Carbon Nanotube Formation on Cr-Doped Ferrite Catalyst during Water Gas Shift Membrane Reaction: Mechanistic Implications and Extended Studies on Dry Gas Conversions

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.

Zeolite-based catalytic membrane reactors for thermo-catalytic conversion of CO2

“…Based on the above discussions, it would be attractive if an integrated reactor system is constructed to form a synergy of physical and chemical processes. One of the representatives is the membrane catalytic reactor, which has been applied in environmental protections and exhaust gas conversions ( Bian et al., 2020b ; Yang et al., 2013 ; Wang et al., 2020a , 2020b ), such as CO oxidation ( Medrano et al., 2018 ; Peng et al., 2019 ; Bernardo et al., 2008 ; Galletti et al., 2008 ), water-gas shift ( Arvanitis et al., 2018 ; Tang et al., 2010 ; Kim et al., 2012 , 2013a , 2013b ; Wang et al., 2014 ; Zhang et al., 2012 ; Dong et al., 2015 ; Yin et al., 2019 ; Bang et al., 2021 ; Sun et al., 2020 ) and volatile organic compound (VOC) combustion ( Liu et al., 2020c ; Chen et al., 2014 ; Yan et al., 2014 ). Specifically, membrane catalytic reactors are also deemed as a promising candidate for the conversion of CO 2 into value-added products.…”