Identification and Design of Super-Active Zr

1
Identification and Design of Super-Active ZrWOx
Nano-Clusters for Solid Acid Catalysis (NSF NIRT
0609018 ) Wu Zhou1, Elizabeth I.
Ross-Medgaarden2, William V. Knowles3, Michael S.
Wong3, Israel E. Wachs2 Christopher J. Kiely1
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
Characterizationof WO3/Zirconia Catalysts

• Active Catalysts WO3/ZrOx(OH)4-2xDenoted
  WZrOH, on metastable zirconium oxyhydroxide
  support
• Inactive Model Catalysts WO3/ZrO2 Denoted
  WZrO2, on heat-treated stable Degussa ZrO2
  support
• Incipient Wetness Impregnation with Ammonium
  Metatungstate (NH4)10W12O415H2O
• Calcination Temperatures WZrOH 773-1173K
• 
  Model WZrO2 723K
• Catalyst Activity Testing 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
  

B
Bulk WO3
The starting low activity 2.5WZrO2 model catalyst
exclusively shows highly dispersed surface mono-
and poly-tungstate species. Post-impregnation
with ZrOx alone results in a catalyst displaying
only surface mono- and poly-tungstate species no
clusters were formed and the apparent WOx surface
coverage was comparable to that of the starting
material. 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.
A
C
Dominant surface WOx species
HRTEM
HAADF
HAADF
Intensity Profiles
mono-tungstate(isolated WOx unit)
Low activity 2.9WZrOH-773K TOF1.410-2 sec-1
A
poly-tungstate (2-D network structure having 2-6
WOx units)
HRTEM
HAADF
HAADF
0.8-1nm 3-D Zr-WOx mixed oxide clusters (10-15
inter-linked WOx units) co-exist with
mono-tungstate and poly-tungstate. Contrast
variation within the clusters suggests possible
incorporation of Zr atoms in the WOx cluster
structure.
High activity 6.2WZrOH-1073K TOF6.910-1 sec-1

• Important Temperatures
• Tammann temperature of ZrO2 (1494K) gt
  calcination temperature (973K) unlikely for
  Zr-species to diffuse from the bulk ZrO2 crystal
  into the surface WOx clusters.
• Hüttig temperature of ZrO2 (896K) lt 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.

B
Table 1 Steady-state turnover frequency (TOF)
values for the methanol dehydration to DME
reaction at 573K.
BF-TEM
HAADF
HAADF
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
(3.5W3.5Zr)/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
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.
Inactive model catalyst 5.9WZrO2-723K TOF3.110-3
sec-1
C
TOF 1.2 10-3 s-1.

• 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)