Imaging of activated nitrogen adsorption
5 Oct 2015 NUS scientists report for the first time the adsorption of a single nitrogen atom on individual manganese phthalocyanine, through in situ LT-STM characterization.
A team led by Prof CHEN Wei from the Departments of Physics and Chemistry in NUS has demonstrated at the single-molecule level, the binding of activated nitrogen species on the single Mn atom contained within the manganese phthalocyanine (MnPc) molecule supported on an inert graphite surface. Through the combination of in situ low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy and density functional theory calculations, the active site and the binding configuration between the activated nitrogen species (neutral nitrogen atom) and the Mn center of MnPc is investigated at the atomic scale.
An atomic-scale understanding of gas adsorption mechanisms on metal-porphyrins or metal-phthalocyanines is essential for their practical application in biological processes, gas sensing and catalysis. Intensive research efforts have been devoted to the study of coordinative bonding with relatively active small molecules, such as CO, NO, NH3, O2 and H2. However, the binding of single nitrogen atoms has never been addressed, which is both of fundamental interest and indeed essential for revealing the elementary chemical binding mechanism in nitrogen reduction processes.
The dinitrogen molecules were first activated by a hot filament and then selectively coordinatively bonded with the center Mn atom. In the next step, they look for the catalyst on which the uptake and activation dinitrogen already occurs at room temperature and low pressure. This heterogeneous process on surface can be extended to other well characterized transition metal complexes, such as Mo and Fe, which are responsible for the biological nitrogen fixation and Haber-Bosch process.
Figure (a) shows the schematic of the MnPc molecule before and after nitrogen atom adsorption. (b) and (c) are the dI/dV spectra taken at the center and lobe positions of MnPc and N-MnPc. It was found that the selective bonding between the activated nitrogen atom and the center Mn atom in MnPc significantly modifies the local density of states on the Mn center of the N-MnPc. [Image credit: Chen Wei]
Zhang JL, Wang ZZ, Zhong JQ, Yuan KD, Shen Q, Xu LL, Niu TC, Gu CD, Wright, CA, Tadich A, Qi DC, Li HX, Wu K, Xu GQ, Li ZY, Chen W. “Single-Molecule Imaging of Activated Nitrogen Adsorption on Individual Manganese Phthalocyanine” Nano Letter 15 3181 (2015) DOI: 10.1021/acs.nanolett.5b00290