Electroreduction of nitrate anions on cubic and polyoriented platinum nanoparticles modified by copper adatoms

Accepted Manuscript Electroreduction of nitrate anions on cubic and polyoriented platinum nanoparticles modified by copper adatoms

Maria R. Ehrenburg, Alexey I. Danilov, Inna G. Botryakova, Elena B. Molodkina, Alexander V. Rudnev PII: DOI: Reference:

S1572-6657(17)30620-3 doi: 10.1016/j.jelechem.2017.08.051 JEAC 3492

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

21 July 2017 30 August 2017 31 August 2017

Please cite this article as: Maria R. Ehrenburg, Alexey I. Danilov, Inna G. Botryakova, Elena B. Molodkina, Alexander V. Rudnev , Electroreduction of nitrate anions on cubic and polyoriented platinum nanoparticles modified by copper adatoms, Journal of Electroanalytical Chemistry (2017), doi: 10.1016/j.jelechem.2017.08.051

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

Electroreduction of nitrate anions on cubic and polyoriented platinum nanoparticles modified by copper adatoms

Maria R. Ehrenburga,*, Alexey I. Danilova†, Inna G. Botryakovaa, Elena B. Molodkinaa,

Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,

RI

a

PT

Alexander V. Rudneva,b,*

Leninskii pr. 31, Moscow, 119071 Russia

University of Bern, Freiestrasse 3, 3012 Bern, Switzerland

NU

SC

b

*

Corresponding authors

AC

CE

PT E

D

MA

E-mail address: [email protected] (M.R. Ehrenburg); [email protected] (A.V. Rudnev)

†

Deceased.

ACCEPTED MANUSCRIPT 2 Abstract In this work, electroreduction of nitrate anions on bare and copper–modified platinum nanoparticles (Pt NPs) supported on glassy carbon is studied using cyclic voltammetry. Two types of Pt NPs are chosen for this purpose: unshaped (polyoriented) NPs and cubic NPs displaying the preferential (100) orientation of faces. The modification of cubic and

PT

polyoriented Pt NPs by copper adatoms with submonolayer coverages is performed in a controlled way in solutions containing small concentrations of Cu2+ ions, 10–5 M. Nitrate

RI

reduction is studied first in copper–free solutions and then in the presence of 10–5 M Cu2+.

SC

The transmission electron microscopy and voltammetric measurements of the cubic NPs indicate the presence of a significant amount of Pt(100) terraces on the surface of these NPs.

NU

In perchloric acid solutions containing 0.02 M NaNO3 and 10–5 M Cu2+, accumulation of

MA

copper adatoms on the NPs results in a fast increase in the currents of nitrate electroreduction. These reduction currents on the cubic NPs are up to three times higher than

D

on the polyoriented NPs at Cu coverages of 0.20–0.35. The comparison of the data on Pt

PT E

NPs with the data for single crystal electrodes with (100) terraces of different width (Pt(610), Pt(210)) shows that the behavior of NPs can be simulated on the basis of the data for single crystal faces with wide (cubic NPs) and narrow (unshaped NPs) (100) terraces.

CE

Thus, cubic NPs manifest rather a high electrocatalytic activity in the studied reaction of

AC

nitrate anion electroreduction, which is typical for single crystal surfaces with relatively wide Pt(100) terraces. At the same time, in comparison with macro single crystalline electrodes, these NPs are characterized by sufficiently higher stability, larger specific surface area, and flexibility in application.

Key words: shape-controlled nanoparticles, platinum, stepped single crystals, copper UPD, nitrate electroreduction, voltammetry.

ACCEPTED MANUSCRIPT 3 1. Introduction Control of nanosystem surface morphology and structure keeps gaining in importance, as variation of the size, crystallographic surface orientation, and shape (from zero– to three– dimensional) [1] of nanostructures allows performing fine tuning of their physical and chemical properties. Although synthesis of unshaped (polyoriented) nanoparticles (NPs) has first been carried

PT

out a long time ago, in the recent decade, ever greater attention has been paid to synthesis and studies of NPs with a tailor–made shape (nanocells, nanorods, nanowires, nanotubes, nanorings,

RI

nanoframes, cubic NPs, multipodes etc. [2-14]) that are simultaneously characterized by a larger

SC

reaction surface and by the presence of relatively stable in air (at least, in the dry state) faces with predominant structural orientation. One of the vivid advantages of such systems is the possibility of

NU

using them in structure–sensitive reactions. It has already been shown that the activity of shape–

MA

controlled NPs in different electrocatalytic reactions, such as oxygen electroreduction, formic acid oxidation etc., is much higher than the activity of commercial catalysts based on platinum NPs

D

applied onto carbon supports [3-7]. The higher activity of the synthesized shape–controlled

PT E

platinum NPs is usually attributed to the very structural sensitivity of these reactions and the high concentration of surface sites with the given orientation (e.g., Pt(100) facets) on the surface of these NPs [6, 15, 16]. Yet another method of enhancing the electrocatalytic activity of platinum catalysts

CE

is their modification by adatoms of foreign metals [17-21]. This allows decreasing the surface

AC

poisoning by the reaction products, changing the adsorption energy of the reaction components, decreasing the initial potential of the reaction, and/or changing the reaction path. One of the electrochemical reactions, for which the catalyst surface structure and the possibility of its modification by foreign metals are of particular importance, is the reaction of nitrate anion electroreduction. This is related both to its scientific (multistage path) and its practical importance [22-24]. Electrocatalysis of reduction of nitrate anions on polycrystalline platinum and on platinum modified by some metals has been studied in much detail [22]. It was shown that the principal final product in the reaction on the platinum group metals is ammonia [25-27]. On a

ACCEPTED MANUSCRIPT 4 copper electrode, the final product of nitrate electroreduction can also be NO [28], which the authors explain by weak adsorption of NO on the surface. The final reaction product on copper– modified platinum electrodes can be both ammonia and compounds with more positive nitrogen oxidation degrees, such as N2O [29]. We have studied nitrate electroreduction in much detail on a number of single crystal platinum surfaces [30, 31] and also on copper–modified Pt(111) and

PT

Pt(100) single crystal electrodes [29, 31]. Several works have studied electrochemical reduction of nitrate anions and other nitrogen-containing species (nitrite, NO) on bare shape-controlled Pt NPs

RI

[32-36]. It was shown in [34] that Pt NPs with a high concentration of (100) oriented surface

SC

domains manifest higher electrocatalytic activity in the nitrate electroreduction reaction in perchloric acid solutions. The initial nitrate reduction occurred on the short–range order Pt(100)

NU

domains, leading to formation of NOads and the further reduction to ammonia, while Pt(110) facets

MA

was hardly involved in nitrate reduction under such conditions. A study of electroreduction of nitrate on preferentially oriented (100) Pt NPs in phosphate buffer solutions demonstrated a similar

D

reactivity of such NPs to that of single crystal Pt surfaces with short (100) terraces [36].

PT E

Another work [37] states that the adsorption and reduction of nitrate on Pt nanostructured films with close to 50% of (100) surface sites modified by a full Cu underpotential deposition (UPD) monolayer occur preferentially on Cu(100) surface sites in a sulfate solution. The effect of

CE

functionalization of copper electrodes by platinum (including supported systems) on reduction [38]

AC

and electroreduction [39] of nitrate has also been studied. However, there are no works on nitrate electroreduction of shape–controlled platinum NPs modified by controlled amounts of copper. The aim of our work is to study the regularities of nitrate anion electroreduction on the initial cubic platinum NPs and also on cubic platinum NPs modified by copper submonolayers, as compared to unshaped (polyoriented) NPs. Also, attention is paid to the effect of the presence of nitrate anions in the solution on modification of platinum NPs by copper adatoms.

