Catalytic dechlorination of 1,2-DCA in nano Cu0-borohydride system: effects of Cu0/Cun+ ratio, surface poisoning, and regeneration of Cu0 sites

Aqueous-phase catalyzed reduction of organic contaminants via zerovalent copper nanoparticles (nCu⁰), coupled with borohydride (hydrogen donor), has shown promising results. So far, the research on nCu⁰ as a remedial treatment has focused mainly on contaminant removal efficiencies and degradation mechanisms. Our study has examined the effects of Cu⁰/Cuⁿ⁺ ratio, surface poisoning (presence of chloride, sulfides, humic acid (HA)), and regeneration of Cu⁰ sites on catalytic dechlorination of aqueous-phase 1,2-dichloroethane (1,2-DCA) via nCu⁰-borohydride. Scanning electron microscopy confirmed the nano size and quasi-spherical shape of nCu⁰ particles. X-ray diffraction confirmed the presence of Cu⁰ and Cu₂O and x-ray photoelectron spectroscopy also provided the Cu⁰/Cuⁿ⁺ ratios. Reactivity experiments showed that nCu⁰ was incapable of utilizing H₂ from borohydride left over during nCu⁰ synthesis and, hence, additional borohydride was essential for 1,2-DCA dechlorination. Washing the nCu⁰ particles improved their Cu⁰/Cuⁿ⁺ ratio (1.27) and 92% 1,2-DCA was removed in 7 h with k_obs = 0.345 h^-1 as compared to only 44% by unwashed nCu⁰ (0.158 h^-1) with Cu⁰/Cuⁿ⁺ ratio of 0.59, in the presence of borohydride. The presence of chloride (1000-2000 mg L^-1), sulfides (0.4-4 mg L^-1), and HA (10-30 mg L^-1) suppressed 1,2-DCA dechlorination; which was improved by additional borohydride probably via regeneration of Cu⁰ sites. Coating the particles decreased their catalytic dechlorination efficiency. 85-90% of the removed 1,2-DCA was recovered as chloride. Chloroethane and ethane were main dechlorination products indicating hydrogenolysis as the major pathway. Our results imply that synthesis parameters and groundwater solutes control nCu⁰ catalytic activity by altering its physico-chemical properties. Thus, these factors should be considered to develop an efficient remedial design for practical applications of nCu⁰-borohydride.

Structure sensitivity reaction of chloroform hydrodechlorination to light olefins using Pd catalysts supported on carbon nanotubes and carbon nanofibers

The upgrading of wasted chloroform in hydrodechlorination for the production of olefins such as ethylene and propylene is studied by employing four catalysts (PdCl/CNT, PdCl/CNF, PdN/CNT, and PdN/CNF) prepared by different precursors (PdCl₂ and Pd(NO₃)₂) supported on carbon nanotubes (CNT) or carbon nanofibers (CNF). TEM and EXAFS-XANES results confirm that Pd nanoparticle size increases in the order: PdCl/CNT < PdCl/CNF ∼ PdN/CNT < PdN/CNF, descending the electron density of Pd nanoparticles in the same order. It illustrates that PdCl-based catalysts show donation of electrons from support to Pd nanoparticles, which is not observed in PdN-based catalysts. Moreover, this effect is more evident in CNT. The smallest and well-dispersed Pd nanoparticles (NPs) on PdCl/CNT with high electron density favor an excellent and stable activity and a remarkable selectivity to olefins. In contrast, the other three catalysts show lower selectivity to olefins and lower activities which suffer strong deactivation due to the formation of Pd carbides on their larger Pd nanoparticles with lower electron density, compared to PdCl/CNT.

Effect of Pretreatment with Acids on the N-Functionalization of Carbon Nanofibers Using Melamine

Nowadays, N-functionalized carbon nanomaterials attract a growing interest. The use of melamine as a functionalizing agent looks prospective from environmental and cost points of view. Moreover, the melamine molecule contains a high amount of nitrogen with an atomic ratio C/N of 1/2. In present work, the initial carbon nanofibers (CNFs) were synthesized via catalytic pyrolysis of ethylene over microdispersed Ni-Cu alloy. The CNF materials were pretreated with 12% hydrochloric acid or with a mixture of concentrated nitric and sulfuric acids, which allowed etching of the metals from the fibers and oxidizing of the fibers' surface. Finally, the CNFs were N-functionalized via their impregnation with a melamine solution and thermolysis in an inert atmosphere. According to the microscopic data, the initial structure of the CNFs remained the same after the pretreatment and post-functionalization procedures. At the same time, the surface of the N-functionalized CNFs became more defective. The textural properties of the materials were also affected. In the case of the oxidative treatment with a mixture of acids, the highest content of the surface oxygen of 11.8% was registered by X-ray photoelectron spectroscopy. The amount of nitrogen introduced during the post-functionalization of CNFs with melamine increased from 1.4 to 4.3%. Along with this, the surface oxygen concentration diminished to 6.4%.

Influence of Surface Chemistry of Carbon Nanofibers on the Hydrodechlorination of Chloroform to Olefins

Functionalized carbon nanofibers (CNF) are fascinating materials to be used as supports in Pd-based catalysts for the treatment of waste chloroform (TCM) to produce light olefins through the catalytic hydrodechlorination (HDC). The CNF were functionalized by HNO3, HCl, and urea. Compared to the Pd supported on un-treated CNF, all the catalysts using functionalized CNF as support showed lower turnover frequency values with higher stability, owing to their smaller Pd nanoparticles (NPs). These smaller Pd NPs are formed due to the stronger metal–support interactions promoted by the higher concentration of surface groups on the functionalized catalysts. Since the smaller Pd NPs could hinder the hydrogenation of olefins to paraffins, the selectivity to olefins increased on the functionalized catalysts. Moreover, the N-doped CNF was successfully formed on the catalyst functionalized by urea. Since the nitrogen functional groups (pyridinic N and pyrrolic N) could provide much stronger metal–support interactions compared to the oxygen functional groups on the other catalysts, the catalyst functionalized by urea showed the smallest Pd NPs among the four catalysts, leading to the highest selectivity to light olefins.

Progress in Catalytic Hydrodechlorination

Catalytic hydrodechlorination (HDC) is a technology with great potential for the treatment and valorization of organic chlorinated wastes [...]