On the dynamical nature of the active center in a single-site photocatalyst visualized by 4D ultrafast electron microscopy

Abstract

Understanding the dynamical nature of the catalytic active site embedded in complex systems at the atomic level is critical to developing efficient photocatalytic materials. Here, we report, using 4D ultrafast electron microscopy, the spatiotemporal behaviors of titanium and oxygen in a titanosilicate catalytic material. The observed changes in Bragg diffraction intensity with time at the specific lattice planes, and with a tilted geometry, provide the relaxation pathway: the Ti 4+ =O 2− double bond transformation to a Ti 3+ −O 1− single bond via the individual atomic displacements of the titanium and the apical oxygen. The dilation of the double bond is up to 0.8 Å and occurs on the femtosecond time scale. These findings suggest the direct catalytic involvement of the Ti 3+ −O 1− local structure, the significance of nonthermal processes at the reactive site, and the efficient photo-induced electron transfer that plays a pivotal role in many photocatalytic reactions.ultrafast electron microscopy | single-site photocatalysis | titanium based photocatalyst | ulatrafast phenomena | time-resolved microscopy S ingle-site catalysts of both the thermally and photoactivated kind now occupy a prominent place in industrial-and laboratory-scale heterogeneous catalysis (1-8). Among the most versatile of these are the ones consisting of coordinatively unsaturated transition metal ions (Ti, Cr, Fe, Mn. . .) that occupy substitutional sites in well-defined, three-dimensionally extended, open-structure silicates of the zeolite type. The well-known and most widely used are the 4-or 5-coordinated Ti(IV) ions accommodated within the crystalline phase of silica, silicalite (9-14).Titanosilicates, especially, are used extensively both industrially and in the laboratory for a wide range of chemo-, regio-, and shapeselective oxidations of organic compounds (15-18). These single-site heterogeneous photocatalysts are quite distinct from those typified by TiO 2 , SrTiO 3 , and other titaniferous photocatalysts where the Ti(IV) ions are in 6-coordination; and where, in interpreting the processes involved in harnessing solar radiation, electronic band structure considerations hold sway in preference to the localized states (see, e.g., refs. 19, 20). It has been demonstrated (16-18, 21, 22) that single-site, coordinatively unsaturated Ti(IV)-centered photocatalysts are especially useful in the aerial oxidation of environmental pollutants in the photodegradation of NO (to N 2 and O 2 ), of H 2 O (to H 2 and O 2 ), and in the photocatalytic reduction of CO 2 to yield methanol. There is an exigent need to explore the precise nature of the electronic, temporal, and spatial changes accompanying the initial act of photoabsorption that sets in train the ensuing elementary chemical processes that are of vital environmental significance in, for example, the utilization of anthropogenic CO 2 as a chemical feedstock (23).Here, we report the use of 4D ultrafast electron microscopy (UEM) (24-26) to trace the spatiotemporal behavior of the Ti(IV) and O ...

Charge migration and charge transfer in molecular systems

“…It was found that the trapping of the electron occurs in less than 200 fs, implying that it is trapped at or near the unit cell where it is created. This trapping time was later confirmed by ultrafast electron diffraction studies of the Zewail group 260 on photoexcited titanosilicate materials consisting exclusively of pentacoordinated Ti centres. Very recently, Obara et al 261 repeated the experiment described in Ref.…”

Reflections on the value of electron microscopy in the study of heterogeneous catalysts

“…Now the territory of catalytic significance that needs to be charted is that involving progressively faster time resolution. In this connection, it is instructive to recall the very last joint work that Zewail and I did on elucidating the nature of the catalytically active site in a multiply-bonded Ti[iv]oxo unit in a low-coordinate titanosilicate photocatalyst [98,99].…”

“…Zewail and I and our colleagues, using his classic 4DEM approach [98], activated this catalytically active centre by laser-induced electron transfer to form a stretched [single bonded] Ti[iii]oxo unit. The femtosecond dynamics revealed exceptional Ti-O bond disruption, making…”

On choosing the most appropriate catalysts for the conversion of carbon dioxide to fuels and other commodities, and on the environmentally benign processing of renewable and nonrenewable feedstocks

“…Recently, one of the present authors (JMT) has been associated with a fundamental study [27] of a single-site Ti (IV) -based photocatalyst, a titanosilicate, known as JDF-L1 [28], with the formula: Na 4 Ti 2 Si 8 O 22 4H 2 O. The essential features of the active centre in this open-structure solid, a TiO 5 arrangement, square pyramid, are shown in Fig.…”

“…6, in which there is one Ti=O apical bond and four Ti-O bonds. 4D ultrafast electron microscopy showed [27] that upon photoexcitation, the Ti (IV) =O bond is transformed to a single bond Ti (III) -O -with a consequential dilation of the length of the double bond from 1.7 Å -2.5 Å . This occurs on the femtosecond time scale, and a schematic illustration of this process as well as the ensuing photochemical possibilities are shown in Fig.…”