Y. Wang, et al.
CatalysisCommunications142(2020)106043
hydrogen, Cu/Co3O4 showed excellent catalytic oxidation performance
and could be reused 10 times without a significant loss in its catalytic
performance [20]. Cu NP catalysts supported on Co3O4-ZrO2 may be an
ideal dual-active catalyst with an SMSI effect for the dehydrogenation
of diethanolamine.
Therefore, Cu/Co3O4-ZrO2 was designed and synthesized with an
additional active component of Co3O4 to form a catalyst with two active
components. Small Co3O4 particles were grown on the surface of ZrO2
by reduction and self-oxidation. Moreover, irregular and fine Co3O4
particles confined the Cu NPs to the surface of ZrO2. Cu/Co3O4-ZrO2
showed good catalytic activity and stability when used to catalyze the
dehydrogenation of diethanolamine. Cu/Co3O4-ZrO2 is an excellent
catalyst with controllable morphology and provides an example of a
new bicomponent dehydrogenation catalyst.
2. Experimental section
2.1. Synthesis of Cu/Co3O4-ZrO2
Fig. 1. XRD patterns of Cu/Co3O4-ZrO2, Cu/Co3O4, and Co3O4-ZrO2 catalysts.
ZrOCl2·8H2O (6.44 g) was dissolved in 500 mL ultra-pure water, and
5 g NaOH was dissolved in 200 mL water. The NaOH solution was
slowly added to the Zr salt solution and continuously stirred. After
addition, the solution was stirred for 3 h to form a Zr(OH)2 colloidal
solution. CoCl6·2H2O (1.19 g) and 2.42 g Cu(NO3)2·3H2O were dis-
solved in 200 mL ultrapure water and stirred and then slowly added to
the Zr(OH)2 colloidal solution and continuously stirred. After addition,
the mixture was stirred for 12 h and aged for an additional 4 h. The
precipitate was filtered, washed, and dried in an oven at 50 °C for 12 h.
Then, the dried solid was ground and roasted in a muffle furnace at
550 °C for 4 h to obtain the catalyst precursor CuO/Co3O4-ZrO2. The
precursor CuO/Co3O4-ZrO2 was placed into a tubular furnace and re-
duced at 240 °C for 4 h at a heating rate of 3 °C/min. When cooled to
room temperature, the catalyst was removed from the tube furnace, and
CoO was allowed to self-oxidize for 1 h to obtain Cu/Co3O4-ZrO2 cat-
alyst.
[21]. The characteristic peaks of Co3O4 were observed at 31.3°, 36.8°,
38.5°, 44.8°, 59.5°, and 65.2° [22], which were respectively indexed to
the (111), (220), (311), (222), and (400) planes. The Cu nanoparticle
size in Cu/Co3O4-ZrO2 was 4.7 nm as calculated from the Scherrer
equation. In addition, no characteristic diffraction peaks associated
with Co3O4 were observed for Cu/ Co3O4-ZrO2, possibly because it was
amorphous and highly dispersed on ZrO2. By comparing Cu/Co3O4 and
Co3O4-ZrO2, it can be seen that a wider FWHM (full width at half
maximum) of the Co3O4 (220) diffraction peak was observed in Co3O4-
ZrO2, indicating a smaller particle size. The results show that the size of
Co3O4 was greatly reduced when Co3O4 was grown on the surface of
ZrO2 by reduction and self-oxidation.
Fig. 2(a) shows the SEM images of Cu/Co3O4-ZrO2. The Co3O4
prepared by reductive self-oxidation had a small size, consistent with
the XRD results. Fig. 2(b) are the TEM images of Cu/Co3O4-ZrO2. Ac-
cording to Fig. 2(b), the diameter of Co3O4 was about 10 nm, and Cu
and Co3O4 were dispersed on the surface of ZrO2.
