Photoconversion of 2-Propanol on Rutile Titania: A Combined Liquid-Phase and Surface Science Study

Abstract

A fundamental reaction in industries for producing aldehydes and ketones is the partial oxidation of alcohols. As a model reaction, we investigated the photo-oxidation of 2-propanol on rutile titania, which is a promising chemically nontoxic photocatalyst. Photochemical infrared reflection absorption spectroscopy (PC-IRRAS) was used to study the reaction on powder catalysts in the liquid phase (neat liquid and dissolved in dichloromethane). We compare these results with polarized Fourier transform (FT)-IRRAS and temperature-programmed desorption (TPD) experiments on rutile TiO2(110) single crystals in ultrahigh vacuum (UHV). Our in situ liquid-phase experiments showed that 2-propanol converts into acetone on rutile powders, which is in accordance with previous ex situ studies. Mass transport limitations are the key to avoid total oxidation. However, the yield of acetone is limited. We identified water formed as a byproduct and suspected that water might block the active sites. To elucidate possible reaction mechanisms, further experiments were performed on rutile TiO2(110) single crystals in the presence and absence of oxygen and UV irradiation under UHV conditions. Here, we obtained further insights into the elementary steps of the different 2-propanol reactions. We demonstrated that acetone desorbs from a diolate species, which forms in the presence of oxygen under UV irradiation at temperatures around 200 K. Furthermore, propane was identified for the first time as a new thermally activated deoxygenation product besides the simultaneously formed, formally reported, propene. Propene formation is quenched by UV irradiation. Active site blocking by water is confirmed by TPD and polarized FT-IRRAS measurements.

Conversion of methanol on rutile TiO2(110) and tungsten oxide clusters: 2. The role of defects and electron transfer in bifunctional oxidic photocatalysts

“…Under the chosen conditions, we expect the maximal conversion for the first oxidation step based on similar experiments for isopropanol photo-oxidation on preoxidized rutile TiO 2 with a small amount of Ti 3+ . 3 However, it may not be excluded that extended irradiation times may lead to the formation of subsequent products or increased total oxidation.…”

“…1,2 However, not only total oxidation can occur but a controlled photochemistry yielding valuable products can be realized by adjustment of the reaction properties (such as irradiation wavelength and power, mass transport etc.). 1,3 Here, surface science studies under well-defined conditions are of particular interest when aiming for a comprehensive understanding. For instance, we have recently investigated the photo-oxidation of isopropanol towards acetone.…”

“…For instance, we have recently investigated the photo-oxidation of isopropanol towards acetone. 3 Several other examples including the photo-oxidation of ethanol to acetaldehyde, 4 the photooxidation of carbon monoxide, 5 and the photochemistry of methanol at TiO 2 single crystals [6][7][8][9] and titania nanomaterials 10 are available in the literature. 11,12 By now, defects such as oxygen vacancies, Ti 3+ interstitials and other reduced sites are recognized to be an essential key to increase and further control the reactivity in thermal reactions towards oxygen and oxygen-containing molecules.…”

Conversion of methanol on rutile TiO2 (110) and tungsten oxide clusters: 1. population of defect-dependent thermal reaction pathways

“…11,12 The chemistry of methanol and other alcohols on stoichiometric rutile TiO 2 (110) was already in the focus of several studies, especially by the groups of Henderson and Dohna ´lek. [13][14][15][16][17][18][19][20][21][22][23] In addition, from reports of the group of Madix and our own work it is known that the chemistry of methanol is sensitive to the presence of defects, such as oxygen vacancies or Ti 3+ interstitials. [24][25][26] Therefore, methanol is a perfect probe molecule for testing the conversion in different reaction pathways involving reduced transition metal oxide centers.…”

Multidentate Interaction of Methylamine with Rutile TiO2 (110)

“…Obviously, the physisorbed multilayer is fully desorbed around 149 K. In analogy to the well-understood adsorption of oxygen-containing molecules on rutile TiO 2 (110), we can assign the other steps to different adsorption sites on the surface. 1,15,20,48 The signal decrease at 190 K is therefore allocated to the desorption of methylamine from bridging oxygen sites via hydrogen bonds. The decrease steps at 297 and 365 K are assigned to desorption of methylamine from fivefold-coordinated titanium sites (Ti 5c , Figure 3, m1−m2).…”