E. Kertalli et al. / Applied Catalysis A: General 524 (2016) 200–205
203
However, we obtain a slight increase of the conversion value. This
trend, up to now, is still an open question that cannot be addressed
with a meaningful explanation. Further investigation on the effect
of the systematic increase of O2 concentrations on the Pd–Pt/TS-1
catalyst is required.
Small amounts of Pt influence also the intermediate H O2 (or
2
OOH species) synthesis by increasing the rate of formation [34].
The excess of oxygen influences the catalyst behaviour. The pres-
ence of a higher oxygen concentration affects the oxidation state of
the metal catalyst [35–38]. This is shown to happen for the direct
synthesis of H O , where operating the system at higher oxygen
2
2
content resulted in a loss of the catalytic activity [38]. The activity
could be fully recovered if the catalyst was flushed with hydrogen,
underlining the change of the catalyst state from reduced to oxi-
dised (higher oxygen content) and from oxidized to reduced (higher
hydrogen content) [38]. However, another possible explanation for
the reduced activity of the catalyst in the presence of oxygen can
be the coverage effect of oxygen on the active catalytic sites. In lit-
erature, it is well established that oxygen has a saturating effect on
metallic nano-particles particles (Pd, Pt) [39–42]. This may result
on hiding the catalytic sites responsible for the PO synthesis.
Thus, the combination of a higher oxygen concentration and
metallic species (Pd–Pt) leads to a catalyst which shows a lower
activity towards propane formation. Additionally, the increased
oxygen concentration reduces the further reactivity of H O during
Fig. 4. Effect of different oxygen conditions on H2 conversion and H O2 pro-
2
ductivity (wt%) for 1 wt% Pd/TS-1 and 1 wt% Pd
T = 40 C. P = 5.5 bar. Methanol = 1.5 ml/h. Gasflow = 2 ml/min. Catalyst = 50 mg. Parti-
cle size = 250–400 m. 10% O2 = 10/10/80: H2/O2/N2. 50% O2 = 10/50/40: H2/O2/N2.
- 0.1 wt%Pt/TS-1 catalysts.
◦
of the catalyst oxidation state in the presence of oxygen. In this
case, the catalyst becomes less active towards the decomposition
reaction. However, for the Pd–Pt/TS-1 catalyst, the small difference
between the H2O2 produced with and without the excess of oxygen
makes it difficult to draw any reliable conclusion.
By direct comparison of PO and H2O2 productivity (Figure 5), we
observe that the amount of PO is always higher than the amount of
H2O2 produced under the same conditions.
This trend qualitatively agrees with the data reported by
Hölderich [16]. However, his experiments for H2O2 synthesis were
conducted at different temperatures compared to the PO synthesis
making the comparison more difficult. Moreover, since no stabili-
zers were added to the reaction of H2O2, his work suggested that the
H2O2 decomposes over the metal catalyst before being detected. In
our experiments, although H2SO4 and NaBr are added to the experi-
ments to reduce the hydrogenation and the decomposition of H2O2,
the PO productivity is always higher than the H2O2 productivity.
This suggests an important role of the propylene in the direct PO
synthesis. In fact, the oxidation of propylene with in-situ produced
H2O2 to PO is a much faster reaction compared to the decompo-
sition and the hydrogenation of the H2O2 (Fig. 5). The continuous
2
2
its direct synthesis [43,44] by lowering the decomposition and the
hydrogenation to water. A similar effect of the catalyst on the H O
2
2
site reactions is also observed for the hydrogenation of propylene
in the direct PO synthesis. The water formation was calculated for
all the experiments conducted in the present work. The amount of
water produced was constant indicating that the presence of Pt and
the excess of oxygen have a direct effect on the hydrogenation of
propylene and do not directly affect the rate limiting step of the
reaction.
3
.3.2. H O2 formation
2
H O synthesis (or OOH species formation) is the rate limiting
2
2
step in the direct PO formation from hydrogen, oxygen and propyl-
ene [6]. This intermediate step strongly influences the performance
of the PO production. To study the influence of the catalyst and the
excess of oxygen on H O2 production, the experiments conducted
2
for the PO synthesis were repeated for H O , and therefore com-
2
2
pared with the results for PO. The experiments were performed in
the same operation conditions, without the feeding of propylene.
To avoid the decomposition of H O (or reduce the reactivity of
2
2
the OOH intermediate species), promoters and halides are neces-
sary [45,46]. Therefore, the reaction medium for H O2 synthesis
2
was modified in comparison to the PO by adding H SO4 (0.05 M)
2
and NaBr (9 ppm). In order to quantify the concentration of H O2
2
(
wt%), the samples were titrated with cerium sulphate. The con-
version was calculated from the continuous measurements of the
hydrogen concentrations in the gas phase. The obtained results are
shown in Fig. 4.
A different trend with respect to the PO formation is observed
for the H O synthesis. The presence of Pt has a negative effect on
2
2
the H O formation. Indeed, the amount of H O2 or OOH species,
2
2
2
being these species indispensable for the consecutive formation
of hydrogen peroxide, decreases at similar conversions indicating
that the water formation increases during these reactions. In our
experiments, the presence of Pt increases the total metal content
of the catalyst by 10% with respect to the simple Pd/TS-1 catalyst.
Therefore, the decomposition of H O2 (or OOH species) and the
2
hydrogenation reactions are affected by the high reactivity of Pt
to water formation [47]. Operating the catalyst with a high oxy-
gen concentration shows a drop in the catalyst activity towards the
H O productivity (Fig. 4). This may be attributed to the change
Fig. 5. Comparison of PO and H2O2 productivity in time for 1 wt% Pd/TS-1
and 1 wt% Pd–0.1 wt%Pt/TS-1 catalysts. T = 40 C. P = 5.5 bar. Methanol = 1.5 ml/h.
Gasflow = 2 ml/min. Catalyst = 50 mg. Particle size = 250–400 m. 50% O2.
◦
2
2