Mechanism of the Periodic Unsteady-State Water–Gas Shift Reaction on Highly Dispersed Cu-Loaded CeO₂ Catalysts

Abstract

Kinetic analyses of Ce 4+ ↔ Ce 3+ redox and CO 2 /H 2 formation for the unsteady-state water−gas shift (WGS) reaction under periodic CO ↔ H 2 O feeds to Cu/CeO 2 catalysts are carried out by in situ/operando ultraviolet−vis and infrared studies at 350 °C. Under CO, the Ce 4+ −OH species are reduced to produce H 2 , CO 2 , and Ce 3+ −□ (oxygen vacancy). Under the subsequent feed of H 2 O, Ce 3+ −□ is reoxidized by H 2 O to yield H 2 and Ce 4+ −OH species. The rates of Ce 4+ reduction/ Ce 3+ reoxidation are close to those of CO 2 /H 2 formation for various Cu/CeO 2 catalysts with different Cu loadings, providing quantitative evidence of the redoxbased mechanism of the unsteady-state WGS reaction. Ce 3+ −□ reoxidation by H 2 O has a lower apparent barrier than the Ce 4+ −O reduction step. The H 2 O-promoted desorption of the adsorbed carbonates is responsible for CO 2 formation under H 2 O.The characterization results suggest that the number of interfacial sites between the CeO 2 and Cu species increases with decreasing Cu loading. Turnover frequencies per surface Cu site for the Ce 4+ ↔ Ce 3+ redox reaction and CO 2 /H 2 formation increase with the number of interface sites. An associative redox mechanism based on the redox reaction between the oxidized state (Cu 2+ −OH adjacent to Ce 4+ and Ce 4+ −OH) and the reduced state (Cu + −□−Ce 3+ ) is proposed as the main catalytic cycle of the unsteady-state WGS reaction.

Mechanism of NH3-SCR over P/CeO2 Catalysts Investigated by Operando Spectroscopies

“…The relative KM values with respect to the initial KM value (ΔKM) at 570 nm were also plotted versus NO feed time. Exposure of the preoxidized P(3)/CeO 2 to NO resulted in an increase in the absorption bands in the range of 500–800 nm, assignable to Ce 3+ -□. , The following stoichiometric scheme may account for this result and the IR results (Figure ) obtained under the same conditions. …”

Continuous Unsteady-State De-NOx System via Tandem Water–Gas Shift, NH3 Synthesis, and NH3–SCR under Periodic Lean/Rich Conditions

“…Ceria-supported Cu (Cu/CeO 2 ) catalysts are among the most widely studied catalysts for the WGS reaction. − Moreover, Manaka et al , and Bhan et al reported that a series of catalysts could be used for selective NH 3 formation from the NO–CO–H 2 O reaction, but the Cu/CeO 2 was the most effective one. To find the optimal catalyst for NH 3 synthesis using the NO–CO–H 2 O reaction, we compared the activity and selectivity of Cu­( x )/CeO 2 catalysts with different Cu loadings ( x = 2, 5, and 10 wt %) under the conditions of 0.1% NO, 0.5% CO, and 1% H 2 O (Figure ).…”

“…In our previous work, 29 the number of surface Cu sites (Cu dispersion) of Cu/CeO 2 catalysts (after H 2 reduction) was estimated using the N 2 O pulse method. The Cu dispersion decreased with an increasing Cu loading.…”

“…The catalysts were pretreated in situ under 10% O 2 /He at 500 °C for 0.5 h before the measurements. The Cu dispersion of the H 2 -reduced Cu/CeO 2 samples was measured by N 2 O pulse experiments . The sample was oxidized with 21% O 2 /He at 500 °C (0.5 h) and then reduced by 5% H 2 /Ar at 350 °C (0.5 h).…”

“…The Cu dispersion of the H 2reduced Cu/CeO 2 samples was measured by N 2 O pulse experiments. 29 The sample was oxidized with 21% O 2 /He at 500 °C (0.5 h) and then reduced by 5% H 2 /Ar at 350 °C (0.5 h). Then, the N 2 O pulse measurements were conducted at 50 °C with BELCat II, and the consumption of N 2 O and production of N 2 were recorded using a MS apparatus (BELMass, MicrotracBEL Corp.).…”