Preparation And Catalytic Properties Of Ni-based Catalyst For Carbon Dioxide Reforming Of Methane

Carbon dioxide and methane,two main greenhouse gases,can be converted simultaneously through dry reforming of methane to obtain syngas with a H₂-to-CO ratio of 1.Compared with the traditional methane steam reforming,the dry reforming of methane has the following advantages: low cost of energy,and suitable H₂-to-CO ratio which can be directly used as the feed gas for downstream synthesis gas conversion,such as Fischer-Tropsch synthesis.The catalysts used for the carbon dioxide reforming of methane can be divided into two kinds: the noble metal catalyst and nickel-based catalyst.The major problem for the nickel catalysts is catalyst deactivation due to carbon deposition on the catalysts.Therefore,it's necessary to modulate catalyst compositions and preparation methods to obtain high reactivity and stability of catalysts.The traditional nickel-based catalysts for carbon dioxide reforming of methane mostly are in pellet form inside which hot spots are easily formed when on stream and further carbon deposition are favored.In recent years,monolithic catalysts have gained comprehensive attention due to their special multifunctional structure and high operational advantages.Catalysts with high activity and stability are intended to be prepared in this paper for carbon dioxide reforming of methane.We choose Ni as the active component,to carry out the work from three aspects: the modulation of promoters,carriers and the preparation methods.Ni-CaO-ZrO₂ granular catalyst,Ni-CaO-ZrO₂/pseudo-boehmite granular catalyst and Ni-CaO-ZrO₂/pseudo-boehmite monolithic catalyst were prepared.The main contents of this work are listed as follows:?1?A series of Ni-CaO-ZrOgranular catalysts were prepared by co-precipitation?1?A series of Ni-CaO-ZrO₂ granular catalysts were prepared by co-precipitation method,in which CaO was chosen as the alkaline additives,while thermally stable ZrO₂ acting as redox oxide to release and restore lattice oxygen.The effects of Nickel content and precipitant on the performance of the catalyst have been studied.Ammonia was determined as the precipitant for catalyst preparation.From the results of BET,XRD characterization,the catalyst with Ni/Zr atomic ratio of 0.8 has the biggest specific surface area and therefore shows the best activity.?2?Peseudo-boehmite,montmorillonoid,and ?-Al₂O₃ were investigated as supports to0.8Ni-0.2CaO-ZrO₂ catalyst.BET,XRD characterization show that after pseudo-boehmite was introduced as the support,specific surface area of catalyst is doubled,yet mesopore structure remains,and the activity is obviously promoted.Moreover,the effects of different basic promoters and calcination temperature on performance of Ni-CaO-ZrO₂/pseudo-boehmite were studied.Furthermore,the addition of La to the catalyst can further improve the reactivity and stability simultaneously.?3?0.8Ni-0.2CaO-ZrO₂ monolithic catalyst and 0.8Ni-0.2CaO-ZrO₂/pseudo-boehmite monolithic catalyst were both prepared.0.8Ni-0.2CaO-ZrO₂ monolithic catalyst with ?-Al₂O₃ as the binder to honeycomb ceramics is less active than corresponding granular catalyst,maybe because of acidic residue on catalyst surface during preparation.Otherwise,0.8Ni-0.2CaO-ZrO₂/pseudo-boehmite monolithic catalyst without ?-Al₂O₃ binder has remarkably high catalytic activity and stability,compared by corresponding granular catalyst.The mass loss test by ultrasonic concussion also shows the active component of monolithic catalyst prepared by “first coating and then calcination” method combines with the honeycomb ceramics tightly,and doesn't easily fall off.?4?The influences of reaction temperature and space velocity on the activity of 0.8Ni-0.2CaO-ZrO₂/pseudo-boehmiteparticle monolithic catalyst and 0.8Ni-0.2CaOZrO₂/pseudo-boehmite granular catalyst were investigated.Besides,the stability of both catalysts were also tested comparatively.It was found that activity of the monolithic catalyst is much higher than that of granular catalyst under different reaction temperatures and the greatest superiority over granular catalyst was observed under 750 ?.And because of the internal regular micro-channel structure of ceramic honeycomb substrate and superiority in hydrodynamics,the monolithic catalyst is not so sensitive to the change in space velocity and therefore can handle more feed gas per hour.Stability test showed that on the monolithic catalyst,CH4 conversion of above 80 % can be maintained within 30 hours without remarkable deactivation under 750 ? with a space velocity of 48L·g-1h-1.With the advantages of regular micro-channel structure containing small-size channel and thin wall,large open frontal area,low heat capacity and high thermostability,honeycomb ceramics supported monolithic catalyst possesses excellent mass and heat transfer capability to make full use of catalytic component,and shows much higher activity than granular catalyst.Furthermore,this microchannel structure design can also decrease the carbonization of catalyst surface and avoid aggregation of active species caused by internal hot spots formation,therefore promote the stability of catalyst.