Facile synthesis of spinel Cu1.5Mn1.5O4 microspheres with high activity for the catalytic combustion of diesel soot

Article abstract of DOI:10.1039/c7ra01971h

A series of Cu-Mn mixed oxides were prepared via a facile co-precipitation method and used as catalysts for diesel soot combustion in NO_x/O₂/N₂. It is found that the chemical composition in the Cu-Mn mixed oxides has a significant influence on both morphology and catalytic activity. When the Cu/Mn atomic ratio was optimized to be 1, a distinctive pure spinel phase of Cu_1.5Mn_1.5O₄ was obtained (named Cu1Mn1), which exhibited superior catalytic activity (e.g., finishing combustion temperature (T_f) = 360 °C, in loose contact mode). The excellent catalytic activity of Cu1Mn1 was mainly attributed to the following aspects: (1) distinct morphological features: the well-dispersed Cu1Mn1 microspheres with a rough surface could contact soot particles sufficiently, and the stacked pores between the loosely packed nanoparticles within the microspheres could facilitate the diffusion of gaseous O₂, NO and NO₂; and (2) the high intrinsic activity of the Cu_1.5Mn_1.5O₄ phase: the abundant adsorbed oxygen species (O_ads) is beneficial to the direct oxidation of soot into CO₂, and the enormous Cu⁺-Mn^λ+ (λ = 3, 4, hereinafter inclusive) cation pairs could facilitate the production of the strong oxidant NO₂, and the interaction between Cu²⁺ and NO₂ would lead to high enhancement efficiency of NO₂ for soot combustion. This facile strategy for the high activity spinel phase towards catalytic soot combustion shows great promise for practical applications.

Full text of DOI:10.1039/c7ra01971h

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Facile synthesis of spinel Cu1.5Mn1.5O4
microspheres with high activity for the catalytic
Cite this: RSC Adv., 2017, 7, 20451
combustion of diesel soot†
ac
H. Zhao,ab X. X. Zhou,a L. Y. Pan,ab M. Wang,ab H. R. Chen
and J. L. Shiac
*
A series of Cu–Mn mixed oxides were prepared via a facile co-precipitation method and used as catalysts
for diesel soot combustion in NOx/O2/N2. It is found that the chemical composition in the Cu–Mn mixed
oxides has a significant influence on both morphology and catalytic activity. When the Cu/Mn atomic
ratio was optimized to be 1, a distinctive pure spinel phase of Cu1.5Mn1.5O4 was obtained (named
Cu1Mn1), which exhibited superior catalytic activity (e.g., finishing combustion temperature (Tf) ¼ 360 ꢀC,
in loose contact mode). The excellent catalytic activity of Cu1Mn1 was mainly attributed to the following
aspects: (1) distinct morphological features: the well-dispersed Cu1Mn1 microspheres with a rough
surface could contact soot particles sufficiently, and the stacked pores between the loosely packed
nanoparticles within the microspheres could facilitate the diffusion of gaseous O2, NO and NO2; and (2)
the high intrinsic activity of the Cu1.5Mn1.5O4 phase: the abundant adsorbed oxygen species (Oads) is
beneficial to the direct oxidation of soot into CO2, and the enormous Cu+–Mnl+ (l ¼ 3, 4, hereinafter
inclusive) cation pairs could facilitate the production of the strong oxidant NO2, and the interaction
between Cu2+ and NO2 would lead to high enhancement efficiency of NO2 for soot combustion. This
facile strategy for the high activity spinel phase towards catalytic soot combustion shows great promise
for practical applications.
Received 16th February 2017
Accepted 4th April 2017
DOI: 10.1039/c7ra01971h
rsc.li/rsc-advances
Hence, reforming of DPF with a catalyst coating has been
proposed to reduce the ignition temperature of the trapped
Introduction
soot. Many kinds of catalysts have been proposed and investi-
gated, e.g. the noble metals,5 transition metal oxides,6 perov-
skite and/or perovskite-like mixed oxides,7 spinel mixed oxides,8
alkaline/alkaline-earth metal oxides,9 ceria-based oxides,10 etc.
