Metal nanostructures supported on carbon nanomaterials as a novel catalyst for heterogeneous reactions.

Abstract

Carbon nanomaterials offer a sufficient surface area for hosting highly-dispersed catalytically active metal nanostructures, for an effective diffusion of reactant/product molecules in gas- phase heterogeneous reactions. This thesis has a main general objective to develop a novel copper (Cu)-based catalyst supported on multi-walled carbon nanotubes (MWCNT) for reactions that involve carbon monoxide (CO) conversion. A family of novel catalysts was generated using chemically synthesised, atomically precise hexameric Cu nanoclusters (Cu6) deposited on carboxyl (–COOH)-pre-functionalised MWCNT (MWCNTCOOH); Cu6/MWCNTCOOH. The fabrication, characterisation, and activity study of the synthesised catalysts were developed in three stages: Catalyst Development; Catalyst Fabrication and Hypothesis Development; and Catalytic Assessment and Feasibility Studies, with a specific objective to produce a finalised novel catalyst that: (1) is optimal in terms of its synthesis parameters, Cu loading content, Cu nanostructure precursor, and catalyst support; (2) possesses structural characteristics of reasonably high dispersion of Cu nanostructures of sufficiently small crystallite size and high surface area; and (3) exhibits satisfactory level of catalytic activity in the CO conversion reaction.

The Cu6 cluster complex has a crystal structure of [(PPh3)CuH]6.0.75THF as determined by single-crystal X-ray diffraction (SCXRD). When deposited on the MWCNT support, the hexameric copper hydride, (CuH)6 changed its phase to Cu2O by oxidation with air at ambient as determined by powder X-ray diffraction (PXRD). The active site characterisation analysis based on interpretation of the CO temperature-programmed desorption (CO-TPD) profiles elucidated by Langmuir–Hinshelwood (L–H) mechanism for low-temperature water– gas shift (LTWGS) reaction predicted that the optimal catalyst, 1%Cu6/MWCNTCOOH was active for LTWGS reaction from 120 °C (governed by dew point of water) up to well below 190 °C (constrained by temperature rise due to the exothermic reaction that leads to Cu sintering).

To experimentally validate the hypothetical prediction, actual activity tests were conducted in CO oxidation, a reaction of which the elementary steps in its mechanism involve the same

surface-reaction elementary step as in the LTWGS reaction mechanism where the CO molecules scavenge the surface-adsorbed oxygen molecules and react to produce carbon dioxide (CO2). It was demonstrated that the bare pristine MWCNT by itself was catalytically active for CO oxidation from approximately 150 °C, and a CO conversion of greater than 98% was reached at approximately 230 °C. However, in these temperatures, the bare MWCNT modified with the –COOH groups adsorbed significant quantities of CO molecules without further catalytic converting. The –COOH groups degraded thermally progressively from approximately 250 °C. The Cu2O species from the Cu6 cluster species precursor was demonstrated to have formed CuCO3 under the CO oxidation reaction conditions of below 330 °C. The CuCO3 decomposed effectively from 400 °C to release CO2 product. A minimum working temperature of 400 °C was required to allow an effective conversion of CO into CO2 on the Cu6/MWCNTCOOH catalysts. On the other hand, a reference MWCNTCOOH-supported catalyst prepared using copper nanoparticles (CuNP) precursor that was not covered by any surfactants or capping agents was demonstrated to be active from 190 °C and approached a full CO conversion at slightly below 330 °C.