(337e) Mechanism for Carbide Supported Water Gas Shift Catalysts | AIChE

(337e) Mechanism for Carbide Supported Water Gas Shift Catalysts


King, T. E. - Presenter, University of Michigan
Bej, S. K. - Presenter, University of Michigan

water-gas-shift (WGS) reaction is an important industrial reaction used to
remove CO from synthesis gas during H2 production.  The reaction is gaining importance as a
consequence of increased interest in producing H2 for fuel
cells.  The widely used low
temperature WGS catalyst formulation, Cu-Zn-Al, has low intrinsic activity
resulting in relatively large reactor volumes.  Recently we discovered that the introduction of Group VIII
metals onto Mo2C significantly improved the WGS activity.  In fact, some of the carbide-based
formulations were more than an order of magnitude more active than a commercial
Cu-Zn-Al catalyst.  In this paper
we discuss the WGS mechanisms for Mo2C and Pt/Mo2C
catalysts.  The data was fitted to
the power law, Langmuir-Hinshelwood-Hougen-Watson (LHHW), and redox
models.  The results suggested that
mechanisms for the carbide-supported materials were different from that for the
Cu-Zn-Al catalyst.  The WGS rate
for Mo2C was nearly first order in CO concentration and almost zero
order with respect to H2O. 
These orders were essentially reversed with the introduction of Pt.  The Cu-Zn-Al catalyst was nearly first
order in both CO and H2O. 
The zero order dependency on CO for the Pt/Mo2C makes it
superior to Cu-Zn-Al at low CO concentrations.  The reaction over Mo2C proceeds through a
mechanism in which CO and H2O compete for adsorption sites. Results
for the Pt/Mo2C catalyst suggested a dual site mechanism involving
the non-competitive adsorption of CO and H2O.  The reaction rate for Pt/Mo2C
was best described using a LHHW model with surface reaction being the
controlling step.  The beneficial
role of Pt was also evident from the CO and H2O chemisorption
data.  Diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS) and in-situ pulse reaction studies were also used to provide
additional mechanistic understanding. 
In addition, the models were used to estimate sizes for the WGS
reactors.  Reactors incorporating
the Pt/Mo2C catalysts are projected to be an order of magnitude
smaller than that for a Cu-Zn-Al catalyst.  These and other results will be discussed.