Is Charles Peirce an existentialist

Peirce is a partial-existentialist, which is to say that existence is only 'secondness' in his metaphysics. Existence is involved in reality, which is thirdness (there are no categories higher: see, the Reduction Thesis). Firstness, the remaining category, is that of feeling, possibility. So, in a genuine triadic relationship involving all Three Universes of Experience, existence is but brute action-reaction, so that reality cannot be reduced to it. Materialism, reductionism, etc. are denied by his philosophy. Peirce holds an evolutionary view of cosmology.