This work investigated the effect of the defect structure referred to as Ti3+ present in titania
support of supported titania on characteristics and activity of the Co/TiO2 catalyst. Titania
supports were prepared by sol−gel and then calcined under N2 plus increasing the amount of
O2 to change the surface defect concentration. The surface defect of titania support was increased
by increasing the oxygen percent in feed during the calcination process. This defect was monitored
using the CO2-temperature program reduction (CO2-TPD) and electron spin resonance (ESR).
Cobalt was impregnated onto titania supports containing different defect structures. XRD, SEM-EDX, TPR, and H2-chemisorption were used to characterize Co/TiO2. It was found that dispersion
of cobalt and reducibility increased with the amount of surface defect present. Based on high
dispersion and reducibility results this catalyst showed a high conversion for methanation without
changing CH4 selectivity.
Samples of the anatase phase of titania were treated under vacuum to create Ti(3+) surface-defect sites and surface O(-) and O(2) (-) species (indicated by electron paramagnetic resonance (EPR) spectra), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti(3+)-OH groups (indicated by IR spectra). EPR spectra showed that the probe molecule [Re(3)(CO)(12)H(3)] reacted preferentially with the Ti(3+) sites, forming Ti(4+) sites with OH groups as the [Re(3)(CO)(12)H(3)] was adsorbed. Extended X-ray absorption fine structure (EXAFS) spectra showed that these clusters were deprotonated upon adsorption, with the triangular metal frame remaining intact; EPR spectra demonstrated the simultaneous removal of surface O(-) and O(2) (-) species. The data determined by the three complementary techniques form the basis of a schematic representation of the surface chemistry. According to this picture, during evacuation at 773 K, defect sites are formed on hydroxylated titania as a bridging OH group is removed, forming two neighboring Ti(3+) sites, or, when a Ti(4+)-O bond is cleaved, forming a Ti(3+) site and an O(-) species, with the Ti(4+)-OH group being converted into a Ti(3+)-OH group. When the probe molecule [Re(3)(CO)(12)H(3)] is adsorbed on a titania surface with Ti(3+) defect sites, it reacts preferentially with these sites, becoming deprotonated, removing most of the oxygen radicals, and healing the defect sites.
The use of solid−state molecular organometallic chemistry (SMOM−chem) to promote the efficient double bond isomerization of 1-butene to 2-butenes under flow−reactor conditions is reported. Single crystalline catalysts based upon the σ-alkane complexes [Rh(R 2 PCH 2 CH 2 PR 2 )(η 2 η 2 -NBA)][BAr F 4 ] (R = Cy, t Bu; NBA = norbornane; Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ) are prepared by hydrogenation of a norbornadiene precursor. For the t Busubstituted system this results in the loss of long-range order, which can be re-established by addition of 1-butene to the material to form, in an order/disorder/order phase change. Deployment under flow-reactor conditions results in very different on-stream stabilities. With R = Cy rapid deactivation (3 h) to the butadiene complex occurs, [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(butadiene)][BAr F 4 ], which can be reactivated by simple addition of H 2 . While the equivalent butadiene complex does not form with R = t Bu at 298 K and on-stream conversion is retained up to 90 h, deactivation is suggested to occur via loss of crystallinity of the SMOM catalyst. Both systems operate under the industrially relevant conditions of an isobutene co-feed. cis:trans selectivites for 2-butene are biased in favor of cis for the t Bu system and are more leveled for Cy.
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