Second, perhaps the Japanese were hoping to be taken up on their request to negotiate a peace.
The peace talks had already been going on before the Potsdam declaration, with Japan still not accepting unconditional surrender.
There was no negotiating. The Potsdam declaration was an ultimatum, and when the Japanese tried to back channel through the Soviets, they were stone-walled.
The wikipedia page says that Japanese attempts to deal with the Soviet Union began June 30th, and the last date it gives for that topic is July 21st, where Tōgō says:
With regard to unconditional surrender we are unable to consent to it under any circumstances whatever. The Potsdam Declaration was issued July 26th.
EDIT: If what you're saying is that the Japanese tried to contact the Soviet Union after the declaration and were "stonewalled", of course they were. It had already been established that only unconditional surrender was acceptable, and the terms of unconditional surrender were not unreasonable. We were discussing why the Japanese would choose the less favorable of two options.
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