Catalytic activity and stability of multiwalled carbon nanotubes in catalytic wet air oxidation of oxalic acid: The role of the basic nature induced by the surface chemistry

Tuning CNT Properties for Metal-Free Environmental Catalytic Applications

“…Another N-precursor (urea) was also studied in the same work; however, lower incorporation of N-groups was achieved. Table 2 compares melamine and urea treated samples (CNT-BM-M and CNT-BM-U samples, respectively) using the ball milling approach, with a conventional hydrothermal treatment, where urea was used as N-precursor after the pre-oxidation of CNTs (CNT-NUT) [31]. The modified samples show differences regarding the surface area (S BET ) lower than 100 m 2¨g´1 , comparing with the pristine CNTs.…”

“…The non-catalytic process operates at high temperatures (200-320˝C) and pressures (20-200 bar), but the process efficiency can be increased under less severe operating temperatures (130-250˝C) and pressures (5-50 bar) by using homogeneous or heterogeneous active catalysts [31]. The problems associated with the leaching of the metals to the liquid phase can be avoided by replacing the catalysts based on noble metals or metal oxides by metal-free carbon materials [60,61].…”

“…In-situ (during synthesis) or ex-situ (post-treatments) approaches are used to incorporate nitrogen onto the carbon structure [25]. Regarding post-treatments, the incorporation of nitrogen can be achieved either by treating the carbon materials with ammonia or using nitrogen-containing carbon precursors, such as urea, melamine, polyacrylonitriles, polyvinylpyridine, and quinoline-containing pitch [31,46,47]. Generally, the incorporation of pyridinic-like (N-6), pyrrolic-like (N-5), and graphitic-like (N-Q) structures enhances the basicity of the CNTs [10,11,48,49].…”

“…Traditionally, CNTs are boiled in a 7 M HNO3 solution for 3 h, as used in ref. [31], incorporating high amounts of O-containing groups: acidic oxygen groups include carboxylic acids, anhydrides, lactones and phenols, and carbonyls and quinones, as basic or neutral oxygen functional groups, which can be identified by deconvolution of the profiles of CO and CO2 released during TPD analysis ( Figure 2). (PH-phenols; CAn-carboxylic anhydrides; CQ-carbonyl/quinones; LC-lactones; CAc-carboxylic acids; SA-strong acidic CAc; WA-weakly acidic CA) (adapted from [32]).…”

An overview on the advanced oxidation processes applied for the treatment of water pollutants defined in the recently launched Directive 2013/39/EU

“…More recently, metal-free carbon materials as catalysts on their own have gained increasing attention due to actual concerns with metal scarcity and catalyst deactivation by metal leaching. Some exam-ples are activated carbons (Morales-Torres et al, 2010), carbon xerogels (Apolinário et al, 2008;Gomes et al, 2008), carbon nanotubes Rocha et al, 2011Rocha et al, , 2014, carbon foams and fibers (Sousa et al, 2010) and graphite (Domínguez et al, 2014). Fig.…”