A secondary effect of hydrogen sulfide on the HDN reaction was found to come from a non-catalytic acid-base reaction between basic saturated nitrogen compounds and hydrogen sulfide (or sulfur). This side-effect may strongly bias the results of catalytic tests concerning HDN reactions.
Yang, M., Grange, P., & Delmon, B. (1997). Secondary effect of hydrogen sulfide on HDN reaction. Applied Catalysis A: General, 154(1-2), L7-L15. https://doi.org/10.1016/S0926-860X(96)00412-7 (Original work published 1997)