We have a 15 hr layover in Beijing from Houston to Manila on December 30.

Well, as I know, there is no tourist bus at the airport.
Upon landing, you can take airport express to get to Dongzhimen Station. It takes around 30 mins.
Then transfer subway lines to get to main popular scenic spots in this city, such as Tiananmen Squre, Forbidden City, Wangfujing Street and Silk Road.
Or you can join one day tour of TravelChinaGuide. They can pick you up from the airport and send you back after sightseeing. If interested, the contact information is traveltravelchinaguide.com.