(722b) A Kinetic Study On the Structural and Functional Roles of Lanthana in Iron-Based Catalysts for High-Temperature Water-Gas Shift Reaction

Hydrogen is an invaluable highly-reactive chemical that is used in numerous industrial processes, including ammonia synthesis, fuel cells, and hydrocarbon cracking. This study focuses on the effects of lanthana addition to iron-based catalysts (8 wt% Cr₂O₃, 4 wt% CuO, (88-x) wt% Fe₂O₃, x wt% La₂O₃) used to catalyze the high-temperature water-gas shift reaction for a better hydrogen yield. Lanthana is well known for its thermal stability and carries +3 charge in its more stable oxide form. Therefore, La³⁺ can integrate into the iron-chromium spinel crystal structure as it replaces some Fe³⁺ cations to enhance the structural and functional properties of the catalyst.

Different batches of catalysts were prepared via co-precipitation with varying wt% of lanthana (0, 0.5, 1, 2, and 5) added at the expense of the Fe₂O₃ phase. They were tested for water-gas shift activity at four different reaction temperatures (350, 375, 400, and 425°C). The results show that addition of 0.5 wt% lanthana increased the water-gas shift activity from 27 to 31 mmol of CO/ g_cat·min and reduced the deactivation rate of the catalyst from 23% to 11% at 400°C after 10 days on-stream when compared to the catalyst with no lanthana in it. Compared to literature, this catalyst performed more actively at 425°C than chromia-free catalysts operating at 550°C.Further additions of lanthana (>0.5 wt%) appeared to impose a negative effect on the activity and stability of the catalysts which becomes worse at higher temperatures. According to characterization by X-ray diffraction, the enhanced catalytic stability with the addition of 0.5 wt% lanthana is due to the stabilization of the iron-chromium spinel structure. Further loadings of lanthana (> 0.5 wt%) apparently disrupt the spinel structure due to the larger size of La³⁺ compared to Fe³⁺. BET surface area measurements show that the most stable spent catalyst with 0.5 wt% lanthana had the highest surface area with 67 m²/g_cat compared to 40 m²/g for the catalyst with no lanthana. Scanning electron micrographs of the spent catalysts confirm that this catalyst is the most physically stable with the smoothest surface and least cracks. Temperature-programmed reduction with hydrogen shows that 0.5 wt% lanthana facilitates the reduction of Fe₂O₃ to the active iron oxide phase, Fe₃O₄, by lowering the reduction temperature 15°C.

The kinetic data were fit to three rate law models (power-law, Langmuir-Hinshelwood, and redox) to acquire a mechanistic insight that reasons how the addition of lanthana affects the functionality of the catalysts and makes the catalyst with 0.5 wt% lanthana the most active catalyst. Statistical analysis results showed that the power-law model provides the best fit and that the reaction follows an absorptive mechanism that is well explained by a Langmuir-Hinshelwood rate model with CO adsorption being the rate-determining step. 95% joint confidence regions were obtained for the fitting parameters (adsorption equilibrium constants for water and CO) to show that the adsorption equilibrium constant for CO is largest for the catalyst with 0.5 wt% lanthana (940 L/mol), indicating that the adsorption of CO is facilitated by this minor addition of lanthana. Furthermore, the modeling showed that water adsorbs more strongly on the catalyst surface than CO and therefore inhibits the reaction from taking place.

In summary, 0.5 wt% addition of lanthana increases the activity and stability of iron-based catalysts for high-temperature water-gas shift reaction by stabilizing the iron-chromium spinel structure and facilitating the adsorption of CO on the catalyst surface.