H2O2 Hydrogen Research Articles

Theoretical Study of the Decomposition and Hydrogenation of H2O2 on Pd and Au@Pd Surfaces: Understanding toward High Selectivity of H2O2 Synthesis

Three possible pathways for the conversion of hydrogen peroxide to water on Pd and Au@Pd catalysts are investigated with periodic density functional theory calculations: (1) the decomposition of H2O2 (H2O2 → H2O + O), including the dissociation of H2O2 to two OH groups (H2O2 → 2OH) and the disproportionation of two OH groups to water and oxygen (OH + OH → H2O + O); (2) the hydrogenation of the OH group to water (OH + H → H2O); and (3) the direct hydrogenation of H2O2 to water (H2O2 + 2H → 2H2O). The results show that the decomposition of H2O2 and the hydrogenation of OH groups are two available channels for the formation of water, and the former plays a main role. A key step in the overall process is the dissociation of H2O2, which is facile and irreversible. The direct hydrogenation of H2O2 to water has a very high activation barrier and is unlikely to occur. The competitions between the dissociation of H2O2 and the release of H2O2 on Pd and Au@Pd surfaces are analyzed. The high selectivity of H2O2 synthesis cannot be explained simply by the relatively increased barrier for H2O2 dissociation on the Au@Pd surface. Actually, the less active Au atoms on the Au@Pd surface weaken the interaction of the metal surface with H2O2, and thus suppress the dissociation of H2O2, and, on the other hand, facilitate the release of H2O2. The opposite effects of Au atoms on the dissociation and release of H2O2 move the balance to the release side, which is responsible for the high H2O2 selectivity of the Au@Pd catalysts. The effects of the unreacted H atoms are also considered. It is found that the H atoms coadsorbed on Pd and Au@Pd surfaces can decrease the interaction between the metal surfaces and H2O2 as well and, consequently, facilitate the release of H2O2 and suppress the dissociation of H2O2.

The Effect of Bromide Pretreatment on the Performance of Supported Au–Pd Catalysts for the Direct Synthesis of Hydrogen Peroxide

AbstractThe effect of pretreating Au–Pd catalysts on MgO and C supports with aqueous bromide solution, prior to using them for the direct synthesis of hydrogen peroxide, has been investigated. These two supports were selected since the parent materials exhibit contrasting microstructures and activities. The carbon‐supported catalysts comprise homogeneous Au–Pd alloy nanoparticles, which give high activity, whereas the MgO‐supported catalyst has Au–Pd alloys with a Pd‐rich surface and a Au‐rich core, which result in lower activity. Pretreatment of these catalysts with bromide was found to enhance H2O2 productivity and the degree of enhancement was largely dependent on the nature of the Au–Pd nanoparticles. Whereas bromide pretreatment significantly enhanced H2O2 productivity over the MgO‐based catalysts, the carbon‐based catalyst only showed a subtle promotional effect. Very low loadings of bromide (0.00034–0.044 wt %) were required to yield a significant positive effect. Higher bromide loadings (0.5–8.3 wt %) proved deleterious. The promotional effect has been correlated to selective poisoning of sites responsible for H2O2 hydrogenation and decomposition. In view of the limited effect of bromide pretreatment on the yield of H2O2 coupled with the effective performance of the carbon supported Au‐Pd catalysts in the absence of halides, for practical processes the addition of halides is not considered advisable with this catalyst system.

