In-situ XPS of Catalytic Reactions
Assoz. Prof. Dr. Bernhard Klötzer
Institute of Physical Chemistry, University of Innsbruck, Innrain 52A, Innsbruck, Austria
17:00 - 18:00 Tuesday 20 November 2012 KFU HS 05.01 The applicability of synchrotron radiation for in-situ studies of "living" catalyst surfaces under realistic reaction conditions is exemplified on the basis of the technologically relevant methane oxidation and methanol reforming reactions.
In contrast to flame combustion, catalytic methane combustion allows to operate gas turbines at low thermal NOx levels, but process stability is hampered by kinetic instabilities and transient catalyst deactivation. The rate instabilities are induced by the kinetic hysteresis of Pd oxidation in O2, which was shown by in-situ XPS to originate from the distinct influence of intermediate surface oxide species existing in between Pd metal and bulk PdO. The most active state of Pd is characterised by a complex interplay between Pd metal loaded with dissolved oxygen, Pd5O4 surface oxide and bulk PdO clusters.
CO2-selective methanol steam reforming is an important process to generate CO-depleted hydrogen for mobile fuel cell applications. A ~1:10 diluted Zn-in-Cu near surface alloy covered by a thin wetting layer of interfacial Zn(ox) represents the most active state of an inverse CuZn model catalyst. The bifunctional action of the mixed Cu(Zn)0/Zn(ox) surface allows for selective dehydrogenation of methanol to formaldehyde and for optimized water activation, providing the required source of oxygen for total oxidation of CH2O to CO2. In the "in-operando" monitored active state, (bi)metallic Cu(Zn)0 centers and wetting interfacial Zn(ox) species are simultaneously present at the surface to allow for a mechanistic CuZn-Zn(ox) "bifunctional synergism" (Figure 1). A dynamic redox cycle Zn0/Zn(ox) is proposed as an integral component of water splitting and hydrogen formation [1]. Similar results for a PdZn surface are presented.
Figure 1: Upper left: pre-catalytic "as-grown" 10:1~CuZn bimetallic state. Middle left: superior "in-operando" active surface state with maximum wetting of Cu(Zn)0 by interfacial Zn(ox). Lower left: High temperature state after major Zn intensity loss by 3D Zn(ox) clustering and/or bulk diffusion/ desorption of Zn. Right panel: deconvolution of the in-operando measured Zn3d region (at 0.12 mbar methanol + 0.24 mbar water) into bimetallic and oxidised Zn components as a function of reaction temperature. Blue lines: bimetallic Zn in Cu, red lines: surface-segregated wetting Zn(ox) component.
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