3 Catalysts via Atomic Layer Deposition
83
interaction between Ru and Pt in the alloy nanoparticles was confirmed by the Ru
K-edge X-ray absorption spectroscopy.
Co/Ni catalysts were fabricated via ALD for the dry reforming of methane reaction (DRM) with enhanced coking resistance and activity as schematically shown in
Fig. 3.6f [77]. Co coating layer on Ni NPs exhibited a meshed-like structure confirmed
with CO chemisorption FTIR measurements (Fig. 3.6g). This structure was realized
by a post-reduction treatment after CoO x ALD on Ni. The meshed coating structure enabled the formation of Co–Ni interfaces and simultaneously exposed the Ni
surface, which was beneficial to break C–H bonds and enhance CO 2 activation.
Furthermore, the coating layer with meshed-like configuration partitioned the Ni
surface that suppressed continuous carbon tube formation and enhanced coking resistance. The coking amount at 650 °C could be reduced to 2.9%. The Co/Ni catalyst
exhibited sintering resistance and good durability (Fig. 3.6h).
3.3 Metal Oxide-Modified Catalysts
Besides metal ALD processes, ALD of metal oxides has demonstrated a great potential in fabricating catalysts with enhanced activity, selectivity, and stability. The
oxides can also modulate the electronic properties and morphology of active metal
sites via interfacial electron donation and metal oxide interaction. Some oxides also
act as active component in catalytic reactions. Thus, oxide coatings can provide
additional opportunities to further promote the catalytic performance. The coating
structures of oxide on metal nanoparticle also improve the catalytic stability with
confinement effect against sintering and leaching under harsh reaction conditions.
3.3.1 ALD Recipes of Metal Oxides
The atomic-level precision control over oxide film thickness with high uniformity
on high-surface-area materials makes ALD an ideal tool in metal oxide composite
catalysts engineering. We have summarized the oxides that can be grown by ALD
reported in literatures as listed in Fig. 3.7, most of which have been used as catalytic
components to modulate the catalytic performances in terms of activity, selectivity,
and stability (Table 3.2). In the following paragraphs, we introduce several metal
oxides grown by ALD in detail according to their specific applications.
Al 2 O 3
Al 2 O 3 ALD using precursors of trimethylaluminum (TMA) and H 2 O is one of the
most commonly used ALD procedures and has been thoroughly investigated. The
mechanism for Al 2 O 3 ALD on oxide surfaces is well understood: In the first half
reaction, TMA reacts with hydroxyl groups on the substrate to form Al(CH 3 ) x *
(x = 1–2) surface species and CH 4 gaseous product; in the second half reaction,
83
interaction between Ru and Pt in the alloy nanoparticles was confirmed by the Ru
K-edge X-ray absorption spectroscopy.
Co/Ni catalysts were fabricated via ALD for the dry reforming of methane reaction (DRM) with enhanced coking resistance and activity as schematically shown in
Fig. 3.6f [77]. Co coating layer on Ni NPs exhibited a meshed-like structure confirmed
with CO chemisorption FTIR measurements (Fig. 3.6g). This structure was realized
by a post-reduction treatment after CoO x ALD on Ni. The meshed coating structure enabled the formation of Co–Ni interfaces and simultaneously exposed the Ni
surface, which was beneficial to break C–H bonds and enhance CO 2 activation.
Furthermore, the coating layer with meshed-like configuration partitioned the Ni
surface that suppressed continuous carbon tube formation and enhanced coking resistance. The coking amount at 650 °C could be reduced to 2.9%. The Co/Ni catalyst
exhibited sintering resistance and good durability (Fig. 3.6h).
3.3 Metal Oxide-Modified Catalysts
Besides metal ALD processes, ALD of metal oxides has demonstrated a great potential in fabricating catalysts with enhanced activity, selectivity, and stability. The
oxides can also modulate the electronic properties and morphology of active metal
sites via interfacial electron donation and metal oxide interaction. Some oxides also
act as active component in catalytic reactions. Thus, oxide coatings can provide
additional opportunities to further promote the catalytic performance. The coating
structures of oxide on metal nanoparticle also improve the catalytic stability with
confinement effect against sintering and leaching under harsh reaction conditions.
3.3.1 ALD Recipes of Metal Oxides
The atomic-level precision control over oxide film thickness with high uniformity
on high-surface-area materials makes ALD an ideal tool in metal oxide composite
catalysts engineering. We have summarized the oxides that can be grown by ALD
reported in literatures as listed in Fig. 3.7, most of which have been used as catalytic
components to modulate the catalytic performances in terms of activity, selectivity,
and stability (Table 3.2). In the following paragraphs, we introduce several metal
oxides grown by ALD in detail according to their specific applications.
Al 2 O 3
Al 2 O 3 ALD using precursors of trimethylaluminum (TMA) and H 2 O is one of the
most commonly used ALD procedures and has been thoroughly investigated. The
mechanism for Al 2 O 3 ALD on oxide surfaces is well understood: In the first half
reaction, TMA reacts with hydroxyl groups on the substrate to form Al(CH 3 ) x *
(x = 1–2) surface species and CH 4 gaseous product; in the second half reaction,