One-step elimination of Cr(VI) by catalytic hydrogenation of Cr(VI) and simultaneous Cr(OH)3 recovery on Pt catalysts encapsulated in N-doped mesoporous carbon

Hexavalent chromium Cr(VI) is a highly toxic heavy metal, which is commonly eliminated by stepwise reduction at acidic pH and precipitation of Cr(OH)₃ at alkaline pH. A unique Pt catalyst with Pt particles embedded in the framework of N-doped mesoporous carbon CMK-3 (denoted as Pt@NCMK-3) was designed and fabricated to one-step eliminate Cr(VI) pollution at near neutral pH via simultaneous Cr(VI) reduction by catalytic hydrogenation and Cr(OH)₃ recovery. Structural characterization showed that Pt particles of Pt@NCMK-3 were effectively embedded in the carbon rods of NCMK-3. Batch experiments revealed that Pt@NCMK-3 exhibited a higher catalytic activity and stability than other test catalysts. Fixed-bed column reaction results indicated that under the experimental conditions Pt@NCMK-3 had better breakthrough performances than other catalysts. Additionally, after 4 treatment-recovery cycles Pt@NCMK-3 maintained nearly identical breakthrough performance, whereas other catalysts displayed markedly decreased breakthrough bed volumes, reflecting a substantially higher stability of Pt@NCMK-3.

Cycloaddition between nitrogen-doped graphene (6π-component) and benzene (4π-component): a theoretical approach using density functional theory with vdW-DF correction

The interaction between nitrogen-doped graphene defects (N3V1 and N4V2 pyridinic, and N3V1 and N3V3 pyrrolic) and benzene have been investigated by applying density functional theory (DFT), together with the vdW-DF correction. We discovered that only the N3V3 pyrrolic defect is a reactive site (6π-component), forming a cycloadduct with benzene (4π-component) that has energy barriers below 154.38 kJ mol-1 (1.60 eV). The conduction and valence bands (HOMO and LUMO) for N3V3 form a degenerate pair of orbitals at the gamma point, with the same ionization potential (IP) and electron affinity (EA). Likewise, inspection of the orbital symmetries for both systems confirms that these must undergo concerted reactions based on the Woodward and Hoffmann principles of orbital symmetry, with the appropriate orbital occupancies. This is the first time that substitutionally doped graphene has been demonstrated to participate as a 6π-component for cycloaddition reactions with benzene.

Effect of the supports on catalytic activity of Pd catalysts for liquid-phase hydrodechlorination/hydrogenation reaction

Carbon nanotubes (CNTs), activated carbon (AC), graphene, and aluminum oxide (Al₂O₃) supported 5% Pd catalysts were prepared by the conventional impregnation method, and catalytic activity was tested in the hydrogenation of 4-chlorophenol (4-CP) and nitrobenzene (NB) under ambient conditions (313 K and atmospheric pressure). It was found that catalytic activity was greatly affected by the supports. Moreover, Pd/CNTs catalyst exhibited much higher catalytic activity than the other three supported Pd catalysts. The mechanism of this phenomenon was studied through catalyst characterization (ICP-MS, Brunauer-Emmett-Teller [BET], TEM, and SEM). It was found that the mean particle size of Pd nanoparticles for Pd/CNTs (4.3 nm) was smaller than that for Pd/AC (6.9 nm), Pd/Al₂O₃ (5.0 nm), and Pd/graphene (5.2 nm). Moreover, the actual loading amounts of Pd and BET surface areas were not the main reasons for the different catalytic activity of the four supported Pd catalysts. Above all, the smaller Pd particles of Pd/CNTs enabled the Pd/CNTs catalyst to exhibit much higher catalytic activity for the hydrogenation reactions.

