Resonance Raman effects in TS-1: the structure of Ti(IV) species and reactivity towards H 2 O, NH 3 and H 2 O 2 : an in situ studyy S. Bordiga,* abc A. Damin, ab F. Bonino, ab G. Ricchiardi, ab A. Zecchina, abc R. Tagliapietra d and C. Lamberti abc a Department of Inorganic, Physical and Materials Chemistry, University of Turin, Via P. Giuria 7, 10125, Torino, Italy. E-mail: silvia.bordiga@unito.it.; Fax: +39011 6707855; Tel: +39011 6707860 b INSTM UdR Torino Universita ` , Via P. Giuria 7, 10125, Torino, Italy c INFM UdR Torino Universita ` , Via P. Giuria 7, 10125, Torino, Italy d RENISHAW S.p.A., Viale dei Prati 5, I-10044, Pianezza (Torino), Italy Received 28th May 2003, Accepted 16th July 2003 First published as an Advance Article on the web 11th August 2003 The isomorphous insertion of 1–2 wt% of Ti into the MFI framework leads to a Titanium silicalite-1 (TS-1) material, which is an active and highly selective catalyst in a remarkable number of low-temperature oxidation reactions with aqueous H 2 O 2 as oxidant. Such Ti(IV) species exhibit a local T d -like symmetry, forming [TiO 4 ] units, and induces to the hosting MFI matrix two Ti-specific vibrational modes at 960 and 1125 cm 1 . We report a Raman study on the perturbation caused by interaction with H 2 O, NH 3 and H 2 O/H 2 O 2 on the vibrational modes of the [TiO 4 ] unit embedded in the MFI framework. The selective use of different excitation laser sources in the near-IR (1064 nm; 9398 cm 1 ), visible (442 nm; 22 625 cm 1 ), near-UV (325 nm; 30 770 cm 1 ) and far-UV (244 nm; 40 985 cm 1 ) allowed us to progressively enter into the oxygen to titanium charge transfer transition and thus to switch on the resonance effects on the 1125 cm 1 mode, which is the only Ti-specific mode exhibiting the same symmetry of the charge transfer transition. Interaction with both water and ammonia causes the formation of [Ti(H 2 O) 2 O 4 ] or [Ti(NH 3 ) 2 O 4 ] complexes which destroy the T d -like symmetry and thus the Raman enhancement of the 1125 cm 1 mode. Upon dosing a H 2 O/H 2 O 2 to TS-1, the powders turn yellow as a consequence of the appearance a new charge transfer transition around 385 nm (26 000 cm 1 ). In order to single out the vibrational mode of the active peroxo complex formed on Ti, we have performed Raman experiments using a visible laser source (442 nm; 22 625 cm 1 ). In these conditions we have observed the strong enhancement of a mode at 618 cm 1 , which has been attributed to the symmetric breathing mode of the Ti(O) 2 ring. 1. Introduction Titanium silicalite-1 (TS-1) 1 is a synthetic zeolite in which a small number of Ti atoms substitute tetrahedral Si atoms in a purely siliceous framework with the MFI structure (i.e. that of ZSM-5 zeolite). It is an active and highly selective catalyst in a number of low-temperature oxidation reactions with aqu- eous H 2 O 2 as the oxidant, such as the conversion of ammonia to hydroxylamine, of secondary alcohols to ketones, of second- ary amines to dialkylhydroxylamines, or reactions such as phenol hydroxylation, olefin epoxidation, or cyclohexanone ammoximation. 2 This explains why it has been one of the most studied materials in heterogeneous catalysis in the last two dec- ades on both experimental 3–19 and computational 6,17–22 levels. TS-1 is characterized by an increase of the unit cell volume proportional to the Ti content. 1,8–10 The UV-Vis spectra of TS- 1 in vacuo gives a simple and clear proof of the presence of tet- rahedral Ti(IV) in the zeolite framework: a Ti 4+ O 2 ! Ti 3+ O  ligand to metal charge transfer (LMCT) located around 208 nm (48.000 cm 1 ) can be unambiguously assigned to the charge transfer transition from the oxygen ligand to an unoc- cupied orbital of a Ti(IV) ion tetrahedrally coordinated in iso- lated [TiO 4 ]. 7,11,13,14,17,19 Ab initio calculations have proven that this LMCT is totally symmetric charge release from the four neighbour oxygen atoms to the central titanium atom. 17 TS-1 is characterized by the presence of a ‘‘ fingerprint ’’ IR absorption component 1,3,5–7,15–20,22 centred at 960 cm 1 the intensity of which is proportional to the Ti content. 17 This band is well visible in the IR spectra since it appears on the low energy tail of the very strong absorption due to n asym (Si–O–Si) modes appearing in the broad 1250–1030 cm 1 interval. Beside the 960 cm 1 component, Raman experi- ments 5 have successively revealed also the presence of a second fingerprint band at 1125 cm 1 not obscured by the n asym (Si–O– Si) modes, which are Raman inactive. Raman spectroscopy is, however, a very critical technique when applied to these systems, due to both the high dilution of Ti centres in the MFI framework, less than 3 wt% in TiO 2 , 8 and the presence of a strong fluorescence background. In this work we investigate the vibrational modes of the [TiO 4 ] groups in TS-1 and their perturbation upon inter- action with water, ammonia and H 2 O/H 2 O 2 , which are the key reactants in partial oxidation reactions. 1,2 The selec- tive use of different excitation laser sources in the near-IR (1064 nm; 9398 cm 1 ), visible (442 nm; 22 625 cm 1 ), near-UV (325 nm; 30 770 cm 1 ) and far-UV (244 nm; 40 985 cm 1 ) has allowed us to progressively enter into the oxygen to titanium charge transfer transition around 208 nm (48 000 cm 1 ) and thus to switch on the resonance y Presented at the International Congress on Operando Spectroscopy, Lunteren, The Netherlands, March 2-6, 2003. 4390 Phys. Chem. Chem. Phys., 2003, 5, 4390–4393 DOI: 10.1039/b306041c This journal is # The Owner Societies 2003 PCCP