AVS2012 Session SS-WeA: Catalysis on Metals and Alloys

MoS₂ is a semiconducting material consisting of sulfur-molybenum-sulfur tripledecker layers loosely bound by van der Waals interactions. Single layer MoS₂ can be exfoliated mechanically similar to graphene. This presentation shows an alternative avenue for the fabrication of MoS₂ monolayers at comparatively low temperature and mild conditions through sulfur loading of a copper substrate using thiophenol followed by the evaporation of Mo atoms and annealing. Here we also demonstrate that another MoS_x structure can be formed in this fashion, which has a far higher affinity to adsorbate interaction. Using anthraquinone and formic acid as test molecules, we titrate the various MoS_x and copper-based structures presented on our substrate in order to determine the relative strength of adsorbate interaction.

The deposition of Co on Cu has been studied extensively due to the use of layered Co/Cu systems in giantmagnetoresistance devices and the application of magnetic Co nanostructures in spintronics. Co deposited on Cu follows a Volmer-Weber type growth mechanism, forming bilayer-high, triangular islands. These islands grow in two orientations that are rotated 60˚ with respect to each other. The formation of triangular islands is dictated by the six-fold symmetry of the underlying Cu lattice, and triangular growth is preferred to hexagonal growth due to favored diffusion of Co from the (100) to the (111) facet of the islands during deposition. The consequence of this diffusion is that there must be a difference in the crystallographic packing between the two orientations of the islands. It is thought that one packing configuration of the Co islands is fcc, in which Co follows the stacking of the underlying Cu, but the second packing structure is still debated. Here we use low-temperature scanning tunneling microscopy to explore the stacking of these islands through adsorption of hydrogen on their surfaces.

Hydrogen adsorbs dissociatively on Co, and prefers to bind to fcc three-fold hollow sites, although it is calculated that there is only a 0.01 eV difference in the binding energy of hydrogen to fcc hollows vs. hcp hollows. We show that at 80 K, hydrogen is present on the Co surface in both adsorption sites, and that there is an electronic difference between the two states that is apparent in our STM images. Through high-resolution imaging of the hydrogen at the boundary between the two adsorption sites, we have been able to deduce the stacking of the underlying Co island lattice. We confirm that the majority orientation of the islands is indeed fcc stacking, and the minority of the islands follow Co’s native hcp stacking. This has important ramifications in the development of Co/Cu/Co systems for GMR devices, as the interface between the two metals can significantly affect electron scattering.

The nucleation, growth and surface composition of bimetallic clusters on titania have been investigated as model systems for understanding how surface chemistry can be controlled by bimetallic composition and interactions between the clusters and the oxide support. Specifically, we have focused on Au-based bimetallic systems (Au-Pt, Au-Ni, Au-Co) as well as Co-Pt systems. Scanning tunneling microscopy studies demonstrate that bimetallic clusters can be formed via sequential deposition when there is a difference in mobility between the two metals; the more mobile atoms (i.e. Au) can be nucleated at existing Ni, Pt and Co clusters. The surfaces of the Au-containing bimetallic clusters are almost pure Au at Au compositions greater than 50% due to the lower surface free energy of Au compared to other metals. In contrast, the Co-Pt clusters are rich in Pt despite the lower surface energy of Co. In the Au-containing bimetallic clusters, CO and methanol adsorbates induce diffusion of Pt and Ni to surface, whereas this effect is not observed for CO on Au-Co. The activity of these model surfaces have been studied in a prototype recirculating loop microreactor.

Bimetallic catalysts involving Sn and Pt have important applications in hydrocarbon conversion catalysis. We have performed experiments probing chemisorption and reaction kinetics on well-defined, ordered Pt-Sn surfaces in order to aid developments needed for improving catalyst selectivity and overall performance. One specific example from investigations of benzene formation from acetylene on Pt-Sn alloys with HREELS, TPD, and DFT calculations will be discussed. On Pt(111), μ₃-η²-acetylene chemisorbed in a three-fold site is the most stable configuration, as indicated by DFT calculations, and has a C-C stretching frequency (ν_CC) of 1310 cm^-1. This configuration becomes less thermodynamically favorable in the presence of Sn compared to the bridge-bonded μ₃-η² configuration (ν_CC = 1495 cm^-1). The ν_CC peaks at ~1600 cm^-1 are assigned to π-bonded acetylene, which dominate the spectra collected at 90 K. Absence of three-fold Pt sites on the Pt₂Sn alloy inhibits the transformation of acetylene to CCH₂, and the ν_CC peak at 1412 cm^-1 assigned to CCH₂ appears only in the spectra of the Pt₃Sn alloy. DFT calculations show that the destabilizing effect of Sn alloying is more significant for CCH₂ and CCH + H than for acetylene. This change in relative stability increases the barrier for acetylene decomposition and makes associative reactions more likely. Results from DFT calculations indicate that benzene formation on the Pt-Sn alloys proceeds through the formation of an upright cyclic C₄H₄ intermediate, which is predicted to produce benzene by reacting with an additional surface acetylene. This closely integrated experimental-computational study has enabled us for the first time to characterize the adsorption modes of acetylene on Pt-Sn alloys. In addition, we developed a molecular level reaction mechanism for benzene formation by consolidating HREELS and TPD experimental results with DFT calculations. The presence of Sn changes the preferential adsorption sites for hydrocarbons, decreases the stability of adsorbed species to varying degrees, and favors associative reactions, thus, enabling benzene production by cyclotrimerization of acetylene.

B.E.K. acknowledges support by NSF Grant No. CHE-1129417.