Modeling and Experimental Examination of the Solonitsyn Memory Effect on the Surface of Wide Band Gap Metal Oxides § S. A. Polikhova, ² N. S. Andreev, ² A. V. Emeline, ‡ V. K. Ryabchuk, ² and N. Serpone* ,‡,⊥ Department of Photonics, Institute of Physics, St. Petersburg State UniVersity, St. Petersburg, Russia, Department of Chemistry & Biochemistry, Concordia UniVersity, 7141 Sherbrooke Street West, Montreal, Quebec, Canada H4B-1R6, and Dipartimento di Chimica Organica, UniVersita di PaVia, Via Taramelli 10, 27100 PaVia, Italy ReceiVed: August 21, 2003; In Final Form: NoVember 10, 2003 When the surface of a solid semiconductor or dielectric metal oxide (or other) specimen is preirradiated, the solid often retains its photochemical activity after termination of irradiation through formation of long-lived surface-active adsorption centers. This effect has two origins, viz., the so-called Kugel’sberg memory effect and the Solonitsyn memory effect. The former denotes preirradiation in the presence of the adsorbate molecules, whereas the latter refers to preirradiation in vacuo followed by subsequent introduction of adsorbate molecules into the reactor. This article reports results of detailed studies on the Solonitsyn memory effect in gas/solid heterogeneous systems with respect to photostimulated adsorption (i.e. reductive or oxidative adsorption) of molecular oxygen, molecular hydrogen, and methane on the surface of a dielectric metal oxide such as zirconia. The memory effect has been quantified for several metal oxides and alkali halides by means of an experimentally determined postadsorption memory coefficient, η(t), which defines the fraction of long-lived photoadsorption centers with respect to the total number of both long-lived and short-lived surface centers of photoadsorption generated for a time of irradiation, t. A simple model is proposed to explain the experimental data. Introduction The preirradiated surface of a solid photocatalyst often retains, albeit partly, its photoinduced chemical activity. 1 This effect is described as a memory effect with regard to surface photo- chemical processes. Adsorption of gas molecules on a photo- excited solid surface represents a simple example of a surface photochemical reaction. The memory effect is observed as postadsorption of gas molecules on the preirradiated surface, the prefix “post” reflecting the situation when adsorption occurs after irradiation of the surface is terminated. There are two different manifestations of the memory effect: 2 (i) the Kugel’sberg memory effect and (ii) the Solonitsyn memory effect. 3 The former is observed as a residual chemical activity of the preirradiated surface in the presence of the adsorbate molecules, whereas the latter effect manifests itself as a conservation of photoinduced surface activity after preirradiation of the sample (typically in vacuo) in the absence of adsorbate molecules. Photostimulated adsorption is a complex multistep process (typically chemisorption) described as a stoichiometric reaction between adsorbate molecules and the solid surface driven by light absorbed either by the adsorbate or by the solid adsorb- ent. 1,4 Often, the process is considered a chemical step (primary) in a photocatalytic reaction. Characteristically, photoadsorption begins from electronic photoexcitation of the solid adsorbent, which leads to generation of free charge carriers (electrons and holes) followed by their migration toward the surface, and subsequent trapping by surface defects (i.e. surface adsorption centers). The process converts the defects into an active state of the solid in terms of the ability to form chemical bonds with adsorbate molecules. 1,5 The final step of photoadsorption is the chemical interaction of adsorbate molecules with surface centers in the active state. An essential feature of a surface-photogenerated adsorption center is that its active state can decay through different physical pathways. Consequently, the active state of the photoinduced adsorption center can be characterized by its lifetime, τ. 1,6,7 The lower limit of τ can be estimated from the Langmuir- Hinshelwood (LH)-like dependence of the initial (quasi- stationary) rate of photoadsorption on the pressure (p; concen- tration) of the adsorbate molecules (eq 1) where (dp/dt)| tf0 is the initial rate of photoadsorption; k exc is an apparent rate constant of photoexcitation of surface centers, which converts the centers into their active state; F is the photon flow of the incident light; S 0 is the initial concentration of the surface centers of photostimulated adsorption; p is the concen- tration (pressure) of adsorbate molecules; k ad is the rate constant of adsorption (interaction of adsorbate molecules with surface centers in the active state); and k dec is the apparent pseudo- first-order rate constant of decay of the active state of the surface centers through physical pathways, with k dec ) 1/τ. Typical estimates of the lifetime of the active state of adsorption centers obtained from eq 1 yield lifetimes in the range 1 to 10 -5 s. 4,6-9 At the same time, results of experimental studies of post- adsorption effects indicate that the lifetime of a photoinduced active state of adsorption centers can be as much as 10 5 s, and perhaps even longer. 4,7-12 In the latter case, the lower limit of * Address correspondence to this author. Fax: (+1) 514-848-2868. E-mail: serpone@vax2.concordia.ca. § Dedicated to the memory of Yu. P. Solonitsyn. ² St. Petersburg State University. ‡ Concordia University. ⊥ Universita di Pavia. dp dt | tf0 ) k exc FS 0 k ad p k dec + k ad p (1) 2354 J. Phys. Chem. B 2004, 108, 2354-2361 10.1021/jp0310044 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/23/2004