The diesel particulate lter (DPF) has been developed for many
years as an effective device to trap over 90% of diesel soot, which
would otherwise lead to a rise in PM2.5 and thus serious health
problems in human beings.1,2 DPF with trapped soot is usually
regenerated in a positive way, such as fuel post-injection, which
is applied to increase the exhaust temperature (usually below
400 ꢀC) up to the soot ignition temperature (ꢁ600 ꢀC).3,4 But
there remain several limitations for industrial application, such
as fuel overconsumption and thermal damage to the lter.
Thereinto, noble metal-based catalysts with
a
three-
dimensional ordered macroporous (3DOM) support11 or
a cordierite support12 are among the best candidates with
outstanding catalytic performance, which can decrease the Tf of
soot combustion to as low as 400 ꢀC. Nevertheless, the high cost
and sulfur sensitivity of noble metal,13,14 would impose a severe
restriction on its industrial application.
aState Key Laboratory of High Performance Ceramics and Superne Microstructure,
Shanghai Institute of Ceramics Chinese Academy of Sciences, 1295 Ding-Xi Road,
Shanghai 200050, P. R. China. E-mail: hrchen@mail.sic.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing, 100049, P. R. China
cJiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),
Nanjing, P. R. China
As one of the promising substitutes for noble metal based
catalysts, copper manganese spinel-type mixed oxides have been
reported to be well suitable for various catalytic processes, e.g.,
total oxidation of volatile organic compounds,15 steam reform-
ing of methanol,16 water gas shi reaction,17 and low tempera-
ture reduction of NO,18 demonstrating versatile application
potential. The physicochemical properties of these spinel-type
compounds mainly depend on the type, charge and distribu-
tion in tetrahedral/octahedral sites of the cations.19 Spinels can
be divided into stoichiometric (AB2O4) and non-stoichiometric
spinels (AxB3ꢂxO4) types, where A, B stand for two kinds of
cations, and the subscripts are the atomic ratio of A, B and O
atom. The ion distribution in AxB3ꢂxO4 is rather complex, e.g.,
† Electronic supplementary information (ESI) available: Characteristic
temperatures (Ti, Tm and Tf) and selectivity to CO2 for catalytic soot combustion
over all studied catalysts and non-catalyzed reference; N2 adsorption/desorption
isotherms & pore size distributions of all studied catalysts; specic surface
areas, pore volumes and average pore diameters of all studied catalysts; SEM
image and corresponding EDS spectrum of Cu1Mn2; SEM image and
corresponding EDS spectrum of Cu1Mn1; STEM image of Cu1Mn1; catalytic
soot oxidation in O2/N2 under loose contact mode over all studied catalysts;
catalytic NO oxidation over CuO, Cu1Mn1 and MnOd; catalytic soot combustion
over CuO, Cu1Mn1 and MnOd with or without NO. See DOI: 10.1039/c7ra01971h
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CuxMn3ꢂxO4 with four kinds of cations, shows a conguration 500 C for 2 h with a heating rate of 1 ꢀC minꢂ1. For ease of
ꢀ
3+
of [Cua+Mnb2+Cuc2+Mn1ꢂaꢂbꢂc3+]a[Cuxꢂaꢂc2+Mn3ꢂ2xꢂb+c
reference, the prepared catalysts are designated CuxMny, where
Mnx+a+bꢂ14+]bO4, where the subscripts except a and b are for the x : y stands for the Cu : Mn atomic ratio in the recipe. In addi-
atomic ratios of the ions in specic sites.20 When x in Cux- tion, single oxide CuO or MnOd for reference was also prepared
Mn3ꢂxO4 is adjusted, different structural, electrical or magnetic with the same synthesis procedure in the absence of Mn(NO3)2
properties can be realized. For example, Cu1.5Mn1.5O4 or Cu(NO3)2$3H2O.
