Umit B. Demirci

Sodium borohydride versus ammonia borane, in hydrogen storage and direct fuel cell applications

Since the late 1990s, sodium borohydride (NaBH4, denoted SB) is presented as a promising hydrogen storage material and an attractive fuel (aqueous solution) of the direct fuel cell (or direct liquid-feed fuel cell). In 2007, the U.S. Department of Energy recommended a no-go for SB for vehicular applications and suggested work on ammonia borane (AB), another promising hydrogen storage material, which is also considered as a fuel for the direct fuel cell. Both boron hydrides in hydrogen and fuel cell applications are the topics of the present paper. The basics, issues, solutions to the issues and state-of-the-art are tackled but the discussion aims to compare the hydrides for either application. It is shown that there are many similarities between SB and AB in their features and applications. Nevertheless SB and AB as hydrogen storage materials do not compete. Rather, SB is intended more to portable technologies while AB to vehicular applications. Otherwise, when these hydrides are utilised as fuels of direct fuel cell, one question arises: what can be the advantage of developing the AB-powered fuel cell when it seems to be less effective, practical, and more complex than the SB-powered fuel cell? These aspects are discussed. However that may be, it is concluded that both SB and AB are not mature enough for the applications considered.

Kinetic study of n-heptane conversion on sulfated zirconia-supported platinum catalyst: the metal–proton adduct is the active site

Abstract Isomerization and cracking reactions of n-heptane on 0.2xa0wt.% platinum supported on sulfated zirconia (Pt/SZ) are investigated in such a way to get maximum kinetic data. The experimental conditions were varied as follows: (i) two ranges of hydrogen pressure, low pressure (LP) 190–760xa0Torr and high pressure (HP) 900–3000xa0Torr were studied; (ii) reaction temperatures of 150, 200 and 250xa0°C were investigated; and (iii) hydrocarbon partial pressures from 5 to 40xa0Torr were analyzed. Reaction orders versus hydrogen pressure and hydrocarbon pressure, and apparent activation energy values have been determined. The Pt/SZ catalyst displays an original catalytic behavior: (i) at LP, the hydrogen reaction order is negative, while it is positive at HP, and in both cases, these orders tend towards zero when the reaction temperature increases; (ii) the apparent activation energy values are lower at HP than at LP; and (iii) the reaction orders versus hydrocarbon are equal to 1 whatever the experimental conditions are. The changes in the experimental conditions involve modifications of the kinetic parameters. Such results are explained by the fact that the relative proportions of metallic sites or acid sites are influenced by the experimental conditions. The metal–proton adduct site [Hue5f8(Mm)(H+)x]x+ explains these experimental results. We interpret that at hydrogen pressures higher than 760xa0Torr, the excess of hydrogen provokes a shift in the reactivity from the metallic part of the adduct to the acid one. In fact, at HP and at high reaction temperature, the reactivity is controlled by the acid sites, while at LP and low temperature, it is managed by the metallic and the acid sites.

How green are the chemicals used as liquid fuels in direct liquid-feed fuel cells?

How green are the chemicals used as liquid fuels in the direct liquid-feed fuel cells intended to mobile, portable applications? Is there any risk of using such fuels? The present paper tries to give indications of answer to these questions while tackling the green chemistry concept, the liquid fuels (i.e. anodic oxidation, inherent hazards and renewability), the reaction products and by-products. The discussion especially focuses on green chemists critical remarks and the green chemistry principles. Globally none of the liquid fuels appears to be green in terms of toxicity even if most could be produced from renewable raw materials. However this does not necessarily mean that the direct liquid-feed fuel cell technology is not green. The greenness of the direct liquid-feed fuel cell will also depend on its reliability and safety.

From Bifunctional Site to Metal–Proton Adduct Site in Alkane Reforming Reactions on Sulphated-Zirconia-Supported Pt or Pd or Ir Catalysts

Isomerization reactions of n-heptane, n-octane and n-nonane are studied on sulphated-zirconia-supported 0.2 wt% Pt, Pd or Ir catalysts. Evolutions of isomer selectivity versus total conversion and reaction temperatures are analysed. When total conversion (αT) is increased, isomer selectivity (%Sisom) is decreased and the slope of the curve %Sisom=f(αT) is more pronounced when the carbon number in the alkane is more important. At isoconversion, around 20%, below 473 K, cracking is favoured over isomerization reaction, and above 473 K it is the reverse. Moreover, with n-heptane, when the catalytic reaction occurred at 423 K and at low conversion, αT≤20%, we observed a large decrease in the isomer selectivity percentages on Pd/SZ and Ir/SZ compared to Pt/SZ. What is remarkable is that, at this low temperature, both metals are inactive in the carbon–carbon bond rupture. To explain these results the following points are raised: (i) an associative mechanism is proposed for the adsorption step of the alkane involving an agostic intermediate species where the carbon–hydrogen bonds act as ligands to the transition metal centres forming covalent C–H⋅⋅⋅M systems, and (ii) a metal–proton adduct site, which gathers metallic and acidic sites is suggested. This approach seems to better explain our results than the “traditional” bifunctional mechanism.