In the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold-palladium nanoparticles as catalysts, two pathways have been identified as the sources of the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O(2), whereas the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. Herein we report that by changing the metal oxide used to support the metal-nanoparticles catalyst from titania or niobium oxide to magnesium oxide or zinc oxide, it is possible to switch off the disproportionation reaction and thereby completely stop the toluene formation. It has been observed that the presence of O(2) increases the turnover number of this disproportionation reaction as compared to reactions in a helium atmosphere, implying that there are two catalytic pathways leading to toluene.
Highly
active and stable bimetallic Au–Pd catalysts have been extensively
studied for several liquid-phase oxidation reactions in recent years,
but there are far fewer reports on the use of these catalysts for
low-temperature gas-phase reactions. Here we initially established
the presence of a synergistic effect in a range of bimetallic Au–Pd/CeZrO4 catalysts, by measuring their activity for selective oxidation
of benzyl alcohol. The catalysts were then evaluated for low-temperature
WGS, CO oxidation, and formic acid decomposition, all of which are
believed to be mechanistically related. A strong anti-synergy between Au and Pd was observed for these reactions, whereby the
introduction of Pd to a monometallic Au catalyst resulted in a significant
decrease in catalytic activity. Furthermore, monometallic Pd was more
active than Pd-rich bimetallic catalysts. The nature of the anti-synergy
was probed by several ex situ techniques, which all indicated a growth
in metal nanoparticle size with Pd addition. However, the most definitive
information was provided by in situ CO-DRIFTS, in which CO adsorption
associated with interfacial sites was found to vary with the molar
ratio of the metals and could be correlated with the catalytic activity
of each reaction. As a similar correlation was observed between activity
and the presence of Au0* (as detected by XPS), it is proposed
that peripheral Au0* species form part of the active centers
in the most active catalysts for the three gas-phase reactions. In
contrast, the active sites for the selective oxidation of benzyl alcohol
are generally thought to be electronically modified gold atoms at
the surface of the nanoparticles.
A mixed oxide support containing Ce, Zr and Al was synthesized using a physical grinding method and applied in the oxidative dehydrogenation of propane using CO2 as the oxidant. The activity of the support was compared with that of fully-formulated catalysts containing palladium. The Pd/CeZrAlOx material exhibited long-term stability and selectivity to propene (during continuous operation for 140 h) which is not normally associated with dehydrogenation catalysts. From temperature-programmed desorption of NH3 and CO2 it was found that the catalyst possessed both acidic and basic sites. In addition, temperature programmed reduction showed that palladium promoted both the reduction and re-oxidation of the support. When the role of CO2 was investigated in the absence of gas-phase oxidant, using a Temporal Analysis of Products (TAP) reactor, it was found that CO2 dissociates over the reduced catalyst leading to formation of CO and selective oxygen species. It is proposed that CO2 has the dual role of regenerating selective oxygen species, and shifting the equilibrium for alkane dehydrogenation by consuming H2 through the reverse water-gas-shift reaction. These two mechanistic functions have previously been considered to be mutually exclusive.
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