Combining IR Spectroscopy and Monte Carlo Simulations to Identify CO Adsorption Sites on Bimetallic Alloys

Abstract

The atomic distribution on the surface of alloys dictates the nature of the ensembles available as possible active sites during catalytic reactions. In the present work, an infrared spectroscopic study of carbon monoxide adsorption on the surface of AuPd/Pd(111) alloys, combined with Monte Carlo simulations of the surface and bulk atomic distribution, identifies the correct distribution of available surface adsorption sites. For gold coverages >0.9 monolayers (ML), CO adsorbs weakly on top of Au atoms and with higher adsorption energy on top of Pd atoms (CO top ), distributed mostly as monomers on the surface. For θ Au = 0.8−0.4 ML, Pd− CO top is the predominant species, even though several other sites with multiple coordination are available. The simulations show no perfect ordering of the surface but a slight tendency to form lines of Pd atoms, thus favoring the appearance of bridge but not 3-fold hollow sites. Using 13 CO: 12 CO isotopic mixtures, the frequency shifts due to chemical and intermolecular coupling effects has been determined for the CO top IR signal. These effects mostly cancel each other out, so that only small frequency shifts are seen, implying the presence of significant electronic/ligand effects. At θ Au < 0.5 ML, hollow sites are experimentally observed in agreement to the simulated model surfaces. Their IR absorption bands are tentatively distinguished as fcc and hcp hollow sites by correlating with the simulated distribution of Au and Pd atoms on subsurface sites, where for θ Au < 0.5 ML an enrichment by Au atoms is seen in the near-surface region.

Optimizing Formic Acid Electro-oxidation Performance by Restricting the Continuous Pd Sites in Pd–Sn Nanocatalysts

“…Actually, CO could not be removed at 0.4 V and the conformation of CO adsorbed on Pd changed with the number of continuous Pd atoms and CO coverage . Generally, threefold hollow CO (CO T ) formed on the Pd surface with continuous sites at first because of the thermal stability.…”

“…Actually, CO could not be removed at 0.4 V and the conformation of CO adsorbed on Pd changed with the number of continuous Pd atoms and CO coverage. 18 Generally, threefold hollow CO (CO T ) formed on the Pd surface with continuous sites at first because of the thermal stability. When the Pd surface was decorated by adatoms, the distribution of continuous Pd sites was such that the adsorption of CO T was limited.…”

“…45 Theoretically, the adsorbed CO and bidentate formate intermediates preferentially formed on three and two adjacent active sites, respectively, during FAOR, which are more stable than the monodentate formate intermediate formed on a single active site. 18,46 Therefore, restricting the continuous Pd sites by SnO x could promote the selectivity through the monodentate formate intermediate pathway to enhance the FAOR performance.…”

“…Different from the electronic effects, ensemble effects, resulted by adatoms decorated on the Pd surface with discontinuous Pd sites, mainly influence the conformation of adsorbed HCOOH and CO. , When the surface of Pd or Pt crystals are decorated with other atoms, the charge transformation between heterogeneous atoms may also alter the electronic state of Pd atoms at different levels . The threefold hollow CO (CO T ) and bridged CO (CO B ) are formed on the continuous Pd sites, so these conformations of CO, which are more stable than the linear CO, could be restricted when modified by adatoms. , Besides, the CO ads species on the surface of Pd has been detected and poisoned the active sites. , Therefore, tailoring ensemble effects could not only promote the FAOR activity but also enhance the antipoisoning capability.…”

“…17 The threefold hollow CO (CO T ) and bridged CO (CO B ) are formed on the continuous Pd sites, so these conformations of CO, which are more stable than the linear CO, could be restricted when modified by adatoms. 18,19 Besides, the CO ads species on the surface of Pd has been detected and poisoned the active sites. 20,21 Therefore, tailoring ensemble effects could not only promote the FAOR activity but also enhance the antipoisoning capability.…”