ACCEPTED MANUSCRIPT 5 2. Experimental Formation of platinum NPs with the predominant face orientation of (100) was carried out by synthesis from aqueous–organic microemulsions [40, 41]. For this, synthesis was performed in microemulsions containing 80.5 vol % of n-heptane (for analysis, Merck) and 3 vol.% of the aqueous phase and also 16.5 vol. % of Brij30® (C12H25(OCH2CH2)4OH) (polyoxoethylene(4)lauryl

PT

ether, Acros Organics) that determined the size of microemultion drops (see, e.g., [42]). The predominant cubic shape and the corresponding (100) orientation of the cubic faces was provided

RI

by choosing the corresponding concentration of the adsorbate (capping agent) that blocked crystal

SC

growth in the (100) direction [2, 40, 43]. The adsorbate used was the chloride anion; as shown in [40], the optimum concentration of the chloride anion in the solution was 25% HCl (37% HCl (for

NU

analysis, Merck) was used for preparation of the dilute solution). H2PtCl6 (Vokhimfarm, Russia)

MA

was used for synthesis of NPs. The primary emulsion in heptane was reduced by addition of a tenfold exceed of NaBH4 (>98%, Merck). The whole process was carried out in air. To remove

D

surfactants introduced at the synthesis stage, NPs were successively washed in the following

PT E

solvents and their mixtures: acetone (high–purity grade), ethanol (rectificate cleaned by addition of KMnO4), water (Milli-Q, 18.2 M cm, 5 ppb total organic content). In the course of the washing, acetone was added to the NP suspension in heptane at the ratio of 1:1. Then, after NP

CE

sedimentation, the solvent mixture was decanted. The process was repeated several times; then

AC

ethanol was added instead of acetone. Finally, at the last stage of the washing, ethanol was replaced by water. If NPs remained suspended in the solution, several pellets of NaOH (p.a., Merck) were added for a short time to the suspension. Then the alkaline solution was replaced by water by means of successive decantation, as described above. Further, the aqueous NP suspension was drop cast onto a base of a glassy carbon (GC) cylindrical rod with the diameter of 1–2 mm. The GC rod was previously polished by the ASM 5/3 diamond powder with the particle size of 3–5 m and then treated by the mixture of H2SO4+H2O2. To achieve uniform NP distribution over the GC surface, the NP suspension was sonicated (40 kHz)

ACCEPTED MANUSCRIPT 6 for 10–20 s each time before NP deposition. Electrochemical measurements were performed in the meniscus configuration. The pretreatment also included the cycling of the GC electrode potential in the range of 0.05 to 1.30 V in 0.5 M H2SO4 in order to obtain a uniformly rough surface (10–15 cycles) [44, 45]. Drops of the suspension on the electrode were dried in the flow of argon under an unsealed hood. After the suspension was dried, the electrode was transferred, without additional

PT

washing, into the cell with the sulfuric acid solution. Estimation of the working surface area on the basis of CV in sulfuric acid is described in detail further in the paper. Before the electrode was

RI

transferred into the cell with the 0.1 М HClO4 solution, it was thoroughly washed by Milli-Q water

SC

to avoid contamination of the perchloric acid solution by sulfate anions.

Transmission electron microscopy (TEM) was used to obtain information on the shape and

NU

size of NPs: the microscope was FEI133 Morgagni 268, the operating voltage was 80 kV. Fig. 1a

MA

shows the obtained NPs that contain a significant fraction of cubic–shaped NPs. One can also observe a certain amount of NPs with a more complicated shape (the so called "tetrapods" and

D

"octapods", see, e.g., [2]), but the actual fraction of the cubic NPs is rather large. Such NPs are

PT E

further denoted as "cubic" NPs or Pt(100)NPs. One can see that the average NP size is 10–20 nm. Fig. 1b shows unshaped NPs used for reference experiments; they were obtained using a method similar to that described above, but without addition of hydrochloric acid into the synthesis

CE

microemulsion. One can see that these unshaped, or polyoriented, NPs (further denoted as

AC

Pt(poly)NPs) have no predominant faceting. The average size of such particles does not exceed 10 nm, but they are noticeably aggregated even after sonication.

ACCEPTED MANUSCRIPT

PT

7

RI

Fig. 1. TEM images of nanoparticles: (a) Pt(100)NP and (b) Pt(poly)NP.

SC

Reference voltammetric measurements were performed using the following platinum single

NU

crystal electrodes: a Pt(100) basal face, stepped Pt (610) and Pt(210) faces with hexatomic and diatomic Pt(100) terraces, respectively, and Pt(110) steps. The electrodes were made in University

MA

of Alicante (Spain) using the method of Clavilier et al. [46-48]. The working surface area was 0.03– 0.04 cm2. Before each experiment, the electrodes were annealed in Bunsen flame for 20–40 s to

D

remove impurities and order the surface structure, were cooled in a 3 : 1 argon–hydrogen gas

PT E

mixture, washed in Milli-Q water saturated by this mixture, and transferred with a water drop to protect the surface from impurities into a cell with the working solution degassed by argon. Then an

CE

electrode/solution meniscus was formed (the hanging meniscus configuration provides contact of the solution solely with the working single crystal face) and cyclic voltammograms were registered

AC

to control the purity degree of the system and quality of the electrode annealing/cooling. A three–electrode glass cell was used for electrochemical measurements. The auxiliary electrode was a platinum wire in the same solution, in the main cell compartment. The reference electrode was a reversible hydrogen electrode (RHE) in 0.5 M H2SO4 or in 0.1 M HClO4, as dependent on the solution in the main cell compartment. All potentials in the paper are presented vs. RHE. The solutions were made on the basis of CuO, NaNO3, and H2SO4 (p.a., Merck), HClO4 (suprapure, Merck), and Milli-Q water. Also, only this water was used for washing the cells, the

ACCEPTED MANUSCRIPT 8 electrodes, etc. GeO2 (99+, Merck), NaOH (p.a., Merck), HClO4 (suprapure, Merck), and Bi2O3 (99.9% extra pure, Acros Organics) were used for characterization of NP surface structure using adsorption of germanium and bismuth [49]. High–purity argon (99.993% Ar, Tsentrogaz) was used for solution deaeration; the inert gas was blown over the solutions in the course of the experiments. To introduce the additives of sodium nitrate or copper perchlorate, the meniscus was broken and the

PT

electrode was placed above the solution in an argon atmosphere. After the sample aliquot was added, argon was bubbled through electrolyte for several minutes to level the additive bulk

RI

concentration and remove oxygen traces from the system. Then the meniscus was formed, potential

SC

cycling was resumed and cyclic voltammograms (CVs) were obtained while argon was continuously passed over the working solution.

NU

The CV data were measured using the potentiostat and software developed in Frumkin

AC

CE

PT E

D

recorded at the scan rate of 0.05 V s–1.

MA

Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences. All CVs were

ACCEPTED MANUSCRIPT 9 3. Results and discussions 3.1. Electrochemical characterization of nanoparticles supported on GC electrodes

Cyclic voltammetry is one of the most reliable methods for characterization of the platinum NP surface structure [49]. Especially well pronounced peaks can be observed in sulfuric acid

PT

solutions. The shape of the peaks at the potentials of underpotential deposition of hydrogen adatoms (H UPD region) unambiguously points to the presence of particular adsorption sites on the surface

RI

([36, 50-52] and references therein). Each type of active sites is characterized by its specific range

SC

of potentials of H UPD. Hence the CVs allow characterizing in detail the surface structure of NPs as compared to the CVs of well-studied Pt single crystal electrodes.

NU

In order to characterize the surface structure of NPs we first present the CVs of model

MA

surfaces, i.e., the single crystal electrodes Pt(100), Pt(610) and Pt(210), obtained in sulfuric and perchloric acid solutions (Figs. 2a,b). These CVs display current regions related to

D

adsorption/desorption of hydrogen adatoms and anions on different surface sites [53]. The step

PT E

density of the electrode surface increases from Pt(100) with large (100) terraces [54] to Pt(610) and Pt(210) with the (100) terraces of hexa- and di-atomic width, respectively, and with (110) steps. It is reflected in the diminishing of the peak in the range of 0.3–0.5 V (region H1), while the peak at

CE

0.28 V (region H2) and the currents in the range of 0.05–0.2 V (region H3) grow (see CVs in Fig.

AC

2a for 0.5 M H2SO4 solution). It was shown in [49] that these regions H1, H2 and H3 correspond to H UPD (and partially to the specific adsorption/desorption of sulfate anions) at Pt(100) terraces, at the step edges, and at the Pt(110) steps, respectively. In perchloric acid, these features in CVs are much less pronounced (Fig. 2b), but a similar pattern is observed in accordance with [49, 55]. In contrast to sulfate, perchlorate anions are not chemisorbed on the platinum surface [56].

