It is generally accepted that the Catalytic active surface area (CASA) is closely related to the particle size of active metal only except for the presence of strong-metal-support- interaction (SMSI). However, the formation rate of methane over Ni/γ-Al2O3 in CO methanation is higher than that over Ni/κ-Al2O3 even though the former has smaller CASA than the latter. How can we explain for that? I wonder about the CO hydrogenation was also dependent on the other parameter besides CASA.
Temperature is important as is the stoichiometry of the reaction - CO can form either a linear (1 CO = 1 metal atom) or bridged (1 CO = 2 metal atoms) form and thus the concentration of active metal will dictate the likelihood of one or both of these forms. 'Sparse' metal may also be preferential in poisoning or coke formation issues.
Thank your answer so much, Dr. Alan. The reason is so sensitive that to be confirmed. May you recommend any kinds of analysis technique to verify the phenomenon? Thank Sir.
For linear/bridged CO, see: http://bfy.tw/Cgk9
For stoichiometry see (for example): C L M Joyal J B Butt Chemisorption and disproportionation of carbon monoxide on palladium/silica catalysts of differing percentage metal exposed J. Chem. Soc., Faraday Trans. 1, 1987,83, 2757-2764 DOI: 10.1039/F19878302757
See also: Catalysis: Science and Technology J R Anderson, M Boudart Springer, (2012)
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