What are some verses that are dedicated to nirguna Brahman from the Upanishads?
Here are the few verses which prove Brahman as Nirguna from Upanishads
He is bright, formless, all-pervading, existing without and within, unborn, without prana, without mind, pure and beyond the avyakrita, which is beyond all.(Mundaka Upanishad 2.1.2)
Which is soundless, touchless, formless, undecaying, so tasteless, eternal and scentless, beginningless, endless, beyond the Mahat, and constant, knowing that, man escapes from the mouth of Death.(Katha Upanishad 1.3.15)
That which is beyond this world, is without form and devoid of suffering. Those who know this become immortal. Others are drawn to suffering only.(Svetasvatara Upanishad 3.10)
He is the unthinkable, unmanifest, of endless forms, the good, the peaceful, Immortal, the origin of the worlds, without beginning, middle, and end, the only one, all-pervading, Consciousness, and Bliss, the formless and the wonderful.(Kaivalya Upanishad 6)
This Brahman is birthless, free from sleep and dream, without name and form, ever-effulgent and omniscient. Nothing has to be done in any way with respect to Brahman. (Mandukya Karika 3.36)