Design of bimetallic nanoparticles for sustainable catalysis

Abstract

The generation of nanoparticles by extrusion of [MCl₄] ^n- anions (where M=Au, Pd, Pt) from within the 1D microporous architecture of copper chloropyrophosphate (CuClP) materials is a recently emerging approach towards highly functional nanoparticle catalysts. For the first time, these structures have been developed to generate a series examples of bimetallic nanoparticle systems by extrusion. Previous work on these materials showed that the activation conditions that are required in order to generate the active nanoparticles need to be optimised to prevent sintering of the parent CuClP framework. Interestingly, the Au/CuClP system demonstrated a large temperature window within which the catalyst could be activated with no sintering. The work described here details the targeted effort to impart stability into the CuClP systems via the incorporation of Au. As a by-product of its inclusion in the framework, Au was also able to be extruded and form nanoparticles. In the bimetallic systems, Au therefore served a role as a framework stabilising agent, as well as forming bimetallic nanoparticles with Pd and Pt. In the case of Pt, an increase in activation temperature from 200 °C to 300 °C resulted in noticeable evidence of framework sintering. This was shown by PXRD, TGA and XAS data. The sintering was only observed in the monometallic Pt catalyst; the incorporation of Au in the catalyst showed much improved stability parameters. In addition to the spectroscopic and physicochemical evidence, the catalytic activity validates the assertion that Au imparts a stabilising effect. The aerobic dehydrogenative oxidation of KA oil (a mixture of cyclohexanol and cyclohexanone) to form cyclohexanone is a key step in the production of caprolactam; essential in the synthesis of nylon-6. This reaction was used to probe the catalytic efficacy of the aforementioned structural modification. As expected from the characterisation, the increase in activation temperature damaged the Pt catalyst, while the bimetallic analogue maintained consistent activity over the same conditions. To provide a comparison of the CuClP system with a conventional nanoparticle catalyst system, bimetallic AuPt nanoparticles were fabricated by the colloidal route, immobilised on mesoporous SiO₂ and tested in the KA oil oxidation. The activity of the catalysts was critically dependent on the ratio of Au to Pt, albeit not as high as that of the CuClP systems. In the case of Pd, which was understood to be the least resistant of the CuClP materials to sintering under activation conditions, the activation temperature was not changed, instead the Au:Pd ratio was the principal factor. As with the Pt case, the incorporation of Au had a stabilising effect on the framework, as evidenced by PXRD, XPS and XAS. Although there was incomplete extrusion of either metal, the surface was populated with a significant amount of metallic Au and Pd. To evaluate the catalytic activity of the bimetallic systems, the Suzuki coupling reaction was chosen. It is an important reaction in industry as it facilitates the formation of C-C bonds. It was demonstrated for the first time, that a CuClP catalyst system can be used in the formation of C-C bonds. As with the Pt case, the activity of the catalyst was dependent on the ratio of Au to Pd. For the most active catalyst, the activity was comparable with the literature values, the substituent effects study also indicated that the catalyst was facilitating the reaction by expediting the normally rate-determining oxidative addition step.

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

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