Carbon nanotubes as catalysts for catalytic wet peroxide oxidation of highly concentrated phenol solutions: towards process intensification

Abstract

Commercial multi-wall carbon nanotubes with different properties (2 samples from Sigma-Aldrich, SA1 and SA2; 1 sample from Nanocyl, NC; and 2 samples from Shenzhen Nanotech, SZ and LSZ) were tested as catalysts in wet peroxide oxidation. Phenol was selected as model compound since represents a class of noxious compounds for human health and for the environment and, due to this, phenol is typically considered in wastewater treatment studies. The experiments were carried out under the following intensified conditions: phenol concentration = 4.5 g L-1 , hydrogen peroxide concentration = 25 g L-1 , catalyst load = 2.5 g L-1 , pH = 3.5, T = 353 K and 24 h. The results demonstrated that phenol is poorly adsorbed in this type of carbon materials (11 % as maximum when using the NC sample). However, in the catalytic experiments, complete removal of phenol is achieved when using some of the carbon nanotubes (SA1, NC and SA2), together with a remarkable total organic carbon removal (77, 69 and 67 %, respectively). These materials have the most pronounced basic character, which is often considered benefic for oxidation reactions in advanced oxidation processes and may explain the higher performance of SA1, NC and SA2 regarding the other materials. Leaching of Fe species into the solution was also observed in all cases, SA1 leading to the highest concentration of Fe species leached (26 mg L-1), followed by SA2 (2 mg L-1) and NC (1 mg L-1). Considering the lower Fe leaching levels observed for SA2 and NC, these catalysts were then tested in consecutive reusability cycles. SA2 showed a superior performance than NC, however, temperature programmed desorption, as well as thermogravimetric analysis, suggested that the carbon material is oxidized by hydrogen peroxide at the employed conditions and/or that carboxylic acids are adsorbed on the catalyst surface after consecutive runs (mainly after the first use), slightly reducing the catalyst activity.

Heterogeneous Fenton-Like Degradation of p-Nitrophenol over Tailored Carbon-Based Materials

“…The decrease in efficiency of CWPO between reutilizations can be due to the porosity blocking and/or deactivation of carbons due to the oxidation of the carbon surface by the hydrogen peroxide, forming oxygen surface groups that have a negative influence in the electronic interactions between the carbon surface and PNP or other organic compounds. This deactivation of carbons was also reported in literature by other authors [25,48,49]. the occurrence of radical-mediated surface reactions because PNP and TOC removals obtained in these experiments were identical to those reached by simple adsorption.…”

“…Among the carbon materials, activated carbon (AC) is the most reported in the literature as iron support to be used as a catalyst for the Fenton reaction [13,14,[17][18][19][20]. Recently, carbon nanotubes (CNT) [21][22][23] and carbon xerogels (XG) [24] have received intensive attention as iron supports due to their interesting properties when compared with other carbon-based materials, such as mechanical resistance, low limitations to mass transfer, high thermal stability in oxidation conditions, high electronic properties for CNT [25], high purity, mesoporous structure with good pore distribution, and high surface area for XG [26].The presence of nitrogen groups on the surface of carbon-based materials improves their catalytic performance for advanced oxidation processes. This is explained by the increase of the electronic density, and consequently the surface basicity, which favors the interaction between the organic compounds and the support [17].…”

Mined pyrite and chalcopyrite as catalysts for spontaneous acidic pH adjustment in Fenton and LED photo‐Fenton‐like processes

“…5(d)). The efficiency of H 2 O 2 consumption (X TOC /X H2O2 ) 39 determined for this particular experiment is c. 0.90. Thus, in the best-performing system (photo-Fenton-like experiment with 0.50 mmol L -1 of TY at 19.0 mmol L -1 of H 2 O 2 ), the magnitude of H 2 O 2 consumption was quite high as the TOC removal, indicating that TY is mostly converted to CO 2 and H 2 O without generating significant amount of reaction by-products (Fig.…”

“…(d)). The efficiency of H 2 O 2 consumption ( X TOC / X H2O2 ) determined for this particular experiment is c . 0.90.…”

Role of Nitrogen Doping on the Performance of Carbon Nanotube Catalysts: A Catalytic Wet Peroxide Oxidation Application

“…In previous studies, the behavior of different commercial CNTs in the CWPO process was analyzed. These materials were found to be highly effective for the removal of different phenolic compounds, even without added metal species.…”