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Catalysis is inherently driven by the interaction of reactants, intermediates and formed products with the catalyst's surface. In order to reach the desired transition state and to overcome the kinetic barrier, elevated temperatures or electrical potentials are employed to increase the rate of reaction. Despite immense efforts in the last decades, research in thermo- and electrocatalysis has often preceded in isolation, even for similar reactions. Conceptually, any heterogeneous surface process that involves changes in oxidation states, redox processes, adsorption of charged species (even as spectators) or heterolytic cleavage of small molecules should be thought of as having parallels with electrochemical processes occurring at electrified interfaces. Herein, we compare current trends in thermo- and electrocatalysis and elaborate on the commonalities and differences between both research fields, with a specific focus on the production of hydrogen peroxide as case study. We hope that interlinking both fields will be inspiring and thought-provoking, eventually creating synergies and leverage towards more efficient decentralized chemical conversion processes.

Palladium nanoparticles have been studied extensively as catalysts for the direct synthesis of hydrogen peroxide, where selectivity remains a key challenge. Alloying Pd with other metals and using acid and halide promoters are commonly employed to increase H2O2 selectivity; however, the sites that can selectively produce H2O2 have not been identified and the role of these additives remains unclear. Here, we report the synthesis of atomically dispersed PdClx/C as a model catalyst for H2O2 production without the presence of extended Pd surfaces. We show that these isolated cationic Pd sites can form H2O2 with significantly higher selectivity than metallic Pd nanoparticles in both the reaction of H2 and O2 and the electrochemical oxygen reduction reaction. These results demonstrate that catalysts containing high populations of isolated Pd sites are selective catalysts for this two-electron reduction reaction and that the performance of materials in the direct synthesis reaction and electrocatalytic oxygen reduction reaction has many similarities.

Hydrogen peroxide is considered one of the most important commodity chemicals worldwide but its main production method, the anthraquinone process, poses serious logistical, environmental and safety challenges. Electrocatalytic synthesis through the reduction of molecular oxygen is a promising H2O2 production route. However, the reduction of molecular oxygen is kinetically hindered and stable electrocatalysts with a high activity and selectivity towards the 2-electron transfer reaction are needed. In this work, we evaluated the influence of chloride on catalysts with low palladium loadings on the ORR selectivity towards H2O2. We report the factors and dynamics that influence H2O2 production and highlight synthesis strategies to obtain close to 100% selectivity. By probing the electrode surface after various degradation cycles, we evaluate the role of adsorbing species and the catalysts oxidation states on the hydrogen peroxide selectivity. We systematically modified the catalyst synthesis using different Pd-precursors that were reduced and supported on high surface area graphene nanoribbons. Identical location transmission electron microscopy was used to probe catalyst dynamics during reaction and the activities and selectivities were measured by a rotating ring disk electrode. We probe the potential boundary conditions that lead to catalyst degradation during accelerated stress tests and potentiostatic polarisation and demonstrate how the catalytically active surface can be revived after degradation. The obtained insights can be used as guideline for the development of active, selective and stable catalysts with low noble metal loadings.

Increasing the catalyst's stability and activity are one of the main quests in catalysis. Tailoring crystal surfaces to a specific reaction has demonstrated to be a very effective way in increasing the catalyst's specific activity. Shape controlled nanoparticles with specific crystal facets are usually grown kinetically and are highly susceptible to morphological changes during the reaction due to agglomeration, metal dissolution, or Ostwald ripening. A strong interaction of the catalytic material to the support is thus crucial for successful stabilization. Taken both points into account, a general catalyst design is proposed, combining the enhanced activity of shape-controlled nanoparticles with a pore-confinement approach for high stability. Hollow graphitic spheres with narrow and uniform bimodal mesopores serve as model system and were used as support material. As catalyst, different kinds of particles, such as pure platinum (Pt), platinum/nickel (Pt3Ni) and Pt3Ni doped with molybdenum (Pt3Ni-Mo), have exemplarily been synthesized. The advantages, limits and challenges of the proposed concept are discussed and elaborated by means of time-resolved, in and ex situ measurements. It will be shown that during catalysis, the potential boundaries are crucial especially for the proposed catalyst design, resulting in either retention of the initial activity or drastic loss in shape, size and elemental composition. The synthesis and catalyst design can be adapted to a wide range of catalytic reactions where stabilization of shape-controlled particles is a focus.

