Unpromoted Oxide Research Articles

Effect of potassium on the active phases of Fe catalysts for carbon dioxide conversion to liquid fuels through hydrogenation

The effect of potassium was tested with unpromoted and K-promoted Fe-Al-O oxide and Fe5C2 carbide materials, formed during CO2 hydrogenation from Fe-Al-O spinel. Each one of the tested catalysts contained a single phase (oxide or carbide). 2wt% potassium enhanced the RWGS rate of reaction on the oxide phase tenfold compared with the unpromoted oxide. This correlated with increase of Fe2+/Fe3+ ratio (XPS) determining oxygen vacancies as active sites for RWGS. Potassium suppressed the methanation rate on the carbide catalyst by a factor of five and increased the CO FTS rate to C2+ hydrocarbons by a factor of 1.4. EFTEM images and elemental profiles of unpromoted carbide nanocrystals measured after testing in CO hydrogenation displayed an amorphous surface layer enriched with oxygen. Potassium stabilized the surface iron atoms in reduced form. The observed effects of potassium on carbide phase were explained by a model implementing carbon vacancies for methanation and near-metallic iron atoms for FTS.

ChemInform Abstract: B‐Site Metal (Pd, Pt, Ag, Cu, Zn, Ni) Promoted La1‐xSrxCo1‐yFeyO3‐ δ Perovskite Oxides as Cathodes for IT‐SOFCs

AbstractReview: 77 refs.

B-Site Metal (Pd, Pt, Ag, Cu, Zn, Ni) Promoted La1−xSrxCo1−yFeyO3–δ Perovskite Oxides as Cathodes for IT-SOFCs

Perovskite oxides La1−xSrxCo1−yFeyO3–δ (LSCF) have been extensively investigated and developed as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) due to mixed ionic–electronic conductivity and high electrooxygen reduction activity for oxygen reduction. Recent literature investigations show that cathode performances can be improved by metal surface modification or B-site substitution on LSCF. Although the specific reaction mechanism needs to be further investigated, the promoting effect of metal species in enhancing oxygen surface exchange and oxygen bulk diffusion is well recognized. To our knowledge, no previous reviews dealing with the effect of metal promotion on the cathodic performances of LSCF materials have been reported. In the present review, recent progresses on metal (Pd, Pt, Ag, Cu, Zn, Ni) promotion of LSCF are discussed focusing on two main aspects, the different synthesis approaches used (infiltration, deposition, solid state reaction, one pot citrate method) and the effects of metal promotion on structural properties, oxygen vacancies content and cathodic performances. The novelty of the work lies in the fact that the metal promotion at the B-site is discussed in detail, pointing at the effects produced by two different approaches, the LSCF surface modification by the metal or the metal ion substitution at the B-site of the perovskite. Moreover, for the first time in a review article, the importance of the combined effects of oxygen dissociation rate and interfacial oxygen transfer rate between the metal phase and the cathode phase is addressed for metal-promoted LSCF and compared with the un-promoted oxides. Perspectives on new research directions are shortly given in the conclusion.

00/02380 The effect of boron on the catalyst reducibility and activity of Co/TiO 2 Fischer—Tropsch catalyst

The effect of potassium was tested with unpromoted and K-promoted Fe-Al-O oxide and Fe5C2 carbide materials, formed during CO2 hydrogenation from Fe-Al-O spinel. Each one of the tested catalysts contained a single phase (oxide or carbide). 2 wt% potassium enhanced the RWGS rate of reaction on the oxide phase tenfold compared with the unpromoted oxide. This correlated with increase of Fe2+/Fe3+ ratio (XPS) determining oxygen vacancies as active sites for RWGS. Potassium suppressed the methanation rate on the carbide catalyst by a factor of five and increased the CO FTS rate to C2+ hydrocarbons by a factor of 1.4. EFTEM images and elemental profiles of unpromoted carbide nanocrystals measured after testing in CO hydrogenation displayed an amorphous surface layer enriched with oxygen. Potassium stabilized the surface iron atoms in reduced form. The observed effects of potassium on carbide phase were explained by a model implementing carbon vacancies for methanation and near-metallic iron atoms for FTS.