A Titanium Disilicide Derived Semiconducting Catalyst for Water Splitting under Solar Radiation—Reversible Storage of Oxygen and Hydrogen

Abstract

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...

Tuning of photocatalytic activity by creating a tridentate coordination sphere for palladium

“…Therefore, to a solution of bmpphenO 2 in dry methanol, phen(NH 2 ) 2 was added in portions to avoid a possible self-condensation of phen(NH 2 ) 2 to tpphz. Finally, the complexation of the obtained ligand bmptpphz with [(tbbpy) 2 RuCl 2 ] (tbbpy = 4,4′-di-tert-butyl-2,2′- bipyridine) was carried out in ethanol-water at reflux to obtain the brownish target molecule Ru(bmptpphz) in 78% yield. In comparison, the complexation of the unsubstituted tpphz with [(tbbpy) 2 RuCl 2 ] has to be performed at 200°C in refluxing ethylene glycol with a three-fold excess of the ligand to guarantee a sufficient solvation of tpphz and to avoid a complexation of the ruthenium moiety on both coordination sites.…”

“…The ligand bmptpphz (0.49 g, 0.82 mmol) was suspended in ethanol-water (100 ml/100 ml) and heated to reflux. Afterwards [(tbbpy) 2 RuCl 2 ] (0.60 g, 0.85 mmol) in ethanol-water (50 ml/50 ml) was added during 1 hour. The orange-brown suspension was refluxed for 18 hours more and subsequently filtered while hot to remove unreacted ligand.…”

“…catalytic efficiency can be tuned. [20][21][22][23][24][25] The intensively studied intramolecular photocatalyst [(tbbpy) 2 Ru(tpphz)PdCl 2 ](PF 6 ) 2 (Fig. 1, Ru(tpphz)PdCl 2 ; tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2′′′,3′′′-j]phenazine) is an active hydrogen evolving PDM generating H 2 with a maximum turnover number (TON) of 238.…”

Advances in the application of nanotechnology in enabling a ‘hydrogen economy’

“…Incorporating Pt or RuO 2 nanoparticles on the surface as suggested by Shangguan [14] could be a method of increasing the density of such sites. Ritterskamp et al [58] reported the use of TiSi 2 as a catalyst for photocatalytic water splitting based on its broadband absorption characteristics (band gap range is 3.4-1.5 eV). TiSi 2 is however unstable in water due to oxide formation.…”

Ti-Si photocatalyst for producing hydrogen synthesized by shock wave

“…Titanium silicides are potential candidates for field emission devices due to their relative low resistivity (10-28μΩ cm), high thermal stability, and low work function. Ritterskamp et al reported that the light-absorption characteristics in UV-visible region (ca.360∼800nm) of TiSi 2 are ideal for solar applications and have a good photocatalytic activity of splitting water into hydrogen [3]. There are a lot of the reports on the shock-induced chemical reaction for Ti-Si concerning to the role of morphologies of the powder mixtures, shock energy and the presence of oxygen [4,5].…”