Vehicle exhaust catalysis: I. The relative importance of catalytic oxidation, steam reforming and water-gas shift reactions

Spectrokinetic Investigation of Reverse Water-Gas-Shift Reaction Intermediates over a Pt/CeO2 Catalyst

“…35 These data show that, while being close to the thermodynamic equilibrium conversion, the rate of the RWGS was still significantly higher than that of the WGS in the present case. Figure 7 shows typical normalized 13 CO 2 (g), 13 CO(g), and He MS traces obtained after an isotopic switch from a RWGS feed containing 12 CO 2 to one containing 13 CO 2 (and the He tracer) at 498 K. The concentration of the tracer reached 50% of its steady-state value in less than 3 s. As the total flowrate was 100 cm 3 min -1 , this indicated that the dead-volume of the system between the 4-way valve and the mass spectrometer was lower than 5 cm 3 (a detailed description of the cell and its flow response will be reported elsewhere). The time response of the 13 CO 2 species was significantly slower (i.e., about 18 s for 50% exchange), indicating a strong adsorption of this reactant.…”

“…The simultaneous combination of mass spectrometry and DRIFT spectroscopy during SSITKA-type experiments has yielded crucial information about the operando rate of exchange of gas-phase reaction products and that of reaction intermediates present on the surface of a 2%Pt/CeO 2 during the RWGS at 498 K. On one hand, the mass spectrometry results showed that the characteristic exchange time τ CO(g) (defined here as the time needed to achieve 50% exchange) between the two isotopes (i.e., 12 CO(g) and 13 CO(g)) of the main reaction product was 54 ( 6 s. It is important to note that τ CO(g) relates to the ratedetermining-step of the reaction.…”

“…The IR bands of these species exhibited a red-shift when the 12 C isotope was replaced with the heavier 13 C isotope. Since the shift in frequency was usually not sufficient to allow a complete resolution of the 12 C and 13 C-IR bands, the integration of the intensity of the 12 C-bands had therefore to be limited to the high-frequency part of the band where there was no contribution from the 13 C-species.…”

“…Blank experiments without catalyst were also performed in order to determine the fragmentation coefficients of 12 CO 2 with respect to the mass 28 and that of 13 CO 2 with respect to the mass 29. These coefficients were used to subtract the contribution of CO 2 to mass 28 and 29 and extract the actual 12 CO(g) and 13 CO(g) concentration for each experiment. Note that the concentrations of the reactants and products were also determined using an on-line gas chromatograph (Perkin-Elmer 8700), equipped with a HayeSep Q column and a FID-fitted with a methanator to allow the detection of CO and CO 2 .…”

“…[13][14][15][16][17][18][19][20][21][22][23][24][25][26] However, it appears that the activity and stability of ceria-based formulations depends markedly on the preparation methods. 20,27,28 Therefore, there is a need to better understand the way ceria-based materials operate (i.e., in terms of reaction mechanism) to allow further improvements in the formulation and preparation of catalysts.…”