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Thermal control of selectivity in photocatalytic, water-free alcohol photo-reforming Sebastian L. Kollmannsberger, Constantin A. Walenta, Carla Courtois, Martin Tschurl, and Ueli Heiz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03479 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018
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Thermal control of selectivity in photocatalytic, water-free alcohol photo-reforming Sebastian L. Kollmannsberger,†,¶ Constantin A. Walenta,†,‡,¶ Carla Courtois,† Martin Tschurl,† and Ueli Heiz∗,†,‡ †Chair of Physical Chemistry, Department of Chemistry & Catalysis Research Center, Technische Universität München, Lichtenbergstr. 4, 85748 Garching, Germany ‡Nanosystems Initiative Munich, Schellingstr. 4, 80799 München, Germany ¶The authors contributed equally to this work E-mail:
[email protected] Phone: +49 (0) 89 289 13391. Fax: +49 (0) 89 289 13389
Abstract The selective oxidation of alcohols has not only recently attracted great 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 Ptx /TiO2 (110) in the 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
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release of H2 , another reaction pathway is detected for methanol (and to 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 strongly be controlled by temperature, by changing the reaction regime from adsorption- to desorption-limitation. The high selectivities toward aldehydes are attributed to the absence of O2 and water, which inhibits an unwanted over-oxidation 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.
Keywords titania, alcohol oxidation, photocatalysis, selectivity, photoreforming, hydrogen production
Introduction Selective oxidation of alcohols has attracted extensive attention in the last decade. 1–3 In this regard tuning the selectivity of photocatalytic reactions is in the focus of current research. 4,5 A key step to high selectivities is the fundamental understanding of photocatalytic mechanisms. Therefore, studies on perfectly defined semiconductor single crystals are of utmost importance. Titania based systems do not only represent the most heavily used semiconductors in photocatalytic applications, but TiO2 (110) is also a well-available, heavily studied single crystaline material. Therefore, it represents an ideal material to elucidate fundamental mechanisms in photocatalysis. Similarly, Pt is a very prominent and often used co-catalyst
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for the photocatalytic hydrogen evolution. 6 In alcohol reforming, the photocatalytic synthesis of highly industrial relevant products as benzaldehyde, formaldehyde or cyclohexanone is particularly attractive. Benzaldehyde is among the most important molecules in cosmetics and flavor industries. 7 Formaldehyde is, besides of its application as building block chemical, also needed as precursor for potential ultra-low emission fuels. 8 In particular the formation of water-free formaldehyde as an alternative to the commonly used formox process, which requires a complex procedure to be freed from water and other impurities, is of great interest. 9 Cyclohexanone is a precursor molecule for the production of nylon-6 and is industrially synthesized by either a high temperature and high pressure oxidation of cyclohexane or a two step process starting from phenol. 10 Consequently, the development of a one step photoprocess may be observed with great attention. Ethanol oxidation is highly relevant, as it is easily available in large amounts from biomass conversion. Its photo-oxidation to produce solar hydrogen from bio ethanol is environmentally benign. 11 As it is generally known that photo-oxidation of primary and secondary alcohols proceeds via an abstraction of the α-H, 12–15 the investigation of a tertiary alcohol like tert-butanol is therefore mechanistically interesting, because the conventional pathway is not feasible. Therefore, we have chosen these four alcohols, whose conversions are industrially relevant, and the mechanistically interesting tert-butanol for the photocatalytic reforming. The vast majority of studies concerning alcohol photoreforming address the evolution of hydrogen and less focus on the oxidation products; also for rather defined systems. 16 When the selectivity of the reaction is considered, it is usually observed that photocatalysis of alcohol with co-catalyst loaded TiO2 leads to several different oxidation products, even to CO and CO2 (examples are e.g. shown in the microreview of Cargnello et al. 17 ). However, in the quantitative analysis of photoreaction products in an aqueous solutions of methanol, it was found that under shorter illumination times (15 min in comparison to 3 hours) high selectivities toward formaldehyde were achieved. 18 For gas-phase reactions, Selli and co-workers have performed several studies. For methanol, they found varying selectivity for different mole
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fractions of the alcohol and water. One result from these studies was, that water serves as an oxygen donor for the oxidation of formaldehyde. 19 Consequently, the highest selectivity was achieved for pure methanol, but only a value of 60% with respect to the H2 yield was found. 20 While for all these studies principle mechanisms have been formulated to explain the observed reaction behavior, detailed insights of the parameters governing the selectivity are still missing and foster research activities in this field. 21 In this regard surface science studies may be helpful, because they enable investigations of highly defined materials under highly defined conditions. While alcohol oxidations have been explored in great depth on bare TiO2 (110) single crystal surfaces in single coverage experiments, 14,15,22–32 investigations with co-catalyst loaded semiconductors under catalytic conditions are scarce. However, such studies enable unique insights into mechanistic details. In the following we demonstrate the determination of stoichiometric mechanisms for the photo-oxidation of alcohols, photo-oxidative coupling reactions, the formation of hydrogen and temperature induced selectivity changes.