ACCEPTED MANUSCRIPT

SC

RI

PT

10

NU

Fig. 2. (а, b) CVs of single crystal macroelectrodes in (a) 0.5 M H2SO4 and (b) 0.1 M HClO4. (c, d) CVs of Pt(100)NPs/GC and Pt(poly)NPs/GC (c) 0.5 M H2SO4 and (d) 0.1 M HClO4. Before the

MA

measurement, the electrodes were subjected to the HER treatment at –0.05 V. (e) CVs of

D

Pt(100)NPs/GC before and after HER cleaning treatment.

PT E

The CVs of clean Pt(100)NPs and Pt(poly)NPs supported on GC in the sulfuric acid and perchloric acid solutions are presented in Figs. 2c,d. Fig. 2e shows the CVs of Pt(100)NPs/GC

CE

before and after cleaning procedure performed in our study. The necessity of additional cleaning of NPs from traces of surfactants remaining on their surface after synthesis is discussed in detail in the

AC

literature [41, 57-59]. Here, we introduce a novel procedure for removing impurities and oxides from the surface of Pt NPs. The NPs were cleaned by electrochemical hydrogen evolution reaction (HER) at –0.05 V for 5 min. At this potential, HER proceeds at a rather moderate rate. Application of higher cathodic potentials is undesirable, as vigorous gas evolution may result in removal on NPs from the GC surface. Between HER treatments, we employed the potential cycling in the range of 0.05–0.80 V, until a stable CV was obtained. One can see from the CVs of Pt(100)NPs/GC before and after the HER treatment (Fig. 2e) that such cleaning provides pronounced development of peaks in the range of 0.35–0.40 V. These peaks, as pointed out above for single crystalline

ACCEPTED MANUSCRIPT 11 macroelectrodes, are attributed to H UPD on Pt(100) terraces. The surface cleaning yields good results both in sulfuric and perchloric acids. Further, all the CVs correspond to NPs cleaned according to the described procedure. Comparison of the CVs of Pt(100)NPs/GC and Pt(poly)NPs/GC electrodes (Figs. 2c,d) shows that the currents in the range of 0.3–0.4 V (region H1, H UPD on (100) terraces) are much

PT

higher for cubic NPs both in sulfuric and perchloric acid solutions. Besides, the peaks at 0.27 V (region H2, terrace edges) in sulfuric acid are much better pronounced for cubic NPs than for

RI

polyoriented ones (Fig. 2c). The CVs of Pt(poly)NPs contain much more distinct peaks in the range

SC

of 0.10–0.15 V corresponding to H UPD at (110) sites (H3 region), which is typical for the CVs of a polycrystalline platinum macroelectrode.

NU

The estimation of the respective amount of (100) and (111) domain sites on NPs was carried

MA

out on the basis of the strategy proposed in [49, 60, 61]. This strategy includes the evaluation of charges involved in the redox process of irreversibly adsorbed germanium (determination of (100)

D

sites) and bismuth (determination of (111) sites). Our estimates demonstrate that the obtained cubic

PT E

NPs have a negligibly small amount of (111) domain sites and above 30% of (100) domain sites (for further details see Supplementary Data (SD) and Figs. S1, S2). The active surface area of NPs supported on GC was determined on the basis of the

CE

desorption charge of hydrogen adatoms at 0.05–0.50 V in the CVs of NPs in sulfuric and perchloric

AC

acid (210 C/cm2 for a hydrogen adatom monolayer in the case of polycrystalline Pt and Pt(100) electrodes) with account for the currents of electric double layer charging. Thus, here and further, the current densities for NP electrodes are given per active platinum surface area.

3.2. Modification of nanoparticles by copper adatoms

The modification of platinum electrodes by copper noticeably affects the rate of nitrate anion electroreduction. Modification of cubic and unshaped NPs by copper adatoms (Cu UPD) in

ACCEPTED MANUSCRIPT 12 the absence of nitrate anions in the perchloric acid solution with a low copper concentration (10–5 M) is shown in Fig. 3. Using dilute solutions of copper ions allows smoothly changing the surface coverage by adatoms. The scheme used to study copper deposition on the NP electrodes was as follows. The electrode potential was cycled in the range of 0.85–0.60 V; then the potential was shifted to 0.35 V

PT

with the following cycling in the range of 0.35–0.05 V for accumulation of copper adatoms on the surface (solid curves in Fig. 3a,b). After the given number of accumulation cycles, the potential was

RI

swept to 0.85 V to desorb copper adatoms (dashed curves). Before the next accumulation round, the

SC

electrode potential was again cycled in the range of 0.60–0.85 V until the concentration of Cu2+ in the near–electrode region became equal to that in the bulk. The integration of the peak of oxidative

NU

UPD copper desorption (after subtraction of the background curve) was used to estimate the amount

MA

of copper deposited onto the electrode, as per 420 C/cm2 of platinum for a complete copper monolayer (the value corresponding to both the polycrystalline platinum surface and to Pt(100)).

AC

CE

PT E

D

For details, see SD, Fig. S3.

Fig. 3. (a, b) Accumulation of copper adatoms on the surface of (а) Pt(100)NPs/GC and (b) Pt(poly)NPs/GC for the given number of cycles in the range of 0.05–0.35 V (solid curves) with the further adlayer desorption during the potential sweep to 0.85 V (dashed curves). The solution is 0.1 M HClO4 + 10–5 M Cu2+. (c) Surface coverage of copper adatoms on NPs as dependent on the number of copper accumulation cycles in the range of 0.05–0.35 V.

ACCEPTED MANUSCRIPT 13 As copper adatoms are accumulated on the surface, the currents of H UPD decrease (solid curves in Figs. 3a,b), while the characteristic narrow peak D1 of copper desorption is gradually developed in the range of 0.75–0.80 V (dashed curves). Fig. 3c shows the dependence of platinum surface coverage by copper adatoms Cu on the number of copper accumulation cycles. Cu UPD occurs more slowly on Pt(poly)NPs. At the chosen Cu2+ concentration, the amount of copper

PT

adatoms is twice as large for cubic NPs as compared to polyoriented ones. Peak D1 of copper desorption in the case of Pt(100)NPs, as opposed to Pt(poly)NPs,

RI

gradually shifts to the right with the increasing copper coverage, which implies enhancement of

SC

stability of the copper submonolayer. The stabilization can result from formation of adatom islands that are more stable with respect to desorption than individual adatoms. This agrees well with the

NU

data of [62] on Pt single crystal surfaces with the (100) terraces of different width manifesting a

MA

similar peak shift at an increase in the terrace width, i.e. with the increasing perfection of the Cu adlayer.

D

he potential sweep range for copper desorption curves was limited by 0.85 V for

PT E

Pt(100)NPs/GC, as the Pt(100) terraces are unstable due to adsorption of oxygen–containing species (OCSs) [63, 64]. By contrast, the surface of Pt(poly)NPs/GC is stable in the potential range of at least up to 1.2 V, which allows using a wider cycling range. The peak at ~0.90 V observed for

CE

Pt(poly)NPs/GC (Fig. 3b) corresponds to adsorption of OCSs on Pt NPs that takes place on the

AC

copper–free sites. OCSs adsorbed on Pt(poly)NPs are reductively removed in the cathodic scan without any change (detectable by CV) in the surface structure.

3.3. Electroreduction of nitrate anions on copper-modified platinum NPs

The concentration of nitrate anions chosen for this work was 20 mM. This is a high enough concentration to avoid serious diffusion limitations and at the same time not too high, which prevents formation of large amounts of nitrate reduction products in the meniscus.

ACCEPTED MANUSCRIPT 14 Fig. 4 shows steady–state CVs of NPs obtained by cycling in a wide range of potentials (0.85 to 0.05 V) in acidic nitrate–containing solutions. One can see that under the chosen conditions cubic NPs manifest much higher cathodic currents of nitrate anion reduction (at 0.05–0.35 V) during the cathodic potential sweep, as compared to the corresponding currents on Pt(poly)NPs. The anodic CV branch contains a cathodic peak at ~0.34 V: very well pronounced in the case of

PT

cubic NPs and only outlined in the case of polyoriented ones. This peak was previously attributed to reduction of the nitrate anion to ammonia and is characteristic precisely for the wide Pt(100) [29] or

RI

Pt(111) [31] terraces. Thus, its presence on the CVs of Pt(100)NPs/GC indicates existence of long–

SC

range order (100) domains on the surface of cubic NPs. Finally, there is a peak in the range of 0.78 V in the CVs of cubic NPs that was attributed in [29, 30] to oxidation of ammonia adsorbed on

MA

NU

wide Pt(100) terraces to NO. This peak is absent from the CVs of Pt(poly)NPs.

