HIGH-TEMPERATURE SUPERCONDUCTORS IN CATALYSIS

The paper summarizes experimental results on the activity and selectiv ity of high-temperature (T-c) superconductors and some related cuprate s in different heterogeneous catalytic reactions (ortho-para hydrogen conversion, iso-propanol and methanol dehydrogenation, nitric oxide de composition and reduction with carbon monoxide, ammonia oxidation, sel ective oxidation of methanol and ethanol, partial oxidation of methane and of toluene, ammoxidation of toluene, oxidation of carbon monoxide ). It contains also information for the structural peculiarities of YB a2Cu3O7-x superconductors and data for oxygen reactivity from temperat ure-programmed reduction, oxygen isotopic exchange and oxygen desorpti on measurements. A distinct correlation between the superconductivity of YBa2Cu3O7-x and its catalytic activity in ortho-para hydrogen conve rsion is observed. Data in the literature show an influence of the oxy gen vacancies concentration in these solids and their activity and sel ectivity. However, in many cases no relationship seems to exist betwee n the superconducting properties and the catalytic activity and select ivity. This is explained by: (i) the experiments were performed at tem peratures much higher than T-c, and (ii) significant surface and bulk alterations in high-T-c superconductors during the catalytic reactions , as concluded from X-ray photoelectron spectroscopy and X-ray diffrac tion data (appearance of Cu+ and Cu-0 states in the surface layers, or thorhombic/ tetragonal phase transition, and, in some cases, decomposi tion and phase segregation). The outstanding importance of oxygen bind ing energy for complete oxidation reactions is already shown. From the practical point of view, the high activities and selectivities should be noted of high-T-c superconductors in selective oxidation of alipha tic alcohols to the corresponding aldehydes, in nitric oxide reduction by carbon monoxide and in ammoxidation of toluene to benzonitrile. Sp ecial attention should be paid to some related non-superconducting cup rates, which show high performances for complete oxidation reactions, with specific catalytic activities commensurable to those of the best catalysts known until now (e.g., Pt, Co3O4). Some possible future stud ies on the catalytic properties of high-T-c superconductors and relate d cuprates are outlined.