Phonon Anharmonicity of PdO Studied by Raman Spectrometry

Abstract

Raman spectra were measured for palladium oxide (PdO) at temperatures from 298 to 973 K. The first-order scattering B 1g and E g phonon modes show prominent frequency and line width dependencies on temperature. The phonon behaviors are described well by a model considering contributions from thermal expansion related quasiharmonicity and explicit lattice anharmonicity from both cubic and quartic phonon decay processes. Compared to the quasiharmonicity for the line shift, the explicit anharmonicity is found to be relatively smaller for the B 1g mode with the cubic anharmonicity leading the quartic term. The E g mode has a smaller line shift which mostly follows a quartic phonon decay mechanism while the cubic contribution is canceled by thermal expansion effects. The cubic anharmonicity dominates mostly the line broadening with more prominent quartic effects responsible for the larger broadening of the E g mode. Multiple potential asymmetric cubic and quartic anharmonic decay channels including down-conversion and up-conversion are identified with their characteristics discussed and correlated to the positive and negative anhamonicities found for the two phonon modes studied. ■ INTRODUCTIONPdO is a catalytically active material and has found extensive use in heterogeneous and homogeneous catalysis for chemical conversions, energy production, and pollution mitigation based on the reversible and energetic creation of Pd 2+ or oxygen vacancies in the oxide. 1−6 The material is also an appreciable source of oxygen because of its reversible equilibrium transformation with Pd at elevated temperatures within a sealed cavity. This characteristic has been employed as an oxygen reference material in the development of harsh environment oxygen sensors. 7 Owing to the technological interest in the oxide as a critical component in a variety of applications, the material has been actively studied. Crystalline PdO has a tetragonal cooperite structure (space group D 4h 9 -P4 2 /mmc) and is composed of edge-sharing chains of PdO 4 planes. 8,9 Strong resonance Raman scattering has been observed from PdO at room temperature, with the resonance enhancement possibly correlated to an exciton model of the electronic structure. 10 The temperature dependence of Raman scattering carries a great deal of information on the interactions of the vibrational phonon modes at the Brillouin zone center. The phonon line shift and broadening with temperature can be understood through a determination of the lattice anharmonicity properties. It has been shown in the literature that for a variety of materials studied the temperature-dependent phonon properties can be modeled by the effects of thermal expansion, phonon−phonon coupling, and perhaps electron−phonon coupling, depending on the specific material and electronic structures. 11−16 Anharmonic effects in crystals strongly affect a number of processes including energy transport, 17,18 structural phase transformation, 19 optical properties, 20 and electronic transport in devices, 21,22 which contrib...

Surface structure of cobalt, palladium, and mixed oxide‐based catalysts and their activity in methane combustion studied by means of micro‐Raman spectroscopy

“…No band shift in comparison to the reference material has been observed. In fact, as noted in the literature, PdO is a very Raman‐active oxide, but this is easily detectable only if the thickness of an oxide is greater than 1 nm . The dominant band at 647 cm −1 (B 1g mode) is symmetric and sharp, suggesting the formation of larger, bulk‐like PdO particles in the sample.…”

“…These are a weak band at 275, 337, 439, and 571 cm −1 ; a strong band at 646 cm −1 ; a broad band between 1,000 and 1,200 cm −1 ; and a weak band at 1282 cm −1 . The presence of many bands can be explained by the strong resonance Raman scattering that led to the appearance of many extra lines in the spectrum . The resonance signal enhancement observed at acquisition temperature can be correlated to an exciton model of the electronic structure, as discussed in the literature .…”

Glycerol valorization to lactic acid catalyzed by hydroxyapatite‐supported palladium particles

“…Fresh Pd 3 /HAP exhibits vibrational modes in the 645–651 cm −1 region, which is the main Raman mode of palladium oxide (PdO). However, this characteristic peak is absent in those catalysts with lower Pd loading . The peak corresponds to the first‐order scattering from the Raman‐active B 1g phonon mode, which involves only O motion along the c ‐axis .…”

“…Due to the carbon film used to support the sample, C manifests strongly in the spectrum. The results confirm the presence of Pd 0 and/or PdO x species on the catalyst surface, with a Pd content of 0.5, 1.0 and 3.3 wt% for Pd 0.75 /HAP, Pd 1.5 /HAP and Pd 3 /HAP catalysts, respectively . However, the ICP results are higher than the SEM‐EDS results, with the Pd loading amounts of 0.75, 1.5 and 3.0 wt% in Pd/HAP catalysts (Table : ICP).…”

Nanostructured Ceria-Based Materials: Effect of the Hydrothermal Synthesis Conditions on the Structural Properties and Catalytic Activity

“…A red-shift of the F 2g mode occurs, together with an asymmetric broadening of its line shape and an intensity decrease, when the samples are heated. This phenomenon has been observed for several materials [27][28][29] as well as for ceria nanocrystals [30,31], and has been explained considering two different contributions: thermal expansion and lattice anharmonicity [29]. While the former only influences the peak position, both Raman shift and linewidth variations are consequences of the phonons anharmonic decay at higher temperatures, which may involve three-or four-phonon interactions [31].…”