(571a) Carbide and Nitride Supported Water Gas Shift Catalysts

The water-gas-shift (WGS) reaction is
an important industrial reaction used to remove CO from synthesis gas during H₂
production.  There is growing
interest in its use in fuel processing applications.  Molybdenum carbide and nitride has been demonstrated to be
an active WGS catalyst.  Recently
we discovered that the introduction of a metal onto these carbide and nitride
supports significantly improved the WGS activity.  In fact, some formulations were more active than a
commercial Cu-Zn-Al catalyst.  Of
the several factors affecting the performance of these materials, the amount
and type of metal, and pretreatment conditions were the most influential.  In this paper, we discuss the effect of
these variables on selected carbide and nitride supported Ni and Pt
catalysts.  The pretreatment gas consisted
of 100% H₂/He, 15% CH₄/H₂, or 100% NH₃
with temperatures ranging from 200-700 °C.  The metal loading was varied between 0-8 wt %.  Results were compared with those for
unsupported Mo₂C and Mo₂N catalysts, and a commercial
Cu-Zn-Al catalyst.

            Catalyst
activation occurred at temperatures in excess of 300 °C all gases.  The mixture containing 15% CH₄/H₂
produced materials with the highest WGS activities.   Pretreatment in H₂ at temperatures in
excess of 450 °C resulted in a loss of activity. This loss in activity was a
function of the Pt content; the higher the Pt content, the lower the activity
loss.  For catalysts pretreated in
15%CH₄/H₂, there was no appreciable loss in WGS
activity.  Similar results were
seen when Mo₂N was used as a catalyst support.

            Following
pretreatment in 15%CH₄/H₂ at 590 °C, the WGS rates for
the Pt/Mo₂C and Pt/Mo₂N catalysts increased with Pt
loading.  The results suggest a
greater synergistic interaction was seen between Pt and the Mo₂N
support than with the Mo₂C support at high loadings.  For both carbide and nitride supports,
the WGS rates saturated with a Pt loading of 7.5 wt. %. Unlike the supported Pt
catalysts, the maximum loading possible for Ni on either Mo₂C or Mo₂N
was approximately 1 wt. %.

            The
addition of a metal facilitated activation of the catalyst.  Hydrogen TPR results indicated that the
addition of Pt to Mo₂C lowers the reduction temperature by 50 °C
when compared to unsupported Mo₂C.  The addition of Pt or Ni lowered the recarburization
temperature necessary to obtain the maximum WGS rate by 100 °C.  Similar results were observed with the
supported Ni catalysts.   One explanation is that Pt and Ni participated in the
dissociation of H₂ and this dissociated hydrogen spilled-over
facilitating reduction of the support. 
Another possibility is that the metal assists in the dissociation of CH₄.

            X-ray
diffraction, temperature programmed reduction, pulsed chemisorption, thermal
gravimetric analysis and BET surface area experiments were used to characterize
the materials to gain further understanding.   These and other results will be discussed.