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).