The Art of Decoration: Rhodium-Modified Platinum Films with

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The Art of Decoration: Rh-modified Pt Thin Films with Preferential (100) Orientation as Electrocatalysts for Nitrate Reduction and Dimethyl Ether Oxidation Matteo Duca, Jonathan Kightley, Sebastien Garbarino, and Daniel Guay J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04332 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

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The Journal of Physical Chemistry

The Art of Decoration: Rh-modified Pt Thin Films with Preferential (100) Orientation as Electrocatalysts for Nitrate Reduction and Dimethyl Ether Oxidation Matteo Duca*, Jonathan Kightley†, Sébastien Garbarino, Daniel Guay* Institut National de la Recherche Scientifique, INRS-Énergie, Matériaux et Télécommunications, 1650, Boulevard Lionel-Boulet, Varennes, J3X 1S2 (Canada)

Abstract

Nanostructured platinum films with a preferential (100) orientation were decorated with a rhodium adlayer through potentiodynamic electrodeposition. The Rh surface coverage was controlled by varying the number of electrodeposition cycles, and the growth of the Rh layer was followed by means of the distinct hydrogen adsorption/desorption peaks pertaining to Rh. The bimetallic electrodes were tested for their catalytic activity towards a set of electrochemical reactions of the carbon and nitrogen cycles (nitrate reduction, CO oxidation and DME oxidation) in H2SO4. At the same time, these reactions provided further insight into the surface structure of the Rh deposits: thus, it was found that Rh deposition occurs mostly at (100) sites, creating

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ordered Rh islands. It was shown that DME oxidation at Pt(100) can be further promoted by minimum Rh coverages, at which the highly active (100) terraces remain mostly free. On the other hand, the activity towards nitrate reduction can be enhanced by increasing amounts of Rh. Finally, we succeeded in directing Rh deposition through the partial stripping of adsorbed CO, using this molecule as a site-blocking adlayer protecting wide (100) terraces, thus achieving the maximum enhancement for DME oxidation.

Main text 1 Introduction Bimetallic Pt-Rh electrodes have attracted considerable interest with an eye to improving the electrocatalytic performance of the individual metals. In a chronological sequence, the focus has gradually shifted from the preparation of (polycrystalline) Pt-Rh alloys1 towards the modification of Pt surfaces with Rh2. In this respect, the controlled deposition – also known as decoration – of Rh adlayers onto a Pt substrate is suitable for well-ordered model surfaces such as single-crystal electrodes, thus allowing the determination of the surface arrangement of Rh adlayers and their effect on catalytic activity. In turn, this has taken the control and the enhancement of electrochemical reactions to a whole new level. Directing and tracking the growth of Rh adlayers on Pt has always represented a primary concern since the first studies of Rh deposition on Pt single-crystals published in the 1990s

3-4

.

The “electrosorption”, or potentiodynamic, method5-10 emerged as a very effective technique to prepare Rh deposits on Pt substrates. In this method, the Pt electrode is immersed under potential control in an electrolyte containing a suitable Rh precursor at a low concentration (10−6 – 10−4 M), usually in the form of a Rh(III) salt. Then, the potential is repeatedly swept into the potential


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range in which Rh(III) can be reduced to its elemental form at the Pt surface, allowing a finetuned, dynamic control of the cycle-by-cycle growth of the Rh adlayer. The scan rate plays a key role, determining the amount of time spent in the region where Rh deposition occurs, and hence the extent of Rh reduction during each cycle. Obviously, the choice of the Rh precursor will also contribute to establishing the pattern of the growth of the Rh adlayer. Gomez et al.

5

first employed this method for Rh deposition at a Pt(111) electrode, using a

H2SO4 solution containing Rh2(SO4)3. It was shown that the deposition of the first Rh layer proceeds pseudomorphically through the formation of small, compact 2D islands entailing slightly strained Rh-Rh distances, which was confirmed in a following study11. A deposition rate of 0.03 ML min−1 was reported, which corresponds to a very slow process. The Rh incomplete adlayer gave rise to two sharp peaks at 0.16 V (positive-going scan) and 0.13 V (negative-going scan),

associated

with

hydrogen

adsorption/desorption

overlapped

to

bisulphate

adsorption/desorption. The presence of Rh enhanced the dissociation of adsorbed NO, as shown by spectroelectrochemical IR studies. Additionally, sub-monolayer amounts of Rh greatly modified the voltammetric features associated with the oxidation of adsorbed CO, leading to a lower onset potential and to the splitting of the oxidation peak. A bifunctional mechanism was proposed, Rh adatoms providing adsorbed OH for the oxidation of CO. On the other hand, the Rh decoration of Pt(100) is of greater relevance to this paper. In a first report by de Dios et al. 6, a potentiostatic method was instead used, and Rh was deposited from a 0.1 M HClO4 solution containing Rh(III) salt and an excess of chloride. Voltammetric measurements highlighted a pair of well-defined reversible peaks located at 0.18 V vs RHE (in 0.5 M H2SO4): their intensity increased with growing Rh coverages, while the features of bare Pt(100) decreased. This evidence suggested, once more, the formation of compact Rh islands.


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The voltammetric charges were used to quantify the Rh coverage, and two alternative methods to compute this parameter were discussed. In a follow-up paper8, the potentiodynamic method was instead employed to decorate Pt(100) with Rh. The voltammetric evidence suggested that the operative deposition mode yielded disordered clusters and islands, leaving a significant amount of the surface unaffected by the Rh deposition. Finally, a recent report on the Rh modification of Pt nanoparticles by means of potentiodynamic deposition in acidic Rh(III) solutions can be seen as the demonstration that this method has proven broadly applicable to Pt surfaces of varying orientations9. Among the surface-sensitive electrochemical reactions, the oxidation of dimethylether (DME) assumes a special importance in that this molecule displays a significant potential as a candidate to replace alcohols in fuel cells12. In a series of experimental studies13-16, a Pt(100) electrode was shown to be the most active towards DME oxidation in acidic media. The voltammetric features and the kinetic data were interpreted by analogy with methanol oxidation, since DME can be regarded as a combination of two methoxy moieties. Despite remaining a slow process, the dissociative adsorption of DME at Pt(100) occurred faster than for the other Pt basal planes. Inhibited by adsorbed hydrogen, but favoured by the introduction of steps, the dissociative adsorption proceeds through DME dehydrogenation to give adsorbed CH3OCH2− at low potentials. This adsorbate, visualised by IR spectroelectrochemistry, undergoes further stepwise dehydrogenation to –COCHads, finally breaking down into COads and CHads 17. Eventually, CHads is converted into COads and then to the final product, CO2. The origin of the enhanced catalytic activity of defect-free (100) terraces was first attributed to the direct oxidation of DME through the formation of adsorbed CH3O− as intermediate16. Instead, electrochemical and DFT studies indicated that surface-sensitivity rather arises from the rate-determining step involving the


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cleavage of the C-O bond in –COCHads

17-19

. On the other hand, some fcc metals (Ni, Ir, Rh)

seem to be limited by the oxidative removal of CHx,ads and CHyOads, which preferentially occurs at (111) facets

20-21

. When designing bimetallic surfaces, the best results should be obtained by

depositing promoters in step and defect sites while preserving pristine and highly active (100) Pt domains. In this respect, Pt(100) single-crystals decorated with a very low coverage of Ru featured a lower onset potential for DME oxidation22. This is in agreement with the known ability of Ru to minimise CO poisoning by facilitating its oxidation, which is the rationale behind the widespread research on PtRu catalysts for methanol oxidation23. Concerning the nitrogen cycle, nitrate reduction has attracted special interest for its relevance to environmental issues, nitrate being an undesirable pollutant in agricultural runoffs and wastewater24-25. Nitrate reduction at Pt is particularly hindered by the co-adsorption of other species, i.e. hydrogen or electrolyte anions such as (bi)sulphate. Despite this, Pt(100) still stands out as a peculiar crystal orientation, giving rise to a voltammetric signal for nitrate reduction located at a relatively high potential (0.32 V vs RHE in 0.5 M H2SO4) 26 and sensitive to terrace length27 . On the other hand, Rh and Ir tend to be more active than Pt towards nitrate reduction; in particular, Rh is able to reduce both HNO3 and NO3−, performing nitrate reduction even in the absence of solution protons, while the reactivity of Pt seems to be restricted to HNO3 28. This can be understood from various points of view: the lower potential of zero charge of Rh

29

(which

facilitated the adsorption of anions), or the number of valence electrons of Rh30. Therefore, the surface modification of Pt(100) with Rh could result in the enhancement of nitrate reduction, provided that there is a trade-off between loss of surface order and increasing coverage of Rh. As discussed in this introduction, all papers addressing Rh deposition on ordered Pt surfaces have either dealt with single-crystal electrodes4-6, 8, 11 or preferentially-oriented nanoparticles9. In


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this work, we instead focus on a different type of Pt surface: electrodeposited thin film having a preferential (100) orientation31-38. These electrodes are significantly more inexpensive than single-crystal model surfaces, while retaining a high proportion of (100) sites which are extremely active towards several electrochemical reactions. However, the very structure of the electrodeposited Pt films represents a significant challenge in terms of the controlled deposition of Rh adlayers and site-selective decoration. We shall address this topic in Section 3.1, in which we will discuss the characterisation of the Rh adlayers with electrochemical probe reactions, including the reduction of nitrite in alkaline media and the oxidation of adsorbed CO. It will be shown that Rh deposition occurs more readily on Pt(100) domains, leading to the formation of an ordered Rh adlayer, as demonstrated by voltammetric features. Secondly, Section 3.2 will focus on two reactions, DME oxidation and nitrate reduction, showing the trends in catalytic activity as a function of Rh coverage. Rh decreases the onset potential for DME oxidation, most likely according to a ‘bifunctional’ mechanism, and a very low Rh coverage enhances the current associated with DME oxidation at Pt. Instead, higher coverages lead to a rapid loss in activity due to the blockage of (100) Pt terraces by Rh adatoms. However, Rh deposition onto the highly active (100) terraces can be avoided by using adsorbed CO as a protective group. The bimetallic surface obtained with this modified protocol displays the most significant enhancement towards DME oxidation. On the other hand, Rh dominates the nitrate reduction performance of a bimetallic surface: increasing amounts of Rh lead to voltammetric features typical of this metal, while the response of pure Pt disappears. 2 Experimental Section All glassware was cleaned by immersing it in an aqueous H2SO4/KMnO4 solution overnight (this cleaning bath was recycled and renewed every month), in order to remove organic


