Mechanical Mixtures of Me (Ni, Pd) Ce Oxides and Silica-Supported Heteropolyacids: Role and Optimal Concentration of Each Active Species in n-Hexane Isomerization

Catalytic properties of silica-supported heteropolyacids (HPA) in a mechanical mixture with reduced Me–Ce oxides (Me = Ni, Pd) in n-hexane isomerization are studied. The role of each component of the mixed oxides (Ce and, typically, Ni and Pd) and their optimum content has been illuminated: cerium is not only beneficial for eliminating or preventing coke deposition but is also effective for maintaining the Keggin structure of the highly-organized HPA during the reaction and probably allows a better dispersion of the second metal species. Nickel and palladium, present as Ni0 and Pd0, reinforce the activation of the alkane, which is difficult to obtain by means of a direct attack by an acid, and, thus, enhance noticeably the activity of the catalyst. The best mechanical mixtures are obtained with 30–70 wt % NiCeO–HPW/SiO2 and 50–50 wt %Pd0.1CeO–PW/SiO2. These mixtures have the highest efficiency for a Ni/(Ni + W) atomic ratio of 0.66 and a Pd/(Pd + W) ratio of 0.40, respectively. Finally, the conversion of n-hexane is in the order HPW > HSiW >HBW, which seems to be consistent with the order of their acid strength as per the literature, but the isomerization selectivity appears to be slightly higher on HSiW

A general route via formamide condensation to prepare atomically dispersed metal-nitrogen-carbon electrocatalysts for energy technologies

Single-atom electrocatalysts (SAECs) have gained tremendous attention due to their unique active sites and strong metal–substrate interactions. However, the current synthesis of SAECs mostly relies on costly precursors and rigid synthetic conditions and often results in very low content of single-site metal atoms. Herein, we report an efficient synthesis method to prepare metal–nitrogen–carbon SAECs based on formamide condensation and carbonization, featuring a cost-effective general methodology for the mass production of SAECs with high loading of atomically dispersed metal sites. The products with metal inclusion were termed as formamide-converted metal–nitrogen–carbon (shortened as f-MNC) materials. Seven types of single-metallic f-MNC (Fe, Co, Ni, Mn, Zn, Mo and Ir), two bi-metallic (ZnFe and ZnCo) and one tri-metallic (ZnFeCo) SAECs were synthesized to demonstrate the generality of the methodology developed. Remarkably, these f-MNC SAECs can be coated onto various supports with an ultrathin layer as pyrolysis-free electrocatalysts, among which the carbon nanotube-supported f-FeNC and f-NiNC SAECs showed high performance for the O2 reduction reaction (ORR) and the CO2 reduction reaction (CO2RR), respectively. Furthermore, the pyrolysis products of supported f-MNC can still render isolated metallic sites with excellent activity, as exemplified by the bi-metallic f-FeCoNC SAEC, which exhibited outstanding ORR performance in both alkaline and acid electrolytes by delivering ∼70 and ∼20 mV higher half-wave potentials than that of commercial 20 wt% Pt/C, respectively. This work offers a feasible approach to design and manufacture SAECs with tuneable atomic metal components and high density of single-site metal loading, and thus may accelerate the deployment of SAECs for various energy technology applications

New perspectives for the isomerization of light n-alkanes

Isomerization of light n-alkanes into the high-octane number branched alkanes is an important challenge for petroleum refining industry. According to the literature, such a transformation needs two different active centres: one having a hydrogenating/dehydrogenating ability and another, a protonic acid function. From this point of view, heteropolyacids (HPAs) in mixing with one or more transition metal appear promising. Mechanical mixtures of alumina-supported platinum or of mixed palladium (or nickel) -cerium oxides and silica-supported HPAs have been tested for the isomerization of n-hexane. The catalysts were characterized before and after tests. Although pure silica-supported HPAs have an initial activity, they are very easily to deactivate by coke deposition. Presence of Pt, Pd, and Ni gives rise to a better activity and a higher selectivity for di-branched isomers, and is really beneficial for retaining the Keggin structure of silica-supported HPAs. The catalytic performances are strongly depending on the composition of the mechanical mixtures. Roughly, the activity is in the same order as the acid strength. 1H and 31P solid-state NMR studies of trimethyl phosphine adsorbed on heteropolytungstate put in the evidence of the acidic sites. It is noteworthy that the presence of metallic Pt, Pd, or Ni metal after the reduction, instead of the original metallic ions, reinforces the generally admitted mechanism with an activation of the alkane. The activation is difficult to obtain by direct attack by an acid (proton), but it can be achieved by the metallic species to the corresponding alkene before formation of a carbenium intermediate on the acid site. However, the formation of alkenes directly by the HPAs and the participation of metallic species for limiting the alkenes concentration are not excluded

Structure and reactivity of silica-supported 12-tungstophosphoric acid

International audienc

Silica-supported heteropoly acids promoted by Pt/Al2 O3 for the isomerization of n-hexane

International audienc

Isomerization of n-Hexane over Silica-Supported Heteropoly Acids Promoted by the Reduced Ce–Ni Oxides

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