In this work, it has been shown that structural changes of an as-prepared catalyst enable the assignment of the catalytic centers responsible for the direct and indirect oxygen reduction reaction, respectively, of porphyrinbased electrocatalysts. An iron porphyrin (FeTMPPCl)-based catalyst as well as a catalyst based on H 2 TMPP were prepared using the so-called foaming agent technique (FAT). The obtained iron catalyst was used as a generic material for the post-treatments. Structural changes were analyzed by 57 Fe Mo ¨ssbauer spectroscopy. The catalytic activity toward the oxygen reduction reaction (ORR) was determined using rotating (ring) disc electrode (R(R)DE) experiments. The catalysts exhibit a variation in the iron content between 2.9 and 4.5 wt % caused by the post-treatments. It has been found that the Mo ¨ssbauer spectra of all catalysts can be fitted assuming two different ferrous Fe-N 4 centers, a CFeN 2 center (Fe 2+ , S ) 2) and an Fe 3 C center (Fe 0 ). After the intensities found in the Mo ¨ssbauer spectra were normalized relative to the iron content, a linear correlation between the kinetic current density related to the direct oxygen reduction and the amount of in-plane Fe-N 4 centers is found. Beside this, there is evidence for a correlation between the kinetic current density related to the hydrogen peroxide formation and CFeN 2 centers. Heat-treated carbon-supported iron porphyrin, prepared as reference material, exhibits the same behavior as our FAT catalysts. The correlation enables us to obtain the turnover frequencies for both the direct and the indirect oxygen reduction reaction and to determine the site densities, in which we reach a third of the target value published by Gasteiger et al. (Appl. Catal., B 2005, 56, 9).

Abstract:Experience gained during efforts towards optimization of noble-metal-free electrocatalysts for oxygen reduction is simultaneously used to understand the chemical and morphological necessities for inducing efficient multi-electron transfer catalysis. The analysis of many preparative experimental steps between the moderately performing metal porphyrines and the highly efficient transition metal-and sulfur-containing pyrolised catalyst material contributes to the following model of the catalyst: The metals function enclosed in nitrogen or graphitic environment where they are shielded against irreversible oxidation. The metals can be exchanged but are not identical in their efficiency. Higher efficiency is achieved, when the function of a binary reaction center is warranted. The carbonization of the environment is critical and provides intercalated metal centers and attached metal complexes in graphite environment for interaction with the nitrogen-chelated partner center in the simultaneously obtained graphene layers. Three alternatives for the binary catalytic center are presented and their relevance discussed on the basis of EXAFS, RAMAN, EPR, Mössbauer and X-ray spectroscopy. A parallel is drawn with the cytochrome oxidase oxygen reduction catalysis, which is proposed to proceed according to roughly the same mechanism.

Porphyrin based catalysts were prepared by the pyrolysis of transition metal-macrocycles in the presence of iron oxalate as foaming-agent and by the addition of sulfur. Characterization was performed using N2 and H2O sorption measurements, XPS, Raman and 57Fe-Mößbauer spectroscopy, RRDE and PEM Fuel Cell experiments. ORR-activity of already pyrolysed material was found to be significantly enhanced by a secondary heat-treatment in either N2, CO2 or NH3. Additionally, the specific surface area was found to increase and in case of NH3, catalytic centers were also generated. The final catalysts exhibit 2-5 wt% metal, 3-6 wt % nitrogen and 1-5 wt% sulfur integrated in an in-situ formed graphene network. However, despite the high efficiency in RDE measurements, in FC tests only 2 % of electrical power compared to a 45 wt% Pt/C catalyst is reached. An unsuitable morphology of the GDE and a high hydrophilicity of the catalyst were identified as limiting factors.

The following work presents the preparation and investigation of pyrolysed Co-porphyrine as an electrocatalyst for the oxygen reduction reaction (ORR) in acid media. A new preparation method which needs no addition of carbon supports, allows the structural investigation of the pyrolysis products by XPS, Raman spectroscopy and X-ray diffractometry without any interferences. Rotating Disc Electrode measurements reveal the high ORR activity which is mainly caused by a well developed porosity and a suitable molecular structure of the formed carbon. Thermogravimetric investigation of the pyrolysis process shows that the addition of sulphur influences the carbonization of the porphyrine in a favorable way. It has been found that extended graphene layers present a particularly suitable matrix for highly active catalytic centers.

Nanocrystalline titanium oxinitride (TiO2−2xNx) thin films (0⩽x⩽1) were prepared by reactive dc magnetron sputtering from a titanium target in an argon-oxygen-nitrogen atmosphere. By increasing the reactive gas component nitrogen the phases changed from TiO2 to TiO2−2xNx and finally to TiN. The reactively sputtered films were characterized by elastic recoil detection analysis, Raman spectroscopy, and resistivity measurements. The crystallographic structures of TiO2 were maintained up to a nitrogen concentration of about 20at.%; however, the crystallite size decreased significantly and a transition from the low-temperature anatase to the high-temperature rutile phase of TiO2 occurred. Optical transmission measurements revealed that the indirect energy band gap can be reduced from 3.2to2.6eV for nitrogen concentrations up to 20at.%. The film properties for photocatalytic water oxidation were investigated by differential electrochemical mass spectroscopy. While the photoactivity in the visible increases, the overall photoelectrochemical activity appears to be deteriorated significantly by nitrogen doping, most probably due to the formation of defect states near to the valence band of TiO2.