New model catalysts (platinum nanoparticles) and new techniques (SFG and STM) for studies of reaction intermediates and surface restructuring at high pressures during catalytic reactions

Abstract

Single-crystal surfaces have long served us well as model catalysts; however, a new type of model catalyst has been prepared using electron beam lithography. Ordered arrays of platinum nano-particles in the 2.5–50 nm size range are deposited on oxide substrates (silica, alumina, and titania) of 1 cm 2 surface area, and are used in catalyzed surface reactions at high pressures (atmospheres). Their preparation, cleaning, and reactivity is discussed. Scanning tunneling microscopy (STM) and vibrational spectroscopy by sum frequency generation (SFG) can be utilized to monitor the substrate and adsorbate structure, respectively, over a fourteen order of magnitude pressure range (10 −10–10 4 Torr). As a consequence, we can monitor the surface structure and reaction intermediates during high pressure catalytic reactions. An STM that operates at both high pressure (atmosphere) and high temperature has been constructed and utilized to monitor platinum (111) and (110) surface structure during chemisorption of H 2, O 2 and CO, and during catalytic reactions of olefin hydrogenation and hydrogenolysis. Changes of surface structure upon chemisorption and during reactions have been monitored. Catalysis by the platinum tip was also detected in the presence of H 2 or O 2 at high pressures and 300 K, leading to hydrogenation or oxidation of carbonaceous deposits with nanometer spatial resolution. Vibrational spectroscopy using SFG has been used to monitor pressure dependent changes in the chemisorption of CO and NO over Pt(111). Bonding — which is similar to that in Pt m (CO) n (where n m > 1 ) clusters and for an incommensurate CO overlayer — is observed above 100 Torr. Reaction intermediates that form during ethylene, propylene, and isobutene hydrogenation, as well as CO oxidation, at atmospheric pressures and 300 K were monitored by SFG. The dominant reacting species that hydrogenate are the weakly π-bonded olefins, while the strongly chemisorbed alkylidyne and di-σ bonded species are spectators during the reaction. From quantitative measurement of coverages, the absolute turnover rates can be determined.