Fourier Transform Infrared Study of 1,1-Dichlorotetrafluoroethane Adsorption on an Alumina-Supported Pd Catalyst

Abstract

An FTIR investigation of CF3CFCl2 adsorption on 5% Pd/γ-Al2O3 at room temperature shows that three types of surface species form. Two of the species are identified as trifluoroethylidyne (CF3C⋮) and the third species is assigned to tetrafluoroethylidene (CF3CF). All three species are on the metal sites, not on the γ-Al2O3. The surface of the reduced fresh catalyst favors the formation of trifluoroethylidyne species. However, CF3CFCl2 adsorption on a catalyst preexposed to the CFC + H2 mixture at 473 K for 1 h, followed by reduction with H2 at 573 K for 1 h, results in the predominant formation of CF3CF. Both surface CF3CF and CF3C⋮ species readily interact with H2 at room temperature to form CF3CFH2 and CF3CH3.

A review of CFC and halon treatment technologies – The nature and role of catalysts

“…Direct spectroscopic evidence of dissociative adsorption of CFCs on Pd catalysts was obtained by Borovkov et al, who carried out a FTIR investigation of CF 3 CFCl 2 adsorption on 5% Pd/Al 2 O 3 at room temperature [101,115]. Figure 3 shows a typical FT-IR spectrum of a Pd/Al 2 O 3 catalyst exposed to gaseous CF 3 CFCl 2 at room temperature.…”

The catalytic transformation of chlorofluorocarbons in hydrogen on metal-based catalysts supported on inorganic fluorides

“…This transient reaction yielded products typical of C 2 H 4 addition on the surface carbene =CH 2 and fluorocarbene =CF 2 . A FTIR investigation of CF 3 CFCl 2 adsorption on Pd/Al 2 O 3 has shown different types of adsorbed species [33]. The fluorocarbene CF 3 -CF= was identified in great proportion on the Pd surface upon contacting a CFC+H 2 mixture at 473 K; on the fresh catalyst the trifluoroethylidyne CF 3 -C≡ was also present.…”

“…Many studies propose that the breaking of the first C-Cl bond is involved in the rate determining step [4,22,26,30,33]. In molecules containing several Cl atoms, after the first C-Cl has been broken the second C-Cl cleavage is very fast [26,33], reversible and this second chlorine may be exchanged with a pool of surface Cl atoms, as shown by isotopic labeling studies using 37 Cl [26]. The C-Cl bond cleavage is homolytic with very little charge redistribution, and occurs very early in the reaction coordinates [30,33].…”

“…In molecules containing several Cl atoms, after the first C-Cl has been broken the second C-Cl cleavage is very fast [26,33], reversible and this second chlorine may be exchanged with a pool of surface Cl atoms, as shown by isotopic labeling studies using 37 Cl [26]. The C-Cl bond cleavage is homolytic with very little charge redistribution, and occurs very early in the reaction coordinates [30,33]. Globally, the rate is dominated by the C-Cl bond strength [22,35], which increases upon substituting H and/or F for Cl.…”

“…However, the adsorbed species may exchange various bond multiplicity with the metal surface which leads to different ensemble size requirements [47]. It was thus shown that triply bonded ≡C-CF 3 species exists in larger proportion on fresh Pd/Al 2 O 3 than on the passivated one [33]. One might also anticipate that the C 3v symmetry of Pd(1 1 1) facet would favor the formation of ≡C-CF 3 .…”