Kevin Kähler

Stable Performance of Ni-Catalysts in Dry Reforming of Methane at High Temperatures for an Efficient CO2-Conversion into Syngas

The catalytic performance of a Ni/MgAlOx catalyst was investigated in the high temperature CO2 reforming of CH4. The catalyst was developed using a Ni, Mg, Al hydrotalcite‐like precursor obtained by co‐precipitation. Despite the high Ni loading of 55 wt%, the synthesized Ni/MgAlOx catalyst possessed a thermally stable microstructure up to 900 °C with Ni nanoparticles of 9 nm. This stability is attributed to the embedding nature of the oxide matrix, and allows increasing the reaction temperature without losing active Ni surface area. To evaluate the effect of the reaction temperature on the reforming performance and the coking behavior, two different reaction temperatures (800 and 900 °C) were investigated. At both temperatures the prepared catalyst showed high rates of CH4 consumption. The higher temperature promotes the stability of the catalyst performance due to mitigation of the carbon formation.

Probing the Reactivity of ZnO and Au/ZnO Nanoparticles by Methanol Adsorption: A TPD and DRIFTS Study†

The adsorption of methanol on pure ZnO and Au-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C-O stretching bands. During the subsequent TPD experiments two different H(2) peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJ mol(-1) and 127 kJ mol(-1) for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO(10 10) surface and on the highly reactive polar ZnO(000 1) surface. The simultaneous formation of H(2), CO, and CO(2) at about 550-600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO(2) ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies.

High-Pressure CO Adsorption on Cu-Based Catalysts: Zn-Induced Formation of Strongly Bound CO Monitored by ATR-IR Spectroscopy

CO adsorption at 1 MPa on Cu-Zn stearate colloids and supported Cu catalysts was studied in situ by attenuated total reflection infrared (ATR-IR) spectroscopy. Subsequent to thorough reduction by H(2), the IR band at 2110-2070 cm(-1) due to linearly adsorbed CO on clean metallic Cu was always observed initially on all Cu catalysts. During the exposure of Zn-containing samples to CO at high pressure, a new IR band at ca. 1975 cm(-1) appeared in addition and increased in intensity even at room temperature. The detailed analysis of the IR spectra showed that the new IR band at ca. 1975 cm(-1) was not related to coadsorbed carbonate/formate-like species, but to the content of Zn in the samples. This IR band was found to be more stable than that at 2110-2070 cm(-1) during purging with inert gas. It disappeared quickly in synthetic air, pointing to a strongly reduced state of the Zn-containing Cu catalysts achieved during high-pressure CO exposure. It is suggested that CO can reduce ZnO to Zn in the presence of Cu, resulting in the formation of a CuZn(x) surface alloy. As the CO species with the characteristic IR band at ca. 1975 cm(-1) binds more strongly to this CuZn(x) alloy than the linearly adsorbed CO to pure Cu, it is suggested to be adsorbed on a bridge site.

Methanol Synthesis from Industrial CO2 Sources: A Contribution to Chemical Energy Conversion

CO2 hydrogenation as a route for the chemical energy storage over a commercial Cu/ZnO/Al2O3 catalyst has been studied. To check the optimal conditions for an efficient methanol production the influence of temperature and space velocity on the catalytic performance has been demonstrated. Time-on-stream measurements in the absence and the presence of benzene in the gas feed mixture were performed to investigate the possibility to use alternative carbon sources, which contain traces of aromatics. The catalyst can operate in a stable way without the presence of carbon monoxide in the feed, which means that increased water contents in the product gas cannot destroy the catalyst’s performance completely. The presence of benzene in the feed does not lead to a deactivation of the catalyst. With these findings methanol production starting from exhaust gases from steel mills seems to become an interesting alternative for sustainable methanol production.Graphical Abstract

Ammonia Decomposition and Synthesis over Multinary Magnesioferrites: Promotional Effect of Ga on Fe Catalysts for the Decomposition Reaction

Magnesioferrite (MgFe2O4)‐derived Mesoporous spinels of the type MgFeM3+O4 with M3+=Fe, Al, and Ga obtained upon calcination of hydrotalcite‐like compounds were investigated in the ammonia decomposition reaction at 1 bar and the synthesis of ammonia at 90 bar. The corresponding precursors were synthesized by co‐precipitation at 50 °C and constant pH of 10.5. N2 physisorption, PXRD, HR‐TEM, H2‐TPR, and NH3‐TPD were applied in order to obtain information about the textural, (micro‐)structural, solid‐state kinetics in reducing atmosphere, and adsorption properties of the samples. While phase‐pure layered double hydroxides (LDHs) were obtained for Al and Ga, magnesioferrite as the desired oxide phase and a low fraction of magnetite were formed besides the targeted precursor phase during co‐precipitation in the presence of Fe2+ and Fe3+ species. Reduction of the binary and ternary magnesioferrites occurs via two consecutive reactions. Only the second stage is shifted towards higher temperatures after incorporation of Al and Ga. The latter element boosts the catalytic decomposition of ammonia, yielding a 2‐fold and 5‐fold higher conversion at 500 °C compared to the samples containing Fe3+ and Al3+ species, respectively. In situ XRD measurements showed that this unprecedented promotional effect is related to the generation of (Fe, Ga)Fe3N. This phase, however, is detrimental for the synthesis of ammonia at elevated pressures in which the binary system outperforms the ternary spinels, yielding 30 % of the activity obtained with a highly promoted Fe‐based industrial catalyst.