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Can water be converted into hydrogen and oxygen using sunlight and a specific catalyst? I can imagine a electrolysis reaction using a catalyst that uses light to release electrons. The goal is to find a process which uses only sunlight as positive source of energy.

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Probably several of them. The one I found at the top with a search for "photocatalytic water decomposition" is Ref. [1]:

The photocatalytic activity for water decomposition of alkaline earth metal and alkaline metal antimonates, $\ce{M2Sb2O7}$ (M=Ca, Sr), $\ce{CaSb2O6}$ and $\ce{NaSbO3}$, was investigated. These antimonates were photocatalytically active when combined with $\ce{RuO2}$: both $\ce{H2}$ and $\ce{O2}$ were produced from the initial stage of reaction under UV irradiation, and the photocatalytic activity became stable as the reaction proceeded. The photocatalytic properties are discussed based on the distorted structures of $\ce{SbO6}$ octahedra.

Reference

  1. J. Sato, N. Saito, H. Nishiyama, Y. Inoue (2002). "Photocatalytic water decomposition by RuO2-loaded antimonates, M2Sb2O7 (M=Ca, Sr), CaSb2O6 and NaSbO3, with d10 configuration", Journal of Photochemistry and Photobiology A: Chemistry, 148, 1–3, 85-89, ISSN 1010-6030, https://doi.org/10.1016/S1010-6030(02)00076-X.
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    $\begingroup$ Here you can find a second example of photocatalytic decomposition of water: Ruinan Zhang, Song Liu, Hongyan Yuan, Dan Xiao, and Martin M. F. Cho (2012) "Nanosized TiO2 for Photocatalytic Water Splitting Studied by Oxygen Sensor and Data Logger", Journal of Chemical Education,89 , 1319-1322. doi.org/10.1021/ed1009283 $\endgroup$
    – PAEP
    Oct 27, 2022 at 13:37
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For the following reaction:

$$\tag{1} \ce{H2O ->[$h\nu$] 2H + O} $$

you could shine light on the water with energy equal to the atomization energy of water: 917.8 kJ/mol (130.34 nm, or ultraviolet light). This is called "photodissociation" (among other names for it) and when dealing with a molecule that contains more than two atoms, we may call it "photoatomization".

For the following reaction:

$$\tag{1} \ce{H2O ->[$h\nu$] H2 + O} $$

you can shine light on the water with energy equal to the difference between the energy of the water molecule and the sum of the energies of the $\ce{H2}$ and $\ce{O}$ species. Similar reactions can be found in our atmosphere and in space.

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  • $\begingroup$ However, if powered by sunlight this is not so useful for producing hydrogen and oxygen at the surface of the Earth. $\endgroup$
    – D Duck
    Oct 28, 2022 at 17:10
  • $\begingroup$ @DDuck The title asked if it was possible with "light" (not just sunlight) and this answer immediately jumped at my mind. Some of the similar reactions mentioned in the link I provided, are in fact powered by sunlight in the atmosphere, but I wouldn't say this reaction involving H2O happens as often as the ones described in that link. $\endgroup$ Oct 28, 2022 at 17:14
  • $\begingroup$ Interesting that 130 nm (short UV) may be considered the "uncatalyzed" energy requirement. $\endgroup$ Oct 28, 2022 at 19:55
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You could indirectly use sunlight to generate electricity with a common solar panel. It only takes around 1.23 volts to split water into H2 at the cathode and O2 at the anode. The gasses can then be collected in separate tubes for storage and use.

Interestingly, solar panels do not require higher energy UV radiation to generate electricity, only needing inexpensive and naturally available sunlight. This would be an important consideration for large scale production.

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    $\begingroup$ technically this doesn't just use light and a catalyst. $\endgroup$ Oct 27, 2022 at 16:30
  • $\begingroup$ @SafdarFaisal: …in a sufficiently broad sense, the solar panel and the electrolysis equipment could be considered catalysts. After all, they do facilitate the reaction but are not consumed in it. $\endgroup$ Oct 28, 2022 at 0:13
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    $\begingroup$ @IlmariKaronen Well, if you're going that far, you might as well broaden it only slightly further and call a synthetic chemist a catalyst for all sorts of reactions... which is a notion that I rather like! $\endgroup$ Oct 28, 2022 at 10:41

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