The Obama administration is preparing to introduce major steps to phase out production of a popular chemical coolant used in refrigerators and air conditioners, citing growing evidence that the substance is contributing to the warming of the planet. The White House will announce on Tuesday a series of voluntary commitments by some of the country’s largest chemical firms and retailers to move rapidly away from R-134a and similar compounds used in nearly every office, home and automobile in the country, according to current and former U.S. officials familiar with the effort.