Impact of Palladium Loading and Interparticle Distance on the

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C: Energy Conversion and Storage; Energy and Charge Transport

The Impact of Palladium Loading and Interparticle Distance on the Selectivity for the Oxygen Reduction Reaction Towards Hydrogen Peroxide Guilherme V Fortunato, Enrico Pizzutilo, Andrea M. Mingers, Olga Kasian, Serhiy Cherevko, Eduardo S. F. Cardoso, Karl J.J. Mayrhofer, Gilberto Maia, and Marc Ledendecker J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04262 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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The Journal of Physical Chemistry

The Impact of Palladium Loading and Interparticle Distance on the Selectivity for the Oxygen Reduction Reaction towards Hydrogen Peroxide

Guilherme V. Fortunatoa,b, Enrico Pizzutilob, Andrea M. Mingersb, Olga Kasianb, Serhiy Cherevkob,c, Eduardo S. F. Cardosoa, Karl J. J. Mayrhoferb,c,d, Gilberto Maiaa* and Marc Ledendeckerb*

a

Institute of Chemistry, Universidade Federal de Mato Grosso do Sul; Av. Senador Filinto Muller, 1555; Campo Grande, MS 79074-460, Brazil b

Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany

c

Forschungszentrum Jülich, Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstr. 3, 91058 Erlangen, Germany d

Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany

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ABSTRACT: A successful market introduction of electrocatalytically produced hydrogen peroxide (H2O2) requires catalysts that are highly selective, active and economically suitable. Here, we present important insights on tuning the selectivity towards H2O2 and elaborate on the opportunities opened for high catalytic performance. Especially the metal loading, the accompanied interparticle distance and catalyst-support interaction were identified as key contributor for high selectivity and activity. We focused on the design of model catalysts with different Pd loadings and distinct interparticle distances and their dependency on the selectivity towards H2O2. The gained understandings can be used as guidelines for the development of highly active and selective catalysts while simultaneously reducing the noble metal loading and the associated costs.

1. Introduction

Hydrogen peroxide (H2O2) is listed among the 100 most important chemical compounds and is used in several important industrial processes such as pulp and paper bleaching, watercleaning, as well as reactant in chemical synthesis.1-3 Nowadays, highly concentrated H2O2 (up to 70 wt%) is produced almost exclusively in large scale centralized reactors through the well-established anthraquinone process using O2 and H2.1,2,4 However, for small scale applications, such as water treatment and as antimicrobial agent, diluted concentrations (2-8 wt%) are typically favored.3 Another promising production method of H2O2 encloses the electrochemical synthesis in a fuel cell or an electrolyzer via the 2-electron oxygen reduction reaction (ORR). The flexibility of such electrochemical reactors puts it at an advantage over large scale production sites due to its easy adjusted scalability and the possibility of being operated on-site under ambient conditions.5-10 However, the industrial viability to produce H2O2 electrochemically requires catalysts that are highly selective, active and economically suitable.7-18 Especially acidic media offers the advantage of a higher H2O2 stabilization compared to alkaline media.19 Here, electrocatalysts 2 ACS Paragon Plus Environment

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based on noble metals such as Pt and Pd are able to withstand the harsh conditions posed by the electrolyte and the high potentials reached during the start-up/shut-down phases. Pt and Pd interact strongly with O2 resulting in a complete reduction to water through the transfer of four electrons. When only two electrons are transferred, H2O2 is selectively obtained.20-34 Selective catalysts such as Au, Hg or Ag interact weakly with O2 and are not able to break the intermolecular O-O bond, thus showing enhanced H2O2 production at the cost of slow reaction kinetics and high overpotentials.15,16,35,36 An ideal selective catalyst towards H2O2 should bind O2 sufficiently strong that the reaction can take place on the catalysts surface while being unable to break the intermolecular O−O bond (O2(g) + 2H+(aq) + 2e˗ H2O2(aq), / = 0.69 V).9,11-13,18

