Effect of Al2O3 phase on the catalytic performance for HCHO oxidation over Ag/Al2O3 catalysts

Article abstract of DOI:10.1016/j.apcata.2020.117705

The effect of different crystalline phases of Al₂O₃ (γ-Al₂O₃, γ, θ-Al₂O₃) on HCHO oxidation over Ag catalysts is investigated, and the silver state and particle size strongly depend on the Al₂O₃ crystalline phase. The various coordinatively unsaturated Al³⁺ (Al_Ⅲ, Al_Ⅴ) involves in the formation of Ag-O-Al entities. Ag species is mainly anchored on Al_Ⅴ sites for γ-Al₂O₃, whereas mainly on Al_Ⅲ sites for γ, θ-Al₂O₃. The weaker interaction of Ag, O and Al_Ⅴ on γ-Al₂O₃ compared with that of Ag, O and Al_Ⅲ on γ, θ-Al₂O₃ and the larger surface area of γ-Al₂O₃ result in the formation of more Ag⁰ with high dispersion. The higher fraction of coordinated Al_Ⅵ on γ-Al₂O₃ provides more adjacent hydroxyl groups for HCHO adsorption and formation of intermediates. And the synergistic effect of OH group and the high activation ability for oxygen on smaller Ag⁰ particles give the high HCHO oxidation activity on Ag/γ-Al₂O₃ (T₁₀₀ = 85 °C).

Full text of DOI:10.1016/j.apcata.2020.117705

Journal Pre-proof
Effect of Al O phase on the catalytic performance for HCHO oxidation
2
3
over Ag/Al O catalysts
2
3
Yi Wang (Conceptualization) (Methodology) (Formal analysis) (Data
curation) (Investigation) (Writing - original draft), Zhenping Qu
(
(
Supervision) (Funding acquisition) (Project administration)
Resources) (Validation) (Conceptualization) (Writing - review and
editing), Jing Xu (Visualization) (Investigation) (Writing - review and
editing), Bin Huang (Software) (Data curation)
PII:
S0926-860X(20)30298-2
DOI:
https://doi.org/10.1016/j.apcata.2020.117705
APCATA 117705
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
11 March 2020
26 May 2020
15 June 2020
Please cite this article as: Wang Y, Qu Z, Xu J, Huang B, Effect of Al O phase on the
2
3
catalytic performance for HCHO oxidation over Ag/Al O catalysts, Applied Catalysis A,
2
3
General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117705

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Effect of Al
2
O
3
phase on the catalytic performance for HCHO
oxidation over Ag/Al catalysts
2
O
3
*
Yi Wang, Zhenping Qu , Jing Xu, Bin Huang
Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of
Environmental Science and Technology, Dalian University of Technology, Linggong Road 2,
Dalian 116024, China
*
Corresponding author. E-mail address: quzhenping@dlut.edu.cn (Z P. Qu)
Graphical Abstract
1

Highlights

Ag species is selectively anchored on coordinated AlⅢ and AlⅤ sites in
Ag/Al catalysts.
2
O
3


The silver state and particle size strongly depends on the Al3+ chemical
environment.
OH groups adjacent to the coordinated Al Ⅵ involve the formation of
intermediates.

Ag/γ-Al
θ-Al
2
3
O exhibits higher activity for HCHO catalytic oxidation than Ag/γ,
2
O
3.
Abstract
The effect of different crystalline phases of Al
2
O
3
(γ-Al
2
O
3
, γ, θ-Al
2
3
O ) on HCHO
oxidation over Ag catalystsis investigated, and the silver state and particle size strongly
crystalline phase. The various coordinatively unsaturated Al3+
depend on the Al
2
O
3
(
Al
on Al
interaction of Ag, O and Al
Al and the larger surface area of γ-Al
high dispersion. The higher fraction of coordinated Al
Ⅲ
, Al
Ⅴ
) involves in the formation of Ag-O-Al entities. Ag species is mainly anchored
sites for γ-Al , whereas mainly on Al sites for γ, θ-Al . The weaker
on γ-Al compared with that of Ag, O and Al
Ⅴ
2
O
3
Ⅲ
2
O
3
Ⅴ
2
O
3
Ⅲ
on γ, θ-
0
2
O
3
2
O
3
result in the formation of more Ag with
Ⅵ
on γ-Al
2
3
O provides more
adjacent hydroxyl groups for HCHO adsorption and formation of intermediates. And
the synergistic effect of OH group and the high activation ability for oxygen on smaller
0
o
Ag particles give the high HCHO oxidation activity on Ag/γ-Al
2
O (T100=85 C).
3
2

Keywords: Ag/Al
2
3
O , Crystalline phase, Coordinatively unsaturated Al, HCHO
catalytic oxidation
1
. Introduction
Exposure to indoor air containing formaldehyde to a certain extent may cause
serious health hazards [1, 2]. Much effort has been devoted to eliminate HCHO, among
which catalytic oxidation is one of the most effective methods [3-6]. Nobel metal-based
catalysts, especially Pt and Au catalysts, exhibit high activities on formaldehyde
oxidation at ambient temperature [7-10]. Recently supported Ag catalyst has been
proved effective for HCHO catalytic oxidation by microstructure adjustment of metal
particles and supports [9, 11-18]. However, it is still an inevitable challenge for practical
application respect to the desires for lower temperature and economy. Therefore, to
enhance the activity at low temperatures much study has focused on the microstructure
and properties of the supports and active components.
Various supports have been employed on the HCHO catalytic oxidation, such as
CeO
properties, reactivity of surface hydroxyls or the strong interaction between supports
and silver [3, 11, 13, 15, 18]. Transitional Al , in recent years, is widely used as
2
, TiO
2
, Co
3
O
4
, MnO
2
, Fe
2
3
O etc., due to their oxygen storage capacity, acid-base
2
O
3
catalytic support in heterogeneous catalysis due to its surface acid-base properties and
high surface area. It shows a significant effect on the stability of the active species
involving in the catalytic process. Ag/Al
than platinum group metals based catalysts for oxidant-free alcohol dehydrogenation
19]. The role Ag-O-Al entities over Ag-alumina catalysts for the SCR reaction reveals
2
O
3
catalyst exhibits the higher activity even
[
that the reaction rate is correlated with the amount of Ag-O-Al sites [20], and the effect
3

