Capitals in Dutch last names that include "de" or "van" or "van der" are handled differently depending on the presence or absence of the first name. You write "This seems to have been written by Jan de Leeuw" but "I can't believe this was written by De Leeuw". Thus references should be to "De Leeuw (1945)", although the bibliography could contain "Jan de Leeuw: He Wrote This, Horticultural News, Volume 611, 1945" or "J. de Leeuw, Item 1, Provo, Volume 7, 1968".