At first I thought that you were wrong. We are constantly told that Hydrogen is pollution free. Indeed a Google still shows this until you dig a little deeper. Turns out you are correct!!
I was not able to find any figures to compare NOx between Petrol and Hydrogen.
Lots of misinformation out there.
John
Indeed - lots and lots of misinformation.
As you will have discovered, the NOx in exhaust fumes doesn't come from the fuel at all, it's a byproduct of combustion using air. The hotter the burn, the more NOx is released. Hydrogen burns very fast and very hot, and can lead to much higher NOx emission than petrol or diesel engines, but is very dependant on the lean-ness of the mix and how loaded the engine is.
Either way, if you have hydrogen as a fuel source available, and have a choice between and fuel cell and a combustion engine, you would be mad to pick combustion. Fuel cells (while not 100% efficient) are far far more efficient than a combustion engine, and have no particulate emissions at all.
In a choice between combustion and fuel cell, for efficiency and emissions - Fuel cells win.
But if you want over all efficiency from wind to wheel, a battery electric vehicle will outperform a fuel cell by a long way too.
Of course - pure efficiency is not the only game in town. Convenience also plays a role, As of today, lots of people are looking at the fuelling model of hydrogen ("just" pump it into a tank) vs charging a battery (spend 10s of minutes at a rapid charger) as a much simpler and more recognisable way of fuelling a car. But either don't realise or forget that H2 is twice as expensive as diesel, and you can only buy it in about 6 places in the whole country. Thats kind of inconvenient too...