The effect of chlorine treatment on the dispersion of platinum metal particles supported on silica and γ-alumina

Abstract

The reaction of Pt metal particles on SiO 2 (Aerosil) and γ-Al 2O 3 supports with Cl 2 (0.2–100% by vol.) in N 2 or He in a flow system at temperatures ( T r) in the range 320–700 K has been studied with XRD, TEM, uv-diffuse reflectance spectroscopy, and temperature-programmed reduction (TPR) of the products. On SiO 2, for 5–25% Cl 2 only PtCl 2 results, forming large (up to 500 nm diam.) singlecrystal sheets for 500 < T r ≤ 700 K and increasingly fine grained to amorphous aggregates for 500 > T r ≥ 320 K. For 25–100% Cl 2 various products form depending on T r: (i) 320 ≤ T r < 520 K a Pt(IV) chloride/support-surface complex [ Pt( IV) Cl x ]; (ii) 520 ≤ T r < 590 K PtCl 4, (needles); (iii) 590 ≤ T r < 670 K PtCl 3 (needles); and (iv) 670 ≤ T r ≤700 K PtCl 2 (chunky). For T r ≥ 600 K Pt is increasingly lost from the support for any concentration of Cl 2. On γ-Al 2O 3, only support-surface bound [Pt(IV)Cl x ] is formed and no Pt is lost from the support for T r ≤ 700 K. On both supports, Pt metal is redispersed—its mean particle size is decreased on Cl 2 treatment followed by reduction—only if [Pt(IV)Cl x ] is formed. Physically distinct chlorides, crystalline or amorphous, always reduce to low surface area aggregates.