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Abstract: The 3-dimensional atom probe (3DAP) is a unique instrument providing chemical analysis at the atomic scale for a wide range of materials. A dedicated 3DAP has been built specifically for analysing reactions at metal surfaces, called the catalytic atom probe (CAP). This paper presents an overview of results from the CAP on structural and chemical transformations to surface layers of Pt and Pt–17.4at.%Rh catalysts following exposure to a number of gases typically emitted by vehicle engine exhausts, normally for 15min at pressures of 10mbar. Following exposure to the oxidising gases NO on Pt, and NO, O2 or N2O on Pt–Rh, both surfaces appear disrupted, while for Pt–Rh, Rh enrichment of the surface atomic layer is noted over the entire specimen apex for exposure temperatures up to 523K. However, for oxidising exposures at 573–773K relatively clean, Rh-depleted surfaces are observed on {001}, {011} and {012} crystallographic regions of Pt–Rh. It is suggested that this result is due to surface diffusion of oxide species over the specimen apex, towards the {111}-orientated areas where the oxides appear to be stabilised. In contrast, CO exposure appears to have little effect on the either the surface structure or composition of the Pt–Rh alloy. Finally, combinations of two gases (NO+CO, O2 +NO) were also dosed onto Pt–Rh alloys in the same exposure. These revealed that while NO and CO can co-adsorb without interference, CO prevents the build up of oxide layers and reduces the extent of Rh segregation seen under NO exposure alone. On exposing Pt–Rh to NO after an oxygen exposure, heavily oxidised surfaces, Rh segregation and no intact NO molecules were seen, confirming the ability of oxidised Pt–Rh to dissociate nitric oxide. [Copyright &y& Elsevier]

Abstract: A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of “nano-engineered” catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673–1073K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773K. During oxidation at 873K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt–Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt–Pd with Pt–Rh and Pd–Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys. [Copyright &y& Elsevier]

Abstract: Using a purpose-built 3D atom probe, we have previously shown that exposure to oxidising gases (NO, N2O, O2) induces Rh surface segregation in Pt–Rh alloys, the extent of which is strongly dependent on treatment temperature, crystallographic plane and the presence of ternary alloy additions. In this paper, the segregation trends identified on three different crystallographic surfaces of Pt–Rh are analysed using thermodynamic and kinetic arguments. The segregation model we present is generic for diffusion on alloy surfaces in the presence of active gases. From it we obtain activation energies and diffusion coefficients for the processes of metal-oxide species diffusion both perpendicular to and laterally across the surface. Using these we propose a simple model for the interaction of chemically active gases with the surfaces of such alloys. Applying this understanding to sequential oxidation/reduction treatments would in principle allow improved control of the surface composition of alloy catalysts. Related applications of this model include optimisation of core–shell catalyst nanoparticles. [Copyright &y& Elsevier]

Abstract: The surface composition of an Au-62at%Pd alloy has been studied by means of a catalytic atom probe (CAP) before and after exposures to nitric oxide (NO) at temperatures ranging from 300 to 573K for 20min. Subsequent CAP analysis at 100K revealed a considerable surface enrichment in Pd (to ∼80at%) after exposure at 573K. This is correlated with the occurrence of NO dissociation, and the formation of strong Pd–O bonds at the surface. Blank experiments in ultra-high vacuum reflect the surface composition of the bulk material, in excellent agreement with electron microprobe analysis. At 573K, no detectable surface segregation occurs in the absence of NO adsorption for the times and temperatures studied. However, classical Metropolis Monte-Carlo simulations performed with a semi-empirical potential on the Au40Pd60 (111), (110) and (100) systems show surface enrichment of gold at equilibrium. This suggests that the temperatures of the clean surface segregation experiments are too low to reach equilibrium within times of the order of hours. [Copyright &y& Elsevier]

Abstract: A versatile gas reaction cell has been developed for a 3D atom probe system to provide a new method for studying the reaction mechanisms of heterogeneous catalysis. We describe the features and advantages of this new system. The reaction cell enables exposure of metal specimens in high temperature (⩽873K) and high pressure (⩽1bar) environments to a wide range of single gases or gas mixtures. Following treatment, specimens are transferred into the UHV analysis chamber of the 3D atom probe, which provides atomic-scale positional and chemical information for adsorbed gas and metal atoms/oxides from selected regions of the specimen apex. Results demonstrating the effects of heating alloy catalysts in vacuo and in air are presented, which validate the instrument design, alongside a sample of data for exposing Pt and Pt-Rh alloys to NO, revealing the power of the 3D atom probe technique in this field. [Copyright &y& Elsevier]