Dealloyed Core-Shell Fuel Cell Electrocatalysts

We review our recent work on dealloyed nanoparticle electrocatalysts and address their synthesis, structural characterization and surface catalytic performance in low-temperature Polymer Electrolyte Membrane fuel cells (PEMFCs). The active form of the catalyst is obtained by voltammetric dealloying of non-noble metal rich Pt alloy precursors. In the dealloying process, the less noble precursor component, here Cu, is selectively removed from the surface of the precursor alloy particles and hence a Pt enriched particle shell is formed. Single fuel cell tests showed that, when used on the cathode of PEMFCs, dealloyed Pt catalysts show reactivities for the oxygen reduction reaction (ORR) which are up to 6 times higher than those of conventional pure Pt fuel cell catalysts. Similarly, the stability of dealloyed nanoparticle catalysts is superior to that of pure Pt particles. X-ray based structural and compositional studies suggested a core—shell particle structure as the active form of the catalyst consisting of a Pt enriched particle shell surrounding a Pt alloy core. At the present time, this catalyst system constitutes one of the most active fuel cell catalyst system reported in the literature. I. THE CONCEPT OF DEALLOYED NANOPARTICLE CATALYSTS Despite much recent focus on the development of advanced Li ion batteries for use as power source for short-range inner city transportation 255 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Vol. 25. No. 4. 2009 Dealloyed Core-Shell Fuel Cell Eleclrocatalysts applications, low temperature fuel cells continue to be the solution of choice for medium and long range transportation technologies based on their gravimetric power density as well as the gravimetric energy density of commonly used fuels. Wider use of low-temperature fuel cell technology is hampered by performance, cost, and durability issues associated with materials and components of a single fuel cell. Figure 1 displays a cross section of the layered structure of a low temperature PEMFC showing the anode (left) and cathode (right) gas diffusion layers (GDLs), which sandwich the anode and cathode catalyst layers and the proton exchange membrane. Figure 1 also schematically shows the molecular as well as electrical pathways of hydrogen fuel molecules, oxygen molecules, protons as well as of electrons. The overall performance of a PEMFC in terms of its practical cell voltage is limited by kinetic, ohmic, and mass transport processes for low, medium and high current densities, respectively. Of these, the kinetic surface catalytic reactions cause the most severe fuel cell voltage losses. * Anode fl( / Cathode ·>' '..·· Electrical | O2+*H* + 4e -> 2 H2O Energy Fig. 1: Cross sectional SEM micrograph through a membrane electrode assembly sandwiched between gas diffusion layers. Reaction processes at anode and cathode, mass and charge flows are indicated (from ref) In particular, the electrocatalytic Oxygen Reduction Reaction (ORR) at the cathode according to O2 + 4H + +4e->2H 2 O E°=+1.23 V/RHE (1) represents the key challenge to improved PEMFCs. In acidic media, Pt catalysts offer the highest catalytic activities which made first unsupported, later high surface area carbon-supported Pt particles the ORR catalyst of 256 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Peter Strasser Reviews in Chemical Engineering choice· . Figure 2 illustrates the associative (via O2H) as well as the dissociative (via O) pathways of reaction (1) from molecular oxygen to water. Key for the catalysis is the chemisorption energy of the adsorbed oxygenated intermediates, such as Pt-O2H, Pt-O, and Pt-OH. On pure Pt, atomic oxygen is bonded too strongly and requires considerable overpotentials to react to Pt-OH. As a result of this, Pt is covered by (hydr)oxide adsorbates near the equilibrium potential of reaction (1). There is a consensus within the fuel cell catalysis community that a moderate reduction of the Pt-O chemisorption energy would result in significant ORR activity increases. H2O2 Hydrogen peroxide 0=0 Oxygen adsorption Oxygenated OH intemediates

The role of the support in achieving high selectivity in the direct formation of hydrogen peroxide

Pd-only, Au-only and bimetallic AuPd catalysts supported on a range of supports (Al2O3, TiO2, MgO, and C) have been prepared by impregnation and tested for the hydrogenation and decomposition of hydrogen peroxide under conditions similar to those used in direct synthesis of hydrogen peroxide. Hydrogenation and decomposition are the main pathways for loss of selectivity and yield in the direct synthesis reaction, and the support is found to be a crucial parameter with respect to hydrogenation and decomposition activity. We show that by making the right choice of support for both the monometallic and bimetallic Au and Pd catalysts, it is possible to achieve very low hydrogen peroxide hydrogenation and decomposition activity, thus enhancing hydrogen peroxide productivity during synthesis. Carbon is found to be the optimal support for both monometallic Au and Pd catalysts as well as Au–Pd alloys, since carbon-supported catalysts gave the lowest hydrogenation and decomposition activities. Au-only catalysts were generally less active than Pd-only catalysts when utilizing the same support and metal loading. The addition of Au to Pd catalysts supported on TiO2 and carbon resulted in a decrease in both H2O2 hydrogenation and decomposition while the reverse effect was observed for the Al2O3 and MgO-supported catalysts. These effects are discussed in terms of the basicity of the support, and in particular the isoelectronic point of the support, which is a major factor in controlling the stability of hydrogen peroxide under reaction conditions.

Decomposition and/or hydrogenation of hydrogen peroxide over Pd/Al2O3 catalyst in aqueous medium: Factors affecting the rate of H2O2 destruction in presence of hydrogen

Hydrogen peroxide destruction by its decomposition and/or hydrogenation to water in the presence of H2 over Pd(5 wt.%)/Al2O3 catalyst in an aqueous reaction medium, similar to that used in the H2-to-H2O2 oxidation, has been thoroughly investigated at different reaction conditions. The H2O2 destruction is strongly influenced by the oxidation state of Pd in the catalyst and also by the presence of different halide anions (viz. F−, Cl−, Br− and I−) in the acidic aqueous medium or in the catalyst, depending upon the concentration of halide anions. The cations associated with halide anions have, however, a little or no influence on the H2O2 destruction. The iodide anions strongly poisoned the catalyst, even at their very low concentration. The fluoride anions have only a small effect on the H2O2 destruction. The chloride or bromide anions drastically inhibit the rapid H2O2 decomposition, but promote the slower H2O2 hydrogenation. The H2O2 destruction reactions are strongly influenced by the halide anion and acid concentrations. The presence of acid (protons) plays a very important role in drastically reducing the H2O2 destruction, particularly in the presence of chloride anions. Both in the presence and absence of Cl− or Br− anions, the H2O2 destruction activity of the catalyst is markedly reduced because of the oxidation of Pd but it is drastically increased due to the presence of H2. A plausible reaction mechanism for the rapid H2O2 decomposition and slower H2O2 hydrogenation reactions, prevailing under the different reaction conditions, has also been discussed.

