Interrogating site dependent kinetics over SiO2-supported Pt nanoparticles

Abstract

A detailed knowledge of reaction kinetics is key to the development of new more efficient heterogeneous catalytic processes. However, the ability to resolve site dependent kinetics has been largely limited to surface science experiments on model systems. Herein, we can bypass the pressure, materials, and temperature gaps, resolving and quantifying two distinct pathways for CO oxidation over SiO2-supported 2 nm Pt nanoparticles using transient pressure pulse experiments. We find that the pathway distribution directly correlates with the distribution of well-coordinated (e.g., terrace) and under-coordinated (e.g., edge, vertex) CO adsorption sites on the 2 nm Pt nanoparticles as measured by in situ DRIFTS. We conclude that well-coordinated sites follow classic Langmuir-Hinshelwood kinetics, but under-coordinated sites follow non-standard kinetics with CO oxidation being barrierless but conversely also slow. This fundamental method of kinetic site deconvolution is broadly applicable to other catalytic systems, affording bridging of the complexity gap in heterogeneous catalysis.

A well-defined supported Pt nanoparticle catalyst for heterogeneous catalytic surface science

“…The modelling of the TAP experiments uses Multi-Zone TAP Reactor Theory 32,33 (MZTRT) coupled with our previously published 3-pathway reaction model for CO oxidation over 2 nm Pt nanoparticles. 22 The one-dimensional MZTRT model has been proven to be valid for our given reactor setup with a reactor L / R (length/radius) > 3.5, no pre-volume, and an idealised vacuum at the reactor exit. 31,34,35 The MATLAB script used to perform the simulation and curve fitting is provided in our previous publication, 22 with the model regression performed using the lsqcurve fit function and the 95% confidence intervals estimated using the nlparci function.…”

“…22 The one-dimensional MZTRT model has been proven to be valid for our given reactor setup with a reactor L / R (length/radius) > 3.5, no pre-volume, and an idealised vacuum at the reactor exit. 31,34,35 The MATLAB script used to perform the simulation and curve fitting is provided in our previous publication, 22 with the model regression performed using the lsqcurve fit function and the 95% confidence intervals estimated using the nlparci function. The data is reported as inlet normalised exit flux, where the exit flux curves are normalised to the amount of reactant (or inert in the case of Ar) pulsed into the reactor.…”

“…The synthesis procedure and extensive characterisation of the 2 nm Pt/SiO 2 catalysts used in this work are outlined in our previous publication, 22 but the synthesis procedure is expanded upon here. The synthesis of the 2 nm Pt nanoparticles follows a typical colloidal method, 30 and is as follows.…”

“…After the O 2 and H 2 treatments no production of CO 2 ( m / z 44) or methane ( m / z 16) can be recorded during O 2 and H 2 pulsing respectively indicating a clean metallic (pristine) surface. 22 Between experiments the pristine state of the catalyst is regenerated using a milder version of the pretreatment with smaller pulses of O 2 and H 2 (∼108 μs).…”

“…Further, we demonstrate how this catalyst can be used to identify and quantify active sites on the surface using the kinetic site deconvolution technique. 22 This catalyst can be synthesised using well-established synthesis techniques, and as such, can act as a benchmarking catalyst, alongside other benchmarking catalysts such as EUROPt-1, 23 to understand structure–activity relationships and resolve reaction mechanisms using techniques such as TAP, 24 DRIFTS coupled with modulation excitation spectroscopy, 25,26 and steady-state isotopic transient kinetic analysis. 27 Further, the weak interaction between Pt and SiO 2 means that the nanoparticles remain spherical, 28,29 which allows for simpler comparison with theoretical simulations over freestanding nanoparticles.…”