Steam Reformers Catalyst Activity

Because of a great practical importance of SMR as a major industrial process for manufacturing H2, the development of efficient steam reforming catalysts is a very active area of research. Nickel and noble metals are known to be catalytically active metals in the SMR process. The relative catalytic activity of metals in the SMR reaction (at 550°C, 0.1 MPa and steam/carbon ratio of 4) is as follows [12] ... 

There is a need for low-cost methane steam reforming catalysts that are active at low temperature and resistant to coke formation under membrane reactor conditions. Low-cost (Ni-based) catalysts are also needed that can withstand regeneration conditions in a sorption-enhanced reformer. 

Nickel-based steam reforming catalysts are very efficient for the decomposition of tars and ammonia in biomass gasification gas. Ceramic candle filters can be applied to remove particles at high temperature. It is proposed to use nickel-activated alumina candle filters for the simultaneous removal of tar, ammonia and particles from biomass gasification gas [12]. 

All steam reforming catalysts in the activated form contain metallic nickel as active component, but the composition and structure of the support and the nickel content differ considerably in the various commercial brands. Thus the theoretical picture is less uniform than for the ammonia synthesis reaction, and the number of scientific publications is much smaller. The literature on steam reforming kinetics published before 1993 is summarized by Rostrup - Nielsen [362], and a more recent review is given by K. Kochloefl [422]. There is a general agreement that the steam reforming reaction is first order with respect to methane, but for the other kinetic parameters the results from experimental investigations differ considerably for various catalysts and reaction conditions studied by a number of researchers. 

In the extrinsic reaction rate, mass transfer plays a dominant role. The combined effect of the molecular diflusion of the reactants from the bulk gas through the gas film around a catalyst particle to the geometrical surface of the particle, and to some extent the Knudsen diflusion within the catalyst pores, are the limiting factors. As the intrinsic reaction is fast, the reactants will have reacted before they travel down the lenght of the pore. The effectivity of a steam reforming catalyst, that is, how much of the catalyst particle is utilized, varies with the reaction conditions and is only about 1 % at the exit. Because of this, the apparent activity increases with decreasing particle size, and the geometrical shape of the catalyst particle also has a distinct influence (Section 4.1.1.3). 

Most commercial steam reforming catalysts contain metaUic Ni supported on an oxide such as AljOj, MgO, and MgAl O. Ni is the catalyst of choice because of its high activity toward C-H bond cleavage and low cost. The oxide supports offer superior mechanical and thermal stability under the reaction conditions [18],... 

Most steam-reforming catalysts are based on nickel as the active material. Also, cobalt and noble metals catalyze the steam-reforming reaction, but they are generally too expensive to find widespread use. A number of different carriers including alumina, magnesium-aluminum spinel, zirconia, and calcium aluminate are employed. 

Further extension of the concept seems possible, based on two ideas. Firstly, it is known that rhodium and ruthenium are better steam reforming catalysts than nickel [1]. It could be possible to dope nickel with a precious metal to the extent that ensemble size control is achieved using an additive which is active for steam reforming in its own right- In this case, both activity and selectivity should be high. 

Various solid state reactions are involved in the process. In addition to those governing sintering, catalyst-support interactions may alter the nature and catalytic activity of the solid and may stabilise or destabilise the solid towards sintering. Thus, for example, the formation of nickel aluminate, NiAlgO, is well established in steam reforming catalysts [21,22], and this compound is catalytically inactive. However, its presence may affect the thermal stability of the solid [23], as is the case in cobalt-molybdenum and nickel-molybdenum based catalysts supported on alumina and used for hydro-treating [24]. 

The steam reforming catalyst is normally based on nickel. The properties are dictated by the severe operating conditions. The activity depends on the nickel surface area and particle size. The shape should be optimized to achieve maximum activity with minimum increase in pressure drop. 

From consideration of the thermodynamics of sulfur chemisorption on ruthenium (ref. 6), the gas phase sulfur activities (llgS/l ) of the lightly and moderately sulfur-poisoned Ru catalysts in equilibrium with the adsorbed sulfur at the process temperature (190 C), were approximately 0.02 and 1 ppb+ respectively. On the basis of these results, the equivalent partial pressure ratio for critical sulfur coverage is about 1 ppb at 490 C. This level is well below that attainable by conventional sulfur removal methods. Thus our result confirms the need for high performance desulfurization technology (ref 3) that can reduce sulfur contaminants in feedstocks to a sufficiently low sulfur level to avoid carbon fouling cf Ru/A Og steam reforming catalysts. 

Thorium and uranium are used in cotmnercial catalytic systems. Industrially, thorium is used in the catalytic production of hydrocarbons for motor fuel. The direct conversion of synthetic gas to liquid fuel is accomplished by a Ni-Th02/Al203 catalyst that oxidatively cracks hydrocarbons with steam. The primary benefit to the incorporation of thorium is the increased resistance to coke deactivation. Industrially, UsOs also has been shown to be active in the decomposition of organics, including benzene and butanes and as supports for methane steam reforming catalysts. Uranium nitrides have also been used as a catalyst for the cracking of NH3 at 550 °C, which results in high yields of H2. 

---