([Cu+]a[Cu0.52+Mn1.54+]bO4) contains abundant Cu+ and Mn4+
cations (arising from reaction (1)),21,22 which are respectively
Characterizations
stabilized in the tetrahedral and octahedral sites without the
Powder X-ray diffraction (XRD) patterns of the prepared
need of a stabilizing agent.20 This Cu1.5Mn1.5O4 phase is ex-
samples were obtained using a Rigaku D/Max 2200 PC diffrac-
pected to be much benecial for diesel soot combustion, since
tometer with Cu Ka radiation (40 kV and 40 mA). The nitrogen
the high concentration of Cu+ and Mn4+ cations could activate
adsorption and desorption curves were recorded using Micro-
O2 and NOx in the diesel exhaust, respectively, thus facilitating
meritics Tristar 3000 at 77 K, and the specic surface area (SSA)
NO2 production.
and the pore size distribution were calculated using the Bru-
nauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
Cu2+ + Mn3+ / Cu+ + Mn4+
(1)
methods, respectively. Scanning transmission electron micro-
scopic (STEM) and elements mapping imaging were performed
using Hitachi S-4800. X-ray photoelectron spectroscopy (XPS)
signals were recorded using an ESCAlab250 instrument. The
tting of the Mn element was carried out using Gaussian tting
parameters. The diffuse reectance infrared Fourier transform
(DRIFT) spectra were obtained at different temperatures in the
range of RT-400 ꢀC, using a Thermo-Scientic Nicolet FT-IR
model iS10 equipped with an in situ DRIFTS cell (Harric
Scientic Inc.) and a gas ow system. The pre-mixed catalyst/
soot (mass ratio ¼ 10 : 1, in total of 3 mg) was diluted with
KBr powder (80 mg) and then lled into the DRIFTS cell. The
DRIFTS cell was then exposed to certain reactant gas mixtures
(350 ppm NO, 10% O2, and N2 as balance, total ow ¼ 200 mL
minꢂ1), and the spectra were recorded aer the temperature
was stable for 20 min for each temperature. The temperature-
programed reduction with hydrogen (H2-TPR) experiments
were performed using a Micromeritics Chemisorb 2750 instru-
ment in a 5% H2/N2 ow (25 mL minꢂ1) using 50 mg catalyst
Our previous work conrmed that the spinel Cu1.5Mn1.5O4
loaded on zeolite beta could exhibit excellent activity for catalytic
soot combustion, which was partially attributed to the synergetic
catalytic effect between the valence-changeable cooper and
manganese cations within the spinel phase.23 However, there
still remain several drawbacks, such as complexity in preparation
and the limited loading amount of active spinel Cu1.5Mn1.5O4. In
this work, a facial, scalable and environmentally-friendly co-
precipitation method was utilized to prepare a series of Cu–Mn
mixed oxides with different Cu/Mn atomic ratios as catalysts for
soot combustion. Thereinto, Cu1Mn1 (atomic ratio of Cu : Mn
equals to 1 : 1) with a pure spinel Cu1.5Mn1.5O4 phase exhibits
the best activity, showing a much low Tf value of 360 ꢀC even
compared to the reported 400 ꢀC.11,12 Multiple measurements
have been conducted to reveal the reason for the high activity of
Cu1.5Mn1.5O4 phase. A plausible mechanism for soot combus-
tion over Cu1Mn1 is also proposed.
with a heating rate of 10 C minꢂ1. The uptake amount of H2
ꢀ
Experimental
Catalyst preparation
was measured using a thermal conductivity detector (TCD). Na
content was analyzed by means of ICP-AES.
All the chemicals used in this study were of analytical grade, and
were used without further purication. Cu–Mn mixed oxides
Catalytic activity measurements
were prepared by the co-precipitation method as reported Carbon black from Degussa (Printex U, diameters: 10–50 nm)
before with some modication.24 The whole procedure was was used as the model soot particles. Following a well-accepted
performed at room temperature expect the drying and calcina- procedure,23 the model soot particles (10 mg) and catalyst (100
tion steps. A typical preparation procedure was as follows: mg) were carefully mixed for 20 min with a spatula to simulate
rstly, 7.5 mmol of nitrates including Cu(NO3)2$3H2O and the loose contact mode. Then, the mixture was loaded in
Mn(NO3)2 (50 wt% in aqueous solution) with different Cu/Mn a quartz tube (i.d. 6.0 mm), aer mixing with 1 g silica pellets to
atomic ratios were thoroughly dissolved in 30 mL of deionized avoid pressure drop and favor heat transfer. A gas mixture of
water under stirring. Then, another 30 mL of deionized water 350 ppm NO, 10% O2 and balance N2 was feed with a ow rate of
with 30 mmol of NaHCO3 dissolved in it was quickly poured 200 mL minꢂ1, leaving the space velocity to be 120 000 mL gꢂ1
into the nitrate solution and was le to react for 30 min with the hꢂ1. The mixture was then heated at a rate of 5 ꢀC minꢂ1 in
pH of the solution reaching a nal value of 8–9. The precipitates a tube furnace equipped with a thermal couple. To minish the
were centrifuged and rinsed thoroughly with deionized water error of the measurement to a least extent, every gas ow was
and absolute ethyl alcohol alternately for several times until the conrmed by a mechanical gas owmeter and the concentra-
residual Na content in the supernatant was below 0.5 wt% tion of NOx was veried by a NOx analyzer (Thermo Fisher 42i-
(measured by inductively coupled plasma-atomic emission LS). The analysis of the emissions from the reactor was per-
spectrometry, ICP-AES). Aer drying at 100 ꢀC, the as-prepared formed using an online GC-FID analyzer equipped with
samples were grinded and then calcined under static air at a methane converter. The catalytic activities (conversion and
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selectivity) were calculated as indicated in previous reports in could be mainly attributed to the inevitable catalyst mass loss.
