Chemical vapor deposition-prepared sub-nanometer Zr clusters on Pd surfaces: promotion of methane dry reforming.
An inverse Pd-Zr model catalyst was prepared by chemical vapor deposition (CVD) using zirconium-t-butoxide (ZTB) as an organometallic precursor. Pd-Zr interaction was then investigated with focus on the correlation of reforming performance with the oxidation state of Zr. As test reactions, dry reforming of methane (DRM) and methanol steam reforming (MSR) were chosen. Depending on treatments, either ZrOxHyor ZrO2overlayers or Zr as sub-nanometer clusters could be obtained. Following the adsorption of ZTB on Pd(111), a partially hydroxylated Zr4+-containing layer was formed, which can be reduced to metallic Zr by thermal annealing in ultrahigh vacuum, leading to redox-active Zr0sub-nanometer clusters. Complementary density functional theoretical (DFT) calculations showed that a single layer of ZrO2on Pd(111) can be more easily reduced toward the metallic state than a double- and triple layer. Also, the initial and resulting layer compositions greatly depend on gas environment. The lower the water background partial pressure, the faster and more complete the reduction of Zr4+species to Zr0on Pd takes place. Under methanol steam reforming conditions, water activation by hydroxylation of Zr occurs. In excess of methanol, strong coking is induced by the Pd/ZrOxHyinterface. In contrast, dry reforming of methane is effectively promoted if these initially metallic Zr species are present in the pre-catalyst, leading to a Pd/ZrOxHyphase boundary by oxidative activation under reaction conditions. These reaction-induced active sites for DRM are stable with respect to carbon blocking or coking. In essence, Zr doping of Pd opens specific CO2activation channels, which are absent on pure metallic Pd.
Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, Innsbruck, Austria. simon.penner@uibk.ac.at and Birck Nanotechnology Center, Purdue University, 1205 West State Street, West Lafayette, IN 47907, USA.
Lukas
L
Mayr
Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, Innsbruck, Austria. simon.penner@uibk.ac.at.
Xue-Rong
XR
Shi
Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, Innsbruck, Austria. simon.penner@uibk.ac.at.
Norbert
N
Köpfle
Birck Nanotechnology Center, Purdue University, 1205 West State Street, West Lafayette, IN 47907, USA and School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, USA.
Cory A
CA
Milligan
Birck Nanotechnology Center, Purdue University, 1205 West State Street, West Lafayette, IN 47907, USA.
Dmitry Y
DY
Zemlyanov
Department of Inorganic Chemistry, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, D-14195 Berlin, Germany.
Axel
A
Knop-Gericke
Department of Inorganic Chemistry, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, D-14195 Berlin, Germany.
Michael
M
Hävecker
Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, Innsbruck, Austria. simon.penner@uibk.ac.at.
Bernhard
B
Klötzer
Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, Innsbruck, Austria. simon.penner@uibk.ac.at.
Simon
S
Penner