D. Wang, Y. Du, X. Wang et al.
Chinese Chemical Letters 32 (2021) 21–24
Fig. 2. (a) XRD patterns and (b) Raman spectra of all these samples.
lamellar structure with inadequate growth and dense distribution,
and its specific surface area has been improved to some extent. For
MnOx-NiCo2O4-2:2 sample (Fig.1c), lamellae are well developed in
vertical direction and the spaces between the lamellae are
abundant. Naturally, it has the largest specific surface area and
the widest pore distribution of these monolithic catalysts,
moreover, its specific surface area of the active phase component
is up to 201.5 m2/g. According to Fig. 1d, the growth direction of
MnOx-NiCo2O4-1:3 sample is chaotic, which is manifested as
three-dimensional disordered stacking of lamellae structures. The
SEM element distribution mapping of MnOx-NiCo2O4-2:2 sample
is shown in Fig. 1g, the uniform distribution of the four elements
indicates that nickel-cobalt spinel grows indiscriminatingly on the
surface of manganese oxide. In order to evaluate the actual
element mole ratios of these multi-stage catalysts, ICP results were
shown in Table S1 (Supporting information). The actual ratio of
manganese and cobalt is basically the same as the theoretical
value, while the higher nickel content may be due to the partial
stripping of the carrier (nickel foam).
The XRD patterns of these monolithic catalysts were shown in
Fig. 2. Unfortunately, due to the extremely low loading of the active
phase (5% ꢃ 0.2%), only the diffraction peaks of nickel foam were
detected in the XRD patterns. According to the previous report
(PDF 01ꢁ070-0989), diffraction peaks at 44.6ꢀ, 52.0ꢀ and 76.6ꢀ can
be attributed to (111), (200) and (220) crystal faces of metallic
nickel, respectively [11]. Raman spectrum is also a powerful tool
for determining the phase structure on the premise of defining
elements, especially some metallic oxides with low crystallinity,
therefore, Raman spectra of these monolithic catalysts were
detected and shown in Fig. 2 [12–14]. The characteristic peaks of
spinel structures were detected in single nickel-cobalt spinel
sample, 658 cmꢁ1 peak can be attributed to cobalt-oxygen bond
vibrations at octahedron sites (CoO6) and 188 cmꢁ1 peak can be
attributed to cobalt-oxygen bond vibrations at tetrahedron sites
(CoO4) [15]. The monolithic catalyst of single manganese oxide
shows unique characteristic peak at 510 cmꢁ1, which is the
deformation modes of Mn-O-Mn chains in the MnO2 phase [16].
For MnOx-NiCo2O4-3:1 catalyst, a wide peak which can be
attributed to the stretching vibration of Mn-O bond appeared at
580 cmꢁ1, and no characteristic peak of nickel-cobalt spinel was
detected, which indicated that its growth was not sufficient, which
was consistent with SEM results [17,18]. Of course, both manga-
nese oxide and nickel-cobalt spinel characteristic peaks were
detected in MnOx-NiCo2O4-2:2 and MnOx-NiCo2O4-1:3 samples. It
is worth mentioning that the nickel-cobalt spinel characteristic
peak of MnOx-NiCo2O4-2:2 catalyst has an obvious red shift,
indicating that the cobalt-oxygen bond weakens, which is very
conducive to the transformation of cobalt ions between bivalent
and trivalent, thus accelerating the catalytic oxidation process.
Fig. 3a exhibits the H2-TPR patterns of these monolithic
catalysts, which may clarify the role of manganese oxide and
Fig. 3. (a) H2-TPR patterns, (b) benzene conversions over prepared catalysts at
12,000 hꢁ1. (c) Schematic diagram of reaction mechanism.
nickel-cobalt spinel loading ratio for catalysts reducibility. The
oxidation of hydrogen continued in single manganese oxide
monolithic catalyst over a wide temperature range, which reflects
a low crystallinity and/or rich defects characteristic of MnOx active
phase. However, single nickel-cobalt spinel monolithic catalyst has
two characteristic peaks with an area ratio of approximately 1:3,
which also conforms to the reduction process of spinel material,
namely the phase transition from NiCo2O4 to NiCoO2, and from
NiCoO2 to metal phase respectively [19]. It is worth mentioning
that MnOx-NiCo2O4-2:2 sample shows the first characteristic peak
at the lowest temperature of 283 ꢀC, which can be attributed to the
proper ratio of manganese oxide and nickel-cobalt spinel phase
resulting in a rich phase interface. In Fig. 3b, 100 ppm benzene was
selected to evaluate the VOCs removal capability of the monolithic
catalyst at a gaseous hourly space velocity (GHSV) of 12,000 hꢁ1
.
The benzene catalytic activity of the prepared catalysts is basically
consistent with H2-TPR reducibility. MnOx-NiCo2O4-2:2 sample
completely catalyzed 50% and 90% of benzene to H2O and CO2 at
176 ꢀC and 198 ꢀC, respectively. As can be seen from Fig. S2
(Supporting information), MnOx-NiCo2O4-2:2 sample possesses
excellent long-term stability, degradation ratio of benzene was still
as high as 95% after 48 h test. The carbon balance of all the samples
was greater than 96% during the reaction above T90 temperature,
indicating that the by-products could be ignored. A detailed
analysis of the reactivity sequence of MnOx-NiCo2O4 catalysts for
T
10, T50 and T90 were exhibited in Supporting information.
Furthermore, we have calculated the reaction rate of each sample
at 150 ꢀC (benzene conversion of all the samples is between
5%–20%), 198 ꢀC (T90 of MnOx-NiCo2O4-2:2 sample) and drawn the
Arrhenius plots for the oxidation of benzene over these catalysts,
all these results were presented in Table S1 and Fig. 1. In
combination with the structural analysis and catalytic perfor-
mance results, a schematic diagram of benzene reaction mecha-
nism with the participation of MnOx-NiCo2O4 monolithic catalyst
is presented in Fig. 3c. It is well known that active oxygen species
play an important role in thermocatalytic VOCs removal, and its
content directly affects the reaction rate of benzene combustion
[9,20]. In addition, compared with bulk phase, the phase interface
23
Wang, Dongdong
