Can you say "The NOₓ are"?
Wikipedia says
[...] Within the SCR catalyst, the NOxare catalytically reduced by the ammonia [...]
NO2 is nitrogen dioxide, which is singular. x can take any one of a range of values, so NOx means
nitrous oxide or nitrogen dioxide or dinitrogen pentoxide or other alternatives
and so is, logically, singular. If we regard NOx as a chemical of variable composition then this makes sense. x cannot logically have different values at once, but I think NOx is being used in this sentence to mean
nitrous oxide and nitrogen dioxide and dinitrogen pentoxide and other alternatives, all mixed together
It is possible the grammar is also influenced by NOx sounding plural as it is pronounced "knocks".
So can we say "The NOx are" or not?
Solution 1:
Yes, but only if you are referring to Nitrogen Dioxide as a category. If you are referring specifically to the gas, the answer is no.
For example, "All types of NOx are converted in the reaction" - valid because it is referring to the types of NOx you have, which is multiple.
"All of my NOx is converted in the reaction" is referring to NOx as a mass noun, so there is no plural in that sense.
Solution 2:
Yes, I figure you can. It just means the different nitrogen oxides. I was surprised to read that it also includes Nitrous oxide, considering the 2 Nitrogen atoms, but in any case 'oxides' would take a plural form.
Wikipedia says the following:
In atmospheric chemistry, NOx is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide (NO) and nitrogen dioxide (NO2).[1][2] These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.
NOx gases are usually produced from the reaction among nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as occur in car engines.[1][2][3] In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. NOx gases are also produced naturally by lightning.
The term NOx is chemistry shorthand for molecules containing one nitrogen and one or more oxygen atom. It is generally meant to include nitrous oxide (N2O),[1] although nitrous oxide is a fairly inert oxide of nitrogen that has many uses as an oxidizer for rockets and car engines, an anesthetic, and a propellant for aerosol sprays and whipped cream. Nitrous oxide plays hardly any role in air pollution, although it may have a significant impact on the ozone layer,[4] and is a significant greenhouse gas.
In the same article NOx is used both in plural and in singular forms depending on whether it refers to various nitrogen oxides or to singular nitrogen oxide, e.g.
Ott noted that the lightning-produced NOx is typically found at altitudes greater than 5 km, while combustion and biogenic (soil) NOx are typically found near the sources at near surface elevation (where it can cause the most significant health effects).
Solution 3:
NOx is an abbreviation for what is referred to as "nitrogen oxides" in environmental regulations (e.g., 40 CFR 51.165(a)(1)(v)(E), 42 USC 7403(c)(3)(A)), so you can see why the Wikipedia entry chose this formulation. However, it's awkward. In actual usage in the environmental profession, people just use NOx like a mass noun (no article, singular agreement).