Study On Catalytic Carbon Dioxide Reforming Of Methane Over Ni-Zr Catalysts

The increase of anthropogenic CO₂ emissions is encouraging extensive research on CO₂ recycling processes.Dry reforming of methane(DRM:CH₄（g）+CO₂（g）（?）2CO（g）+2H₂（g）,Δ₂₉₈₆⁰)=+247·8)7)^-1)represents an attractive way to reduce the CO₂.Because the feed gases CO₂ and CH₄ are greenhouse gases and the products H₂ and CO are more valuable.Transition metal Ni is universally considered as an alternative to noble metal for DRM,due to its high activity and low cost.However,Ni-based catalysts were suffered deactivation resulted from sintering of the active phase and carbon deposition,thus its practical application was not actually possible.So,there is a need to develop a cost-effective catalyst,able to withstand high temperatures and able to resist deactivation.In our previous work,the promoter Zrcould enhance the activity for low temperature DRM reaction,however,its stability was very low and the catalyst almost was deactivation within 2 hours.In this dissertation,combining the character of promoter Zr,a series of Ni-Zrmaterial was prepared to obtain a better catalytic performance of Ni-Zrcatalyst for DRM reaction.Herein,this research concentrates on new strategies towards improving the sintering and carbon deposition over nickel-based catalysts.These strategies include the use of different supporters and promoters,and different methods to prepare catalysts.The relationship between the structure of catalyst and activity was discussed in depth by combining various characterization methods,further providing theoretical basis for industrial application.1.The study on low temperature of dry reforming of methane over Ni-Si/ZrO₂ catalystThe Ni-Si/ZrO₂ catalyst with better catalytic performance in low temperature DRM was obtained by the adjusting the electronic donor ability and enhancing the dispersity of nickel species through addition of Si promoter and using ZrO₂ supporter.The mechanistic study was researched by in situ XPS and DRIFTS.The Ni-Si/ZrO₂ showed higher initial TOF of CH₄(0.50 s^-1)and CO₂(0.44 s^-1),and stability for low temperature（400°C）DRM reaction compared to Si O₂-supported-nickel catalyst promoted by zirconia（Ni-Zr/Si O₂）.The designed Ni-Si/ZrO₂catalyst was featured by the formation of active nickel particle with small size of 7-9nm and with slightly strong electronic donor ability,the stabilization of the initial metal nickel state under the reaction conditions.Besides,at 400℃the carbon formed on Ni-Si/ZrO₂ catalyst was easy to be removed C₁ coke.Whereas,at 450°C DRM reaction,the coke was mainly difficult to be removed C₂ coke,because the CO₂ preferred to combine with H species rather than react with the coke,namely,the adsorption and activation of CO₂ could be influenced by the temperature,thereby affecting the ability of elimination of carbon deposition,which resulted in carbon deposition.2.The study on the oxygen cycle and the stability of catalyst for low temperature dry reforming of methaneThe introduction of oxygen vacancies to adjust the electron cloud density of active sites and the utilization of the Ni-O-Zrinterface to enhance the electronic transmission capacity,which will increase the activity and stability of Ni/ZrO₂ catalyst for 300℃DRM reaction.The Ni/ZrO₂ catalyst exhibited stable activity for 10 h DRM reaction with CH₄ and CO₂ turnover frequencies of 0.13 s^-1 and 0.02 s^-1,respectively.After CH₄ decomposition on the Ni⁰ site,the resultant C would react with nearby O to form CO,simultaneously forming oxygen vacancy,namely the Ni-Ov-Zrinterface（the vacancy nearby Ni⁰ species）.The oxygen vacancy,which can trap the electron from metallic nickel,would activate CO₂ and then regenerate the new Ni-O-Zrinterface to complete an oxygen cycle and recover the metallic nickel species.This oxygen cycle,dependent on the oxygen supply.3.The study on the competition of adsorption between CO₂ and CH₄ in dry reforming of methaneThe catalysts are pretreated under different condition of adsorption（CO₂ or CH₄）and then apply to DRM reaction to understand the effect of the adsorption relationship between CO₂ and CH₄ on catalytic performance for DRM reaction.The existence of competition between adsorption CO₂ and CH₄.The ability of adsorption of CO₂ at low temperature is higher than that of CH₄ over Ni/ZrO₂ catalyst.The temperature of the initiation of the DRM reaction would be influenced by the competition between CO₂ and CH₄,because the methane decomposition was the key role in the initiation of reaction.The active sites would be covered by CO₂,which would restrain initiating the DRM reaction.Thus,after adsorption of CO₂,the initiation of the DRM reaction increased from 377℃to 400℃compared with after the adsorption of CH₄.Besides,the relationship of cooperation between CO₂ and CH₄ is found on Ni/ZrO₂ for dry reforming of methane.After the reaction initiation,the presence of CO₂ might promote the decomposition of methane,also,the existent of CH₄ would improve the activation of CO₂.4.The influence of NiO-ZrO₂ solid solution on catalytic performance for dry reforming of methaneThe nanoscale nickel species are dispersed into the ZrO₂ support by controlling the content of nickel species to obtain the NiO-ZrO₂ solid solution.10Ni-Zrcatalyst prepared by the one-step method exhibited higher activity,long-run stability,and selectivity of H₂ compared to 10Ni/ZrO₂catalyst prepared by traditional method.At 750℃,the methane and CO₂ conversion,and the H₂/CO were 96.7%,96%and 0.95,respectively.The NiO–ZrO₂ solid solution promoted the formation of smaller Ni crystallite size,increased specific surface area and enhanced weak and medium-strength basic sites over reduced 10Ni-Zrcatalysts.A higher amount of adsorbed oxygen species on used 10Ni-Zrcatalyst improved the ability of carbon removal and further enhanced the stability of catalyst for dry reforming of methane.5.The influence of the promoter（Al and Mn）over Ni-Zrcatalyst on catalytic performance for dry reforming of methane.The utilization of different promoters（Al and Mn）modified the Ni-Zrcatalysts and the applied for dry reforming of methane to understand the effect of the promoter on Ni-Zrcatalyst.The Al-modified Ni-Zrcatalyst exhibited higher selectivity,activity and stability for DRM reaction.Because the Al promoter could modify the structure of ZrO₂,leading to an increase of its pore volume and pore diameter.Moreover,in the presence of aluminum can improve the interaction between nickel and support,thereby enhancing the ability for the sintering resistance and high suppression to loss of nickel.Mn promoter catalyst might promote the redox of nickel species during the reaction,and further promote the redistribution of nickel species.Meanwhile,this redistribution may contribute to forming a smaller nickel particle,thus,the catalytic activity increased at first 300 min.Both Al and Mn promoters can enhance the stability of ZrO₂ and further improve the stability of the activity for dry reforming of methane.