Diplomat National Bank announced yesterday that it had received 10 firm offers to purchase at least $1 million in the troubled bank's capital shares.

Although Diplomat, in a brief statement, did not disclose the identities of the possible purchasers, the bank said it would begin negotiating immediately with the groups to determine which offer to accept and submit to the Comptroller of the Currency for approval.

The announcement, which followed by one day approval by the bank's shareholders of a new capital plan proposed by management, is considered a major step in a recovery plan put together by the bank's new chairman, Soo Young Oh, and its president, John Ricche.

Diplomat never has made a profit since it opened 4 1/2 years ago, but after losing $1.08 million last year the bank announced it had cut its losses for the first quarter of 1980 to just over $70,000.

Under an agreement with the Comptroller of the Currency, the bank is required to raise not less than $1 million in new capital by July 15.

The new capital plan required the approval of two-thirds of the bank's outstanding shares, and about 78 percent of the shareholders voted to authorize the new shares, the bank said. The bank said it "anticipated that at least $1 million in new capital will be available for use by the bank by July 15."