He was promoted to department head due to his outstanding performance.

promoted 是动词 promote 的过去分词形式，在句中表示被动。promote 由前缀 pro-（表示"向前"）和词根 mote（移动）组成，本义是"向前推动"，引申为"晋升、提拔"的意思。promoted 在这里表示"被提拔"的被动状态。