-0.05 -0.10

D

0.00

PT E

j / mA cm

-2

0.05

Pt(100)NP/GC Pt(poly)NP/GC

-0.15

0.2

0.4

0.6

0.8

E/V

CE

0.0

solution.

AC

Fig. 4. Steady–state CVs of Pt(100)NPs/GC and Pt(poly)NPs/GC in 0.1 M HClO4+0.02 M NaNO3

Further, we study the reduction of nitrate anion on Pt NPs from the solution containing 10–5 M Cu2+. Under these conditions, nitrate anions are reduced and copper adatoms are simultaneously accumulated on the surface in the course of potential cycling. Such an approach allows direct monitoring of the nitrate reduction rate upon copper adatom accumulation on the NP surface (see our recent work [31]). Moreover, it eliminates the stage of transferring electrodes modified by

ACCEPTED MANUSCRIPT 15 copper adatoms from one cell to another (in case we perform the modification step and nitrate reduction test in different cells), which can result in uncontrollable copper adlayer etching [65]. The measurement procedure is described in detail in SD, section 3. First, the meniscus was formed at 0.85 V in 0.1 M HClO4+0.02 M NaNO3+10–5 M Cu2+ solution (there is no deposition of Cu adatoms at this potential). Afterwards, the potential was changed to 0.35 V, and then the given

PT

number of cycles was registered in the potential range of 0.35 to 0.05 V. Figs. 5a,b show examples of such voltammetric cycles for Pt(100)NPs/GC and Pt(poly)NPs/GC. Upon cycling, copper

RI

adatoms are gradually accumulated on the surface replacing hydrogen adatoms (as shown for the

SC

nitrate–free solution in Fig. 3). At the same time, we observe a significant increase in the cathodic current with potential cycling. This current is attributed to nitrate reduction on copper–modified

NU

NPs. After a certain amount of cycles in the range of 0.35 to 0.05 V, copper was desorbed from the

MA

NP surface by potential sweep from 0.35 V to 1.00 V in order to control the copper coverage (Figs. 5c,d). In nitrate solutions, the Pt(100) terraces are more stable at higher anodic potentials than in

D

nitrate–free solutions, as their surface is protected by a partial or complete NOad adlayer that hinders

PT E

adsorption of OCSs and reorganization of the terrace surface [30], as NOad oxidation with formation of nitrite and nitro species occurs only at 0.9–1.1 V [30, 66]. The Cu coverage is estimated by integration of the respective peaks after subtraction of the background current (see SD for details,

CE

section 3, Fig. S4b). Then three additional 0.35 to 0.83 V cycles were added to allow for the Cu

AC

concentration leveling in the meniscus and removal of the accumulated products of nitrate reduction. Afterwards, the electrode potential was shifted to 0.35 V again and a new series of potential cycles for Cu accumulation and nitrate reduction in the range of 0.35–0.05 V was recorded (see SD section 3, Fig. S4a).

ACCEPTED MANUSCRIPT

SC

RI

PT

16

NU

Fig. 5. (a,b) CVs of copper accumulation and nitrate reduction on (a) Pt(100)NPs/GC and (b) Pt(poly)NPs/GC in the 0.1 M HClO4+0.02 M NaNO3+10–5 M Cu2+ solution for a given number of

MA

cycles (number near the curves) in the potential range of 0.05–0.35 V. The inset in (a) shows evolution of the peak at ~0.34 V in the course of cycling in the potential range of 0.05–0.35 V. (c,d)

D

Potential sweep to 1.00 V for copper adlayer desorption from (c) Pt(100)NPs/GC and (d)

PT E

Pt(poly)NPs/GC after the given number of cycles in the range of 0.05–0.35 V (the numbers are indicated in the figures). (e) Coverage of copper adatoms on Pt(100)NPs/GC and Pt(poly)NPs/GC

CE

as dependent on the number of accumulation cycles in the range of 0.05–0.35 V. (f) Dependence of

AC

the current density at 0.05 V (CVs in panels (a) and (b)) on the copper coverage.

The voltammograms of copper desorption (Figs. 5c,d) after different series of copper accumulation in the nitrate-containing solution (Figs. 5a,b) display two Cu desorption peaks D1 and D2. Peak D1 is similar to that detected in the nitrate–free solution (Figs. 3a,b). The peak–like feature D2 in the range of 0.45–0.70 V appears at Cu>0.17 (>12 accumulation cycles for Pt(100)NPs and >40 accumulation cycles for Pt(poly)NPs) and corresponds to desorption of less stable copper adatoms. The appearance of peak D2 is systematically studied by us using a series of stepped Pt(hkl) electrodes [67]. We suggest that this peak-like feature corresponds to adsorption of

ACCEPTED MANUSCRIPT 17 copper at some sites with a rather high number of copper neighbors (see, e.g., [68]), which results in considerable repulsion interactions. Interestingly, the main peak D1 of Cu desorption from the surface of cubic NPs noticeably shifts in the anodic direction with accumulation of copper adatoms on the surface. While the potentials of copper adlayer dissolution in the absence of nitrate anions manifest a shift within the

PT

range of 0.76–0.78 V (Fig. 3a), the peak of the copper adlayer dissolution in nitrate–containing solutions is located at 0.78 V already at low surface coverages (about 0.10 ML). This effect is

RI

explained by formation on the NP surface of coadsorption layers containing copper adatoms, nitrate

SC

anions, and adsorbed NO, with coulombic interaction between the coadsorbed species (copper adatoms deposited on the platinum surface bear a partial positive charge due to electron density

NU

transfer from copper atoms to more electronegative platinum atoms [68-70]). In the case of

MA

polyoriented NPs with virtually no Pt(100) terraces/domains and therefore with adsorption of nitrate anions being hindered, this effect is necessarily weaker. Similarly, as shown in Fig. 5d, the anodic

D

shift in the Cu desorption peak potential with an increase in copper coverage on Pt(poly)NP/Cu ad

PT E

after copper accumulation in the nitrate–containing solution (Fig. 5b) is much smaller, though the peak is located at 0.78 V already at low copper surface coverages (cf. Fig. 3b). Some more detailed information on this effect of peak potential shift is provided in SD, Fig. S5.

CE

Fig. 5e shows estimated Cu coverages as functions of the number of accumulation cycles.

AC

Clearly, at low Cu coverages, the rate of Cu UPD on Pt(100)NPs is significantly higher than that on Pt(poly)NPs. However, it slows down noticeably after 20 cycles, when Cu on Pt(100)NPs reaches ~0.2. On the contrary, the Cu coverage on Pt(poly)NPs continues increasing almost linearly with potential cycling. We suggest that the observed decrease in the Cu accumulation rate on Pt(100)NPs can be related to competitive coadsorption of nitrate reduction products/intermediates (such as NO, N2O [28, 29]) on free platinum sites. Although adsorption of the products/intermediates on copper is weak (e.g., [28, 71-73]), they can adsorb on platinum, thus to a greater extent hindering further copper adsorption on the surface. Since nitrate reduction becomes very intensive on

ACCEPTED MANUSCRIPT 18 Cuad/Pt(100)NPs at Cu ~0.15–0.20 (Fig. 5e, see the discussion below), such mechanism of the blocking of Pt sites for Cu adatoms by nitrate reduction products seems very plausible. Also, one can see that the accumulation rate of copper on both Pt(100)NPs and Pt(poly)NPs is higher as compared to nitrate–free solutions (cf. figs. 5e and 3c). As Cu adatoms on the platinum surface bear a partial positive charge [68-70], Cu UPD on the Pt surface appears to be enhanced due

PT

to coadsorption of negatively charged nitrate anions due to electrostatic effects (induced adsorption).

