Abstract
Platinum group metals (PGMs) serve as highly active catalysts in a variety of heterogeneous chemical processes. Unfortunately, their high activity is accompanied by a high affinity for CO and thus, PGMs are susceptible to poisoning. Alloying PGMs with metals exhibiting lower affinity to CO could be an effective strategy toward preventing such poisoning. In this work, we use density functional theory to demonstrate this strategy, focusing on highly dilute alloys of PGMs (Pd, Pt, Rh, Ir and Ni) with poison resistant coinage metal hosts (Cu, Ag, Au), such that individual PGM atoms are dispersed at the atomic limit forming single atom alloys (SAAs). We show that compared to the pure metals, CO exhibits lower binding strength on the majority of SAAs studied, and we use kinetic Monte Carlo simulation to obtain relevant temperature programed desorption spectra, which are found to be in good agreement with experiments. Additionally, we consider the effects of CO adsorption on the structure of SAAs. We calculate segregation energies which are indicative of the stability of dopant atoms in the bulk compared to the surface layer, as well as aggregation energies to determine the stability of isolated surface dopant atoms compared to dimer and trimer configurations. Our calculations reveal that CO adsorption induces dopant atom segregation into the surface layer for all SAAs considered here, whereas aggregation and island formation may be promoted or inhibited depending on alloy constitution and CO coverage. This observation suggests the possibility of controlling ensemble effects in novel catalyst architectures through CO-induced aggregation and kinetic trapping.
Highlights
IntroductionUnder micro-reactor conditions, it was shown that in the presence of 200 ppm CO (a typical industrial concentration in H2 streams) the activity of Pt/Cu single atom alloys (SAAs) nanoparticle catalysts for acetylene hydrogenation is reduced twofold, when compared to monometallic Pt nanoparticles there was a 15-fold activity decrease [18]
The platinum group metals (PGMs), including Pd, Pt, Rh and Ir, as well as Ni, are well-known for their excellent activity in a wide variety of heterogeneous catalytic systems; Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Thomas Young Centre and Department of Chemical Engineering, University College London, Roberts Building, Torrington Place, London WC1E 7JE, UKDepartment of Chemistry, Tufts University, 62 Talbot Ave., Medford, MA 02155, USAThomas Young Centre, London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK these metals suffer from CO poisoning as a consequence of their high reactivity [1,2,3]
The rest of the paper is organised as follows: we first present the setup of the density functional theory (DFT) and kinetic Monte Carlo (KMC) calculations in Sect. 2, continuing with Sect. 3 where we explore the interactions of CO with single atom alloys (SAAs), in the context of poisoning resistance and adsorbate-induced structural changes
Summary
Under micro-reactor conditions, it was shown that in the presence of 200 ppm CO (a typical industrial concentration in H2 streams) the activity of Pt/Cu SAA nanoparticle catalysts for acetylene hydrogenation is reduced twofold, when compared to monometallic Pt nanoparticles there was a 15-fold activity decrease [18] It follows that the weak binding of CO to single, isolated Pt atoms in Pt/Cu SAAs compared to that on pure Pt is sufficient to give this material notable resistance to CO poisoning, despite a relatively low number of active sites compared to monometallic Pt [18]. Our study should provide a valuable guide for the choice of catalytically active and selective binary alloy combinations that exhibit improved CO tolerance and structural stability
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