No abstract

A multistep synthesis procedure for the homogeneous coating of a complex porous conductive oxide with small Ir nanoparticles is introduced to obtain a highly active electrocatalyst for water oxidation. At first, inverse opal macroporous Sb doped SnO 2 (ATO) microparticles with defined pore size, composition, and open-porous morphology are synthesized that reach a conductivity of ≈3.6 S cm −1 and are further used as catalyst support. ATO-supported iridium catalysts with a controlled amount of active material are prepared by solvothermal reduction of an IrO x colloid in the presence of the porous ATO particles, whereby homogeneous coating of the complete outer and inner surface of the particles with nanodispersed metallic Ir is achieved. Thermal oxidation leads to the formation of ATO-supported IrO 2 nanoparticles with a void volume fraction of ≈89% calculated for catalyst thin films based on scanning transmission electron microscope tomography data and microparticle size distribution. A remarkably low Ir bulk density of ≈0.08 g cm −3 for this supported oxide catalyst architecture with 25 wt% Ir is determined. This highly efficient oxygen evolution reaction catalyst reaches a current density of 63 A g Ir −1 at an overpotential of 300 mV versus reversible hydrogen electrode, significantly exceeding a commercial TiO 2 -supported IrO 2 reference catalyst under the same measurement conditions.

The dissociation of CO, a crucial step in the cobalt-catalyzed Fischer–Tropsch synthesis, was investigated by in situ scanning tunneling microscopy on Co(0001) at CO pressures up to 0.25 mbar and temperatures up to 493 K. A new type of surface reconstruction was observed that is formed by carbidic carbon, as shown by X-ray photoelectron spectroscopy. Using 13CO and titrating 13C with O2, the surface carbon is shown to originate from CO. The results are in contrast to predictions by theoretical work. However, the dissociation rates are 3 orders of magnitude lower than the previously measured methanation rates under equivalent conditions.

The cobalt catalyst used in the Fischer–Tropsch synthesis, under the conditions of the reaction, is covered by a dense adsorption layer in which CO is the main constituent. To obtain insight into the structure of this layer and possible surface phases, CO structures on a Co(0001) model have been investigated by high-pressure scanning tunneling microscopy (STM). The experiments were performed in situ, at CO pressures between 10–9 and 800 mbar and at a temperature of 300 K. Under these conditions, an adsorption–desorption equilibrium was established, reflecting the situation of the reaction. A series of CO surface phases were observed; from 10–9 to 10–7 mbar, a (√3 × √3)R30° structure; between 10–6 and 10–3 mbar, a disordered phase representing a nonstoichiometric, fluctuating (√7 × √7)R19.1° structure; and between 10–2 and 100 mbar, a (2√3 × 2√3)R30° structure. Between 100 and 800 mbar, three moiré structures were observed that were analyzed by a recently developed method. All phases were formed by intact CO molecules. A phase diagram was obtained that allowed us to extrapolate the stability regions of the CO phases to industrial Fischer–Tropsch conditions. We conclude that the reaction operates in the region of the disordered phase at a coverage between 0.43 and 0.50 monolayers of CO.