RI

As seen in Figs. 5a,b, the currents of nitrate reduction increase upon accumulation of copper

SC

adatoms on the surface of both types of NPs as compared to the steady–state curve of nitrate reduction in copper–free solutions (Fig. 4). Thus, the electrocatalytic activity of the modified Pt

NU

NPs is the higher, the higher the copper surface coverage (in the studied range of coverages). A

MA

substantial increase in the activity of copper–modified Pt NPs towards nitrate reduction as compared to unmodified NPs can be explained by a number of reasons. 1) Nitrate reduction can

D

freely proceed on Cu adatoms, while H UPD atoms compete with nitrate anions for the Pt surface

PT E

sites; 2) Pztc of the platinum surface is at about 0.2 V RHE [74], and copper adatoms deposited on the platinum surface bear a partial positive charge [68], hence the electrode surface becomes less negatively charged, which facilitates approach of nitrate anions from the solution to the surface; 3)

CE

As copper adatoms continue accumulating on the platinum surface, they start displacing

AC

intermediates of electroreduction of nitrate anions. Since adsorption of intermediates on copper is much weaker than on platinum [28, 75], the observed growth of currents at an increase in copper surface coverage can be attributed, among other factors, to acceleration of desorption of intermediates and occurrence of the reaction on the thus newly available surface sites. Hence, the number of the overall "acts" of nitrate electroreduction increases, while the reduction depth may decrease. In case of cubic NPs, the current density increases significantly faster with cycling than in the case of polyoriented NPs (Figs. 5a,b). For instance, after 125 potential cycles, the current

ACCEPTED MANUSCRIPT 19 density j0.05V at 0.05 V (left scanning potential limit) for Pt(100)NPs/GC is more than twice the value of j0.05V for Pt(poly)NPs/GC (cf. Figs. 5a,b). This effect is primarily related to the faster copper accumulation on Pt(100)NPs. Cu UPD is probably induced by nitrate coadsorption due to local electrostatic effects, as described for coadsorption of copper with other anions capable of specific adsorption on the surface of Pt [76-78]. Also, as nitrate anions are adsorbed predominantly

PT

on the (100) terraces [36, 37], the effect of the coverage growth is pronounced much better in the

cubic NPs upon potential cycling in the range of 0.35–0.05 V.

RI

case of cubic NPs. This leads to a more pronounced acceleration of nitrate reduction reaction on

SC

Comparison of the nitrate reduction currents on copper–modified Pt(100)NPs and Pt(poly)NPs at similar coverages (Fig. 5f displays j0.05V as a function of copper coverage) under the

NU

chosen conditions shows a very interesting pattern. While the currents on unshaped NPs increase

MA

almost linearly, those on cubic particles manifest an S–shaped curve. At small Cu surface coverages (Cu < 0.12), the electrocatalytic activity of Cu–modified Pt(100)NPs and Pt(poly)NPs is rather

D

similar. The activity of Cuad/Pt(100)NPs rapidly increases at Cu = 0.15–0.20 and becomes

PT E

significantly higher than that of Cuad/Pt(poly)NPs. This observation demonstrates a clear catalytic effect of the Pt(100) domains existing in much larger amounts on Pt(100)NPs as compared to

CE

Pt(poly)NPs. The significant increase in the activity of Cuad/Pt(100)NPs at Cu = 0.15–0.20 suggests that there should be a minimal threshold amount of Cu adatoms on the Pt(100)NPs surface in order

AC

to noticeably facilitate nitrate reduction. One must also take into account that according to the literature data the mechanism (and end products) of the nitrate reduction reaction on platinum and on copper are different: while the end product of nitrate reduction on platinum at potentials close to hydrogen evolution is ammonia [28, 30, 79], the reaction on copper is interrupted at the stage of NO formation due to weak NO adsorption on Cu [28, 75], which is also supported by the FTIRS data in [29] for nitrate reduction on Cu–modified Pt(100). Also, as indicated in [28, 30, 79], nitrate reduction on platinum at potentials close to hydrogen evolution is hampered by hydrogen adatoms blocking the electrode surface for nitrate anions. Thus, it is reasonable to assume that after a certain

ACCEPTED MANUSCRIPT 20 Cu coverage is reached, the mechanism of the reaction on the modified electrode largely changes with the reaction occurring preferably on Cu adatoms and the reduction being interrupted

NU

SC

RI

PT

terminated at stage of NO, before NH3 is formed, as discussed above.

Fig. 6. (a,b) CVs of copper accumulation and nitrate reduction on (a) Pt(610) and (b) Pt(210) in the

MA

0.1 M HClO4+0.02 M NaNO3+10–5 M Cu2+ solution for a given number of cycles (number near the curves) in the potential range of 0.05–0.35 V. The inset shows evolution of the peak at ~0.34 V in

PT E

D

the course of cycling in the potential range of 0.05–0.35 V.

The further increase in the amount of copper adatoms on Pt(100)NPs did not improve the

CE

nitrate reduction rate in our system, but rather led to its slight decrease (Fig. 5f). One of the reasons of such saturation of the reaction rate can be related to nonstationary nitrate diffusion limitations in

AC

the meniscus. Moreover, in the course of prolonged cycling, accumulation of the products of nitrate electroreduction in the meniscus may also eventually lead to slowdown of the reaction and/or partial poisoning of the electrode surface. No such pronounced effects are observed on Pt(poly)NPs/GC, which is probably related to the slower reaction rate on the polyoriented Pt surface. In order to model the processes occurring on Pt NPs, similar experiments on electroreduction of nitrate anions in the presence of 10–5 M Cu2+ were carried out on the stepped Pt(610) and Pt(210) single crystal electrodes (Figs. 6a,b). One can see that the described electrocatalytic effect of copper adatoms with respect to nitrate electroreduction is to an even

ACCEPTED MANUSCRIPT 21 greater extent characteristic for single crystal electrodes with wider (100) terraces. In nitratecontaining solutions, copper is at first less readily accumulated on narrow terraces than on wider ones (see Table 1), which is probably related to the above–described induced adsorption of UPD copper in the presence of nitrate anions on wide terraces. The effect of acceleration of nitrate anion electroreduction on narrow terraces of the Pt(210) electrode becomes observable much later and is

PT

much less pronounced than on wider ones (compare: j0.05V in the fifth cycle is less than 0.1 mA/cm2 in the case of Pt(210) and more than 2 mA/cm2 in the case of Pt(610), while the Cu coverage is the

RI

same (θCu = 0.18). These results confirm that the presence of relatively wide Pt(100) domains plays

SC

a crucial role in acceleration of nitrate reduction on Cuad–modified Pt surfaces.

NU

Table 1. Copper coverages θCu on Pt(610) and Pt(210) single crystal electrodes after different

θCu on Pt(210)

0.17

0.09

0.18

0.18

0.52

0.45

CE

20

PT E

2 5

θCu on Pt(610)

D

Number of cycles

MA

amount of accumulation cycles in the potential range of 0.05–0.35 V.

AC

Finally, let us also consider evolution of the cathodic peak at 0.30–0.35 V in the anodic branch of the CV of Cu–modified cubic Pt NPs (the insets in Figs. 5a and 6a) corresponding to electroreduction of the nitrate anion to ammonia on wide Pt(100) terraces [29]. As follows from comparison with the insets in Figs. 6a,b, the presence of this peak in the case of Pt(100)NPs/GC points to the presence of relatively wide Pt(100) terraces on the surface of these NPs. Same as in the case of single crystal electrodes, this peak disappears rather fast in the course of copper adatom accumulation, while in the case of the Pt(poly)NP/Cuad electrode, its behavior being to a certain extent similar to that of the Pt(210) single crystal, there is practically no such peak in the range of

ACCEPTED MANUSCRIPT 22 0.30–0.35 V in the anodic CV branch. The disappearance of this peak is related to the fact that accumulation of copper adatoms on the surface of platinum results in a decrease in the surface coverage by hydrogen adatoms, the presence of which is critical for occurrence of the reaction of nitrate anion electroreduction to ammonia. Besides, as shown in [28, 75], the presence of copper adatoms on the surface leads to weaker adsorption of the reduction intermediate, NO, that can be

PT

too quickly desorbed from the surface, failing to be reduced to ammonia. The absence of the peak at 0.33–0.34 V in the case of Pt(poly)NPs and Pt(210) also confirms the absence of wide Pt(100)

SC

RI

terraces/domains on these surfaces.

