Is diatribe a predicate

No, it's a noun. A "predicate" is just another name for a verb, an action Word like run, jump, sit, stand, dance, etc. But the Word "diatribe" refers to a bitter or angry verbal attack, like you might hear on some talk radio stations. This Word carries a meaning of an emotional and outraged opinion. For example: On his talk show today, Rush Limbaugh got into a diatribe about why he dislikes President Obama.