Ανάπτυξη καταλυτών κοβαλτίου για αναμόρφωση μεθανόλης

Abstract

Methanol is an attractive option for storage and transportation of chemical energy. In thiscontext, methanol steam reforming is being considered as an interesting route for hydrogenproduction, since it can take place at lower temperatures in comparison with reforming of otherorganic compounds. Copper-based catalysts are considered as reference catalysts for methanolsteam reforming because they combine high activity and selectivity towards hydrogen production.On the other hand, cobalt-based catalysts have not been studied extensively for this reaction.The present work refers to the investigation of cobalt–manganese catalysts prepared throughpyrolysis of the corresponding fumarate or gluconate salts in the reaction of methanol reforming.The corresponding catalysts were prepared either by pyrolysis of the salts under inert gas flow inthe temperature range of 500-700oC or via oxidative treatment of the salts at 500oC.Catalysts characterization was performed by in situ XRD, BET, SEM and TPR techniques.It was found that mixed cobalt-manganese fumarate salts are useful precursors leading to catalystswith different structure depending on the type of surrounding atmosphere during activation.Activation in air leads to formation of CoMn spinel oxides, while activation in inert gas(pyrolysis) leads to structures containing metallic cobalt, MnO and residual carbon. Controlledoxidation of pyrolyzed samples leads to surface oxidized materials, which get reduced at muchlower temperatures compared to spinels.Combination of in-situ XRD, H2-TPR and methanol-TPR has led to identification of thevarious stages of catalyst reduction. These are: surface reduction, spinel reduction to (Co2+,Mn2+)O and reduction of Co2+ to Co0. It was also found that catalysts produced by pyrolysis arealmost fully reduced. The specific surface area of catalysts prepared from pyrolysis of fumaricsalts was ~200 m2 g-1 regardless of pyrolysis temperature, while the specific surface area of preoxidizedcatalysts was significantly lower in then range 16-34 m2 g-1.Methanol steam reforming products were CO, CO2, H2, while CH4 was also produced withselectivity less than 4%. Increase of cobalt loading leads to increase of methanol conversion, whilepre-oxidized catalysts get activated in the presence of the reaction mixture. Pyrolyzed catalysts aremore active than pre-oxidized ones and lead to product distributions according to thermodynamicpredictions at high reaction temperatures. Addition of potassium does not influence catalyticactivity but improves CO2 selectivity. Experimental findings indicate that methanol is initiallydecomposed to CO and H2 followed by the water-gas shift reaction.CO, H2, CO2, H2O and CH3OH adsorption was studied by TPD. No significant adsorption ofCO was observed on the catalysts following exposure to CO at room temperature, and this was~ vi ~also confirmed by FTIR. CO2 adsorption, on the other hand, was significant showing both weakand strongly-bound states. Hydrogen adsorption was found to be activated leading to both weakand strongly bound species on metallic cobalt and MnO. Water adsorption on pyrolyzed catalystsat 300oC is dissociative with simultaneous hydrogen production. Methanol TPD showed moleculardesorption at low temperatures and methanol decomposition towards CO and H2 at temperatureshigher than 125oC. CO2 desorption is also noticed at temperatures higher than 225oC. CO2originates either from CO oxidation with lattice oxygen or from the Boudouard reaction.Temperature programmed surface reaction of methanol in the absence of water shows initialadsorption of methanol at 30-90oC and methanol decomposition at temperatures higher than250oC. CO2 production is also observed at 250-350oC with simultaneously CO decrease. COproduction increases again above 350oC with simultaneously CO2 decrease. For highly reducedcatalysts CO2 production is attributed to the Boudouard reaction. When water is present, methanoldecomposition is accompanied by the WGS reaction, while carbon formation is prevented and thereaction activation energy decreases by 15-20 kJ mol-1.