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 into tuning the selectivity toward 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 contributors 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 toward 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.

International audience; Reducing the noble metal loading and increasing the specific activity of the oxygen evolution catalysts are omnipresent challenges in proton-exchange-membrane water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements. However, proper verification of the stability of these materials is still pending. Here we introduce a metric to explore the dissolution processes of various iridium-based oxides, defined as the ratio between the amounts of evolved oxygen and dissolved iridium. The so-called stability number is independent of loading, surface area or involved active sites and provides a reasonable comparison of diverse materials with respect to stability. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to the formation of highly active amorphous iridium oxide, the instability of which is explained by the generation of short-lived vacancies that favour dissolution. These insights are meant to guide further research, which should be devoted to increasing the utilization of highly durable pure crystalline iridium oxide and finding solutions to stabilize amorphous iridium oxides.

International journal of hydrogen energy 42(40), 25359-25371 (2017). doi:10.1016/j.ijhydene.2017.08.108, Published by Elsevier, New York, NY [u.a.]

During application, electrocatalysts are exposed to harsh electrochemical conditions, which can induce degradation. This work addresses the degradation of AuPd bimetallic catalysts used for the electrocatalytic production of hydrogen peroxide (H2O2) by the oxygen reduction reaction (ORR). Potential-dependent changes in the AuPd surface composition occur because the two metals have different dissolution onset potentials, resulting in catalyst dealloying. Using a scanning flow cell (SFC) with an inductively coupled plasma mass spectrometer (ICP-MS), simultaneous Pd and/or Au dissolution can be observed. Thereafter, three accelerated degradation protocols (ADPs), simulating different dissolution regimes, are employed to study the catalyst structure degradation on the nanoscale with identical location (IL) TEM. When only Pd or both Au and Pd dissolve, the composition changes rapidly and the surface becomes enriched with Au, as observed by cyclic voltammetry and elemental mapping. Such changes are mirrored by the evolution of electrocatalytic performances toward H2O2 production. Our experimental findings are finally summarized in a dissolution/structure/selectivity mechanism, providing a clear picture of the degradation of bimetallic catalyst used for H2O2 synthesis.

This study focuses on the synthesis and electrochemical performance (i.e, activity and stability) of advanced electrocatalysts for the oxygen reduction reaction (ORR), made of Pt–Ni nanoparticles embedded in hollow graphitic spheres (HGS). The mechanism of the confined space alloying, that is, the controlled alloying of bimetallic precursors with different compositions (i.e., Pt3Ni, PtNi, and PtNi3) within the HGS mesoporous shell, was examined in detail. It was found that the presence of platinum during the reduction step, as well as the application of high annealing temperatures (at least 850 °C for 3.5h in Ar), are necessary conditions to achieve the complete encapsulation and the full stability of the catalysts. The evolution of the activity, the electrochemical surface area, and the residual alloy composition of the Pt–Ni@HGS catalysts was thoroughly monitored (at the macro- and nanoscale level) under different degradation conditions. After the initial activation, the embedded Pt–Ni nanoparticles (3–4 nm in size) yield mass activities that are 2- to 3.5-fold higher than that of pure Pt@HGS (depending on the alloy composition). Most importantly, it is demonstrated that under the normal operation range of an ORR catalyst in PEM-FCs (potential excursions between 0.4 and 1.0 VRHE) both the nanoparticle-related degradation pathways (particle agglomeration) and dealloying phenomena are effectively suppressed, irrespectively of the alloy composition. Thus, the initial enhanced activity is completely maintained over an extended degradation protocol. In addition, owing to the peculiar configuration of the catalysts consisting of space-confined nanoparticles, it was possible to elucidate the impact of the dealloying process (as a function of alloy composition and severity of the degradation protocols) separately from other parallel phenomena, providing valuable insight into this elusive degradation mechanism.

