What noble metal is most resistant to oxidation by diatomic oxygen in air at room and elevated temperatures?

Question

To clarify, by noble metals in this case I mean all platinum metals + gold, no copper, silver or rhenium. These noble metals are described as having low reactivity, but which one is least reactive or most "noble" of them all?

I am chemistry noob and I have feeling this might greatly depend on what type of element/molecule is trying to react with given noble metal so to make this simple I will ask only about the element most resistant to *oxidation by diatomic oxygen in air at room and elevated temperatures.

If we had all these noble metals side by side in identical shape, let's say cube with highly polished sides, and we heated them in air atmosphere to 100, 500, and 1000 degrees Celsius, which one would oxidise least?

Background and current understanding:

Here is my current knowledge and thoughts on why some noble metals might be more noble than other.

This is just window into my head so you can see what I think so you are able to correct what is wrong, approve what is right and expand on what is missing.

My current understanding of what makes noble metal noble, is that it's somewhere in middle of periodic table, their electron shells are half full. Elements on the left side are strongly reactive because their shells are only sparsely filled, so they give away electrons easily, they have low ionisation energy.

Elements on the right side, excluding noble gasses, are strongly reactive because they have almost full electron shell and they have tendency to tear out and "steal" electrons from less electronegative elements. That is why noble metals are in middle, they don't want to give away or get inside electrons very much.

That would be my first trait of noble metal, the more in middle it is, the more noble it is. Since periodic table is 18 numbers wide, 18 is even number so there is no integer exactly in middle, the middle is between 9 and 10.

That means elements in group 9 and 10 should be more noble than those in 8 and 11 group because they are closer to middle, so rhodium, palladium, iridium, and platinum should be more noble than gold, ruthenium, and osmium.

Another thing that I believe is that the heavier elements don't hold their electrons as strongly as lighter elements because the inner electron shells shield the valence electron shell from attractive force of nucleus. If this is true, then noble metals in period 5 should be more noble than those in period 6, that is ruthenium, rhodium, and palladium more noble than osmium, iridium, platinum, and gold.

Third factor in my opinion is first ionisation energy, the higher it is, the more it should be resistant to reacting with oxygen. From highest to lowest first ionisation energy: Gold = 9.2255 eV, iridium = 8.967 eV, platinum = 8.9587 eV, osmium = 8.4382 eV, palladium = 8.3369 eV, rhodium = 7.4589 eV, ruthenium = 7.3605 eV. If this is true, gold should be most noble, and ruthenium least noble.

I read one study about oxidation resistance at high temperatures, the study said rhodium was most resistant, even more than iridium or platinum. I also heard Platinum doesn't oxidise in air at any temperature bellow melting point, that seems contradictory to the paper I read.Furthermore, spark plugs are mode from iridium and platinum but not rhodium.

Answer 1

Ruthenium, Rhodium, Palladium, Osmium, Iridium, Platinum, Gold

... I will narrow this question down to oxidation by diatomic oxygen in air at room and elevated temperatures.

According to this criterion, gold is the most oxidation resistant of the above metals.

First of all, gold oxide cannot be obtained by reaction of gold and oxygen ($\ce{O2}$) at standard pressure. Gold oxide ($\ce{Au2O3}$) decomposes to gold and oxygen above $\pu{160°C}$.

• Ruthenium reacts with oxygen (air) above $\pu{700°C}$ to give $\ce{RuO4}$. Ruthenium powder burns in air.
• Rhodium reacts with oxygen (air) above $\pu{700°C}$ to give $\ce{Rh2O3}$.
• Palladium powder burns in air.
• Osmium powder reacts with oxygen (air) at room temperature to give $\ce{OsO4}$.
• Iridium reacts with oxygen at high temperatures to give $\ce{IrO2}$ or $\ce{IrO3}$. Iridium powder burns in air.
• Platinum powder burns in air.

Answer 2

Using your comment clarifying that the conditions include high temps of 100, 500, and 1000 degrees celsius, this is my answer:

At high temperatures, the noble metal that is most resistant to oxidation by diatomic oxygen in air is ruthenium. It can resist oxidation at temperatures up to 1000°C in air.

Ruthenium is a noble metal that has a very high resistance to oxidation, due to its stability and chemical properties. It has a high melting point of around 3900°C and low coefficient of thermal expansion which makes it to withstand high temperatures without showing degradation. Additionally, ruthenium has a relatively high level of resistance to chemical reactions and corrosion. These properties make ruthenium particularly useful in high-temperature applications, such as aerospace, electronic and chemical industries.

Other noble metals such as platinum, palladium, rhodium can withstand lower temperatures compared to ruthenium, for instance, platinum and palladium resist oxidation up to about 900°C, while rhodium can resist up to 800°C.

It's important to note that the oxidation resistance of a metal at high temperature is also affected by the specific conditions of the environment, such as the presence of other gases and the oxygen concentration.