Dynamics of acetone photooxidation on TiO2(110): State-resolved measurements of methyl photoproducts

Abstract

State-resolved laser techniques were used to study the internal state distributions of gas phase methyl radicals which are produced during the photooxidation of acetone on TiO2(110). This approach was used as a means of understanding the nature of the bimodal kinetic energy distributions for these radicals. Specifically, we investigated the population of the ν2 "umbrella mode" which has been shown to be important in similar photodissociation reactions where methyl radicals are liberated. We observed that for methyl radicals undergoing prompt dissociation (EK = 0.15 eV), the vibrational population in the umbrella mode is quite cold and can be characterized by a Tvib = 151 ± 15 K. Methyl radicals in this channel were also characterized by a rotational energy distribution of Trot = 325 ± 25 K which is comparable to the gas phase value obtained by acetone photolysis. State-resolved energy distributions also show that methyl radicals which are vibrationally excited have an overall kinetic energy distribution which is ~35 meV less than those which are in their vibrational ground state. This value is comparable to, but not exactly in agreement with, the known vibrational spacing of the ν2 mode and suggests that vibrationally excited methyl radicals have less energy available for translation.

Insights into Acetone Photochemistry on Rutile TiO2(110). 1. Off-Normal CH3Ejection from Acetone Diolate

“…During UV irradiation, the desorbing CH 3 radicals follow specific off-normal trajectories consistent with the C–CH 3 bond orientation in the η 2 -acetone diolate molecule on TiO 2 (110) surface based on theoretical calculations . These angle-resolved PSD measurements indicate ejection of hyperthermal CH 3 radicals, which is consistent with the pump–delayed-probe laser PSD studies. − IRAS spectra of the irradiated samples show new peaks consistent with the η 2 -acetate oriented along the Ti rows on the TiO 2 (110) surface in agreement with the theoretical calculations . Acetate was proposed earlier from high-temperature thermal desorption measurements, ,, and we show here that it is a direct product of the acetone diolate photodecomposition at low temperature (∼30 K).…”

“…Photon-stimulated desorption (PSD) of the methyl radicals has been used to study the photochemistry of acetone on oxidized TiO 2 (110). The ejection of methyl radicals was proved using isotopically labeled acetone and later confirmed with UV pump, vacuum ultraviolet (VUV) probe mass spectrometry and state-resolved, resonance-enhanced multiphoton ionization (REMPI) measurements . Recent pulsed pump–probe laser excitation measurements discovered two components in the translational energy distributions of the ejected CH 3 radicals with average energies of 0.19 eV (“fast”) and 0.03 eV (“slow”), representing different final states of the methyl fragments .…”

“…TiO 2 is a broadly used material and one of the most-studied oxide photocatalysts. − On single crystal, rutile TiO 2 (110), the thermal chemistry and photochemistry of the adsorbed acetone and other ketones has been investigated by one of us (M.A.H.) and co-workers − and by White and co-workers. − Acetone adsorbs molecularly in an η 1 configuration at Ti 5c sites on reduced TiO 2 (110) (see Figure a) and desorbs over a broad temperature range due to repulsive interactions among the adsorbates . Acetone is not photochemically active on reduced TiO 2 (110), but it is photoactive when coadsorbed with oxygen. , Henderson proposed a two-step reaction mechanism to explain the observed photochemistry .…”

Single Molecule Photocatalysis on TiO2 Surfaces

“…Among various organic molecules, formaldehyde (CH 2 O) and acetone ((CH 3 ) 2 CO) are the simplest aldehyde and ketone, respectively. Therefore, the adsorption and photolysis of CH 2 O − and (CH 3 ) 2 CO − have been widely studied on TiO 2 model surfaces (mainly on R-TiO 2 (110)) as prototypes for photocatalytic degradation VOCs. Thus, in this part, we will mainly focus on the photochemistry of CH 2 O and (CH 3 ) 2 CO on R-TiO 2 (110).…”

Elementary photocatalytic chemistry on TiO2surfaces

“…81,83 The formation of the Z 2 acetone-oxygen complex was consistent with STM 250,251 and DFT studies. 252 The photochemistry of acetone on the r-TiO 2 (110) surface has been investigated systematically by Henderson and White et al [253][254][255][256][257][258][259] The Z 1 bounded acetone was inert on r-TiO 2 (110) when exposed to UV light, while the Z 2 acetone-oxygen complex, i.e., the acetone diolate, was photoactive. 253 Post-irradiation TPD results suggested that acetone diolate was photocatalytically converted into acetate (Fig.…”