of the support on the formation and distribution of Ag species of Ag-based catalysts for
NH
3
selective catalytic oxidation (NH
3
-SCO) has been documented [21] that the
support exhibit the
0
smaller Ag particles (5 nm) with high dispersion obtained on Al
2
O
3
highest performance for NH
3
conversion to N .
2
It is found that alumina phases take a crucial role in the formation of active
components of alumina-supported catalysts and then influence their performance [22,
2
3]. The surface structure of alumina usually changes with the phase determines the
location of the active components. The terminal hydroxyl groups on the surface of γ-
Al has been deduced to be responsible for Ag species anchoring [24]. Generally, for
transitional alumina, the mixed crystalline phases are always observed since the
preparation of Al is a process of phase transformation, for example, in which γ-
Al and θ-Al may coexist in the structure of core and shell [25]. Alumina with
2
O
3
2
O
3
2
O
3
2
O
3
different coordinatively unsaturated Al (three-, four-, five-coordinated) [26] possess
various capacity for anchoring metal components into different coordinated Al sits,
which will induce the variation of the metal nanostructure and properties [27]. However,
there are few reports on the influence of different phases of Al
HCHO catalytic oxidation over Ag/Al up to now. Thus, it is essential to reveal their
effects on the formation of Ag species and the performance of HCHO catalytic
oxidation over Ag/Al catalysts.
In this work, we emphasize the necessity to understand different Al
phases and their effects on the state and structure of Ag particles. The surface structure
of Al supports and their influences on the performance for HCHO oxidation over
Ag/Al catalysts are investigated.
2
O
3
on the activity of
2
O
3
2
O
3
2
3
O crystalline
2
O
3
2
O
3
4

2
. Experimental Section
2
.1 Catalyst preparation
Two kinds of Al
2
O
3
supports (γ-Al
2
O
3
, γ, θ-Al
2
O
3
) were prepared by hydrothermal
o
o
procedure according to the previous study [28, 29] and calcined at 500 C and 900 C for
2
h respectively. Ag/Al
2
O
3
catalysts were prepared by impregnating Al
2
O
3
with an
o
o
aqueous solution of silver nitrate and dried at 80 C, followed by calcination at 500 C
for 2 h. The obtained powder was squashed and crushed to get samples with mesh sizes
of 20-40. The amounts of Ag loading were 8wt% nominally in as-prepared catalysts.
2
.2 Characterization
Specific surface areas were obtained from the isotherms by employing the
Brunauer–Emmett–Teller (BET) method in Quantachrome quadrasorb SI. The pore
diameter distributions were calculated from desorption isotherms using the BJH
(Barrett–Joyner–Halenda) model. Powder X-ray diffraction (XRD) patterns were
recorded on a Rigaku powder diffract meter using Cu Kα (λ = 0.1542 nm) radiation and
operating at 40 kV and 200 mA. The patterns were estimated by scanning 2θ angles
o
ranging from 20°to 80°for wide-angle XRD with a step size of 0.02 . Transmission
electron microscopy (TEM) images were recorded using a Tecnai G2F30 S-Twin
operating at 100 kV. UV–vis diffuse reflectance spectra were collected with
SHIMADZU UV–2600 UV–vis spectrophotometer. X-ray photoelectron spectroscopy
were measured on an ESCALAB250 X-ray photoelectron spectrometer (Thermo VG)
using monochromatic X-ray source of Al Kα (1486.6 eV) as excitation source under
ultra-high vacuum. The binding energies were corrected by the carbon C (1s) binding
energy at 284.6 eV.
HCHO-TPD (Temperature programmed desorption) and HCHO-TPSR
(Temperature programmed surface reaction) tests were carried out on the in-situ
5

OmniStar 320 mass spectrometer. Prior to the adsorption of HCHO, the catalysts were
o
pretreated by pure helium at 300 C for 30 min, and cooled to room temperature under
helium. HCHO was introduced by helium for a certain time, and then the catalysts were
purged with helium to remove the physisorbed HCHO molecules on the surface for 90
o
o
−1
minutes. Subsequently, the temperature was raised up from RT to 450 C at 10 C min
o
o
−1
for HCHO-TPD in a continuous flow of helium, and from RT to 300 C at 10 C min
for HCHO-TPSR in a continuous flow of helium and oxygen. The effluents from quartz
reactor were detected on-line by mass spectrometer.
The catalytic oxidation of HCHO was tested in a fixed-bed flow reactor with 0.2
g of the catalyst under atmospheric pressure. By introducing nitrogen gas to a sealed
bottle with trioxymethylene (99.5%, AcrosOrganics) in an incubator with an ice water
mixture, gaseous HCHO was generated. The feed gas mixture consisting of 20 vol. %
oxygen, ~ 200 ppm HCHO and balanced N
total flow rate of 30 mL/min. The apparent activation energy (E
kinetic experiments, and the HCHO conversion was controlled within 16%. The
2
was introduced to the catalysts bed at a
a
) was obtained from
conversion of HCHO to CO
function as follows:
2
was evaluated according to the formation of CO as a
2
[
CO2]out
XHCHO% =
×100%
[
CO2]total
The effluent gas CO was analyzed by on-line gas chromatograph (GC 7890II, Tech
2
comp, China) with FID detectors and a nickel catalyst converter.
27Al MAS NMR experiments were performed on the Agilent DD2-500 MHz
spectrometer at 130.2 MHz using a 4-mm MAS NMR probe with a spinning rate of 14
kHz. The spectra were accumulated for 120 scans with 2 s recycle delay. The chemical
shifts were referenced to 1% Al (NO
3
3
) aqueous solution. The samples were not
pretreated before NMR measurements in order to well elucidate the effect of
6

