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

AuPt core–shell nanocatalysts with bulk Pt activity

In the presented work, the evaluation of an unsupported AuPt core–shell catalyst for the oxygen reduction reaction is introduced. Applying only basic chemicals in an upscalable synthesis route, it is demonstrated that uniform, flat, and complete Pt layers around a spherical Au core are obtained. The electrocatalytic measurements show that the surface area specific activity of the AuPt core–shell catalyst towards the important oxygen reduction reaction equals the one of polycrystalline bulk Pt. To our knowledge, this is the first time that the unfavorable particle size effect of Pt nanoparticles could be by-passed for a nanoscale catalyst. Keywords: Core–shell catalyst, Oxygen reduction reaction, Fuel cel

Coupling of a high throughput microelectrochemical cell with online multielemental trace analysis by ICP-MS

In this work, the successful coupling of a specially designed microelectrochemical cell and direct online multi-elemental trace analysis by ICP-MS is presented. The feasibility of this method is demonstrated by the example of copper dissolution in HCl (1 and 10 mM), showing very high sensitivity and an excellent congruency between electrochemical experiments and copper concentrations detected downstream. The complementary data allows for a precise determination of the valence of Cu ions released during anodic dissolution, which undergoes changes depending on the electrolyte and the applied current density. Moreover it provides a means of quantification of processes without net external currents that are not readily accessible by plain electrochemical techniques, in particular the exchange current densities at the open circuit potential, i.e. corrosion rate, and the dissolution of native oxides. The system presented combines full spectrum electrochemical capabilities, convection control, and highly sensitive electrolyte analysis in an integrated, miniaturized arrangement under full computer control and automation. Keywords: Online electrolyte analysis, Copper dissolution, Inductively coupled plasma-mass spectrometry, Micro electrochemistr

Stability of nanostructured iridium oxide electrocatalysts during oxygen evolution reaction in acidic environment

The electrochemical stability of thermally prepared Ir oxide films is investigated using a scanning flow cell (SFC)–inductively coupled plasma mass-spectrometer (ICP-MS) setup under transient and stationary potential and/or current conditions. Time-resolved dissolution rates provide important insights into critical conditions for material breakdown and a fully quantitative in-situ assessment of the electrochemical stability during oxygen evolution reaction (OER) conditions. In particular, the results demonstrate that stability and OER activity of the IrOx catalysts strongly depend on the chemical and structural nature of Ir oxide species and their synthesis conditions. Keywords: Oxygen evolution, Energy conversion, Iridium oxide, IrO2, Dissolution, Corrosio

Alkaline manganese electrochemistry studied by in situ and operando spectropscopic methods - metal dissolution, oxide formation and oxygen evolution

Manganese-based systems are considered as candidate electrocatalysts for the electrochemical oxygen evolution reaction (OER), because of their abundance in biochemical oxygen producing catalyst systems. In this work, the surface of metallic manganese was investigated in situ and operando in potentiodynamic cyclic voltammetry (CV) experiments and potentiostatic chronoamperometry (CA) experiments in NaOH. In both cases, the surfaces were initially reduced. At corresponding potentials, no oxide species can be detected by Raman spectroscopy, though electrochemical data and the absence of dissolution above the reversible potential for reactions of type Mn → MnII indicate that the material is passive. The CV shows anodic peaks at potentials in line with expectations on the basis of thermodynamic data for the oxidation to Mn3O4 and Mn2O3; the thickness of the surface layer increases by a few nm during these peaks, as evidenced by spectroscopic ellipsometry. Dissolution of Mn as evidenced by downstream electrolyte analysis by inductively coupled plasma mass spectrometry in a scanning flow cell (SFC-ICP-MS) of the electrolyte is negligible in the range of electrode potential vs. Ag|AgCl|3 M KCl, EAg|AgCl, up to 0.3 V. Remarkably, Raman spectra already show the occurrence of α-MnO2 at EAg|AgCl > −0.25 V, which is ca. 0.5 V below the potential at which oxidation to MnO2 is expected. This observation is attributed to disproportionation above a certain level of MnIII. For EAg|AgCl > 0.4 V, dissolution sets in, at a constant layer thickness. Above the onset potential of the OER, at EAg|AgCl ≈ 0.6 V, SFC-ICP-MS analysis shows fast dissolution, and the oxide layer thickness is constant or increases. CA experiments during the OER show strong dissolution, and the re-formation of a strongly disordered, β-MnO2-like oxide, which exists in a quasi-stationary state at the interface. Several CV cycles increase the dissolution per cycle and the fraction of α-MnO2 on the surface which cannot be reduced. The high dissolution currents show that metallic Mn is hardly suitable as an OER catalyst, however, at least the MnIV oxides remain stationarily present in the system

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