Atom probe analysis of Pt alloy catalysts for nitric acid production

This project is in close collaboration with our industrial partner, Yara Interational, who are the world's biggest producer of the vital chemical nitric acid. This is a key intermediate in the production of highly efficient nitrate fertilizers, which are critical to continue feeding the world’s growing population. Producing this requires an ammonia combustion catalyst composed of Pt/Rh alloys, configured as stacked metal gauzes or meshes, which has been used in the Ostwald process for over 100 years to selectively oxidise ammonia to NO. 

The reaction is highly exothermic, and the upstream Pt/Rh gauzes undergo significant reconstruction within the first week of use generating what are known as ‘cauliflower growths’ on the wire surface. This process involves massive transport of metal (e.g. vaporisation, diffusion, and mechanical degradation) and is associated with undesirable metal losses. However, it also affords an increase in chemical selectivity, suggesting that the surface of these cauliflower formations are the active sites for ammonia combustion. Despite the ubiquitous use of Pt/Rh catalysts in industry, the mechanism by which these growths form, and their subsequent composition, is still unknown. The spatial resolution afforded by conventional tomographic techniques (μm) does not afford sufficient detail to determine whether the surface is composed of a random alloy, or clustering, which could be used to facilitate the development of a kinetic model for ammonia oxidation. A novel approach to map the compositional gradients in cauliflower growths at the atomic scale, and asses the impacts of impurities, is to use Atom Probe Tomography (APT). The atomic resolution afforded by APT will offer insights into the difference in surfaces for catalysts which afford a typical 96% efficiency as compared to an exceptional efficiency of 98%, for example.

Furthermore, by developing APT alongside other advanced characterisation techniques available in the Department, such as 3D FIB slicing, the student will be able to build a full image of the complex compositional and morphological changes that these Pt/Rh alloys undergo during activation and operation. The generated information will provide key insights into the active sites of current catalysts and the roles of Pt and Rh, alongside that of other trace metallic species, in ammonia combustion. Additionally, it will validate DFT models of the alloy surface and contribute to advanced CFD modelling of an ammonia oxidation kinetic model within Yara. Ultimately, this will enable us to design a more efficient catalyst. The student will therefore be at the cutting edge of efforts to design critically important catalysts with improved efficiency, and will have the opportunity to interact with a wide network of scientists working on surface characterisation, collaborate closely with industrial partner Yara and visit their facilities.

This project also has had full funding agreed by Yara.