A permanent flow of nitrogen of 50 ml STP/min (all
flowrates are measured at STP) was used as protective gas for the
balance. In addition to this flow, the reacting gas (50 ml/min)
was fed to the TGA reactor during the operation. Prior to reacting
with the solid sample, the reactive gas was mixed with the
protective gas. The initial sample mass used in the TGA
experiments was always 20–21 mg.
During the reduction period, 10% of carbon monoxide in nitrogen
was introduced in the TGA for 30 min. This time was selected
as a compromise to perform as many tests as possible while the
duration was sufficient to study the kinetics of the reaction. After
this, nitrogen was fed for 15 min to avoid the mixing of the
reducing and the oxidising gases. The oxidation took place in two
different stages. First, the particles were oxidized by 10% carbon
dioxide in nitrogen for 20 min. Following a 15 min nitrogen purge,
samples were fully oxidized with air for 10 min in order to return
to the initial state.