Alloying of two elements with opposite O2 adsorption binding strengths (i.e. Pt, Pd with Hg, Au) is nowadays a well-known and common strategy to tune the selectivity while still maintaining a high activity.8,9,15,16 Despite their enhanced activity/selectivity, Au/Pd alloys change their surface composition (crucial for the interaction with O2) during operation depending on the posed conditions.14,16 From a stability point of view, Hg-based alloys are less investigated and expected to be more stable but their high toxicity make them rather undesirable for a green production of H2O2. Alternatively, as suggested in recent studies on Pt, the activity and selectivity might be also influenced by the loading of Pt.29-34,37 At low loadings, the selectivity towards H2O2 increases and the breakup of the intermolecular O-O bond is hindered. This was also suggested for Pd by Mittermeier et al. who observed an increase in selectivity of carbon supported Pd triggered by nanoparticle degradation after accelerated stress tests.29 They hypothesized that the dissolution of Pd leads to a lower Pd loading and a higher interparticle distance (ipd)) that goes in hand with a higher selectivity towards H2O2. Nevertheless, a detailed and systematic analysis of Pd loading, ipd and its influence on the selectivity is still pending. To address this gap in literature, this study focuses on the design of 3 ACS Paragon Plus Environment


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model catalysts with different Pd loadings and distinct ipd and its dependency on the selectivity towards H2O2. Special emphasis will be laid on different support materials (graphene

nanoribbons

vs.

Vulcan)

and

synthesis

procedures

(hydrothermal

vs.

electrochemically deposited) and their implications towards activity and selectivity. The gained understandings are used as guidelines for the development of highly active and selective catalysts while simultaneously reducing the noble metal loading and the associated costs.

2. Experimental details 2.1 Reagents and instruments The compounds employed were PdCl2, KOH, H2SO4, K2S2O8, P2O5, H2O2, ascorbic acid, acetone, and ammonia (28 wt.% in water) (all Vetec),Vulcan Carbon XC-72 (C) (Cabot) NaNO3 and HCl (both Merck), KMnO4 (Nuclear), and hydrazine sulfate (Dinâmica). Multiple wall carbon nanotube (MWCNT) of 10 ± 1 nm o.d. × 4.5 ± 0.5 nm i.d.× 3−6 µm-long, 6−8 tube walls was from Aldrich. The supporting electrolyte was HClO4 (Merck) 0.1M. All electrochemical measurement were performed in a three-electrode Teflon cell either with a rotating disk electrode (RDE) consisting of in-house-built Teflon-embedded glassy carbon (GC) disk or a rotating ring-disk electrode (RRDE) consisting of Teflon-embedded GC disk/Pt ring assembly (0.196 and 0.11 cm2 in geometric area, respectively) (Pine Research Instrumentation), with a collection efficiency of N = 0.26. Separated by a Nafion membrane, a graphite rod and a saturated Ag/AgCl electrode (Metrohm) served as the counter-electrode and reference electrode, respectively. All potentials of the electrochemical results are shown with respect to the reversible hydrogen electrode (RHE) potential. Prior to each measurement its potential vs. an Ag/AgCl electrode was measured in the corresponding electrolyte after hydrogen saturation.


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2.2 Materials characterization For TEM and scanning transmission electron microscopy (STEM) experiments, diluted catalysts aqueous solutions collected from the GC surface were dripped onto ultrathin carbon films supported on a lacey carbon film on a 400-mesh gold grid (Ted Pella). TEM and STEM investigations were performed on a JEOL JEM 2200FS operating at 200 kV. The loading of Pd on catalysts was determined by ICP-MS (NexION 300X, Perkin Elmer). Prior to ICP-MS measurements, in case of the catalysts produced by EDCs, the catalyst film was removed from the RDE electrodes by dissolution in boiling aqua regia (4 mL solution, Merck Suprapur) under ultrasonication. For S1, the carbon support was removed by heating the catalyst powder in porcelain crucibles at 650 °C in air. The metal oxide residues were dissolved by boiling in quartz beakers in agua regia (20 mL) for 45 min followed by the transfer into 100 mL volumetric flasks. The support surface area (GNR: 269.301 m² g-1) was obtained from nitrogen sorption experiments employing the Brunauer–Emmett–Teller (BET) model at −196.277 °C using a Micromeritics ASAP 2020 V3.00. A PHI Quantera II Surface Analysis Equipment was used to perform the XPS measurements. The Al Kα line (1486.6 eV) was used as the ionization source operating at 15 kV and 25 W. The spectra were deconvoluted using a Voigt-type function with Gaussian (70%) and Lorentzian (30%) combinations after the background subtraction.