the literature,10,25 in terms of Ti, Tm and Tf, which were dened Additionally, the selectivity of CO2 was kept ꢁ95% in the
as the temperatures at 10%, 50% and 90% of soot conversion, repeated tests, also demonstrating the stable and excellent
respectively, as well as the selectivity to CO2 (SCO ), which was catalytic activity. The effect of the residual Na species in
2
dened as the integrated CO2 outlet amount divided by inte- Cu1Mn1 on the catalytic activity for soot combustion has also
grated COx (containing CO and CO2) outlet amount during the been investigated. As shown in Fig. S2 and S3,† the residual Na
whole combustion process. For the repeated activity tests, the species (0.32 wt% by ICP-AES) impose no obvious inuences on
spent catalyst, still mixed with silica pellets, was recharged with the catalytic activity of the spinel Cu1.5Mn1.5O4 phase.
fresh soot particles (10 mg), followed by a thorough mixing
To get a kinetics insight into the catalytic soot combustion
using a spatula to simulate the loose soot–catalyst contact. over CuO, CuxMny and MnOd, the corresponding Arrhenius
Following the identical procedure to that described above, the curves (Fig. 1b) were plotted from the COx concentration–
activity tests were repeated 5 times.
temperature curves shown in Fig. S4† using an integral
method.26 The kinetics parameters, such as apparent activation
energy (Ea) and frequency factor (A) are derived from the
Arrhenius plots and summarized in Table 1, according to the
Arrhenius equation: ln k¼ ln A ꢂ Ea/RT, where k is specic
reaction rate, R is molar gas constant, T is temperature in
Kelvin. As can be seen in Table 1, the value of Ea gradually
decreases with increasing Mn content, reaching the highest in
CuO (140.0 kJ molꢂ1) and the lowest in MnOd (48.1 kJ molꢂ1),
while keeping roughly constant in CuxMny (100–116 kJ molꢂ1),
Results and discussion
Soot combustion catalytic activity
Fig. 1a shows the catalytic activities of different CuxMny cata-
lysts and reference CuO and MnOd towards soot combustion in
NOx/O2/N2 (catalyst–soot in loose contact mode). It is clear that
all the catalysts can greatly promote soot combustion, and
improve the selectivity to CO2 formation from ꢁ60% to over
90% (Table S1†). Especially, Cu1Mn1 exhibits the best catalytic
activity with the lowest characteristic temperatures (tempera-
ture of maximum soot combustion rate (Tm) ¼ 330 ꢀC, and Tf ¼
360 ꢀC), which are desirably within the temperature range of
diesel exhaust (175–400 ꢀC).3 Additionally, it is worth noting
that both Cu2Mn1 and Cu1Mn2 show lower catalytic activity
than CuO, indicating that suitable Cu/Mn ratio (1 : 1 in this
case), which could lead to a strong synergetic effect, is crucial to
the enhanced catalytic activity. Moreover, the reusability of the
optimized Cu1Mn1 is observed to be of excellence (Fig. S1†),
with Tf and Tm below 380 and 370 ꢀC, respectively, even aer 5
runs of repeated catalytic soot combustion tests. It should be
noted that the slight loss of catalytic activity in repeated tests
Table 1 Apparent activation energy, “ln A” values and lattice constant
derived from XRD patterns of different catalysts
Catalyst
CuO
Cu2Mn1 Cu1Mn1 Cu1Mn2 MnOd
Ea (kJ molꢂ1
)
140.0
27.37
115.8
21.97
115.6
23.70
8.283
100.7
18.49
8.266
48.1
9.16
9.414
[a]
ln A[a]
Lattice constant[b] a ¼ 4.687 8.279
˚
(A)
b ¼ 3.433
c ¼ 5.134
a
Derived from the ln k ꢂ 1/T plots in Fig. 1b according to Arrhenius
b
equation: ln k ¼ ln A ꢂ Ea/RT. Derived from XRD patterns shown in
Fig. 2.
