Why do some transitive verbs take に instead of を?


> ヘスティアはせっせと残りの「神聖文字」を付け足し、己のしてやれる **ことに努める。**

I don't really understand why に used instead を even though 努める is transitive?

Thanks!




I think you're mixing up two different つとめる with different kanji. 努める is
always intransitive.

  * [**務【つと】める**](http://dictionary.goo.ne.jp/leaf/jn2/147797/m0u/) ( _transitive_ ; ～を務める): be in charge, play a role. [Examples](http://eow.alc.co.jp/search?q=%E5%8B%99%E3%82%81%E3%82%8B).

  * [**努【つと】める**](http://dictionary.goo.ne.jp/leaf/jn2/147796/m0u/) ( _intransitive_ ; ～に努める): make an effort, endeavor. [Examples](http://eow.alc.co.jp/search?q=%E5%8A%AA%E3%82%81%E3%82%8B).

And there is also
[勤める](http://dictionary.goo.ne.jp/leaf/jn2/147798/m0u/%E5%8B%A4%E3%82%81%E3%82%8B/).




I think you're mixing up two different つとめる with different kanji. 努める is
always intransitive.

  * [**務【つと】める**](http://dictionary.goo.ne.jp/leaf/jn2/147797/m0u/) ( _transitive_ ; ～を務める): be in charge, play a role. [Examples](http://eow.alc.co.jp/search?q=%E5%8B%99%E3%82%81%E3%82%8B).

  * [**努【つと】める**](http://dictionary.goo.ne.jp/leaf/jn2/147796/m0u/) ( _intransitive_ ; ～に努める): make an effort, endeavor. [Examples](http://eow.alc.co.jp/search?q=%E5%8A%AA%E3%82%81%E3%82%8B).

And there is also
[勤める](http://dictionary.goo.ne.jp/leaf/jn2/147798/m0u/%E5%8B%A4%E3%82%81%E3%82%8B/).