2.3 Electrode preparation and electrochemical measurements The GC disk working electrode was polished with an alumina slurry (0.05 mm) and cleaned by sonication in ultrapure water twice (PureLab Plus system from Elga, 5 min). Subsequently, the GC disk electrode underwent 10 cycles at a scan rate of 50 mV s−1 within the potential range 0.05−1.0 V, and the Pt ring underwent 200 cycles at 900 mV s−1 in the potential range of 0.05−1.4 V (changing the Ar-saturated 0.1 M aqueous solution of HClO4 whenever necessary to ensure a clean surface finish on both electrodes). A uniform thin film was 5 ACS Paragon Plus Environment


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generated by dripping 20 µL of an aqueous solution of catalysts or supports (GNR or C) using a concentration of 1.5 mg mL−1 (loading: 152.7 µg cm−2 or 0.8 mg mL−1 for a loading of 81.5 µg cm−2) onto a GC disk surface and allowing it to dry at room temperature. The electrodes were placed in an electrochemical Teflon cell containing a 0.1 M aqueous solution of HClO4, which was subsequently saturated with Ar (5.0 purity) or O2 (4.0 purity). Cyclic voltammetry (CV), hydrodynamic linear potential scans and impedance spectroscopy (EIS) were run on a potentiostat Gamry Reference 600 coupled to an AFMSRCE modulated speed rotator from Pine Research Instrumentation. EIS experiments were conducted at opencircuit potential of around 0.91 V (on average). EIS results were collected within a frequency range of 10 mHz to 100 kHz with a potential amplitude of 10 mV (rms) at 10 points per decade. Each hydrodynamic linear potential scan curve was corrected for ohmic drop resistance measured and determined from the fitted high-frequency intercept measured using EIS. The average value of measured ohmic drop resistance in 0.1 M HClO4 was 34 Ω. In order to monitor H2O2 generated during ORR on the disk electrode, the Pt ring electrode was kept constant at 1.2 V, which is a favorable potential to oxidation of peroxide, and their responses (positive currents) were normalized by the collection efficiency (N). Additionally, the selectivity of the catalysts to peroxide formation during ORR ( ) were calculated for various potentials using Ir and Id (data from RRDE curves, multiplied by the GC disk geometric area and N in the case of ring current densities, and by the GC disk geometric area in the case of disk current densities). The was calculated from Equation (1)38,39: = ((2 ⁄)(( + )⁄)) 100.

2.4 Synthesis of GNR GNR synthesis from MWCNTs, as well as the surface and physical properties of these GNRs, is described elsewhere.40 6 ACS Paragon Plus Environment

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2.5 Pd electrodeposition on GNR and C The support material (GNR/C) was dispersed in H2O and put onto the GC disk surface. The modified electrode was placed in an electrochemical cell that contained a 1 mM (or 0.2 mM) solution of PdCl2 in a 0.1 M aqueous solution of HClO4, and electrodeposition (via CV) was performed. Here, four different electrodeposition conditions (EDC1-EDC4) were used as presented in Scheme 1. Interested readers are also forwarded to the SI for more details. After deposition, the modified electrode was thoroughly washed with ultrapure water and placed in an electrochemical cell that contained a 0.1 M aqueous solution of HClO4.

