J.A.R. van Veen

On the effect of a heat treatment on the structure of carbon-supported metalloporphyrins and phthalocyanines

Abstract The four models proposed up till now to explain the beneficial effect of a heat treatment on the O 2 -reduction activity of transition-metal chelate/carbon catalysts are discussed in the light of available evidence. Two recent experiments designed to discriminate between the various possibilities are treated in some detail. It is concluded that the continued existence of the MeN 4 moiety of the chelate is the structural feature which is associated with the high activity of heat-treated materials.

Oxygen reduction on monomeric transition metal phthalocyanines in acid electrolyte

The characteristics of oxygen reduction on high surface-area carbon supported metallophthalocyanines have been studied in 8 N H2SO4 and also in 6 N KOH. Catalysts were prepared by the usual H2SO4-precipitation method or by adsorption from pyridine solution. The order of activity in both electrolytes is: Fe > Co ≈ Ru > Mn > Pd ≈ Pt > Zn. It was found that basic surface groups have a beneficial effect on the activity of CoPc/C. A heat treatment in N2 or Ar atmosphere at temperatures up to 800°C enhances the stability and activity of CoPc/C, the activity of RuPc/C and the stability of FePc/C. Some electrocatalytic aspects are discussed; it is shown that FePc, CoPc and RuPc are real oxygen reduction catalysts in H2SO4. The activity of CoPc/C is proportional to CoPc loading, but the activity of FePc/C seems to be determined by a small fraction of the FePc present. The oxygen reduction and H2O2 decomposition activities are found to be correlated. For CoPc/C the first electron transfer seems to be rate-limiting in H2SO4; in KOH the second electron transfer seems to be rate-limiting in all cases.

On the formation of type I and type II NiMoS phases in NiMo/Al2O3 hydrotreating catalysts and its catalytic implications

Two active phases can exist in sulfided NiMo/Al2O3 catalysts. Impregnation of an ammoniacal NiMo solution leads eventually to NiMoS phase I, which is strongly bonded to the alumina, while the use of nitrilotriacetic acid gives rise to the totally sulfided NiMoS phase II. Phase I is less active in gas-phase thiophene hydrodesulfurization (HDS) and trickle-flow quinoline hydrodenitrogenation (HDN), but more active in trickle-flow dibenzthiophene HDS. The presence of phosphoric acid in the impregnation solution results in partial formation of NiMoS II upon sulfidation; P itself does not have a direct effect in HDS, but does influence HDN performance, mainly through increasing the catalysts affinity for quinoline. Extended X-ray absorption fine structure analysis data are consistent with this picture. In-situ Raman spectroscopy and NO chemisorption/infra-red spectroscopy cannot distinguish between NiMo, NiMoP, and NiMo(nitrilotriacetic acid) catalysts. In-situ fluoridation of the sulfided NiMo/Al2O3 catalyst leads to a fairly complete phase I a phase II transformation. Again, however, in-situ Raman spectroscopy does not detect the difference. The implications of these findings for catalyst design are discussed.

Catalysts for second-stage deep hydrodesulfurisation of gas oils

Abstract The interest for new deep hydrodesulfurisation (HDS) processes is expected to rise since more stringent legislation for the maximum sulfur concentration in automotive diesel fuel has been proposed. A realistic option is the application of a separate deep HDS reactor following the existing HDS process in which alternative catalysts may be applied. It was shown that ASA-supported Pt and PtPd catalysts are very active in model feed deep HDS reactions. Moreover, in the deep HDS of a pre-hydrotreated straight-run gas oil (P-SRGO) under relevant industrial conditions, PtPd/ASA showed a very promising performance. The applicability of ASA-supported noble metal catalysts in practice will be largely determined by the H 2 S concentration in the second-stage reactor and the price of noble metals. In addition, NiW/γ-Al 2 O 3 is also considered to be a promising catalyst for second-stage deep HDS. From the differences in the relative performance between model and real feed experiments, it is found that the suitability of a catalyst for deep HDS of gas oils cannot be evaluated by single-component model studies alone. The H 2 S concentration and the presence of other competing reactants largely determine the outcome of model experiments and should therefore be chosen carefully.

On the difference between gas- and liquid-phase hydrotreating test reactions

Abstract In industrial practice, hydrotreating of oil fractions is carried out in either a gas-phase process or a trickle flow process. We previously noticed that a remarkable difference exists between the relative activity of mixed sulfide catalysts in gas-phase and liquid-phase hydrodesulfurization (HDS) reactions. In the literature, however, no satisfying explanation with respect to the possible fundamental differences between these reactions can be found. In this paper, we report an elaborate investigation on the effect of reaction conditions, type of reactant and type of the catalyst on the occurrence of differences between the relative activity, i.e. ranking, of mixed sulfide catalysts in gas- and liquid-phase reactions. Striking differences were observed between the ranking of nitrilo-triacetic acid (NTA) and conventionally prepared NiMo catalysts in thiophene gas-phase HDS and liquid-phase dibenzothiophene (DBT) HDS. Importantly, these differences did not depend on the nature of the reacting sulfur-containing compound. This allows the generalisation that NTA-based Ni(Mo) catalysts are relatively more active in gas-phase HDS reactions, whereas conventionally prepared NiMo catalysts are relatively more active in liquid-phase HDS reactions. An analogous behaviour was observed for low- and high-temperature sulfided NiW/γ-Al 2 O 3 catalysts, of which the latter is much more active in gas-phase HDS reactions and the former is more active in liquid-phase HDS reactions. It is concluded that this so-called ‘gas–liquid-phase controversy’ is a generic phenomenon in hydrotreating reactions over metal sulfide catalysts. It was verified that mass transfer limitations do not play a role in this matter. The active sites of stacked slabs of the type II catalysts are more affected than those of type I catalysts, in which the active phase is in a more close interaction with the support. It is proposed that the phenomenon is related to a non-selective competitive adsorption of the a-polar solvent molecules on sites protruding from the catalyst surface. Apparently, the proximity of the ionic surface of the alumina support hinders the adsorption of the a-polar hydrocarbon molecules on the non-stacked systems, whereas the sulfur- and nitrogen-containing molecules are not so much affected in their adsorption behaviour on these active sites.

