Aspects of temperature-programmed analysis of some gas–solid reactions. Part 1.—Dispersion effects in temperature-programmed bulk reduction and temperature-programmed desorption

Abstract

The reduction of hexachloroplatinic acid and the desorption of adsorbed hydrogen have been followed by temperature-programmed methods. Both the temperature-programmed bulk reduction (t.p.b.r.) and the temperature-programmed desorption (t.p.d.) profiles reveal several maxima at temperatures which vary with the ultimate Pt dispersion and the support used. Reductions commenced at subambient temperatures and good separation of all three t.p.b.r. peaks was only achieved on starting t.p.b.r. at least as low as 223 K. T.p.b.r. temperatures of maximum reduction rates for samples supported on porous silica decreased with increasing dispersion but were higher for the supported rather than the unsupported state. The maximum quantities of hydrogen consumed in t.p.b.r. exceed that expected for complete reduction in some silica-supported samples, but the contribution of the high-temperature γ-peak (possibly associated with spillover) increased with increasing Pt dispersion. T.p.d. liberated less gaseous hydrogen (41–84%) from silica-supported samples than the total quantity of preadsorbed hydrogen, but the percentage increased (and the temperatures at which desorption rates were a maximum decresed) as the Pt dispersion increased. Alumina- and titania-supported samples produced more complex t.p.d. and t.p.b.r. profiles than those of silica. It is argued that it is not justified to interpret these or other equivalent temperature-programmed profiles in terms of processes occurring on chemically distinct and distinguishable solid or surface states. The activation energy for diffusion is critically important and diffusion may be too slow in t.p.b.r. and too fast in t.p.d. to allow temperature-programmed profiles to arise from processes occurring to such distinguishable and identifiable species. In t.p.d. surface residence times are also important in defining the fraction of adsorbate subsequently desorbed.