Enrique García-Bordejé

Toward Practical Application Of H2 Generation From Ammonia Decomposition Guided by Rational Catalyst Design

This perspective article focuses on the recent advances in the rational design of catalyst for NH3 decomposition at different length scales, from catalyst nanoparticles to reactor design. For an overall optimum performance, all scales should be optimized in a concerted manner. The study of the mechanism of reaction has given guidelines for the catalyst design. As far as reactor design is concerned, a structured reactor is beneficial for this reaction because it minimizes the inhibiting effect of produced H2, besides other inherent advantages. This holistic optimization of catalysts is expected to pave the way for application of NH3 as H2 storage media.

Control of nitrogen insertion during the growth of nitrogen-containing carbon nanofibers on cordierite monolith walls

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.

Support‐Induced Oxidation State of Catalytic Ru Nanoparticles on Carbon Nanofibers that were Doped with Heteroatoms (O, N) for the Decomposition of NH3

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.

Catalytic ozonation of metolachlor under continuous operation using nanocarbon materials grown on a ceramic monolith.

The catalytic ozonation of the herbicide metolachlor (MTLC) was tested using carbon nanomaterials as catalysts. Multiwalled carbon nanotubes were used in semi-batch experiments and carbon nanofibres grown on a honeycomb cordierite monolith were tested in continuous experiments. The application of the carbon catalyst was shown to improve the mineralization degree of MTLC and to decrease the toxicity of the solution subject to ozonation. Degradation by-products were also followed in order to compare the two processes. The application of the carbon coated monolith to the continuous ozonation process was shown to have potential as it improved the TOC removal from 5% to 35% and decreased the inhibition of luminescent activity of Vibrio Fischeri from 25% to 12%.

Biobased catalyst in biorefinery processes: Sulphonated hydrothermal carbon for glycerol esterification

Sulphonated hydrothermal carbon (SHTC), obtained from D-glucose by mild hydrothermal carbonisation and subsequent sulphonation with sulphuric acid, is able to catalyse the esterification of glycerol with different carboxylic acids, namely, acetic, butyric and caprylic acids. Product selectivity can be tuned by simply controlling the reaction conditions. On the one hand, SHTC provides one of the best selectivity towards monoacetins described up to now without the need for an excess of glycerol. On the other hand, excellent selectivity towards triacylglycerides (TAG) can be obtained, beyond those described with other solid catalysts, including well-known sulphonic resins. Recovery of the catalyst showed partial deactivation of the solid. The formation of sulphonate esters on the surface, confirmed by solid state NMR, was the cause of this behaviour. Acid treatment of the used catalyst, with subsequent hydrolysis of the surface sulphonate esters, allows SHTC to recover its activity. The higher selectivity towards mono- and triesters and its renewable origin makes SHTC an attractive catalyst in biorefinery processes.

Carbon Nanofibers Modified with Heteroatoms as Metal‐Free Catalysts for the Oxidative Dehydrogenation of Propane

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.

Catalytic ozonation of oxalic acid using carbon nanofibres on macrostructured supports

Carbon nanofibres (CNFs) were grown on different macrostructured supports such as cordierite monoliths, carbon felts and sintered metal fibres. The resulting composites exhibited excellent resistance to attrition/corrosion and its porosity is mainly due to mesoporous structures. The CNF/structured materials were tested in the ozonation of oxalic acid in a conventional semi-batch reactor after being crushed to powder form, and in a newly designed reactor that may operate in semi-batch or continuous operation. The CNFs supported on the different structured materials exhibited high catalytic activity in the mineralization of oxalic acid.

Self-assembled graphene aerogel and nanodiamond hybrids as high performance catalysts in oxidative propane dehydrogenation

Graphene aerogels and graphene aerogel–nanodiamond hybrids have been fabricated by a mild reduction/self-assembly hydrothermal method using graphene oxide dispersion as a precursor. The aerogels have been used as metal-free catalysts for oxidative dehydrogenation of propane. Reduced graphene oxide (RGO) aerogels without nanodiamonds outperformed carbon nanotubes in terms of propene productivity and selectivity, which is correlated with a higher content of accessible carbonyl–quinone groups and more defective structures of reduced graphene oxide. Graphene aerogels loaded with low amounts of nanodiamonds (2 wt%) by a one-pot strategy provided 18% higher activity than RGO aerogels, ascribed to the increase of the sp3/sp2 ratio. For nanodiamond contents higher than 2 wt%, the productivity and selectivity drops, which can be explained by a dramatic decrease of carbonyl–quinone groups, an increased content of unselective oxygen species and clustering of nanodiamonds for the highest loadings. Hybrid aerogels are freestanding, robust and highly porous monoliths; thereby a suitable platform to be used as catalysts or adsorbents in flow systems.

Function of the Support and Metal Loading on Catalytic Carbon Dioxide Reduction Using Ruthenium Nanoparticles Supported on Carbon Nanofibers

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.

Process design for wastewater treatment: catalytic ozonation of organic pollutants

Emerging micropollutants have been recently the target of interest for their potential harmful effects in the environment and their resistance to conventional water treatments. Catalytic ozonation is an advanced oxidation process consisting of the formation of highly reactive radicals from the decomposition of ozone promoted by a catalyst. Nanocarbon materials have been shown to be effective catalysts for this process, either in powder form or grown on the surface of a monolithic structure. In this work, carbon nanofibers grown on the surface of a cordierite honeycomb monolith are tested as catalyst for the ozonation of five selected micropollutants: atrazine (ATZ), bezafibrate, erythromycin, metolachlor, and nonylphenol. The process is tested both in laboratorial and real conditions. Later on, ATZ was selected as a target pollutant to further investigate the role of the catalytic material. It is shown that the inclusion of a catalyst improves the mineralization degree compared to single ozonation.