14164
J. Phys. Chem. C 2010, 114, 14164–14172
Characterization of Hydrous Palladium Oxide: Implications for Low-Temperature Carbon Monoxide Oxidation Stewart F. Parker,* Keith Refson, Alex C. Hannon, and Emma R. Barney ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom
Stephen J. Robertson AdVanced Material Group, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom
Peter Albers AQura GmbH, Rodenbacher Chaussee 4, D-63457 Hanau, Germany ReceiVed: April 28, 2010; ReVised Manuscript ReceiVed: July 21, 2010
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 · H2O 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, RuO2 · xH2O, 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 CO2. 1. Introduction Supported palladium catalysts are widely used for a variety of oxidation reactions including the following: complete oxidation of hydrocarbons (for elimination of volatile organic compounds from aerial effluents),1–3 alcohols and polyols4 (particularly glycerol5), hydrogen peroxide synthesis,6 and carbon monoxide oxidation.7–10 Palladium catalysts are often used in combination with other metals such as gold11 and the choice of support strongly influences the catalytic properties.12 To isolate the effect of palladium, in a recent paper13 Oh and Hoflund investigated the low-temperature catalytic oxidation of carbon monoxide over hydrous and anhydrous palladium oxide powder. On the basis of X-ray photoelectron spectroscopy (XPS) and microreactor testing they proposed that the formulation of hydrous palladium oxide as PdO · xH2O, x ≈ 0.80, was incorrect and that the material was better described as Pd(OH)2 with a small admixture of anhydrous PdO. In this paper, we use a combination of diffraction methods, vibrational spectroscopy, electron microscopy, and periodic density functional theory (periodic-DFT) to investigate the structure of hydrous palladium oxide and to discriminate between the possible formulations as a hydrous oxide, a hydrate, or a hydroxide. 2. Experimental Section Materials. Hydrous palladium(II) oxide (PdO · xH2O, Pd 77.81%, purity 99.9%) and anhydrous palladium(II) oxide (PdO, Pd 85%, purity 99.9%) were purchased from Alfa Aesar. The * To whom correspondence should be addressed. E-mail: stewart.parker@ stfc.ac.uk.
water content of the as received material and after heating under vacuum at 100 °C was determined by thermogravimetric analysis (TGA, Perkin-Elmer Pyris 1, Ar purge gas at 20 cm3 min-1 and ramped at 5 deg min-1 from 50 to 800 °C). This gave values of x ) 0.92 and 0.31 for the as received and the 100 °C treated materials, respectively. Particularly for the as received material this is a lower bound, since it was losing water immediately on loading into the TGA apparatus. X-ray fluorescence measurements (XRF, Panalytical MiniPal4) were performed on both the as received sample and after heating. Surface area measurements were carried out with the single point Brunauer-Emmett-Teller (BET) method (Micromeritics Tristar 3000 with high purity N2 as the adsorbate) and gave values of 93 ( 3 m2 g-1 for the as received hydrous PdO and