In this work we employ the advanced scanning flow cell based analytical techniques, viz. inductively coupled plasma mass spectrometry (SFC-ICP-MS) and on-line electrochemical mass-spectrometry (SFC-OLEMS) to directly detect the amounts of dissolved platinum and evolved carbon dioxide in two protocols that are commonly used in the fuel cell community to simulate load cycle and start-stop conditions in proton exchange membrane fuel cells (PEMFCs). In contrast to previous assumptions, claiming a separation between carbon corrosion and platinum dissolution, in both standard protocols platinum dissolution and carbon corrosion are present at low rates, which is also reflected by a comparably low ECSA decrease. On the other hand, a huge increase in rate of both processes is observed during transitions from low to high potential regimes experienced by a PEMFC in operation, here studied in a third protocol covering the whole potential range from 0.6 to 1.5 VRHE. The latter is typically not addressed in literature. This finding is explained by taking into account platinum catalyzed carbon corrosion and transient platinum dissolution. Based on the obtained results, the question is raised on the practical adequacy of the standard protocols for differentiation of degradation processes and simulation of the degradation processes occurring in PEMFCs.

Palladium (Pd) is considered as a possible candidate as catalyst for proton exchange membrane fuel cells (PEMFCs) due to its high activity and affordable price compared to platinum (Pt). However, the stability of Pd is known to be limited, yet still not fully understood. In this work, Pd dissolution is studied in acidic media using an online inductively coupled plasma mass spectrometry (ICP-MS) in combination with an electrochemical scanning flow cell (SFC). Crucial parameters influencing dissolution like potential scan rate, upper potential limit (UPL) and electrolyte composition are studied on a bulk polycrystalline Pd (poly-Pd). Furthermore, a comparison with a supported high-surface area catalyst is carried out for its potential use in industrial applications. For this aim, a carbon supported Pd nanocatalyst (Pd/C) is synthesized and its performance is compared with that of bulk poly-Pd. Our results evidence that the transient dissolution is promoted by three main contributions (one anodic and two cathodic). At potentials below 1.5 V RHE the anodic dissolution is the dominating mechanism, whereas at higher potentials the cathodic mechanisms prevail. On the basis of the obtained results, a model is thereafter proposed to explain the transient Pd dissolution.

International audience; For a successful replacement of Pt, tremendous efforts have hitherto been made to develop high-performing Fe-N-C catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). In comparison to the remarkable progress in activity, the stability of Fe-N-C catalysts still remains critical, however. Fe demetallation in acidic medium is hypothesized to be one critical factor for the overall lifetime. In contrast to the general belief, we herein demonstrate using an operando spectroscopic analysis that catalytically inactive Fe particles exposed to acid electrolytes cannot be fully removed by acid washing due to a relatively high open circuit potential (ca. 0.9 VRHE) leading to the formation of insoluble ferric species, whereas these particles dissolve under PEMFC operating conditions (Ecathode < 0.7 VRHE) due to operando reduction to soluble ferrous cations. To overcome this issue, we demonstrate two approaches: (i) synthesis of Fe-N-C catalysts free of Fe particles and (ii) postsynthesis removal of exposed Fe particles through the control of potential using an external potentiostat or an internal reducing agent (i.e., SnCl2). Operando spectroscopic analyses verified that Fe demetallation during a given voltammetric protocol was dramatically decreased for both synthetically and postsynthetically modified Fe-N-C catalysts, while the initial ORR activity did not significantly change. However, all of these catalysts showed similar performance decay over short-term PEMFC durability tests, demonstrating the lack of a role played by ferrous cations leached from inactive Fe particles on catalyst deactivation. This supports the view that the activity is mainly imparted by FeNxCy moieties. Nevertheless, the presented guidelines are generally applicable to the whole class of Fe-N-C catalysts in order to minimize Fe demetallation in PEMFCs, which provides important advances for the future design of stable electrocatalytic systems for long-term operation

