This article says that a tiny section in the U.S. code allows the Treasury secretary to "mint and issue platinum bullion coins and proof platinum coins" of a size and denomination of "the secretary's discretion." This trillion dollar coin could then be used to pay America's debts. The idea is being floated by A Democratic congressman, a Nobel-winning economist and several prominent writers. They're absolutely serious.
I don't think China would accept a trillion dollar coin as payment.