The effect of chlorine treatment on the dispersion of platinum metal particles supported on silica and $gamma;-alumina
1985; Elsevier BV; Volume: 92; Issue: 1 Linguagem: Inglês
10.1016/0021-9517(85)90237-4
ISSN1090-2694
Autores Tópico(s)nanoparticles nucleation surface interactions
ResumoThe reaction of Pt metal particles on SiO2 (Aerosil) and γ-Al2O3 supports with Cl2 (0.2–100% by vol.) in N2 or He in a flow system at temperatures (Tr) 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 SiO2, for 5–25% Cl2 only PtCl2 results, forming large (up to 500 nm diam.) singlecrystal sheets for 500 < Tr ≤ 700 K and increasingly fine grained to amorphous aggregates for 500 > Tr ≥ 320 K. For 25–100% Cl2 various products form depending on Tr: (i) 320 ≤ Tr < 520 K a Pt(IV) chloride/support-surface complex [Pt(IV)Clx]; (ii) 520 ≤ Tr < 590 K PtCl4, (needles); (iii) 590 ≤ Tr < 670 K PtCl3 (needles); and (iv) 670 ≤ Tr ≤700 K PtCl2 (chunky). For Tr ≥ 600 K Pt is increasingly lost from the support for any concentration of Cl2. On γ-Al2O3, only support-surface bound [Pt(IV)Clx] is formed and no Pt is lost from the support for Tr≤ 700 K. On both supports, Pt metal is redispersed—its mean particle size is decreased on Cl2 treatment followed by reduction—only if [Pt(IV)Clx] is formed. Physically distinct chlorides, crystalline or amorphous, always reduce to low surface area aggregates.
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