High chemical activity and passivation of ultradispersed particles synthesized under the conditions of electric-arc low-temperature plasma

Abstract

Under conditions of electric-arc low-temperature plasma (LTP), various ultradispersed particles (UDP) have been synthesized. UDP are characterized by their specific surface (from several tens to several hundred m 2 g-l), particle size (10-1 O0 nm), phase composition, etc. The most efficient passivation agents preventing the high chemical activity of pyrophore, plasmachemically synthesized UDP have been experimentally established. These are: pure nitrogen for manganese, CO for molybdenum, N2(0.5% 02) for iron, N2(0.5%-2% 02) or C02( < 2% 0=) for the reduced catalyst for ammonia synthesi s, N2(1% 02) for the reduced catalyst for natural gas reforming, and N2(1% 02) for the reduced catalyst for low-temperature conversion of CO with H20. The high chemical activity of AIN and Mg3N= was decreased by slow annealing at about 900-1000 K in a nitrogen atmosphere or by thermal treatment in an inert gas flow. Passivation was enhanced by the ability of some metals to form surface complexes (due to chemosorption of some gases) of composition similar to that of chemical compounds, e.g. metal carbonyls (Fe, Mo, W, etc.). The sorption (chemisorption)of gases on the surface of UDP can only decrease the oxidation rate rather than prevent oxidation.

Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts

“…Deactivation studies carried out on ceria-supported precious metal catalysts indicate that CO and CO 2 can adsorb at catalyst surfaces during reactor shutdown forming stable surface carbonates [55]. Vissokov [29] claims that the rate of oxidation could be lowered, hence the passivation improved, by the ability of certain metals to form surface complexes (e.g., metal carbonyl), but oxidation could not be prevented completely under the conditions used. These findings are in line with the results shown in figure 3.…”