Structure and chemical properties of Pt nitrate and application in three-way automotive emission catalysts

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 Pt-O bond distance of 1.99(5) Angstrom. Each Pt center has an average of 2.9 neighboring Pt atoms with a Pt...Pt non-bonding distance of 3.08(3) Angstrom. A Pt[mu -O(H)](2)Pt ring with two Pt atoms linked by two O or OH ligands is;proposed to be the primary structure moiety and buildin g blocks fur more complex oligomeric structures. Pt(IV) nitrate solution is prone to hydrolysis upon dilution forming an amo rphous reddish-brown precipitate PtO2. xH(2)O. Pt nitrate has more complete adsorption on alumina than H2PtCl6 with no significant change of Pt coordi nation and oxidation state observed upon adsorption. Upon calcination to 50 0 degreesC, Pt complexes undergo structural changes but remain as Pt(IV). H -2 TPR of Pt nitrate on alumina is consistent with Pt(IV) to Pt metal reduc tion. 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 d istribution from H2PtCl6. Pt/Rh three-way catalysts made from Pt nitrate/Rh nitrate have equivalent light-off but better hydrocarbon, CO and NOx conve rsions than those made from H2PtCl6/RhCl3 solutions. (C) 2001 Elsevier Scie nce B.V. All rights reserved.