Dedicated to Professor Kurt Schaffner on the occasion of his 75th birthdayHydrogen and oxygen evolution from water using semiconductors and light is an important issue in the exploitation of solar radiation as a sustainable energy. [1,2] However, a major drawback of most of the research in this field relates to the fact that appropriate semiconductors either are not readily accessible, absorb solar radiation inefficiently, [3][4][5][6][7][8] or produce hydrogen in a sacrificial manner only (i.e. the catalyst is degraded). We present titanium disilicide (TiSi 2 ) as a prototype for the promising new class of silicide semiconductors, [9] which have not, to date, been used for water splitting.[1]These semiconducting materials are inexpensive and abundant. One disadvantage might be their poor stability (in particular of TiSi 2 in water). However, we anticipated that sufficient passivation of TiSi 2 by limited oxide formation might render this project successful.[10]The light-absorption characteristics of TiSi 2 are ideal for solar applications: broad-band reflectance measurements show a band-gap range from 3.4 eV (ca. 360 nm) to 1.5 eV (ca. 800 nm) for TiSi 2 (Figure 1). This behavior is atypical of semiconductors since these materials usually exhibit small band-gap spreads. Determination of the quasi Fermi level of electrons at pH 7 of our catalyst showed values of À0.43 eV and À0.41 eV before and during reaction, respectively;[11] the latter energy level still fulfils the physical requirement for the reduction of protons to form hydrogen. [12] The broad-band water-splitting capacity has been ascertained by experiments run at individual wavelength ranges. For this purpose, Rayonet photoreactors were equipped with RUL 350 or 540 nm lamps (l max ; AE 60 nm emission range). Comparable water-splitting kinetics were obtained.Two phases (A and B) of hydrogen evolution are observed when dark grey TiSi 2 powder (ca. 325 mesh, Alfa) is allowed to react at 55 8C under standard conditions (see the Experimental Section and Figure 2). Phase A starts at t = 0 with [H 2 ] = 0 and shows a nonlinear dependence. Phase B is characterized by the nearly linear part of the hydrogen evolution curve. We interpret the time dependence of hydrogen evolution to be a consequence of simultaneously occurring reactions [Eqs. (1)- (3)]. The course of the reaction inEquation (1), which sacrifices the TiSi 2 by oxide formation, has been verified by runs in the dark at 50-85 8C. Initially, it shows a similar dependence as the reactions in light, but it levels off and does not show a linear dependnce as in phase B, which is attributed to water splitting, and hydrogen production comes to a stop after approximately 150 h ( Figure 6 in the Supporting Information). The course of hydrogen evolution in phase A depends strongly on the quality and composition of the catalyst, its particle size, the reaction temperature, and the pH value.[13] Figure 1. Band-gap range of the TiSi 2 -based semiconducting catalyst employed in this work. h + = hole with positive ch...
Static headspace gas chromatography-ion mobility spectrometry (SHS GC-IMS) is a relatively new analytical technique that has considerable potential for analysis of volatile organic compounds (VOCs). In this study, SHS GC-IMS was used for the identification of the major terpene components of various essential oils (EOs). Based on the data obtained from 25 terpene standards and 50 EOs, a database for fingerprint identification of characteristic terpenes and EOs was generated utilizing SHS GC-IMS for authenticity testing of fragrances in foods, cosmetics, and personal care products. This database contains specific normalized IMS drift times and GC retention indices for 50 terpene components of EOs. Initially, the SHS GC-IMS parameters, e.g., drift gas and carrier gas flow rates, drift tube, and column temperatures, were evaluated to determine suitable operating conditions for terpene separation and identification. Gas chromatography-mass spectrometry (GC-MS) was used as a reference method for the identification of terpenes in EOs. The fingerprint pattern based on the normalized IMS drift times and retention indices of 50 terpenes is presented for 50 EOs. The applicability of the method was proven on examples of ten commercially available food, cosmetic, and personal care product samples. The results confirm the suitability of SHS GC-IMS as a powerful analytical technique for direct identification of terpene components in solid and liquid samples without any pretreatment. Graphical abstract Fingerprint pattern identification of terpenes and essential oils using static headspace gas chromatography-ion mobility spectrometry.
In this paper we demonstrate that the choice of an appropriate non-polar modifier which can provide sufficient chemical interactions with the target analytes may lead to the improvement of the selectivity and sensitivity of differential ion mobility spectrometric (DMS) methods.
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