If the idea of a catalyst is an agent added to a particular combination in order to speed up a reaction and a result, then I think that the events of September 11 do serve as one in enabling Changez to question his own sense of identity in America. Changez had some semblance of questioning that was percolating under the surface, ever so slightly. Consider the awkwardness he experienced in determining what to wear to meet Erica's parents for the first time and the spike of intensity he experienced when Erica's father spoke about issues of cultural identity that his Changez in a square manner. Changez could not put his finger but there was something that was fundamentally "off" about his place in America. We see this in his reaction to the towers falling, the "smile" at America "being brought to her knees." In the events of September 11 and the patriotic zeal that followed, it became clear that his perception as an "outsider" was on full display for all, including Changez, to see. September 11 was a catalyst because it served to speed up the reaction of how Changez viewed himself and how he was viewed. It was not until then that he had engaged in a serious and existential reflection of his place in the world and how he viewed himself. September 11 serves as the catalyst for the "fundamentalist" that Changez believes himself to become.