer AuPd electrode. As seen before
for the bulk Pd electrode the current is a little higher and shows
fluctuations due to the sample collection. On both electrodes
hydroxylamine and ammonia are formed. The Sn-modified
0
-1
-2
-3
Blank
Acknowledgements
NO3- on AuPd
-4
-5
-6
NO3- on Sn-modified AuPd
The authors acknowledge a Chinese Scholarship Council (CSC)
grant awarded to Jian Yang.
HONH3+
NH4+
0.3
0.2
0.1
0.0
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Please cite this article in press as: Y.Y. Birdja, et al., Electrocatalytic Reduction of Nitrate on Tin-modified Palladium Electrodes, Elec-
trochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.06.011