Investigation of platinum and platinum/rhodium hydrogen cyanide catalyst recrystallization with scanning tunneling microscopy

Kathleen Ann Jurman, Purdue University

Abstract

In the Andrussow process, a mixture of air, NH$\sb3$ and CH$\sb4$ ($\rm O\sb2$:NH$\sb3$:CH$\sb4$, 1.3:1:1) is ignited over 90%Pt-10%Rh gauze resulting in a combustion-driven synthesis of HCN at 1100$\sp\circ$C and a dramatic restructuring of the catalyst as it activates during the first 24 to 72 hours. Scanning Tunneling Microscopy (STM), first reported in 1982, offers a new approach in analyzing initiation of the recrystallization process by providing quantitative information with submicron scale resolution about topography and the surface electronic state of conducting samples. In order to apply this technique to the study of HCN catalysts, an isothermal laboratory reactor has been constructed which produces a recrystallization gradient down the length of a single polycrystalline wire and STM reregistry, the capability to return to the same submicron target area on these wires with the STM probe, has been developed to follow specific structural features through a sequence of reaction treatments. We have demonstrated that it is possible to preserve the high temperature reaction morphology of platinum HCN metal catalysts by rapid cooling and have observed that at 1100$\sp\circ$C the surface morphology in the presence of an inert environment is smooth and glassy, while exposure to the Andrussow reaction mixture results in crystal-orientation-dependent facetting. Consistent with the literature, recrystallization is driven by Andrussow reaction. Treatment of samples in O$\sb2$ at 1100$\sp\circ$ and 1600$\sp\circ$C does not result in recrystallization. Treatment of samples in 6.2% NH$\sb3$ in He at 1100$\sp\circ$C reveals a variety of facet patterns related to the crystal orientation of the exposed grain surface, as confirmed by electron channeling microscopy. Neighboring grains show significant variation in the extent of recrystallization under the influence of five hours of Andrussow reaction, and recrystallization does not originate exclusively at grain boundaries. Ten percent Rh alloyed with Pt also appears to slow the recrystallization process. Pt and Pt-Rh do not recrystallize in the presence of the endothermic Degussa reaction, which indicates oxygen, and presumably oxidation reactions, are involved in recrystallization. The interplay between homogenous and heterogeneous oxidation appears to be an additional factor in platinum recrystallization.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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