If you are looking for the equation of the surface generated by the rotation of a catenary around its axis of symmetry (taken as $z$ axis), then it is simply: $$ z=a\cosh\left({\sqrt{x^2+y^2}\over a}\right). $$
If you are looking for the equation of the surface generated by the rotation of a catenary around its axis of symmetry (taken as $z$ axis), then it is simply: $$ z=a\cosh\left({\sqrt{x^2+y^2}\over a}\right). $$