Michael M. Forde

Selective oxidation of methane using AuPdCu/TiO₂ catalysts.

The selective oxidation of linear alkanes with molecular oxygen under mild conditions remains a challenging topic in the field of catalysis. In this study we investigate the co-oxidation of C–H bonds in substrates with different relative reactivities, the aim being to couple the oxidised products in situ to form the corresponding esters. Initial attempts were made to co-oxidise octane with toluene to form octyl benzoate using Au–Pd catalysts. During the study the oxidation of octane in the presence of benzaldehyde, an oxidation product of toluene, was also investigated in order to demonstrate the potential feasibility of the reaction. This work summarises our attempts to show whether a co-oxidation system could be an effective way to oxidise linear alkanes.

As co-catalyst materials, metal nanoparticles (NPs) play crucial roles in heterogeneous photocatalysis. The photocatalytic performance strongly relies on the physical properties (i.e., composition, microstructure, and surface impurities) of the metal NPs. Here we report a convenient chemical vapour impregnation (CVI) approach for the deposition of monometallic-, alloyed, and core–shell structured metal co-catalysts onto the TiO₂ photocatalyst. The as-synthesised metal NPs are highly dispersed on the support and show narrow size distributions, which suit photocatalysis applications. More importantly, the surfaces of the as-synthesised metal NPs are free of protecting ligands, enabling the photocatalysts to be ready to use without further treatment. The effect of the metal identity, the alloy chemical composition, and the microstructure on the photocatalytic performance has been investigated for hydrogen production and phenol decomposition. Whilst the photocatalytic H₂ production performance can be greatly enhanced by using the core–shell structured co-catalyst (Pd_shell–Au_core and Pt_shell–Au_core), the Pt_shell–Au_core modified TiO₂ yields enhanced quantum efficiency but a reduced effective decomposition of phenol to CO₂ compared to that of the monometallic counterparts. We consider the CVI approach provides a feasible and elegant process for the decoration of photocatalyst materials.

Fe/ZSM-5(30) catalysts have been prepared by chemical vapour impregnation (CVI) using iron(III) acetylacetonate as the precursor. These materials have been used for the oxidation of methane and ethane using aqueous hydrogen peroxide as oxidant. Heating in air leads to materials that exhibit high catalytic activity and give formic and acetic acid with high selectivity from methane and ethane respectively. Heat treatment of the uncalcined materials under a reducing atmosphere results in partial reduction of iron from the Fe^III to Fe^II oxidation state with the majority of the iron being present as isolated octahedral extra-framework species having oxygen neighbours and showing no evidence of a coordination shell containing Al or Fe, as evidenced from studies using X-ray absorption and UV-Vis spectroscopies. These hydrogen treated catalysts show the same catalytic activity as their analogues formed by heating in air, but in contrast exhibit higher alcohol selectivities for both methane and ethane conversion to oxygenates and are reusable. Our findings for both the oxidation of methane and ethane indicate that the selectivity to the oxidation products, i.e. acids or alcohols, can be controlled by tuning the active site structure and/or oxidation state of the Fe species in Fe/ZSM-5.

The use of precious metals in heterogeneous catalysis relies on the preparation of small nanoparticles that are stable under reaction conditions. To date, most conventional routes used to prepare noble metal nanoparticles have drawbacks related to surface contamination, particle agglomeration, and reproducibility restraints. We have prepared titania-supported palladium (Pd) and platinum (Pt) catalysts using a simplified vapor deposition technique termed chemical vapor impregnation (CVI) that can be performed in any standard chemical laboratory. These materials, composed of nanoparticles typically below 3 nm in size, show remarkable activity under mild conditions for oxidation and hydrogenation reactions of industrial importance. We demonstrate the preparation of bimetallic Pd–Pt homogeneous alloy nanoparticles by this new CVI method, which show synergistic effects in toluene oxidation. The versatility of our CVI methodology to be able to tailor the composition and morphology of supported nanoparticles in an easily accessible and scalable manner is further demonstrated by the synthesis of Pd_shell–Au_core nanoparticles using CVI deposition of Pd onto preformed Au nanoparticles supported on titania (prepared by sol immobilization) in addition to the presence of monometallic Au and Pd nanoparticles.

Iron and copper containing ZSM-5 catalysts are effective for the partial oxidation of ethane with hydrogen peroxide giving combined oxygenate selectivities and productivities of up to 95.2% and 65 mol kg_cat^–1 h^–1, respectively. High conversion of ethane (ca. 56%) to acetic acid (ca. 70% selectivity) can be observed. Detailed studies of this catalytic system reveal a complex reaction network in which the oxidation of ethane gives a range of C₂ oxygenates, with sequential C–C bond cleavage generating C₁ products. We demonstrate that ethene is also formed and can be subsequently oxidized. Ethanol can be directly produced from ethane, and does not originate from the decomposition of its corresponding alkylperoxy species, ethyl hydroperoxide. In contrast to our previously proposed mechanism for methane oxidation over similar zeolite catalysts, the mechanism of ethane oxidation involves carbon-based radicals, which lead to the high conversions we observe.

Trimetallic Au–Pd–Pt nanoparticles have been supported on activated carbon by the sol-immobilisation method. They are found to be highly active and selective catalysts for the solvent-free aerobic oxidation of benzyl alcohol. The addition of Pt promotes the selectivity to the desired product benzaldehyde at the expense of toluene formation. Detailed aberration corrected STEM-XEDS analysis confirmed that the supported particles are indeed Au–Pd–Pt ternary alloys, but also identified composition fluctuations from particle-to-particle which vary systematically with nanoparticle size.