Highly effective bromate reduction by liquid phase catalytic hydrogenation over Pd catalysts supported on core-shell structured magnetites: Impact of shell properties

Liquid phase catalytic reduction of bromate with supported noble metals as the catalysts is a promising method to remove bromate in water. Magnetic supports provide a feasible way to recover catalysts whose surface properties also strongly influence the catalytic efficiency. In this study, Pd nanoparticles supported on core-shell structured magnetites with varied shells (e.g., carbon, SiO₂, polypyrrole, polyaniline, polydopamine and chitosan) were prepared and catalytic reduction of bromate on the catalysts was investigated. The results showed that in comparison with other catalysts Pd/(Fe₃O₄@polyaniline) exhibited a higher catalytic efficiency due to its higher point of zero charge and surface hydrophilicity. In parallel, bromate reduction on Pd/(Fe₃O₄@polyaniline) followed the Langmuir-Hinshelwood model, confirming the crucial role of bromate adsorption. At pH 5.6 and a catalyst dosage of 0.05 g/L, 0.4 mM bromate could be completely reduced into bromide within 120 min. Furthermore, the magnetic catalysts could be effectively separated and recovered under an external magnetic field within 3 min. The results of catalyst reuse showed that after five consecutive catalytic reduction cycles Pd/(Fe₃O₄@polyaniline) retained 87% of its fresh catalyst activity. The present findings indicate that Pd/(Fe₃O₄@polyaniline) with polyaniline as the shell is a highly active, stable and recyclable catalyst for liquid phase catalytic hydrogenation of pollutants in water.

Catalytic Performance of Nitrogen-Doped Activated Carbon Supported Pd Catalyst for Hydrodechlorination of 2,4-Dichlorophenol or Chloropentafluoroethane

Nitrogen-doped activated carbon (N-AC) obtained through the thermal treatment of a mixture of HNO₃-pretreated activated carbon (AC) and urea under N₂ atmosphere at 600 °C was used as the carrier of Pd catalyst for both liquid-phase hydrodechlorination of 2,4-dichlorophenol (2,4-DCP) and gas-phase hydrodechlorination of chloropentafluoroethane (R-115). The effects of nitrogen doping on the dispersion and stability of Pd, atomic ratio of Pd/Pd²⁺ on the surface of the catalyzer, the catalyst’s hydrodechlorination activity, as well as the stability of N species in two different reaction systems were investigated. Our results suggest that, despite no improvement in the dispersion of Pd, nitrogen doping may significantly raise the atomic ratio of Pd/Pd²⁺ on the catalyst surface, with a value of 1.2 on Pd/AC but 2.2 on Pd/N-AC. Three types of N species, namely graphitic, pyridinic, and pyrrolic nitrogen, were observed on the surface of Pd/N-AC, and graphitic nitrogen was stable in both liquid-phase hydrodechlorination of 2,4-DCP and gas-phase hydrodechlorination of R-115, with pyridinic and pyrrolic nitrogen being unstable during gas-phase hydrodechlorination of R-115. As a result, the average size of Pd nanocrystals on Pd/N-AC was almost kept unchanged after liquid-phase hydrodechlorination of 2,4-DCP, whereas crystal growth of Pd was clearly observed on Pd/N-AC after gas-phase hydrodechlorination of R-115. The activity test revealed that Pd/N-AC exhibited a much better performance than Pd/AC in liquid-phase hydrodechlorination of 2,4-DCP, probably due to the enhanced stability of Pd exposed to the environment resulting from nitrogen doping as suggested by the higher atomic ratio of Pd/Pd²⁺ on the catalyst surface. In the gas-phase hydrodechlorination of R-115, however, a more rapid deactivation phenomenon occurred on Pd/N-AC than on Pd/AC despite a higher activity initially observed on Pd/N-AC, hinting that the stability of pyridinic and pyrrolic nitrogen plays an important role in the determination of catalytic performance of Pd/N-AC.

