Some FeTPP-Cl/carbon electrocatalysts, heat-treated at temperatures up to 800 °C, have been studied with cyclic voltammetry, x-ray photoelectron spectroscopy, extended X-ray absorption fine structure, and in situ Mo ¨ssbauer spectroscopy. It appears that the heat treatment induces considerable site heterogeneity in electronic terms, although structurally the Fe-N4 moiety seems persistent. The data indicate that only part of these Fe-N4 sites contributes to the activity for the electrochemical reduction of O 2 but that they operate according to the well-known redox mechanism.

No abstract

Surface characterization of amorphous silica− alumina (ASA) by CO ads IR, pyridine ads IR, alkylamine temperature-programmed desorption (TPD), Cs + and Cu-(EDA) 2 2+ exchange, 1 H NMR, and m-xylene isomerization points to the presence of a broad range of Brønsted and Lewis acid sites. Careful interpretation of IR spectra of adsorbed CO or pyridine confirms the presence of a few very strong Brønsted acid sites (BAS), typically at concentrations lower than 10 μmol/g. The general procedure for alkylamine TPD, which probes both Brønsted and Lewis acidity, is modified to increase the selectivity to strong Brønsted acid sites. Poisoning of the m-xylene isomerization reaction by a base is presented as a novel method to quantify strong BAS. The surface also contains a weaker form of BAS, in concentrations between 50 and 150 μmol/g, which can be quantified by CO ads IR. Cu(EDA) 2 2+ exchange also probes these sites. The structure of these sites remains unclear, but they might arise from the interaction of silanol groups with strong Lewis acid Al 3+ sites. The surface also contains nonacidic aluminol and silanol sites (200−400 μmol/g) and two forms of Lewis acid sites: (i) a weaker form associated with segregated alumina domains containing five-coordinated Al, which make up the interface between these domains and the ASA phase and (ii) a stronger form, which are undercoordinated Al sites grafted onto the silica surface. The acid catalytic activity in bifunctional nheptane hydroconversion correlates with the concentration of strong BAS. The influence of the support electronegativity on the neopentane hydrogenolysis activity of supported Pt catalysts is considerably larger than that of the support Brønsted acidity. It is argued that strong Lewis acid sites, which are present in ASA but not in γ-alumina, are essential to transmit the Sanderson electronegativity of the oxide support to the active Pt phase.

No abstract

Monomolecular cracking of propane was used to investigate the activity of the Brønsted acid sites in amorphous silica−aluminas with three different Si/Al ratios. The reaction rates increased with increasing aluminum content, but the apparent activation energies were identical. In comparison to zeolite ZSM5, the ASA catalysts showed much lower activity, both per weight and per total aluminum content. However, after correcting for the heat of adsorption, the intrinsic activation energies of ASA and H-ZSM5 were similar. This indicates that the ability of the active Brønsted acid sites to protonate propane is similar in amorphous and crystalline structures and that the much lower activity of ASA is due to the lower heat of adsorption and the small number of active sites. Few Brønsted acid sites were detected by means of pyridine adsorption followed by infrared spectroscopy; a broad band at around 3600 cm-1 was observed in the infrared region of the hydroxyl stretch vibrations. It is unclear whether this band was related to the catalytically active sites.

No abstract

It is shown that on A1203, SiOz, and C, a fully sulphided CoMoS phase can be prepared with similar degrees of dispersion, and that the specific activity of this phase for the hydrodesulphurization of thiophene is higher when this phase is supported on carbon than when it is supported on alumina or silica.

l~lb~ el bu~rgrmic Chemi,~lr~' aml Catra!vsix, EindhoPen tJni,~6(1~) MB Eimlhocen. Netherlands Received 16 December 1Ot)0; revised 31 January 1997 AhsiraelThe: c~,clroca|alyliqr |vdBcIion of uilrale has been investigated on Pt, Pd and Pl k Pd elecu~od¢,~ covered wi|h a ,~ubmonolayeJ of gern~aniunk PI + Pd eleo,~des were prepared by electrol¢~s deposition of stubmnnolayer~ of Pd on Pl by exchange of PdCI~ for preadsod~ed eopl~,r, Underpote~ ~i,dly deposited germanium enhances the reduction rale of nitrate strongly. The reduction of nitrite i~ euhanced to a lesser extent, whezeas germanium is inactive for NO and hydroxylamine reduction. Furlher. cyclic voltammetry shows that the well known inhibition of the nitrate reduction at low potentials is absent fi~r germaniumomodified electrodes. Amperometry shows lhat the cun~nt densities for nitrate reduction at 0.1 V depend strongly on the composition of the electrode surface. The activities increase in the order Pd, PI and Pt + Pd and all electrodes display a plopurtional relation between the activity and the germanium coverage. This shows that gemmnium is involved in the rate determining step, which is the reduction of nitrate to nitrite and its role is to bind the oxygen atom of nitrate, The higher activities for Pt + Pd electrodes can be understood in terms of changes in the electronic structure of the metals as a result of alloying. Selectivity measurements with a rotating ring-disk electrode have shown for all electrodes that the hydroxylamine selectivity increases for increasing germanium coverage. Pd displays higher hydroxylamine ,;electivides than Pt and Pt + Pd electrode~. No gaseous products were observed for Pi. whereas for Pt + Pd and Pd N~O selectivitie~ up to 8% were found. © 1997 Elsevier Science S.A.