3. Results and discussion 3.1 Electrodeposited Pd/GNR The first step was to bring Pd onto a carbon support. To achieve a higher dispersion, a crucial parameter for controlling the ipd, GNRs were used as support. Straight strips contain oxygenated41,42 and nitrogenated groups that form during the production of graphene strips.23,43-49 The presence of such groups on the surface is important for the extent of dispersion and the interaction with the metal catalysts.42,50 Alternatively, Vulcan Carbon XC72 (C) as reference support material with similar BET surface area (254 m² g-1) as GNR (269 m² g-1) was used. In order to avoid impurities during synthesis, Pd was electrodeposited directly onto GNR and Vulcan.23,24,51,52 The loading of Pd was tuned systematically by changing the electrodeposition conditions (EDC) as shown in Scheme 1 (c.f. Tables S1 and S2, and EDCs section in SI). Applying less and shorter cycles, less Pd nucleates on the surface and less time is left for the particles to grow. Consequently, for less and shorter cycles, particles become less frequent as shown by the changing ipd (Scheme 1 and Table S1) calculated by Equation S1. The slightly deviating particle size distribution is not expected to 7 ACS Paragon Plus Environment


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change the selectivity as shown before by Yano et al..53 The electrochemical Pd deposition profiles on GNR and Vulcan Carbon XC-72 can be found in Figure S1.

Scheme 1. Electrodeposition of Pd on GNR using different synthesis parameters (left), the corresponding bright-field STEM images (middle), the size distribution histograms and ipd (right). The synthesized materials were first characterized by cyclic voltammetry in 0.1 M HClO4 scanning the potential from 0.1 V to 1.4 V vs. RHE as shown in Figure 1 and S2. As expected, EDC1 with the highest Pd concentration shows the characteristic Pd features starting with the hydrogen underpotential deposition (HUPD) region between ca. 0.1 and 0.4 V vs. RHE, Pd oxidation between ca. 0.9-1.4 V vs. RHE, and the corresponding oxide back reduction between ca. 0.8-0.6 V vs. RHE. When reducing the Pd loading, the classical oxide formation/reduction as well as the HUPD become less pronounced until none of these features are visible on the cyclic voltammograms. With the lowest loading (i.e. for EDC4), only the characteristic quinone/hydroquinone peaks from the carbon support appear.51,54 As discussed by others authors for the case of carbon-supported Pt, we also observed a slight potential shift for the oxide reduction peak (EDC1, EDC2) to more negative values as 8 ACS Paragon Plus Environment

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the Pd loading was decreased.32-34 The potential shift is related to a change in oxophilicity due to varied adsorption energies of O*, HO* and HOO* intermediates (the * relates to surface bound adsorbates) resulting in an overall lower ORR activity. The activity of the described catalysts toward the ORR was evaluated by using RRDE linear potential scan in an O2-saturated 0.1 M aqueous solution of HClO4. The ORR responses are shown in Figure 1, S3 and S4.

Figure 1. (A) Cyclic voltammograms of synthesized EDC1-EDC4 in Ar-saturated 0.1 M HClO4 between 0.1 and 1.4 V using a scan rate of 50 mV s–1. The currents were normalized to 9 ACS Paragon Plus Environment


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the total carbon loading (LC). (B) Selectivity towards H2O2 ( ) during the ORR at varying potentials and the corresponding RRDE results for Pd supported GNR in O2-saturated 0.1 M HClO4 between 0.1 and 1.0 V at ν = 50 mV s−1 rotating the WE at 900 rpm. High Pd loadings and the consequent low ipd of Pd on GNR (EDC1, EDC2, Figure 1B) leads to the expected full reduction towards H2O with low selectivity towards H2O2 (< 2% in average) which is in line with literature.21,23,24,29,38,39,43,51,52,54,55 The obtained diffusion limiting current densities (4.6 and 4.1 mA cm-² for EDC1 and EDC2) and the corresponding Tafel slopes are within the expected theoretical values (c.f. Figure S5). At potential around the HUPD (< 0.3 V vs. RHE), the ORR current density slightly decreases (more evidently for EDC2) as the adsorbed hydrogen decreases the O adsorption on Pd.55,56 When reducing the Pd content further (EDC3, EDC4, Figure 1B), however, a different scenario is obtained. In case of EDC3, a selectivity of around 42.7% (potential range: 0.5-0.2 V) toward H2O2 was achieved. The current density between 0.1 and 0.5 V exceeds the theoretical diffusion limiting current density for a 2-electron transfer (~2.3 mA cm-2, see horizontal black dashed line in Figure 1B) but is still lower than the theoretically expected value for a 4-electron transfer (~4.5 mA cm-2, see horizontal black dashed line in Figure 1B). In case of EDC4, the H2O2 selectivity increases up to 93% (potential range: 0.5-0.2 V) with the onset potential very close to the thermodynamically expected O2/H2O2 potential (/ = 0.69 V). This potential is concentration dependent and since there is no H2O2 in

