N-doped carbon nanofibers with various nitrogen contents
and chemistries
have been grown on cordierite monoliths by varying the synthesis conditions.
Ru has been impregnated on the N-doped CNFs, on undoped CNFs, H2O2-treated CNFs, and alumina coated monoliths.
These catalysts have been characterized (XPS, TPR, STEM, CO chemisorption)
after several preparation stages and they have been tested in ammonia
decomposition. It has been found that nitrogen doping contributes
to stabilize Ru to a small particle size and in a reduced state. The
strong interaction of the precursor with the nitrogen groups enables
the preparation of small Ru nanoparticles, uniformly distributed throughout
all the CNFs coating the monolith. Catalysts supported on N-doped
CNFs have exhibited NH3 decomposition activities higher
than catalyst on N-free CNFs. It has been evidenced that the catalyst
with the highest amount of nitrogen facilitates keeping Ru in reduced
state upon air exposure. This enhanced reducibility correlates with
substantially higher TOF in ammonia decomposition than for the other
catalysts supported on CNFs with various functionalizations.
Ru nanoparticles were supported on monoliths that were coated with variously functionalized carbon nanofibers (CNFs), that is, un‐doped CNFs, CNFs that had been post‐treated with H2O2, and CNFs that had been doped with nitrogen during their growth. The Ru uptake (by equilibrium adsorption) onto N‐doped CNFs was larger compared to that on their un‐doped and O‐doped counterparts. The functionalization of the CNF support did not play a significant role in determining the size of the deposited Ru nanoparticles, but it substantially impacted on the sintering under the reaction conditions and on the electron density of the reduced metal. Among the catalysts on the different CNF supports, Ru on N‐CNF exhibited the highest H2 productivity from ammonia decomposition, which pointed to electronic effects that were induced by functionalization of the support.
A well attached coating of nitrogen-functionalised carbon nanofibers (N-CNFs) has been prepared on the walls of cordierite monolith channels. It is formed via concurrent decomposition of ethane and ammonia catalysed by nickel nanoparticles dispersed on alumina coated cordierite monolith. N-CNF/monoliths synthesis employing several growth temperatures and NH(3) compositions was exhaustively characterised by Raman, XPS, elemental analysis and TEM. Synthesis conditions affected profoundly content and type of nitrogen functionality, enabling its fine tuning. N-CNFs surface chemistry and microstructure differed remarkably from its N-free counterparts.
Carbon materials have rarely been used as support for CO methanation, which is usually carried out using catalysts supported on metal oxides. Here, it is shown that Ru nanoparticles supported on nitrogen-doped carbon nanofibers (NCNF) provide competitive CH production rate and stability compared to Al O -supported catalysts. Contrary to the general belief about the inert nature of carbon supports, it is demonstrated that NCNF is a non-innocent spectator in CO methanation due to its ability to store a high amount of CO reaction intermediates. This explains the excellent catalytic behaviour afforded by this unconventional catalyst support.
Catalytic CO2 reduction has been performed using carbon nanofibers or nitrogen‐doped carbon nanofibers as a support for Ru nanoparticles. The catalyst that consists of 5 wt % Ru on nitrogen‐doped carbon nanofibers exhibited the highest conversion at a relatively low temperature, complete selectivity to CH4, and stable catalytic performance. The catalytic performance was substantially superior to catalysts supported on carbon nanotubes and akin to the best metal‐oxide‐supported catalyst in the literature. The characterization of the prepared catalyst by transient experiments (CO2 temperature‐programmed desorption, temperature‐programmed surface reaction, and transient response to CO2 removal) revealed that the catalyst support participates actively in the reaction. The Ru content governed the selectivity, which either favored CO formation for lower Ru contents (0.5–2 wt %) or CH4 formation for 5 wt % Ru loading. The mean Ru particle size determined by TEM was similar for each of the metal loadings. Therefore, the substantially different selectivity patterns cannot be attributed to structure sensitivity. The higher selectivity to CH4 can be explained by the enhanced supply of adsorbed hydrogen to the activated adsorbed CO intermediate, which was demonstrated to be the rate‐determining step.
Carbon nanofibres (CNFs) were modified with B and P by an ex situ approach. In addition, CNFs doped with N were prepared in situ using ethylenediamine as the N and C source. After calcination, the doped CNFs were used as catalysts for the oxidative dehydrogenation of propane. For B-CNFs, the effects of boron loading and calcination temperature on B speciation and catalytic conversion were studied. For the same reaction temperatures and conversions, B- and P-doped CNFs exhibited higher selectivities to propene than pristine CNFs. The N-CNFs were the most active but the least selective of the catalysts tested here. Our results also show that the type of P precursor affects the selectivity to propene and that CNFs modified using triphenylphosphine as the precursor provided the highest selectivity at isoconversion.
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