4. Conclusions

NU

The regularities of electroreduction of nitrate anions on cubic and polyoriented platinum

MA

NPs are studied in the presence of very low concentrations of copper ions in the solution. In copper– and nitrate–containing solutions, growth of the currents of nitrate anion electroreduction

D

with accumulation of copper adatoms on the electrode is observed on both types of Pt NPs under

PT E

cycling in the range of 0.05–0.35 V. Comparison of the nitrate reduction currents on copper– modified Pt(100)NPs and Pt(poly)NPs at similar coverages under the chosen conditions shows that while the currents on unshaped NPs increase almost linearly, those on cubic particles manifest an

CE

S–shaped curve reaching the state of saturation due to diffusion limitations by nitrate anions. The

AC

activity of Cuad/Pt(100)NPs is significantly higher than that of Cuad/Pt(poly)NPs at copper coverages θCu > 0.15. This is due to the favorable geometry of nitrate adsorption on planar Pt(100) terraces that promotes Cu UPD; this, in its turn, reduces coulombic repulsion of anions from the surface. Such induced copper adsorption on Pt(100)NPs and facilitation of approach to the surface for nitrate anions on the Pt(100) terraces result in formation of a more stable Cu adlayer, as indicated by a significant anodic shift in the potential of the copper adlayer desorption peak as compared to nitrate–free solutions. Another important cause for higher nitrate reduction currents on Cu–modified Pt NPs is the lesser electrode surface poisoning due to weaker adsorption of the nitrate

ACCEPTED MANUSCRIPT 23 electroreduction intermediates and products on the copper adatoms as compared to the unmodified platinum surface. The pattern of electroreduction current growth on Cu–modified cubic Pt NPs is similar to the pattern of evolution of nitrate electroreduction currents from copper–containing solutions on the Pt(100) terraces of stepped Pt(610) single crystals. The behavior of polyoriented NPs under these conditions to a certain extent resembles the behavior of the Pt(210) single crystal

PT

electrode. The results obtained for NPs and Pt(hkl) single crystals demonstrate the key role of (100) terraces/domains in the acceleration of nitrate reduction on platinum surfaces modified by copper

RI

adatoms. At the same time, in comparison with macro single crystalline electrodes, NPs under study

SC

are characterized by sufficiently higher stability (under ambient atmosphere and in the electrolyte

NU

solution at rather high positive potentials), larger specific surface area, and flexibility in application.

MA

Acknowledgments

The work was supported by the Russian Foundation for Basic Research (project no. 14-03-

D

00530a). A.R. acknowledges financial support by the CTI Swiss Competence Center for Energy

PT E

Research (SCCER Heat and Electricity Storage). We are also grateful to Prof. Juan Feliu for providing platinum single crystals and advice in the preparation of shaped nanoparticles.

CE

Appendix A. Supplementary data

AC

Supplementary data to this article can be found online.

ACCEPTED MANUSCRIPT 24 References

[1] J.N. Tiwari, R.N. Tiwari, K.S. Kim, Zero-dimensional, one-dimensional, two-dimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices, Prog. Mater. Sci. 57 (2012) 724-803.

PT

[2] T. Herricks, J. Chen, Y. Xia, Polyol Synthesis of Platinum Nanoparticles:  Control of Morphology with Sodium Nitrate, Nano Letters 4 (2004) 2367-2371.

RI

[3] X. Wang, L. Figueroa-Cosme, X. Yang, M. Luo, J.Y. Liu, Z.X. Xie, Y.N. Xia, Pt-Based

SC

Icosahedral Nanocages: Using a Combination of {111} Facets, Twin Defects, and Ultrathin Walls to Greatly Enhance Their Activity toward Oxygen Reduction, Nano Letters 16 (2016) 1467-1471.

NU

[4] T. Bian, H. Zhang, Y.Y. Jiang, C.H. Jin, J.B. Wu, H. Yang, D.R. Yang, Epitaxial Growth of

MA

Twinned Au-Pt Core-Shell Star-Shaped Decahedra as Highly Durable Electrocatalysts, Nano Letters 15 (2015) 7808-7815.

D

[5] L. Du, S. Zhang, G.Y. Chen, G.P. Yin, C.Y. Du, Q. Tan, Y.R. Sun, Y.T. Qu, Y.Z. Gao,

PT E

Polyelectrolyte Assisted Synthesis and Enhanced Oxygen Reduction Activity of Pt Nanocrystals with Controllable Shape and Size, ACS Appl. Mater. Interfaces 6 (2014) 14043-14049. [6] M. Subhramannia, V.K. Pillai, Shape-dependent electrocatalytic activity of platinum

CE

nanostructures, J. Mater. Chem. 18 (2008) 5858-5870.

AC

[7] C. Wang, H. Daimon, Y. Lee, J. Kim, S. Sun, Synthesis of Monodisperse Pt Nanocubes and Their Enhanced Catalysis for Oxygen Reduction, J. Am. Chem. Soc. 129 (2007) 6974-6975. [8] L. Han, H. Liu, P.L. Cui, Z.J. Peng, S.J. Zhang, J. Yang, Alloy Cu3Pt nanoframes through the structure evolution in Cu-Pt nanoparticles with a core-shell construction, Sci Rep 4 (2014). [9] H.L. Liu, F. Nosheen, X. Wang, Noble metal alloy complex nanostructures: controllable synthesis and their electrochemical property, Chem. Soc. Rev. 44 (2015) 3056-3078. [10] J. Park, H.L. Wang, M. Vara, Y.N. Xia, Platinum Cubic Nanoframes with Enhanced Catalytic Activity and Durability Toward Oxygen Reduction, ChemSusChem 9 (2016) 2855-2861.

ACCEPTED MANUSCRIPT 25 [11] X. Wang, M. Luo, H.W. Huang, M.F. Chi, J. Howe, Z.X. Xie, Y.A. Xia, Facile Synthesis of Pt-Pd Alloy Nanocages and Pt Nanorings by Templating with Pd Nanoplates, ChemNanoMat 2 (2016) 1086-1091. [12] X.F. Yu, L.L. Li, Y.Q. Su, W. Jia, L.L. Dong, D.S. Wang, J.L. Zhao, Y.D. Li, PlatinumCopper Nanoframes: One-Pot Synthesis and Enhanced Electrocatalytic Activity, Chem.-Eur. J. 22

PT

(2016) 4960-4965. [13] L. Zhang, L.T. Roling, X. Wang, M. Vara, M.F. Chi, J.Y. Liu, S.I. Choi, J. Park, J.A. Herron,

SC

well-defined, controllable facets, Science 349 (2015) 412-416.

RI

Z.X. Xie, M. Mavrikakis, Y.N. Xia, Platinum-based nanocages with subnanometer-thick walls and

[14] N. Zhang, L.Z. Bu, S.J. Guo, J. Guo, X.Q. Huang, Screw Thread-Like Platinum-Copper

NU

Nanowires Bounded with High Index Facets for Efficient Electrocatalysis, Nano Letters 16 (2016)

MA

5037-5043.

[15] L.M. Falicov, G.A. Somorjai, Correlation between catalytic activity and bonding and

D

coordination-number of atoms and molecules on transition-metal surfaces - theory and

PT E

experimental-evidence, Proc. Natl. Acad. Sci. U. S. A. 82 (1985) 2207-2211. [16] A. Ruditskiy, H.C. Peng, Y.N. Xia, Shape-Controlled Metal Nanocrystals for Heterogeneous Catalysis, in: J.M. Prausnitz (Ed) Annual Review of Chemical and Biomolecular Engineering, Vol

CE

7, Annual Reviews, Palo Alto, 2016, pp. 327-348.

AC

[17] J.S. Spendelow, A. Wieckowski, Noble metal decoration of single crystal platinum surfaces to create well-defined bimetallic electrocatalysts, Physical Chemistry Chemical Physics 6 (2004) 5094-5118.

[18] L. Ma, X. Luo, A.J. Kropf, J. Wen, X. Wang, S. Lee, D.J. Myers, D. Miller, T. Wu, J. Lu, K. Amine, Insight into the Catalytic Mechanism of Bimetallic Platinum–Copper Core–Shell Nanostructures for Nonaqueous Oxygen Evolution Reactions, Nano Letters 16 (2016) 781-785. [19] J. Wu, H. Yang, Platinum-Based Oxygen Reduction Electrocatalysts, Accounts of Chemical Research 46 (2013) 1848-1857.

ACCEPTED MANUSCRIPT 26 [20] W. Huang, H. Wang, J. Zhou, J. Wang, P.N. Duchesne, D. Muir, P. Zhang, N. Han, F. Zhao, M. Zeng, J. Zhong, C. Jin, Y. Li, S.-T. Lee, H. Dai, Highly active and durable methanol oxidation electrocatalyst

based

on

the

synergy of

platinum–nickel

hydroxide–graphene,

Nature

Communications 6 (2015) 10035. [21] F. Gauthard, F. Epron, J. Barbier, Palladium and platinum-based catalysts in the catalytic

PT

reduction of nitrate in water: effect of copper, silver, or gold addition, Journal of Catalysis 220 (2003) 182-191.

