Objection 1. It would seem that those who are not priests cannot excommunicate. Because excommunication is an act of the keys, as stated in Sent. iv, D, 18. But those who are not priests have not the keys. Therefore they cannot excommunicate.
Objection 2. Further, more is required for excommunication than for absolution in the tribunal of Penance. But one who is not a priest cannot absolve in the tribunal of Penance. Neither therefore can he excommunicate.
On the contrary, Archdeacons, legates and bishops-elect excommunicate, and yet sometimes they are not priests. Therefore not only priests can excommunicate.
I answer that, Priests alone are competent to dispense the sacraments wherein grace is given: wherefore they alone can loose and bind in the tribunal of Penance. On the other hand excommunication regards grace, not directly but consequently, in so far as it deprives a man of the Church's prayers, by which he is disposed for grace or preserved therein. Consequently even those who are not priests, provided they have jurisdiction in a contentious court, can excommunicate.
Reply to Objection 1. Though they have not the key of orders, they have the key of jurisdiction.
Reply to Objection 2. These two are related to one another as something exceeding and something exceeded [Cf. 1, a2; 24, 1, ad 1, and consequently one of them may be within the competency of someone while the other is not.
Source: Summa Theologica (New Advent)