What might serve as an initial starting photocatalyst for this large water-splitting solar simulator?
Vitamin B-12 is a photocatalyst, see, for example, https://pubs.rsc.org/en/content/articlelanding/2018/ra/c7ra13037f#!divAbstract and also https://www.ncbi.nlm.nih.gov/pubmed/29271445 , but it has not been applied to water splitting!

What has been used is TiO2, see this source to split H2O into H2 and O2.

Mechanics: The action of light on TiO2 creates electrons (e-) and electron holes (h+). Some reactions:

$\ce{H+ + e- -&gt; .H}$

$\ce{.H + .H -&gt; H2}$

$\ce{OH- + h+ -&gt; .HO}$

$\ce{.HO -&gt; H+ + .O-}$

$\ce{.O- + h+ -&gt; .O}$

$\ce{.O + .O -&gt; O2}$

One of the less desirable reaction is:

$\ce{.H + .HO -&gt; H2O}$