A research team led by Prof. WANG Zhandong from the University of Science and Technology of China (USTC), observed a series of covalent cluster intermediates in resonantly stabilized free radical gas-phase reactions with a synchrotron radiation vacuum ultraviolet photoionization mass spectrometry experimental platform, revealing the role of resonantly stabilized free radicals in the growth of particulate matter mass. The study was published in Journal of the American Chemical Society.

Carbonaceous particles are produced in transportation, industrial emissions, and combustion. Apart from causing haze, they can induce respiratory and cardiovascular diseases if inhaled by humans; also, they can affect the performance and lifespan of engines. Therefore, it is urgent to reduce these particles in the air. To realize this, revealing how carbonaceous particles are formed helps.

Resonantly stabilized radicals (RSRs) are a special class of radicals detected in carbonaceous particles, which obtain higher stability and longer lifetime through electron delocalization. Due to the limitations of experimental methods, research on the reaction mechanism of RSRs mainly focuses on the reaction path, and there is no direct experimental evidence on how RSRs achieve particle mass growth.

A series of regularly growing covalently bound clusters (CBCs), intermediates in the evolution from small RSRs, is observed in experiments by using in situ synchrotron radiation ultraviolet photoionization mass spectrometry (SVUV-PIMS) at the BL09U beamline of the National Synchrotron Radiation Laboratory. The undulator-based beamline was designed with an energy range of 7–124 eV to study the spectroscopy and dynamics of atoms, molecules, and clusters. Pyrolyze halogenated radical precursors in a laminar flow reactor were chosen to conduct the experiment since RSRs could be exclusively produced via cleavage of the C-Br bond at a high temperature in this reactor.

SVUV-PIMS mass spectra of products from the pyrolysis of selected precursors in a flow reactor at a pressure of 20mbar (Image by WANG Zhandong et al.)

Combined with quantitative calculations, hydrogen atom abstraction and multiple RSRs additions were found to promote the formation and rapid growth of covalent clusters. These clusters gradually dehydrogenated to produce large-sized peri-condensed aromatic and polycyclic aromatic radicals and form white smoke-like carbonaceous particles.

Thess progressive H-losses from CBCs (PHLCBC) elucidated an important relationship among RSRs, covalent clusters, and peri-condensed aromatics, bridging small-sized gas-phase radical intermediates with large-sized nanocarbon particles.

This study provides important insights for the multi-scale simulation of carbonaceous particulates and the synthesis of carbon nanomaterials, reveals the formation mechanism of fullerene clusters in flames, and develops new coking inhibitors.