3
+
coordinatively unsaturated Al on the anchoring of silver species.
In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS)
were recorded on a Thermos Nicolet iS 10 equipped with an MCT detector which is
cooled by liquid nitrogen. Prior to the experiments, all the samples (ca. 10 mg in weight)
o
o
were purged by N for 30 min at 300 C, and then cooled to 30 C. The spectra of
2
Ag/Al
2
O
3
served as background were monitored in N
2
at different temperatures. The
mixture for 30
/N at
o
adsorption spectra of HCHO at 30 C were recorded in the HCHO/N
2
min. Then the spectra for HCHO oxidation were collected in the flow of 20% O
a rate of 30 ml/min at different temperatures.
2
2
3
. Results and discussion
3
.1 Structure and surface properties
The XRD patterns of different Al
2
O
3
and Ag/Al
phase (JCPDS Card No.10-0425), with peaks at
1.9°, 37.6°, 39.4°, 45.8°, 60.9°, 67.0°, denoted as γ-Al . For the other sample,
phase, there are some peaks at 27.6°, 31.2°, 32.7°,
phase [30]. Thus, this sample is a hybrid of θ-
. When the silver is loaded, the peaks at
θ=38.2°, 44.3°, 64.5°, and 77.6° assigned to metallic silver emerge on the patterns of
XRD, revealing the existence of metallic Ag in both Ag/Al samples. The diffraction
peak intensity of Ag for Ag/γ, θ-Al is much stronger than that for Ag/γ-Al
2
O
3
samples are shown in Fig.1.
One of the Al
2
O
3
is typical of γ-Al
2
O
3
3
2
O
3
besides the peaks assigned to γ-Al
2
O
3
o
4
6.5 , 50.4°, 60.1° assigned to θ-Al
2
O
3
Al
2
O
3
and γ-Al
2
O
3
, denoted as γ, θ-Al
2
O
3
2
2
O
3
2
O
3
2
3
O ,
indicative of higher crystallinity of metallic Ag or larger Ag particle size on the former.
Solid–state nuclear magnetic resonance (NMR) technique as a sensitive probe for
selected elements, that can detect the local structures despite of the crystallinity, has
been employed to identify the local environment, e.g., coordinative numbers and bond
7

distance recent years [31-33]. The local coordination geometry information of the two
kinds of Al supports was obtained to elucidate the distribution of Al atoms according
to the empirical correlations between the Al NMR parameters and structure features
2
O
3
2
7
2
7
of aluminum compounds. The isotropic chemical shift, δiso of Al nuclei is known to
be strongly correlated to aluminum coordinated number. The fractions of Al , Al and
were evaluated by comparing of the peak areas of Al MAS NMR spectra, and the
results are shown in Fig.2. The chemical shifts at ca. 4, 11 ppm are assigned to A
ca.36 ppm is assigned to Al , and 60, 69 ppm are assigned to Al , respectively [31, 34].
The fractions of three kinds of coordinated Al on the two supports are listed in Table 1.
Compared with γ, θ-Al , γ-Al possesses higher proportions of Al (4.60% vs.
(68.0% vs.65.4%), and lower proportion of Al (27.4% vs. 32.5%). When
wt% silver is loaded on Al , the fraction of Al on Ag/γ-Al catalyst decreases
increases from 68.0% to 70.6%. The
to form Ag-O-Al entities [35]. No obvious
fraction is observed (27.4% to 27.2%). Differently, for Ag/γ, θ-Al
the spectrum shows fraction drops of Al (2.12% to 1.18%) and Al (65.4% to 61.9%),
but a significant fraction increase of Al (32.5% to 37.2%).
Ⅳ
Ⅴ
2
7
Al
Ⅵ
Ⅵ
,
Ⅴ
Ⅳ
2
O
3
2
O
3
Ⅴ
2
8
.12%), Al
Ⅵ
Ⅳ
2
O
3
Ⅴ
2
O
3
from 4.60% to 2.20%, while the fraction of Al
certain amount of Ag is anchored on Al
change for Al
Ⅵ
Ⅴ
Ⅵ
Ⅳ
2
3
O ,
Ⅴ
Ⅵ
Ⅳ
2
7
It has been known that three-coordinated Al is not detectable by AlNMR despite
the fact that the three-coordinated Al on the surface is expected based on the bulk
structure. F.R. Chen [31] have ascribed this to the high quadrupole coupling constants
(
QCCs) of the species, and concluded the conversion of Al into a higher coordination,
Ⅲ
such as tetra-coordinative Al when exposing to traces of oxygen or of water vapor.
Similarly, Karl Sohlberg etc. [36] have proposed that when the structure is relaxed, it
undergoes a reconstruction, in which the tri-coordinated Al atoms initially exist in the
surface layer and then drop into the vacant octahedral interstices of the first subsurface
8

