You ask – we answer
During our webinars we recieve a lot of good questions. We have summarized some of them below and hope you find your answer! The answers are provided by professor and presenter of the webinars – Christian Hulteberg.
Q: Is there a possibility that the porosity will decrease after coating? And what are the tricks to avoid that?
A: Porosity almost always decrease with impregnation. Drying temperature is a strongly influencing factor, the higher the temperature the more the decrease. On fast drying material it is accumulated at the pore mouths and can block the pore below.
Q: Are there more effects of catalyst morphology during the tableting process? And how can that influence the selectivity?
A: During tableting the material is strongly compressed and will during this lower in macro porosity (the space between the powder particles) and this introduces more severe mass transfer and can influence selectivity depending on the reactions occurring, but there is no general rule.
Q: What are the differences between an adsorption site and an active site?
A: All reaction sites are adsorption sites, but not vice versa. For getting a catalytic effect, you have to have adsorption (interaction with the surface) but if the adsorption is to strong the site will not be freed for reaction again. Likewise, if the adsorption is to weak, there is not enough change to the adsorbing molecule to alter it to promote reaction.
Q: Are there any general rules that should be obeyed when combining active phases i.e. transition metals onto a support?
A: There are no general rules that I know of, the normal modus operandi is to do intelligent guesswork with support from literature and then evaluate your experimental work and iterate towards a solution.
Q: What really contributes to the catalyst deactivation?
A: The catalyst is deactivated by three main mechanisms, poisoning, sintering and fouling. Which of these mechanisms dominate depend on what reaction is considered, which conditions are used and what catalyst we employ.
Q: What’s the difference between incipient wetness and wet impregnation?
A: During incipient wetness impregnation, the same amount of impregnating liquid as the catalyst pore volume is used. After impregnation, the catalyst pore system is filled with the liquid, but the catalyst is still dry to the touch. In wet impregnation, a large surplus of liquid is used, and the active phase pre-cursors are interacting with the surface through adsorption.
Q: Generally, in what cases a bulk catalyst would be preferred over a supported catalyst?
A: Bulk catalysts are preferred when we catalyse reactions that are limited by the surface reaction (and not mass transfer), e.g. equilibrium limited reactions, and want as much catalyst mass in the reactor as possible. Also, it is preferred when the catalyst material is not to costly, it would not be used for noble metals as part of the active phase will be entrapped in the material.
Did you miss our webinars on catalysis? Don’t worry – you can still watch them on our channel