RI

[22] V. Rosca, M. Duca, M.T. de Groot, M.T.M. Koper, Nitrogen Cycle Electrocatalysis, Chemical

SC

Reviews 109 (2009) 2209-2244.

[23] H. Bothe, S. Ferguson, W.E. Newton, The Biology of the Nitrogen Cycle, 2006.

NU

[24] L.W. Canter, Nitrates in Groundwater, 1997.

MA

[25] K. Nishimura, K. Machida, M. Enyo, On-line mass spectroscopy applied to electroreduction of nitrite and nitrate ions at porous Pt electrode in sulfuric acid solutions, Electrochimica Acta 36

D

(1991) 877-880.

PT E

[26] G. Schmid, M.A. Lobeck, Das Verhalten von salpetriger Säure und Salpetersäure an der rotierenden Scheibenelektrode. I. HNO2 in schwefelsauren Lösungen ohne HNO3-Zusatz, Berichte der Bunsengesellschaft für physikalische Chemie 73 (1969) 189-199.

CE

[27] R.R. Gadde, S. Bruckenstein, The electroduction of nitrite in 0.1 M HClO4 at platinum,

AC

Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 50 (1974) 163-174. [28] G.E. Dima, A.C.A. de Vooys, M.T.M. Koper, Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions, Journal of Electroanalytical Chemistry 554 (2003) 15-23. [29] E.B. Molodkina, I.G. Botryakova, A.I. Danilov, J. Souza-Garcia, J.M. Feliu, Kinetics and mechanism of nitrate and nitrite electroreduction on Pt(100) electrodes modified by copper adatoms, Russian Journal of Electrochemistry 49 (2013) 285-293.

ACCEPTED MANUSCRIPT 27 [30] E.B. Molodkina, I.G. Botryakova, A.I. Danilov, J. Souza-Garcia, J.M. Feliu, Mechanism of nitrate electroreduction on Pt(100), Russian Journal of Electrochemistry 48 (2012) 302-315. [31] E.B. Molodkina, M.R. Ehrenburg, Y.M. Polukarov, A.I. Danilov, J. Souza-Garcia, J.M. Feliu, Electroreduction of nitrate ions on Pt(1 1 1) electrodes modified by copper adatoms, Electrochimica Acta 56 (2010) 154-165.

controlled Pt nanoparticles, Chem. Commun. (2005) 3730-3732.

PT

[32] A. Miyazaki, T. Asakawa, Y. Nakano, I. Balint, Nitrite reduction on morphologically

RI

[33] Q.A. Cheng, Y.X. Jiang, N. Tian, Z.Y. Zhou, S.G. Sun, Electrocatalytic reduction of nitric

SC

oxide on Pt nanocrystals of different shape in sulfuric acid solutions, Electrochimica Acta 55 (2010) 8273-8279.

NU

[34] Q.F. Wang, X.B. Zhao, J.F. Zhang, X.W. Zhang, Investigation of nitrate reduction on

MA

polycrystalline Pt nanoparticles with controlled crystal plane, Journal of Electroanalytical Chemistry 755 (2015) 210-214.

D

[35] L.A. Estudillo-Wong, G. Santillan-Diaz, E.M. Arce-Estrada, N. Alonso-Vante, A. Manzo-

Acta 88 (2013) 358-364.

PT E

Robledo, Electroreduction of NOxz species in alkaline medium on Pt nanoparticles, Electrochimica

[36] M.C. Figueiredo, J. Solla-Gullon, F.J. Vidal-Iglesias, V. Climent, J.M. Feliu, Nitrate reduction

AC

202 (2013) 2-11.

CE

at Pt(100) single crystals and preferentially oriented nanoparticles in neutral media, Catalysis Today

[37] C. Roy, J. Deschamps, M.H. Martin, E. Bertin, D. Reyter, S. Garbarino, L. Roué, D. Guay, Identification of Cu surface active sites for a complete nitrate-to-nitrite conversion with nanostructured catalysts, Applied Catalysis B: Environmental 187 (2016) 399-407. [38] A. Miyazaki, K. Matsuda, F. Papa, M. Scurtu, C. Negrila, G. Dobrescu, I. Balint, Impact of particle size and metal-support interaction on denitration behavior of well-defined Pt-Cu nanoparticles, Catal. Sci. Technol. 5 (2015) 492-503.

ACCEPTED MANUSCRIPT 28 [39] N. Vila, M. Van Brussel, M. D'Amours, J. Marwan, C. Buess-Herman, D. Belanger, Metallic and bimetallic Cu/Pt species supported on carbon surfaces by means of substituted phenyl groups, Journal of Electroanalytical Chemistry 609 (2007) 85-93. [40] R.A. Martinez-Rodriguez, F.J. Vidal-Iglesias, J. Solla-Gullon, C.R. Cabrera, J.M. Feliu, Synthesis of Pt Nanoparticles in Water-in-Oil Microemulsion: Effect of HCl on Their Surface

PT

Structure, J. Am. Chem. Soc. 136 (2014) 1280-1283. [41] C. Coutanceau, P. Urchaga, S. Brimaud, S. Baranton, Colloidal Syntheses of Shape- and Size-

RI

Controlled Pt Nanoparticles for Electrocatalysis, Electrocatalysis 3 (2012) 75-87.

SC

[42] J. Solla-Gullon, V. Montiel, A. Aldaz, J. Clavilier, Electrochemical characterisation of platinum nanoparticles prepared by microemulsion: how to clean them without loss of crystalline

NU

surface structure, Journal of Electroanalytical Chemistry 491 (2000) 69-77.

MA

[43] R.A. Martinez-Rodriguez, F.J. Vidal-Iglesias, J. Solla-Gullon, C.R. Cabrera, J.M. Feliu, Synthesis and Electrocatalytic Properties of H2SO4-Induced (100) Pt Nanoparticles Prepared in

D

Water-in-Oil Microemulsion, ChemPhysChem 15 (2014) 1997-2001.

PT E

[44] A. Dekanski, J. Stevanovic, R. Stevanovic, B.Z. Nikolic, V.M. Jovanovic, Glassy carbon electrodes I. Characterization and electrochemical activation, Carbon 39 (2001) 1195-1205. [45] J. Mattusch, K.H. Hallmeier, K. Stulik, V. Pacakova, Pretreatment of glassy-carbon electrodes

CE

by anodic galvanostatic pulses with a large-amplitude, Electroanalysis 1 (1989) 405-412. .

AC

[46] J. Clavilier, Interfacial electrochemistry : theory, e periment, and applications, in:

i ckowski (Ed) Flame-annealing and cleaning technique, Marcel Dekker, New York :, 1999, pp. 231.

[47] J. Clavilier, D. Armand, S.G. Sun, M. Petit, Electrochemical adsorption behaviour of platinum stepped surfaces in sulphuric acid solutions, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 205 (1986) 267-277.

ACCEPTED MANUSCRIPT 29 [48] J. Clavilier, R. Faure, G. Guinet, R. Durand, Preparation of monocrystalline Pt microelectrodes and electrochemical study of the plane surfaces cut in the direction of the {111} and {110} planes, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 107 (1980) 205-209. [49] J. Solla-Gullon, P. Rodriguez, E. Herrero, A. Aldaz, J.M. Feliu, Surface characterization of platinum electrodes, Physical Chemistry Chemical Physics 10 (2008) 1359-1373.

PT

[50] P. Rodriguez, E. Herrero, J. Solla-Gullon, F.J. Vidal-Iglesias, A. Aldaz, J.M. Feliu, Specific surface reactions for identification of platinum surface domains - Surface characterization and

RI

electrocatalytic tests, Electrochimica Acta 50 (2005) 4308-4317.

SC

[51] J. Solla-Gullon, E. Lafuente, A. Aldaz, M.T. Martinez, J.M. Feliu, Electrochemical

Electrochimica Acta 52 (2007) 5582-5590.

NU

characterization and reactivity of Pt nanoparticles supported on single-walled carbon nanotubes,

MA

[52] F.J. Vidal-Iglesias, J. Solla-Gullon, J.M. Campina, E. Herrero, A. Aldaz, J.M. Feliu, CO monolayer oxidation on stepped Pt(S) [(n в€’ 1)(1 0 0) Г— (1 1 0)] surfaces, Electrochimica Acta

D

54 (2009) 4459-4466.

