Characterization of Hydrous Palladium Oxide: Implications for Low-Temperature Carbon Monoxide Oxidation

Abstract

The structure of hydrous palladium oxide has been investigated by powder diffraction with both X-rays and neutrons, transmission electron microscopy, inelastic neutron scattering spectroscopy, infrared spectroscopy, and periodic-DFT calculations. The results clearly show the formulation as PdO • H 2 O is correct rather than as Pd(OH) 2 as has been proposed previously. The material is best described as ∼18 Å diameter particles of nanocrystalline PdO with a monolayer or so of hydroxyls capped by 4-7 layers of water. The very small nanocrystallites are agglomerated into larger clusters which reduces the effective surface area. This structure is very similar to that proposed for hydrous ruthenium oxide, RuO 2 • xH 2 O, x ) 2, and for water on anatase and rutile. This model provides a quantitative explanation for why hydrous palladium oxide deactivates three times faster at 25 °C than at 100 °C in the low-temperature oxidation of CO to CO 2 .

The fine structure of Pearlman's catalyst

“…The difference spectra show a broad band centred at ~500 cm -1 and a feature at 960 cm -1 that is poorly defined for the 5 wt% Pd/C sample but sharp for the 20 wt% Pd/C sample. Comparison with the spectrum of hydrous palladium oxide dried at 100 °C [8], Figure 6, shows a striking Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 2012 similarity and the spectra are assigned accordingly: the broad feature is the librational modes of a disordered water layer and the 960 cm -1 feature is the Pd-O-H bending mode of surface hydroxyls. The assignment to surface hydroxyls is supported by periodic-DFT calculations of hydroxyls on PdO 8 and that the bending mode of hydroxyls on alumina 25 and silica 26 occur in this region where water does not have any modes.…”

“…The catalyst is commonly written as palladium (II) hydroxide supported on carbon, Pd(OH) 2 /C. However, we have recently shown 8 that a typical commercial sample of unsupported hydrous palladium oxide consists of a poorly crystalline core of PdO ~1.8 nm in diameter capped with a monolayer of hydroxyls and all covered with several layers of water. Thus it is a core-shell structure rather than uniform stoichiometric Pd(OH) 2 .…”

Neutron Scattering Investigations of Hydride Species in Heterogeneous Catalysis

“…[44][45][46] Advancesi nn eutron spectroscopy also allow characterizationo f the interaction of species with ac atalyst under operando conditions. [47,48] INS spectra of Ni/Al 2 O 3 catalysts tested for dry reforming and steam reforming of methane showedv ast differences in the concentration of surfaceh ydroxyl groups,a nd the type of hydrocarbonaceous overlayers. Dry reforming of two differently treated Ni/Al 2 O 3 catalysts showedm ostly aliphatic carbon on one sample, whereas the other sample presented mostly aromatic carbon.Q uantification of hydrogen in the hydrocarbonaceous overlayers of Ni/Al 2 O 3 catalysts showedC /H ratios ranging from 160:1 to 11 690:1;t hus highlighting the good hydrogen cycling capabilitieso ft he catalysts, butp oor (and differ-ent) carbon cycling capabilities.…”

“…As reviewed by Parker and co‐workers, since neutron scattering can probe the interaction of hydrogen with catalysts, one can perform unique characterization, such as observation of the C−H/N−H/O−H stretching region, and the quantification of adsorbed hydrogen . Advances in neutron spectroscopy also allow characterization of the interaction of species with a catalyst under operando conditions …”

Catalysts, neutrons and calculations

“…H 2 O nothing was known about the material. We investigated [7] hydrous palladium-oxide with a combination of powder diffraction (neutron and X-ray), vibrational spectroscopy (infrared and INS on TOSCA and MARI) and electron microscopy with assignments provided by periodic-DFT calculations. Comparison of the INS spectra of hydrous palladium-oxide with those of a hydroxide and the phases of ice showed a striking resemblance to high-density amorphous ice with an additional peak at 962 cm -1 .…”