Nasikin, M., and A. Wahid. “Effect of Ultrasonic During Preparation on Cu-Based Catalyst Performance for Hydrogenation of C02 to Methanol”. ASEAN Journal of Chemical Engineering, Vol. 5, no. 2, Dec. 2005, pp. 111-5, https://jurnal.ugm.ac.id/v3/AJChE/article/view/7639.