Hydrogen Spillover
Hydrogen spillover arises in hydrogen catalysed reactions on supported metal catalysts.
Dihydrogen molecules dissociate on the metal part of the catalyst. Some hydrogen atoms
remain attached to the metal, whilst others diffuse to the support and are said to spillover.
Spillover hydrogen has often been inferred from hydrogen adsorption and reactivity
studies.
In the case of interaction with supported catalysts (metals of the platinum group) H
atoms bound to the surface atoms migrate to the carrier. This effect is called hydrogen
spillover. Spillover hydrogen (SH) is capable of entering into almost all reactions typical
of hydrogen activated on the surface of a platinum group metal. SH ensures
hydrogenation of solid unsaturated organic compounds at room temperature. Solid-state
reactions between organic compound and SH have found a wide application in the
synthesis of labeled compounds.
There are hypotheses that SH can be either a solvated proton or a proton-electron pair.
Over metal oxide, SH seems to occur by moving over –OH surface group with hydrogen
exchange as this can be proved by D2 experiment. However, hydrogen spillover over a
carbon surface is accompanied by simultaneous electron and ion conductivity.
Example 1: Jung KD, Bell AT, J. Catal, 193, 207-223 (2000)
Recent studies have shown that the synthesis of methanol from both CO and CO2 over
Cu/ZrO2 involves the spillover of H atoms formed on Cu to the surface of ZrO2. The
atomic H then participates in the hydrogenation of carbon-containing species (i.e.,
HCOO-Zr and HCO3-Zr) to methanol. The present study examines the dynamics of H/D
exchange of HO-Zr, HCOO-Zr, and CH3O-Zr groups present on the surface of ZrO2 and
Cu/ZrO2 by means of in situ infrared spectroscopy. The rate of H/D exchange of HO-Zr
groups is much more rapid in the presence than in the absence of Cu dispersed on the
surface of ZrO2. This effect is attributed to the higher effectiveness of Cu to dissociate
H-2(D-2). While adsorbed water significantly inhibits the rate of H/D exchange on ZrO2,
the opposite effect is observed for Cu/ZrO2. The reason is that adsorbed water inhibits
the dissociation of H-2(D-2) on the surface of ZrO2 but not on the surface of Cu.
Adsorbed water facilitates the transport of H(D) atoms formed on the surface of Cu
across the surface of ZrO2 as a consequence of hydrogen bonding between adsorbed
H2O and HO-Zr groups. Formate groups are formed on the surface of ZrO2 primarily via
the process CO(g) + HO-Zr --> HCOO-Zr. Formate groups can also form on the surface
of Cu and spill over onto the surface of ZrO2. As in the absence of formate groups,
adsorbed water inhibits the rate of HID exchange for ZrO2 but enhances it for Cu/ZrO2.
The dynamics of H/D exchange are compared with the dynamics of methanol formation
as measured by the rate of CH3O-Zr formation on Cu/ZrO2. On the basis of this analysis
it is concluded that the rate of hydrogen spillover from Cu is more than an order of
magnitude faster than the rate of methanol formation, and, hence, not a rate-limiting step
in the synthesis of methanol over Cu/ZrO2.
Example 2: Investigation on Hydrogen Spillover Part I: Electrical Conductivity Studies
on TiO2 and II: Hydrocarbon Conversion on Bifunctional Catalysts, U. Roland, R. Salzer,
T. Braunschweig, F. Rößner, H. Winkler, J. Chem. Soc., Faraday Trans. I, 91, (1995)
1539 and 11091