(67e) UV-Visible Spectroscopic Studies of High Temperature Water Gas Shift Catalysts

High temperature water gas shift catalysts (WGS) synthesized
by co-precipitation with typical industrial formulations containing iron
oxides, chromia, and copper were augmented by small additions (1 to 20 wt%) of
rare earth oxides, such as ceria or lanthana, at the expense of the active iron
phase.  The catalysts were characterized by standard techniques and kinetically
evaluated under WGS conditions.  The catalysts were evaluated using operando
UV-visible spectroscopy to correlate reaction rate with absorption response and
calibrated extent of reduction measurements.  Although these catalysts appear
black, the absorption spectra respond sensitively to the reaction conditions
and provide useful information on the extent of catalyst reduction.

Figure 1 shows preliminary results from in-situ UV-visible
spectroscopic experiments on a catalyst with a fully oxidized composition of 1
wt% La₂O₃ with 87 wt% Fe₂O₃, 8 wt%
Cr₂O₃, and 4 wt% CuO.  The absorbance spectra were
obtained during a series of temperature programmed reductions (TPR's) and
temperature programmed oxidations (TPO's) that were followed by water gas shift
conditions for increasing amounts of time.  The sensitive response near 12,500
cm^-1 has been correlated to the extent of catalyst reduction and
reaction rate.  Based on a calibration from the hydrogen TPR's, all of the Fe₂O₃
in the catalyst appears to be reduced to Fe₃O₄ after
approximately 20 hours under WGS conditions.

Figure 1.  In-situ UV-visible absorbance spectra obtained
following TPR's and TPO's and then at specific times during WGS conditions (T =
400°C, 2:1 H₂O:CO, catalyst initial composition of 87 wt% Fe₂O₃,
8 wt% Cr₂O₃, 4 wt% CuO, 1 wt% La₂O₃).