layer.
+
Ag can be anchored on the Al
selective catalytic reduction of NO
are the precursors of the Ag-O-Al
proportion for Ag/γ, θ-Al probably should be originated from the conversion of a
certain amount Al into Al , forming Ag-O-Al entities after Ag addition. A little
increment in proportion of Al on γ-Al indicates the little Ag anchoring on Al sites
for γ-Al . Thus more coordinatively unsaturated Al exists on γ, θ-Al than on γ-
Al . To further understand this locating mechanism, Ag loading was decreased to 4
wt.% on γ-Al
possesses 27.8% Al
that of γ-Al support and 69.5% Al
concluded that apart from a little fraction of Ag anchoring on Al
located on Al sites, forming Ag-O-Al
Moreover, the fraction of Al on 8wt.% Ag/γ-Al
wt.% Ag/γ-Al (27.8%), indicative of the possibility that Ag-O-Al
to form (Ag-O)
fraction of Al
more Ag is anchored on Al
to get the conclusion that Ag is mainly located on Al
sites for γ, θ-Al . Kwak, J. H. and An-Hui Lu etc. [39-41] have also concluded that
the active component Pt is primarily anchored on coordinated Al sites of γ-Al
Ⅲ
sites as the most reactive Ag species [37] for
by ethanol. Deng, H. etc. proposed that Al sites
entities [38]. Thus, in this work, the increase of A
x
Ⅲ
Ⅳ
Ⅳ
2
O
3
Ⅲ
Ⅳ
Ⅳ
Ⅳ
2
O
3
Ⅲ
2
O
3
Ⅲ
2
O
3
2
O
3
2
7
2
O
3
and the results of Al NMR spectra (Table 1) show that Ag/γ-Al
, little increase compared to γ-Al support; 2.7% Al , less than
, higher than that of γ-Al support. It can be
sites, Ag is primarily
2
O
3
Ⅳ
2
O
3
Ⅴ
2
O
3
Ⅵ
2
O
3
Ⅲ
Ⅴ
Ⅵ
, and give rise to the increment of Al
(27.2%) is a bit lower than that on
bind Ag again
should result in the increase of the
drops from 2.7% to 2.2%, suggesting
Ⅵ
.
Ⅳ
2
O
3
4
2
O
3
Ⅳ
2
-Al
. However, the fraction of Al
sites than on Al
Ⅴ
. The formation of (Ag-O)
2
-Al
Ⅴ
Ⅴ
Ⅴ
Ⅴ
Ⅳ
sites. Therefore, the above data lead us
sites for γ-Al , while on Al
Ⅴ
2
O
3
Ⅲ
2
O
3
Ⅴ
2
3
O .
Summarily, the interactions between Ag, O and differently coordinated Al are the
basic factor for determining the Ag anchoring sites, and the various fractions of
3
+
3+
coordinated Al affect the distribution of Ag on coordinated Al . Based on above
9

results, Ag is preferentially anchored on Al
are mainly dependent on the fractions of Al
more proportion of Al and less proportion of Al
anchored on Al sites. While γ, θ-Al support has more proportion of Al
proportion of Al , therefore, more Ag species are anchored on Al sites.
Fig. 3a shows the N sorption isotherm of different Al supports. Both Al
samples yielded type IV curves with H3-shaped hysteresis loops, indicative of the
presence of closely packed slit pores. Compared to γ-Al , γ, θ-Al has a sharp
capillary condensation step at slightly higher relative pressures P/P =0.8-1.0, indicating
the existence of larger sized pores than those of γ-Al . Correspondingly, the size
distribution of the two samples centers at 5.7 nm for γ-Al
respectively (Fig. 3b). Table 2 displays the structure properties of the two Al
Ⅲ
and Al
Ⅴ
sites, and the Ag anchoring sites
. Since γ-Al support possesses
, and then more Ag species are
and less
Ⅲ
and Al
Ⅴ
2
O
3
Ⅴ
Ⅲ
Ⅴ
2
O
3
Ⅲ
Ⅴ
Ⅲ
2
2
O
3
2
O
3
2
O
3
2
O
3
0
2
O
3
2
O
3
and 9.7 nm for γ, θ-Al
2
O
3
,
2
O
3
supports and Ag/Al
2
O
3
catalysts. It is found that γ-Al
2
O
3
support exhibits the larger
2
2
BET specific surface area (262 m /g) than γ, θ-Al
2
O
3
(158.1 m /g). When Ag is loaded
2
2
on alumina, the BET surface area becomes 308.0 m /g for Ag/γ-Al
2
O
3
and 146.2 m /g
for Ag/γ, θ-Al
2
O
3
. The slight increase of BET of Ag/γ-Al
2
3
O after Ag addition is
probably due to the formation of interparticle pores [42] or the presence of some Ag
particles on the surface [13].
Transmission electron micrographs of the Ag/Al
Both the alumina supports appear the similar lamellar morphology. 80 particles were
selected for particle size statistical analysis. Compared with Ag/γ-Al , a handful of
larger silver particles are observed on Ag/γ, θ-Al . Ag particles on Ag/γ-Al are
better controlled, displaying a narrower size distribution of 9-17 nm than those on Ag/γ,
θ-Al (11-23 nm). The mean particle size of Ag is around 13.0 nm for Ag/γ-Al
and 16.2 nm for Ag/γ, θ-Al respectively (listed in Table 2). Moreover, it is found
2
O
3
samples are shown in Fig.4.
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
10