Formation from direct oxidation of H2 and destruction by decomposition/hydrogenation of H2O2 over Pd/C catalyst in aqueous medium containing different acids and halide anions

Abstract Direct oxidation of H2 by O2 to H2O2 and decomposition/hydrogenation of H2O2 (at 27 °C and atmospheric pressure) over Pd/C catalyst in an aqueous acidic reaction medium have been thoroughly investigated using different mineral acids (viz. 0.1N H2SO4, H3PO4, H3BO3, HNO3, HCl, HBr or HI) and/or halide anions (viz. F−, Cl−, Br− or I−), covering their wide concentration range (0–24 mmol/dm3), in the reaction medium. In the absence of any externally added halide, appreciable H2O2 formation in the H2 oxidation occurred only when the medium contained HCl. However, the rates of H2O2 decomposition and hydrogenation are greatly reduced because of the presence of any acid; the influence on the H2O2 decomposition was, however, much larger for the halo acids. In the presence of acid (0.1N H3PO4 or H2SO4), the H2-to-H2O2 formation reaction is greatly enhanced by the externally added halide anions up to their optimum concentrations. However, above the optimum halide anion concentration, it is inhibited more than the H2-to-water formation (parallel) reaction, causing a decrease in both the H2O2 yield and selectivity. Whereas, the H2 conversion and H2O2 destruction activities of the catalyst are decreased continuously with increasing the halide anion (except F−) concentration. Among the halides, chloride is the best halide promoter for Pd/C catalyst in the H2-to-H2O2 oxidation. At the optimum Cl− concentration (5.4 mmol/dm3), both the H2 conversion and H2O2 yield are passed through a maximum and the H2O2 decomposition is greatly inhibited with increasing the phosphoric acid concentration. In the absence of either the chloride anions or the acid (or both) in the reaction medium, only a little or no H2O2 is formed in the H2 oxidation and also the rate of H2O2 destruction is very fast, particularly in the presence of H2; the rapid destruction of H2O2 is mainly due to its decomposition rather than its hydrogenation. The best H2-to-H2O2 oxidation results are obtained at the optimum concentrations of both the acid and halide anions. In the presence of acid and chloride (or bromide) promoter, the H2O2 hydrogenation dominates the H2O2 destruction and hence the net H2O2 formation is mainly controlled by the H2O2 hydrogenation.

Hydrogenation of Hydrogen Peroxide over Palladium/Carbon in Aqueous Acidic Medium Containing Different Halide Anions under Static/Flowing Hydrogen

Kinetics of the hydrogenation of hydrogen peroxide (at 278 and/or 300 K) over Pd(5 wt%)/carbon in aqueous medium with or without containing acid or different halide anions (F-, Cl-, Br-, or I-) under static H2 (i.e. batchwise) or flowing H2 (i.e. semi-batchwise) at atmospheric pressure have been thoroughly studied. In the absence of protons or halide anions and also in the presence of F- anions, a very rapid decomposition of H2O2 (H2O2 → H2O + 0.5O2) predominates over the H2O2 hydrogenation (H2O2 + H2 → 2H2O). However, in the presence of both the protons and Cl- or Br- anions, the H2O2 conversion occurs mainly by the hydrogenation reaction. The hydrogenation is found to be a zero-order reaction with respect to the H2O2 concentration, and its activation energy and frequency factor are strongly influenced by the halide anions. The hydrogenation is found to be more and more inhibited with increasing halide (Cl- or Br-) anion concentration. The cations associated with the halide anions have, however, only a very little influence on the hydrogenation. Iodide anions act as a very strong catalyst poison for the hydrogenation. In the presence of acid, the hydrogenation is very strongly influenced by the different halides, but, in the presence of halide anions (5.4 mmol/dm3), it is very mildly influenced by the different mineral acids (except HI). In the absence of protons, the Cl- and Br- anions are ineffective for inhibiting the rapid H2O2 decomposition. A role played by protons and halide anions for drastically inhibiting the rapid H2O2 decomposition is discussed.

Reaction of stannous fluoride in hydrogen peroxide

The reaction of SnF2 stannous fluoride with aqueous solutions of H2O2 hydrogen peroxide was studied as a function of the molar ratio H2O2/SnF2 in the range 0.02 to 5.00. The products were characterized by thermal analysis, X-ray diffraction and tin119 Mossbauer spectroscopy. The X-ray diffraction pattern of all samples shows only highly broadened lines, characteristic of microcrystalline SnO2 (average particle diameter: 39 A). Thermal analyses show that the material is hydrated. Mossbauer spectroscopy gives a broad single line at approximately 0 mm/s, characteristic of SnO2 for all samples, and in some cases a tin(II) doublet with δ=3.1 mm/s and Δ=1.9 mm/s.