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contaminants. Residues of permanganate were then removed with a dilute H2O2/H2SO4 solution. Finally, boiling ultrapure water (Millipore Gradient, MilliQ®, resistivity > 18.2 MΩ cm) was employed to eliminate ions left from previous cleaning steps. All electrochemical experiments were carried out in a three-electrode configuration and in standard one-compartment glass vessels. A mercury-mercury sulphate reference electrode (MMSE) was employed throughout this paper, unless otherwise stated. Its potential was calibrated with respect to a RHE electrode and found to be constant in time and equal to E = 700 mV vs RHE. Potential values are always expressed with respect to the RHE scale. The counter electrode was a platinum gauze, which was cleaned by flame-annealing, followed by quenching in MilliQ water, prior to immersion in the working electrolyte. Cyclic voltammetry and chronoamperometry measurements were all performed with a BioLogic VSP potentiostat. Electrolyte and stock solutions were prepared with ultrapure water. 96 % H2SO4 (TraceMetal Grade, Fisher Scientific) and NaOH pellets (99.99% metal basis, Alfa Aesar) were employed to prepare the supporting electrolytes. Chemicals and gases were used as received and provided by various suppliers, as follows: Ar (AirLiquide, UHP, 5.0), CO (Praxair, 5.0), NaNO3 (99.995%, Aldrich), NaNO2 (99.999% trace metal basis, Sigma Aldrich), dimethyl ether (DME, 99.9 % , Air Liquide) and CH3OH (spectroscopy grade, Uvasol ®, EMD Millipore). NaNO3 and NaNO2 were stored in a desiccator. Details about Pt and Rh electrodeposition can be found in the following paragraphs. Pt electrodes were prepared by electroplating onto strips of 0.5 mm thick Ti foil (99%, Alfa Aesar) from a plating bath containing 0.5 mM Na2PtCl6 (31.30-36.00 % w/w Pt, Alfa Aesar) in 0.01M HCl (TraceMetal Grade, Fisher Scientific), as described previously31-34, 36-37, 39. The main steps of the electroplating procedure are as follows: first, care was taken to remove oxides from


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the Ti surface immediately prior to the deposition of Pt: Ti foils were coarsely polished with sandpaper and subsequently boiled in 10% oxalic acid (anhydrous, 98%, Acros Organics) for an hour. The geometric surface area was kept constant at 1 cm2 for all electrodes by covering the Ti foil with Teflon tape. The reference electrode employed for Pt electrodeposition was a standard calomel electrode (SCE). The electrodes were held at a potential of −0.285 V vs SCE for an hour each, then rinsed and stored under water; Pt electrodes were prepared in small batches to minimise storage prior to use. Characterisation and electrocatalytic experiments involved two cells filled with the same supporting electrolyte, 0.5 M H2SO4. The electrodeposited Pt film was characterised by cyclic voltammetry between 0.045 V and 0.85 V at 20 mV s−1 ; the upper potential was limited at 0.85 V so as to avoid the loss of preferential orientation of the Pt surface. CO adsorption was performed by bubbling CO through the working electrolyte for 10 minutes while the potential was held at 0.08 V, followed by bubbling Ar for at least 45 minutes to purge CO. Oxidative removal of CO, or “CO stripping”, was performed using the same conditions as the first. The blank profile was recovered after the first scan. Immediately after CO stripping, Rh deposition was carried out in the same cell following the potentiodynamic procedure described by Buso-Rogero et al.9 for the decoration of Pt nanoparticles, which employed RhCl3 as Rh precursor. In fact, the alternative protocols described by the Alicante group for Pt(100) (see Introduction) are instead more suitable for the preparation of well-ordered adlayers, which was not the primary goal of our study. An aliquot of a 0.01 M RhCl3 solution prepared from rhodium trichloride hexahydrate (38% Rh, Acros Organics) was added to the cell under vigorous Ar bubbling to attain a concentration of 6·10−5 M, and Rh3+ was electrodeposited over the course of several cycles at 50 mV s−1 from 0.8 V (the


8

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contact potential) to 0.045 V. The Rh concentration, though very low, was sufficient to provide a large excess of Rh: the molar ratio between Rh atoms available for deposition and Pt surface atoms (measured by hydrogen adsorption) is 50, obviously varying as a function of the roughness of the Pt deposits. The use of a chloride salt was motivated by the known beneficial effect of this anion, favouring the growth of more ordered Rh adlayers 6. The Rh coverage could be controlled by monitoring the voltammogram and varying the number of cycles for different electrodes. The cycle count ranged from 10 to 200. After the deposition, the electrodes were transferred to a cell containing deoxygenated clean 0.5 M H2SO4, and contact was made with the solution under potential control at E = 0.08 V. A second CO stripping experiments (identical conditions as the first, except for the upper potential limit, which was extended to 0.9 V) was then performed with the Rh-decorated Pt electrode. A reactant, either DME, CH3OH (see Supporting Information, section S2), or NaNO3 was introduced to the cell. It should be emphasized that a fresh electrode was employed for every single electrocatalytic experiment involving a different reactant: no electrode was used more than once. This was explicitly done to ensure the cleanliness of initial Pt surface – which could not be cleaned by flame-annealing. Sodium nitrate was added as a 1 M stock solution directly to the cell while the electrode was held at 0.8 V. The solution was homogenized by bubbling Ar, and then a voltammetric cycle was taken from 0.8 V to 0.045 V at 5 mV s−1 while blanketing with Ar. The nitrate concentrations used were 1 mM, 2.5 mM, 5 mM, 7.5 mM, and 10 mM. DME was bubbled through the electrolyte for 20 minutes to ensure that the electrolyte be saturated with the gas16, and bubbling continued during the reaction under Ar blanketing. The voltammogram was taken from 0.08 V to 0.9 V at 50 mV s−1.


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For the experiment shown in Figure 3 (reduction of nitrite in alkaline media), the second cell contained 0.1 M NaOH. The reduction of NaNO2 was carried out under the same conditions as those of previous studies of this reactions at Pt nanocrystals40 and Pt single-crystal electrodes4142

: NaNO2 was added to the working electrolyte to achieve a concentration equal to 2 mM. A

single voltammetric scan was then performed at 20 mV s−1 while blanketing with Ar. 3. Results This section is divided into two parts: firstly, we shall address the preparation and the characterisation of Rh adlayers. Section 3.1 will present the voltammetric features in blank electrolyte, and the response for the reduction of nitrite in NaOH and for the oxidation of COads in H2SO4. Later, Section 3.2 will address the electrocatalytic activity of bimetallic Rh-Pt surfaces towards the oxidation of DME and the reduction of nitrate. 3.1 Rh adlayers: preparation and characterisation 3.1.1 Rh deposition and estimation of Rh coverage Figure 1A displays the voltammetric profile of the pristine Pt surface (recorded in clean 0.5 M H2SO4) along with selected cycles recorded during Rh deposition in 0.5 M H2SO4 containing 6·10−5 M RhCl3 (Figure 1B). These are representative of different stages in the Rh decoration of the Pt surface. In the first place, the blank voltammogram of the as-prepared Pt electrode is in very good agreement with previously published results

31-37

; it displays four main features,

labelled h1, h2, h3, h4 from lower to higher potentials33, 37. These signals arise from the various surface domains that constitute the electrodeposited Pt film43. The largest voltammetric peak is h2 (Eh2 = 0.26 V) which is associated with hydrogen and (bi)sulphate adsorption/desorption at (100) steps and narrow terraces. To its left (Eh1 = 0.1 V), h1 can be observed, which arises from


10

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(110) steps, also called “defects” throughout this paper. On its right, h2 is flanked by a shoulder with a small peak at 0.38 V: this is h3, and evidences the presence of larger (100) terraces. Finally, the broad, flat feature at 0.48 V can be ascribed to bisulphate adsorption/desorption on well-ordered 2D (111) domains. The quantification of the proportion of (100) sites as a function of the potential applied during Pt plating was carried out elsewhere37-38 by combining a site-specific probe (Bi irreversible adsorption onto (111) sites) and deconvolution of the hydrogen underpotential deposition peaks. The latter provided the proportion of short and wide (100) terraces. For electrodes deposited under the same conditions as in the present study, (100) domains accounted for 40% of the surface. The deposition time, and in turn the recorded current, allowed us to control the roughness of the Pt film; the average roughness factor (Rf) of the electrodes used in this paper was 35. In a previous publication37, it was shown that the proportion of wide (100) terraces increases as a function of the roughness factor, but, at the same time, rougher electrodes are affected from more severe mass transport limitations. It was suggested that a trade-off between larger amount of (100) sites and minimisation of mass transfer issues was achieved at Rf =38.


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Figure 1 Panel A: Blank voltammetric profile (0.5 M H2SO4) of a Pt electrode obtained by electrodeposition onto a Ti plate, v = 20 mV s−1. Peak labels refer to the main text. Dotted line: voltammetric profile corresponding to the fifth cycle recorded for the same Pt electrode after adding 6 ·10−5 M RhCl3 to the H2SO4 solution. Panel B: Evolution of the voltammetric profile for the same Pt electrode in the same Rh-containing 0.5 M H2SO4 solution, as a function of the


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number of cycles, v = 50 mV s−1. The y-axis expresses current density with respect to geometric area (1 cm2).

