Influence of Cs Loading on Pt/m-ZrO2 Water–Gas Shift Catalysts

Abstract

Certain alkali metals (Na, K) at targeted loadings have been shown in recent decades to significantly promote the LT-WGS reaction. This occurs at alkali doping levels where a redshift in the C-H band of formate occurs, indicating electronic weakening of the bond. The C-H bond breaking of formate is the proposed rate-limiting step of the formate associative mechanism, lending support to the occurrence of this mechanism in H2-rich environments of the LT-WGS stage of fuel processors. Continuing in this vein of research, 2%Pt/m-ZrO2 was promoted with various levels of Cs in order to explore its influence on the rate of formate intermediate decomposition, as well as that of LT-WGS in a fixed bed reactor. In situ DRIFTS experiments revealed that Cs promoter loadings of 3.87% to 7.22% resulted in significant acceleration of the forward formate decomposition in steam at 130 °C. Of all of the alkali metals tested to date, the redshift in the formate ν(CH) band with the incorporation of Cs was the greatest. XANES difference experiments at the Pt L2 and L3 edges indicated that the electronic effect was not likely due to an enrichment of electronic density on Pt. CO2 TPD experiments revealed that, unlike Na and K promoters, Cs behaves more like Rb in that the decomposition of the second intermediate in LT-WGS, carbonate species, is hindered due to (1) increased basicity of Cs, (2) the tendency of Cs to cover Pt sites that facilitate CO2 decomposition, and (3) the tendency of Cs to increase Pt particle size as shown by EXAFS results, resulting in fewer Pt sites that facilitate CO2 decomposition. As such, the LT-WGS rate was hindered overall and the rate-limiting step shifted to carbonate decomposition (CO2 removal). Like its Rb counterpart, low levels of added Cs (e.g., 0.72%Cs) were found to improve the stability of the catalyst relative to the unpromoted catalyst; the stability comparison was made at similar CO conversion level as well as similar space velocity.

Influence of Cs Promoter on Ethanol Steam-Reforming Selectivity of Pt/m-ZrO2 Catalysts at Low Temperature

“…One additional piece of evidence is that the difference in position of the ν(OCO) asymmetric and symmetric bands changes after adding Cs, and there are further changes with increasing loading of Cs. Such a trend was also observed for the ν(OCO) asymmetric and symmetric bands of formate in our recent WGS work [20]. Examining Table 3, for the unpromoted catalyst, this difference is ∆ 112 cm −1 ; for example, adding Cs, the difference changes as follows: ∆ 130 cm −1 for 2.9% Cs; ∆ 136 cm −1 for 3.9% Cs; and ∆ 142 cm −1 for 4.8% Cs.…”

“…The underlying reason for the selectivity changes may stem from electronic and/or geometric modifications to the catalyst surface structure. In our recent work on WGS with Cs addition [20], there is clearly an electronic modification of the formate intermediate, as the ν(CH) band is shifted to lower wavenumbers by addition of Cs, as well as with increasing Cs loading (Figure 13). This shift in band position is consistent with a weakening of the C-H bond.…”

“…Figure 14 was discussed in our prior work on WGS [20]. The possibility that the alkali may be donating electron density to Pt has been proposed in the past for certain catalysts.…”

“…It was also of interest to examine Cs as a potential promoter. In a recent WGS investigation examining the loading effect [20], the addition of Cs led to greater shifts in the formate ν(CH) stretching band to lower wavenumbers than observed previously with Na. As such, at loadings of 3.9-7.2% Cs, in situ infrared studies revealed that formate decomposed in the forward direction (i.e., to H 2 and CO 2 ) more rapidly in steam than previously observed with Na.…”

“…Figure 3. TCD signal (black) and MS signals of (red) H 2 , (blue) CO, and (green) CO 2 during temperature-programmed reduction in 10% H 2 /Ar for (a) 2% Pt/ZrO 2 , (b) 2.9% Cs-2% Pt/ZrO 2 , (c) 3.9% Cs-2% Pt/ZrO 2 , (d) 4.8% Cs-2% Pt/ZrO 2 , (e) 5.8% Cs-2% Pt/ZrO 2 , (f) 7.2% Cs-2% Pt/ZrO 2 , (g) 10.4% Cs-2% Pt/ZrO 2 , and (h) 14.5% Cs-2% Pt/ZrO 2 (reprinted from[20]). …”