Fig. 1 Soot combustion performance without catalyst, or with different catalysts under loose contact mode (a), and the corresponding
Arrhenius plots (b) (reaction conditions: 350 ppm NO and 10% O2 in N2 with a total flow of 200 mL minꢂ1, mass of soot–catalyst–silica is
10–100–1000 mg).
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indicating the similar catalysis mechanism for soot combustion Furthermore, the characterization of morphology and chemical
over CuxMny. Additionally, as shown in Table 1, CuO and composition of these catalysts was conducted for comparison.
Cu1Mn1 show higher (ln A) values among all the catalysts, while
MnOd possesses the lowest one. Therefore, according to the
Arrhenius equation, it can be inferred that Cu1Mn1 could show
the best catalytic activity, as conrmed in Fig. 1b, where
Cu1Mn1 can achieve high (ln k) value even at the largest 1/T
(i.e., the lowest temperature). More detailed characterizations
were conducted and analyzed as follows.
Morphology
The different morphology and elemental distribution images of
Cu2Mn1, Cu1Mn1 and Cu1Mn2 are depicted in Fig. 3. It is clear
that well-dispersed microspheres and nanoparticles co-exist in
Cu2Mn1, and the microspheres show uniform distribution of
Cu and Mn species while the nanoparticles mainly consist of
copper oxide species. Whereas, Cu1Mn2 presents a unique
core–shell structure, with uniformly-distributed Cu and Mn
species in the core and excessive Mn species in the shell
(Fig. S6,† Mn/Cu > 1.3).
Physical and textural properties
The XRD patterns of different CuxMny oxides and reference
MnOd and CuO are displayed in Fig. 2, and the corresponding
lattice constant data are listed in Table 1. Apparently, all the
samples show well crystallization. Thereinto, CuO presents pure
phase of monoclinic CuO, while MnOd exhibits mixed MnO2–
Mn2O3 phases, suggesting the co-existence of Mn species with
different valence states. Interestingly, only Cu1Mn1 possesses
pure spinel phase of Cu1.5Mn1.5O4. Whereas, Cu2Mn1 shows
a secondary CuO phase besides the predominant spinel
Cu1.5Mn1.5O4; and in Cu1Mn2, some amorphous or highly
dispersed manganese oxides co-exist with the main spinel
Cu1.5Mn1.5O4. More interestingly, it is worth noting that the
crystallinity of Cu1.5Mn1.5O4 spinel phase decreases in the order
of Cu1Mn1 > Cu2Mn1 > Cu1Mn2, which presents the same
tendency with the order of catalytic activity (Fig. 1a). Thus it is
reasonable to assume that a well crystalized Cu1.5Mn1.5O4 spinel
phase could be mainly responsible for the improved catalytic
activity towards soot combustion over CuxMny. Detailed infor-
mation about the SSA and pore structure for each catalyst can be
found in Fig. S5 and Table S2.† Thereinto, Cu1Mn1 shows the
lowest SSA value among CuxMny, consistent with its highest
crystallinity (Fig. 2). Nevertheless, Cu1Mn1 exhibits the highest
activity towards catalytic soot combustion, demonstrating the
high intrinsic activity of the spinel Cu1.5Mn1.5O4 phase.
