CO temperature-programmed desorption of a hexameric copper hydride nanocluster catalyst supported on functionalized MWCNTs for active site characterization in a low-temperature water-gas shift reaction

Abstract

A family of novel catalysts was generated using chemically synthesised, atomically precise hexameric copper hydride nanoclusters (Cu6) deposited on carboxyl-pre-functionalized multi-walled carbon nanotubes (MWCNTCOOH). The Cu6/MWCNTCOOH catalysts were synthesized by wet impregnation of MWCNTCOOH with varying copper loading contents (0.5–15 wt%). The study of the interaction between active sites in these materials with CO at low temperatures using CO temperature-programmed desorption in conjunction with the elementary steps in the Langmuir-Hinshelwood mechanism of low-temperature water–gas shift (LTWGS) reaction allowed us to predict the potential catalytic performance of the synthesized catalysts in the LTWGS. The hypothetical activities were correlated with the catalyst surface characterization by CO chemisorption (Cu dispersion, crystallite size and surface area) and characterization of the active phase composition by XRD, showing good agreement. Optimal copper loading was identified to be 1 wt% based on the highest Cu surface area per sample weight and dispersion, and the amount of CO adsorbed per sample weight. The predicted catalytic performance was analysed as a function of support type: MWCNTCOOHcf. non-functionalized MWCNTs and alumina with fixed Cu loading of 1 wt%. The CO reactivity was analysed on Cu2O crystallites as the active phase, with a focus on the most dominant facets: (1 1 0), (1 1 1), (2 0 0) and (2 2 0). A comparison was made with a sample consisting of Cu nanoparticles (CuNP) supported on MWCNTCOOH, and a reference commercial catalyst, 51%CuO/31%ZnO-Al2O3. It is expected that the optimal catalyst, 1%Cu6/MWCNTCOOH, is active for LTWGS reaction from temperatures as low as 120 °C (governed by dew point of water) up to temperatures well below industrial operating temperatures (constrained by temperature rise due to the exothermic reaction that leads to Cu6 sintering).