Today, the Hereditary Grand Duke accompanied Luxembourg's Foreign Affairs minister Jean Asselborn to a meeting of the Foreign Affairs council at the EU Headquarters in Brussels. During the meeting it was decided that the impose a formal oil embargo and to freeze the Iranian central bank's EU accounts in order to force negotiations over the Iranian nuclear program. Other matters on the agenda included the current situations in Syria and Egypt.
Photograpic evidence and further information can be found over at Tageblatt, Wort, Belga and Daylife.
For further information on Guillaume's stay in Brussels, have a look here.