Abstract
Rh-Fe catalysts supported on Ca-Al2O3, MgO and ZrO2 were evaluated in ethanol steam reforming at 623 K and compared to Rh catalysts on the same supports without iron promotion. The metal-support interaction among the three entities, i.e. Rh ↔ Fe2O3 ← support (ZrO2, MgO and Ca-Al2O3) was investigated using H2-chemisorption, TEM, XPS and in situ techniques such as DRIFTS, temperature-resolved XRD and XAS. As compared to the unpromoted Rh catalysts on the same supports, the CO selectivity is depressed in the presence of iron on Rh/MgO and Rh/Ca-Al2O3, the latter being significantly superior. The role of metal-support interaction for CO-free hydrogen generation was unravelled using a combination of techniques. It was found that the reducibility of iron oxide determines the extent of the strong metal support interaction between Rh and Fe2O3 and the reducibility of iron oxide was affected by the support. On Rh-Fe/Ca-Al2O3, a good balance of the interaction between Rh, Fe2O3 and Ca-Al2O3 prevents strong metal support interaction between Rh and Fe2O3 and thus promotes CO elimination via water-gas-shift reaction on Rh-FexOy sites.
Highlights
CH3CH2OH + 3H2O 2 2CO2 + 6H2 (R1)C2H5OH 2 CH4 + CO + H2 (R2)C2H5OH 2 C2H4 + H2O (R3)C2H5OH 2 CH3CHO + H2 (R4)CH3CHO 2 CO + CH4 (R5)CO + H2O 2 CO2 + H2 (R6)
The results from the current study clearly show that iron promotion completely removes the undesired CO by-product on Rh–/Ca–Al2O3 (47.0% vs. 0.0%) while a less effective promotion effect is observed on Rh/MgO (35.9% vs. 4.1%)
A thorough in situ characterization of the promoted catalysts indicates that the interaction between the support and iron oxide plays a crucial role in determining the chemical states of Rh and iron oxides, thereby affecting the chemical nature and the availability of the active sites Rh–FexOy present on the catalyst surface for ESR
Summary
An iron oxide promoted Rh catalyst supported on Ca–Al2O3, reported by our group, is the only noble metal based catalyst to date, which can effectively produce CO-free hydrogen from ESR for 288 h without deactivation at low temperature of 623 K.6 It has been identified that the close proximity between Rh, iron oxide and Ca–Al2O3 support is required to reduce CO selectivity via water–gas shift reaction (WGSR, eqn (R6)) during ESR.[6,7] The synergistic effect between Rh and iron oxide leads to partially reduced iron oxides FexOy species, which are the precursors for. SMSI effect has been reported over Rh and some reducible oxides, such as TiO2, V2O5 and CeO2.13,14 On Rh/V2O5 and Rh/CeO2, a sub-monolayer of VOx and CeOx promote CO hydrogenation, but the reaction rates depends on the extent of SMSI and reduction temperature.[14] Studies on Rh/Fe2O3 interaction have been, less often reported. In this contribution, a series of Rh–Fe catalysts supported on Ca–Al2O3, MgO and ZrO2 was studied for low temperature ESR. The support effect on the formation of the Rh–FexOy active sites is unraveled and discussed
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