The catalytic removal of nitrate (NO3−) in water using hydrogen as a reducing agent was studied using palladium-copper bimetallic catalysts in different supports. Commercial carbon nanotubes (CNTs), used as received and with different mechanical (CNT (BM 2h)) and chemical modifications (CNT (BM 4h)-N), titanium dioxide (TiO2) and composite materials (TiO2-CNT) were considered as main supports for the metallic phase. Different metal loadings were studied to synthesize an optimized catalyst with high NO3− conversion rate and considerable selectivity for N2 formation. Among all the studied support materials, the milled carbon nanotubes (sample CNT (BM 2h) was the support that showed the most promising results using 1%Pd-1%Cu as metallic phases. The most active catalysts were 2.5%Pd-2.5%Cu and 5%Pd-2.5%Cu supported on CNT (BM 2h), achieving total conversion after a 120 min reaction with N2 selectivity values of 62% and 60%, respectively. Reutilization experiments allowed us to conclude that these catalysts were stable during several reactions, in terms of NO3− conversion rate. However, the consecutive reuse of the catalyst leads to major changes concerning NH4+ selectivity values.

Multiwalled carbon nanotubes (MWCNTs) are known to have great potential to be used as catalysts in the ozonation of organic pollutants in water. However, solutions are required toward their practical application to overcome difficulties with the handling of nanosized powders. One such alternative is their coating on macrostructured ceramic supports. The majority of instances of such applications are based on the in situ formation of a nanocarbon layer by chemical vapor deposition. With recent advances in the modification of MWCNTs by mechanical methodologies showing that these can enhance their catalytic activity, there is an interest in the coating of ceramic macrostructures with a premodified MWCNTs because mechanical methods are not applicable to in situ grown materials. The coating of a MWCNTs using a conventional dip-coating technique would allow for premodification of the carbon by mechanical means. However, several obstacles in the formation of the slurry and nanostructured layers exist because of the behavior of the MWCNTs in suspension. In this work, the textural and morphological modification of MWCNTs by ball milling and subsequent interaction with different organic binders and surfactants in slurries was investigated. The main characteristics influencing the slurry stability and its use in the dip-coating of cordierite macrostructures were identified. Different modes of nanostructured layer formation were observed depending on the particle size distribution of the slurry, which is influenced by the surface chemistry and morphology of the MWCNTs. A correlation between the nanostructured layer homogeneity and adhesion and the slurry particle size distribution was established. This understanding was applied to form nanostructured layers with a pretreated nitrogen-containing MWCNTs. The material's basic character resulted in larger slurry particle sizes and consequently poorly adhered coatings. An approach using a premixed MWCNTs with a nitrogen precursor was shown to be able to produce nanostructured coatings with a nitrogen-doped MWCNTs and good adherence. The resulting nanostructured layers of MWCNTs were found to be catalytically active in the ozonation of a model organic pollutant (oxalic acid).

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