Different from either Cu2Mn1 or Cu1Mn2, only homoge-
neous microspheres occur in Cu1Mn1. Combined with the XRD
results in Fig. 2, it can be inferred that the microspheres in
Cu2Mn1 and Cu1Mn1, as well as the inner core in Cu1Mn2 all
belong to the spinel Cu1.5Mn1.5O4 phase. The typical SEM image
and corresponding EDS result of Cu1Mn1 shown in Fig. S7,†
conrm that Cu/Mn atomic ratio in Cu1Mn1 is close to 1 : 1,
which is in good consistence with the composition of
Cu1.5Mn1.5O4 phase. In addition, Cu1Mn1 microspheres present
rather rough surface, different from the smooth surface in
Cu1Mn2. The STEM image with high magnication of Cu1Mn1
(Fig. S8†) reveals that the microsphere rough surface is derived
from the loosely-packed nanoparticles with particle size of
around 20–30 nm and the stacked pores within them. Therefore,
the following two aspects are believed to mainly contribute to the
excellent catalytic activity of Cu1Mn1: (1) the pure Cu1.5Mn1.5O4
phase without extra components (e.g., CuO nanoparticles in
Cu2Mn1 and the Mn-rich shell in Cu1Mn2) benets the suffi-
cient contact between soot and the active sites on catalyst
surface, and (2) the well-dispersion and rough surface of
Cu1Mn1 microspheres both favor the sufficient contact between
catalyst and soot particles, and the presence of stacked pores
within the microspheres is also helpful for the diffusion of
gaseous reactant or product, such as O2, NO and NO2.
Surface composition measured with XPS
The deconvolutions of the O 1s, Mn 2p and Cu 2p spectra of
CuxMny oxides and the reference MnOd and/or CuO are shown
in Fig. 4. The O 1s spectrum can be well tted into a main peak
located at 529.6–529.7 eV that is assigned to the lattice oxygen,
and several peaks with higher binding energies corresponding
to different kinds of absorbed oxygen species.27 The Mn 2p3/2
XPS spectrum is composed of three peaks, i.e., peaks at 640.4–
640.6, 641.7–642.1 and 643.8–644.0 eV corresponding to Mn2+
,
Mn3+ and Mn4+, respectively,28,29 and a satellite peak at 646.8–
648.4 eV belonging to the MnO satellite feature.30 As for the Cu
2p XPS spectrum, it is clearly found that the CuxMny mixed
oxides show great difference from pure CuO. Cu 2p3/2 spectrum
of CuO exhibits two peaks, i.e., the main one at 932.7 eV cor-
responding to Cu+ and another one at 934.0 eV due to Cu2+
;
31
Fig. 2 XRD patterns of different samples, (1) CuO, (2) Cu2Mn1, (3)
Cu1Mn1, (4) Cu1Mn2 and (5) MnOd. C CuO (JCPDS 80-1916), -
Cu1.5Mn1.5O4 (JCPDS 70-0260), ; MnO2 (JCPDS 72-1982), A Mn2O3
(JCPDS 73-1826).
while the Cu 2p3/2 spectra of CuxMny show peaks at much lower
binding energies of 930.7–930.8 eV (Cu+ in Cu1.5Mn1.5O4), and
933.4–933.7 eV (Cu2+ in Cu1.5Mn1.5O4).32 The remarkable
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Fig. 3 Typical STEM images (a1, b1, c1), SEM images (a2, b2, c2) and the corresponding elemental mapping (a3–a5, b3–b5, c3–c5) of Cu2Mn1
(a1–a5), Cu1Mn1 (b1–b5) and Cu1Mn2 (c1–c5) samples. Scale bars: 400 nm.
difference in the peak position highlights the great difference in the generation of oxygen vacancies (VO).33 The interaction
chemical environment between CuO and Cu1.5Mn1.5O4.
between Cu and Mn species through electron exchange is also
Table 2 lists the quantied results of the XPS analysis for veried by H2-TPR measurement, as shown in Fig. S9,† where
CuxMny oxides, MnOd and CuO. It is clearly observed that H2 reduction of CuxMny occurs at much lower temperatures
it
is
Mn3+
instead
of
Mn4+
(Cu1.5Mn1.5O4
¼
than that for either CuO or MnOd. It should be mentioned that
Cu1.0+Cu0.52+Mn1.54+O42ꢂ), that dominates in the Mn 2p spectra there is inconsistence between the activity order and reduc-
for all the CuxMny catalysts, conrming the existence of the ibility order, as previously reported,34 which could be attributed
redox equilibrium (Cu+ + Mn4+ 4 Mn3+ + Cu2+) in the to the difference in chemical and structural properties between
Cu1.5Mn1.5O4 phase.21,22 As for Cu species, the Cu+ proportion H2 and soot, e.g., reactive species at the inner pores surface can
on catalyst surface rises with increasing Mn content in CuxMny, be attacked by H2 molecule, but are unavailable for soot parti-
reaching a maximum value in Cu1Mn2 (69.6%), which is, cles. Nevertheless, H2-TPR results rmly prove the existence of
however, still lower than that in reference CuO (78.7%). This Cu–Mn synergetic effect in the prepared CuxMny catalysts.
