Abstract
γ-Alumina is widely used as an oxide support in catalysis, and palladium nanoparticles supported by alumina represent one of the most frequently used dispersed metals. The surface sites of the catalysts are often probed via FTIR spectroscopy upon CO adsorption, which may result in the formation of surface carbonate species. We have examined this process in detail utilizing FTIR to monitor carbonate formation on γ-alumina and zirconia upon exposure to isotopically labelled and unlabelled CO and CO2. The same was carried out for well-defined Pd nanoparticles supported on Al2O3 or ZrO2. A water gas shift reaction of CO with surface hydroxyls was detected, which requires surface defect sites and adjacent OH groups. Furthermore, we have studied the effect of Cl synthesis residues, leading to strongly reduced carbonate formation and changes in the OH region (isolated OH groups were partly replaced or were even absent). To corroborate this finding, samples were deliberately poisoned with Cl to an extent comparable to that of synthesis residues, as confirmed by Auger electron spectroscopy. For catalysts prepared from Cl-containing precursors a new CO band at 2164 cm−1 was observed in the carbonyl region, which was ascribed to Pd interacting with Cl. Finally, the FTIR measurements were complemented by quantification of the amount of carbonates formed via chemisorption, which provides a tool to determine the concentration of reactive defect sites on the alumina surface.
Highlights
Introduction γAlumina is widely used as oxide support in catalysis
In a previous report [26] based on isotopic IR studies, we proposed a mechanism of carbonate formation (Scheme 1), which will serve as starting point for the current contribution
temperature programmed reduction (TPR) measurements of calcined Pd–alumina catalysts displayed a maximum in hydrogen consumption at 400 K, indicating that Pd is fully reduced at about 400 K
Summary
Commercial γ-alumina obtained from Sasol Germany GmbH (Puralox SBA 200, specific surface area 226 m2/g) was used in this study. For preparing Pd on zirconia catalysts zirconium hydroxide (MEL, ZXO 880/01) was calcined in static air at 773 K for 3 h (heating ramp 2°/min) to obtain monoclinic zirconia (specific surface area 82 m2/g). Three Pd–ZrO2 catalysts with a 5 wt% Pd loading were again prepared by incipient wetness impregnation using as precursors either an aqueous solution of Pd(II)nitrate dihydrate (Fluka), Pd(II)chloride (Fluka) dissolved in water, or Pd(II)acetate (Fluka) dissolved in toluene. The pure Al2O3 and ZrO2 supports were deliberately poisoned with chloride via impregnation with 1 N aqueous HCl solution, leading to approximately 2 wt% of Cl present at the surface, as determined by AES. The following denomination will be used below for alumina supported catalysts: Pdx with x = Pd loading (in wt%), “nt” for the nitrate precursor, “cl” for chloride precursor, “ac” for acetate precursor, “/cl” for samples treated with Cl upon deliberate poisoning. The 5% Pd–zirconia catalysts were labeled as PdZnt, PdZac and PdZcl for the samples prepared from different precursors, and “/cl” for the deliberately Cl-poisoned PdZac sample
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