that there are 55.6 percentage of Ag particles with size below 13 nm on Ag/γ-Al
2
3
O ,
which is much higher than that on Ag/γ, θ-Al
accordance with that of XRD patterns. Ag/γ-Al
10.3%) of Ag particles than that of Ag/γ, θ-Al
Previous study [43] has proved that the large external surface area benefits the
dispersion of active components. The study from Park et al. [44] reveals that, among
the various crystalline Al supportedPt catalysts, the highest and lowest Pt dispersion
can be got from Pt/γ-Al and Pt/α-Al with the largest and smallest BET surface
area, respectively. The crystalline degree of Al increases with the decrease of the
BET surface area [45]. Additionally, it has been known that the interaction of Pt and
Al contributes to small Pt particle size with high dispersion [39-41]. In comparison to
γ, θ-Al , the formation of more proportion of Al on γ-Al should be conductive
to the higher dispersion of Ag on Ag/γ-Al . Therefore, the higher BET surface area
of the lower crystalline support and the interaction between Ag and Al are responsible
for well controlling Ag particles size and high dispersion of Ag on Ag/γ-Al
XPS spectra of the two catalysts are displayed in Fig. 5. The deconvolution results
of XPS spectra suggest the co-existence of Ag O and metallic Ag species. The Ag 3d5/2
binding energy (BE) at 368.7 eV for both Ag/γ-Al
characteristic of metallic Ag [46], and BE at 367.7 eV or 367.8 eV belongs to Ag
2
O
3
(only 28.4%). This result is in
2
O
3
possesses a higher dispersion
(
2
3
O (8.27%).
2
O
3
2
O
3
2
O
3
2
O
3
Ⅴ
2
O
3
Ⅴ
2
O
3
2
O
3
Ⅴ
2
3
O .
2
2
O
3
and Ag/γ, θ-Al
2
O
3
is
O
2
[
47]. The Al 2p XPS peaks are both symmetric, centered at 74.1eV with a FWHM of
.20 eV and 74.0 eV with a FWHM of 2.06 eV for catalysts Ag/γ-Al and Ag/γ, θ-
Al respectively. The binding energy shifts 0.2 and 0.3 eV to lower BE compared
2
2
O
3
2
O
3
with the standard binding energy (74.3 eV)，indicative of the change of electronic
valence of Al-O bond due to the interaction between Al, O and Ag [48], and the electric
density is transferred from Ag to Al. Correspondingly, Ag shows an increase in ionic
11

character. The stronger the interaction of Al, O and Ag, the lower BE the Al-O bond
shifts to. Thus, it can be concluded that the interaction of Al, O and Ag on Ag/γ, θ-
Al
2
O
3
is stronger than that on Ag/γ-Al
2
3
O . Additionally, the width of the half peak
(FWHM) of the both catalysts become larger than that of the standards (1.41 eV), which
might be due to the incorporation of Ag particles into the alumina lattice [42] or Ag
clusters screening on the alumina surface within a depth of about 20 Å [49]. These
results also provide the valid evidences for the strong interaction between Ag and Al
supports. In the deconvoluted O 1s XPS spectra, the peak at 531.2 eV with FWHM of
.6 eV, is attributed to the lattice oxygen of Al , and the peaks at 532.4 and 532.5 eV
belong to the oxygen of surface hydroxyl (OOH) groups [50]. Ag/γ-Al shows a higher
molar ratio of OOH /Ototal (77.9%) than that of Ag/γ, θ-Al (73.5%).
2
O
3
1
2
O
3
2
O
3
2
O
3
UV-vis spectra shown in Fig. 6 reveal the valence state of Ag supported on Al
2
3
O .
+
For the both samples, the obvious peaks at 220 nm is ascribed to Ag [20, 21, 51] and
the bands ranging from ca. 270 to 335 nm are ascribed to Ag δ+
[20, 21]. The bands at
50 nm and 450 nm are due to Ag clusters [43, 49]. Thus, it can be concluded that both
Ag oxide and metallic Ag exist on Ag/Al combining XPS and UV-vis results. The
between 350 and 450 nm is owing to the
n
0
3
2
O
3
broad and asymmetric peak of Ag/γ-Al
2
O
3
0
multiformity of Ag particles [21]. It can be found from the deconvolution results that
0
the percentage of Ag in total Ag species is 83.8% and 33.6% for Ag/γ-Al
2
O
3
and Ag/γ,
possesses higher proportion of metallic silver than
; whereas the latter has higher proportion of Ag oxide.
θ-Al
Ag/γ, θ-Al
It has been observed from Al NMR that silver species are anchored on different
2
O
3
, respectively. Ag/γ-Al
2
O
3
2
O
3
27
3
+
coordinated Al sites for different Al
electron properties of silver species due to the different interaction between metal and
Al supports [52]. Ag/γ-Al possesses more fractions of Ag-O-Al , while Ag/γ, θ-
2
O
3
, which will result in the different structure and
2
O
3
2
O
3
Ⅵ
12