extremely high Cu+ proportion in CuO, which can be veried by
As reported by previous works, Cu+–Mnl+ cation pair can
the rather weak Cu2+ satellite peak for CuO in Fig. 4c, corre- serve as active site for NO2 production, since Cu+ and Mnl+ can
sponds well with its highest ratio of Oads/O (shown in Table 3), facilitate the adsorption and activation of O2 and NO, respec-
since the reduction of Cu2+ in CuO is always accompanied by tively.35,36 Therefore a parameter “U” was introduced (dened in
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Fig. 4 The XPS spectra of different catalysts, O 1s (a), Mn 2p (b) and Cu 2p (c).
Table 2 XPS surface composition analysis of different catalysts
by XPS measurement. The U values as well as the Oads propor-
tions for different catalysts are summarized in Table 3.
Mnx+/Mn[a]
Mn2+ (%)
Cux+/Cu[b]
Cu+ (%)
Catalytic activity measurement in the absence of NOx (O2/N2)
was also conducted in an attempt to nd the key factors
determining the catalysis performance. The catalytic behavior
in O2/N2 (represented by Tm0) was shown in Table 3. It's clearly
observed that the higher ratio of Oads/(Oads + Olatt), the lower
Catalysts
Mn3+ (%)
Mn4+ (%)
Cu2+ (%)
CuO
78.7
23.6
60.0
69.6
21.3
76.4
40.0
30.4
Cu2Mn1
Cu1Mn1
Cu1Mn2
MnOd
23.3
19.7
14.2
11.5
65.4
66.8
68.0
69.9
11.2
13.5
17.7
18.6
0
value of Tm (the better catalytic activity), which is in good
coincidence with the widely accepted “Active Oxygen Mecha-
nism”,37 wherein soot is directly oxidized by the reactive oxygen
species on catalyst surface. Additionally, it is worth noting that
CuO with a slightly higher Oads proportion than Cu1Mn1,
possesses much better catalytic activity in O2/N2, indicating that
the highest Cu+ proportion in CuO could make extra contribu-
tion to its superb activity.
Additionally, a new parameter “DTm” was introduced to
quantify the enhancement efficiency of NOx for catalytic activity
of different CuxMny, as shown in Table 3. It is interesting to
nd that Cu1Mn1 with the highest U value (2.14) shows the
greatest NOx enhancement efficiency (corresponding to the
largest DTm value of 73 ꢀC), suggesting that in addition to
“Active Oxygen Mechanism”, the Cu+–Mnl+ pairs as active sites
could also promote the catalytic soot combustion by improving
NO2 production. Therefore, it is believed that the catalytic soot
combustion in NOx/O2/N2 over Cu1Mn1 follows both the “Active
Oxygen Mechanism” and the “NO-aided Oxidation
Mechanism”.37
a
b
The whole Mn species with various valence states. The whole Cu
species with various valence states.
0
Table 3 Comparison of Tm vs. Oads proportion, and DTm vs. U
[a]
[b]
[c]
0
Catalysts
Tm (ꢀC)
Oads/(Oads + Olatt
)
DTm (ꢀC)
U
CuO
379
414
397
403
413
0.50
0.40
0.49
0.46
0.43
Cu2Mn1
Cu1Mn1
Cu1Mn2
MnOd
41
73
17
1.42
2.14
1.08
a
b
0
Tm is for the catalytic soot combustion in O2/N2 (Fig. S10). Olatt
:
c
0
lattice oxygen. DTm ¼ Tm ꢂ Tm, where Tm is for the catalytic soot
combustion in NOx/O2/N2 (Fig. 1a).
eqn (2)) to quantify the concentration of Cu+–Mnl+ cation pair
in different CuxMny catalysts.
