Alkali metals (Na, K) have been shown to promote the low temperature water-gas shift (LT-WGS) reaction, and one proposal is that the role of the alkali promoter is to accelerate the decomposition of a WGS intermediate, formate, by weakening its C-H bond.According to the reactant-promoted mechanism, C-H bond breaking is the rate-limiting step of the mechanism. based on in-situ DRIFTS experiments, Cs promoter loadings in the range of 3.87% to 7.22% added to 2%Pt/m-ZrO2 resulted in significant acceleration of the forward formate decomposition to H2 and carbonate. However, based on CO2 TPD experiments, the decomposition rate of the second intermediate in LT-WGS, a 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, thus decreasing Pt sites that facilitate CO2 decomposition at the Pt/ZrO2 interface where the catalysis occurs. As such, unlike Na and K studied previously, Cs was not an effective promoter for improving initial CO conversion in LT-WGS. Interestingly, an improvement in catalyst stability was observed over that of the undoped catalyst at a low loading of 0.72%Cs.Because an optimized amount of Cs to Pt/m-ZrO2 catalysts promoted formate decarboxylation in LT-WGS, we examined Cs as a potential promoter for ethanol steam reforming (ESR), where an analogous intermediate, acetate, is proposed for the catalytic cycle. In this case, we examined low temperature ESR for producing CO2, H2, and CH4, with the methane being assumed to be fed to a downstream commercial reformer. Cs-doped Pt/m-ZrO2 catalysts with optimum loading were found to offer the potential to produce hydrogen with higher H2 selectivity by favoring the selective decarboxylation pathway over that of the unselective decarbonylation route. For example, addition of 2.9%Cs was able to stave off the decarbonylation pathway completely at an ethanol conversion of 73%, whereas the undesired CO selectivity of the unpromoted catalyst was greater than 20%. On an atomically equivalent basis, Cs was found to destabilize formate and acetate more than Na. The higher basicity of Cs at all loadings tended to decrease ethanol conversion once again due to CO2 product inhibition. At the optimum Cs loading of 2.9% for selectivity, ethanol conversion was 14% lower (absolute basis); nevertheless, the improvement in selectivity outweighs the slight decrease in conversion.

Potential of Cs as a promoter of Pt/m-ZrO2 catalysts in the low temperature water-gas shift and ethanol steam reforming reactions

ACS Spring 2022

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