Water Chemistry on Model Ceria and Pt/Ceria Catalysts

Abstract

We have studied the interaction of water with stoichiometric CeO 2 (111)/Cu(111), partially reduced CeO 2−x /Cu(111), and Pt/CeO 2 / Cu(111) model catalysts by means of synchrotron−radiation photoelectron spectroscopy (SRPES), resonant photoemission spectroscopy (RPES) at the Ce 4d edge, infrared reflection absorption spectroscopy (IRAS), and density functional (DF) calculations. The principal species formed during adsorption of water at 160 K on CeO 2 (111) films is chemisorbed molecular water. On the surface of CeO 2−x water partially dissociates yielding hydroxyl groups. By use of core-level PES, differentiation between chemisorbed water and hydroxyl groups is complicated by the overlap of the corresponding spectral features. Nevertheless, we determined three characteristic indicators for OH groups on ceria: (i) the presence of 1π and 3σ states in valence band (VB) PES; (ii) an increase of the binding energy (BE) separation between the O 1s spectral components of lattice oxygen and OH/H 2 O; (iii) an increase of the amplitude of the Ce 3+ resonance in RPES. Chemisorbed water desorbs below 400 K and hydroxyl groups vanish at 500 K. The most favorable configurations of chemisorbed water and hydroxyl groups have been investigated by DF calculations. Both CeO 2 (111) and CeO 2−x involve strongly tilted H 2 O and OH species which complicate their detection by IRAS. On Pt/CeO 2 , water adsorbs molecularly at 160 K but undergoes partial dissociation during annealing. The dissociation of water is accompanied by spillover of hydrogen to ceria and formation of hydroxyl groups between 180 and 250 K. Above 250 K, decomposition of hydroxyl groups and reverse spillover of hydrogen from ceria to Pt occurs, followed by desorption of molecular water.

Fast vacancy-mediated oxygen ion incorporation across the ceria–gas electrochemical interface

“…To the best of our knowledge, this is the first direct experimental evidence showing the fast vacancymediated ion incorporation reaction. This observation also agrees with the density-functional theory calculations under equilibrium conditions 35,[40][41][42][43][44] Migration and segregation of oxygen vacancies. Having shown quantitatively that OH O incorporates into surface oxygen vacancies in ceria and that the reaction is fast for the range of overpotentials probed, we now turn to the near-surface transport of oxygen vacancy from the bulk to the surface (Fig.…”

“…This impurity peak, which reaches a maximum atomic fraction of 14% of the oxygen photoelectron spectra, does not change appreciably with bias over the course of the experiment. Finally, the photoelectron peak with the highest binding energy (B2.2 eV higher than the lattice oxygen peak) is assigned to adsorbed OH À groups, consistent with prior photoemission and temperature-programmed desorption investigations [34][35][36][37] . We note, however, the binding energy is considerably larger than that reported by Zhang et al 32 Moreover, we observed that the binding energy difference between the OH and the lattice O changed with bias ( Supplementary Fig.…”

“…We identified a total of three oxygen species at an information depth of B0.6 nm (for the definition of surface sensitivity see Supplementary Methods). Based on earlier photoemission studies, we assigned the most intense peak (shown in blue and denoted as O) to the bulk-like lattice oxygen 34,35 . A smaller peak B1.3 eV higher in binding energy (shown in grey and denoted as O') is attributed to oxygen bonded to segregated silicon impurities ( Supplementary Fig.…”

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

“…101,141,142 DFT calculations were employed to investigate water dissociation at the ceria (111) surface 143 and at the ceria/liquid interface 144 as well as to elucidate the charge transfers between Pt particles and the ceria support resulting from water dissociation. The influence of the aqueous environment on the reaction mechanism, thermodynamics and kinetics was investigated by means of ab initio molecular dynamics (MD) simulations.…”

Structuring Pt/CeO2/Al2O3 WGS catalyst: Introduction of buffer layer

“…However, on partially reduced CeO2(111) surfaces water dissociation takes place readily [52]. A significant enhancement of the water splitting process on partially reduced ceria has been already reported [38,[54][55][56][57].Recently, Anarifard et al [53] found for Pt/CeO2…”