This work aims to clarify the nanostructural transformation accompanying the loss of activity and selectivity for the hydrogen peroxide synthesis of palladium and gold-palladium nanoparticles supported on N-functionalized carbon nanotubes. High-resolution X-ray photoemission spectroscopy (XPS) allows the discrimination of metallic palladium, electronically modified metallic palladium hosting impurities, and cationic palladium. This is paralleled by the morphological heterogeneity observed by high-resolution TEM, in which nanoparticles with an average size of 2 nm coexisted with very small palladium clusters. The morphological distribution of palladium is modified after reaction through sintering and dissolution/redeposition pathways. The loss of selectivity is correlated to the extent to which these processes occur as a result of the instability of the particle at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to palladium clusters with a modified electronic structure compared with palladium metal or palladium oxides. These beneficial species are formed and stabilized on carbons modified with nitrogen atoms in substitutional positions. The formation of larger metallic palladium particles not only reduces the number of active sites for the synthesis, but also enhances the activity for deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of palladium clusters by all means is thus the key to better performing catalysts.

Pd nanoparticles deposited by sol-immobilization on N-doped nanocarbon (carbon nanotube-like from Pyrograf Products; N-CNT-like) is studied in the direct synthesis of H2O2 and compared with those of the undoped catalysts or prepared by the same Pd deposition method on active carbon (Vulcan XC-72) as the support. The catalytic tests were carried out using a slurry batch-or semi-continuous reactor at room temperature and a total pressure of 10 bar using CO2-expanded methanol as the solvent. The Pd on N-CNT-like gives high productivities to H2O2, comparable to the best literature results, while the selectivity to H2O2 is low due to the low oxygen partial pressure and likely low oxygen coverage on Pd particles. The introducing of nitrogen in the CNT-like material favors not only the dispersion of Pd (with a consequent improvement of the activity), but also the specific turnover, due to probably the electronic effect of pyridine-like nitrogen sites present in the N-CNT-like support favoring the O2 surface coverage. However, the introduction of these N functionalities on the surface has also a negative effect on the rate of H2O2 consecutive conversion to water.

Herein, the development of an improved Pd on carbon nitride catalyst for direct H2O2 synthesis from the elements is reported. Microcalorimetric CO chemisorption is used to characterize the chemical speciation of the Pd-selective and -unselective sites. Selectivity trends among the samples suggest that a bare metal surface with a differential heat of CO chemisorption ranging between 140 and 120 kJ mol–1 is responsible for the total O2 hydrogenation, while a maximum threshold value of differential heat of CO chemisorption of approximately 70 kJ mol–1 is necessary for the partial hydrogenation of O2 to H2O2. Such a low differential heat of CO chemisorption indicates a low exposed metallic Pd surface subjected to electron withdrawal from the surrounding ligands: i.e., the N functional groups on the carbon support. With respect to N-containing carbon nanotubes, carbon nitrides provide the following: a higher concentration of N sites, a flexible network of π-conjugated polymeric subunits with sp3 linking subunits, and a flakelike morphology with high exposure of reactive C edge terminations. This results in a more effective kinetic stabilization of the electronically modified Pd species.

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A series of new tubular catalytic membranes (TCM's) have been prepared and tested in the direct synthesis of H 2 O 2 . Such TCM's are asymmetric a-alumina mesoporous membranes supported on macroporous a-alumina, either with a subsequent carbon coating (CAM) or without (AAM). Pd was introduced by two different impregnation techniques. Deposition-precipitation (DP) was applied to CAM's to obtain an even Pd particles distribution inside the membrane pore network, whereas electroless plating deposition (EPD) was successfully applied to AAM's to give a 1-10 mm thick nearly-dense Pd layer. Both type of membranes were active in the direct synthesis of H 2 O 2 . Catalytic tests were carried out in a semi-batch re-circulating reactor under very mild conditions. Concentrations as high as 250-300 ppm H 2 O 2 were commonly achieved with both CAM's and AAM's after 6-7 h time on stream, whereas the decomposition rate was particularly high in the presence of H 2 . Important features are the temperature control and pre-activation. In order to slow down the decomposition and favor the synthesis of H 2 O 2 a smooth metal surface is needed. #

Advantages of hydrotalcite-like precursors and the synergistic effect of bimetallic Ni–Fe alloys are combined and the most appropriate amount of Fe identified with respect to activity, selectivity and stability.

Two nickel−aluminum hydrotalcite samples (HTLCs) were prepared by a coprecipitation method at different pH values and investigated as catalysts for the hydrogenation of carbon dioxide. The newly synthesized samples have been compared with a reference alumina supported nickel-based commercial catalyst, with equal nickel content. The as-prepared and commercial samples were characterized by BET analysis, atomic adsorption spectroscopy (AAS), X-ray diffraction (XRD), and temperatureprogrammed techniques (H 2 -TPR and CO-TPD). Catalytic activity of the analyzed samples was investigated toward hydrogenation of CO 2 at atmospheric pressure by varying reaction temperature between 250 and 400 °C. The maximum CO 2 -to-CH 4 conversion value achieved by hydrotalcyte was ≈86% at 300 °C. The superior performance of HTLCs has been put in relationship with the major catalysts reducibility nature and with the higher metal surface area and metal dispersion. The stability of the HTLCs was investigated through long-term tests, resulting in good stability in the reported reaction conditions.

Composite oxide supported Ni-based catalysts were prepared by a wet impregnation technique and applied to the methanation of carbon dioxide. The composite oxide supports were prepared by an impregnation−precipitation method using commercial γ-Al 2 O 3 powder as a host with variation of the percentage of loading of ZrO 2 , TiO 2 , and CeO 2 promoters from their respective salt precursors. NH 4 OH was used as the precipitating agent. The as-prepared catalysts were characterized by Brunauer−Emmet−Teller surface area analysis, atomic absorption spectroscopy, X-ray diffraction, temperature-programmed reduction by H 2 (H 2 -TPR), and CO chemisorption. Catalytic activity of the newly synthesized catalysts was investigated toward hydrogenation of CO 2 at atmospheric pressure by varying reaction temperature between 250 and 400 °C (with increasing step equal to 25 °C). Experimental results revealed that the composite oxide supported Ni-based catalysts showed performance superior to that of the γ-Al 2 O 3 only supported Ni-based catalyst (which was synthesized using the same procedure for comparison). Among the investigated catalysts, the Ni/C15 catalyst with composite oxide support (55% of γ-Al 2 O 3 loading and 15% equivalent loading of ZrO 2 , TiO 2 , and CeO 2 ) showed the best activity: 81.4% conversion of CO 2 to CH 4 at 300 °C. Better performance of the composite oxide supported Ni-based catalysts was achieved because of the improvements in the reducibility nature of the catalysts (investigated using H 2 -TPR).