The adsorption and reaction of maleic anhydride and deuterated
maleic anhydride on Mo(110), monolayer
Pd/Mo(110), and multilayer Pd(111)/Mo(110) surfaces have
been studied using temperature-programmed
desorption (TPD) and high-resolution electron energy loss spectroscopy
(HREELS). Maleic anhydride
adsorbs irreversibly on the Mo(110) surface at 100 K. Heating
to 1200 K yields adsorbed carbon (Cads)
and gas-phase CO and H2. In contrast, the adsorption
of maleic anhydride on monolayer Pd(111)/Mo(110)
and multilayer Pd(111)/Mo(110) surfaces is largely reversible
with the chemisorbed maleic anhydride
desorbing at 365 and 375 K, respectively. Approximately 15% of
the chemisorbed maleic anhydride
decomposes upon heating to 400 K, forming CO, CO2, and
C2H2; C2H2 further
dehydrogenates upon heating
to Cads and gas-phase H2. The HREELS
measurements indicate that maleic anhydride is bonded to
multilayer Pd(111)/Mo(110) through the olefin bond in a
di-σ configuration, while on monolayer Pd(111)/Mo(110), the maleic anhydride is bonded to the surface through the
olefin via a π-bond. On the Mo(110)
surface, maleic anhydride is bonded to the surface through the ring
oxygen with the molecular plane
perpendicular to the surface. As a result of this modified
adsorption geometry, the carbonyl stretching
mode is red-shifted ∼150 cm-1 on the monolayer
Pd(111)/Mo(110) surface, unshifted on the
multilayer
Pd(111)/Mo(110) surface, and blue-shifted by ∼100
cm-1 on the Mo(110) surface.