Testing and characterisation of Pt/ASA for deep HDS reactions

Abstract In the search for active catalysts for the conversion of refractory sulfur compounds in diesel fuel, the activity, the role of the support, and the nature of the active sites on Pt/ASA catalysts in deep HDS reactions were studied and compared to Pt/γ-Al 2 O 3 and Pt/XVUSY (stabilised Y zeolite). Pt/ASA appears to be much more active than Pt/γ-Al 2 O 3 but is initially less active than Pt/XVUSY. The latter however, showed strong deactivation after short reaction times. It is concluded that an appropriate tuning of the support acidity is crucial for this reaction. In contrast to the activity, the H 2 S sensitivity of the tested Pt based catalysts is hardly influenced by acidity of the support. Adsorption of H 2 S on these sulfur vacancies leads to strong competitive adsorption with the reacting sulfur compound. It is proposed that the stabilisation of small platinum clusters in the presence of H 2 S is an important effect of acidic supports. In addition, the strength and nature of the acidic sites on the support may affect the Pt–S bond strength of the active sites on small platinum particles. It is concluded that no sulfur-free platinum metal sites are present under the applied reaction conditions. It is therefore proposed that the conversion of 4-E,6-MDBT over Pt/ASA proceeds over sulfur vacancies on small platinum particles. The creation of sulfur vacancies on these small platinum particles may be related to their electron-deficient character on acidic supports.

First-stage hydrocracking: process and catalytic aspects

Abstract First-stage hydrocracking is a fixed-bed, catalytic process, which is implemented industrially with the prime objective of reducing organonitrogen, organosulfur and organooxygen compounds from the feedstock, and for lowering the aromatics concentration. Additionally, a part of the feedstock may be cracked. The most difficult step is generally the hydrodenitrogenation (HDN) reaction. The essential factors in HDN, and the consequences for other hydrotreating and hydrocracking reactions, are discussed. Considerable attention is paid to inhibition effects of nitrogen compounds. These effects are not only important from a process point of view, but also influence developments of first-stage hydrocracking catalysts. Recent trends in catalysts are reviewed with an emphasis on those showing improved cracking activity compared to conventional NiMo/alumina systems.

A thermoanalytic―mass spectrometric study of oxide-supported acetyl acetonates

The present DTA/MS study concerns M(acac)n/support materials, in which M = Pd2+, Pt2+, MoO2+2, Fe3+, Ru3+ or Ni2+ and support = SiO2, γ-Al2O3 or TiO2. The thermal behaviour depends, in the first place, on whether the complex is chemisorbed, i.e. has reacted in the adsorption step, or is simply physisorbed. Then, there is the influence of M, even in the case of M(acac)n chemisorbed on γ-Al2O3, where the acac is no longer associated with M, but with the surface Al ions. Lastly, the support also plays a role, and it is only with SiO2 that the simplest decomposition reaction, H transfer from surface OH groups to adsorbed M(acac)n to form acacH, is consistently observed.

The adsorption of heptamolybdate ions on oxidic surfaces

The adsorption of ammonium heptamolybdate (AHM) on γ-Al2O3 is governed by the basic surface hydroxyl groups. Having determined the number of these groups on the γ-Al2O3 used, we can give a quantitative description of the adsorption isotherm. When too highly concentrated AHM solutions are used, a (surface) precipitate is formed as well. On TiO2, only the formation of a surface precipitate is observed, even at low [AHM]; this effect is probably due to the acidic surface OH groups. On SiO2, not much adsorption takes place, but AHM reacts with the oxide to form a molybdosilicate solution species.

Selective electro-oxidation of carbon monoxide with carbon-supported Rh- and Ir-porphyrins at low potentials in acid electrolyte

During our study of carbon-supported Group VIII metalloporphyrins as oxygen reduction electrocatalysts it was found that catalysts based on Rh and Ir are very active in the electrochemical oxidation of CO, particularly when pretreated at elevated temperatures in an inert atmosphere. The active site appears to be an isolated metal ion in a (thermally modified) porphyrin framework. During this CO-electro-oxidation reaction the following three stages can be envisaged: (i) adsorption of CO onto an isolated metal centre, (ii) nucleophilic attack of H2O on the adsorbed CO, and (III) decarboxylation. The Rh- and Ir-porphyrin-Norit BRX catalysts have the unique feature of selectively oxidizing the CO component of COue5f8H2 gas mixtures.