(691e) Differences in Reactivity of Hydrogen in/on Early Transition-Metal Nitrides/Carbides for Hydrogenation Reactions

Hydrogenation
and dehydrogenation reactions are critical steps in the production of a variety
of important chemicals and fuels. Research described in this paper investigated
the use of early transition-metal nitrides and carbides as catalysts for
selective hydrogenation reactions with a focus on understanding the chemistry
and reactivity of hydrogen. These materials have been reported to have
properties similar to platinum-group metals [1] and are interstitial compounds,
providing the potential for hydrogen population of both surface and subsurface sites.
It was hypothesized that hydrogens located on versus in
the metal nitrides and carbides will have differing reactivities,
similar to previous reports for the reactivity of surface versus bulk hydrogen
in and on Ni [2] and Pd [3]. In this work, the nitride and carbide catalysts are
used in the liquid phase hydrogenation of crotonaldehyde
(CH₃CH=CHCHO); the selectivity for C=O and C=C hydrogenation was
probed.

Our thermal desorption
spectroscopic experiments indicate that early transition-metal nitrides and carbides
(Mo, W, V, Nb, Ti-based) have different binding sites
for hydrogen. For some of the nitrides (e.g. Mo₂N, W₂N,
VN) two distinct sites are populated following reduction in H₂ (Figure
1). Conversely, carbides were found to have predominantly one type of site. Results
from thermal desorption spectroscopy and theoretical calculations suggest that the
two types of sites for Mo₂N arise from a surface site (low
temperature site, "site A") with hydrogen bound to terminal surface N atoms and
a subsurface site (high temperature site, "site B") in which hydrogen
intercalates into octahedral sites. These sites were populated to directly evaluate
the reactivity of each hydrogen for hydrogenation of crotonaldehyde. The relative density of hydrogen in the
subsurface site (site B) correlated with the selective hydrogenation of crotonaldehyde
to crotyl alcohol achieving nearly 100% selectivity. Decreasing selectivity to butanal and butanol products was observed with increasing
site B density. The observed trend suggests that site B is chemoselective
to C=O bond hydrogenation over hydrogenation of the C=C bond.

Ongoing work includes the
extension of these analyses to new hydrogenation model substrates with aromatic
C=C or -NO₂ functionality. 
Further, these materials are being explored for dehydrogenation of model
compounds such as N-methylpyrrolidine.

References

1.    
R. B. Levy, M.
Boudart, Science, 181, 547 (1973).

2.    
S.P. Daley, A.L. Utz,
T.R. Trautman, S.T. Ceyer, J. Am. Chem.
Soc., 116, 6001 (1994).

3.    
M. Shirai, Y. Pu,
M. Arai, Y. Nishiyama, Appl. Surf. Sci.,
126, 99 (1998).

4.    
M.K. Neylon, S.
Choi, H. Kwon, K.E. Curry, L.T. Thompson, Appl.
Cat. A: General, 183 253 (1999).

Figure  SEQ Figure \* ARABIC 1. Thermal
desorption spectra for various, early transition-

metal nitride catalysts following exposure to 10% H₂/Ar at 500°C

for 0.5hr.  Heating rate (β)
was set to 15 K min^-1.