I'll be arriving at PEK at midnight on Saturday. My plane will be landing in terminal 2.

I am afraid that Bank of China will be closed at midnight. You can convert some currency at Travelex Foreign Currency Exchange Self-service Machine near exit of the International Arrival Hall at 1F; in the Customs area of the International Departure Hall at 2F; or in the Security Check area of the International Departure Hall at 2F. :)