Al
2
O
3
Ⅳ
possesses more Ag-O-Al entities after Ag addition. It has been known that five-,
3
+
four- and three-coordinate Al ions are assigned to Lewis sites in an order of increasing
acid strength [53]. When Ag is stabilized on coordinatively unsaturated Al via O, the
electrons density would flow from Ag toward O and Al, therefore, the transferring
electron density should be greater on stronger acid sites than on weaker ones. This
means the transferring electron density is higher on Al
Ⅲ
than that on Al
Ⅳ
Ⅴ
or Al . The
study of Hua Deng [38] on the DFT of structure of Ag/γ-Al
2
O
3
revealed that in
comparison with Ag-O-Al
Ⅳ
, Ag-O-Al
Ⅵ
has the weaker Ag-O bonds, which meant
+
metallic Ag is liable to present on the Al
Ⅴ
sites, and Ag tends to form on the Al
Ⅲ
sites.
Therefore, Al electronic environment affects the Ag valence state due to the different
3
+
interaction intensity between Ag and various coordinative Al (described in scheme 1).
For the two supports, the weaker interaction between Ag, O and Al of γ-Al benefits
Ⅴ
2
O
3
the formation of more metallic Ag (Fig.5 and Fig.6 ), while the stronger interaction
+
between Ag, O and Al
Ⅲ
of γ, θ-Al
2
O
3
results in more Ag formation (Fig.5 and Fig.6)
during the process of calcination of the catalysts.
3
.2 HCHO adsorption and catalytic activity over Ag/Al
HCHO-TPD profiles (m/z=30) of both catalysts derived from different Al
supports are shown in Fig. 7. Ag/γ-Al shows higher HCHO adsorption capacity than
Ag/γ, θ-Al , which should be related with the stronger interaction between HCHO
and the catalyst as well as the surface structures of Al . The larger BET surface area
benefits exposing the more adsorption sites, and the appropriate pore volume are critical
to allow guest reactant molecules to diffuse into the host Al [29]. Little CO is
2
O
3
catalysts
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
generated during the process of HCHO-TPD experiments (not shown here). Obviously,
the whole process of HCHO adsorption depends on the physical and chemical
13

properties of the surface of Al
During the HCHO-TPSR experiments, no CO
observed. HCHO-TPSR over Ag/Al catalysts profiles are shown in Fig. 8. When O
is introduced, a large amount of CO is observed on Ag/Al catalysts. In
consideration of little CO generation during the process of HCHO-TPD, it is deduced
2
O
3
supports.
2
generation on Al
2
3
O supports is
2
O
3
2
2
2
O
3
2
that the gas oxygen species plays an essential role in HCHO oxidation and the activation
of gas oxygen species should be related with silver species. It is noticed that the amount
of CO
2
generation on Ag/γ-Al
2
O
3
is much larger than that on Ag/γ, θ-Al
2
3
O , which
should be owing to the larger amount of adsorbed HCHO and easy activation of oxygen
species by more small-sized metallic Ag species on the former.
The catalytic performance of Ag/Al
2
O
3
catalysts for HCHO oxidation on fixed bed
increases with
is shown in Fig. 9, and it is found that the conversion of HCHO to CO
2
the temperature. Ag/γ-Al
2
O
3
catalyst exhibits the higher catalytic activity than Ag/γ, θ-
o
Al
Al
2
2
O
3
3
and reaches 100% of conversion at 85 C (Fig. 9(a)). The long-term tests of Ag/γ-
o
o
O
and Ag/γ-Al
2
O
3
catalysts for HCHO oxidation were carried out at 75 C and 85 C
catalysts exhibit
remains 36% conversion of HCHO and Ag/γ,
remains 25% conversion of HCHO for 24 h. Fig. 9 (c) shows the Arrhenius
plots of HCHO oxidation over Ag/Al samples. The activation energies (E ) are
obtained based on the slopes of the curves between ln(R) versus 1000/T. The activation
energy of Ag/γ-Al is 56.0 KJ/mol, much lower than that of Ag/γ, θ-Al (100.5
respectively, and the results are shown in Fig. 9 (b). Both the Ag/Al
the excellent reaction stability. Ag/γ-Al
θ-Al
2
O
3
2
O
3
2
O
3
2
O
3
a
2
O
3
2
O
3
KJ/mol). The activities of both catalysts dramatically decrease when the reaction is
undertaken with water vapor (50% RH), which is probably owing to the adsorption
competition between water and HCHO. The activity of Ag/γ-Al
2
3
O is still higher than
o
that of Ag/γ, θ-Al
2
O
3
at lower temperatures (<120 C).
14

It is reported that the addition of metallic Ag contributes to the activation of
oxygen species and then improves HCHO catalytic oxidation activity at low
temperature [3], and the formation of small and highly dispersed Ag particles will
enhance the catalytic activity for HCHO oxidation [17]. Therefore, the large amount of
relatively smaller metallic Ag particles with narrow size distribution on Ag/γ-Al
2
O
3
should be responsible for its higher activity.
3
.3 in situ DRIFTS study of HCHO catalytic oxidation
The adsorption spectra of complexes formed on various Ag/Al
2
O
3
catalysts were
mixture gas
catalysts at 3019,
recorded by in situ DRIFT spectrometer. After exposed to HCHO and N
2
o
for 30 minutes at 30 C, the similar bands appear on the both Ag/ Al
2
O
3
2
945, 2867, 2848, 2795, 2743, 2234, 2200, 2130, 1690, 1551/1585, 1492, 1403, 1308,
-1
-1
1
229, 1167, 1067, 963cm (Fig.10). The negative features at 3648-3788 cm for Ag/γ-
-1
Al
2
O
3
and 3648-3775 cm for Ag/γ,θ-Al
2
3
O should be due to the consumption of the
OH groups [53, 54] on the surface arising from the interaction of adsorbed HCHO with
the catalysts surface. These OH groups are corresponded to the different terminal or
bridging species bound to aluminum cations in different coordination [53, 54]. The
intensity of the negative peaks for Ag/γ-Al
Al catalyst, indicating that a larger amount of OH group is consumed on the former
for the formation of the intermediates. The strongest negative band is observed at ca.
2
O
3
catalyst is stronger than that of Ag/γ, θ-
2
O
3
-1
-1
3
730-3745 cm . E. V. Kul’ko [54] declared that the bands of 3730-3735cm are
corresponded to the bridging OH groups on octahedral Al ions. Liu etc. [53] concluded
that pyridine adsorbed on the weak Lewis acid sites is interacting with neighbor OH
-1
Ⅵ
groups at 3729 cm , which belong to bridging OH groups on Al ; Lundie etc. [55]
-1
ascribed 3730 cm to a hydroxyl bridging Al
Ⅳ
Ⅵ
and Al . Although the assignment of the
bands at 3730-3745 cm-1 remains controversial, the accordant result shows its
15

