The Earth also includes vast quantities of reduced metals like iron, more than enough to react with all the oxygen in the atmosphere. Perhaps some way could be found to exploit that, at least a little bit.
What's weird is that, as far as I know, there's no feedback mechanism that's been identified that keeps the atmosphere's O2 level stable. It may have been stable since the Cambrian just because if it hadn't been, we wouldn't have evolved, an anthropic argument.
And we don't know AFAIK that it's been entirely stable. There's some debate over what the level was in the Cretaceous for example.
https://en.wikipedia.org/wiki/Redox
Here, the metal is the reducing agent. If it were somehow reduced in redox reactions, there not much chance of it being oxidized as that would make the metal an oxidizing agent that wants electrons, not a reducing agent that gives electrons.
That said, these things have already been oxidized (not reduced) and thus there is no chance to have them consume oxygen. You need the pre-oxidized material in order to be able to consume oxygen.
Finally, you failed to answer my question regarding the relevance of these metals to atmospheric oxygen. They should be inert having been oxidized long ago. That is why rocks are full of oxides, such as silicon dioxide and aluminum oxide, despite being composed of metal.
Iron in the Earth is mostly in a reduced state (either Fe(+2) or even elemental iron). Upon exposure to the atmosphere it is oxidized to Fe(+3), changing from a more reduced to a more oxidized state (and similarly for other things in reduced states, such as manganese and sulfur and organics).
It's very common in weathered rock to see the red color of ferric iron.
Ferrous iron occurs in common minerals like pyroxene, hornblende, pyrite, and olivine. Olivine is the most common mineral in the upper mantle.
The fully oxidized iron, i.e. ferric Fe(III) oxide, i.e. rust, forms a negligible fraction of the iron in the Earth, being restricted almost entirely to the upper crust of the Earth. A small amount of ferric Fe(III) oxide (which forms magnetite with the remainder of ferrous oxide) is formed at great depths by the reaction with water, where this is present in the rocks, which releases some free dihydrogen, which may remain trapped in the rocks and which is the source of the hydrogen discussed in this article.
Here, on the surface of the Earth, almost all iron is oxidized as a consequence of being exposed to the air, but when iron comes from the deeper regions of the Earth, through volcanic eruptions, it is reduced and it begins to oxidize after being exposed to the air. So the volcanic rocks with iron are unstable in the air, they transform slowly into rocks with oxidized iron, in the same way as the artificial objects made of metallic iron, which rust.
In general, saying just "reduced" and "oxidized", is ambiguous for many chemical elements, because, like iron, they can have several degrees of reduction or oxidation. Most frequently, using "reduced" and "oxidized" without any other qualifier is taken to mean unstable or stable in the presence of an oxidizer. Some oxidizers are stronger than others, so a given degree of reduction/oxidation may be stable or unstable depending on the oxidizer that is used. However, without other mentions, it is understood that the implicit oxidizer is air. In the presence of air, metallic iron and Fe(II) are reduced, while Fe(III) and Fe(VI) are oxidized. Fe(VI) can be created only by a stronger oxidizer than air, but once created it can no longer be oxidized by air, like the Fe(III) of rust, so both are oxidized forms of iron that are stable in air.
In the presence of air, oxidized substances, like silicates and the other abundant components of stones and soil, are the substances that are non-reactive and stable.
For many billions of years, even long before the evolution of the kind of phototrophy (a.k.a. photosynthesis) that produces free dioxygen by oxidizing water, the living beings had to continuously produce reduced (and reactive) forms of carbon, nitrogen and sulfur from the oxidized (and non-reactive) forms of carbon, nitrogen and sulfur from the environment.
These biological reduction processes have also used solar energy a very long time before the evolution of the variant that produces free oxygen, and before that they have used free dihydrogen, which is produced naturally by the reaction between the partially reduced iron, Fe(II), from volcanic rocks, with water, which oxidizes it to Fe(III), releasing reduced free dihydrogen as a consequence of the reaction. Here the origin of the energy that powers this process is the internal heat of the Earth, because at the higher internal temperatures the substances that are in chemical equilibrium are not the same that are in chemical equilibrium at low temperatures. So when surface rocks are created by volcanism, they are not in chemical equilibrium and the reaction between their reduced components with water can produce the energy that has fed the first forms of life until they have evolved the means for capturing solar energy.
Probably the most important development in human technology has been the discovery of how to transform the non-reactive oxidized forms of metals from the environment into reduced forms of metals, which are reactive, therefore they are easily corroded, but they are very useful materials. For many millennia, the reduced metals have been produced with the help of another reduced substance, i.e. charcoal, whose ultimate origin is in the reducing processes by which living beings produce reduced carbon from the oxidized carbon dioxide.