High-throughput surface science and catalysis across ternary alloy composition space
Transition metals alloys have been widely used in catalysis for over a hundred years. Their advantages of having high activity, stability, and allowing flexibility in catalyst design, etc., also bring challenges to researchers due to the vast parameter space to explore and optimize. High-throughput experimentations developed in recent decades have greatly accelerated the exploration and screening of materials, and their potential have been demonstrated in numerous cases. Among these, Composition Spread Alloy Films (CSAFs) and Surface Structure Spread Single Crystals (S4Cs) are two types of high-throughput libraries suitable for the exploration of the alloy composition space and the surface structural space. Coupling these techniques with high-throughput surface characterization tools, such as spatially resolved Energy Dispersive X-ray Spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS), Low Energy Ion Scattering Spectroscopy (LEIS), Ultraviolet Photoelectron Spectroscopy (UPS), and Mass Spectroscopy (MS), we can perform a comprehensive analysis of an alloy system or a surface structural space.
The equilibrium surface segregation property was first studied on three alloy systems, including CuAuPd, NiPdAu, and CuAgAu. The mapping of LEISderived surface compositions with high compositional resolution showed the effects of temperature, crystalline structure, long-range ordering, and adsorbates on surface segregation. A segregation model was derived to describe the segregation in ternary alloys. Rich datasets were generated, which could be used for benchmarking computational works.
Inspired by the interesting behaviors that we observed during the equilibrium segregation studies, the kinetics of surface segregation under UHV and oxygen environment was then examined in detail on CuAu alloys and a CuAgAu alloy using LEIS and Near Ambient Pressure XPS (NAP-XPS). A series of experiments were designed to properly account for the damage of ion scattering caused to the surfaces, and the time scale of segregation kinetics was measured across the binary alloy composition space and a temperature range of 300-500 K.
The structural dependence of initial-stage oxygen adsorption on Cu surfaces has been investigated. High-throughput characterization of oxygen adsorption on a Cu(111)-14° S4C at 500 K was performed by XPS. The kinetic behavior of oxygen adsorption was examined using several kinetic models, including 1st order, LH 2nd order, and precursor-state 2nd order adsorption models. At least three types of sites were identified on surfaces vicinal to Cu(111) during oxygen adsorption and oxygen-induced surface reconstruction, and the distribution of these sites was compared to the structure of ideal clean surfaces.
Next, the performance of alloys in isopropyl alcohol (IPA) partial oxidation reaction was studied using CSAFs and a high-throughput catalytic microreactor array. Mo showed high activity in converting IPA to propene at above 500 K, and Mo oxide was identified to be the active species. While other metals such as Cu, Ag, Pd, and Pt all showed activity in IPA oxidation, either to propene or to acetone, their activities were too low to be studied using the high-throughput microreactor setup limited by the small contact area and short contact time.
Furthermore, the potential of Angle-resolved XPS (AR-XPS) being used in quantifying the top-surface alloy compositions was explored on a CuAgAu CSAF. With the parallel data acquisition mode in ThetaProbe, the concentration information at various depths was collected by measuring XPS at four different emission angles with respect to the surface normal. Three lines with different binding energies were chosen for each element to collect information at different effective depths below the surface. The segregation in 20 CuAu alloys, 20 CuAg alloys, and 20 AgAu alloys was measured and compared to the compositions determined by EDX and LEIS.
All in all, in this study, we have explored the applications of several highthroughput experimental methodologies in surface science and catalysis, including topics such as surface segregation, oxygen adsorption, and catalytic evaluation.
- Doctor of Philosophy (PhD)