Upon Rh deposition, a gradual decrease in the intensity of all four h peaks was observed, although the loss of peak area occurs at different stages for the various peaks. After 5 cycles in the Rh-containing solution, peaks h2 and h3 ((100) sites) decrease most rapidly, while h4 (associated with (111) sites) and h1 (a proxy for “defect” sites) are much less affected. Eventually, the Pt peaks arising from long, ordered domains (h3 and h4) disappear completely, while peaks h1 and h2 are still present. At the same time, Rh deposition gives rise to a couple of peaks, labelled Rh1 and Rh1*, located at E = 0.16 V (Rh1, oxidation) and 0.11 V (Rh1*, reduction), along with a broad oxidation plateau (RhOX) starting from 0.5 V and its counterpart, an asymmetric reduction peak at 0.55 V (RhOX*). Rh1 and Rh1* peaks can be assigned to hydrogen and anion adsorption/desorption, while RhOX peaks are associated with the formation of Rh oxide44. For higher Rh coverages, Rh1 and Rh1* develop into sharp peaks and their position remains unchanged. This can be compared to previous reports on the growth of ordered Rh adlayers on Pt(100) single-crystals6, 8. In fact Rh1 and Rh1* are surprisingly close to their counterparts for a well-ordered flame-annealed Rh(100) electrode (0.16 and 0.12 V, respectively)45-46, testifying to the fact that the Rh deposits have a significant degree of order. This is further confirmed by the position and the well-defined shape of the oxide reduction peak RhOX*. Multilayer Rh deposition on Pt, signalled by the appearance of a couple of hydrogen adsorption/desorption peaks located at a potential 50 mV negative of Rh1 and Rh1*

5, 47

, was

never observed. Deposition of a Rh biatomic adlayer can also be ruled out, because Rh1 and Rh1* are expected to shift to more negative values in response to the formation of such adlayer 8.


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Figure 2 Graphical representation of the measured charge associated with the reduction of Rh oxides (qRh,OX*, squares), or the desorption of hydrogen (qRh,H, circles), as a function of the duration of Rh deposition, expressed as number of cycles (see Figure 1). The plot in Figure 2 reports charges recorded in Rh-free electrolyte (see also Supporting Information, section S1).


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Figure 2 addresses the issue of the evaluation of the Rh coverage. Literature reports on the estimation of the real surface area of Rh electrodes show that both the hydrogen/adsorption desorption5-6,

8

(Rh1 and Rh1*, respectively) and the oxide reduction48 (RhOX*) features can

provide useful insight. In this respect, we chose to focus on the hydrogen adsorption/desorption signals. This is because the formation of a complete Rh oxide layer, a necessary requirement for the accurate determination of the real area, depends on the upper potential limit of the voltammetric scan. A full monolayer is only formed at 1.25 V 49. In our case, this limit was kept as low as 0.8 V (blank voltammograms) or 0.9 V (in the presence of electroactive molecules) to prevent the potential loss of surface order of the Pt electrode31, or of the Rh adlayer4, 44. As a consequence, only an incomplete RhOx layer was formed, which could not be employed to quantify the Rh coverage. This is demonstrated by the comparison of the voltammetric charges for Rh1 and RhOX* as a function of the number of cycles, displayed in Figure 2. The data reported in this figure refer to blank voltammograms recorded in clean 0.5 M H2SO4, thus in the absence of RhCl3 (see Supporting Information, section S1 for: examples of blank voltammograms of Rhdecorated electrodes; details on baseline, peak integration and fitting). The charge associated with the reduction of RhOx (henceforth qRh,OX* squares in Figure 2) increases with the number of cycles. This trend was fitted to an exponential function of the form

y = y0 − a ⋅ ebx (1) where y0, a and b are fitting parameters. Since the slope of the equation can be seen as a deposition rate, Figure 2 suggests that Rh deposition tends to slow down as it progresses (see Figure S2 for a plot of the fitting curves of Figure 2 and their derivatives), tending asymptotically to a maximum Rh coverage. Similarly, the charge corresponding to hydrogen desorption (henceforth qRh,H, circles) follows an exponential trend like Equation 1. The ratio


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rRh =

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q Rh,H q Rh,OX*

(2)

remains equal to 0.39 regardless of the cycle number, whereas rRh = 0.33 would be expected on the basis of the oxide reduction charge (660 µC cm−2) and the hydrogen adsorption charge (220 µC cm−2) reported for Rh49. This evidences that the RhOx layer formed in the voltammetric sweep is incomplete, as discussed above. The Rh coverages reported throughout this paper were calculated with the following formula:

θ Rh =

qRh,H qPt,H

(3)

in which qRh,H is the charge under the Rh1 peak, and qPt,H is the total hydrogen adsorption/desorption charge measured for the pristine Pt electrode prior to Rh deposition. For ease of reading, the Rh coverages have been expressed in terms of percentages. 3.1.2 Nitrite reduction at Rh-decorated Pt electrodes. The study of NO2− reduction in alkaline media, which is a surface-sensitive probe reaction40-42, helped to corroborate the results described in Figures 1 and 2. Figure 3 compares the voltammetric profiles for NO2− reduction at a Pt electrode before and after Rh deposition. For bare Pt, the voltammetric features agree well with previous results on the same electrodeposited Pt films36 and Pt cubic nanocrystals40. The voltammograms highlight the presence of a significant amount of (100) sites (giving rise to the large reduction peak at 0.38 V); some of these sites are large (100) terraces, which are able to reduce NO2− to N2 (smaller peak at 0.6 V). Upon deposition of 3% Rh, this contribution is lost, indicating that Rh disrupts the long-range


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order required for N2 to form42. Accordingly, the loss of free Pt(100) sites is reflected in the decrease in the peak at 0.38 V. Interestingly, Rh deposition does not seem to affect the large signal at 0.16 V, which is a proxy for (110) or “defect” sites. These observations corroborate the evidence presented in sections 3.1.1 (blank voltammogram) and 3.1.3 (oxidation of adsorbed CO), which underscores the marked preference of Rh for (100) terraces. Finally, the bold arrow in Figure 3 highlights a peculiar reduction signal recorded for the Rh-decorated Pt electrode in nitrite-containing NaOH. This feature arises from the reduction of NO2− at polycrystalline Rh in alkaline media, an intense reduction peak located at 0.09 V and reported previously in the literature50.

Figure 3 Voltammetric profiles for the reduction of 2 mM NaNO2 in 0.1 M NaOH, v = 20 mV s−1. Thin line: pristine Pt electrode; thick line: the same Pt electrode after decoration with 3 % Rh. The current density is expressed with respect to geometric area (1 cm2). 3.1.3 CO stripping at Rh-decorated electrodes


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The adsorption and the oxidative desorption (“stripping”) of CO is a widely used method to clean the metal surface40 and determine the electrode surface area. Although CO is not –strictly speaking – a surface-sensitive probe, CO stripping also provides useful insight into the orientation of the Pt surface51. Figure 4 summarises the CO stripping profiles for a series of RhPt electrodes, as compared to the pristine Pt films. In the absence of Rh (panel A), the signal recorded for CO stripping is in very good agreement with previous reports on electrodeposited Pt films33, 37, and it features three main contributions: a plateau starting at 0.35 V (henceforth “prepeak”), and a doublet of peaks, one centred at 0.71 V, the other at 0.75 V, of a similar intensity. The pre-peak was also observed for cubic Pt nanocrystals40 and controversy exists in the literature as to its origin51-56; here, suffice it to say that it signals the presence of steps or defects within –or at the outskirts of – (100) domains. On the other hand, the peak at 0.71 V arises from CO oxidation at defects and low-coordination sites51, while the signal at 0.75 V is ascribed to (100) domains51, 56.


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Figure 4 Collection of voltammetric profiles for the stripping of adsorbed CO as a function of Rh coverage in CO-free 0.5 M H2SO4; v = 20 mV s−1 in all cases. Panel A: Thin line: Rh-free Pt electrode. Thick line: θRh = 1.7%. Panel B: Thick line θRh = 3,0%, thin line: θRh = 13.7%..The current is normalised with respect to the electrode ‘real’ area measured by hydrogen adsorption/desorption. The deposition of 1.7% Rh already brings about a clear-cut decrease in the intensity of the CO oxidation signal associated with Pt(100) domains, indicating that – as pointed out in section 3.2 –


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Rh nucleates on (100) terrace sites since the very beginning of Rh deposition. At the same time, the pre-peak also shrinks and is shifted to a higher potential, suggesting that defects within or at the border of (100) domains are decorated with Rh, too. As for the main CO stripping peak – the signal located at E = 0.71 V for pristine Pt, we observed the following features in the presence of increasing Rh coverages: •

Full width at half maximum - FWHM (peak fitting performed with Origin, Gaussian amplitude peak function), which is a proxy for CO oxidation kinetics39: the FWHM varies from 87 ± 3 mV (pristine Pt) to 83 ± 3 mV (Rh = 13.7%), error expressed in terms of 3σ. This decrease is therefore not statistically significant.

•

Peak centre (from as-recorded voltammograms): shift towards lower potentials: Epeak = 0.69 V for Rh = 13.7%.

•

Peak onset (from peak fitting, see above): shift towards lower potentials which parallels the shift in peak centre. The onset changes from E = 0.60 V (pristine Pt) to E = 0.58 (Rh = 13.7%).

•

Peak intensity: increase. This can be due to a combination of phenomena. Firstly, the uncorrected charge of CO oxidation on Rh is larger than the theoretical value of 440 µC cm−2, and this is because of interference of Rh oxide formation coupled to CO stripping45. The practical consequence of this effect can be readily seen in Figure 4: the CO stripping peak tail is located at higher currents for increasing Rh coverages. Secondly, if we assume a constant total CO coverage for all electrodes in Figure 4, the oxidation charge gained in the main CO peak seems to be balanced by the shrinking of the CO oxidation pre-peak typical of Pt.


