The catalytic nanodiode. Its role in catalytic reaction mechanisms in a historical perspective

Abstract

Catalytic nanodiodes, Pt/TiO 2 or Pt/GaN produce continuous flow of hot electron current during carbon monoxide oxidation for hours in the 80-150°C temperature range at pressures of 100 Torr of O 2 and 40 Torr of CO. These observations provide proof of the Schottky diode model of oxide supported metal catalysis of many reactions that have been proposed by Schwab, Solymosi and others since the 1960s. The flow of hot electrons should influence chemistry at oxide-metal interfaces and the metal particle size dependence of catalytic activity and selectivity.KEY WORDS: catalytic nanodiode; electron flow in catalysis; hot electrons.We recently reported the continuous flow of hot electrons through a thin platinum film through semiconductor Schottky diodes of TiO 2 and GaN during the steady state catalytic oxidation of carbon monoxide (100 Torr of O 2 and 40 Torr of CO) in the 80-250°C range [1,2]. The scheme of the nanodiodes is shown in figure 1 along with the cross sections of the nanodiodes used successfully in these experiments. Platinum acts both as an electrode and as a catalyst, and its film thickness has to be of the order of the elastic mean free path of electrons or smaller so that the charges could arrive at the metal-semiconductor interface without attenuation. The measured barrier height at the metalsemiconductor interface of both Pt/TiO 2 and Pt/GaN nanodiodes is 1.2 eV, and thus the electrons must have kinetic energies greater than 1.2 eV to be collected by the electrode on the semiconductor side. High conversion efficiency was reported for Pt/TiO 2 (three electrons for every four CO 2 molecules formed) and lower efficiencies for the Pt/GaN diode that was explained by the greater thickness of the Pt film and its discontinuity.These definitive experimental confirmations of the flow of hot, high kinetic energy electrons away from the platinum surface where the catalyzed exothermic reaction occurs to the metal-semiconductor interface brings into focus the early suggestions of Schwab [3,4], Szabo and Solymosi [5] as well as Langenbeck et al.[6] who argued for the importance of the Schottky diode model of electron flow at oxide-metal interfaces for catalyzed reactions ranging from carbon monoxide oxidation, sulfur dioxide oxidation, formic acid decomposition, ethylene hydrogenation, cyclohexene hydrogenation/ dehydrogenation to ammonia synthesis [5,7]. Their experimental strategies were through the use of ''inverse catalyst'' systems that were fabricated by depositing oxides on films of transition metals that induced large changes in activation energies as a function of oxide thickness along with changes of turnover rates. However, these ideas of the importance of electron flow at the oxide-metal interfaces of these catalysts were strongly questioned [8] and found unacceptable by most catalyst researchers, mostly because of the absence of any experimental proof of current flow and its correlations to catalytic activity.In light of the high hot electron currents that were detected through the oxide-metal interfa...

Modeling gold/iron oxide interface system

“…In our case these types of growth mode can be excluded. We believe that we have a similar system to those discovered by Somorjai for nano diod [26][27][28].…”

Charge Transport in Metal–Oxide Interfaces: Genesis and Detection of Hot Electron Flow and Its Role in Heterogeneous Catalysis

“…Early experiments utilizing a metal-semiconductor Schottky diode to elucidate fundamental insights into the dynamics of chemical processes at metal surfaces were conducted at low-temperature and ultrahigh vacuum conditions [41]. Since 2005, however, Somorjai's group has extended this approach to high-temperature and -pressure systems that are identical to practical catalysis conditions [49,50]. As a model system, oxidation of carbon monoxide Since hot electrons generated on the Pt surface during exothermic reactions have enough kinetic energy to overcome the Schottky barrier (1.2 eV), a steady-state current was observed, implying that the reaction occurred and that the product rapidly left the surface; whereas a transient current was generated from the chemisorption reaction because of the eventual surface saturation.…”

The Catalytic Nanodiode: Detecting Continous Electron Flow at Oxide–Metal Interfaces Generated by a Gas‐Phase Exothermic Reaction

“…[14][15][16][17] Here, we describe the detection scheme of hot electron flow. We discuss the correlation between the hot electron and the turnover rate of the chemical reaction.…”