2.2. Catalytic activity evaluation
10 g Diethanolamine, 2 g catalyst, and 8.5 g sodium hydroxide
(dissolved in 80 mL deionized water) were added to a high-pressure
reactor. The airtightness of the reactor was checked, and then N2 was
passed and emptied 5–6 times before pressurizing to 1 MPa. The tem-
perature was raised to 160 °C at a heating rate of 4 °C/min, and stirring
was carried out at a speed of 400 rpm. The volume of hydrogen dis-
charged was measured by a rotameter. The exhaust valve was opened
when the air pressure reached 1.5 MPa and then closed when the
pressure dropped to 1 MPa. The volume and exhaust temperature of
each exhaust were recorded. During reactions, if the pressure in the
reactor did not change for 20 min, the reaction was regarded as com-
plete. Liquid chromatography was used for qualitative analysis. The
yield of iminodiacetic acid was calculated using a strong anion ex-
change column (Hypersil SEX, 5 μm, 4.6 mm × 250 mm). The calcu-
lation of the yield of iminodiacetic acid is described in detail in the
Supporting Information.
The H2-TPR profiles of Cu/ZrO2, Cu/Co3O4-ZrO2, and Cu/Co3O4 are
displayed in Fig. 3 Cu/ZrO2 samples showed two low-temperature re-
duction peaks near 130 °C and 179 °C. The reduction peak at 130 °C was
caused by the reduction of highly-dispersed CuO on the surface of the
catalyst, while the reduction peak at 179 °C was caused by the reduc-
tion of crystalline CuO. Another reduction peak observed at 200 °C was
caused by the reduction of Cu2+ in the ZrO2 lattice. The two low-
temperature reduction peaks of Cu/Co3O4-ZrO2 samples near 126 °C
and 135 °C were caused by the reduction of highly-dispersed CuO on
the catalyst surface, while the reduction peaks at 162 °C were caused by
the reduction of crystalline CuO. The reduction temperature stability of
Cu species in Cu/Co3O4-ZrO2 decreased significantly compared with
Cu/ZrO2. This indicates that the introduction of Co3O4 improved the
dispersion of Cu NPs and that the particle size was small. The overall
reduction peak temperature of Cu/Co3O4 Cu NPs was higher than that
of Cu/Co3O4-ZrO2 due to the strong interaction between Cu and the
Co3O4 carrier. The peak at 200–260 °C was attributed to the reduction
of Co3+ to Co2+ in Cu/Co3O4-ZrO2 and Cu/Co3O4 [23]. During this
process, Co3O4 in Cu/Co3O4-ZrO2 was reduced to CoO, thus decreasing
the amount of Co3O4. During self-oxidation, CoO self-oxidized to small
Co3O4 particles, which grew on the surface of ZrO2.
3. Results and discussion
Fig. 1 shows the XRD patterns of the catalysts. The Cu grain size in
the catalyst was calculated using the Scherrer equation and the half-
peak width of the highest diffraction peak of each material. Diffraction
peaks of ZrO2 were observed at 30.5°, 34.7°, 50.8°, and 60.9° (JCPDS
no. 37–1484). Metallic Cu peaks appeared at 2θ values of 43.3° and
50.4°, which corresponded to the crystal planes of Cu(111) and Cu
(200), respectively (JCPDS no. 04–0836). However, no metallic Cu
peaks were detected in the pattern of Cu/Co3O4, indicating that the
copper species may be amorphous and highly dispersed on the carrier
The influence of the surface elemental composition and chemical
state on catalyst properties was studied using XPS. Fig. 4(a) shows the
bimodal Co(2p) spectrum of Co3O4. In particular, peaks of Co 2p3/2 and
Co2p1/2 were found at 779.2–780.2 eV and 794.7–796.6 eV, respec-
tively. The energy differences between the peak values of Co 2p3/2 and
Co 2p1/2 of Cu/Co3O4-ZrO2, Cu/Co3O4, and Co3O4-ZrO2 were
2