Abstract
X-ray Absorption Near Edge Spectroscopy (XANES) shows that Pt in Pt nitrate solutions has +4 oxidation state. Extended X-ray absorption fine structure (EXAFS) reveals each Pt is coordinated to an average of 5.4(±0.5) oxygen atoms with a PtO bond distance of 1.99(5)Å. Each Pt center has an average of 2.9 neighboring Pt atoms with a Pt···Pt non-bonding distance of 3.08(3)Å. A Pt[μ-O(H)]2Pt ring with two Pt atoms linked by two O or OH ligands is proposed to be the primary structure moiety and building blocks for more complex oligomeric structures.Pt(IV) nitrate solution is prone to hydrolysis upon dilution forming an amorphous reddish-brown precipitate PtO2·xH2O. Pt nitrate has more complete adsorption on alumina than H2PtCl6 with no significant change of Pt coordination and oxidation state observed upon adsorption. Upon calcination to 500°C, Pt complexes undergo structural changes but remain as Pt(IV). H2 TPR of Pt nitrate on alumina is consistent with Pt(IV) to Pt metal reduction.The rapid and strong adsorption of Pt nitrate results in Pt depositing only at the outer surface of catalyst washcoats, in sharp contrast to uniform distribution from H2PtCl6. Pt/Rh three-way catalysts made from Pt nitrate/Rh nitrate have equivalent light-off but better hydrocarbon, CO and NOx conversions than those made from H2PtCl6/RhCl3 solutions.
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