Isolated Pd Sites as Selective Catalysts for Electrochemical and Direct Hydrogen Peroxide Synthesis

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.</p

Isolated Pd Sites as Selective Catalysts for Electrochemical and Direct Hydrogen Peroxide Synthesis

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 the use of acid and halide promoters are commonly used 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 Pd/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 (ORR). This demonstrates 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 ORR have many similarities

Addressing stability challenges of using bimetallic electrocatalysts: the case of gold?palladium nanoalloys

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 restructuring, 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 O-2), 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

Palladium electrodissolution from model surfaces and nanoparticles

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 VRHE 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.(C) 2017 Elsevier Ltd. All rights reserved

Gold–Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity

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

Analysing the relationship between the fields of thermo- and electrocatalysis taking hydrogen peroxide as a case study

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

Towards on-site production of hydrogen peroxide with gold-palladium catalysts in electrocatalysis and heterogeneous catalysis

Wasserstoffperoxid (H$_2$O$_2$) gilt als umweltfreundlichstes Oxidationsmittel, da als Nebenprodukt nur Wasser entsteht. Der Herstellungsprozess beruht auf dem "Anthraquinone" Verfahren, das noch immer Sicherheitsrisiken birgt. Alternative Herstellungsstrategien basieren auf heterogener Katalyse oder elektrokatalytischen Verfahren, die beide das Potential besitzen leistungsfähiger und umweltfreundlicher als die klassischen Herstellungsprozesse zu sein. Trotz hohen Interesses sind beide Technologien noch in der Entwicklungsphase. In dieser Dissertation wurden die Stabilität und Aktivität von verschiedenen Gold-Palladium Katalysatoren untersucht und beide H$_2$O$_2$ Herstellungsmethoden verglichen. Hierdurch konnten neue Einblicke in die katalytischen Prozesse gewonnen werden, welche gezielt genutzt wurden, um einen neuen katalytischen Mechanismus zu beschreiben

The oxygen reduction reaction on palladium with low metal loadings:The effects of chlorides on the stability and activity towards hydrogen peroxide

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.</p

Electrocatalytic synthesis of hydrogen peroxide on Au-Pd nanoparticles: From fundamentals to continuous production

The electrochemical synthesis of hydrogen peroxide (H2O2) represents a promising alternative to the anthraquinone process, as it combines on-site chemical and electrical production. The design of selective electrocatalysts is challenging and is commonly based on the alloying of elements to generate a synergistic effect and increase activity. In the present work, we report the electrochemical activity of Au-Pd nanoparticles immobilized directly onto an electrode as a model to study H2O2 electrochemical synthesis from fundamentals to continuous production. The impact of composition on the oxygen reduction reaction (ORR), the selectivity, as well as the peroxide reduction and oxidation reactions (PROR) are studied

On the Need of Improved Accelerated Degradation Protocols (ADPs): Examination of Platinum Dissolution and Carbon Corrosion in Half-Cell Tests

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 V$_{RHE}$. 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