Monometallic and bimetallic catalysts with different proportions of Pd and Pt prepared by co-impregnation on activated carbon have been deeply characterized by inductively coupled plasma-mass spectroscopy, temperature-programmed reduction, 77 K N2 adsorption-desorption, CO chemisorption, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. They have been tested in the gas-phase hydrodechlorination of dichloromethane (DCM) at atmospheric pressure, reaction temperatures of 150-200 ºC, and a space-time of 0.6 kg h mol-1. The presence of Pd and Pt in the catalysts produces a synergistic effect observed in terms of dichloromethane conversion and overall dechlorination, especially when both metals are in similar proportions. The results from catalysts characterization suggest that the enhanced activity is due to a significant decrease of the metallic particles size and an optimum ratio of electro-deficient to zero-valent species in the bimetallic catalysts. The catalyst with 0.90 wt.% of Pt and 0.50 wt.% of Pd yielded the best results. Under intensified conditions, viz. 250 ºC and 1.73 kg h mol-1 , 100% DCM conversion and 98.6% overall dechlorination were obtained. This catalyst had most of its metallic particles within the range of 0.5 to 1 nm.

Bimetallic Pt:Pd catalysts with different molar ratios and 0.5 wt% overall metal load supported on sulfated zirconia catalysts (SZ) were synthesized and tested in the gas-phase hydrodechlorination (HDC) of chloromethanes and their mixtures. The catalysts were characterized by adsorption-desorption of N2 at-196 ºC, X-ray diffraction, X-ray photoelectronic spectroscopy, temperature programmed reduction and aberration corrected scanning transmission electron microscopy (STEM). The effect of the Pt:Pd molar ratio on the activity, stability and selectivity was analyzed. The high acidity of the sulfated zirconia results in metal particles of small size (mainly < 5 nm) as confirmed by STEM. The bimetallic catalysts showed higher stability than the monometallic ones as demonstrated in long term experiments (80 h on stream), confirming the positive effect 2 of combining both metallic phases. TOF values in the range of 0.0007 to 0.0168 s-1 and apparent activation energies between ≈ 41-44 kJ•mol-1 were obtained. TOF values for dichloromethane HDC increased at increasing mean metal particle size within the range of this work (≈1.2-2.3 nm). The catalysts with Pt:Pd molar ratios of 1:3 and 1:1 showed significantly better performance than the 3:1 one for overall dechlorination due to their higher atomic metal content and TOF at the same total metal weight load (0.5%).

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This study analyzes the effect of the reduction temperature on the properties of Rh, Pt and Pd catalysts supported on activated carbon and their performance in the hydrodechlorination (HDC) of dichloromethane (DCM). The reduction temperature plays an important role in the oxidation state, size and dispersion of the metallic phase. Pd is more prone to sintering, followed by Pt, while Rh is more resistant. The ratio of zero-valent to electro-deficient metal increases with the reduction temperature, with that effect being more remarkable for Pd and Pt. The higher resistance to sintering of Rh and the higher stability of electro-deficient species under thermal reductive treatment can be attributed to a stronger interaction with surface oxygen functionalities. Dechlorination activity and a TOF increase with reduction temperature (250–450 °C) occurred in the case of Pt/C catalyst, while a great decrease of both was observed for Pd/C, and no significant effect was found for Rh/C. Pt0 represents the main active species for HDC reaction in Pt/C. Therefore, increasing the relative amount of these species increased the TOF value, compensating for the loss of dispersion. In contrast, Pdn+ appears as the main active species in Pd/C and their relatively decreasing occurrence together with the significant decrease of metallic area reduces the HDC activity. Rh/C catalyst suffered only small changes in dispersion and metal oxidation state with the reduction temperature and thus this variable barely affected its HDC activity.

Dichloromethane (DCM) is an important pollutant with very harmful effects on human health and the environment. Catalytic hydrodechlorination (HDC) is an environmentally friendly technology for its removal from gas streams; it avoids the formation of hazardous pollutants like dioxins and phosgene (produced by other techniques), and the products obtained can be reused in other industries. When compared to other precious metals, Pd/C catalyst exhibited a better catalytic activity. However, the catalyst showed a significant deactivation during the reaction. In this study, the oxidation state and particle size of Pd was monitored with time on stream in order to elucidate the transformations that the catalyst undergoes during HDC. The deactivation can be ascribed to the formation of a new PdC x phase during the first hour of reaction. Carbon atoms incorporated to Pd lattice come from (chloro)-hydrocarbons adsorbed in the metallic species, whose transformation is promoted by the HCl originating in the reaction. Nevertheless, the catalyst was regenerated by air flow treatment at 250 • C, recovering the catalyst more than 80% of initial DCM conversion.

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