P. Gao et al. / Catalysis Communications 50 (2014) 78–82
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Table 2
The basicity and distribution of basic site over reduced CHTx-CO3 and CHTx-F samples.
Sample
Number of total basic sites
Number of basic sites (μmol g−1) and contributiona
(μmol g−1
)
Site α
Site β
Site γ
CHT0.08-CO3
CHT0.24-CO3
CHT0.08-F
200.0
234.0
210.9
257.4
78.1 (39.0)
85.8 (36.7)
56.5 (26.8)
59.8 (23.2)
87.5 (43.8)
88.9 (38.0)
92.9 (44.1)
106.7 (41.5)
34.4 (17.2)
59.3 (25.3)
61.5 (29.2)
90.9 (35.3)
CHT0.24-F
a
The value in the parenthesis is the contribution of single basic site to the number of total basic site.
Table 3
The catalytic performance over the CHTx-CO3 and CHTx-F catalysts.
Sample
CO2 conversion
(%)
CH3OH selectivity
(%)
TOF1 × 103
(s−1
TOF2 × 103
(s−1
TOF3 × 103
(s−1
CH3OH yield
)
)
)
(g gcat−1 h−1
)
CHT0.08-CO3
CHT0.24-CO3
CHT0.08-F
21.9
22.5
20.5
21.1
45.3
46.9
52.3
53.5
8.77
9.11
12.75
12.90
11.02
10.64
8.00
4.88
4.91
4.92
4.76
0.39
0.41
0.43
0.44
CHT0.24-F
7.55
Reaction conditions: T = 523 K, P = 5.0 Mpa, GHSV = 4000 h−1, H2:CO2 (molar ratio) = 3:1.
Gaussian peaks, which could be assigned to the weakly (α peak), mod-
erately (β peak) and strongly (γ peak) basic sites, respectively. It could
be seen from Table 2 that the number of total basic sites increased with
the introduction of fluorine. The increase was also observed for the
number of basic sites β and γ, which can be explained as follows:
For CHTx-CO3 samples, the weakly basic sites were related to the OH−
groups, the moderately basic sites were ascribed to the metal–oxygen
pairs and the strongly basic sites were associated with the coordinatively
unsaturated O2− ions [20]. Upon introduction of fluorine, the metal–fluo-
rine pairs and the unsaturated F− ions were formed after calcinations [13,
14]. The weakly, moderately and strongly basic sites in CHTs-F might be
then attributed to the OH− groups, metal–oxygen and metal–fluorine
pairs, and coordinatively unsaturated O2− and F− ions, respectively. The
amount of metal–oxygen pairs was not affected by the formation of
metal–fluorine pairs, and the strongly basic sites (O2− or F−) in CHTx-F
were mainly related to the cleavage of metal–oxygen and metal–fluorine
pairs [13]. Therefore, the formation of metal–fluorine pairs and the unsat-
urated F− ions could increase the amounts of the moderately and strongly
basic sites.
higher than that for CHTx-CO3. Therefore, the introduction of fluorine
into Cu/Zn/Al/Zr mixed oxides greatly improved the strongly basic sites,
and then significantly increased the CH3OH selectivity. Moreover, in
spite of the lower CO2 conversion, CH3OH yields over CHTx-F catalysts
were higher than those on CHTx-CO3 catalysts due to the much higher
CH3OH selectivity.
The turnover frequency was also calculated for various catalysts. As
depicted above, there are two kinds of active sites (Cu and basic sites)
participating in the CO2 hydrogenation to methanol. Therefore, two
sets of TOF were calculated via dividing the reaction rate by the number
of surface metallic Cu sites (TOF1) as well as by the total amount of
moderately and strongly basic sites (TOF2). As shown in Table 3, the
values of TOF1 increased with the introduction of fluorine, while the
TOF2 decreased. The result reveals that the Cu or basic sites alone cannot
explain the differences in the activity of catalysts, because the value of
TOF should be a constant if only one active site is involved in the rate-
determining step and the active sites are all equally active [9,21].
Some authors believed that there was a synergy between the metal Cu
and the oxides and the metal/oxides interface played an important
role in the CO2 hydrogenation to methanol [22,23]. In addition, it is
generally accepted that the rate limiting step is the formation and
hydrogenation of the reactive intermediates presented on the basic
sites [9,22]. Accordingly, the CO2 hydrogenation reaction possesses the
structure-sensitive character, which was also proposed by other
researchers [9,22,24]. In our case, TOF3 was also calculated via dividing
the reaction rate by the total number of surface Cu sites and basic sites
(Table 3). It is found that the change of TOF3 is much smaller comparing
with the date of TOF1 or TOF2, suggesting that the activity of catalysts is
more closely related to the synergy between the Cu and basic sites.
The contributions of single basic site to the total basic sites are also
listed in Table 2. It could be seen that the proportion of strongly basic
sites to the total basic sites increased remarkably with the introduction
of fluorine, while the change of the contribution of moderately basic
sites was small.
3.3. Catalytic performance
The catalytic performance of the CHTx-CO3 and CHTx-F catalysts for
CO2 hydrogenation to methanol is summarized in Table 3. The conversion
of CO2 for CHTx-F was slightly lower than that for CHTx-CO3 despite the
significant decrease of SCu (Table 1), indicating that the relationship
between activity and SCu was not simple linear. Similar results were also
obtained in the recent works of Sun et al. and Arena et al. [4,21]. It is note-
worthy that the CH3OH selectivity for CHTx-F is markedly higher than
that for CHTx-CO3 sample. Based on the reaction mechanism proposed
in our previous research [5,15], both methanol and CO were produced
mainly via the hydrogenation and decomposition of intermediate species
formaldehyde (H2CO). Compared with the H2CO species adsorbed on
moderately basic sites, those species adsorbed on strongly basic sites
preferred to be hydrogenated into methanol rather than dissociate to
form CO; thus the methanol selectivity was related to the proportion of
strongly basic sites to the total basic sites. According to the results of
CO2-TPD, the proportion of strongly basic sites for CHTx-F was much
4. Conclusions
The fluorine-modified Cu/Zn/Al/Zr catalysts were successfully
synthesized by calcination of the fluorine-containing Cu/Zn/Al/Zr
hydrotalcite-like compounds. The introduction of fluorine showed a
significant influence on the physicochemical and catalytic properties of
the catalysts. With the introduction of fluorine, the BET specific surface
area and the copper surface area decreased remarkably, while the
number and the proportion of strongly basic sites increased significantly.
For the fluorine-modified Cu/Zn/Al/Zr catalysts, the CO2 conversion
decreased slightly, while the CH3OH selectivity increased significantly
due to the remarkable increase of the proportion of strongly basic sites,
and the CH3OH yield was higher compared with the fluorine-free