the electrolyte in the beginning, the thermodynamic potential is shifted to more positive values.26 The limiting current densities were also close to the theoretically expected current density for a 2-electron transfer. 3.2 Electrodeposited Pd/C In order to investigate if the observed results are specific to GNR, Vulcan XC-72 (C) with comparable surface area as GNR was used as reference. The electrodeposition profile can be 10 ACS Paragon Plus Environment

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found in Figure S1. Here, the same catalytic trend was observed (Figure S4) as for Pd/GNR, where a high Pd loading leads to total four electron transfer while reducing the Pd loading leads to a 2-electron path. The selectivity for EDC4 on Vulcan, however, was lower compared to GNR (~87 % vs. 93 %, 0.3 µg cm-2 vs. 1.5 µg cm-2) with an accompanied lower onset potential (~0.70 V vs. 0.61 V). Moreover, the observed current density was far from the theoretically expected value owing to an insufficient Pd loading (0.3 µg cm−2). The changes in selectivity and activity between GNR and Vulcan are explained by 1) the higher dispersion of Pd on GNR due to (mainly) oxygenated and nitrogenated anchor groups and 2) the different porosity (determinant for the diffusivity and residence time of peroxide on the catalysts) and mass transport within layers, also related to different layer thicknesses or loadings. 3.3 Hydrothermally synthesized Pd/GNR and Pd/C Electrodeposition of Pd was used as proof of concept in a systematic and controlled manner in an environment where impurities can be avoided. In order to confirm further the validity of our findings, two samples with low Pd loading were also synthesized on GNR and Vulcan through a classical hydrothermal synthesis (S1) using ascorbic acid as reducing agent. Unlike electrodeposition, such synthesis route can be easily scaled up for applications that require large amounts of catalyst. The experimental details can be found in the SI. The structure of Pd NPs supported on GNR produced by hydrothermal synthesis (S1) (Figure S7) is very similar to electrodeposited Pd (EDC4 on GNR), exhibiting similar size distributions and ipd distances (Figure S8). For Pd/GNR and Pd/C produced via S1, the ORR (Figure S3 and S4) starts at ~0.70 V, with diffusion-limited current densities close to the theoretical value for a 2-electron transfer. When taking the selectivity into account, we confirm that for low Pd loading, a high H2O2 selectivity can be achieved. Similarly to EDC, we observe differences in selectivity (~78 % and 46% for Pd/GNR and Pd/C respectively, Figure 2A). Intriguingly, a higher selectivity was observed for Pd/GNR (S1) despite the higher Pd loading (2.0 µgPd cm-2 for Pd/GNR and 1.2 11 ACS Paragon Plus Environment


