Are there certification requirements for an emergency descent?
There is a descent requirement, but it's perhaps not in the way you imagine as it has to do with oxygen supply. FAR 122.333 subpart K states:


  (b) Crewmembers. When operating at flight altitudes above 10,000 feet,
  the certificate holder shall supply enough oxygen to comply with
  §121.329, but not less than a two-hour supply for each flight
  crewmember on flight deck duty. The required two hours supply is that
  quantity of oxygen necessary for a constant rate of descent from the
  airplane's maximum certificated operating altitude to 10,000 feet in
  ten minutes and followed by 110 minutes at 10,000 feet. The oxygen
  required in the event of cabin pressurization failure by §121.337 may
  be included in determining the supply required for flight crewmembers
  on flight deck duty.


This specifies that a turbine aircraft must be able to descend from it's maximum certificated altitude to 10,000ft in 10 minutes or less. This means the maximum descent speed is variable to maximum certificated altitude, so if an aircraft's ceiling is 42000ft it must be able to descend at least 3200fpm, it it's 25000ft then 1500fpm. 

As for how that's done I don't think the regulations say, it's up to the manufacturers to develop aircraft that can meet the requirement.