Thermal Control of Selectivity in Photocatalytic, Water-Free Alcohol Photoreforming

Abstract

The selective oxidation of alcohols has attracted a great deal of attention. While most photocatalytic studies focus on the generation of hydrogen from alcohols, there is also a great potential to replace inefficient thermal reaction pathways (as e.g. the formox process) by light-driven reactions. In this work we focus on the photoreforming of methanol, ethanol, cyclohexanol, benzyl alcohol, and tert-butanol on well-defined Pt x /TiO2(110) under UHV. It is found that, with the exception of tert-butanol, alcohol oxidation can produce the respective water-free aldehydes and ketones along with the formation of stoichiometric molecular hydrogen with 100% selectivity. While α-H-containing alcohols usually exhibit only a disproportionation reaction with the release of H2, another reaction pathway is detected for methanol (and to a much lower extent benzyl alcohol) to yield the respective ester, methyl formate (or benzyl benzoate, respectively). The formation of this product occurs via a consecutive photoreaction and is strongly influenced by temperature. In general, higher temperatures lead to a higher selectivity toward formaldehyde, as product desorption is favored over the consecutive photoreaction. For tert-butanol two parallel photoreactions occur. In addition to the splitting of a C–C bond yielding a methyl radical, hydrogen, and acetone, dehydration to isobutene is observed. The branching ratios of both reaction pathways can be strongly controlled by temperature, by changing the reaction regime from adsorption to desorption limited. The high selectivities toward aldehydes are attributed to the absence of O2 and water, which inhibits an unwanted overoxidation to acids or CO/CO2. This study shows that photocatalysis under such conditions provides a prospective approach for a highly selective and water-free aldehyde production under mild conditions.

Photocatalytic production of H2 is a multi-criteria optimization problem: Case study of RuS2/TiO2

“…Activation energy measurements were performed for the photocatalytic oxidation of alcohols with Pt/TiO 2 [35,69,71] or noble-metal-free TiO 2 [70,72]. Because all the reported The measurement of , and provides two additional kinetic parameters.…”

Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO2(110)

“…[2,16] Therefore,the ejection of radicals and concomitant stoichiometric production of H 2 is expected in analogy to tert-butanol photoreforming. [17] Neither radical abstraction nor significant molecular hydrogen formation is detected for prolonged reaction times.I nstead, this reaction, which to the best of our knowledge has never been described before,c an be viewed as ap hotocatalytic disproportionation yielding higher alkanes and the respective ketones.I nterestingly,r eaction products originating from the cleavage of the methyl group are not observed ( Figure S9). In the same way, dehydration reactions,c ommon in thermal reactions,a lso do not occur ( Figure S9).…”

Light‐Driven Alcohol Splitting by Heterogeneous Photocatalysis: Recent Advances, Mechanism and Prospects

“…Report regarding light‐driven splitting of liquid alcohols can be tracked back to the pioneering work of Pichat and co‐workers in 1981 [62] . Since then, various semiconductor‐based photocatalysts have been developed, and most of them are metal‐modified titanium dioxide (TiO 2 ) which are only active under irradiation of ultraviolet (UV) light [54,62–73] . One potential issue regarding using UV light is that undesirable by‐products might be obtained due to the high energy of the UV light.…”