D.P. Dissanayake, J.H. Lunsford / Journal of Catalysis 214 (2003) 113–120
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of H2O2 would not have been significant. At longer times
with larger concentrations of H2O2, the selectivities would
be less.
Choudhary and co-workers [13] have recently shown that
the support strongly influences both the rate of formation and
the rate of decomposition of H2O2, with PdO/fluorinated-
Al2O3 being among the best of the catalysts that they
studied. Fluorination of the alumina significantly decreased
the rate of H2O2 decomposition.
Clearly, the “support” material can play an important
role in the steady state concentration of H2O2, which
makes PdCl2 an attractive source of palladium for colloid
formation. The data of Fig. 2 show that after 15 h the largest
concentration of H2O2 was attained when the palladium was
introduced as PdCl2, and the results of Table 1 confirm
that the system derived from PdCl2 was the only one for
which there was a net increase in the amount of H2O2 when
the original solution contained 2 wt% H2O2. Most supports
are considered to be inert with respect to H2O2 formation;
however, Park et al. [14] reported that an H-beta zeolite had a
small amount of activity, but poor selectivity, for generating
H2O2.
Scheme 1.
are mainly responsible for the induction period (Fig. 3)
and the different rates of reaction as the O2 and H2
partial pressures are changed (Fig. 6). The induction period
depicted in Fig. 3 is more pronounced for Pd/SiO2(03)
than for Pd/SiO2(57) because the pore size of the former
2−
is much smaller. Consequently, the generation of PdCl4
and its transport to the bulk aqueous phase are inhibited.
For the practical utilization of this process, one of the
remaining challenges is to stabilize the colloidal palladium
so that it is not transformed into Pd/frit. Attempts to increase
the lifetime and/or the catalytic activity of the Pd system
by adding reagents that are known to stabilize colloids
were unsuccessful. These reagents included sodium citrate,
poly(vinyl alcohol) and poly(ethylene glycol).
4.3. Effect of pressure
Reactions were not carried out in this study at total
pressures greater than 760 Torr; although, Izumi et al. [2]
have reported in a patent the weight percent of H2O2 and
the selectivity at the end of 20 h for pressure up to 15.2 ×
103 Torr. Their catalyst was Pd/hydrous silicic acid and
the solution was 0.03 N in HCl and 1 N in H2SO4. The
O2/H2 ratio was 2.5. At 760 Torr they observed 0.78 wt%
H2O2, which is comparable to the amount shown in Fig. 2,
but the selectivity was only 30%. The amount of H2O2
increases almost linearly with pressure up to 7.6 × 103 Torr,
while the selectivity increased to a value of 79%. At 15.2 ×
103 Torr the wt% H2O2 and selectivity were 12.45 and 88%,
respectively. These results demonstrate that a significant
advantage in both H2O2 formation and selectivity can be
gained by operating at higher pressures
Although it is not explicitly indicated in Scheme 1, HCl
plays an important role in the formation of PdCl4 from
2−
the reduced forms of Pd. The previous study demonstrated
that > 0.01 M HCl was needed to form the colloid from a
Pd/SiO2(57) catalyst. The importance of protons in inhibit-
ing the decomposition of H2O2 has long been known [11],
and the results of Fig. 5B confirm this observation. In this
case, PdCl4 was used to produce the active phase; there-
fore, HCl was not required to generate PdCl4 as an inter-
mediate in the formation of colloidal Pd.
2−
2−
4.2. Decomposition of H2O2
At the largest acid concentrations used in this study (1 M),
the decomposition of H2O2 probably does not become
significant at H2O2 concentrations less than about 0.5 wt%,
but at larger concentrations the decomposition reaction, as
well as the loss of colloid, may cause the nonlinear behavior
shown in Fig. 2. At considerably larger concentrations, the
decomposition reaction, which is first order [11], becomes a
limiting factor, especially with Pd/SiO2(57) as the catalyst
(Table 1). Thus, the ultimate H2O2 concentration that can
be achieved depends on several factors including the source
of Pd, the concentration and stability of the colloid, and
the HCl concentration. Selectivities > 60% were obtained
after 1 h for the conditions of Fig. 2; hence, the H2O2
concentration was about 0.1 wt% and the decomposition
4.4. Insight into the reaction mechanism
Although important mechanistic questions remain unre-
solved, the Raman results shown in Fig. 8 provide strong ev-
idence that oxygen remains in a diatomic form during hydro-
genation. This is consistent with the mechanism suggested
by Pospelova and Kobozev [11,15], who proposed that O2
is adsorbed in the molecular form on Pd and reacts with ad-
sorbed H to yield HO2 as a surface intermediate. By con-
trast, on Pt, which is a catalyst for H2O production, O2 is
believed to adsorb in a dissociative form. The behavior of
O2 on Pd dispersed in an aqueous phase is different from
O2 on a Pd (111) surface under UHV conditions for which