The activity of a catalyst towards the oxygen reduction reaction (ORR) is determined by its surface composition and structure. Although careful synthesis strategies have managed to improve the ORR activity of Pt by a factor of 40 in recent years, the long-term stability of supported catalysts still poses a challenge.[1] One approach that can be used to supress degradation phenomena, such as particle migration and agglomeration, is incorporation of the active metal species in the pores of a conducting, porous graphitic matrix, in order to hinder the motion of the alloy particles. [2-4] Here, we describe the synthesis, characterization and stability of octahedral Pt3NiMo nanoparticle catalysts supported in hollow graphitic spheres. By combining electrochemically active surface area (ECSA) measurements, identical location scanning transmission electron microscopy (STEM) and measurements performed using an in situ electrocatalytic scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS), we demonstrate the stabilizing effect of the hollow graphitic spheres on the PtNi octahedra. After 10800 cycles, the octahedra are found to have retained their shape, with only a small loss of ECSA. In summary, we demonstrate the successful incorporation of shaped bimetallic nanoparticles into a porous support. This design strategy can be used for other catalytic reactions and opens new ways for achieving space confinement of faceted Pt alloy particles. References: [1] P. Strasser, Science 2015, 349, pp 379-380. [2] C. Galeano, J. C. Meier, V. Peinecke, H. Bongard, I. Katsounaros, A. A. Topalov, A. Lu, K. J. J. Mayrhofer, F. Schüth, J. Am. Chem. Soc. 2012, 134, pp 20457– 20465. [3] S. Mezzavilla, C. Baldizzone, A.-C. Swertz, N. Hodnik, E. Pizzutilo, G. Polymeros, G. P. Keeley, J. Knossalla, M. Heggen, K. J. J. Mayrhofer, F. Schüth, ACS Catal., 2016, 6 (12), pp 8058–8068. [4] E. Pizzutilo, J. Knossalla, S. Geiger, J. P. Grote, G. Polymeros, C. Baldizzone, S. Mezzavilla, M. Ledendecker, A. Mingers, S. Cherevko, F. Schuth, K. J. J. Mayrhofer, Adv. Energy. Mater. 2017, 7(20).

In this paper, we propose to study the impact of very low filler content (0.005 wt.%) of graphite nanoflakes (80 nm), multi-layer (5–20 nm) and few-layer (1–2 nm) graphene on the electrical conductivity of an epoxy nanocomposite. The results highlight that an improvement of the quality of the exfoliation process of graphene, particularly in few-layer graphene/epoxy, allows decreasing the DC electrical conductivity (by a factor 100 compared to neat epoxy) in a large range of electric field from 1 to 10 kV/mm. This novel property could allow decreasing space charge trapping within the insulator bulk at the origin of long-term electrical ageing.

The performance of polymer electrolyte fuel cells is strongly correlated to the electrocatalytic activity and stability. In particular, the latter is the result of an interplay between different degradation mechanisms. The essential high stability, demanded for real applications, requires the synthesis of advanced electrocatalysts that withstand the harsh operation conditions. In the first part of this study, the synthesis of oxygen reduction electrocatalysts consisting of Pt-Co (i.e., Pt5Co, Pt3Co, and PtCo) alloyed nanoparticles encapsulated in the mesoporous shell of hollow graphitic spheres (HGS) is reported. The mass activities of the activated catalysts depend on the initial alloy composition and an activity increase on the order of two to threefold, compared to pure Pt@HGS, is achieved. The key point of the second part is the investigation of the degradation of PtCo@HGS (showing the highest activity). Thanks to pore confinement, the impact of dissolution/dealloying and carbon corrosion can be studied without the interplay of other degradation mechanisms that would induce a substantial change in the particle size distribution. Therefore, impact of the upper potential limit and the scan rates on the dealloying and electrochemical surface area evolution can be examined in detail.