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Experimental viewport Nd:YAG Laser 532 nm, 100 Hz
skimmer octupole
gasline with reactant gases
EI-QMS z,φ - manipulator for sample transfer
mol. beam doser
metal target
PI-TOF-MS sample transfer chamber
Nd:YAG pumped UV/VIS Dye Laser
quadrupole deflector
cluster beam
QMF einzel lense TiO2(110) sample
sputter gun
cluster source
e-gun
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x,y,z,φ - manipulator with sample holder and liquid-N2 cooling Auger spectroscopy and depostion
Nd:YAG pumped UV/VIS OPO Laser
Figure 1: Chamber scheme of the UHV apparatus including the laser vaporization cluster source and the laser system. The experimental setup for photocatalytic measurements, which is shown schematically in figure 1, consists of a UHV chamber with a background pressure better than 9.8×10−11 mbar and a laser vaporization cluster source for the deposition of size selected metal clusters. The main chamber includes a manipulator (VAB Vakuum GmbH), a surface preparation and surface analysis part, a reaction part with an electron ionisation quadrupole mass spectrometer (EI-QMS) (QMA 430, Pfeiffer Vacuum GmbH) and a homebuilt photo ionisation time of flight mass spectrometer (PI-TOF-MS), which enables the selective ionisation of isobar molecules. 33 Furthermore, a sample transfer chamber and a gasline with reactant gases, which contains a leak valve and a molecular beam doser, are attached to the chamber. The sample holder, which is described in detail elsewhere, 14 enables liquid nitrogen cooling and heating. The laser system consists of a Nd:YAG pumped dye laser and a Nd:YAG pumped UV/Vis OPO laser. The photoexcitation of the semiconductor is carried out with a wavelength of 242 nm, obtained by the frequency doubled OPO laser (GWU, premiScan ULD/400), which is pumped by the third harmonic of a Nd:YAG-laser (Innolas Spitlight 5
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HighPower 1200, 7 ns pulse width, 20 Hz repitition rate). If not otherwise noted, illumination intensities in the saturation regime of 4.5 ± 0.7 mW are applied. The rutile TiO2 (110) single crystal (SurfaceNet GmbH) was prepared by cycles of Ar+ sputtering (1.5 keV, 1 × 10−5 mbar, 30 min), oxygen annealing (800 K, 1 × 10−6 mbar, 20 min) and vacuum annealing (800 K, 10 min) until no contamination was detected in Auger electron spectroscopy (AES). The resulting TiO2 (110) has a light blue color with a bridge-bonded oxygen (BBO) vacancy concentration, determined with H2 O TPD, of 6±1% of Ti-lattice sites. 34 The laser vaporization cluster source is operated by a Nd:YAG laser (532 nm, 100 Hz, Innolas), which ablates a Pt target (99.95% purity, ESG Edelmetalle, Germany). The resulting plasma is cooled by a He pulse and expanded into the vacuum. The resulting cationic clusters are guided and bent into a quadrupole mass filter (QMF) (Extrel, USA) to enable cluster size selection. 35 With these settings a size-distribution from Pt7 to Pt35 with a maximum from Pt11 to Pt13 (see SI Fig. S1) results. In this work, the QMF is operated with ACpotential only during deposition and acts as an ion guide discarding all masses lower than Pt8 . The deposition of 1% ML Pt (respective to the surface atoms) onto the TiO2 (110) single crystal occurs under soft-landing conditions (
