Abstract
A nanocrystalline chromium-doped ferrite (FeCr) catalyst was shown to coproduce H2 and multiwalled carbon nanotubes (MWCNTs) during water gas shift (WGS) reaction in a H2-permselective zeolite membrane reactor (MR) at reaction pressures of ~20 bar. The FeCr catalyst was further demonstrated in the synthesis of highly crystalline and dimensionally uniform MWCNTs from a dry gas mixture of CO and CH4, which were the apparent sources for MWCNT growth in the WGS MR. In both the WGS MR and dry gas reactions, the operating temperature was 500 °C, which is significantly lower than those commonly used in MWCNT production by chemical vapor deposition (CVD) method from CO, CH4, or any other precursor gases. Extensive ex situ characterizations of the reaction products revealed that the FeCr catalyst remained in partially reduced states of Fe3+/Fe2+ and Cr6+/Cr3+ in WGS membrane reaction while further reduction of Fe2+ to Fe0 occurred in the CO/CH4 dry gas environments. The formation of the metallic Fe nanoparticles or catalyst surface dramatically improved the crystallinity and dimensional uniformity of the MWCNTs from dry gas reaction as compared to that from WGS reaction in the MR. Reaction of the CO/CH4 mixture containing 500 ppmv H2S also resulted in high-quality MWCNTs similar to those from the H2S-free feed gas, demonstrating excellent sulfur tolerance of the FeCr catalyst that is practically meaningful for utilization of biogas and cheap coal-derived syngas.
Highlights
The water gas shift (WGS) reaction is a key operation in the industrial production of hydrogen from fossil fuel and biomass or biogas-derived syngas
Decomposition is very limited at such a low temperature and, CH4 was unlikely a main source. These results strongly suggest that CH4 decomposition is very limited at such a low temperature and, for the fast multiwalled carbon nanotubes (MWCNTs) growth in the WGS membrane reactor (MR)
The MWCNT formation in the HTP WGS MR apparently involved catalyst surface carbonizations of both CO and CH4, with the latter being a major byproduct of WGS reaction favored at high pressures
Summary
The water gas shift (WGS) reaction is a key operation in the industrial production of hydrogen from fossil fuel and biomass or biogas-derived syngas. The WGS reaction rate is enhanced but the equilibrium CO conversion is lowered when increasing reaction temperature. The concept of high-temperature H2 -permselective WGS membrane reactor (MR) has attracted tremendous interest because of its capability to overcome the limit of equilibrium conversion by instantaneously separating and removing the H2 product from the catalyst bed during reaction. Substantial progress has been made in developing H2 -permselective membranes and WGS catalysts with hydrothermal stability and chemical resistance for processing the H2 S-containing syngas from fossil fuels and biogases. The group of transition metal-doped ferrite catalysts is well known for their excellent activity in catalyzing WGS reaction at high temperatures (350~550 ◦ C)
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