Application of silica-supported Ir and Ir-M (M = Pt, Pd, Au) catalysts for low-temperature hydrodechlorination of tetrachloromethane

Towards High Efficacy of Pd-Au/C Catalyst for Tetrachloromethane Hydrodechlorination

We present an efficient strategy for synthesising the PdAu catalysts with a homogeneous PdAu alloy phase for environmentally important hydrodechlorination of tetrachloromethane in the gas phase. The synthesis of carbon-supported catalysts involved two major steps: (i) incorporation of palladium and gold nanoparticles into carbon support and (ii) activation of the catalysts. The critical part of this work was to find the optimal conditions for both steps. Thus, the incorporation of the nanoparticles was carried out in two ways, by impregnation and direct redox reaction method using acetone solutions of metal precursor salts. The activation was performed either by a conventional thermal reduction in hydrogen or flash irradiation in a microwave oven. The homogeneity and structure of the PdAu alloy were found to depend on the catalyst activation method critically. In all cases, we observed better homogeneity for catalysts that were subject to microwave irradiation. Moreover, the flash microwave irradiation of prepared catalysts provided catalysts of better stability and selectivity towards the desired products (hydrocarbons) in the hydrodechlorination of tetrachloromethane as compared to the catalyst obtained by conventional thermal activation in hydrogen.

Understanding Hydrodechlorination of Chloromethanes. Past and Future of the Technology

Chloromethanes are a group of volatile organic compounds that are harmful to the environment and human health. Abundant studies have verified that hydrodechlorination might be an effective treatment to remove these chlorinated pollutants. The most outstanding advantages of this technique are the moderate operating conditions used and the possibility of obtaining less hazardous valuable products. This review presents a global analysis of experimental and theoretical studies regarding the hydrodechlorination of chloromethanes. The catalysts used and their synthesis methods are summarized. Their physicochemical properties are analyzed in order to deeply understand their influence on the catalytic performance. Moreover, the main causes of the catalyst deactivation are explained, and prevention and regeneration methods are suggested. The reaction systems used and the effect of the operating conditions on the catalytic activity are also analyzed. Besides, the mechanisms and kinetics of the process at the atomic level are reviewed. Finally, a new perspective for the upgrading of chloromethanes, via hydrodechlorination, to valuable hydrocarbons for industry, such as light olefins, is discussed.

Ultrafine Re/Pd nanoparticles on polydopamine modified carbon nanotubes for efficient perchlorate reduction and reusability

This study synthesized nanocomposite catalysts via a modification of Re/Pd codoped carbon nanotubes (CNTs) with different concentrations of polydopamine (PDA), which were used for perchlorate (ClO₄^-) reduction. The loads, dispersion and reducibility of Re/Pd nanoparticles increased yet their particle sizes significantly decreased with the increase of PDA concentrations. The average diameter of Re/Pd codoped D₂CNT (CNT modified by 2 mg/mL PDA) with a narrow size distribution was measured to be 2 nm. The ultrafine Re/Pd codoped D₂CNT catalysts represented outstanding catalytic reduction activity for the conversion of ClO₄^- to Cl^- with TOF of 17.34 h^-1 under the room H₂ atmospheric pressure, which was about 8 times than that of the unmodified catalysts. Furthermore, PDA modification minimized the dissociation of Re by chemical bonding between Re and CNTs carrier and maintained good stability of nanocomposite. This study inspires us to apply green bionic methods to enhance the catalytic reduction of perchlorate by changing the physical properties of Re/Pd nanoparticles.

Tuning Thermal Catalytic Enhancement in Doped MnO2-Au Nano-Heterojunctions

Sodium (Na)- and potassium (K)-doped δ-MnO₂, which presented different band gaps, were synthesized by a hydrothermal method. Then, uniform Au nanoparticles (NPs) were deposited on MnO₂ to form metal-semiconductor nano-heterojunctions (MnO₂-Au). By comparing their temperature-dependent thermal catalytic performances, p-aminothiophenol to p, p'-dimercaptoazobenzene conversion was used as proof-of-concept transformations. MnO₂-Au hybrid materials demonstrated better thermal catalytic performances relative to individual Au NPs. Meanwhile, K-doped MnO₂-Au, with a MnO₂ support displaying a narrower bandgap, displayed superior catalytic activities relative to Na-doped MnO₂-Au. To get the same catalytic performance by individual Au NPs, it can be ∼50 K less by Na-doped MnO₂-Au and ∼100 K less by K-doped MnO₂-Au. The enhancement is mainly attributed to the thermally excited electrons in MnO₂, which were transferred to Au NPs. The additional electrons in Au NPs increase the electron density and thus contribute to the improvement of thermal catalysis. Our findings show that the establishment of a nano-heterojunction formed by metal NPs on a semiconductor support has a significant impact on thermal catalysis, where a narrower band gap can facilitate thermally excited carriers and thus bring about better catalytic performances. Thus, the results presented here shed light on the design of a nano-heterojunction catalyst to approach reactions with superior performance under moderate conditions.