I would like to synthesis Fe-based catalyst by coprecipitation. K and Mo may be added to the solution but I don´t know what oxides I´m going to obtain.
In the precipation reaction an aqueous solution of NaOH are added dropwise up to reach pH 8, the solution is aged for 1 hour and the temperature of the whole process is 70ºC.
Almost FeNO3 is converted into Fe2O3 but I´m not sure about what happens with KNO3. The K phase is important to calculate the metal atomic ratio of the final catalyst.
Yes, of course. The sample is washed 3 or 4 times before the drying and calcination.
First: Control the pH during co-precipitation by conrolling the flow rate of the precipitating agent. Second : After precipitation, aging..wash your precipitate by adding the cake to 1L of distilled water...Then stir gently at R.T. for certain time and filter the cake again...Repeat the step 5 times. Do ICP-Ms to check the absence of cations of the precipitating agent. (5 times wash would be super!!)....Calcination of the dried cake should be controlled 1oC/min. up to 450oC
KNO3 should not precipitate under those conditions, but some K is adsorbed/ocluded during Fe2O3 precipitation. The solution to minimise K impurities is washing the precipitate repeteadly with distilled water until reaching a constant pH of the residual water.
I think I didn´t explain the process correctly. I want obtain a Fe-based catalyst including K, so KNO3 may be added to the solution. The atomic ratio of the catalist would be 10:1 (Fe:K) but I´m not sure about what potassium oxide could be formed in the coprecipitation. I suspect that KOH is formed (in the reaction or calcination).
The sampled will be washed time and again to remove any impurities.
My suggestions is to coprecipitate Fe(OH)3.xH2O first. Then impregnate your material with KNO3 prior to calcination. The advantage of this method is : You control the amoubt of K2O obatined after calcination, the evolution of nitrate during the calcination creates extra pores in the material. The drawback of this method is: K2O will distribute heterogeneously in the matrix and not homogeneously.
Second choice: Mix KNO3 with Fe(NO3)2.xH2O from the begining then start copreciptation using NH4OH. Then wash the precipitate gently and dry the cake at 100 oC over night before calcination at 450oC. Do ICP-Ms and XPS , XRD-EDX to know the exact % of K+ and the distribution in the surface/bulk of the material
Thank you so much for your suggestions. I´ll synthesize the catalyst by two ways (coprecip. and impregnation of dopants) and then they will be compared by caracterization techniques.
Are there any differences between using NaOH or NH4OH as precipitation agents? Different papers suggest many precipitation agents, but there are no explanations about the differences.
There is difference between them due to the Na+ cation is different NH4+ ion. Na+ ions need washing carefully to get rid of excess Na+ ions.
I´m affraid that you could not reach the 10:1 ratio by coprecipitation due to the extreme solubility of KOH in water...
KOH is extremely soluble in water, but in this reference, 10:1 of Fe:K is reached by coprecipitation.
Kumabe K,Sato T, Matsumoto K, Ishida Y, Hasegawa T. "Production of hydrocarbon in FTS with Fe-based catalyst:Investigations of primary kerosene yield and carbon mass balance". Fuel 89 (2010);2088-2095
Assuming that, the answer you wanted is easy: follow the procedure of the cited reference...
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