AVS 62nd International Symposium & Exhibition
    In-Situ Spectroscopy and Microscopy Focus Topic Monday Sessions
       Session IS+AS+SS-MoM

Invited Paper IS+AS+SS-MoM1
Hot Electron In-Situ Surface Chemistry at Oxide-Metal Interfaces. Foundations of Acid-Base Catalysis

Monday, October 19, 2015, 8:20 am, Room 211C

Session: Fundamental Studies of Surface Chemistry of Single Crystal and Nanomaterials under Reaction Conditions
Presenter: Gabor Somorjai, University of California, Berkeley
Correspondent: Click to Email
The development of Catalytic Metal-Semiconductor Nanodiodes (CMSN) to measure the flow of electrons excited during exothermic catalytic reactions at the metal interface proved that oxidation on platinum generates a steady flux of hot electrons [1] Evidence is presented that the steady state of chemicurrent is correlated to the turnover frequency and that the exothermic hot electron production during reactions on transition metal particles may be widespread. The CO/O2 and H2/O2 reactions were studied most frequently by this method and semiconductors included TiO2, GaN, CoOx, NbOx and TaOx Charge transport between the metal and oxide interfaces also influences the product distribution of multipath reactions. These were shown in the hydrogenation of furfural and croton aldehyde at platinum/TiO2 interfaces as compared to the platinum/silica interfaces [2]. The oxide-metal interfaces appear to produce ions, which carry out reactions that have long been called by the organic chemistry community as acid base catalysis. The typical catalytic structure is mesoporous oxide that is produced to hold the metal nanoparticles. The structures produce high surface area oxide metal interfaces and this is a catalytic architecture for acid base catalysis. Studies in changing the transition metal oxide using a single metal of platinum as nanoparticles, shows the tremendous amplification effect of the oxide metal interfaces in the reactions such as the carbon monoxide oxidation. Platinum alone produces on silica three orders of magnitude less CO2 by the CO oxidation process than on cobalt oxide, that is the most active of these acid base oxide metal interface catalytic systems [3]. Nevertheless, not just cobalt oxide, but nickel oxide, manganese oxide, and iron oxide, produces much higher activity for this reaction than platinum alone. In other reactions, when n-hexane isomerization or cyclisation reactions studied, the pure oxides niobium or tantalum do not produce any reaction other than cracking two smaller molecular fragments. However, at the platinum-oxide interfaces with niobium oxide or tantalum oxide, almost 100% selectivity for isomerization could be achieved [4]. Thus it appears that charge catalysis plays a very important role, which is equal in importance to the role of pure metal covalent catalysis that produces molecules without any apparent charge flow. Generation of hot electron flows and the catalytic activity of two-dimensional arrays of colloidal Pt nanoparticles with different sizes are investigated using catalytic nanodiodes. Pt nanoparticles of smaller size lead to higher chemicurrent yield, which is associated with the shorter travel length for the hot electrons, compared with their inelastic mean free path [5]. In many oxide supports microporous sites are used, which are less than 1 nm in size and do not allow the larger platinum nanoparticles inside these pores. In that case, the metal that is used to create the catalysts are deposited on the outside surface of the microporous support. This sort of system, where the metal is outside, but the acidic microporous oxides are inside, can be active only by a spill over of the reaction intermediates from the metal to the oxide - and this is quite well known. However if the micropores are substituted by mesopores in the oxide phase the metal nanoparticles can go inside and then single site oxide-metal interface catalysis commences. These two different catalytic processes, where both the oxide and the metal are catalytically active, deserve attention and distinction.