|Abstract:||The purpose of this master thesis was to investigate the effect of ceria dioxide (CeO2) morphology and palladium (Pd) loading on the catalytic activity of Pd/CeO2 catalysts. For this purpose, we synthesized CeO2 of different shapes (rods, cubes, polyedra, two different nanoparticles) containing different loadings of Pd (0,5, 2 and 5 wt. %). As-synthesised catalysts have been analyzed by various instrumental techniques, such as XRD, SEM with EDX, BET, pyridine-TPD, CHNS and TGA. The activity tests were carried out in a fixed-bed reactor system coupled to a gas chromatograph for online analysis.
SEM images have shown a well-defined structure of CeO2 particles, on the basis of which we can conclude that the chosen synthesis methods were adequate. We have also measured changes in the physical-chemical properties of the Pd-containing catalyst samples and found an interaction of Pd with CeO2. From the activity tests, we further found that the morphology of CeO2 has a strong effect on the activity of the investigated catalysts. The morphology also affects the degree of coking, which reduces the catalytic activity as a function of time on stream.
We found that the most active catalyst is 5Pd/CeO2-P (a CeO2 catalyst with morphology of polyethra with a 5 % loading of Pd), which gave the methanol yield (STY) of 0,090 g/(gcath) at a temperature of 260 °C and pressure of 50 bar, and has a methanol selectivity of 24,9 % with only 1 wt. % of coking. If we calculate the STY per mass of Pd, we can find that the most active catalyst is again 5Pd/CeO2-P with the STY of 2,373 g/(gPdh).
We further found that the percentage of Pd loading affects the catalyst activity, which increases with increasing loading up to 2 % of Pd and the effect then decreases at loadings above 2 %, most probably due to the agglomeration of Pd ensembles. A catalyst stability test was further conducted, revealing a 37 % loss of activity over the period of 50 hours of time on stream with a trend for a constant STY after 40 hours, which indicates the possibility of long-term catalyst use.|