Ni/Al2O3催化剂表面状态对CH4氧化反应的影响

采用瞬变响应技术研究了常压700℃条件下气相O2、Ni/Al2O3催化剂表面上可逆吸附氧物种及催化剂的表面状态对CH4吸附、反应以及CH4部分氧化反应的影响,同时也对CH4部分氧化制合成气反应过程中催化剂表面所处的状态进行了研究.结果表明,如果催化剂表面处于氧化态,CH4不能吸附分解,只能通过RidealEley机理与催化剂表面的吸附氧进行非选择性氧化反应,这将严重影响CH4的转化和目的产物H2、CO的选择性.只有在还原的催化剂上,CH4部分氧化制合成气反应才能高转化、高选择性地进行.在CH4部分氧化制合成气反应过程中,催化剂表面处于还原态,不存在多余的中间氧物种NiO,但存在少量的碳物种,这有利于保持催化剂的还原态和抑制CO2的生成.

The Effect of Surface State of Ni/Al2O3 Catalyst on Oxidation of CH4

The effect of gas phase O 2, reversibly adsorbed oxygen and the surface state of Ni/Al 2O 3 catalyst on the decomposition of CH 4 and partial oxidation of CH 4 were studied using transient response technique at atmospheric pressure and 700 ℃. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H 2 can be produced from non-selective oxidization of CH 4 by reversibly adsorbed oxygen which is more active in oxidizing CH 4 completely than NiO via Rideal-Eley mechanism and both the conversions of CH 4 and O 2 and the selectivities to CO and H 2 are very low. Therefore, keeping the catalyst surface in the reduction state is the prerequi sation of high conversion of CH 4 and high selectivities to CO and H 2.The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH 4. During partial oxidation of CH 4, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduction state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH 4, and indirect oxidation mechanism is impossible to gain dominance in the reaction under the experimental conditions.