Effects of nitrogen-containing functional groups of reduced graphene oxide as a support for Pd in selective hydrogenation of cinnamaldehyde

Abstract

The importance of electronic and chemical properties of nitrogen-doped reduced graphene oxide (NRGO) has attracted more attention in recent years. Various carbon–nitrogen bonds with different chemical properties are formed in the process of nitrogen doping (N-doping) of reduced graphene oxide, such as pyrrolic-N, pyridinic-N, and graphitic-N. These types of carbon–nitrogen bonds in the structure are distinctly effective when they are used as catalyst support. This study reveals that various types of carbon–nitrogen bonds of the support affect activity and selectivity of hydrogenation of cinnamaldehyde to hydrocinnamaldehyde reaction differently by palladium active phase. The catalyst supports were synthesized with thermal treatment at three different heat treatment and atmosphere procedures, i.e., 500 and 900 °C, in the presence of ammonia diluted with argon. Various procedures led to different types of carbon–nitrogen bond and different quantity of nitrogen doping of the support. Following the support synthesis, palladium active phase was deposited on the support by an incipient wetness impregnation method. The catalysts were characterized by ICP-OES, XPS, elemental analysis, Raman spectroscopy, FESEM, BET, TPD, TPR, CO chemisorption, and HRTEM. The characterization results showed that Pd dispersion and its size distribution were affected by the quantity and the type of carbon–nitrogen species of the support. The pyridinic nitrogen, as electron acceptor species, seems to have a positive impact on a more uniform distribution of Pd. The catalytic activity was evaluated in a high-pressure bubble column reactor. In this work, the catalyst containing 4.5 wt% Pd supported on NRGO with 2.4% nitrogen and treated at 500 °C showed a maximum conversion of about 98% with high selectivity (96%) for the reaction.