Transcript Slide 1
Identification and Design of Super-Active Zr–WOx Nano-Clusters for Solid Acid Catalysis (NSF NIRT #0609018 ) 1 2 3 3 2 1 Wu Zhou , Elizabeth I. Ross-Medgaarden , William V. Knowles , Michael S. Wong , Israel E. Wachs & Christopher J. Kiely 1 Dept. of Materials Science & Engineering, Lehigh University, Bethlehem, PA 18015. 2 Operando Molecular Spectroscopy & Catalysis Lab, Dept. of Chemical Engineering, Lehigh University, Bethlehem, PA 18015. 3 Dept. of Chemical & Biomolecular Engineering, Rice University, Houston, TX 77005. Electron Microscopy Characterization of WO3/ZrO2 Catalysts → Directly Imaging the Catalytic Active Species Catalyst Design: To Increase the Number Density of the Catalytic Active Sites and Consequently Improve the Catalyst Performance Synthesis, Activity Testing, and Characterization of WO3/Zirconia Catalysts Active Catalysts: WO3/ZrOx(OH)4-2x inactive model WO3/ZrO2 catalyst 2.5 WZrO2-723K B Denoted: WZrOH, on metastable zirconium oxyhydroxide support intermediate, postimpregnated with WOx only Inactive Model Catalysts: WO3/ZrO2 Denoted: WZrO2, on heat-treated stable Degussa ZrO2 support calcination 973K, 3h Incipient Wetness Impregnation with Ammonium Metatungstate: (NH4)10W12O41*5H2O calcination 973K, 3h calcination 973K, 3h co-impregnated with both WOx & ZrOx (3.5W+3.5Zr)/2.5 WZrO2-973K post-impregnated with WOx only (3.5W)/2.5 WZrO2-973K post-impregnated with ZrOx only (3.5Zr)/2.5 WZrO2-973K Bulk WO3 Calcination Temperatures: WZrOH : 773-1173K Catalyst Activity Testing: Zr[OC(CH3)3]4 impregnation, N2 A Model WZrO2 : 723K Starting Model WO3/ZrO2 C Only WOx Addition Only ZrOx Addition Methanol TPSR Spectroscopy → number of exposed surface acid sites Steady-State Methanol Dehydration → turnover frequency (TOF) Aberration Corrected Electron Microscopy: High-Resolution TEM (HRTEM): morphology and crystal structure High-Angle Annular Dark-Field (HAADF) STEM: atomic structure with Z-contrast HRTEM HAADF Post-impregnation with ZrOx alone results in a catalyst displaying only surface mono- and polytungstate species; no clusters were formed and the apparent WOx surface coverage was comparable to that of the starting material. WO3/ZrO2 2 CH3OH H2O + CH3OCH3 (DME) HAADF The starting low activity 2.5WZrO2 model catalyst exclusively shows highly dispersed surface monoand poly-tungstate species. Dominant surface WOx species: Intensity Profiles mono-tungstate (isolated WOx unit) Low activity 2.9WZrOH-773K TOF=1.4*10-2 sec-1 Both ZrOx & WOx Additions Both ZrOx & WOx Additions Post-impregnation with additional WOx precursor generates an additional population of 0.8-1nm WOx clusters. Co-impregnation with both WOx and ZrOx produces a high density population of sub-nm oxide clusters, and intensity variations in HAADF images indicate the successful inclusion of Zr atoms in the WOx clusters. poly-tungstate (2-D network structure having 2-6 WOx units) HRTEM HAADF HAADF High activity 6.2WZrOH-1073K TOF=6.9*10-1 sec-1 B 5 nm 0.8-1nm 3-D Zr-WOx mixed oxide clusters (1015 inter-linked WOx units) co-exist with monotungstate and polytungstate. Contrast variation within the clusters suggests possible incorporation of Zr atoms in the WOx cluster structure. Important Temperatures: • Tammann temperature of ZrO2 (1494K) > calcination temperature (973K): unlikely for Zr-species to diffuse from the bulk ZrO2 crystal into the surface WOx clusters. • Hüttig temperature of ZrO2 (896K) < calcination temperature (973K): the surface ZrOx species (from post-impregnated ZrOx precursor) have sufficient surface mobility to agglomerate and become intermixed with surface WOx species and incorporated into the sub-nm clusters. Table 1 | Steady-state turnover frequency (TOF) values for the methanol dehydration to DME reaction at 573K. BF-TEM Inactive model catalyst 5.9WZrO2-723K TOF=3.1*10-3sec-1 C HAADF HAADF 0.8-1nm pure WOx clusters co-exist with mono-tungstate and poly-tungstate. The different activities indicate the clusters in sample B and C have different compositions. Samples Total W surface density (W-atoms/nm2) W-atoms/nm2 added Zr-atoms/nm2 added Activity † (normalized) 2.5 WZrO2-723K 2.5 0 0 1* (3.5W+3.5Zr)/ 2.5 WZrO2-973K 6.0 3.5 3.5 167 (3.5W)/ 2.5 WZrO2-973K 6.0 3.5 0 4.8 (3.5Zr)/ 2.5 WZrO2-973K 2.5 0 3.5 1.7 6.2 WZrOH-973K 6.2 NA NA 118 5.9 WZrO2-723K 5.9 NA NA 2.6 * TOF = 1.2 ×10-3 s-1. Larger WOx domains would better disperse the extra electron densities transferred onto the WOx species during the acidic catalytic reaction and, thus, help to stabilize acidic sites in this system. The incorporation of Zr into the WOx structure may further change the electronic structure and enhance the catalytic acidity. Thus, the ~0.8-1nm Zr-WOx mixed-oxide clusters exhibit a greater catalytic activity than the ultra-dispersed species (i.e. poly-tungstate with 2-6 WOx units and mono-tungstate with isolated WOx unit.) These post-impregnation experiments demonstrate that both ZrOx and WOx in an intimately mixed form are crucial in forming the catalytically active sites. The formation of mixed Zr-WOx clusters via co-impregnation of both ZrOx and WOx significantly increase the catalytic acidity of the original inactive model catalyst, and make it comparable to the most active WZrOH-type materials. In contrast, post-impregnation of the ZrOx precursor or WOx precursor alone shows only a minimal improvement in catalytic activity. • 0.8-1nm mixed Zr-WOx clusters constitute the most catalytic active species in the WO3/ZrO2 catalyst system. • The precise role of the small amount of incorporated ZrOx species will be investigated with first-principle calculations informed by direct structure observations from aberration-corrected STEM-HAADF imaging. References: [1] Ross-Medgaarden et al. J. Catal. 256, 108-125 (2008) [2] Zhou et al. Nat. Chem. DOI: 10.1038/NCHEM.433 (2009)