Preparation Of Co-based/Tungsten Carbide-Activated Carbon Catalyst And Its Catalytic Performance For CH₄-CO₂ Reforming

Dry methane reforming(DRM)is the process of converting two major greenhouse gases,methane and carbon dioxide,into syngas with promising industrial value.Syngas is an important feedstock for the synthesis of higher-value liquid-phase products.Thus,carbon dioxide reforming of methane is of great significance to contribute to climate change mitigation and match the target set for carbon neutrality.Unfortunately,the commercial implementation of low-cost non-precious metal catalysts used in this reaction is restricted due to coke-and sintering-induced instability.Since tungsten carbide has a Fermi level similar to that of noble metals,it has performed similarly with the noble metals(like Pt)in various catalytic reactions.In this thesis,tungsten carbide-activated carbon(WC-AC)composite support was prepared by in-situ carburization of ammonium metatungstate on the activated carbon matrix.Tungsten carbide nanoparticles are inlaid or anchored into the carbon support.Based on the precious metal-like properties of tungsten carbide,a non-noble metal Co-based/WC-AC catalyst for DRM reaction was built.Combined with mathematical model analysis and various characterization methods,the effects of the coupling effect of WC-AC composite support and the active metal Co on the catalytic performance were systematically studied in terms of component content,WC-AC preparation conditions and Ce promoter addition.The specific conclusions are as follows:(1)The variance analysis of the quadratic term model obtained by the multi-response surface methodology showed that W content had an antagonistic effect on the catalyst activity,while Co and Ce contents had a significant synergistic effect on the catalytic performance;the interaction of Co,W and Ce contents imposed a significant effect on the CO2 conversion and CO yield,but not on the CH4 conversion and H2 yield.The optimal loading of Co,W and Ce on AC matrix was 10.6 wt.%,6.5 wt.%and 8.6 wt.%,respectively.(2)The C atoms in the AC matrix can be inserted into the W lattice to form WC-AC composite supports with the coexistence of WC and W2C phases at the carburizing temperature of 950℃.The XRD quantitative analysis showed that the composition of W2C and WC in the WC-AC composite supports can be tailored by changing the ratio of CH4 and H2 in the carburizing atmosphere.The textural properties of WC-AC were severely affected by the carburizing atmosphere containing CH4,while the specific surface area and pore structure of WC-AC prepared under the carburizing atmosphere of pure H2 were closest to those of AC.(3)Compared with AC support,the WC-AC support boosted the formation of Co2+species and enhanced metal-support interactions,resulting in a higher TOF value for Co/WC-AC,which promoted the CH4 cracking.The strong coupling between WC-AC and Co led to a small amount of carbon deposition,resulting in the slow deactivation of Co/WC-AC.However,Co/AC was rapidly deactivated in a short time.(4)The stability of the catalyst was further improved by adding Ce promoter to Co/WC-AC.In the 1Co/1Ce catalyst,the Ce promoter was beneficial to improve the dispersion of Co species,enhance the metal-support interaction,and improve the oxidation resistance of Co species,leading to the lowest CH4 initial activation temperature in 1Co/1Ce catalyst.This confirms that the addition of Ce facilitates the CH4 cracking.In the meantime,XPS analysis showed that the addition of Ce increased the chemisorbed oxygen species on the catalyst surface.This favors the decrease of carbon deposition,thus improving the coking-resistance.(5)In the Co-Ce/WC-AC catalysts prepared at the W loading of 6.5 wt.%and 150 mL/min H2,the WC on the catalyst surface accounted for the largest proportion of tungsten species.WC facilitated the exposure of metal Co sites,promoted Co3+→Co2+,and enhanced metal-support interactions,resulting in a striking CH4 conversion(～90%).These indicate that the coupling effect of WC and Co can promote the CH4 cracking.Meanwhile,WC promoted Ce4+→Ce3+,thereby greatly increasing the oxygen vacancies on the catalyst surface and further promoting the adsorption and activation of CO2.This was conducive to the enrichment of chemisorbed oxygen species,which in turn significantly improved the coking-resistance.This resulted in stable operation of the catalyst for more than 50 h.