The electronic influence on the catalytic properties in the semihydrogenation of acetylene is addressed by applying unsupported intermetallic materials from the solid solution Ga 1−x Sn x Pd 2 (0 ≤ x ≤ 1). Due to the only marginal changes in the crystal structure, the whole series shows excellent selectivity to ethylene (∼85%) comparable with the binary GaPd 2 . Moreover, the incremental addition of up to one electron per formula unit reveals a maximum in the specific catalytic activity around the nominal composition with x = 0.28 corresponding to 4.0 atom % tin in the near-surface region, as revealed by operando X-ray photoelectron spectroscopy measurements. The study shows a reliable and quantitative relation between the d-band shift and catalytic activity, revealing once more the potential of intermetallic compounds in heterogeneous catalysis.
Efficient
development of catalytic materials requires knowledge
of the decisive parameters defining the catalytic properties. In multicomponent
metallic catalysts, these are categorized as electronic and geometric
effects, yet they are strongly interrelated. A systematic disentanglement
can be achieved by fixing one parameter while altering the other,
which becomes possible through the substitution in isostructural intermetallic
compounds. This approach enables the evaluation of electronic or geometric
contributions both individually and combined. Herein, this is achieved
by substitution of indium (three valence electrons) with tin (four
valence electrons) in the series In1–x
Sn
x
Pd2, which allows
for a systematic variation of the total number of electrons per unit
cell with only a minor variation of the unit cell parameters and thus
the evaluation of the electronic effect. Geometric effects were evaluated
by substitution of indium with gallium in the Ga1–x
In
x
Pd2 series,
which allows for a systematic variation of the interatomic distances
while maintaining the same number of valence electrons per unit cell
and close atomic coordinates. By substituting gallium with tin in
the Ga1–x
Sn
x
Pd2 series, both effects are combined and addressed
simultaneously. The activity enhancement of the methanol oxidation
reaction on the Ga1–x
Sn
x
Pd2 series is attributed to the synergy
of the combined effects.
The new ternary compound Ga 2+x+y Sn 4-x Pd 9 (x = 0.72, y = 0.06) was synthesized by arc melting of the elements. The crystal structure was solved from the single-crystal X-ray diffraction data [space group Fd3m, a = 12.4048(9) Å, Z = 8, 222 reflections, R F = 0.035]. The crystal structure of Ga 2+x+y Sn 4-x Pd 9 is related to the Ti 2 Ni type and its filled derivatives (η-phases) with additional Pd atoms at octahedrally and tetrahedrally coordinated sites. The electronic density of states of Ga 2+x+y Sn 4-x Pd 9 is similar to those of the binary compounds GaPd and GaPd 2 , [a]
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