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Abstract
The thesis is focused on the study of high temperature water gas shift catalysis, the
identification of new improved catalysts and the study of the kinetics and mechanism
of reaction over these catalysts. Rh-promoted Fe2O3-Cr2O3 was found to offer best
performance which was significantly better than unpromoted catalyst over wide
temperatures range.
An extensive literature survey is first reported. Guidelines to develop new WGS
catalysts are developed. As a result, the activities of precious metals supported on
various oxides for high temperature WGS reaction have been tested.
Rh(1wt%) doped Fe2O3/Cr2O3, exhibits the highest activity for WGS over a wide
temperature range. 5wt%CuO/Fe2O3-Cr2O3, 1wt%Pt/Cr2O3, 1wt%Pt/Fe2O3-Cr2O3,
1wt%Pt/U3O8, 1wt%Pt/10%U3O8-Al2O3 and 1wt%Pt/5%V2O5-TiO2 fall into the
second most active catalysts group, with an improved activity compared to commercial
Fe2O3-Cr2O3 catalyst. It is clear that both the support/catalyst and the promoter can
affect the efficiency of the WGS, leading to the obvious inference that the reaction rate
is controlled at the promoter – support interface.
Further kinetic studies and characterisation (TPR, TPD, pulse-adsorption (reaction)) on
Rh/Fe3O4/Cr2O3 have been conducted. The study, conducted under conditions without
inhibition from products of both forward and backward reactions, shows that the
overall reaction rate expression is described as:
2 22
− =0.0041exp(−4042.6 ) 0.64 0.5 −0.024 exp(−6022.9 ) 0.46 0.73 CO CO H O CO H r PP P P
T T .
Kinetics studies carried out under fuel reforming gas compositions shows that reaction
rate expression changed when the temperature of reaction varied. The reaction rate
equations at temperatures of 573K, 623K and 673K are derived as:
573K: 2 2 2
- 2.84 10-6 0.6 0.12 - 9.08 10-7 0.09 0.52 rCO = × PCO PH O × PCO PH
623K: 2 2 2
- 1.45 10-6 0.99 0.40 - 7.12 10-7 0.11 0.73 rCO = × PCO PH O × PCO PH
673K: -6
2 2 2
- = 4.37 × 10 0.86 0.41 -1.83 ×10-6 0.28 0.66 rCO PCO PH O PCO PH ,
The apparent activation energy was 61.7±2.5 kJmol-1
.
TPR, TPD, TPO characterisation studies and reoxidation of catalysts by CO2 or H2O
show that the active site for high temperature WGS reaction on Rh/Fe2O3/Cr2O3 is
reduced magnetite Fe3O4 which dissociatively breaks down the H2O to form H* and
OH* and adsorbs CO2. The deposited metal, Rh, acts as a promoter by facilitating the
uptake of hydrogen (H2) and carbon monoxide (CO), desorption of H2 (at high
temperature) and CO2.