The
existence of different carbonaceous deposits on the catalysts
is well illustrated by the TPR spectra in Fig. 4. Nearly
the same features were found in all other cases. On Rh
containing samples a small portion of surface carbon was
hydrogenated to methane at 400–550 K and this was followed
by a high temperature peak above 600 K. Similar
TPR curves were found when the surface carbon was
produced in the decomposition of methane at 523 K and
only the ratio of the two peaks were different [43]. The low
and the high temperature TPR peaks were found in the
same temperature range irrespective of the metal or the
support. It means that the reactivity of these species
depends mainly on the composition and the structure of the
carbonaceous deposit.
The amount of surface carbonaceous deposit formed
during the reaction was determined from the TPR peak area
(Table 1). Comparing the data obtained on different samples
we have found that the amount of surface carbon
depends not only on the metal but on the support, too. The
effect of the metal on the amount of surface carbon could
be explained by the different decomposition rate of surface
species, such as formate ion or adsorbed CO, in the presence
of different metals. The amount of deposited carbon
decreased in the order of NT[NW[TiO2, with the
exception of supported Au samples (Table 1). In these