Adsorption and Thermal Decomposition of Methanol on the (100) and (110) Surfaces of Nickel and Iron Aluminides

The adsorption and thermal decomposition of methanol on the (100) and (110) surfaces of nickel and iron aluminides (Ni_{50}Al_ {50} and Fe_{54} Al_{46}) in the ultrahigh vacuum environment have been investigated with temperature programmed desorption, high-resolution electron energy loss spectroscopy, X-ray and ultraviolet photoelectron spectroscopies, and work function measurements. The surface reactivity of NiAl and FeAl toward methanol varies dramatically from one face to the other, exhibiting a strong dependence on the atomic composition of the surface layer. Methanol chemisorption at 120 K is determined to be "primarily dissociative" with the scission of O-H bond on FeAl(110) but "primarily associative" on FeAl(100), NiAl(110), and NiAl(100). Chemisorbed methanol transforms into surface methoxy in the temperature interval of 120 and 200 K on both NiAl(100) and NiAl(110) but of 200 and 270 K on FeAl(100). The primary gaseous products (CH_4 , CH_3, and H_2 ) result from breaking the C-O and C-H bonds in adsorbed methoxy on all surfaces, with the exception of NiAl(110) where a substantial amount of CO product evolves as well. Compared to the Al-terminated NiAl(100) surface, the production of both CH_4 and CO on NiAl(110) is correlated to the presence of Ni atoms in the surface layer. The C-O bond of a fraction of surface methoxy is cleaved below 350 K on NiAl(110), in contrast to monometallic Al and Ni, exemplifying the unique reactivity of the alloy surfaces. On the Al-terminated (100) surfaces of NiAl and FeAl, methanol exhibits similar decomposition channels while its dissociation occurs in different temperature ranges. The differences in surface reactivity between NiAl(100) and FeAl(100) are due to the variant electronic perturbations to the surface aluminum atoms via bimetallic bonding with the underneath transition metal atoms at the second layers of individual surfaces. It is found that the surface reactivity of NiAl toward methanol has no resemblance to that of Cu, in spite of the similar, filled d-bands of both substrates. The relevance of the NiAl alloy to the Ni/Al_2O_3 catalyst is demonstrated as well.