A basic wall surface reaction question !!
 

Hello everyone,
I am having problems understanding how to setup a surface wall reaction.
My geometry consists of a heated tube where jet fuel(C12H23) flows. the tube's walls are at a fixed temperature value of 540K.
I am to study the decomposition of the C12H23 so I can determine the deposition rate of carbon at the wall surface.
Taking into account this reaction:

C12H23 + 5.75O2 --» 12C + 11.5H2O

Should I add a volumetric reaction in my model ? In the species transport model, I cannot select surface reaction without volumetric. I went on and created the above reaction as wall reaction, but I don't know if I should add carbon as site species or solid species. I don't understand the difference between this two. I could not find the species carbon under "solid" materials, but I could find it under "fluid" materials.

Thanks for your help..