and the reduction of 4NP by NaBH4 was negligible in the absence
of catalyst, the reaction could be considered pseudo-first-order
with respect to the concentration of 4NP. GO/Ag and silver
nanoparticles are chosen as catalysts to make a comparison about
the catalytic activity. Time-dependent adsorption spectra of this
reaction mixture show the disappearance of the peak at 400 nm
that accompanied by a gradual development of a new peak at
300 nm corresponding to the formation of 4AP (Fig. 5(a) and (b)).
It should be noted that the quantities of catalyst involved are
maintained the same. It is seen that the reduction of 4NP to 4AP
will be finished within three minutes with catalysts (Fig.5 (a) and
(b)) while the GO/Ag displays a little faster. As the absorbance of
4NP is proportional to its concentration in the medium, the ratio
of absorbance at time t(At) to that at t¼0 (A0), i.e., At/A0, could be
used as the ratio of concentration of 4NP at time tto that att0, i.e.,
At/A0¼Ct/C0. As shown in Fig. 6, ln(Ct/C0) versus time was
obtained. Upon the addition of nanocatalysts, a certain period of
time (defined as tads) was required for the 4NP to adsorb onto the
catalyst’s surfaces before reduction could be initiated. As shown
in Fig. 6, the tad is about the two catalysts while it is almost
undetectable indicating very fast adsorption process. The kinetic
reaction rate constant (defined as kap) is also estimated from the
linear relationship, which is 14.5103 and 7.8103 s1 for
GO/Ag and silver catalysts, respectively. It is seen that the
reaction catalyzed by GO/Ag catalyst shows the much shorter
adsorption time and faster reaction rate [19]