Sequential precursors in dissociative chemisorption:O2on Pt(111)

Abstract

A physisorbed state for 02 on Pt(111) has been identified by XPS spectroscopy and demonstrated to be an intrinsic precursor to molecular chemisorption. Combined with the prior demonstration that the molecularly chemisorbed species is a precursor to dissociation, we propose a model of sequential precursors to describe dissociation.Precursor species have often been invoked to describe the kinetics of adsorption, desorption, and dissociative chemisorption of gases on surfaces. This concept, dating from the seminal papers of Langmuir' and Lennard-Jones, is based on the recognition that there may exist a weakly bound molecular state between the gas phase and the strongly chemisorbed or dissociated state. For example, if an incident gas molecule is first trapped in this weakly bound state, fully accommodates with the surface, and then subsequently chemisorbs or dissociates, then this weakly bound and usually mobile species is labeled a precursor. Kinetic models employing such precursor species have been very successful in describing a wide variety of adsorption phenomena, e.g. , the independence of the sticking coe%cient S with coverage e and its decrease with increasing surface temperature T, and initial gas translational energy E;. Two types of intrinsic precursor species (precursors over a bare metal surface) have often been suggested. First, a physisorbed species may be a precursor to molecular chemisorption. Attempts to isolate such species at low T, have proven both successful ' and unsuccessful, depending upon the exact system. Second, there is accumulating evidence that a molecularly chemisorbed species may act as a precursor to dissociative chemisorption.We will show here that both types of precursors exist for 02-Pt(111). Therefore, the kinetics of dissociation requires a description in terms of two sequential precursors.The adiabatic potential-energy surface (PES) describing the interaction of 02 with a metal surface can be constructed from three separate diabatic interactions. These three diabatic surfaces extrapolate asymptotically to the M +02, M+ 02, and M +20 dissociation limits, where M is the metal. This PES can, in principle, have three distinct wells corresponding to physisorption, molecular chemisorption formed by charge transfer from the metal to the 02, and the atomic well formed by dissociation. Whether barriers actually exist separating these wells depends on the details of the energetics. For some metals, only the dissociated state can be isolated at low T, . ' In other cases, a molecularly chemisorbed state is also observed, while in still other cases, all three states have been isolated.For 02 interacting with Pt(111), both the atomic state and a molecularly chemisorbed state (resembling a negative ion) have been well characterized by a variety of surface spectroscopies.No physisorbed species has been previously observed. The thermal conversion of the molecularly chemisorbed species into the atomic one, the sensitivity of the initial dissociative chemisorption probability So to step density,...

Continuum and Quantum-Chemical Modeling of Oxygen Reduction on the Cathode in a Solid Oxide Fuel Cell

“…Compared to the oxygen interactions with metal-oxide surfaces, theoretical studies on metallic surfaces (2PBs) have been more extensively reported as summarized in [101]. In order to examine the oxygen reduction reaction at the 2PBs in SOFC applications, Pt [133][134][135][136][137][138][139][140][141][142][143][144][145][146][147] and Ag [116,[148][149][150][151][152][153][154][155][156][157][158] cathode materials will be discussed. It is well known that different surface morphologies, subsurface elements and surface charges induce d-band shifting of the metal electrodes and change O 2 dissociation barriers in different surface orientations [130].…”

The Mechanism of Forming H2O from H2 and O2 over a Pt Catalyst via Direct Oxygen Reduction

“…Various phases of atomic and molecular oxygen have been proposed to interpret electron energy loss spectroscopy (EELS) [31][32][33], thermal desorption spectroscopy (TDS) [32][33][34][35][36], near-edge X-ray-absorption spectroscopy [37][38][39], and photoemission spectroscopy [31,34] . This corresponds to the current bridge state, which is considered to form a covalent bond from each O to an on-top site of Pt.…”

Ab Initio Calculations of Intermediates of Oxygen Reduction on Low-Index Platinum Surfaces

“…Adsorbed molecular oxygen is found at 150 K; at higher temperatures dissociation takes place. 7 Previous spectroscopic studies identified three distinct chemisorbed states of oxygen on the Pt͑111͒ surface. Two of them are adsorbed molecular species, the superoxo-like (O 2 Ϫ ) state is formed at bridge sites with an intramolecular bond order of 1.5 and a vibrational frequency of 870 cm…”

“…From the experimental point of view, there is evidence for the dissociative adsorption of O 2 at the platinum-ultrahigh vacuum ͑UHV͒ interface at room temperature, 7 but there is no evidence for the direct cleavage of the O-O bond at the electrochemical interfaces.…”