correlation with OH groups on Al sites. Therefore, it is deduced that formaldehyde
Ⅵ
molecules are adsorbed on the Lewis acid sites via the oxygen on HCHO [56], and the
H of HCHO or the intermediates interacts with adjacent hydroxyl groups mostly on the
coordinated Al
and should provide more adjacent hydroxyl groups during HCHO adsorption process
than Ag/γ, θ-Al
Additionally, several bands belonging to the adsorbed HCHO are detected at 2848,
Ⅵ
sites. Ag/γ-Al
2
O
3
catalyst possesses higher fraction of coordinated Al ,
Ⅵ
2
3
O .
-1
-1
2
795, 1167 cm . Meanwhile the peaks at 2743 cm ascribed to combination of δ(CH)
-1
and υ (OCO) in formate [8, 56-58], and two bands at ca.1551, 1585 cm on the two
a
catalysts respectively due to υ(OCO) of formate species [16, 59, 60] are observed.
-1
Dioxymethylene (DOM) species (2945, 2867, 1492, 1403, 1308, 1067 cm ) [56, 61]
-1
and methoxy group (3019 cm ) form during HCHO adsorption process. It is worthy
-1
noted that the weak bands at ca. 1474, 963 cm are assigned to polyoxymethylene
POM) [56], arising from the polymerization of DOM [51]. The bands at 1690 and 1229
(
-1
cm correspond to the C=O stretch and OH bend of HCOOH respectively [58].
-1
Meanwhile adsorbed CO (2234, 2200, 2130 cm ) appears on the both catalysts. The
-
1
bands at 2234, 2200 cm can be ascribed to CO interacting with Lewis acid sites that
belong to corner of tri-coordinated and tetra-coordinated Al [54, 62, 63], and the band
-
1
at 2130 cm belongs to physisorbed CO molecules or H-bonded (CO)
63, 64].
After the adsorption of HCHO for 30 min, 20%O
temperatures and the spectra are recorded. For Ag/γ-Al
H
, respectively
[
2
/N
2
is introduced at different
2
O
3
catalyst (Fig.11 (a)), the
o
intensities of all the peaks gradually decrease as O
2
is introduced at 30 C. When the
o
-1
-1
temperature rises up to 90 C, methoxy (3019 cm ) and formate (2743 cm ) species
almost disappear. All the peaks disappear with further increasing the temperature to
16

o
1
10 C. That gives an evidence of complete transformation of the intermediates which
-1
originate from HCHO adsorption. In addition, a perturbation at ca.1642 cm attributed
to adsorbed water is observed. This should result from the intermediates conversion to
CO
2
and H O on the surface of the catalyst during the oxidation process. Moreover, it
2
can be concluded that POM, DOM, and adsorbed HCHO species are difficult to be
oxidized than methoxy groups and fomate species.
For Ag/γ, θ-Al
2
O
3
(Fig.11 (b)), when 20%O
2
/N
2
is introduced, the intensities of
-1
-1
methoxy (3019 cm ), DOM (2945, 2867, 1492, 1403, 1308, 1067 cm ), formate (2743
-1
-1
-1
cm ), adsorbed formaldehyde (2795, 1167 cm ) and formic acid (1229 cm ) decrease
with the increase of the reaction temperature. Meanwhile the band at 2965 cm-1
assigned to δ (CH) + υas (OCO) in HCOO(a) of bidentate formate [59, 60] emerges at
o
o
5
0 C, and the intensity reaches to maximum at 80 C, then decreases at higher
o
temperature, remaining even at 150 C. The characteristic bands of POM (2980, 1118,
-
1
-1
1
096 cm ) and H
bands belonging to bidentate formate, POM and H
decrease of those of DOM (2867, 1408, 1308, 1067 cm ). Thus, it can be inferred that
bidentate formate and H CO originate from the DOM and methoxy oxidation
3
CO
2
(2952, 1192 cm ) show the same trend. The intensities of these
3
CO increase at the cost of the
2
-1
3
2
respectively. That means the formation of formate and methoxy is due to a Cannizzaro-
type disproportionation of DOM. These results also prove that DOM species are not
completely oxidized on Ag/γ, θ-Al
2
O
3
when O is introduced and temperature rises up
2
o
(
below 150 C) due to its weak activation ability of oxygen.
Based on the above results and conclusions of previous studies [56, 65], the
possible process of formaldehyde catalytic oxidation on Ag/Al
follow:
2
3
O is speculated as
HCHO + O
s
H
2
CO
2
17