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The shift of CO stripping peak towards lower potentials is in good agreement with the literature: a peak potential of 0.69 V has been reported for the oxidation of a CO adlayer at a Pt(100) surface covered with a Rh multilayer 4, while a Rh(100) single-crystal electrode features a peak potential of 0.68 V 45. 3.2 Electrocatalytic reactions at (100) preferentially-oriented Pt electrodes: effect of Rh decoration 3.2.1 Dimethylether (DME) oxidation. The electrocatalytic oxidation of dimethylether (DME) is a test-reaction of paramount importance, not only from the point of view of its applications in fuel cells, but also because of its peculiar structure-sensitivity towards (100) Pt terraces, as discussed in the Introduction. Figure 5 displays the voltammetric profile for DME oxidation recorded at a pristine Pt electrode, comparing the first and the second cycle. The voltammograms in Figure 5 are in good agreement with the literature on DME oxidation at Pt (100) single-crystal electrodes13-14, 16 and preferentially-oriented cubic (100) Pt nanocrystals40. Focussing on the first cycle, α and β feature in the positive-going sweep, while γ stands out as the only peak in the negative-going direction. All three signals compare well with the published voltammograms for DME oxidation at a Pt(100) single-crystal electrode13,

16

. However, a single-crystal Pt(100) electrode displays an

additional broad and flat oxidation peak centered at 0.6 V, which is missing in our current profiles; this is not at all surprising, since this peak is highly sensitive to the width of (100) domains, disappearing in the case of stepped surfaces16 or cubic Pt nanocrystals40. Peak α arises from the oxidation of the fragments generated by the decomposition of DME at low potentials at Pt, as part of the “indirect” oxidation pathway of DME13-15. This peak is particularly helpful in that its shape and position is a proxy for the preferential orientation of the


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Pt surface. When (110) steps are introduced into a Pt(100) surface, peak α shifts to lower potentials, thus indicating that steps promote DME dehydrogenation16. In the present case, Eα = 0.28 V, which compares well with the peak potential of a Pt(910) single-crystal electrode (having 9-atom (100) terraces separated by (110) monoatomic steps). The current density of peak α shown in Figure 5, jα =80 µA cm−2, is also in very good agreement with the values reported by Lu et al. 16 for stepped [n(100)×(110)], ca. 100 µA cm−2. The following signal, β, is associated with the direct oxidation of DME accompanied by the simultaneous oxidation of COads produced by DME fragmentation. Its position, Eβ = 0.80 V, corresponds to the potential expected at Pt(100) and Pt[n(100)×(110)] single-crystals; its current density (jβ = 170 µA cm−2) compares well with the value recorded for cubic Pt nanoparticles (200 µA cm−2)40 . However, it is almost one order of magnitude smaller than for a single-crystal Pt(100) electrode (1000 µA cm−2) 16. Similarly, the current density of peak γ in Figure 5 (jγ = 55 µA cm−2) is approximately one tenth of the peak current displayed by a single-crystal Pt(100) – 600 µA cm−2 , whereas it is double than the value reported for cubic Pt nanocrystals (ca. 25 µA cm−2)40. Peak γ, which arises from the direct oxidation of DME, is highly sensitive to the length of (100) terraces

16

: it shrinks rapidly, shifting to higher potential values as the step density

increases. In our case, the peak position (Eγ = 0.69 V) compares once more very well with the peak potential of a single-crystal Pt(910) electrode. We point out, however, that the latter features a current density of 300 µA cm−2 for peak γ. The second cycle is particularly remarkable in that peak α is largely suppressed, disappearing almost completely. This observation has been reported and discussed previously in the literature on DME oxidation at Pt(100) single-crystal electrodes13-14, 22; it is a direct consequence of the fact that, after the first cycle, the coverage of CO and other DME moieties reaches a steady-state


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value. Peaks β and γ, on the other hand, undergo marginal changes, their current density increasing slightly with respect to the first cycle. This increase is more noticeable for peak β, in good agreement with the literature16.

Figure 5 Cyclic voltammogram of an electrodeposited Pt film in a 0.5 M H2SO4 solution saturated with dimethyl ether (DME), starting potential 0.08 V vs RHE, v = 50 mV s−1. Thick line: first cycle; thin line: second cycle. Peaks labelled as α and β are recorded in the positivegoing half-cycle, while γ is featured in the return, negative-going sweep.


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The effect of Rh decoration is very informative and most intriguing: it is summarised in the three panels of Figure 6.


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Figure 6 Synopsis of the effect of Rh decoration on the voltammetric profiles for DME oxidation in 0.5 M H2SO4. Panel A: comparison of the voltammograms recorded for a Rh-free electrode (dashed line, same as Figure 5) and in the presence of 1 % Rh (thick line), starting


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potential 0.08 V vs RHE, v = 50 mV s−1. Panel B: voltammetric profiles in the presence of growing Rh coverages (line thickness increases from θRh = 1.9 % through θRh = 3.6 % to θRh = 5.6 %) as compared to Rh-free Pt (dashed line), experimental conditions as in Panel A. The inset highlights the evolution of peak γ with increasing Rh coverage. Panel C: Trends in peak current for peaks β (squares, left y-axis) and γ (open triangles, right y-axis) as a function of Rh coverage. Values of peak currents are measured from data in Panels A and B. Figure 6A, compares the voltammetric profiles for DME oxidation at a bare Pt electrode and at a Pt surface decorated with a Rh coverage of 1%. Peak α remains largely unaffected in the presence of a small amount of Rh, indicating that DME dehydrogenation is neither enhanced nor inhibited. On the other hand, we can observe a noticeable anticipation of the onset potential of peak β (decreasing from 0.62 V to ca. 0.58 V), along with an increase in the peak current (+ 7%). Similarly, peak γ also grows and expands upon Rh decoration, tailing off at a lower potential than on bare Pt. Therefore, the voltammograms in Figure 6A demonstrate that the deposition of a minute amount of Rh turns out to be clearly beneficial for the overall performance towards DME oxidation, effectively extending the potential range of DME – and CO – oxidation with respect to bare Pt. On the other hand, increasing Rh coverages cause a decrease of all three peaks associated with DME oxidation (Figure 6B and 6C). The most abrupt changes occur for θRh > 1.9%: this correlates well with the severe loss of free (100) Pt domains at θRh = 3 % highlighted by nitrite reduction (Figure 3) and CO stripping (Figure 4). Taking a closer look, the three peaks display the following trends for increasing Rh coverages: α. Peak area decreases significantly for θRh > 1.9%. Peak position unaffected.


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β. Peak intensity decreases, the most significant loss of current occurring for θRh > 1.9%. Peak potential slightly shifted to lower potentials; the onset potential remains below 0.6 V for all Rh coverages, leading to a broader peak shape. γ. Similar to peak β: peak current plummets for θRh > 1.9%. The position is instead largely unaffected. Interestingly, the voltammetric profiles of DME oxidation upon Rh deposition are reminiscent of the voltammograms reported for Ru-decorated single-crystal Pt(100) electrodes22. In fact, the deposition of Ru brought about a significant anticipation of the onset potential for DME oxidation (peak β, shifting from 0.72 V at bare Pt(100) to 0.6 V at Ru-modified) and a loss of peak current for both β and γ, with an overall broader peak profile. Finally, we underline the presence of the hydrogen adsorption/desorption peaks (Rh1* and Rh1) for the Rh adlayer, which are not suppressed by DME, contrary to Pt. This is particularly evident in the negative-going, return sweep, clearly indicating that Rh adlayers do not assist in DME dehydrogenation, remaining available for H adsorption instead. On the other hand, adsorbates derived from DME effectively block H adsorption at Pt17.

The oxidation of methanol is often regarded as a counterpart to DME oxidation, because of similarities in the reaction pathways and molecular structures, (H3C-O-CH3 vs H3C-O-H)16. However, the structure sensitivity of methanol oxidation is completely different: the activity increases in the order Pt(111) < Pt(100) < Pt(110) in H2SO4 57. The hysteresis between the peak recorded during the positive-going scan (the more intense signal, analogue to the β peak of DME oxidation) and the peak featuring in the negative-going sweep (analogue to the γ peak of DME oxidation) is also the highest for a Pt(110) electrode39. Therefore, we also investigated the


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electrocatalytic activity of the Pt films towards methanol as a function of Rh coverage. The results, shown in the SI (Figures S3 and S4), can be compared and contrasted to the trends for DME. The highest electrocatalytic activity for methanol oxidation was observed for low Rh coverages, but – in contrast to the results in Figure 6 – the enhancement of methanol oxidation occurs up to θRh = 3.0 %, the current decreasing significantly only for θRh > 8 %. Instead, DME oxidation plummets rapidly as θRh crosses a very low threshold coverage of 1 % (Figure 6). This can be easily explained by recalling the different surface sensitivity of the two reactions: whereas DME oxidation depends on the availability of Pt (100) sites, methanol oxidation is promoted at steps and defects, with (100) terraces playing a minor role. Therefore, the preferential decoration of (100) terraces by Rh does not disable active sites for methanol oxidation and a promotion of this reaction can be observed in a wider range of Rh coverages. The beneficial effect on methanol oxidation resulting from the decoration of Pt with a small amount of Rh is in line with reports of bimetallic Pt-Rh alloys

58-59

: Pt-rich alloys always display the most intense catalytic

activity (the optimum compositions are Pt87Rh13 or Pt90Rh10 for bulk58 and nanoparticle alloys59, respectively). It is likely that Pt defect sites can be further activated towards methanol oxidation by neighbouring Rh atoms (deposited on Pt (100) terraces) according to the aforementioned ‘bifunctional mechanism’. This is reflected by the shift of the main oxidation wave towards lower potentials, observed for all Rh coverages; the maximum anticipation of methanol oxidation was obtained for a θRh = 6.7 %. This is also in very good agreement with previous studies of PtRh alloys: the lowest onset potential is obtained for alloys richer in Rh than the most active alloys. For example, Rodriguez et al. 59 showed that the earliest onset for methanol oxidation was associated with alloy nanoparticles of composition Pt70Rh30 (while the Pt90Rh10 were the most


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active particles). Rh itself is largely inactive towards methanol oxidation due to excessive CO poisoning, as demonstrated in the literature 39, 58-59. 3.2.2 Enhancement of DME oxidation by directing Rh decoration to steps. Finally, we shall report on the protection of Pt(100) terraces during Rh deposition by partial stripping of the adsorbed CO. The voltammetric profiles shown in Figure 4 suggest that, in principle, a careful choice of the upper potential limit during CO stripping can ensure that only selected Pt sites are freed from the adsorbate during the oxidative sweep. A possible limitation affecting this method is CO surface diffusion within the experimental timescale; however, CO diffusion from Pt(100) terraces to low-coordinated sites was reported to be very slow60. The results discussed below show that a surprisingly effective protection of (100) Pt terraces was achieved with this simple protocol regardless of such CO diffusion being operative or not. First of all, CO was adsorbed onto a pristine Pt electrode immersed in clean 0.5 M H2SO4, following the usual procedure (see Experimental section). After purging the electrolyte with Ar, a positive going sweep was started at 20 mV s−1, only to be interrupted at E = 0.72 V, just after the main CO oxidation peak, as shown in Figure 7A. As soon as the potential sweep was reversed into the negative-going direction, RhCl3 was injected into the electrolyte, a gentle Ar bubbling ensuring homogenization of the solution. (The final concentration of RhCl3 was as usual 60 µM). The scan rate was switched to 50 mV s−1, and the upper potential limit was set to the customary value for Rh deposition, E = 0.6 V. A total of 50 cycles were performed in the Rhcontaining H2SO4 solution, achieving a final θRh = 2.6%. Throughout the following discussion, we shall refer to this electrode as ‘protected Pt-Rh’. After transferring the protected Pt-Rh electrode to a second cell containing clean electrolyte, another CO stripping experiment was performed, shown in Figure 7B. For ease of comparison, this panel includes the CO oxidation


