Solid state aspects of oxidation catalysis

The microwave cavity perturbation technique for contact-free and in situ electrical conductivity measurements in catalysis and materials science

“…Upon thermal excitation of an electron from the valence band into the acceptor level electron holes are formed that can move freely in the valence band giving rise to the p-type conductivity found for VPO. 4,5,11,12 If all the aforementioned investigations are performed under operando conditions, which is work in progress, the (bulk and surface) electronic properties can be correlated indeed with the catalytic activity and selectivity.…”

“…It is still controversially debated, if such reactions can be explained on the basis of the single site concept with an isolated active catalytic center large enough to ''store'' reversibly the rather high amount of charge carriers transferred during a catalytic cycle, or if the bulk region underneath the active surface layer with appropriate charge transfer properties ''buffering'' the dynamic charge carrier concentrations on the surface has to be taken into account. [3][4][5][6][7][8][9][10] To identify true correlations between conductivity, or generally any other property, of a sample and its catalytic activity and selectivity to a specific product or its functional capability as a selective gas sensor, the measurements have to be performed under in situ conditions. Usually, in-depth conductivity studies of heterogeneous catalysts under real working conditions are utilized by pressing the powder catalyst between two metal electrode discs or by the deposition of metal electrodes on catalyst pellets by means of physical vapor deposition.…”

“…Usually, in-depth conductivity studies of heterogeneous catalysts under real working conditions are utilized by pressing the powder catalyst between two metal electrode discs or by the deposition of metal electrodes on catalyst pellets by means of physical vapor deposition. In such a way, e.g., the selective oxidation of n-butane on vanadium-phosphorous-oxide (VPO), 4,5,11,12 of propane and propene on MoV mixed oxide catalysts, 13,14 the oxidative coupling of methane 15 or the oxidative dehydrogenation of butene on bismuth molybdates 16 have been studied by measuring the DC or AC electrical resistance up to 10 MHz. However, the two-point contact method, which has been most often used in catalytic experiments, bears the severe disadvantage that it is highly dependent on the quality of the contacts between electrodes and catalyst.…”

Mathematical modelling of the unsteady-state oxidation of nickel gauze catalysts

“…The kinetics of nickel oxidation were described in several papers: (Labohm, Gijzeman, & Geus, 1983;Leclercq, Pietrzyk, Gengembre, & Leclercq, 1986;Hoang-Van, Kachaya, Teichner, Arnaud, & Dalmon, 1989;Vreeburg, vanKooten, Gijzeman, & Geus, 1992a;Vreeburg, Tongeren, Gijzeman, & Geus, 1992b;Stuckless, Wartnaby, Al-Sarraf, Dixon-Waren, Kovar, & King, 1997;Gellings & Bouwmeester, 2000). Most of these studies were carried out on mono-crystals in ultrahigh vacuum at temperatures under 600 K. The results obtained show that the surface coverage by oxygen species is higher than one indicating a multilayer oxygen adsorption.…”

“…Taking into account the subsurface di usion the value of 2:4 mol(O) kg −1 cat is in the range of those observed on monocrystals at lower temperatures (100 -500 K). Typical values reported for oxygen coverage at low temperatures vary in a range from 1 to 7 monolayer (ML) (Labohm et al, 1983;Leclercq et al, 1986;Hoang-Van et al, 1989;Vreeburg et al, 1992a, b;Stuckless et al, 1997;Gellings & Bouwmeester, 2000). 1 ML (monolayer) corresponds to approximately 1:9 × 10 15 atoms=cm 2 .…”