ABSTRACT
Among the multitudinous methodologies to steer on-surface reactions, less attention has been paid to the effect of externally introduced halogen atoms. Herein, highly selective trans-dehydrogenation coupling at the specific meta-C-H site of two poly(p-phenylene) molecules, p-quaterphenyl (Ph4) and p-quinquephenyl (Ph5), is achieved on Cu(111) by externally introduced bromine atoms. Scanning tunneling microscopy/spectroscopy experiments reveal that the formed molecular assembly structure at a stoichiometric ratio of 4:1 for Br to Ph4 or 5:1 for Br to Ph5 can efficiently promote the reactive collision probability to trigger the trans-coupling reaction at the meta-C-H site between two neighboring Ph4 or Ph5 molecules, leading to an increase in the coupling selectivity. Such Br atoms can also affect the electronic structure and adsorption stability of the reacting molecules. It is conceptually demonstrated that externally introduced halogen atoms, which can provide an adjustable halogen-to-precursor stoichiometry, can be employed to efficiently steer on-surface reactions.
ABSTRACT
Ceria has been widely applied as a support in heterogeneous catalysis due to its unique capability to store and release oxygen. As a typical inverse model catalyst, a ceria/Pt(111) system has attracted much attention due to its strong metal-oxide interaction. The structural and electronic properties of the ceria/Pt(111) system can be effectively modified by the introduction of alien K and Rh atoms. Here, the K- and Rh-modified ceria/Pt(111) inverse model catalysts have been investigated with high resolution scanning tunneling microscopy and apparent local work function measurement. The experimental results indicate that the K atoms prefer to occupy the top sites of the stoichiometric ceria, while the Rh atoms are prone to stay at the electron-rich ceria island edges. The K and Rh atoms act as an electron donor and acceptor on ceria/Pt(111), respectively. Such a study on the modification of the ceria-based catalysts should help understand strong metal-oxide interaction in heterogeneous catalysis at the atomic level.
ABSTRACT
A heterogeneous catalytic process involves many surface elementary steps that affect the overall catalytic performance in one way or another. In general, a high-performance heterogeneous catalyst should meet the main criteria: excellent catalytic activity and high selectivity toward target products. Using surface science techniques, the two-sidedness of the surface reaction mediations can be explored, from the perspectives of the surface and the molecule manipulations. The surface manipulation refers to a reaction that is mediated by composition and structure of the substrate as well as surface species, while the molecular manipulation relates to a reaction that is mediated by the reacting molecule via the precursor selection, environmental control, or external excitation. The best catalytic system should consist of the most efficient catalyst and the best suitable reacting molecule, in addition to its economic benefit and environmental amity. Recent research progress in surface reaction mediation is outlined, and its two-sidedness is governed by the Arrhenius equation. This should shed new light on the connection between basic theory and surface reaction mediation strategies. To conclude, challenges and possible opportunities are elaborated for efficient surface reaction mediations.