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profile for the unprotected electrode decorated with θRh = 3% shown in Figure 3B. Subtle as they may be, the differences between the voltammetric profiles provide us with valuable clues as to the effectiveness of the protection of (100) Pt terraces by adsorbed CO. Most importantly, the protected Pt-Rh electrode still features the CO oxidation peak associated with (100) Pt sites, which instead is totally suppressed if Rh is deposited in the absence of protection by COads. Secondly, the protected Pt-Rh electrode displays a less intense CO stripping pre-peak (arising from defects into or next to (100) Pt domains) than in the absence of protection, suggesting that Rh has preferentially decorated these domains. This is completely logical, as such sites were laid bare during the partial CO stripping in Figure 7A, and hence they were available for Rh deposition. To corroborate these pieces of evidence indicating that sites other than (100) terraces were preferentially decorated, the blank voltammogram of the protected Pt-Rh (Figure 7C) can be contrasted with the voltammetric profile of the Pt-Rh electrode prepared in the absence of COads protection. The inset to Figure 7C zooms in on the significant hydrogen desorption signals for (100) domains, highlighting the larger current density (normalised with real area) recorded for the protected Pt-Rh electrode with respect to the unprotected counterpart. Despite small, the observed enhancement of hydrogen desorption at (100) domains on the protected Pt-Rh is indicative of the effectiveness of CO partial stripping to redirect Rh towards other sites.


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Figure 7 Partial COads stripping and ensuing Rh deposition, showing the protection of Pt (100) terraces from Rh decoration in the presence of the residual CO adlayer. Panel A: Voltammetric profile of the partial oxidation of CO adsorbed at a Pt electrode in 0.5 M H2SO4 (thick line), v = 20 mV s−1. The dashed line corresponds to COads stripping for a pristine Pt electrode in the same conditions (profile identical to Figure 1). The arrow indicates the direction of the potential sweep. In the return half-scan, RhCl3 is injected into the electrolyte (dashed arrow) to achieve a total concentration of 60 µM, and Rh decoration is performed as in Figure 1, v = 50 mV s−1. Panel B: voltammetric profiles for CO stripping in clean 0.5 M H2SO4, v = 20 mV s−1. Thick line: protected Pt-Rh (prepared as in Panel A), θRh = 2.6 %; thick line: Rh-decorated Pt electrode without CO protection, θRh = 3 %. Panel C: blank voltammogram in clean 0.5 M H2SO4 of the protected Pt-Rh (prepared as in Panel A), θRh = 2.6 %; the inset highlights the differences in the hydrogen adsorption region for the protected Pt-Rh (thick line) and the Rh-decorated Pt electrode without CO protection, θRh = 3 % (thin line), v = 20 mV s−1. Since (100) Pt sites are crucial to the catalytic activity towards DME oxidation, we carried out a test of the performance of a protected Pt-Rh towards this reaction. The resulting voltammetric profiles are shown in Figure 8, along with the response for DME oxidation recorded for bare Pt; in this figure, only the main DME oxidation peaks are highlighted (see above for a detailed description).


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Figure 8 Comparison of the voltammograms recorded in DME-saturated 0.5 M H2SO4 for a pristine Pt electrode (as of Figure 4, thin line) and for a protected Pt-Rh electrode (thick line), v = 50 mV s−1. The selective decoration of non-(100) Pt sites with Rh brings about a significant enhancement of the current density associated with DME oxidation, corresponding to a 14% increase with respect to bare Pt. More importantly, Rh also contributes to a remarkable anticipation of the onset potential, as indicated in the figure. This evidence indicates that Rh-modification of preferentially-oriented (100) surfaces can indeed be beneficial for DME oxidatation, provided that Rh deposition onto the highly active terraces is avoided. The results presented in Figures 7 and 8 demonstrate that a “protective group” as simple as COads can effectively direct Rh towards non-(100) Pt domains.

3.2.3 Nitrate reduction at Rh-decorated Pt electrodes.


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As explained in the Introduction, Rh is known to be particularly active towards the electrocatalytic reduction of nitrate. Therefore, we studied the electrocatalytic activity of the RhPt electrodes towards this reaction in 0.5 M H2SO4. This electrolyte was chosen because it allowed us to test nitrate reduction activity under demanding conditions, due to the inhibition by co-adsorption of (bi)sulfate 25. The nitrate reduction activity of a pristine Pt electrode, in the presence of 10 mM nitrate, is shown in Figure 9A. Since the reaction is particularly sluggish, a slow scan rate (5 mV s−1) was employed, as in previous publications

26

. The reduction of nitrate is not intrinsically surface

sensitive, because it responds to the surface sensitivity of interfering co-adsorbing species such as hydrogen and bisulfate 25. Therefore, this reaction does not provide as much insight as nitrite reduction or DME oxidation into the orientation of the Pt surface. The most informative reduction signal is the feature observed at 0.31 V during the positive-going sweep, which compares well with the typical sharp nitrate reduction ‘spike’ observed for Pt(100) electrodes 2627

, highlighting once more the presence of (100) domains in the electrodeposited Pt film. (This

peak arises from nitrate reduction at (100) sites after partial desorption of Hads

26-27

). Figure 9

also shows that, when the potential is swept from high to low potentials, a shoulder at 0.45 V and reduction currents below 0.3 V, including a peak located at 0.21 V, can be observed. The latter feature is particularly prominent for polycrystalline Pt electrodes, and can be ascribed to nitrate reduction at (110) sites26, 28. Generally speaking, there is however a certain degree of overlap in the position of the voltammetric response of the three Pt basal planes towards nitrate reduction in the negative-going sweep26. As we shall see, Rh deposition can fortunately provide useful clues about the origin of these voltammetric features of nitrate reduction at Pt.


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Figure 9B, displays the voltammograms for nitrate reduction for three Rh coverages; the profile of the pristine Pt electrode from Figure 9A is added for ease of comparison. The lowest Rh coverage in the plot (2.5% Rh) does not bring about a significant change in the electrocatalytic activity towards nitrate reduction, except for an increase in the region below 0.2 V during the negative-going sweep. This signal is associated with nitrate reduction at Rh itself, which, for a polycrystalline surface, gives rise to a peak located at 0.1 V in H2SO4 solutions28. In fact, increasing Rh coverages lead to larger currents at 0.1 V, while the ‘spike’ at 0.31 V is progressively suppressed. The inset to Figure 9B shows that the latter signal, associated with nitrate reduction at Pt(100) sites, loses intensity with increasing Rh coverages, indicating once more that Rh decorates (100) Pt terraces. On the other hand, the decrease in the Pt peak at 0.2 V is much more gradual, suggesting that this peak is associated with Pt defect sites which remain available even after Rh deposition.


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Figure 9 Panel A: Voltammetric profile of a pristine Pt electrode in blank 0.5 M H2SO4 (thin line) and in the presence of 10 mM NaNO3 in the same electrolyte (thick line), v = 5 mV s−1. Panel B: overview of the effect of Rh decoration on the voltammetric profiles for the reduction of nitrate. All voltammograms were recorded in 0.5 M H2SO4 containing 10 mM NaNO3. The dashed line corresponds to the voltammogram of a Rh-free Pt electrode (the same as in Panel A). Line thickness increases for growing Rh coverages (from θRh = 2.5 % through θRh = 5.6 % to θRh


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= 9.2 %). The inset highlights the so-called ‘spike’ associated with nitrate reduction at Pt (100) sites. The arrow signals the growth of the voltammetric peak of nitrate reduction at Rh. The reaction order of nitrate reduction (Figures S5 and S6) for the highest Rh coverage in Figure 9 is equal to 0.48. Reaction orders lower than 1 are typical for this reaction61 and indicate that the rate-determining step involves adsorbed species.

4. Discussion 4.1 (100) domains in the electrodeposited Pt electrode: insight from surface-sensitive electrochemical probes. A brief description of the pristine Pt surface will lay the foundations for the discussion of Rh deposition and its effects on electrocatalytic reactions. In this section we shall show that (100) domains obtained by electrodeposition are widespread, probably not more than 4-atom large, but well-ordered, though not defect-free. In addition, it will be suggested that our Pt electrodeposited structures are most similar to Pt cubic nanoparticles/nanocrystals. The overall response to surface-sensitive probes indicates that the share of the surface ordered as (100) is large enough for us to describe the electrode as preferentially (100) oriented: this orientation can extend over as much as 45 % of the surface, as thoroughly discussed in previous publications32-34,

37

. However, the (100) domains are undoubtedly small. Their size could be

estimated by referring for instance to the voltammetric features reported for Pt[n(100)×(110)] single-crystals. However, before attempting to do so, we would like to emphasize that such ordered stepped surfaces are just an imperfect reference, since they are structurally very different from the electrodes prepared in this work. In fact, Pt[n(100)×(110)] are composed of ordered


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terraces that extend along the surface parallel to monoatomic steps, which is clearly not the case for electrodeposited electrodes. Bearing in mind these intrinsic limitations, we can draw a comparison between the Pt electrodes we studied and Pt[n(100)×(110)] stepped surfaces62. The following list highlights the single-crystal surface that is most similar to our electrodeposited electrodes, according to the voltammetric profiles reported for several surface-sensitive reactions: •

Hydrogen underpotential deposition62 (focussing only on peaks h2 and h3 in Figure 1): Pt(510), i.e. Pt[5(100)×(110)]

•

Nitrite reduction42 (Figure 3): Pt(410), i.e. Pt[4(100)×(110)]

•

DME oxidation, peak α 16 (Figure 5): Pt(910), i.e. Pt[9(100)×(110)]

•

DME oxidation, peaks β and γ (Figure 5): peak intensity reminiscent of a Pt(310), i.e. Pt[3(100)×(110)], but peak sharpness and position most similar to Pt(910).

