Hi there, can I use a Visa credit card to pay for airport express to downtown Beijing?

I am afraid not. You are advised to get some cash for your convenience. By the way, there are many currency exchange points inside the airport, like Travelex in Domestic Arrival Hall, F1, T1; ICE in International Arrival Luggage Hall, F2, T2; Hang Seng Bank in the restricted area of International Departure Hall, F2, T3-E (opposite to Boarding Gate No. E20), etc. Good luck to you! :)