NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-6, 2007

Grant # : 0609018

Tuning the Electronic and Molecular Structures of Catalytic Active Sites with Oxide Nanoligands

NSF NIRT Grant 0609018

Israel E. Wachs (PI)1*, Christopher J. Kiely (co-PI)2, Michael S. Wong (co-PI)3 and Matthew Neurock (co-PI)4

1 Operando Molecular Spectroscopy and Catalysis Lab., Chemical Engineering Dept., Lehigh University, Bethlehem, PA 18015 USA

2 Center for Advanced Materials and Nanotechnology, Materials Science and Eng. Dept., Lehigh University, Bethlehem, PA 18015 USA

3 Dept. of Chemical and Bimolecular Engineering and Dept. of Chemistry, Rice University, Houston, TX USA

4 Department of Chemical Engineering, University of Virginia, Charlottesville, VA USA

*

Strategically applying nanotechnology synthesis methods facilitates the development of novel materials with enhanced properties [1]. It has been well-established for several decades that synthesizing metal and semiconductor materials as nanoparticles, or nanometric domains, often results in unique chemical and electrical properties that are distinct from their bulk and molecular counterparts [2-4]. Early systematic studies with supported metal catalysts clearly demonstrated that nano-sized metallic catalytic active sites can influence the catalytic performance of some structure sensitive catalytic reactions [5]. For structure-sensitive reactions, the specific reactivity with decreasing domain size in the 1-10 nm range of the catalytic active sites can either increase (e.g., oxidation of H2 to H2O in excess H2 over Pt catalysts) or decrease (e.g., synthesis of NH3 from N2 and H2 over Fe catalysts). For structure-insensitive reactions, however, the specific catalytic activity is independent of the domain size of the metallic catalytic active site (e.g., hydrogenation of cyclohexene over Pt). These reactivity trends have been shown to be dependent on the nature of the interactions of the reactant molecules with the specific surface metal atom arrangements of the catalytic active sites [5-7].

Apart from examination of metal systems, little work has been done to systematically examine the effect of size for metal oxide domains in supported metal oxide catalyst. The surface structure-sensitivity of several specific catalytic reactions over metal oxide surfaces, however, is well documented in literature [5, 8-12]. This suggests that the domain size of metal oxide catalysts should also affect the reactivity of structure-sensitive reactions.

The objective of this NSF NIRT study is to systematically examine and establish the reactivity trends as well as their origins for nanoscale metal oxide catalytic active site domains for redox and acidic reactions. The metal oxide materials chosen for this investigation consist of well defined, model supported metal oxide catalysts. Supported metal oxide catalysts are composed of an amorphous two-dimensional surface metal oxide overlayer on an oxide substrate [13-15], and are extensively employed in the environmental, energy and petrochemical industries [16-19]. For supported metal oxide catalytic materials involved in redox reactions, the most significant parameter affecting the specific reactivity of the catalytic active site is the specific oxide support ligand (e.g., Al2O3, TiO2, ZrO2, etc.). The turnover frequency (TOF), defined as the number of molecules converted per catalytic active site per second, can vary by as much as ~103 for such catalytic systems when the specific oxide support ligand is varied [15,1 9]. Consequently, the influence of oxide nanoligand substrates upon the specific reactivity of redox and acidic catalytic active sites of supported metal oxide catalysts is the focus of this research.

A series of supported 1-50% TiO2/SiO2 catalysts were synthesized and subsequently used to anchor surface VOx redox and surface WOx acid sites. The supported TiOx, VOx and WOx phases were physically characterized with TEM, in situ Raman and UV-vis spectroscopy, and chemically probed with CH3OH-IR, CH3OH-TPSR and steady-state CH3OH dehydration. The supported TiO2 phase was present as fully dispersed surface TiOx species at low titinia loadings and crystalline anatase-TiO2 nanoparticles (~0.5-10 nm) at higher loadings. The domain size variation in the supported TiO2 phase with titania content gives rise to a decreasing UV-vis edge energy decrease from ~4.3 to ~3.2 eV that reflects the increasing electron delocalization for the supported titania phase. The CH3OH chemical probe revealed that the surface VOx sites possess are redox in nature and the surface WOx sites contain acidic character. For the redox surface VOx sites anchored onto the titania component, the redox TOF increased with increasing domain size of the titania phase. For the acidic surface WOx sites anchored onto the titania component, the acidic TOF decreased with increasing domain size of the titania phase. The opposite dependencies of the redox surface VOx sites and acidic surface WOx sites reflect the different electronic requirements of redox and acidic catalytic active sites. This study demonstrates that the catalytic activity of surface redox and acidic sites can be tuned by varying the domain size of oxide support nanoligands. Current studies with zirconia produce smaller oxide nanoligands and show similar results.

These model studies with titania nanoligands in a SiO2 matrix have demonstrated for the first time how oxide support nanoligands can tune the catalytic activity of surface redox and surface acidic catalytic active sites. These new insights can assist in the molecular engineering of novel supported metal oxide catalysts by tuning the redox/acidic surface functionalities.

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