The discrepancies between the various peak features for DME oxidation underscore once more the point that we raised above: Pt stepped surfaces are not entirely appropriate as reference structures when discussing electrodeposited preferentially-oriented electrodes. Two other structural features of Pt electrodeposits can be highlighted by electrochemical probes: •

the sharpness of the β and γ peaks of DME oxidation points to a significant degree of order within the Pt(100) domains…

•

…which however do feature kinks and defects at their border, or within them, as highlighted by the presence of a pre-peak for COads oxidation in Figure 4. Such prepeak is not detected for CO stripping at Pt[n(100)×(110)] stepped surfaces62.


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The latter characteristics have already been reported for Pt cubic nanocrystals40 and suggest that such (100)-preferentially oriented particles represent the structurally most meaningful benchmark when discussing the performance and the features of electrodeposited Pt electrodes.

4.2 Rh deposition: topography and kinetics After briefly surveying the landscape of the Pt electrode, we can now discuss the features of Rh deposition. In terms of spatial location of the Rh deposits, the experimental evidence presented in Section 3 indicates that: •

Rh decorates (100) terraces, and low-coordination sites within them, preferentially, depositing on these locations rather than polyoriented domains. (111) terraces, which occupy a minor share of the surface, show an intermediate tendency to Rh deposition.

•

The position of Rh1 and Rh1* peaks remains constant while their intensity grows as the Rh coverage increases. This is a proxy for the formation of small, uniform (2D) islands8, 63. Also, the sharp profile of peaks Rh1 and Rh1* suggests that Rh deposits are spatially well-ordered4,

6, 8, 63

. These observations clearly point to a growth mode in

which the arrangement of Rh adatoms matches the substrate orientation (pseudomorphic growth), although the nature of the Pt substrate does not allow us to state that we achieve truly epitaxial growth. As suggested elsewhere6, chloride ions can assist in forming an ordered Rh adlayers by adsorbing onto Pt and Rh during deposition. •

The absence of any peak below 0.1 V (observed for H adsorption at polycrystalline Rh) indicates that no Rh multilayer is formed5, 47.


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The preferential deposition of Rh on terraces with respect to defect sites shows an intriguing similarity with Cu underpotential deposition on Pt[n(100) × (111)]: these surfaces, too, display a preferential decoration of (100) terraces by Cu 64. As for the literature on Rh deposition, it has been shown that this metal forms well-ordered deposits onto Pt(100) and Pt(111) single-crystal electrodes5-6, 8, 11, while there is no report addressing Rh decoration of steps in stepped surfaces. Two factors could account for this preference. The first is the Smoluchowski effect, a redistribution of the electron wave function typical of corrugated surfaces. This effect leads to the formation of an excess of positive charge on the rows of atoms jutting out from the average profile of the surface, while a net negative charge is formed in the ‘troughs’. The Smoluchowski effect is most pronounced on stepped surfaces and is minimal on close-packed fcc(111). Since the hexacoordinated rhodium complex [Rh(H2O)xCl6−x]+(x−3) is most likely positively charged, the Smoluchowski effect suggests that surface diffusion of the Rh complex will be fastest at Pt(111) terraces, slower at Pt(100), while negatively-charged defects could effectively capture the Rh complex. This may help explain the preferential decoration of defects near the terraces, as evidenced by the loss of the CO stripping pre-peak (Figure 4A). On the other hand, there is another parameter related to the work function that should be taken into account: the potential of zero (total) charge. As shown in a review by Petrii65, the potential of zero total charge decreases in the order Pt(100) > Pt(111) > Pt(110) in 0.1 M H2SO4. In other words, Pt(100) will be the first surface to become negatively charged during a potentiodynamic cycle, followed by (111). This could help to explain the preference for decoration of Pt(100) terraces displayed by positivelycharged adatoms, while also accounting for the fact that Pt(111) terraces, too, can be decorated by Rh when they are predominant11.


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Figure 2 also provides insight into the kinetics of Rh deposition. The figure shows that the rate of deposition slows down progressively, flattening out as the number of cycles decreases. This cannot be due to a scarcity of available Rh3+ in the solution, because, as mentioned in the Experimental section, the number of moles of dissolved Rh is 50 times larger than the moles of surface Pt atoms. Rather, we believe that the decrease in the deposition rate simply reflects the progressive occupation of the Pt terrace domains, at which deposition is most favourable. Hence, Rh deposition becomes gradually more and more sluggish as these prime sites are being covered by Rh. The stages of Rh deposition can be visualised in the following series of cartoons (Scheme 1):


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Scheme 1 Top cartoon: a schematic depiction of the surface of electrodeposited Pt electrodes, showing the preferential (100) orientation and the presence of defects. Following cartoons: (1)


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nucleation of Rh onto (100) terraces and near-terrace defects; (2) growth of Rh islands, showing their high degree of order; (3) decoration of non-(100) sites such as steps. 4.3 Effects of Rh islands on electrocatalysis Section 3 indicates that the presence of Rh deposits on preferentially-oriented (100) exerts an overall effect arising from the overlap of: •

Topography – Rh decorates the (100) terraces, occupying and disabling well-ordered domains having a specific high catalytic activity towards, for instance, the oxidation of DME. In this sense, the presence of Rh on the (100) terraces inhibits the oxidation of DME by a pure steric effect. The magnitude of such inhibition will increase as a function of increasing Rh coverages.

•

Intrinsic catalytic properties – Rh is more active than Pt towards the reduction of nitrate, as discussed in the Introduction. On the other hand, Rh can enhance the performance of Pt towards the oxidation small organic molecules such as DME or methanol by ensuring a larger availability of adsorbed OH, and this at a lower potential than on bare Pt. This is possible when Pt is combined with a second metal located to its left in the periodic table, such as Rh and Ru, which are more “oxophilic” (i.e. more oxidisable) than Pt. These properties can be better described in terms of the d-band centre, which increases in the order Pt < Rh < Ru. A classic case of bimetallic combination is the PtRu alloy, which is known for its higher catalytic activity towards the oxidation of CH3OH and its greater CO tolerance than bare Pt23. This is often explained in terms of a bifunctional mechanism, Ru adsorbing OH at an early potential and thus providing the reactant required to oxidise CO which, instead, is preferentially adsorbed at Pt atoms.


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In this section, we shall attempt to disentangle the two contributions while underscoring the particular role that Rh deposits play in individual reactions. Let us start by addressing the role of Rh deposits in the oxidation of CO and DME at preferentially-oriented (100) Pt electrodes. When discussing trends in the electrocatalytic activity towards these reactions, we must bear in mind that cyclic voltammetry can only provide limited kinetic information – chronoamperometry should be used instead13, 22, 39. However, voltammetric data presented in section 3 do offer some valuable insight into the effects of Rh deposits on the oxidation of CO and DME. Let us highlight the most significant observations concerning CO stripping (Figure 4): •

The kinetics of CO oxidation remains largely unchanged upon Rh deposition, as indicated by the fact that the FWHM of the CO stripping peak does not vary in the presence of Rh.

•

On the other hand, the (minor) decrease in the onset and peak potential for CO stripping observed for increasing Rh coverages suggests that the so-called bifunctional mechanism39 is indeed operative, although to a limited extent. In fact, the bifunctional mechanism is most significant at Rh-decorated Pt surfaces when the CO coverage is low, and adsorbed species are able to diffuse across the surface to reach most reactive sites39,

47

, for example reaching the edge of Rh islands, where OH is preferentially

adsorbed. In our case, we always worked under conditions of maximum coverage, which explains the minor anticipation of CO oxidation potential observed for our electrodes. •

Under CO saturation, the topography of Rh deposition also plays a less significant role. Rh deposits do occupy highly-active defects within (100) terraces (which oxidise CO in


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a low-potential pre-peak), but the early stages of Rh deposition leave a large share of Pt “polyoriented” sites uncovered. In summary, Rh deposition exerts a relatively mild effect on the oxidation of a full CO adlayer, with a beneficial, though very small, shift of the oxidation peak towards lower potentials. This can be contrasted with larger effects observed for Ru-decorated single-crystal Pt electrodes66. The lack of a significant enhancement of CO oxidation at Rh-decorated electrodeposited Pt surfaces is not at all surprising, for the following two reasons. The first concerns the known fact that Rh promotes CO oxidation at Pt to a lower extent than Ru

11, 63, 66

; secondly, the

enhancement is maximised by alloying Pt and Rh (compare for instance the well-ordered PtRh{100} alloys reported by Fang et al.63), which does not happen during potentiodynamic Rh decoration. On the other hand, COads oxidation is just one of the multiple adsorbates in the oxidation of DME, and it is likely that the coverage of CO is well below saturation. Thus, it is not surprising to observe a more significant impact of Rh deposition onto DME oxidation: •

Topography plays a crucial role: the electrocatalytic activity of a Pt surface towards DME oxidation decreases with the loss of (100) terraces sites. Rh deposition, occurring preferentially at these domains, contributes negatively to the DME oxidation activity. This is compounded by the fact that Rh(100) is expected to be less active than Rh(111) for DME oxidation, because the (100) surface tends to remain poisoned by DME fragments, as suggested by DFT calculations21.

•

However, Rh can be beneficial to DME oxidation by enhancing the availability of OHads as discussed above for COads stripping. If Rh deposition at substrate (100) domains is avoided, a significant promotion of DME oxidation can be achieved, as demonstrated in


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section 3.2.2. In the absence of protection of (100) domains, a minimal Rh coverage (1%) still ensures that the trade-off between steric effects and bimetallic catalysis is still favourable (Section 3.2.1). Even at higher Rh coverages, the onset potential for peak β at the bimetallic surface remains lower than at pristine Pt. In summary, a fine-tuning of inhibition (loss of (100) domains) and promotion (bifunctional mechanism) can lead to an enhancement of DME oxidation upon Rh-decoration of a Pt electrode at low Rh coverages. This can be further corroborated by trends in the enhancement of methanol oxidation, shown in the Supporting Information (Figures S2 and S3): this reaction, which does not require well-ordered (100) terraces, is enhanced by Rh deposits on Pt within a much broader range of coverages than DME oxidation: in other words, bimetallic catalysis can remain operative for methanol oxidation even as Rh deposits grow and cover (100) domains. Finally, nitrate reduction warrants a separate discussion because, in contrast to CO, DME and methanol oxidation, Rh and Pt show intrinsic differences in terms of mechanistic pathways. As described in the Introduction, the higher catalytic activity of Rh, even in the presence of coadsorbing species (such as HSO4− and H), is ascribed to its ability to activate H+ for the direct reduction of NO3−. Pt, on the other hand, seems to react only with HNO3 28. As a consequence: •

There is no evidence of synergy between the two metals, which perform nitrate reduction independently of each other.

