(734g) Structure Sensitive Phenol Hydrogenation on Pd Nanostructures

Structure Sensitive Phenol Hydrogenation on Pd
Nanostructures

Shelaka
Gupta^a, Sreedhala.S^b, Tuhin Suvra Khan^a,
C.P.Vinod^b and M. Ali Haider^a

^bCatalysis Division,
CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune - 411 008

^aRenewable
Energy and Chemicals Laboratory, Department of Chemical Engineering,

Indian Institute
of Technology Delhi, Hauz Khas, New Delhi-110016

Tel: +91-11-26591016, Fax:
+91-11-2658-2037
haider@iitd.ac.in

Phenol
hydrogenation on unsupported Pd nanostructures showed structure sensitivity
with respect to particle shape and insensitivity with respect to particle size.
The hydrogenation reaction was performed over Pd octadehra (~27 nm, Figure 1(a)),
­­cubes (~25 nm, Figure 1(b)), and spheres (~6nm). While Pd cubes (exposing
only the (100) facets) were observed to selectively (100%) produce
cyclohexanone, octahedra (exposing only the (111) facets) were 100% selective
towards cyclohexanol. Interestingly, Pd spheres of varying size (<5 nm tp 6
nm) and exposing both (111) and (100) facets were observed to produce
cyclohexanone only. Density functional theory (DFT) calculations showed that
the difference in the binding modes of phenol on Pd(111) (Eb = -102 kJ/mol) and
Pd(100) (Eb = -135 kJ/mol), primarily lead to the difference in the selectivity
(Figure 1(a) and 1(b)). On Pd(111), cyclohexanol was produced by phenol
dissociation to phenoxy and subsequent hydrogenation (Figure 1(a)), whereas on
Pd(100) cyclohexanone was produced by direct phenol hydrogenation (Figure 1(b)).
Further hydrogenation of cyclohexanone to produce cyclohexanol on Pd(100) was
observed to be prohibitive (E a = 125 kJ/mol) at reaction conditions.
Interestingly, a mixture of cube and octahedra with 40:60 ratio, resembling a
spherical supported Pd nanoparticle was observed to yield cyclohexanone with
100% selectivity even on increasing the phenol weight in the reaction from 50
mg to 450 mg. However, a small fraction (8% selectivity) of cyclohexanol was
detected when phenol concentration was raised to 600 mg. The shift in
selectivity was attributed to the reduction in the adsorption energy of phenol
on the (100) facets (Eb = -97 kJ/mol at 0.6 ML coverage) which became
comparable to the (111) facet due to increased lateral interactions at high
phenol coverage (Figure 1(c)). The reaction however remained structural
insensitivity on varying the size of the Pd sphere

Figure
1: Reaction energy diagram for phenol hydrogenation on a) Pd (111) and b) Pd
(100) surface. c) Trend in calculated binding energy (E_b)
of phenol and d-band centre (ɛ_d) of Pd (100) versus phenol
coverage (shaded area shows the region where selectivity is likely to shift
towards cyclohexanol).