Matumuene Joe Ndolomingo, studied chemistry at UJ (University of Johannesburg-RSA) and obtained his Ph.D. in 2016 under the guidance of Prof. R. Meijboom. Presently, he is a Post- Doctoral fellow at the same University in chemistry department at Prof. Meijboom’s Research Center for Synthesis and Catalysis. His current research interests include the synthesis and development of mesoporous metal oxides supported metal nanoparticles for industrial oxidation and hydrogenation reactions, and development of a simple general approach for the easy determination of the true surface area and particles sizes of a wide range of metal nanoparticles using organothiols as probe ligand.
The quantification of the ligand packing density of metal nanoparticles is of considerable importance for catalysis and for quality control in many applications. The effective catalytic properties of the metal nanoparticles are mostly based on their inherent high surface area to volume ratio. However, the determination of the true surface area of the metal nanoparticle in catalysis represents one of the greatest challenges due to the nanoparticle surface unevenness and morphological irregularity. In this contribution, we report on the ligand adsorption-based technique for quantification of ligand packing density on copper nanoparticles and determination of specific surface area of copper nanoparticles on gamma alumina supports. 2- mercaptobenzimidazole (2-MBI) was used as probe ligand. The adsorption of 2-MBI on the nanoparticle surface was evaluated by using different supported copper catalysts, and was followed by ultraviolet-visible spectrometry. The amount of ligand adsorbed was found to be proportional to the copper nanoparticles surface area. The ligand packing density was calculated based on the ratio of the mass fractions of 2-MBI and Cu as determined by UV-vis and AAS, respectively. With assumption that the self-assembly of thiol molecules onto the copper surface was completed, the amount of 2-MBI adsorbed per gram of copper nanoparticle was determined as the difference between the amount of 2-MBI added and the amount of unadsorbed 2-MBI. For 2-MBI with concentration of 40-65 µM, the calculated packing density of 2-MBI on copper nanoparticles is independent on the concentration, ranging from 2.88 to 3.26 nm-2, and averaging 3.07 ± 0.13 nm-2 which would be equivalent to the saturation capacity of
0.510 ± 0.02 nmol cm-2. On the basis of the packing density, the average surface area which each
2-MBI molecule occupies is 0.326 ± 0.01 nm2. To validate this method, the quantification of 2- MBI packing density on gold nanoparticles was done under the same conditions. A good agreement was found between the saturation packing density obtained in this study, 0.586 ±
0.04 nmol cm-2 and those reported in the literature; 0.571 ± 0.005 nmol cm-2 and 0.574 ± 0.006
nmol cm-2. A Langmuir–isotherm plot was used in order to determine the effective surface area per gram of the nanoparticles. Thus, the reciprocal of the slope of the Langmuir plot was multiplied by Avogadro’s number and the cross section of the probe ligand. Although the specific surface area is the essential information acquired from the 2-MBI adsorption method, it is worth comparing the obtained particle sizes with those obtained by TEM. A fair agreement
was found between copper particle sizes obtained from ligand adsorption and TEM methods; average sizes 3.0 ± 0.0 and 3.1 ± 0.2, respectively. To validate this method, 2-MBI and H2 chemisorption methods were used for Pt/Al2O3 and Pd/Al2O3 catalysts. A good agreement was found between platinum and palladium specific surface area and particle sizes obtained from thiol adsorption and H2 chemisorption. For palladium, both specific surface area and particle sizes deviate in the order of 5% and 7%, respectively.
Audience Take away:
Despite its significant merits and potential, catalysis by copper nanoparticles is relatively less explored compared to the noble and other base metals. Copper is environment-friendly, easily available and comparatively inexpensive. So, copper is of considerable importance as an alternative to these metals. For the first time, the 2-MBI packing density on copper nanoparticles was quantified using a simple and easy method.
The physisorption method, BET is a reliable method for the surface area determination. The BET theory can be derived similar to the Langmuir theory; however, the BET does not allow the discrimination between a catalytic active species and the supporting material. Commonly, the specific surface area and the size of supported metal nanoparticle are investigated by chemisorption with H2, CO, or O2 as probe molecules. However, those methods are limited by the properties of the support and some metals, and could not be generally used. In addition, the method requires expensive instrumentation.
True surface area and particle sizes determination are key aspects of the activity of metal nanoparticle catalysts. This study demonstrated that, a more easy, simple and precise determination of the specific surface area and particle sizes of metal nanoparticles is possible.