The application of infrared spectroscopy to probe the surface morphology of alumina-supported palladium catalysts

Abstract

Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1-7.3 wt %). Collectively, this series of catalysts provides a range of metal particle sizes (1.2-8.5 nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)(111) and (111)(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1 wt % loaded catalysts, with the relative inactivity of the catalyst prepared from palladium(II) chloride attributed to a diminished hydrogen supply due to decoration of edge sites by chlorine originating from the preparative process. It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.

In situ study of the formation and stability of supported Pd2Ga methanol steam reforming catalysts

“…Reduction of the catalyst at 303 K and subsequent adsorption of CO yielded a spectrum with a C-O stretching region typical of CO adsorbed on metallic Pd particles in the low-nm-size regime [49,50]. This is in agreement with XRD measurements which showed that reduction at room temperature is sufficient to reduce the initially present PdO (from calcination) to metallic Pd.…”

“…Besides the obvious contribution of gas-phase CO with a minimum at 2143 cm -1 , we observed three distinct bands in the carbonyl region. The signal at 2091 cm -1 can be assigned to CO adsorbed linearly on metallic Pd atoms, the one at 1986 cm -1 to bridge-bonded CO on edges/steps or (100) facets and the one at 1926 cm -1 corresponds to hollow-or bridge-bonded CO on (111) facets [42,49]. Reduction of the catalyst at temperatures of 423 and 523 K and subsequent adsorption of CO already indicate pronounced changes in the outermost surface layer of the (inter)metallic particles.…”

The application of a supported palladium catalyst for the hydrogenation of aromatic nitriles

“…A maximum CO capacity of 9.24 x 10 -5 moles CO/g catalyst was obtained. Assuming a CO:Pd (s) ratio of 1:2 [29], this equates to 1.11 x 10 20 Pd(s) atoms/g (cat) , which corresponds to a Pd dispersion of 39 % and a mean particle size of 2.8 nm [29]. TEM analysis (JEOL 1200EX) reveals a narrow particle size distribution centred around 2.5 nm in diameter, in good agreement with the chemisorption measurements.…”

Directing reaction pathways by catalyst active-site selection using self-assembled monolayers

“…To directly test the availability of specific adsorption sites, diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) was performed under variable pressures of carbon monoxide; in these experiments, peak positions are readily attributed to CO binding in threefold, bridging and top sites 24 . We previously found that on unmodified 5 wt% catalyst, increasing thiol density restricted adsorption on terrace sites (Fig.…”