Hydrogen peroxide (H2O2) is an important chemical, commonly employed for disinfection, water treatment and bleaching. Whilst H2O2 itself is considered as an environmental friendly chemical oxidant, whose sole side product is water, its current synthesis method, the anthraquinone process, can be considered less so. Considering that most of end-use applications require concentrations between 2 to 5 wt%, a continuous on-site production of H2O2 would provide an attractive alternative and is therefore highly desirable. Both the electrochemical reduction of oxygen to hydrogen peroxide [1-2] and the direct synthesis in an autoclave [3] have attracted considerable research interest into catalysts which could provide a more efficient alternative to the current industrial process. Since the first patent claiming Pd catalysts active for the peroxide synthesis [4], several studies have been carried out to investigate the role of different acids and halides [5-6] as well of different supports [3] in the productivity [mol/kgcat/h] and hydrogen selectivity for Pd based catalysts. Despite the state of the art characterization techniques, being applied to determine the surface structure of mono and bimetallic catalysts, there is still a lack of detailed understanding of the active sites of the metal nanoparticle. The promotional effect of Au addition to Pd catalysts for the direct synthesis of H2O2in a reaction medium free of acid and halide promoters has been extensively studied by Hutchings and co-workers over the last decade [3]. In particular, one major breakthrough is related to the role of the acid pre-treated activated carbon [7]. Starting from these previous studies, in this work different molar ratio of Au/Pd catalysts were synthesized [8] with a final metal loading of 10 wt% on activated carbon and the influence of bimetallic nanoparticles composition for Oxygen Reduction Reaction (ORR) and Hydrogen Oxydation Reaction (HOR) was studied. The change in activity, selectivity towards hydrogen peroxide as well as in H2O2 decomposition was characterized in both an electrochemical cell, using a Rotating Ring Disk Electrode (RRDE), and in an autoclave. Whilst the addition of Au to Pd increases the overall selectivity, the results for pure Au showed low peroxide productivity during direct synthesis. Furthermore, the stability of these catalysts in acidic media and the catalyst degradation consequences on the performances for H2O2 synthesis represents also a major interest of this work. The online dissolution was characterized with a Scanning Flow Cell and Inductively Coupled Plasma Mass Spectrometer (SFC-ICPMS), while the microscopic stability was studied with Identical Location TEM (IL-TEM). [1] S. Siahrostami, A. Verdaguer-Casadevall, M. Karamad, D. Deiana, P. Malacrida, B. Wickman, M. Escudero-Escribano, E. A. Paoli, R. Frydendal, T. W. Hansen, I. Chorkendorff, I. E. L. Stephens and J. Rossmeisl, Nature Materials 12 (2013), 1137-1143 [2] J. S. Jirkovsky, I. Panas, E. Ahlberg, M. Halasa, S. Romani, and D. J. Schiffrin, J. Am. Chem. Soc. 133 (2011), 19432–19441 [3] J. K. Edwards, S. J. Freakley, R. J. Lewis, J. C. Pritchard, G. J. Hutchings, Catalysis Today 248 (2015) 3–9 [4] H. Henkel, W. Weber, US 1108752, 1914. [5] V.R. Choudhary, C. Samanta, J. Catal. 238 (2006) 28–38 [6] V.R. Choudhary, P. Jana, Appl. Catal. A: Gen. 352 (2009) 35–42. [7] J.K. Edwards, B. Solsona, E. Ntainjua, A.F. Carley, A.A. Herzing, C.J. Kiely, G.J.Hutchings, Science 323 (2009) 1037–1041. [8] J. Pritchard, L. Kesavan, M. Piccinini, Q. He, R. Tiruvalam, N. Dimitratos, J. A. Lopez-Sanchez, A. F. Carley, J. K. Edwards, C. J. Kiely, and G. J. Hutchings, Langmuir 26 (2010), 16568–16577

Bimetallic catalysts are known to often provide enhanced activity compared to pure metals, due to their electronic, geometric and ensemble effects. However, applied catalytic reaction conditions may induce re-structuring, metal diffusion and dealloying. This gives rise to a drastic change in surface composition, thus limiting the application of bimetallic catalysts in real systems. Here, we report a study on dealloying using an AuPd bimetallic nanocatalyst (1 : 1 molar ratio) as a model system. The changes in surface composition over time are monitored in situ by cyclic voltammetry, and dissolution is studied in parallel using online inductively coupled plasma mass spectrometry (ICP-MS). It is demonstrated how experimental conditions such as different acidic media (0.1 M HClO4 and H2SO4), different gases (Ar and O2), upper potential limit and scan rate significantly affect the partial dissolution rates and consequently the surface composition. The understanding of these alterations is crucial for the determination of fundamental catalyst activity, and plays an essential role for real applications, where long-term stability is a key parameter. In particular, the findings can be utilized for the development of catalysts with enhanced activity and/or selectivity.