Photoinduced Processes in Heterogeneous Gas−Solid Systems. Temperature Dependence (100−600 K) and Modeling of a Surface Chemical Reaction on Zirconia that Triggers Photophysical Events in the Solid

Abstract

We report studies on photoinduced processes that take place in gas-solid heterogeneous systems. In particular, we examine the chemical reaction (i.e., chemical relaxation pathway) between hydrogen (or methane) and the surface of ZrO 2 particles which occurs after the solid was preirradiated (vacuo or oxygen) and the intrinsic phosphorescence from zirconia had terminated. Introduction of H 2 yields an after-glow (light pulse referred as photoinduced chesorluminescence, PhICL) that is caused by chemical interaction between the photoactivated surface of the photocatalyst and the H-containing molecules in the heterogeneous gas-solid system. Unlike hydrogen, methane does not yield a detectable PhICL emission. The PhICL emission for hydrogen decays via biphasic kinetics (τ 1 ) 0.9 s and τ 2 ) 10 s; ambient temperature). Reactions were also investigated in the temperature range of 75-600 K. For a nonpreirradiated surface, the H-containing molecules physisorb with binding energy E phys ) 33 kJ mol -1 (0.34 eV) for H 2 and 52 kJ mol -1 (0.54 eV) for CH 4 . After preirradiation of ZrO 2 , the molecules chemisorb dissociatively yielding free radicals (H • for H 2 , and CH 3 • for methane, the other H • is trapped) through an activated process; activation energies E a ) 30 kJ mol -1 (0.31 eV) for H 2 and 32 kJ mol -1 (0.33 eV) for CH 4 if the process occurs by a Langmuir-type pathway; if the process occurs by an Eley-Rideal mechanism, E a ) 41 kJ mol -1 (0.43 eV) for H 2 and 46 kJ mol -1 (0.48 eV) for CH 4 . The data were modeled through a sequence of differential equations. A mechanism is proposed that involves shallow traps and deep energy traps for electrons to explain the photophysical events triggered by the surface chemical reaction. In essence, surface chemical reactions cause the release of the energy stored during the preirradiation stage of the solid. This results in photoinduced formation of metastable defects in the solid (e.g., O s -• ) together with deep electron traps (F centers) that lead to both chemical and physical relaxation of the system.

Photoinduced Radical Processes on the Spinel (MgAl2O4) Surface Involving Methane, Ammonia, and Methane/Ammonia

“…Either or both processes can lead simultaneously to the PhICL effect during the postadsorption of methane on the photoexcited surface of the solid, as observed in various wide band gap metal oxides. 9,10 Clearly, formation of ethane occurs through recombination of the methyl radicals produced in eq 1. Recombination of photogenerated radicals has thus led to formation of a molecule more complex, in this particular instance, ethane, than the initial molecule methane.…”

Photoreactions occurring on metal-oxide surfaces are not all photocatalytic

“…However, this level can differ from the one in vacuo depending on the extent to which the surface reaction affects the concentration of charge carriers. Also, an additional influence on the stationary level of the photoluminescence might originate from the chemiluminescence that accompanies the surface reaction; for example by the photoinduced chesorluminescence, PhICL, phenomenon [48,49].…”

Photochromism of a Spiropyran in the Presence of a Dendritic Fibrous Nanosilica; Simultaneous Photochemical Reaction and Adsorption

“…Photoinduced adsorption, where photochemical reaction was combined with adsorption (diffusion), is a phenomenon with possible practical application. Two types of photoinduced adsorption of organic molecules onto solids have been reported; one is the adsorption of a photoresponsive molecule as adsorbate, − and the other is the adsorption of photochemically inert species onto a photoresponsive adsorbent (composed of photoresponsive molecule stably immobilized on a solid). − We have been interested in the photoinduced adsorption phenomena onto inorganic nanospace materials to find several unique systems. − The quantitative and kinetic evaluation is required in order to understand the phenomena and to find the practical applications. In this study, a photoinduced adsorption phenomenon, which is the reaction based on the photochemical isomerization of a spiropyran in a solution to a merocyanine and the simultaneous adsorption of the photochemically formed merocyanine into dendritic fibrous nanosilica (DFNS), also known as KCC-1, was investigated using 1-(2-hydroxyethyl)-3,3-dimethylindolino-6′-nitrobenzopyrylospiran (SP) as photochromic molecule and DFNS, as the adsorbent.…”