Thermal and Photoreactivity of TiO₂at the Gas−Solid Interface with Aliphatic and Aromatic Aldehydes

Abstract

EPR spectroscopy has been used to characterize the interactions between aldehydes and the surface of rutile TiO2 active toward heterogeneous charge transfer and ultimately the formation of surface peroxyacyl radicals. Paramagnetic centers formed by vacuum reduction and UV irradiation of TiO2 have been detected and characterized at the dehydroxylated particle surface. In low-temperature (100 K) gas−solid studies, reduced TiO2 surfaces have been used as model systems for interfacial charge-transfer processes. Radical products resulting from aldehyde reduction are not detected at 100 K on the reduced surface even under UV irradiation. However, coadsorption of aldehydes with O2 and subsequent UV irradiation on reduced TiO2 surfaces does result in the formation of stable surface radicals at 100 K. When the reactions conditions are varied, such as aldehyde type and UV wavelength, these radicals were identified as peroxyacyl species (RCO3 •) stabilized at the semiconductor surface. The RCO3 • radicals were only formed in coadsorption experiments and decayed irreversibly at T > 250 K. The results clearly demonstrate that the TiO2 surface can participate in both the initiation and propagation stages of aldehyde photooxidation. Similar radical species were not detected in UV-irradiated TiO2−aldehyde suspensions at 100 K.

Free‐Radical Pathways in the Decomposition of Ketones over Polycrystalline TiO2: The Role of Organoperoxy Radicals

“…The signal can be reformed, however, by photoirradiation of the sample (containing 2-butanone:O 2 ) at 77 K. At 298 K, only a weak EPR signal due to the background localised conduction electrons is observed. [22] The formation of similar transient peroxy radicals was also examined with other ketones, including 4-heptanone, cyclopentanone and cyclohexanone. The resulting EPR spectra are shown in Figures 3 c, 3 d and 3 e, respectively.…”

“…In other words, all of the radicals observed in Figure 3 decayed rapidly at temperatures above 250 K. These results therefore indicate that a family of transient organoperoxy radicals (of general formula ROOC) is clearly formed during low-temperature (77 K) photoirradiation of ketones over dehydrated activated TiO 2 in the presence of molecular oxygen. Ionic superoxide radicals can be easily generated on TiO 2 by a number of methods including 1) by contact with H 2 O 2 , [44] 2) by direct O 2 adsorption on thermally reduced samples, [21,22,24] and, by far the most commonly used method, 3) by photoadsorption. [19,27,28,39,[45][46][47] In methods 2) and 3) the radicals are formed at the gas/solid interface and they are reasonably stable at room temperature provided the sample is kept under vacuum and free from other surface adsorbates, [24] particularly OH groups and water.…”

“…= 1.966 and g j j = 1.95 can be attributed to a mixture of bulk and surface Ti 3 + centres, [54] while the sharp signal close to free spin can be assigned to a medium-polarised conduction electron spin resonance signal observed in the reduced sample. [22,55] Acetone was then admitted to the EPR cell at 77 K and the temperature slowly raised. No significant changes were detected in the spectra until 210 K (Figure 5 d); at this temperature, the EPR signal intensity due to Ti 3 + significantly decreased.…”

“…[22][23][24][25][26][27][28][29][30][31][32][33][34][35] In many of these studies ionic oxygen-centred radicals including O À , O 3 À and particularly O 2 À were reported, with specific relevance to photoadsorption phenomena or as probes to identify the distribution of radicals formed on the surface. However, other types of oxygen-based radicals have received far less attention, [22][23] and these include the series of thermally unstable peroxyacyl radicals (of general formula RCO 3 C) [22] and peroxy radicals (of general formula ROOC), [21] which we identified on TiO 2 following low-temperature UV irradiation of coadsorbed aldehyde:O 2 and acetone:O 2 , respectively. Che was one of the pioneers in this field and established that a peroxy-based radical could be formed from coadsorbed ethylene:O 2 mixtures on TiO 2 .…”

“…Che was one of the pioneers in this field and established that a peroxy-based radical could be formed from coadsorbed ethylene:O 2 mixtures on TiO 2 . [23] Because these transient radical species [21][22][23] are so thermally unstable, they are often not easily detected and can be completely missed during experimental observation. Instead the more thermally stable superoxide radical (O 2 À ) may exist as the sole remaining EPR-detectable species at room temperature, which may be only partially responsible for the photooxidation process.…”