WASHINGTON -- Obama administration officials worked Sunday to persuade reluctant private investors to buy as much as $1 trillion in troubled mortgages and related assets from banks, with government help. The talks came a day before the Treasury secretary, Timothy F. Geithner, planned to unveil the details of the administration's long-awaited plan to purchase troubled assets, meant to remove them from the balance sheets of banks and, in turn, spur banks to lend more money to consumers and companies.