H
2
H
2
H
2
CO
CO
CO
2
2
2
+ OH
s
HCOO + H O
2
HCOO + CH
3
O
+ O
s
H
2
O + CO
2
HCOO + O
CH O + O
CO + O
s
CO + H
CO + H
CO
2
O
3
s
2
O
s
2
During the HCHO adsorption and oxidation process, the adsorbed HCHO forms
DOM; DOM is converted to formate by consuming OH groups, or decomposes to
formate and methoxy species via disproportionate reaction; DOM is oxidized to CO ;
2
and formate and methoxy are oxidized to CO and water, finally CO is oxidized to CO .
2
+
The higher ability of the electron-donation of metallic Ag than that of Ag results
in the higher ability of activation and dissociation of molecular oxygen for Ag/γ-Al
66]. Besides, smaller size Ag particles expose more active sites than the larger ones,
which may facilitate the adsorption and activation of oxygen [43], therefore, Ag/γ-
Al with more fraction of small size metallic Ag particles exhibit better catalytic
capacity for HCHO oxidation than Ag/γ, θ-Al
2
3
O ,
[
2
O
3
2
3
O .
To sum up, the coordinatively unsaturated Al on different crystalline phase play
pivotal roles in the state and size of the active components (Scheme 1). The various
3
+
coordinatively unsaturated Al species (Al
process of Ag addition, and determines the interactions between Al, O and Ag. These
interactions are conducive to forming and stabilizing the Ag species. Since Al is more
liable to picking up the transferring electrons than Al , the interaction between Al , O
, O and Ag. The former benefits the formation
Ⅲ
and Al ) acts as electron acceptors in the
Ⅴ
Ⅲ
Ⅴ
Ⅲ
and Ag is stronger than that between Al
Ⅴ
+
0
of Ag , while the latter contributes to the formation of Ag during the process of
impregnation and calcination. Compared to Ag/γ, θ-Al
2
O
3
, Ag/γ-Al
2
3
O possesses
18

higher content of Al
of Ag/γ-Al also contribute to Ag dispersion, formation of smaller size Ag particles
and HCHO adsorption. By consuming more OH group, larger amounts of intermediates
form on Ag/γ-Al than on Ag/γ, θ-Al 3, which results in the higher activity of
Ⅴ
, and thus possess more metallic Ag. Moreover, the higher BET
2
O
3
2
O
3
2
O
HCHO oxidation combining with the higher ability of adsorption and activation of
oxygen on more content of smaller metallic Ag particles with fine dispersion.
4
. Conclusions
In this work, the physicochemical properties of different phase of alumina supports
and Ag/Al
2
3
O samples were remarked, and their catalytic performance for HCHO
oxidation is investigated. Ag/γ-Al
2
O
3
catalyst shows the higher performance for
o
o
formaldehyde oxidation (T100 = 85 C) than Ag/γ, θ-Al
2
O
3
(T100 = 130 C).
The Al chemical environment of different phase Al
2
O
3
determines Ag state and
particles size, and thus the different catalytic activity for HCHO oxidation. Ag species
is mainly anchored on Al
Ⅴ
sites for γ-Al
2
O
3
, forming Ag-O-Al
Ⅵ
entities; while on Al
Ⅲ
sites for γ, θ-Al , forming Ag-O-Al
2
O
3
Ⅳ
entities. The stronger interaction for Ag-O-Al
Ⅳ
+
0
entities leads to the formation of more Ag species, and more Ag with high dispersion
is observed on γ-Al . Additionally, higher fraction of coordinated Al on γ-Al
provides more adjacent hydroxyl groups for HCHO adsorption and formation of
intermediates. The high HCHO oxidation activity on Ag/γ-Al should be related with
2
O
3
Ⅵ
2
O
3
2
O
3
the synergistic effect of OH group and the high activation ability of oxygen species on
0
of smaller Ag particles with high dispersion.
Here we highlight the nature of coordinatively unsaturated Al
and the interaction between Ag and Al with different crystalline phase. The further
investigation and profound understanding are essential for catalyst design in specific
Ⅲ
and Al
Ⅴ
of Al
2
O
3
2
O
3
19

applications.
CRediT author statement
Yi Wang: Conceptualization, Methodology, Formal analysis, Data curation,
Investigation, Writing-Original draft preparation
Zhenping Qu: Supervision, Funding acquisition, Project administration, Resources
Validation, Conceptualization, Writing-Reviewing and Editing
Jing Xu: Visualization, Investigation, Writing-Reviewing and Editing
Bin Huang: Software, Data curation
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Nature Science Foundation
of China (21876019), Fundamental Research Funds for the Central Universities
(DUT19LAB10) and Dalian science and technology innovation Fund (2019J12SN74).
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(
26

Table 1 Al coordinated distribution of different Al
2
O
3
and Ag/Al
2
3
O sample
Samples
γ- Al
%Ag/γ-Al O3
Al
Ⅳ
(%)
Al
Ⅴ
(%)
Al
Ⅵ
(%)
2
O
3
27.4
27.8
27.2
32.5
37.2
4.60
2.70
2.20
2.12
1.18
68.0
4
69.5
70.6
65.4
61.9
2
8
%Ag/γ- Al
2
3
O
3
2
γ,θ- Al O
8
%Ag/γ,θ-Al
2
O
3
27

Table 2 The structure properties of different Al
2
O
3
and Ag/ Al
2
3
O samples
BET surface
Pore
Ag mean
particle
Ag
Average pore
diameter (nm)
Samples
area
volume
dispersion
(%)b
2
3
size(nm)a
(
m /g)
(m /g)
γ-Al
Ag/γ-Al
γ,θ-Al
Ag/γ,θ-Al
2
O
3
262.0
308.0
158.1
146.2
0.537
0.518
0.569
0.634
5.657
4.347
9.663
9.572
2
O
3
13.0
16.2
10.3
8.27
2
O
3
2
O
3
a
b
Determined by HRTEM. Ag crystallites are assumed to be spherical, the relationship between
diameter(d) and dispersion D, is d (nm) =1.34/D [67].
28