Catalysts reforming reactivation

Graf et al.108 performed a comparative study of steam reforming of methane, ethane and ethylene on Pt, Rh, and Pd supported on YSZ. They observed that the reactivity and product distribution depends on the type of noble metal loaded. Over Rh/YSZ catalyst, the reactivity decreased in the order C2H6 > CdE > CH4. On the other hand, over Pt/YSZ, methane reacted much faster than the C2 hydrocarbons and the order of reactivity is CH4 > C2H4 > C2H6 (Fig. 2.8). The higher reactivity of Rh... 

Many late transition metals such as Pd, Pt, Ru, Rh, and Ir can be used as catalysts for steam reforming, but nickel-based catalysts are, economically, the most feasible. More reactive metals such as iron and cobalt are in principle active but they oxidize easily under process conditions. Ruthenium, rhodium and other noble metals are more active than nickel, but are less attractive due to their costs. A typical catalyst consists of relatively large Ni particles dispersed on an AI2O3 or an AlMg04 spinel. The active metal area is relatively low, of the order of only a few m g . ... 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Steady state and non steady state kinetic measurements suggest that methane carbon dioxide reforming proceeds in sequential steps combining dissociation and surface reaction of methane and CO2 During admission of pulses of methane on the supported Pt catalysts and on the oxide supports, methane decomposes into hydrogen and surface carbon The amount of CH, converted per pulse decreases drastically after the third pulse (this corresponds to about 2-3 molecules of CH< converted per Pt atom) indicating that the reaction stops when Pt is covered with (reactive) carbon CO2 is also concluded to dissociate under reaction conditions generating CO and adsorbed... 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

The direct reaction of methane partial oxidation always competes with total oxidation reactions, which are also responsible for O2 consumption, whereas steam and dry reforming and C-forming reactions are also to be considered. All reactions are catalyzed by the materials which are active in partial oxidation, but different scales of reactivity for the catalysts can be estimated from the experimental data. Total oxidation prevails at the light-off of the fuel-rich stream over most catalysts, but precious metals are more active than transition metals. 

Reference Reforming type Catalyst Reaction conditions Relative reactivities... 

Side reactions specific to one component play an important role in the reforming of a mixture. For example, aromatics are more prone to coking upon reforming, so their presence in a mixture can lower syngas yields over time due to catalyst deactivation. Also, the catalyst surface-component interactions may play an important role in the reforming of a mixture. For example, aromatics have an abundance of 71-electrons, so they may occupy active sites for a longer duration, due to 71-complexation between d-orbitals of the metal and 7i-elec-trons. Hence there will not be enough reactive sites available for the desired reaction to occur. 

Of course, this reactive adsorption is favoured by removal of hydrogen from the reaction zone. When 80% of the hydrogen is removed in the membrane reactor, the H2S tolerance of the catalyst is about halve the tolerance when no hydrogen is removed from the reaction zone. A higher degree of sulphur removal from the feed stream should be accomplished when operating a membrane steam reformer. 

Wang LC, et al. Production of hydrogen by steam reforming of methanol over Cu/ZnO catalysts prepared via a practical soft reactive grinding route based on dry oxalate-precursor synthesis. J Catal. 2007 246(1) 193-204. 

Courson, C., Makaga, E., Petit, C., and Kiennemann, A., Development of Ni catalysts for gas production from biomass gasification. Reactivity in steam- and dry-reforming. Catalysis Today 2000, 63 (2 4), 427-437. 

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