Is it permissible for a wife to pay the expiation for sexual intercourse during Ramadan on behalf of her husband without his knowledge, fearing that he would refuse to pay it and thus fall into sin?

What we understand from the question is that your husband owes expiation for sexual intercourse during the daytime in Ramadan (definition), and he believes it is not obligatory for him due to his ignorance of it at the time of the act. This is incorrect; it is indeed obligatory upon him. The expiation is not valid unless performed in sequence, and your payment of it on his behalf is not valid while he is capable of fasting, nor is it valid without his permission, as it is an act of worship that requires intention. Your duty is to persuade him gently, kindly, and wisely of its obligation upon him.