•

Therefore, the distribution of Rh deposits on the Pt surface brings about a progressive shift from a voltammetric profile characteristic of Pt (with the typical ‘spike’ of (100) domains, discussed in Section 3.2.3) to that of a Rh surface, with the growth of a lowpotential reduction peak.

5. Conclusions


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In this work, we showed that preferentially-oriented (100) platinum films can be decorated with sub-monolayer amounts of Rh by means of potentiodynamic electrodeposition. Evidence from cyclic voltammetry in blank electrolyte and from surface-sensitive electrochemical reactions indicated that the Rh adlayers are formed preferentially at (100) domains of the Pt substrate, and that well-ordered Rh deposits were formed. The presence of Rh modifies the electrocatalytic properties of the Pt substrate to varying extents, also reflecting the structure sensitivity of each individual reaction. Pt (100) terraces are highly active towards the oxidation of dimethyl ether (DME): therefore, the most significant enhancement of this reaction was achieved at very low Rh coverages, which ensured that most Pt (100) sites are still Rh-free. As an alternative approach, Rh deposition was re-directed towards defects by partial stripping of adsorbed CO, which was not removed from (100) domains, thus preventing Rh decoration. In both cases, DME oxidation displayed a lower onset and higher peak currents than for pristine Pt: this is most likely due to an increased availability of OHads at a lower potential. Similarly, Rh decoration led to a minor anticipation of the onset for the oxidation of adsorbed CO, without a significant impact on reaction kinetics. On the other hand, the effect of Rh adlayers on the reduction of nitrate was remarkably different: Rh is the dominant metal, giving rise to the most intense reduction signal. In other words, Pt and Rh perform the reduction of nitrate independently of each other.

Corresponding Author *[email protected] *[email protected] Present Addresses


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†Department of Chemistry, Rm D223 - 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Supporting Information. Details on the estimation of Rh coverage, on methanol oxidation and on additional experiments concerning nitrate reductions can be found in the Supporting Information. The following files are available free of charge: PtRh_Decoration_SI.pdf Acknowledgement Ms. N.N. Fomena is gratefully acknowledged for her assistance in implementing Pt electroplating. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chair program. References (1)

Ross, P. N.; Kinoshita, K.; Scarpellino, A. J.; Stonehart, P., Electrocatalysis on Binary

Alloys. J. Electroanal. Chem. 1975, 59, 177-189. (2)

Spendelow, J. S.; Wieckowski, A., Noble Metal Decoration of Single Crystal Platinum

Surfaces to Create Well-Defined Bimetallic Electrocatalysts. Phys. Chem. Chem. Phys. 2004, 6, 5094-5118.


49


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3)

Page 50 of 58

Attard, G. A.; Price, R.; Alakl, A., Electrochemical and Ultra-High-Vacuum

Characterization of Rhodium on Pt(111) - a Temperature-Dependent Growth Mode. Surf. Sci. 1995, 335, 52-62. (4)

Gomez, R.; Rodes, A.; Perez, J. M.; Feliu, J. M.; Aldaz, A., Electrochemical and in Situ

FTIRS Studies of the CO Adsorption at Palladium and Rhodium Multilayers Deposited on Platinum Single Crystal Surfaces .2. Pt(100) Substrate. Surf. Sci. 1995, 344, 85-97. (5)

Gomez, R.; Feliu, J. M., Rhodium Adlayers on Pt(111) Monocrystalline Surfaces.

Electrochemical Behavior and Electrocatalysis. Electrochim. Acta 1998, 44, 1191-1205. (6)

de Dios, F. J. G.; Gomez, R.; Feliu, J. M., Preparation and Electrocatalytic Activity of Rh

Adlayers on Pt(100) Electrodes: Reduction of Nitrous Oxide. Electrochem. Commun. 2001, 3, 659-664. (7)

Bakos, I.; Szabo, S., Electrochemical Adsorption of Rhodium on Platinum. J.

Electroanal. Chem. 2003, 547, 103-107. (8)

de Dios, F. J. G.; Gomez, R.; Feliu, J. M., Preparation and Electrochemical Behavior of

Ordered Rh Adlayers on Pt(100) Electrodes. Langmuir 2005, 21, 7439-7448. (9)

Busó-Rogero, C.; Solla-Gullon, J.; Vidal-Iglesias, F. J.; Herrero, E.; Feliu, J. M., Adatom

Modified Shape-Controlled Platinum Nanoparticles Towards Ethanol Oxidation. Electrochim. Acta 2016, 196, 270-279. (10) Oliveira, R. T. S.; Santos, M. C.; Bulhoes, L. O. S.; Pereira, E. C., Rh Electrodeposition on Pt in Acidic Medium: A Study Using Cyclic Voltammetry and an Electrochemical Quartz Crystal Microbalance. J. Electroanal. Chem. 2004, 569, 233-240.


50

Page 51 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Gomez, R.; de Dios, F. J. G.; Feliu, J. M., Carbon Monoxide Oxidation and Nitrous Oxide Reduction on Rh/Pt(111) Electrodes. Electrochim. Acta 2004, 49, 1195-1208. (12) Semelsberger, T. A.; Borup, R. L.; Greene, H. L., Dimethyl Ether (DME) as an Alternative Fuel. J. Power Sources 2006, 156, 497-511. (13) Tong, Y.; Lu, L.; Zhang, Y.; Gao, Y.; Yin, G.; Osawa, M.; Ye, S., Surface Structure Dependent Electro-Oxidation of Dimethyl Ether on Platinum Single-Crystal Electrodes. J. Phys. Chem. C 2007, 111, 18836-18838. (14) Lu, L.; Yin, G.; Tong, Y.; Zhang, Y.; Gao, Y.; Osawa, M.; Ye, S., Electrochemical Behaviors of Dimethyl Ether on Platinum Single Crystal Electrodes. Part I: Pt(111). J. Electroanal. Chem. 2008, 619–620, 143-151. (15) Zhang, Y.; Lu, L.; Tong, Y.; Osawa, M.; Ye, S., Electrochemical and Infrared Study of Electro-Oxidation of Dimethyl Ether (DME) on Platinum Polycrystalline Electrode in Acid Solutions. Electrochim. Acta 2008, 53, 6093-6103. (16) Lu, L.; Yin, G.; Tong, Y.; Zhang, Y.; Gao, Y.; Osawa, M.; Ye, S., Electrochemical Behaviors of Dimethyl Ether on Platinum Single Crystal Electrodes. Part Ii: Pt(100). J. Electroanal. Chem. 2010, 642, 82-91. (17) Li, H.; Calle-Vallejo, F.; Kolb, M. J.; Kwon, Y.; Li, Y.; Koper, M. T. M., Why (100) Terraces Break and Make Bonds: Oxidation of Dimethyl Ether on Platinum Single-Crystal Electrodes. J. Am. Chem. Soc. 2013, 135, 14329-14338.


51


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 58

(18) Li, H. J.; Li, Y. D.; Koper, M. T. M.; Calle-Vallejo, F., Bond-Making and Breaking between Carbon, Nitrogen, and Oxygen in Electrocatalysis. J. Am. Chem. Soc. 2014, 136, 1569415701. (19) Roling, L. T.; Herron, J. A.; Budiman, W.; Ferrin, P.; Mavrikakis, M., Dimethyl Ether Electro-Oxidation on Platinum Surfaces. Nano Energy 2016, 29, 428-438. (20) Herron, J. A.; Ferrin, P.; Mavrikakis, M., First-Principles Mechanistic Analysis of Dimethyl Ether Electro-Oxidation on Monometallic Single-Crystal Surfaces. J. Phys. Chem. C 2014, 118, 24199-24211. (21) Herron, J. A.; Ferrin, P.; Mavrikakis, M., On the Structure Sensitivity of Dimethyl Ether Electro-Oxidation on Eight Fcc Metals: A First-Principles Study. Top. Catal. 2015, 58, 11591173. (22) Lu, L.; Yin, G.; Wang, Z.; Cai, K.; Gao, Y., Electrocatalytic Oxidation of Dimethyl Ether on Ruthenium Modified Platinum Single Crystal Electrodes. Catal. Commun. 2009, 10, 971-974. (23) Petrii, O. A., Pt-Ru Electrocatalysts for Fuel Cells: A Representative Review. J. Solid State Electr. 2008, 12, 609-642. (24) Rosca, V.; Duca, M.; de Groot, M. T.; Koper, M. T. M., Nitrogen Cycle Electrocatalysis. Chem. Rev. 2009, 109, 2209-2244. (25) Duca, M.; Koper, M. T. M., Powering Denitrification: The Perspectives of Electrocatalytic Nitrate Reduction. Energy Environ. Sci. 2012, 5, 9726-9742. (26) Dima, G. E.; Beltramo, G. L.; Koper, M. T. M., Nitrate Reduction on Single-Crystal Platinum Electrodes. Electrochim. Acta 2005, 50, 4318-4326.


