Preposition + infintive

Pour generally marks the purpose, end, or object of an action. Can be translated to "in order to."

For c'est and il est, if it's a dummy subject:
impersonal expression + de + intransitive infinitive
or
impersonal expression + de + transitive infinitive + direct object

If it's a real subject:
impersonal expression + à + intransitive infinitive

http://french.about.com/od/expressions/a/impersonal.htm


the subject in the impersonal construction il est + adjective + de must be a dummy subject. If it's a real subject, you must use à instead of de.
https://www.duolingo.com/skill/fr/Verbs:-Infinitive-2

Passive infinitive follows the form: noun + à + infinitive

http://french.about.com/od/grammar/a/passiveinf.htm

These have different meanings depending on which preposition they use:

http://french.about.com/library/prepositions/bl_prep_a_vs_de2.htm