Mechanism analysis
U ¼ {(Cu+/Cu)$[Cu/(Cu + Mn)]}/
{(Mn2+/Mn)$[Mn/(Cu + Mn)]}
(2) Based on the above discussion, combined “Active Oxygen
Mechanism” and “NO-aided Oxidation Mechanism” for cata-
lytic soot combustion over Cu1Mn1 was proposed, as shown in
Scheme 1. Therein, the “Active Oxygen Mechanism” shown in
where, Cu (or Mn) stands for all the Cu (or Mn) species with
different valence states, and all the atomic ratios are obtained
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Scheme 1 Schematic illustrations of the possible combined catalytic mechanism, (a) “Active Oxygen Mechanism”, (b) “NO-aided Oxidation
Mechanism”, (c) formation of NO2 in the “NO-aided Oxidation Mechanism”.
Scheme 1a indicates that the active oxygen species (O*) can other to form NO2, as shown in eqn (7).36 The obtained NO2, as
attack the nearby active sites on carbon surface [C(f)], producing a stronger oxidant than O2,40 can effectively accelerate soot
carbon oxides (mostly CO2) and more C(f) (eqn (3)).38 Aer- combustion (Fig. S11†). Besides, it is worth noting that the NO2
wards, the spent oxygen species can be supplemented by molecules are inclined to adsorb on metal ions, e.g., Cu2+,
gaseous O2.37
producing Cu2+$NO2 species.41
The presence of such Cu2+$NO2 species could keep NO2 from
effusion into the gas ow at low temperatures, and ensure the
steady release of NO2 when heated (eqn (8)), thus realizing the
full use of NO2. This can well explain the nding that MnOd,
though with the higher NO2 producing activity than Cu1Mn1
(Fig. S12†), shows lower enhancement efficiency of NOx for
catalytic soot combustion, especially in high temperatures
(Fig. S13†). Aer NO2 releasing, the active sites can be regen-
2O* + CCC(f) / CO2 + 2C(f)
(3)
Scheme 1b shows the schematic illustration of “NO-aided
oxidation mechanism”. Similar to O* in the “Active Oxygen
Mechanism”, NO2 can also directly oxidize C(f) into CO2, and
regenerate more C(f) (eqn (4)).39 The process of NO2 production
over Cu+–Mnl+ cation pairs (herein, Cu+–Mn4+ is taken as an
example) is depicted in Scheme 1c.
erated through reaction equilibrium Mn3+ + Cu2+ 4 Cu+
+
Mn4+, thus completing the catalysis circle.
To sum it up, the presence of enormous Oads and Cu+–Mnl+
cation pairs on Cu1Mn1 surface mainly contributes to the
extremely high intrinsic catalytic activity of the spinel
Cu1.5Mn1.5O4 phase, and the interaction between NO2 and Cu2+
also ensures the sufficient utilization of NO2.
2NO2 + CCC(f) / 2NO + CO2 + 2C(f)
Cu+ + 1/2O2 / Cu2+$Oꢂ
(4)
(5)
(6)
(7)
Mn4+ + NO / Mn3+$NO+
Cu2+$Oꢂ + Mn3+$NO+ / Cu2+$NO2 + Mn3+
Cu2+$NO2 / Cu2+ + NO2(g)
In Situ DRIFT studies of soot combustion over Cu1Mn1
(8) To verify the mechanism shown in Scheme 1, DRIFT measure-
ment was performed to investigate the existence and evolution
of different types of absorbed species over Cu1Mn1 during
catalytic soot oxidation in NOx/O2/N2, as shown in Fig. 5. It is
clearly found that the intensity of the NO2ꢂ peak ca. 1250 cmꢂ1
Cu+ cations in Cu1.5Mn1.5O4 can adsorb, then activate
gaseous O2 to generate active oxygen (eqn (5)).35 Similarly, Mnl+
cations can also adsorb as well as activate NO in the feed gas,
resulting in reactive nitrogen oxide (NO+) species (eqn (6)).36
Then, the produced active species can easily react with each
ꢂ
decreases with increasing temperatures (RT-400 ꢀC),42 and NO3
peak ca. 1404 cmꢂ1 can be obviously observed at 400 C.43 The
ꢀ
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Distinguished Young Scientists (51225202) and National
Natural Science Foundation of China (51502330).
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