Abstract
The decarboxylation pathway in ethanol steam reforming ultimately favors higher selectivity to hydrogen over the decarbonylation mechanism. The addition of an optimized amount of Cs to Pt/m-ZrO2 catalysts increases the basicity and promotes the decarboxylation route, converting ethanol to mainly H2, CO2, and CH4 at low temperature with virtually no decarbonylation being detected. This offers the potential to feed the product stream into a conventional methane steam reformer for the production of hydrogen with higher selectivity. DRIFTS and the temperature-programmed reaction of ethanol steam reforming, as well as fixed bed catalyst testing, revealed that the addition of just 2.9% Cs was able to stave off decarbonylation almost completely by attenuating the metallic function. This occurs with a decrease in ethanol conversion of just 16% relative to the undoped catalyst. In comparison with our previous work with Na, this amount is—on an equivalent atomic basis—just 28% of the amount of Na that is required to achieve the same effect. Thus, Cs is a much more efficient promoter than Na in facilitating decarboxylation.
Highlights
Ethanol has been used as a production automotive biofuel since the late 1970s, and US ethanol production reached approximately 5.8 × 1010 L in 2017 [1]
We found that acceleration of the water–gas shift (WGS) rate occurs as a step change improvement within a certain loading range (e.g., 1.8% to 2.5% Na for 2% Pt/zirconia), which corresponded to (1) a pronounced shift in the formate ν(CH) band to lower wavenumbers in in situ infrared studies and (2) low coverage of Pt by Na, as measured by the intensity of the ν(CO) band for CO adsorbed on Pt
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the temperature-programmed reaction of ethanol steam reforming as well as fixed bed catalyst testing revealed that the addition of Cs promotes the decarboxylation/demethanation pathway over decarbonylation by increasing the basicity of the catalyst and weakening the C–C bond of the acetate intermediate, facilitating its scission
Summary
Ethanol has been used as a production automotive biofuel since the late 1970s, and US ethanol production reached approximately 5.8 × 1010 L in 2017 [1]. First-generation sources, which compete with food production, include primarily sugar cane, sugar beets, and corn, but in recent years, there has been a shift in research focus toward next-generation sources that do not compete with food production, such as cellulosic ethanol from grasses, wood, and algae [2]. The production of cellulosic ethanol increased from 2014 to 2015 from 728,509 gallons to 2,181,096 gallons [3]. Light alcohols are being considered as a chemical carrier for hydrogen for use in polymer electrolyte membrane (PEM) fuel cells, and bioethanol is attractive due to its renewable production as described previously. Decarboxylation routes in the mechanism of ethanol steam reforming improve the H2 selectivity by as much as 50% as compared to decarbonylation routes: C2H5OH + 3H2O → 2CO2 + 6H2 (∆H298◦ = 173 kJ/mol)
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