How NO affects nickel and cobalt nitrates at low temperatures to arrive at highly dispersed silica-supported nickel and cobalt catalysts

Abstract

Impregnation of porous silica supports with cobalt and nickel nitrate precursor solutions is a convenient method to prepare supported nickel and cobalt (oxide) catalysts. However, the metal (oxide) dispersion obtained is highly dependent on the gas atmosphere during thermal treatment to convert the metal nitrate to the metal oxide, where a 1% NO/He gas flow yields the highest and stagnant air the lowest dispersion. The origin of this large difference was previously investigated by studying the decomposition of a silica-supported nickel nitrate precursor dried at 120 °C, focusing on the ability of NO to act as an oxygen scavenger at high temperatures (200−350 °C). In this contribution we show, using in situ DRIFTS and XRD, that NO plays an equally if not more important role in the prevention of agglomeration of nickel and cobalt nitrate at low temperatures (25−150 °C). It was found that in a N2 and O2/N2 flow a mixture of metal hydroxynitrates and metal nitrate hydrates was formed between 100 and 150 °C, whereas in an NO/He flow nearly complete hydrolysis to hydroxynitrates occurred. The hydroxynitrates exhibited a reduced mobility upon decomposition, and therefore, it is proposed that their increased formation is the key to the high dispersions of NiO and Co3O4 obtained in NO.