Author : Michael P. Humbert
Publisher :
ISBN 13 : 9781109671605
Total Pages : pages
Book Rating : 4.6/5 (716 download)
Book Synopsis Catalytic Properties of Platinum and Tungsten Carbide-based Bimetallic Surfaces by : Michael P. Humbert
Download or read book Catalytic Properties of Platinum and Tungsten Carbide-based Bimetallic Surfaces written by Michael P. Humbert and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Bimetallic surfaces often possess catalytic properties that differ from those of their parent metals, often resulting in activities and/or selectivities higher than those achievable with a monometallic catalyst. In addition, it is possible that the properties of bimetallic catalysts can be tuned by controlling their composition. In a fuel cell, for example, modifying Pt with small amounts of Au has the ability to stabilize the Pt while maintaining the activity of unmodified Pt. However, while increasing the amount of Au improves the stability, it does so at the expense of the activity of the underlying Pt. A plethora of additional examples can be given, and it is the wide range of applications that these materials may impact that has been the driving force behind the study of their properties. One class of bimetallic catalysts that has received a great deal of attention is Pt modified by 3d transition metals. A novel low-temperature cyclohexene hydrogenation pathway was found on Ni-modified Pt. Specifically, subsurface Ni modification of Pt(111) resulted in the improved catalytic activity. In order to determine the driving force behind the improvement in activity on Pt-Ni-Pt(111), a study of the interaction of hydrogen and cyclohexene with these surfaces was performed. From these results it was hypothesized that a weaker hydrogen binding energy (HBE) and cyclohexene binding energy were responsible for the novel properties of the Pt-Ni-Pt(111) surface. However, results on Pt-Co-Pt(111) seemed to conflict with this hypothesis as this surface had a weaker HBE and cyclohexene BE than did Pt-Ni-Pt(111), yet the activity on Pt-Ni-Pt(111) remained higher. The aim of this work was to explain this apparent discrepancy. By extending the Ni and Co-modified Pt(111) results to Fe and Cu-modified Pt(111), a volcano plot was found that related hydrogenation activity to the binding energy of cyclohexene. Further, Pt-Ni-Pt(111) had a binding energy near the optimal value. Thus, while the binding energy of hydrogen and cyclohexene on Pt-Co-Pt(111) was lower than that on Pt-Ni-Pt(111), it was too low such that Co modification of Pt led to lower activity than Ni modification. A similar result held for the case of acrolein hydrogenation, for which it was found that Pt-Ni-Pt(111) also provided an optimal binding energy to promote hydrogenation. In addition to the obvious impact of these results, the ability to correlate hydrogenation activity to a value which can be determined from density functional theory calculations leaves open the possibility to predict a priori additional surfaces which may possess attractive catalytic properties. While these results were promising, it was important to determine if Pt surfaces more relevant to industrial catalysts possessed the same activity improvement through modification with Ni. To that end, the single crystal studies were extended to polycrystalline Pt. As on Pt(111), Ni modification of Pt foil increased the activity toward hydrogenation. However, it was found that subsurface Ni was not necessary to promote this reaction since the Ni-Pt-Pt foil surface was also active toward this pathway. This was hypothesized to be due to the decreased binding energy of cyclohexene on the Ni-Pt-Pt(100) facet, or due the ability of Ni to form an alloy with Pt defect sites at lower temperatures than on Pt(111). Thus, the importance of considering the effect of the presence of multiple crystal facets, step edges, grain boundaries, and other defects was demonstrated. While, it was found that subsurface Ni-modification of polycrystalline Pt was not necessary for the production of cyclohexane from cyclohexene, subsurface Ni was necessary for the production of benzene, which was attributed to the lower binding energy of the benzene intermediate on Pt-Ni-Pt foil. Similar results were found for the hydrogenation of acrolein on Nimodified Pt foil. Specifically, surface and subsurface Ni enhanced the C=C hydrogenation pathway while subsurface Ni was required to hydrogenate the C=O moiety. Finally, it was desired to utilize the Pt-like properties of WC to replace the bulk Pt in Pt-Ni-Pt to arrive at an analogous Pt-Ni-WC surface. The reaction pathways of cyclohexene and acrolein were studied on this surface with the result that the activities and selectivities for cyclohexene and acrolein hydrogenation were similar on the Pt-Ni-Pt and Pt-Ni-WC surfaces. This led to the conclusion that it may indeed be feasible to replace the bulk Pt while producing a cheaper, more stable alternative.
