(478c) Tuning the Electronic and Molecular Structures of Catalytic Active Sites with Oxide Support Nanoligands | AIChE

(478c) Tuning the Electronic and Molecular Structures of Catalytic Active Sites with Oxide Support Nanoligands

Authors 

Ross, E. I. - Presenter, Lehigh University
Wachs, I. E. - Presenter, Lehigh University
Burrows, A. - Presenter, Lehigh University


Introduction

Supported metal oxide catalytic materials play a dominant role in catalytic processes of the petroleum, chemical and environmental industries. The catalytic active sites in supported metal oxides are significantly affected by the charge transfer between the surface metal oxide active component and the underlying oxide support ligand. The charge transfer can modify the electron density of the surface metal oxide catalytic active sites and dramatically influence the resultant catalytic performance. Thus, oxide support nanoligands can assist in the design of new and novel catalytic materials.

Materials and Methods

The supported TiO2/SiO2 system was chosen as the support material to synthesize ~1-10 nm titania nanoligands on the relatively inert and amorphous SiO2 matrix by varying the titania content. The electronic structures, local electron density of the titania nanoligands, were determined with in situ UV-Vis spectroscopy. The molecular structures of the surface WOx and VOx species were provided with in situ Raman and also in situ XANES in the case of the surface TiOx species. The crystalline phases were identified with Raman and their dimensions were measured with transmission electron spectroscopy (TEM). The surface chemistry of the surface TiOx, WOx and VOx species was chemically probed with CH3OH-temperature programmed surface reaction (TPSR) spectroscopy and steady-state CH3¬OH oxidation and dehydration studies. The CH3¬OH molecule can discriminate between different types of surface sites since acidic surface sites exclusively yield dimethyl ether, CH3OCH3, and redox surface sites primarily produce formaldehyde, H2CO, as the reaction product. CH3OH-TPSR provided fundamental surface kinetic information and the steady-state experiments provided quantitative catalytic activity (TOF), selectivity and thermodynamic information. The electronic/molecular structures of the catalytic active sites were compared with the corresponding kinetic and thermodynamic parameters to establish electronic/molecular structure-activity/selectivity relationships.

Results and Discussion

Raman and UV-Vis spectroscopy revealed that the dehydrated 1% TiO2/SiO2 sample consists of isolated surface TiO4 species and that the 12% TiO2/SiO2 samples consists of a two-dimensional monolayer of polymeric surface TiO5 species. Bright field images and selected area electron diffraction patterns revealed only the presence of surface TiOx on the amorphous SiO2 particles (< 12% TiO2/SiO2) and crystalline TiO2 (anatase) nano-particles (> 12% TiO2/SiO2) that also consumed the surface TiOx layer on the amorphous SiO2 support. The UV-Vis band gap energy, Eg, of the TiO2 nanoligands on the SiO2 matrix systematically decreased with increasing domain size of the titania nanoligands. Smaller band gap energy values correspond to lower local electron density. Therefore, the local electron density decreases as the titania domain size increases and, consequently, electron delocalization increases.

The supported redox vanadium oxide and acidic tungsten oxide catalytic active sites were synthesized by impregnation onto the SiO2 supported titania substrates. Raman spectroscopy established that the vanadia and tungsten oxide preferentially self-assembled on the surface titania nanoligands as surface VOx/WOx species and that crystalline V2O5 and WO3 particles were not present in any of the final catalysts (100% dispersion of the catalytic active components). The surface vanadia species maintanined a constant molecular structure as a function of TiO2 domain size since the Raman terminal V=O vibration remained constant at ~1031 cm-1. The molecular structure of the surface tungsta species was also constant as a function of TiO2 domain size with a terminal W=O Raman vibration at ~1010 cm-1. These Raman band positions also reveal that the surface VOx and WOx species are not coordinated to the SiO2 matrix since these surface species on SiO2 give rise to different Raman bands on SiO2 (~1040 and 960 cm-1, respectively).

The reactivity of the supported redox surface VOx and acidic surface WOx catalytic active sites was significantly influenced by the domain size of the titania nanoligand. The redox surface VOx and the acidic surface WOx, however, responded inversely to each other to the influence of the titania domain size. The activity of the surface WOx decreased with increasing domain size indicating that smaller titania domain size resulted in more acidic surface WOx sites. This reveals that surface acidic sites are more active when coordinated to oxide support nanoligands with higher band gaps (higher local electron density and less electron delocalization). In contrast, the activity of the redox surface VOx sites increased with increasing domain size of the titania nanoligands. This reveals that surface redox sites are more active when coordinated to oxide support nanoligands with lower band gaps (lower electron density and more electron delocalization). Thus, the activity of redox surface VOx sites and the acidic surface WOx sites can be tuned by controlling the electron density of the oxide support nanoligands.

Significance

These model studies with titania nanoligands in a SiO2 matrix demonstrated for the first time how oxide support nanoligands tune the catalytic activity of surface redox and surface acidic catalytic active sites. Surface acidic sites function more efficiently when their electron density are low and surface redox sites function more efficiently when their electron density are high. These new fundamental insights can assist in the molecular engineering of novel supported metal oxide catalysts by tuning the redox and/or acidic surface functionalities.

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