Actually it is not his personal piano but a Steinway at the Berlin Philharmonic hall that he favors. The proceeds will go to UNICEF/East Africa.
Here is the press release
No artist played on this piano more often than Alfred Brendel: Whenever he appeared in the Philharmonie in Berlin, he always performed on the Steinway D-524780. But other pianists such as Pierre-Laurent Aimard, Daniel Barenboim and Martha Argerich also used this instrument. And with good reason, as the piano is characterized by superior beauty of tone, unique playability and rapid touch reponse. The playing mechanism and acoustic system are in excellent condition.
Since the Berliner Philharmoniker Foundation purchased three new concert grand pianos in the last two years, the idea arose – together with Alfred Brendel – of auctioning off the instrument in aid of UNICEF. The auction will take place as part of the open day on Whit Monday, 28th May 2012 from 6 pm in the concert hall of the Philharmonie in the presence of Alfred Brendel, Sir Simon Rattle and the Berliner Philharmoniker. The auction is going to be carried out free of commission by Andreas Rumbler, managing director of Christie's Germany. The proceeds from the auction will benefit UNICEF’s emergency relief for children in East Africa.
More details here:
http://www.berliner-philharmoniker.de/en/unicef/alfred-brendel-piano/
