Carbon Forming Reactions Over Precious Metal Steam Reforming Catalysts

Steam reforming of CH4 is an established industrial process for the formation of synthesis gas. It takes place in two reversible stages: Cf4 reforming reaction (1) followed by the water-gas shift (2). Reaction (1) takes place at high temperature (approximately 1200K) and pressure (15-30bar) to maximise the efficiency and conversion while reaction (2) takes pl.ace at as Iowa temperature as possible. The biggest worry to the steam reforming industry is the formation ofcarbon which is produced favourably via three side-reactions: (3) Cf4 C+2H2 &=75 kJmorl (4) 2CO C+C02 MIo = -171 kJ morl (5) CO +rr2 , b C+H20 MIo = -131 kJ morl The catalysts currently used for reaction (1) are based on Ni metal however, Ni is prone to formation of a type of carbon known as 'whiskers' (filamental carbon) which form when deposited carbon dissolves into the bulk metal. These structures result in a breakdown of the catalyst particles leading to reactor tube blockages and eventual costly damage. Certain measure can be taken to reduce carbon, namely, use ofexcess steam; partial poisoning of the catalyst with sulfur and alterations to the catalyst formulation such as alkali dopants and noble metals. In this project, the behaviour of eight supported noble metals (Rh, Pt, Ru and Ni supported on La-Zr02 and Ah03) during reactions (3)-(5) at 20bar and 873K was investigated. It has been shown previously that noble metal catalysts can be more active than Ni during steam reforming and are more carbon resistant. More significantly, noble metal catalysts do not form whisker carbon. High pressure reactions (3) and (4) combined with a series of low pressure ~ulses followed by mass spectrometry revealed the greater stability of the La-Zr02 supported catalysts due to strong metal support interactions (SMSI) and a faster rate of O-transfer. Ni was always the more reactive catalyst although it was not compared at high pressure for risk of potential damage. Common mechanisms were proposed for all catalysts involving interaction of surface OH-groups with reactant species on all La-Zr02 and Ah03 catalysts. The reactivity of the Ah03 catalysts was related to percentage metal dispersion although this was not apparent for the La-Zr02 catalysts. Deactivation studies revealed the likely first and second order behaviour of reactions (4) and (3) respectively. These studies also showed how the r!lte of deactivation was greater over Pt than Rh. Theories on the type of carbon produced during reaction (3) were discussed. Each noble metal catalyst produced a unique reaction profile during reaction (5) and the greater reactivity of the La-Zr02 catalysts was attributed to interfacial metalsupport interactions (IMSl). The reaction profile was clearly affected by the nature of .the metal and the support and so a mechanism was proposed with an explanation of each catalysts' behaviour in terms oftype ofmetal and support. .An excess of.steam can be used to prevent carbon formation and likewise, C02 can be used for the same purpose. An investigation of different CO/C02 ratios was made and it was apparent that ratios of approximately 1:1 were necessary for carbon prevention. A pre-treatment of each catalyst with CO/C