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µgPd cm-2 for Pd/C). We attributed these differences to metal/support interactions and the different degree of dispersion as discussed in the previous section emphasizing the importance of the support to the overall selectivity and activity. The XPS results for Pd on GNR and on Vulcan (Figure S9) confirm this assertion. For Pd on GNRs, the lower binding energy doublet at 335.2 eV and 340.7 eV can be assigned to zero-valent Pd – the higher binding energies to oxidized Pd. Under ultra-high vacuum conditions, a peak shift in the Pd 3d region to higher binding energies (~0.6 eV) can be observed when the catalyst is supported on GNR compared to C. Similar effects have been observed for Pt on multiwall carbon nanotubes57 and boron carbide58 where a shift in the Pt 4f region was observed. In order to equilibrate the respective Fermi levels, charge is transferred from the metal clusters to the oxygen and nitrogen moieties present on GNR when in contact.50,59-61 The improved selectivity and the shift in binding energy observed for Pd on GNR strongly suggest favorable electronic metal-support interactions of Pd and GNR and might be an additional tool to fine tune the selectivity/activity parameters. In comparison to Vulcan, a higher concentration of oxygenated groups can be found on GNR. As discussed in the previous section, nitrogen groups on GNR - together with the higher concentration of oxygenated groups – act as anchor groups for Pd favoring a high dispersion. Impurities (especially chlorides) from the synthesis were also reported to change the adsorption properties of oxygen intermediates to the catalyst’s surface.25,26 A survey XPS scan can be found in Figure S9F. Interestingly, almost no chlorides were detected (Figure S9E). It seems that the multiple washing steps remove most chlorides from the catalyst’s surface and that present trace amounts of Cl cannot be detected. 3.4 Comparison and Discussion As stated before, by changing the synthesis conditions, it is possible to tune the Pd loading as well as particles size and the Pd ipd. Figure 2A shows the Pd atomic ratio (%) and its correlation to during the ORR. At Pd atomic ratios less than 0.25 % (on both supports) equivalent to less than 2wt % Pd – H2O2 can be detected. In this region, the ipd is relatively 12 ACS Paragon Plus Environment

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high (> 125 nm) and a high selectivity is obtained. The correlation of to the ipd for Pd/GNR is displayed in Fig 2A (inset) confirming the previously drawn conclusions. These results suggest that 1) the ORR on Pd consist of a 2+2 electron transfer and that 2) below a certain Pd loading, the ORR takes place with partial or complete suppression of the second 2electron transfer step (depending on the loading and ipd). With a low Pd loading, the formed H2O2 intermediate can diffuse into the solution before undergoing further reduction (c.f. Figure 2B). For EDC1 and EDC2, the significant number of large particles and the low ipd favors the O-O bond splitting and oxygen is fully reduced to water. Eventually formed H2O2 can easily be readsorbed on neighboring Pd particles and can be further reduced to H2O resulting in negligible amounts of observed H2O2. When the amount of Pd on the surface is reduced (EDC3, EDC4) and larger particles are less frequent and increasingly spaced, H2O2 as reaction product is favored. Additionally, the selectivity can be tuned by the layer thickness as indicated in Figure S3. Here, the loading of Pd/GNR S1 was increased from 150 to 450 µg cm-2 (Pd loading increased from 2.0 to 6.0 µg cm-2) enhancing the probability of H2O2 encountering another Pd-particle resulting in a change of from 78 % to 62 %. In practical applications, H2O2 can undergo further electrochemical reduction (H2O2 + 2e− + 2H+

2H2O) or suffer electrochemical oxidation (H2O2 O2 + 2e− + 2H+) at more positive potentials. A selective catalyst, however, should not permit the “degradation” of produced peroxide. Therefore, the reduction and oxidation of deliberately added 10 mM H2O2 to an Arsaturated 0.1 M HClO4 was tested as shown in Figure S10. The RDE responses of EDC4 and S1 of Pd NPs supported on GNR and Vulcan presented relatively low current densities over the whole tested potential range indicating low catalytic performance towards the decomposition of H2O2. When increasing the Pd loading (EDC1-EDC3), however, oxidative and reductive current responses speak for the oxidation/reduction of H2O2 which is line to the previously obtained low H2O2 selectivity (Figures 1, S3, and S4. See also the kinetic parameters to Pd loading at GNR and C by EDC1 and EDC2 in Table S3). The highest 13 ACS Paragon Plus Environment


