post_class output in wrong area?

post_class() echos so that’s you’re problem.

From the codex:

If you would prefer to have the post classes returned instead of echoed, you would want to use get_post_class().

So just do this:

$the_post_classes = get_post_class( 'clearfix' );
$the_post_class_string = '';
foreach( $the_post_classes as $post_class ) {
    $the_post_class_string .= $post_class . ' ';
}

$defaults = array (
    'before' => 'article id="post-' .get_the_ID(). '"' .$the_post_class_string. 'role="main">',
    // etc...

UPDATE: Sorry. I forgot this returns an array and not a string. You just need to iterate through the results before spitting them out. Code above is updated and tested.

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