Catalytic investigations on CVD prepared Pd/Zr⁰/ZrO_xH_y inverse model catalysts and Pd/Zr intermetallic compound system in dry reforming of methane are presented. Dry reforming of methane, which produces syngas, is a useful way to run an SOFC without emitting CO₂. In fact, the CO present in syngas is an effective fuel for SOFCs that does not disturb their operation ¹, the catalytic investigations of Pd/Zr system yield important information for novel electrode materials or external reforming catalysts.

From a catalytic perspective the initially bimetallic Pd-Zr precatalyst shows a distinct activity for dry reforming of methane. This activity can be ascribed to synergistic bifunctional cooperation of palladium and zirconium oxycarbide. Moreover, the investigations clearly demonstrate that metallic Zr is crucial to observe any activity.

The evolution of structural changes and activation monitored by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) indicates a transformation of the initially bimetallic Pd₃Zr/Pd₂Zr pre-catalyst into an active, selective and self-stabilizing state with coexistence of Pd₉Zr, Pd₂Zr and partially hydroxylated tetragonal and monoclinic ZrO₂.

Beyond intermetallic co-melting of pure components, inverse Pd/ZrO_xH_y model catalysts were prepared via chemical vapor deposition (CVD). The CVD process is using zirconium-tert-butoxide (ZTB) as an organometallic precursor. Depending on the deposition conditions ZTB could be deposited either via an ALD (atomic layer deposition) or a CVD like process. Characterization of the CVD prepared inverse model catalyst was carried out in an UHV-chamber by XPS, scanning tunnelling microscopy (STM) and low energy ion scattering (LEIS). For catalytic testing the prepared samples were transferred to an attached batch reactor setup with online detection of the gas phase by GC-MS ². The direct connection of the batch reactor to the UHV chamber makes correlation to morphologic or electronic structure changes induced by the catalysed reaction possible. Due to different post-treatments (variation of annealing temperature and gas atmosphere), either ZrO_xH_y or ZrO₂ overlayers, or metallic Zr (Pd/Zr sub surface alloy) could be obtained. Following decomposition of ZTB on Pd(111), a partially hydroxylated Zr⁴⁺-containing layer is formed, which can be reduced to metallic Zr by thermal annealing in ultrahigh vacuum, leading to a redox active Zr⁰ containing surface/subsurface. Stepwise annealing/reduction under UHV is leading to trilayer ZrO₂ islands next to metallic regions on Pd(111). These trilayer islands can be reduced by further annealing until a fully reduced Zr species. Alternatively, a fully oxidised ZrO₂ layer can be prepared by annealing in 5x10^-7 mbar O₂. Complementary DFT calculations showed that a single layer of ZrO₂ on Pd(111) can be more easily reduced than double and triple layers. Also, the initial and resultant layer compositions greatly depend on the oxidation potential of the gas environment: The better the background pressure (especially water and oxygen), the faster and more complete the reduction of Zr on Pd takes place. As a practical consequence of Zr redox activity, the initially bimetallic Pd-Zr precatalyst shows pronounced activity for dry reforming of methane, which is attributed to the synergistic bifunctional cooperation of Pd and ZrO_xC_y surface species. ³

References

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