UC Riverside

One of the major challenges in heterogeneous catalysis is the preparation of highly selective catalysts. Cu catalysts have been ubiquitous in many applications, including for the promotion of the low temperature water-shift reaction and hydrogenation of carbon monoxide. By combining with a second metal, Cu can also assist in the selective hydrogenation of organic molecules with multiple double and triple bonds. In this work, Pt-Cu alloy catalysts have been studied to obtain better understanding of some fundamental questions, such as what the oxidation state of Cu under reaction conditions is, where the atoms of the two metals are.Majority of this work has been focused on the Pt-Cu bimetallic catalyst, so-called single atom catalyst, where Pt atoms diluted in Cu nanoparticles. Pt is presumed to activate H2 while Cu is believed to aid in the selective addition of the resulting H atoms to C=O bonds. In fact, catalysts with Pt content as low as 0.1% of the total composition have shown improved selectivity. In this catalysis, it is reasonable to expect the active catalyst to be metallic copper. However, in situ x-ray absorption spectroscopy (XAS) and CO infrared (IR) titration experiments have suggested that at low temperatures, below 100C, copper-containing nanoparticles dispersed on silica support develop a thin layer copper oxide film. This was found even in the presence of a hydrogen atmosphere for several different experiment setups. However, our latest experiment from ANL indicated oxidation can be prohibited under hydrogen atmosphere with adequate sealing. At high temperatures, Cu is fully reduced to metallic state, and the interconversion is reversible. Nonetheless, plenty of hydrogenation catalysis is carried out under mild temperature, under which reaction may take place on oxidized copper sites. In addition to the oxidation of Cu surface, several CO IR titration experiments were carried out and suggested that Pt sites are not available on the surface until the Pt content surpasses about 10% of the total metal loading. Where are the Pt atoms during reaction, on the surface or inside Cu nanoparticles, or attached to the oxide support? XAS absorption spectroscopies and in-situ IR have been carried out. Both in vacuum and under gaseous atmospheres, Pt atoms are found to be diffusing within the Cu-Pt nanoparticles. From in situ IR in CO atmosphere, for diluted single atom catalysts, Pt was found to reversibly segregate to the surface and diffuse back into the bulk when ramp up the temperature from 295 K to 495 K and cool back. A comparison of the thermodynamics of the adsorption of CO on the metal surface of a Cu/SBA-15 catalysts was made as well, under two different environments: under vacuum versus in the presence of atmospheric pressures of CO. Changes associated with entropic effects introduced by the presence of CO gas such as adsorbate displacement and adsorbate-assisted adsorption steps need to be introduced to explain the differences seen between the two environments. Also, core-shell Pt@Cu catalysts were prepared successful, and it will be utilized to study the behavior of Pt atoms under reaction condition. Future follow-up work of this project might include designing flow reactor for in-site infrared experiments and optimizing current core-shell Pt@Cu catalysts to minimize the amount of single Cu atom site on the surface and test them with in situ IR and XAS absorption spectroscopies.