PT E

[53] R. Francke, V. Climent, H. Baltruschat, J.M. Feliu, Electrochemical deposition of copper on stepped platinum surfaces in the [0 1 ] zone vicinal to the (1 0 0) plane, Journal of Electroanalytical Chemistry 624 (2008) 228-240.

CE

[54] A.V. Rudnev, T. Wandlowski, An influence of pretreatment conditions on surface structure

AC

and reactivity of Pt(100) towards CO oxidation reaction, Russian Journal of Electrochemistry 48 (2012) 259-270.

[55] K. Domke, E. Herrero, A. Rodes, J.M. Feliu, Determination of the potentials of zero total charge of Pt(100) stepped surfaces in the [011М„] zone. Effect of the step density and anion adsorption, Journal of Electroanalytical Chemistry 552 (2003) 115-128. [56] V. Climent, R. Gomez, J.M. Orts, J.M. Feliu, Thermodynamic Analysis of the Temperature Dependence of OH Adsorption on Pt(111) and Pt(100) Electrodes in Acidic Media in the Absence of Specific Anion Adsorption, The Journal of Physical Chemistry B 110 (2006) 11344-11351.

ACCEPTED MANUSCRIPT 30 [57] P.S. Fernandez, D.S. Ferreira, C.A. Martins, H.E. Troiani, G.A. Camara, M.E. Martins, Platinum nanoparticles produced by EG/PVP method: The effect of cleaning on the electrooxidation of glycerol, Electrochimica Acta 98 (2013) 25-31. [58] R.M. Arán-Ais, F.J. Vidal-Iglesias, J. Solla-Gullón, E. Herrero, J.M. Feliu, Electrochemical Characterization of Clean Shape-Controlled Pt Nanoparticles Prepared in Presence of

PT

Oleylamine/Oleic Acid, Electroanalysis 27 (2015) 945-956. [59] J. Solla-Gullon, V. Montiel, A. Aldaz, J. Clavilier, Synthesis and Electrochemical

RI

Decontamination of Platinum-Palladium Nanoparticles Prepared by Water-in-Oil Microemulsion,

SC

2003, pp. E104-E109.

[60] P. Rodriguez, J. Solla-Gullon, F.J. Vidal-Iglesias, E. Herrero, A. Aldaz, J.M. Feliu,

MA

bismuth, Anal. Chem. 77 (2005) 5317-5323.

NU

Determination of (111) ordered domains on platinum electrodes by irreversible adsorption of

[61] P. Rodriguez, E. Herrero, J. Solla-Gullon, E.J. Vidal-Iglesias, A. Aldaz, J.M. Feliu,

D

Electrochemical characterization of irreversibly adsorbed germanium on platinum stepped surfaces

PT E

vicinal to Pt(100), Electrochimica Acta 50 (2005) 3111-3121. [62] E.B. Molodkina, A.I. Danilov, J.M. Feliu, Cu UPD at Pt(100) and stepped faces Pt(610), Pt(410) of platinum single crystal electrodes, Russian Journal of Electrochemistry 52 (2016) 890-

CE

900.

AC

[63] A. Rodes, M.A. Zamakhchari, K. El Achi, J. Clavilier, Electrochemical behaviour of Pt(100) in various acidic media, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 305 (1991) 115-129.

[64] J.M. Feliu, A. Rodes, J.M. Orts, J. Clavilier, The problem of surface order of Pt single-crystals in electrochemistry, Pol. J. Chem. 68 (1994) 1575-1595. [65] A.V. Rudnev, M.R. Ehrenburg, E.B. Molodkina, I.G. Botriakova, A.I. Danilov, T. Wandlowski, CO2 Electroreduction on Cu-Modified Platinum Single Crystal Electrodes in Aprotic Media, Electrocatalysis 6 (2015) 42-50.

ACCEPTED MANUSCRIPT 31 [66] A. Rodes, V. Climent, J.M. Orts, J.M. Pérez, A. Aldaz, Nitric oxide adsorption at Pt(100) electrode surfaces, Electrochimica Acta 44 (1998) 1077-1090. [67] E.B. Molodkina, A.I. Danilov, M.R. Ehrenburg, J.M. Feliu, Reduction of nitrate on Pt[n(100)x(110)] stepped single crystal faces modified by copper adatoms, in preparation (2017). [68] A.I. Danilov, R.R. Nazmutdinov, T.T. Zinkicheva, E.B. Molodkina, A.V. Rudnev, Y.M.

PT

Polukarov, J.M. Feliu, Mechanism of copper underpotential deposition at Pt(hkl)-electrodes: Quantum-chemical modelling, Russian Journal of Electrochemistry 44 (2008) 697-708.

RI

[69] H.D. Abruna, J.M. Feliu, J.D. Brock, L.J. Buller, E. Herrero, J. Li, R. Gуmez, A. Finnefrock,

SC

Anion and electrode surface structure effects on the deposition of metal monolayers: electrochemical and time-resolved surface diffraction studies, Electrochim. Acta 43 (1998) 2899-

NU

2909.

brun˜a,

nion effects and the

MA

[70] R. Gómez, H.S. Yee, G.M. Bommarito, J.M. Feliu, H.D.

mechanism of Cu UPD on Pt(111): X-ray and electrochemical studies, Surf. Sci. 335 (1995) 101-

D

109.

PT E

[71] W. Biemolt, G.J.C.S. van de Kerkhof, P.R. Davies, A.P.J. Jansen, R.A. van Santen, Chemisorption theory of ammonia on copper, Chemical Physics Letters 188 (1992) 477-486. [72] J. Robinson, D.P. Woodruff, The local adsorption geometry of CH3 and NH3 on Cu(1 1 1): a

CE

density functional theory study, Surface Science 498 (2002) 203-211.

AC

[73] K.J. Wu, S.D. Kevan, E.G. Seebauer, A.C.F. Kong, L.D. Schmidt, Isothermal measurements of NH3 and ND3 desorption from Cu(001), The Journal of Chemical Physics 94 (1991) 7494-7498. [74] G.A. Attard, O. Hazzazi, P.B. Wells, V. Climent, E. Herrero, J.M. Feliu, On the global and local values of the potential of zero total charge at well-defined platinum surfaces: stepped and adatom modified surfaces, Journal of Electroanalytical Chemistry 568 (2004) 329-342. [75] M. Neurock, R.A. Vansanten, W. Biemolt, A.P.J. Jansen, Atomic and molecular-oxygen as chemical precursors in the oxidation of ammonia by copper, J. Am. Chem. Soc. 116 (1994) 68606872.

ACCEPTED MANUSCRIPT 32 [76] J.H. White, H.D. Abruna, Coadsorption of copper with anions on platinum (111): the role of surface redox chemistry in determining the stability of a metal monolayer, The Journal of Physical Chemistry 94 (1990) 894-900. [77] D.W. Miwa, M.C. Santos, S.A.S. Machado, A Microgravimetric Study of Simultaneous Adsorption of Anions and Copper on Polycrystalline Pt Surfaces, J. Braz. Chem. Soc. 17 (2006)

PT

1339-1346. [78] E. Herrero, L.J. Buller, H.D. Abruña, Underpotential Deposition at Single Crystal Surfaces of

RI

Au, Pt, Ag and Other Materials, Chemical Reviews 101 (2001) 1897-1930.

AC

CE

PT E

D

MA

NU

electrodes, Electrochimica Acta 50 (2005) 4318-4326.

SC

[79] G.E. Dima, G.L. Beltramo, M.T.M. Koper, Nitrate reduction on single-crystal platinum

ACCEPTED MANUSCRIPT

NU

SC

RI

PT

33

AC

CE

PT E

D

MA

Graphical abstract

ACCEPTED MANUSCRIPT 34 Highlights

Cubic and unshaped Pt NPs are modified by Cu adatoms with different coverages



Nitrate reduction is strongly promoted on Pt NP electrodes modified by Cuad



(100) domains further accelerate nitrate reduction on Pt NPs modified by Cuad



Nitrate reduction on Pt/Cuad NPs is compared to that on Pt(hkl)/Cuad electrodes

AC

CE

PT E

D

MA

NU

SC

RI

PT

