Abstract
Understanding platinum (Pt) speciation on catalysts is crucial for the design of atom-efficient materials and optimized formulations. The adsorption of carbon monoxide (CO) as a probe molecule is widely used to reveal Pt dispersion and structures, yet the assignment of IR bands is not straightforward, hindering determination of the nature of the surface sites or ensemble involved. CO adsorption was studied here over a zirconia-supported Pt catalyst. Specific sites at the interface between Pt and the support were highlighted, giving rise to an unusual band around 1660 cm-1 that could be confidently assigned to a Pt2-CO bridging carbonyl interacting head-on with a support surface hydroxyl. This adduct was yet unstable in the present conditions and was converted into a linear and bridged carbonyl bound only to Pt. Such sites are potentially important for bifunctional reactions requiring both metal and acid/base properties, particularly those occurring at the metal-support perimeter. Such adducts have probably been mistaken for carbonate-type species in many past contributions and could potentially represent crucial reaction intermediates for CO oxidation and the water-gas shift reaction.
Highlights
Pt-based materials are widely used in heterogeneous catalysis and electrochemistry.Understanding metal speciation is crucial for the design of atom-efficient materials and optimized formulations 1,2,3
Lateral interactions imply band intensity transfer that can make spectral interpretation difficult, even at low surface coverage if island formation takes place. 5,6 Lateral interactions were shown to be markedly decreased in the case of intermetallic Pt-Sn nanoalloys, in which Pt atoms are spatially separated by Sn atoms
The Diffuse Reflectance Fourier Transform Infra red Spectroscopy (DRIFTS) spectra of the ZrO2 support and Pt/ZrO2 heated for 30 min in He at 50°C were similar in nature (Fig. 3)
Summary
Pt-based materials are widely used in heterogeneous catalysis and electrochemistry. Understanding metal speciation is crucial for the design of atom-efficient materials and optimized formulations 1,2,3. 4 The stretching frequency ν(CO) is affected, among others, by the nature (e.g. coordination number, oxidation state) of the Pt sites present, CO adsorption modes and lateral interactions between adsorbed CO (so called “dipole-dipole coupling”). = 9, which is 2096 cm-1 as proposed by the model of Kappers and van der Maas.8 This difference is probably due to the occurrence of dipole-dipole effects in the case of the data reported on Pt(111) leading to a large increase of the wavenumber value. CO bridged between Pt and sites of the alumina support that adsorbed at 1756 cm-1 were proposed by Ferri et al 16 Dilara and Vohs assigned bands located at 1680 and 1570 cm-1 to a head-and-tail CO adduct formed between Pt and a Zr4+ cation, while linear and bridged CO solely adsorbed on Pt absorbed at and 1815 cm-1, respectively (Fig. 1.A). Such sites are potentially important for bifunctional reactions requiring both metal and acid/base properties, those thought to be occurring at the metal - support perimeter
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