Modification of the surface chemistry of single- and multi-walled carbon nanotubes by HNO₃ and H₂SO₄ hydrothermal oxidation for application in direct contact membrane distillation

Abstract

A specific methodology based on nitric acid hydrothermal oxidation was used to control the surface chemistry of multi-walled (MWCNTs) and single-walled (SWCNTs) carbon nanotubes (CNTs) with different lengths, and this methodology was adapted to the use of sulphuric acid containing ammonium persulfate as an oxidizing agent. The amount of oxygen-containing surface groups depends on the number and length of the graphene layers of the CNTs, thicker and shorter CNTs having more reactive sites for surface functionalization. In particular, the oxidation of MWCNTs was more pronounced than that of short SWCNTs and less surface groups were introduced into long SWCNTs, regardless of the acid used at any fixed concentration. It was also possible to tailor the surface chemistry of both SWCNTs and MWCNTs by using the adopted methodologies, and the amount of both oxygen- and sulphur-containing functional groups was correlated with the concentration of each oxidizing agent used. Mathematical functions that allow precise control of the amount and type of the surface groups introduced into carbon nanotubes were obtained. Buckypapers were also prepared over a polytetrafluoroethylene commercial membrane. These membranes were tested in direct contact membrane distillation and, under salinity conditions, the membrane prepared using oxidized MWCNTs (instead of SWCNTs) was the most efficient, the permeate flux of the commercial membrane significantly increasing in the presence of these CNTs, while completely rejecting chloride ions. In addition, the permeate flux was precisely correlated with the amount of oxygenated functional surface groups (as well as with the pH of point of zero charge) of the oxidized MWCNTs.

Tuning CNT Properties for Metal-Free Environmental Catalytic Applications

“…Direct functionalization with H 2 SO 4 originates high incorporation of S-containing groups at low temperature (50˝C, [56]) or at high temperature (250˝C [53]); however, in the last case the authors did not show the textural modifications obtained after the treatment. Regarding the hydrothermal methodology, contrary to what was observed with HNO 3 (low acid concentration allowing similar functionalization as the conventional nitric acid boiling approach), the hydrothermal treatment of CNT with H 2 SO 4 revealed a weak incorporation of S-containing groups on the surface [59]. The simultaneous use of H 2 SO 4 /HNO 3 led to higher incorporation of -SO 3 H [56]; however, in this case a large amount of O-functionalities was also incorporated.…”

“…In addition to the use of H 2 SO 4 as S-source, others sulfonating agents as benzenesulfonic acid (C 6 H 5 SO 3 H), p-toluenesulfonic acid (PTSA, CH 3 C 6 H 4 SO 3 H), and "piranha" solution (H 2 O 2 -H 2 SO 4 ) were proposed [29,57,58]. The methodology already presented for HNO 3 controlled functionalization of CNTs by hydrothermal treatment was adapted in a pioneer work to the use of H 2 SO 4 (containing ammonium persulfate-(NH 4 ) 2 S 2 O 8 ) as an oxidizing agent [59]. Table 3 compares selected sulfonated CNTs from the literature.…”

Hydrothermal functionalization of ordered mesoporous carbons: The effect of boron on supercapacitor performance

“…In that regard, hydrothermal treatments (HT) have demonstrated to be advantageous [30][31][32] because the generated high pressures favour the aggregation of molecules. Therefore, the main goal is to achieve a controlled hydrothermal synthesis allowing not only to adjust the boron content, but also to define the chemical state of boron functionalities in ordered mesoporous carbons.…”

Promoting role of sulfur groups in selective catalytic reduction of NO with NH3 over H2SO4 modified activated carbons

“…The newly formed CO and CO 2 peak at 265 o C has two possible assignments: (i) the oxygen groups with less thermal stability; and (ii) the interaction between the sulfur groups and the carbon. However, the CO and CO 2 peak at 265 o C excellently corresponds to the SO 2 release peak (temperature and shape) over the modified carbons, which is attributed to the decomposition of thiols and oxidized sulfur groups [13,24,25], thus prompting us to assign this peak to the interaction between the sulfur groups and the carbon rather than the oxygen groups. In other words, the decomposition of thiols and oxidized sulfur groups and their interaction with the carbon lead to the formation of SO 2 , CO, and CO 2 release peak at 265 o C. Additional information obtained from Fig.…”