Carbon forming reactions over precious metal steam reforming catalyst

Steam reforming of CH4 is an established industrial process for the formation of synthesis gas. It takes place in two reversible stages: CH4 reforming reaction (1) followed by the water-gas shift (2). (Fig. 3599A) The biggest worry to the steam reforming industry is the formation of carbon which is produced favourably via three side-reactions: (Fig. 3599B) In this project, the behaviour of eight supported noble metals (Rh, Pt, Ru and Ni supported on La-ZrO2 and Al2O3) 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 pulses followed by mass spectrometry revealed the greater stability of the La-ZrO2 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-ZrO2 and Al2O3 catalysts. The reactivity of the Al2O3 catalysts was related to percentage metal dispersion although this was not apparent for the La-ZrO­2 catalysts. Deactivation studies revealed the likely first and second order behaviour of reactions (4) and (3) respectively. These studies also showed how the rate of deactivation was greater over Pt than Rh. Each noble metal catalyst produced a unique reaction profile during reaction (5) and the greater reactivity of the La-ZrO2 catalysts was attributed to interfacial metal-support interactions (IMSI). 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 of type of metal and support. An excess of steam can be used to prevent carbon formation and likewise, CO2 can be used for the same purpose. An investigation of different CO/CO2 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/CO2 changed the product selectivity during reaction (5)