How Does Silica Gel or Alumina Accelerate HX Addition to Alkenes in CH2Cl2?

[person_random_normal]

Adding silica gel or alumina to the mixture of the alkene and HCl or HBr in CH2Cl2 increases the rate of addition dramatically
but WHY ?
and why is CH2Cl2 specifically used ?

 

[Bystander]

Google "heterogeneous catalysis," and "surface reactions." https://www.google.com/?gws_rd=ssl#q="heterogeneous+catalysis"