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selectivity and activity towards H2O2 was obtained by using GNR as support whereby electrodeposited Pd (Pd/GNR; EDC4) and hydrothermally obtained Pd (Pd/GNR; S1) show comparable results (Figure 2A). These insights allowed for the identification of high performance catalysts as depicted in Figure 2C. Here, the calculated mass activities (MA) (see details in SI) and the over a potential range between 0.2 and 0.5 V obtained of the best catalysts is shown. To the best of the authors’ knowledge, Pd/GNR, EDC4 exhibits the highest reported MA (~110 A per g Pd) based on monometallic catalysts calculated at an overpotential of 50 mV. Only bimetallic catalysts such as Pd-Hg show a slightly higher MA of ~137 A g-1 based on the noble metal content. The H2O2 selectivity of EDC4 on GNR ( of ~94%) is as high as Pd-Hg ( of ~95%) with the advantage of excluding harmful and toxic Hg.18

Figure 2. (A) Correlation of to Pd atomic ratio obtained from different Pd loadings and synthesis strategies. Inset: Plot of against ipd for Pd/GNR produced by EDC. (B) Schematic display of the influence of ipd and loading on the selectivity. (C) MA at an overpotential of 50 mV of the best catalysts obtained by different synthesis strategies.

4. Conclusions In conclusion, various Pd loadings on different supports were prepared either by electrodeposition or in a hydrothermal synthesis. By tuning the electrodeposition conditions, 14 ACS Paragon Plus Environment

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the Pd loading was - as well as the particles size and their dispersion - systematically altered. Lower Pd loadings lead to a selectivity of almost 100 % towards H2O2 while high Pd loadings favor the full reduction of O2 to H2O. The obtained insights allowed for the successful design of catalysts with H2O2 selectivities close to 100% with almost no kinetic overpotential and high mass activities belonging to the best-known oxygen reduction catalysts. We envision that the gained understanding of ipd, loading of Pd and its impact on H2O2 selectivity contribute to the successful design of more selective and active catalysts. Especially the “homeopathic dose” of noble metal will be beneficial for the implementation of Pd/carbon based catalysts into real devices. By modification and tailoring of the support material, the limits towards higher selectivity might be stretched while simultaneously reducing the noble metal content.

ASSOCIATED CONTENT Supporting Information. The supporting information contains experimental details, figures, equations, and tables concerning supplementary results, and references. This material is available free of charge via the Internet at:

AUTHOR INFORMATION *Corresponding Author. E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes 15 ACS Paragon Plus Environment


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The authors declare no competing financial interest. ACKNOWLEDGMENTS Thanks are also given to CNPq (grants 301403/2011-2, 473991/2012-8, 405695/2013-6, 303759/2014-3,

and

442268/2014-9)

and

Fundect-MS

(grants

23/200.583/2012,

23/200.735/2012, 23/200.246/2014, and 59/300.184/2016) for their financial support. E.S.F.C. thanks Fundect-MS (grant 23/200.675/2014) and as G.V.F. thanks CAPES for the PDSE fellowship (grant 88881.131904/2016-01). We thank the MAXNET Energy for financial support.

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TOC Graphic



Scheme 1. Electrodeposition of Pd on GNR using different synthesis parameters (left), the corresponding bright-field STEM images (middle), the size distribution histograms and ipd (right). 2116x1114mm (96 x 96 DPI)

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Figure 1. (A) Cyclic voltammograms of synthesized EDC1-EDC4 in Ar-saturated 0.1 M HClO4 between 0.1 and 1.4 V using a scan rate of 50 mV s–1. The currents were normalized to the total carbon loading (LC). (B) Selectivity towards H2O2 (S_(H_2 O_2 )) during the ORR at varying potentials and the corresponding RRDE results for Pd supported GNR in O2-saturated 0.1 M HClO4 between 0.1 and 1.0 V at ν = 50 mV s−1 rotating the WE at 900 rpm. 291x446mm (300 x 300 DPI)



Figure 2. (A) Correlation of S_(H_2 O_2 ) to Pd atomic ratio obtained from different Pd loadings and synthesis strategies. Inset: Plot of S_(H_2 O_2 ) against ipd for Pd/GNR produced by EDC. (B) Schematic display of the influence of idp and loading on the selectivity. (C) MA at an overpotential of 50 mV of the best catalysts obtained by different synthesis strategies. 500x197mm (96 x 96 DPI)


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Impact of Palladium Loading and Interparticle Distance on the

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