Identifying the Optimal Pd Ensemble Size in Dilute PdAu Alloy Nanomaterials for Benzaldehyde Hydrogenation

Unraveling metal nuclearity effects is central for active
site
identification and the development of high-performance heterogeneous
catalysts. Herein, a platform of nanostructured palladium (Pd) in
gold (Au) dilute alloy nanoparticles supported on raspberry-colloid-templated
(RCT) silica was employed to systematically assess the impact of the
Pd ensemble size for the low-nuclearity regime in the Au surface layer,
from single atoms to clusters, on the catalytic performance in the
liquid-phase hydrogenation of benzaldehyde to benzyl alcohol. Combining
catalyst evaluation, detailed characterization, and mechanistic studies
based on density functional theory, we show that Pd single atoms in
the Au surface plane (corresponding to samples with 4 atom % Pd in
Au) are virtually inactive in this reaction and benzyl alcohol production
is optimal over small Pd clusters (corresponding to samples with 10–12
atom % Pd in Au) due to superior benzaldehyde adsorption and transition
state stabilization for the C–H bond formation step. For larger
Pd ensembles (samples with ≥10 atom % Pd in Au), C–O
bond hydrogenolysis occurs, promoting toluene formation and decreasing
the selectivity toward benzyl alcohol, in line with a relatively lowered
C–O bond cleavage barrier. Nevertheless, the nanostructured
bimetallic Pd13Au87/SiO2-RCT catalyst
still outperforms monometallic Pd counterparts in terms of selectivity
for benzyl alcohol over toluene at comparable conversion and rate.
Furthermore, the stability is improved compared to pure Pd nanoparticles
due to inhibited particle agglomeration in the RCT silica matrix.