(778a) Hydrodesulfurization On Re Metal and Sulfide Catalysts: Effects of Catalyst Structure and Metal-Sulfur Binding Energy On Reactivity

Hydrodesulfurization on Re Metal and
Sulfide Catalysts:

Effects of Catalyst Structure and
Metal-Sulfur Binding Energy on Reactivity

Edwin Yik^a, Huamin
Wang^b, and Enrique Iglesia^a,*

^aDepartment of Chemical and Biomolecular Engineering,
University of California, Berkeley, CA 94720

^b Pacific Northwest National Laboratory, Energy and
Efficiency Division, Richland, WA 99352

*iglesia@berkeley.edu

Recent
mechanistic studies of thiophene hydrodesulfurization (HDS) [1,2] on silica-supported
Ru and Pt metal clusters, which avoid the structural anisotropy of layered metal
sulfides encountered in commercial Co(Ni)Mo(W) sulfides [3], and on the effects
of their cluster size have shown that HDS turnover rates are lower on surfaces
with higher S-metal binding energies, due to a lower availability of vacant
sites required for reaction at a given sulfur chemical potential, which is set
by varying H₂S:H₂ ratios [4]. We extend those studies to
silica-supported Re metal clusters and layered Re sulfides (ReS_x) and
observe that, despite differences in the catalyst identity and structure, HDS
of thiophene occurs via elementary steps that are similar to those prevalent on
Ru and Pt.  Therefore, the differences in
turnover rates on these materials may be attributed to the inherent properties
of the catalysts rather than to a change in the reaction mechanism. Reactivity
decreases from Pt to Ru to Re metal clusters, consistent with an increase in
the S-metal bond strength [5] and consequent decrease in the density of empty
sites as a result of the higher S-coverage.

HDS
turnover rates on ReS_x/SiO₂ samples were roughly 100
times higher than those on Re/SiO₂, even when the former were
normalized by the total number of Re atoms (because transmission electron
microscopy (TEM) images showed well-dispersed lamellar features) and the latter
by its surface atoms. Both samples were synthesized from different treatments
of a common ReO_x/SiO₂ precursor and remained kinetically
stable during the course of the experiments (1-3 MPa of H₂, 1-10 kPa
of thiophene, and 0.2-3.0 kPa of H₂S) over a period of more than 100
h. X-ray absorption (XAS), TEM, and S-contents (obtained through temperature
programmed reduction (TPR) in H₂) suggested that Re metal retained
its metallic bulk during HDS and exhibited an amount of sulfur corresponding to
near-monolayer surface coverages after HDS. TEM images also showed that Re
clusters did not form lamellar ReS_x structures and that ReS_x
samples did not form isotropic Re clusters during HDS, further indicating an
absence of interconversion between the two phases despite their exposure to
identical experimental conditions. The marked differences in reactivity on
surfaces of ReS_x and on those of S-covered Re metal likely reflect weaker
S-Re bond strengths in the former [4], which lead to a concomitant increase in
S-vacancy surface concentrations. In addition, the structural buckling that may
exist in ReS_x could allow higher accessibility to the vacant sites
on which the reaction may proceed. Thus, the inhibition of turnover rates as a result
of increasing H₂S:H₂ ratios is more pronounced on Re/SiO₂
than on ReS_x/SiO₂.

References

1.     
Wang, H., and
Iglesia, E. J. Catal. 270, 245
(2010).

2.     
Wang, H., and
Iglesia, E. ChemCatChem. 3, 1166
(2011).

3.     
Topsøe, H.,
Clausen, B.S., and Massoth F.E., in ?Catalysis Science and Technology? (J.R.
Anderson and M. Boudart, eds.) Vol. 11. Springer-Verlag, New York, 1996.

4.     
Bartholomew,
C.H., Agrawal, P.K., and Katzer, J.R. Adv.
in Catalysis. 31, 135 (1982).

5.     
Alfonso, D.R. J. Phys. Chem. C. 115, 17077 (2011).