Carbon Monoxide Poisoning Resistance and Structural Stability of Single Atom Alloys

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.

Intermetallic Compounds as an Alternative to Single‐atom Alloy Catalysts: Geometric and Electronic Structures from Advanced X‐ray Spectroscopies and Computational Studies

“…Chemical bonding between the constituents in alloys alters the valence electronic states that are responsible for the catalytic behavior of the metals. Experimental and theoretical works have shown that the unique isolated geometries of metals in SAA's can lead to electronic perturbations, which can in turn lead to changes in adsorption enthalpies and reaction barriers . To determine the effect of atomic ordering on the valence states of Pt, in situ RIXS measurements and DFT calculations were performed.…”

“…Experimental and theoretical works have shown that the unique isolated geometries of metals in SAA's can lead to electronic perturbations, which can in turn lead to changes in adsorption enthalpies and reaction barriers. [12,27,[37][38][39] To determine the effect of atomic ordering on the valence states of Pt, in situ RIXS measurements and DFT calculations were performed. The 2-D RIXS planes show the incident photon energy (x-axis) and energy loss (y-axis) versus the intensity of emitted photons ( Figure 5).…”

“…SAA catalysts have been shown to be active and selective for both the hydrogenation and dehydrogenation of various hydrocarbons and oxygenates . Several SAA's have also been reported to be resistant to poisoning by CO due to greatly reduced adsorption enthalpies on the isolated atoms compared to pure metal surfaces . In all studies, the enhanced catalytic function has been attributed to the unique geometric and electronic properties of the isolated metal atoms.…”

“…In all studies, the enhanced catalytic function has been attributed to the unique geometric and electronic properties of the isolated metal atoms. In reported SAA's, Ni, Pd, or Pt has commonly been chosen as the catalytic component while a coinage metal (i. e. Cu, Ag, Au) was selected as the host . However, mixtures of these elements typically form solid solutions where each metal is randomly distributed throughout the bulk lattice; so high host‐to‐catalyst atomic ratios are required to ensure complete isolation of active sites.…”

“…[7][8][9][10][11] Several SAA's have also been reported to be resistant to poisoning by CO due to greatly reduced adsorption enthalpies on the isolated atoms compared to pure metal surfaces. [12][13] In all studies, the enhanced catalytic function has been attributed to the unique geometric and electronic properties of the isolated metal atoms. In reported SAA's, Ni, Pd, or Pt has commonly been chosen as the catalytic component while a coinage metal (i. e. Cu, Ag, Au) was selected as the host.…”