52

Page 53 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Molodkina, E. B.; Botryakova, I. G.; Danilov, A. I.; Souza-Garcia, J.; Feliu, J. M., Mechanism of Nitrate Electroreduction on Pt(100). Russ. J. Electrochem. 2012, 48, 302-315. (28) Yang, J.; Sebastian, P.; Duca, M.; Hoogenboom, T.; Koper, M. T. M., pH Dependence of the Electroreduction of Nitrate on Rh and Pt Polycrystalline Electrodes. Chem. Commun. 2014, 50, 2148-2151. (29) Petrii, O. A.; Safonova, T. Y., Zero Charge Potentials and Work Function of Rhodium. J. Electroanal. Chem. 2013, 688, 336-348. (30) Calle-Vallejo, F.; Martinez, J. I.; Garcia-Lastra, J. M.; Rossmeisl, J.; Koper, M. T. M., Physical and Chemical Nature of the Scaling Relations between Adsorption Energies of Atoms on Metal Surfaces. Phys. Rev. Lett. 2012, 108, 5. (31) Ponrouch, A.; Garbarino, S.; Bertin, E.; Andrei, C.; Botton, G. A.; Guay, D., Highly Porous and Preferentially Oriented {100} Platinum Nanowires and Thin Films. Adv. Funct. Mater. 2012, 22, 4172-4181. (32) Roy, C.; Bertin, E.; Martin, M. H.; Garbarino, S.; Guay, D., Hydrazine Oxidation at Porous and Preferentially Oriented {100} Pt Thin Films. Electrocatalysis 2013, 4, 76-84. (33) Bertin, E.; Garbarino, S.; Guay, D., Formic Acid Oxidation on Bi Covered Pt Electrodeposited Thin Films: Influence of the Underlying Structure. Electrochim. Acta 2014, 134, 486-495. (34) Bertin, E.; Fleury, A.; Roy, C.; Martin, M. H.; Garbarino, S.; Guay, D., Preferentially (100) Oriented Pt Thin Film with Less Than a Monolayer of Bi, Pd and Sb Adatoms: Application for Formic Acid Oxidation. Electrochim. Acta 2015, 162, 237-244.


53


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 58

(35) Bertin, E.; Garbarino, S.; Guay, D., Formic Acid Oxidation on Antimony-Covered Platinum Films with a Preferential (100) Orientation. J. Power Sources 2015, 299, 315-323. (36) Finkelstein, D. A.; Bertin, E.; Garbarino, S.; Guay, D., Mechanistic Similarity in Catalytic N2 Production from NH3 and NO2− at Pt(100) Thin Films: Toward a Universal Catalytic Pathway for Simple N-Containing Species, and Its Application to in Situ Removal of Nh3 Poisons. J. Phys. Chem. C 2015, 119, 9860-9878. (37) Bertin, E.; Garbarino, S.; Guay, D.; Solla-Gullon, J.; Vidal-Iglesias, F. J.; Feliu, J. M., Electrodeposited Platinum Thin Films with Preferential (100) Orientation: Characterization and Electrocatalytic Properties for Ammonia and Formic Acid Oxidation. J. Power Sources 2013, 225, 323-329. (38) Bertin, E.; Roy, C.; Garbarino, S.; Guay, D.; Solla-Gullon, J.; Vidal-Iglesias, F. J.; Feliu, J. M., Effect of the Nature of (100) Surface Sites on the Electroactivity of Macroscopic Pt Electrodes for the Electrooxidation of Ammonia. Electrochem. Commun. 2012, 22, 197-199. (39) Herrero, E.; Lai, S. C. S.; Koper, M. T. M. In Fuel Cell Catalysis: A Surface Approach; Koper, M. T. M., Ed. ; J. Wiley & Sons, New York: 2009; pp 159-206. (40) Duca, M.; Rodriguez, P.; Yanson, A. I.; Koper, M. T. M., Selective Electrocatalysis on Platinum Nanoparticles with Preferential (100) Orientation Prepared by Cathodic Corrosion. Top. Catal. 2014, 57, 255-264. (41) Duca, M.; Cucarella, M. O.; Rodriguez, P.; Koper, M. T. M., Direct Reduction of Nitrite to N2 on a Pt(100) Electrode in Alkaline Media. J. Am. Chem. Soc. 2010, 132, 18042-18044.


54

Page 55 of 58

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Duca, M.; Figueiredo, M. C.; Climent, V.; Rodriguez, P.; Feliu, J. M.; Koper, M. T. M., Selective Catalytic Reduction at Quasi-Perfect Pt(100) Domains: A Universal Low-Temperature Pathway from Nitrite to N2. J. Am. Chem. Soc. 2011, 133, 10928-10939. (43) Solla-Gullon, J.; Rodriguez, P.; Herrero, E.; Aldaz, A.; Feliu, J. M., Surface Characterization of Platinum Electrodes. Phys. Chem. Chem. Phys. 2008, 10, 1359-1373. (44) Xu, Q. Q.; Linke, U.; Bujak, R.; Wandlowski, T., Preparation and Electrochemical Characterization of Low-Index Rhodium Single Crystal Electrodes in Sulfuric Acid. Electrochim. Acta 2009, 54, 5509-5521. (45) Gómez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J., Validity of Double-Layer ChargeCorrected Voltammetry for Assaying Carbon Monoxide Coverages on Ordered Transition Metals: Comparisons with Adlayer Structures in Electrochemical and Ultrahigh Vacuum Environments. Surf. Sci. 1998, 410, 48-61. (46) Tamura, H.; Sasahara, A.; Tanaka, K., Cyclic Voltammogram of Rh(100), Pt-Deposited Rh(100) and Chemically Reconstructed Pt/Rh(100) Surfaces. J. Electroanal. Chem. 1995, 381, 95-98. (47) Fang, L.; Vidal-Iglesias, F. J.; Huxter, S. E.; Attard, G. A.; Wells, P. B., RhPt/Graphite Catalysts for CO Electrooxidation: Performance of Mixed Metal and Alloyed Surfaces. Surf. Sci. 2015, 631, 258-266. (48) Leontiev, A. P.; Brylev, O. A.; Napolskii, K. S., Arrays of Rhodium Nanowires Based on Anodic Alumina: Preparation and Electrocatalytic Activity for Nitrate Reduction. Electrochim. Acta 2015, 155, 466-473.


55


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 58

(49) Jerkiewicz, G.; Borodzinski, J. J., Studies of Formation of Very Thin Oxide Films on Polycrystalline Rhodium Electrodes: Application of the Mott-Cabrera Theory. Langmuir 1993, 9, 2202-2209. (50) Duca, M.; van der Klugt, B.; Hasnat, M. A.; Machida, M.; Koper, M. T. M., Electrocatalytic Reduction of Nitrite on a Polycrystalline Rhodium Electrode. J. Catal. 2010, 275, 61-69. (51) Urchaga, P.; Baranton, S.; Coutanceau, C.; Jerkiewicz, G., Electro-Oxidation of COchem on Pt Nanosurfaces: Solution of the Peak Multiplicity Puzzle. Langmuir 2012, 28, 3658-3663. (52) Cuesta, A.; Couto, A.; Rincón, A.; Pérez, M. C.; López-Cudero, A.; Gutiérrez, C., Potential Dependence of the Saturation Co Coverage of Pt Electrodes: The Origin of the PrePeak in CO-Stripping Voltammograms. Part 3: Pt(Poly). J. Electroanal. Chem. 2006, 586, 184195. (53) López-Cudero, A.; Cuesta, Á.; Gutiérrez, C., Potential Dependence of the Saturation CO Coverage of Pt Electrodes: The Origin of the Pre-Peak in CO-Stripping Voltammograms. Part 2: Pt(100). J. Electroanal. Chem. 2006, 586, 204-216. (54) Urchaga, P.; Baranton, S.; Coutanceau, C., Changes in COchem Oxidative Stripping Activity Induced by Reconstruction of Pt (111) and (100) Surface Nanodomains. Electrochim. Acta 2013, 92, 438-445. (55) Farias, M. J. S.; Camara, G. A.; Feliu, J. M., Understanding the CO Preoxidation and the Intrinsic Catalytic Activity of Step Sites in Stepped Pt Surfaces in Acidic Medium. J. Phys. Chem. C 2015, 119, 20272-20282.


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(56) Rudnev, A. V.; Wandlowski, T., An Influence of Pretreatment Conditions on Surface Structure and Reactivity of Pt(100) Towards CO Oxidation Reaction. Russ. J. Electrochem. 2012, 48, 259-270. (57) Herrero, E.; Franaszczuk, K.; Wieckowski, A., Electrochemistry of Methanol at LowIndex Crystal Planes of Platinum - an Integrated Voltammetric and Chronoamperometric Study. J. Phys. Chem. 1994, 98, 5074-5083. (58) Ureta-Zanartu, M. S.; Montenegro, M.; Zagal, J. H., Methanol Electro-Oxidation on PtRh Alloys in Acid Medium. Bol. Soc. Chilena Quim. 2001, 46, 209-216. (59) Rodriguez, P.; Tichelaar, F. D.; Koper, M. T. M.; Yanson, A. I., Cathodic Corrosion as a Facile and Effective Method to Prepare Clean Metal Alloy Nanoparticles. J. Am. Chem. Soc. 2011, 133, 17626-17629. (60) Coutanceau, C.; Urchaga, P.; Baranton, S., Diffusion of Adsorbed CO on Platinum (100) and (111) Oriented Nanosurfaces. Electrochem. Commun. 2012, 22, 109-112. (61) Dima, G. E.; de Vooys, A. C. A.; Koper, M. T. M., Electrocatalytic Reduction of Nitrate at Low Concentration on Coinage and Transition-Metal Electrodes in Acid Solutions. J. Electroanal. Chem. 2003, 554-555, 15-23. (62) Vidal-Iglesias, F. J.; Solla-Gullon, J.; Campina, J. M.; Herrero, E.; Aldaz, A.; Feliu, J. M., CO Monolayer Oxidation on Stepped Pt(S) (n−1)(100) × (110) Surfaces. Electrochim. Acta 2009, 54, 4459-4466.


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(63) Fang, L.; Vidal-Iglesias, F. J.; Huxter, S. E.; Attard, G. A., A Study of the Growth and CO Electrooxidation Behaviour of Ptrh Alloys on Pt{100} Single Crystals. J. Electroanal. Chem. 2008, 622, 73-78. (64) Francke, R.; Climent, V.; Baltruschat, H.; Feliu, J. M., Electrochemical Deposition of Copper on Stepped Platinum Surfaces in the [011] Zone Vicinal to the (100) Plane. J. Electroanal. Chem. 2008, 624, 228-240. (65) Petrii, O. A., Zero Charge Potentials of Platinum Metals and Electron Work Functions (Review). Russ. J. Electrochem. 2013, 49, 401-422. (66) Maillard, F.; Lu, G. Q.; Wieckowski, A.; Stimming, U., Ru-Decorated Pt Surfaces as Model Fuel Cell Electrocatalysts for CO Electrooxidation. J. Phys. Chem. B 2005, 109, 1623016243.

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