The Catalyst Review November 2019 - 16

EXPERIMENTAL
electron
microscope
(HAADF-STEM),
as shown in
Figure 1b. The
energy-dispersive
spectrum
elemental
mappings
(Figure1c-f)
and Inductively
Coupled Plasma
(ICP) results
confirm the
existence of
iridium in the
obtained Ir-SAC,
suggesting an
atomic-level Ir
embedment within
the carbon matrix.

Figure 1. a) Scheme showing host-guest strategy for the
fabrication of single atomic site catalysts; b-f) STEM images
and the corresponding elemental mappings for the Ir-SAC;
g) High-resolution HAADF-STEM image of Ir-SAC, with the
distinct bright dots (circled in red) indicating Ir is atomically
dispersed on the nitrogen doped carbon matrix.

Figure 2. a) Cyclic voltammograms with a scanning rate of 50 mVs-1 in
0.1m HClO4 solution; b) ORR polarization curves with a scanning rate of 5
mVs-1 at rotating speed of 900 rpm for the synthesized catalysts; c) Tafel
slopes for these catalysts; d) LSV curves of Ir-SAC with various rotation
rates; e) Mass activity comparison between Ir-SAC and Pt/C-TKK; f) TOF
and E1/2 values of Ir-SAC and other recently reported SACs, where the TOF
values were estimated at the potential of 0.85 V for Ir-SAC and 0.8 V for
the other catalysts.

In order to verify the potential electrochemical improvement, the
ORR performance of the Ir-SAC was evaluated using the rotating disk
electrode (RDE) and rotating ring disk electrode (RRDE) techniques
in 0.1m HClO4 solution in comparison with Ir/N-C, Ir/C, Ir/C-Com and
commercial Pt/C (Pt/C-TKK) catalysts (Figure 2). In accordance with
theoretical predictions, the as-developed Ir-SAC exhibits orders of magnitude higher ORR activity than iridium nanoparticles with a
record-high turnover frequency (TOF) of 24.3 e- site-1 s-1 at 0.85 V vs. RHE) and an impressive mass activity of 12.2 A mg-1Ir, which far
outperforms the previously reported SACs and commercial Pt/C. Atomic structural characterizations and density functional theory
calculations reveal that the high activity of Ir-SAC is attributed to the moderate adsorption energy of reaction intermediates on the
mononuclear iridium ion coordinated with four nitrogen atom sites. Source: Xiao M, Zhu J, Li G, et al. (2019). Angew. Chem. Int. Ed.,
58: 9640-9645.
Evolution of Zn Species on Zn/HZSM-5 Catalyst under H2 Pretreated and its Effect on Ethylene
Aromatization...
Zn-containing HZSM-5 zeolites (Zn/HZSM-5) show excellent performance in catalyzing the conversion of methanol, light alkanes, and
olefins to produce aromatic hydrocarbons. However, Zn species in ZSM-5 zeolites are usually highly dispersed, strongly associated
with protons, and are easily transformed during the reaction, which increases the complexity and difficulty of clarifying the role
and state of Zn species. Herein, the authors set out to study the effect of H2 treatment on the structural change of catalysts and
transformation behavior of Zn species, to better understand the structure-property relationships and rationalize the design of Zn/
HZSM-5 catalysts.
Zn-containing ZSM-5, prepared by impregnation, was treated in an H2 atmosphere at different temperatures and the evolution in
structure, acidity, and Zn species distribution were then studied. The results revealed the presence and transformation of ZnO,
ZnOH+, and Zn+ species during thermal treatment with H2. In the H2-enriched atmosphere at elevated temperature, the location
of Zn species was redistributed. A certain amount of Zn species was reconstructed at the outside surface below 300°C, and the
formation of Brønsted acid sites (BAS) occurred at the intersection of ZnO and the zeolite. The binuclear ZnOZn was subjected to
a reducing atmosphere, starting above 300 °C. In this process, the formation of Zn+ occurred on the Zn0 and BAS of the zeolite,
which accelerated the formation of ZnOH+ species and resulted in the maximum ZnOH+ amount at 300°C. The amount of ZnOH+
species in the Zn/HZSM-5 zeolites, on the other hand, is decisive for the selectivity for aromatics formed from ethylene. Therefore,
the selectivity for aromatics in ethylene aromatization is promoted on the Zn/HZ5-300-R catalysts. When correlating the amount
of ZnOH+ species with the selectivity for aromatics, a linear relation was observed (Figure 1), suggesting that the amount of ZnOH+
species might be readily adjusted by H2 treatment at different temperatures. At low temperatures, ZnO cluster confined on the
surface of zeolite is the main Zn species, as proved by UV-vis DRS and XPS. With an increase in the treatment temperature, the
16

The Catalyst Review											

November 2019



The Catalyst Review November 2019

Table of Contents for the Digital Edition of The Catalyst Review November 2019

The Catalyst Review November 2019 - cover
The Catalyst Review November 2019 - contents
The Catalyst Review November 2019 - 1
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