Nanoparticle deposition onto model AlPO supports for establishing structure-performance relations in catalytic transformations.

Abstract

The work noted throughout is a study of Pt nanoparticle (NP) deposition onto metal doped aluminophospahtes (MAlPOs) as supports for the formation of bifunctional catalysts. MAlPOs are highly versatile materials which can be finely tuned structurally to give access to unique catalytic performance. When coupled with highly active Pt NPs designed by robust deposition techniques, the selectivity and stability of the catalyst can be elevated. NPs lend themselves to having high surface area, varied morphologies, and sizes. By varying the metal and pore architecture of the support, this reveals alternative catalytic activity. Deposition of metal NPs by specialised techniques allows access to size selective effects by partial blocking of pores to then give alternative catalytic activity. Butane isomerisation and ethanol dehydration as test reactions allow an understanding of how the structure and design of the catalyst orchestrates the catalytic behaviour. Analysis of the product distribution can be linked back to characterisation detailing aspects of the catalyst support such as crystallinity and porosity. The NPs oxidation, size and morphology were explored to expose their role in the catalytic performance. To establish the effect of nanoparticle fabrication on the resultant catalyst formation and activity, four deposition techniques were investigated. Wetness Impregnation (WI), Incipient Wetness (IW), Colloidal Deposition (CD), with PVP and PVA as capping agents, and Chemical Vapour Impregnation (CVI). The contrasting parameters of each technique were emphasised by deposition onto hierarchical (HP) AlPO-5. The hierarchical supports contain micropores and mesopores, which were fabricated by a soft templating method. The addition of mesopores to AlPOs is theorised to aid mass transport and extend the lifetime of the catalyst. When implemented in butane isomerisation, the catalysts with higher loading of fully reduced Pt resulted in higher conversion and greater selectivity to isobutane. Application of the same catalyst series in ethanol dehydration supressed conversion due to obscuring weak acidity within the support material. Co, Cu, Mg and Ni were chosen as metal dopants for isomorphous substitution into microporous AlPO-5. This was implemented with a view to exhibit redox capabilities and varying acid site strength. Co, Mg and Ni were also successfully incorporated into hierarchical AlPO-5. The contrast of the strictly microporous (MP) and hierarchical (HP) pore structures were exploited for Pt nanoparticle deposition. The colloidal deposition technique was selected for its superior activity in butane isomerisation on undoped HP-AlPO-5. The more relatively acidic dopants, Mg and Co, within the parent supports yielded superior catalyst activity which was not aided with Pt nanoparticle deposition. The addition of mesopore to form the hierarchical material yielded superior activity when the bifunctional catalyst Pt/HP-CoAlPO-5 was studied. This combination of the moderately acidic Co dopant and Pt nanoparticles facilitates high selectivity to isobutane. Employing the Pt/HP-MAlPO-5 catalyst series in ethanol dehydration showed little effect compared to the parent supports. The Pt/MP-MgAlPO-5 and Pt/HP-MgAlPO-5 catalysts were also selected for a case study into high resolution transmission electron microscopy (TEM), x-ray absorption spectroscopy (XAS), and operando UV-Vis measurements to elucidate key structure-performance relations in butane isomerisation. Co and Mg doped MP and HP AlPO-5 catalysts were also evaluated by magic angle spinning nuclear magnetic resonance (MAS-NMR). To truly assess the role of Pt deposited by colloidal deposition (PVP) the same catalyst series was investigated with a hydrogen co-feed in butane hydroisomerisation. In every instance the Pt containing catalyst demonstrated superior catalyst activity and stability. Thus, confirming the importance of a hydrogen co-feed in achieving superior catalyst activity with Pt containing MAlPO-5 catalysts. Due to the success of implementation of the hydrogen co-feed, the CVI technique was also investigated due to the avoidance of a solvent and capping agent for the technique. The Co, Mg and undoped MP and HP supports were chosen to compare the role of the dopant in relation to Pt deposited by CVI. In each instance the Pt deposited catalysts showed increased catalytic activity compared to the respective parent supports. Compared to the CD deposition method, the catalyst activity varies depending on the dopant and type of porosity. Where the moderately acidic cobalt dopant and higher confinement for the Pt/MP-CoAlPO-5 catalyst favoured isobutane formation. To contrast the effects of confinement, microporous AlPO-18 doped with Mg was synthesised. The atomically tailored supports were utilised in anchoring Pt NPs by CVI. The increased confinement supressed the formation of isobutane to form mainly propane on the parent support under hydroisomerisation conditions. Additionally, the yield of hydrogenolysis products methane and ethane was elevated due to the incorporation of Pt, again with little selectivity to isobutane.

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Identifiers

ORCID for Evangeline Bridget McShane: orcid.org/0000-0002-5935-8328

ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

ORCID for Marina Carravetta: orcid.org/0000-0002-6296-2104

ORCID for Mark Light: orcid.org/0000-0002-0585-0843

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