(Invited) Finite Size Effects – a Guiding Principle in Monolayer Catalyst Design and Synthesis

Abstract

The ultimate configuration of ultra-thin catalyst overlayers in terms of catalyst utilization and specific activity is the 2D monolayer (ML). Catalyst MLs on different substrates have been extensively studied and their catalytic properties depend on the interplay between the host metal and the ML overlayer.1–8 Their overall behavior is well described by the position of the energy level of d-band center (ed ) with respect to the Fermi level.9 In the case of a substrate with weak electronic effect (weak ligand), ed is mainly affected by the coherent strain (coh) caused by the lattice constant mismatch between the overlayer and the substrate.10 For substrates which are stronger ligands, the electronic and strain effects are coupled and ed is a function of both.9,11 Ideal ML catalysts are difficult to synthesize and often exhibit defect structures that may contribute to their catalytic activity.12 The most common example is Pt and, at room temperature, Pt is very difficult to deposit in a true continuous ML configuration.13 Hence, the sub-monolayer (sML) configuration is dominant. Functional Pt sML catalysts have a morphology consisting of compact 2D nanoclusters with certain size distribution and coverage of the substrate.14–17 Due to finite size effects,18,19 each 2D Pt nanocluster experiences a size-dependent compressive stress.20 Thus, the local strain in Pt nanoclusters (εl) is a combination of the coherent strain and the strain caused by the finite size effects. Therefore, the finite size effect represents a phenomenon that can be used as additional knob to fine-tune the d-band energy of the ML catalysts, which inherently controls overall catalytic activity. Turning this knob in desired direction by varying nanocluster sizes requires a better understanding of the conditions during ML catalyst synthesis, which are responsible for nanocluster size control. Recent studies show that finite size effects dominate the Pt ML catalyst activity for both type of substrates i.e. strong ligand (Pd) and weak ligand (Au) 21,22,23. In the first part of the talk we will review several examples of the finite size effects on activity of ML catalyst as a prelude to the discussion about how to control catalyst ML morphology. The special emphasis is on identifying effective approach to control and manipulate conditions during ML catalyst synthesis via SLRR of UPD ML which result in desired nanocluster size, nucleation density and overall ML morphology. The analytical model describing the fundamental link between the nucleation kinetics and reaction kinetics of SLRR is discussed. Further on, the link between the SLRR process parameters such as choice of UPD ML, concentration of metal ions, temperature, mixing and concentration of supporting electrolyte in solution for SLRR will be discussed within the frame work of the models describing the SLRR reaction kinetics and their link to the resulting catalyst ML morphology. References (1) Yu, W.; et al, J. G. Chem. Rev. 2012, 112, 5780. (2) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810. (3) Alayoglu, S.; et al. Nat. Mater. 2008, 7, 333. (4) Knudsen, J.;et al. J. Am. Chem. Soc. 2007, 129, 6485. (5) Xu, Y.; Ruban, A. V; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 4717. (6) Rossmeisl, J.; et al. Energy Environ. Sci. 2012, 5, 8335. (7) Loukrakpam, R.; et al, P. Phys. Chem. Chem. Phys. 2014, 16, 18866. (8) Erini, N.; et al. ACS Catal. 2014, 4, 1859. (9) Ruban, A.; et al. J. Mol. Catal. A Chem. 1997, 115, 421. (10) Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Phys. Rev. Lett. 1998, 81, 2819. (11) Bligaard, T.; Nørskov, J. K. Electrochim. Acta 2007, 52, 5512. (12) Greeley, J.; Mavrikakis, M. Catal. Today 2006, 111, 52. (13) Waibel, H. et al. Electrochim. Acta 2002, 47, 1461. (14) Brankovic, S. R.; et al. Electrochem. Solid-State Lett. 2001, 4, A217. (15) Brankovic, S.; et al. Surf. Sci. 2001, 474, L173. (16) Park, S.; et al J. J. Am. Chem. Soc. 2002, 124, 2428. (17) Liu, Y.; Gokcen, D.; Bertocci, U.; Moffat, T. P. Science 2012, 338, 1327. (18) Li, L.; et al J. Phys. Chem. Lett. 2013, 4, 222. (19) Quantum Phenomena in Clusters and Nanostructures; Khana, S. N.; Castleman, A. W., Eds.; Springer: New York, 2003. (20) Kern, R.; Müller, P. Surf. Sci. 1997, 392, 103. (21) Grabow, L. C.; Yuan, Q.; Doan, H. A.; Brankovic, S. R. Surf. Sci. 2015, 640, 50. (22) Bae, S.-E.; et al. Electrocatalysis 2012, 3, 203. (23) Zhang, J. L.; et al. Angew. Chemie-International Ed. 2005, 44, 2132. (24) Q. Yuan, et al J. Am. Chem. Soc. 139, 13676 (2017)