AVS 69 Session HC+SS-WeA: Advances in Complex Catalytic Systems

Many important catalysts and electrocatalysts for energy and environmental technologies involve late transition metal nanoparticles dispersed across the surface of some oxide or carbon support.The activity and long-term stability of these materials depend strongly on particle size below 7 nm, and, in this size range, upon the composition and atomic-level structure of the support surface. We show here that the chemical potential of the metal atoms in such supported catalysts provides a convenient descriptor of their performance as heterogeneous catalysts that captures many of the effects of particle size, metal-metal alloying and support on catalyst performance. Based on microcalorimetric measurements of metal adsorption energies, the metal chemical potential is shown to be predictable as a function of metal particle size and the adhesion energy of the particle to the support. For oxide supports, this adhesion energy correlates predictably with metal oxophilicity, as we defined based on heats of oxide formation from gaseous metal atoms. For carbon supports, this adhesion energy correlates predictably with metal carbophilicity, as we defined based on DFT estimates of C atom adsorption energies. These correlations provide predictions of metal chemical potential that can enable catalyst design.

Work supported by DOE-OBES under Grant Number DE-FG02-96ER14630.

Materials science in the nanoscale domain has become a reality for several applications, from integrated circuits, sensors, catalysts, medicines, and data-storage devices, among others [1]. We achieved the ability to understand materials and, more importantly, command the materials’ properties at the atomic level using precise synthesis and growth methods. Therefore, during the last decades, enormous efforts have been made to develop new processes for the fabrication, characterization, and manipulation of materials in complex nanoarchitectures with atomic precision, making it possible to express emergent new chemical, electronic, photonic, magnetic, and structural properties. On-surface synthesis becomes a powerful bottom-up technique to fabricate such nanostructures using organic and organometallic precursors as molecular building blocks [2]. In this talk, I will present some strategies we have adopted to produce planar carbon lattices nanostructures, for example, porous nanoribbons and nanomembranes [3-5]. For a complete understanding of the atomic and electronic properties of the materials, we have combined scanning tunneling microscopy and spectroscopy (STM/STS), X-ray photoelectron spectroscopy (XPS), and numerical simulations based on density functional theory (DFT) calculations.

During recent years, 2D Surface Optical Reflectance (2D-SOR) [1,2] microscopy [3] has emerged as a valuable surface characterization tool for model catalysts or electrodes [4] when performing operando investigations in harsh environments. In particular, 2D-SOR microscopy is favorably used as a complementary technique to other photon-in-photon-out techniques which do not carry direct information on the surface 2D morphology. In this presentation we will present the development and examples of 2D-SOR instrumentation and investigations from single and poly-crystalline samples in combination with Planar Laser Induced Fluorescence (PLIF) [2, 3], High Energy Surface X-Ray Diffraction (HESXRD) [5,6,7] and Polarization Modulation-Infrared Reflection Absorption Spectroscopy (PM-IRRAS) [8] coupled to Mass Spectrometry (MS) and Cyclic Voltammetry (CV) in thermal catalysis, electrocatalysis and corrosion. Illustrating examples of the versatility of the technique will be shown including reflectance changes during the thermal CO oxidation over Pd(100) and Pd polycrystalline surfaces. We show that reflectance changes during the reaction can be associated with the formation of thin Pd oxides by the combination of 2D-SOR and Surface X-Ray Diffraction (SXRD). The combined measurements demonstrate a sensitivity of 2D-SOR to the formation of a 2-3 Å thin Pd oxide film. During Cyclic Voltammetry (CV) in an acidic electrolyte using a Au(111) surface as an electrode, we show that the differential of the change in 2D-SOR reflectance correlate to various current features in the CV curve. This observation can be used to differentiate current features in the CV curve from a polycrystalline Au surface, demonstrating that the different grains contribute to the current at different potentials due to the different surface orientations. Finally, we show that 2D-SOR is a cheap and useful technique to investigate the corrosion of applied materials such as duplex stainless steels and Ni alloys.

[1] W. G. Onderwaater et al Rev. Sci. Instrum., 88 (2017) 023704.

[2] J. Zhou et al, J. Phys. Chem. C 121 (2017) 23511.

[3] S. Pfaff et al, ACS Appl. Mater. Interfaces 13 (2021) 19530.

[4] W. Linpe, et al, Rev. Sci. Instrum., 91 (2020) 044101.

[5] S. Pfaff, et al Rev. Sci. Instrum. 90 (2019) 033703.

[6] S. Albertin, et al, J. Phys. D: Appl. Phys. 53 (2020) 224001.

[7] W. Linpé, et al J. Electrochem. Soc. 168 (2021) 096511.

[8] L. Rämisch et al, Appl. Surf. Sci. 578 (2022) 152048

Supported sub-nano clusters are potentially a metals-efficient approach to catalysis, where all the expensive catalytic atoms (Pt, Pd, etc.) are exposed in the surface layer.In addition, because the properties of small clusters are highly size dependent, varying the cluster size provides a parameter that can be used to tune activity and selectivity.The problem with small clusters is that they tend to sinter and poison easily, and much of our work is in developing approaches to stabilize the supported clusters under thermal or electro-catalytic conditions.

Deposition of mass-selected clusters in UHV is used to prepare model catalysts and electrocatalysts with catalytic centers that all start out being the same size.We have developed an in-vacuum ALD-like self-limiting reaction approach to dope or alloy the clusters with elements like B, Sn, or Ge, with the goal of stabilizing the clusters against both poisoning and sintering.Two types of catalysis experiments will be described.

Gas-surface catalysis is studied in the UHV system by mass spectrometric methods, but we also have new microreactor system that allows clusters deposited on alumina or silica surfaces to be exposed to reactant flows at pressures up to 1 atm, with mass spectrometric product detection.This part of the talk will focus on using Ge doping to stabilize small Pt clusters against deactivation by both carbon deposition (coking) and sintering at temperatures up to 700 K.To goal is to make stable and selective alkane dehydrogenation and cracking catalysts.

Electrocatalysis is studied either in situ, using an electrochemical cell housed in an antechamber to the UHV system, or in conventional benchtop electrochemical cells.The in situ experiments allow us to study aqueous electrochemistry with minimal air exposure, while the ex situ setups allow more elaborate types of electrochemical measurements.Results for the hydrogen evolution reaction (HER), oxygen evolution reaction (ORR), and alcohol electro-oxidation will be presented for catalytic Pt_n clusters deposited on indium tin oxide (ITO), fluorine tin oxide (FTO), and graphite (HOPG).For ITO/FTO, electrodes were prepared by soft landing the clusters.For HOPG, the effects of deposition energy on electrocatalytic activity and stability, and on physical properties measured by XPS and ISS will be discussed.