Gas-phase poisons

The use of this probe reaction is currently being extended to other metal/graphite systems and an investigation of the influences of other gas phase poisons including sulfur and phosphorus. 

Gas Phase Poisons. The effectiveness was examined of four sulphur compounds, thiophene, hydrogen sulphide, sulphur dioxide and carbonyl sulphide, as gas phase poisons. 

Nitrogen dioxide, N02 (oxidation number -t-4), is a choking, poisonous, brown gas that contributes to the color and odor of smog. The molecule has an odd number of electrons, and in the gas phase it exists in equilibrium with its colorless dimer N204. Only the dimer exists in the solid, and so the brown gas condenses to a colorless solid. When it dissolves in water, NOz disproportionates into nitric acid (oxidation number +5) and nitrogen oxide (oxidation number +2) ... 

Accordingly, serious commercially oriented attempts are currently being made to develop special gas-phase micro and mini reactors for reformer technology [91, 247-259], This is a complex task since the reaction step itself, hydrogen formation, covers several individual processes. Additionally, heat exchangers are required to optimize the energy balance and the use of liquid reactants demands micro evaporators [254, 260, 261], Moreover, further systems are required to reduce the CO content to a level that is no longer poisonous for a fuel cell. Overall, three to six micro-reactor components are typically needed to construct a complete, ready-to-use micro-reformer system. 

In the gas phase, the reaction of O- with NH3 and hydrocarbons occurs with a collision frequency close to unity.43 Steady-state conditions for both NH3(s) and C5- ) were assumed and the transient electrophilic species O 5- the oxidant, the oxide 02 (a) species poisoning the reaction.44 The estimate of the surface lifetime of the 0 (s) species was 10 8 s under the reaction conditions of 298 K and low pressure ( 10 r Torr). The kinetic model used was subsequently examined more quantitatively by computer modelling the kinetics and solving the relevant differential equations describing the above... 

G) In reality, CO with H20 shifts H2 and C02, and CH4 with H20 reforms to H2 and CO faster than reaction as a fuel at the electrode. CO is a poison for lower temperature fuel cells, but is used as a fuel in the high-temperature cells (e.g., SOFC, MCFC). CO may not actually react electrochemically within these cells. It is commonly understood that CO is consumed in the gas phase through the water-gas shift reaction as CO + H20 = C02 + H2. The H2 formed in this reaction is subsequently consumed electrochemically. 

The philosophy used to develop detailed chemical kinetic mechanisms for gas-phase reactions can, in principle, be extended to treat heterogeneous reactions, provided diffusion is also considered in the final analysis. Clearly, the problem in heterogeneous catalysis is considerably more complex because of the close proximity of a large number of atoms and their collective effect on reaction kinetics and mechanisms, and the inevitable variation of catalyst structure with time—for example, as a result of sintering and poisoning. 

It is important to note that the computed potential energy surface reveals a facile reactivation via Scheme 7, with low energy barriers both in the gas phase and in aqueous solution" . This accords with experimental findings on oximates as reactivators following poisoning by organophosphorus toxics. 

There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M° sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of... 

The rationalization of the structure sensitivity of a certain reaction might also be a problem. First, one has to answer the following question Is the structure sensitivity observed inherent for the reaction studied and therefore related to the varying exposure of different sites to the gas phase, or is the sensitivity induced by side reactions which themselves (and not the reaction followed) are structure sensitive The latter problem was first formulated by Katzer (227), who also demonstrated that an apparent structure sensitivity may be a consequence of a structure-sensitive self-poisoning (228). 

It should be reminded here that the most interesting feature of the original ZGB-model is the existence of kinetic phase transitions. Denoting the mole fraction of CO in the gas phase by Yco (and therefore Yqo2 = 1 - Yco), one finds a reactive interval 0.395 = y < Yco < Vi = 0.525 [2] in which both particle types are coexisting on the surface. For Yco < V and for Yco > 2/2 the surface is completely covered by O2 or CO, respectively. The phase transitions are found to be of the second order at y and of the first order at 2/2 Because of the irreversible character the model describes a poisoned state from which the system cannot escape and reaction comes to a stop. 

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