Changes to the dissociation barrier of H2 due to buckling induced by a chemisorbed hydrogen on a doped graphene surface

An effectiveway of enhancing hydrogen storage on adsorbent materials can be induced by the hydrogen spill-over mechanism, although to date there is no general consensus which satisfactorily explains the mechanism. In this work, a possible reaction path to explain hydrogen adsorption is shown. Density-functional calculations were used to study the dissociation of molecular hydrogen near to a stressed region, as a consequence of chemisorbed hydrogen at the graphene-nitrogen surface. We found that as a result of the buckling induced by the chemisorbed hydrogen, the dissociation barrier of molecular hydrogen diminished by 0.84 eV. The chemisorbed hydrogen is the final state in the spill-over mechanism on a graphene-nitrogen decorated with palladium clusters. This effect helps to create hydrogen nanoislands that may change the diffusion and detrapping of H. An electronic structure analysis suggests that these systems occasionally present metallic or semiconductor behavior. Graphical Abstract Hydrogen dissociation and adsorption process via buckling defect.

Palladium Supported on Titanium Carbide: A Highly Efficient, Durable, and Recyclable Bifunctional Catalyst for the Transformation of 4-Chlorophenol and 4-Nitrophenol

Developing highly efficient and recyclable catalysts for the transformation of toxic organic contaminates still remains a challenge. Herein, Titanium Carbide (Ti₃C₂) MXene modified by alkali treatment process was selected as a support (designated as alk-Ti₃C₂X₂, where X represents the surface terminations) for the synthesis of Pd/alk-Ti₃C₂X₂. Results show that the alkali treatment leads to the increase of surface area and surface oxygen-containing groups of Ti₃C₂X₂, thereby facilitating the dispersion and stabilization of Pd species on the surface of alk-Ti₃C₂X₂. The Pd/alk-Ti₃C₂X₂ catalyst shows excellent catalytic activity for the hydrodechlorination of 4-chlorophenol and the hydrogenation of 4-nitrophenol in aqueous solution at 25 °C and hydrogen balloon pressure. High initial reaction rates of 216.6 and 126.3 min⁻¹·gpd−1 are observed for the hydrodechlorination of 4-chlorophenol and hydrogenation of 4-nitrophenol, respectively. Most importantly, Pd/alk-Ti₃C₂X₂ exhibits excellent stability and recyclability in both reactions without any promoters. The superior property of Pd/alk-Ti₃C₂X₂ makes it as a potential material for practical applications.

Enhancement of the activity of Pd/C catalysts in aqueous phase hydrodechlorination through doping of carbon supports

Improved catalytic activity in the hydrodechlorination reaction at room temperature with N-doped carbon as the support of Pd nanoparticles.

Removal of halogenated emerging contaminants from water by nitrogen-doped graphene decorated with palladium nanoparticles: Experimental investigation and theoretical analysis

The removal performance and mechanisms of halogenated emerging contaminants from water by palladium decorated nitrogen-doped graphene (Pd/NG) were investigated in this study. For comparison, three catalysts of Pd/NG, palladium decorated graphene (Pd/G) and commercial Pd/C were initially explored to degrade tetrabromobisphenol A (TBBPA). After that, the influence of various environmental parameters on TBBPA removal by the Pd/NG catalyst was evaluated. Moreover, both Langmuir-Hinshelwood model and density functional theory (DFT) were adopted to theoretically elucidate the adsorption and the activation of TBBPA on the catalyst. The results show that the apparent rate constant of TBBPA dehalogenation was increased by 26.7% and 39.0% in the presence of the Pd/NG catalyst compared to the Pd/G and Pd/C ones. Higher temperature, catalyst dosage and alkaline conditions resulted in the enhancement of TBBPA dehalogenation by the Pd/NG catalyst, while humic acid in the solution had a negatively effect on the transformation of TBBPA. The corresponding rate constant value exhibited a 2.1- and 1.8-fold increase with the rise of temperature from 298 to 328 K and initial pH from 6.5 to 9.0, respectively. On the contrary, the rate constant was decreased by 78.9% in the presence of 15 mg L(-1) humic acid. Theoretical analysis revealed that both adsorption and activation processes of TBBPA on the Pd/NG catalyst were enhanced through the N doping into graphene framework.

Study of the interplay between N-graphene defects and small Pd clusters for enhanced hydrogen storage via a spill-over mechanism

The hydrogen spill-over mechanism was studied by applying Density Functional Theory.