Catalytic oxidation of ammonium ions with nitrite ions in water over metallic platinum supported on titanium dioxide at a mild reaction temperature

Article abstract of DOI:10.1246/cl.2008.1024

Titanium dioxide-supported platinum catalyst, which was pretreated with H₂ at 353 K in a reactor just before the reaction, exhibited extremely high activity toward the oxidation of ammonium ions with nitrite ions into nitrogen in water at 353 K. Copyright

Full text of DOI:10.1246/cl.2008.1024

1024
Chemistry Letters Vol.37, No.10 (2008)
Catalytic Oxidation of Ammonium Ions with Nitrite Ions in Water over Metallic Platinum
Supported on Titanium Dioxide at a Mild Reaction Temperature
Yoshinori Sakamoto,1 Chengkai Wang,2 Toshio Okuhara,1 and Yuichi Kamiyaꢀ1
1Research Faculty of Environmental Earth Science, Hokkaido University, Kita 10 Nishi 5, Sapporo 060-0810
2Graduate School of Environmental Science, Hokkaido University, Kita 10 Nishi 5, Sapporo 060-0810
(Received July 24, 2008; CL-080721; E-mail: kamiya@ees.hokudai.ac.jp)
Titanium dioxide-supported platinum catalyst, which was
pretreated with H2 at 353 K in a reactor just before the reaction,
exhibited extremely high activity toward the oxidation of
ammonium ions with nitrite ions into nitrogen in water at 353 K.
(BEL-CAT, Bel Japan, Inc.). The CO/Pt stoichiometry was
assumed to be 1. Before the measurement, the catalyst was
reduced under a flow of Hþ2 at 353 K fꢁor 2 h.
The oxidation of NH4 with NO2 in water was performed
using a continuous fixed-bed reactor at 353 K. The reaction mix-
ture was an aqueous solution of NaNO2 (Wako Pure Chem. Ind.
Ltd., 4.30 mmol dmꢁ3) and (NH4)2CO3 (Wako Pure Chem. Ind.
Ltd., 2.15 mmol dmꢁ3). The catalyst powder (2 g) was fixed in
the reactor, and H2 gas was fed into the reactor at 353 K. After
stopping the flow of H2 gas, the aqueous reaction mixture was
fed into the reactor. The liquid at the reactor outlet was intermit-
tently collected, and the concentrations of NO3ꢁ, NO2ꢁ, and
NH4þ were determined using a flow injection analysis (FIA) sys-
tem. Gas products (N2 and N2O) at the reactor outlet were ana-
lyzed by using a gas chromatography equipped with a thermal
conductivity detector (Shimadzu GC-8A). Prior to use, the water
was deoxygenated by bubbling dry N2 (ca. 500 cm3 minꢁ1) into
the water, and just before the reaction, the reaction solution was
deoxygenated also by bubbling dry N2. The concentration of
dissolved oxygen in the reaction solution was measured by using
an oxygen analyzer (YSI, Model DO200 DO). The amount of
dissolved Pt during the reaction was measured by using induc-
tively coupled plasma atomic emission spectroscopy (Shimadzu,
ICPS-7000) on the aqueous solution at the reactor outlet.
Figure 1 shows time courses for the oxidation of NH4þ with
In the biological process anaerobic ammonia oxidation
(Anammox), which was discovered in 1995,1 bacteria þcalled
Candidatus ‘‘Brocadia anammoxidans’’2 oxidizes NH4 into
ꢁ
N2 with NO2 (eq 1).3 Anammox process is attractive as a
means to remove ammonia from wastewater1,4 but has some
problems, including the difficulty in developing enrichment
cultures of the bacteria and in immobilizing them in the reactor.
Thus, if a reaction like that in eq 2, which is similar to the
Anammox process, proceeds over a heterogeneous catalyst,
it should be possible to remove ammonia from wastewater.
NH4þ þ 1.32NO2ꢁ þ 0.066HCO3ꢁ þ 0.13Hþ
!
1.02N2 þ 0.26NO3ꢁ þ 0.066CH2O0:5N0:15 þ 2.03H2O ð1Þ
ꢁ
NH4þ þ NO2 ! N2 þ 2H2O
ð2Þ
In addition, the reaction in eq 2 can apply the decompositioꢁn
þ
of NH4 produced during the catalytic hydrogenation of NO3
in groundwater, because there are only few catalysts with accept-
able selectivity for keeping the concentration of NH4þ below the
allowable level (0.03 mmol dmꢁ3) in drinking water, although a
number of investigatioꢁns have been carried out on the catalytic
hydrogenation of NO3 over bimetallic catalysts.5 Nitrite ions
contributed for this can be produced by the selective hydrogena-
tion of NO3ꢁ to NO2ꢁ over a Cu–Pd bimetallic cluster catalyst at
alkaline pH conditions.6
ꢁ
NO2 over Pt/TiO2. The catalyst, which was not reduced under
the H2 flow at 353 K just before the reaction, showed only a very
low activity (Figure 1, ). In contrast, when the catalyst was
reduced under a H2 flow at 353 K and a deoxygenated reaction
solution (concentration of O2 was 0.003 mmol dmꢁ3) was utiliz-
ed, Pt/TiO2 showed a high activity (Figure 1, ), and the initial
conversion was almost 100%. However, when a reaction solu-
tion with an O2 concentration of 0.1 mmol dmꢁ3 was utilized
with Pt/TiO2, only a moderate conversion even at the initial
stage of the reaction was achieved, and pronounced deactivation
occurred, even if the catalyst was reduced with H2 just before
the reaction (Figure 1, ). Joko and Nakahara have reported
that, in order to perform the reaction in eq 2 over Pt/TiO2, a high
temperature of about 473 K is needed.7 However, our results
þ
Joko and Nakahara have reported the oxidation of NH4
ꢁ
with NO2 (eq 2) over a Pt/TiO2 catalyst.7 However, a high
reaction temperature (433 K) is necessary for the reaction.
If the reaction can proceed under mild reaction temperature
þ
(<373 K), the removal of NH4 from water would become
practical. We report here that metallic platinum þsupported oꢁn
TiO2 effectively catalyzed the oxidation of NH4 with NO2
into N2 in water at 353 K when the catalyst was pretreated
with H2 in the reactor just before the reaction.
ꢁ
clearly show that NH4þ could be effectively oxidized with NO2
Platinum supported on TiO2 (Aerosil P-25, 46 m2 gꢁ1),
Al2O3 (JRC-ALO-4, 166 m2 gꢁ1) or on SiO2 (Tosoh, 262
m2 gꢁ1) was prepared by using incipient wetness impregnation
with an aqueous solution of H2PtCl6 (Wako Pure Chem. Ind.
Ltd., 96 mmol dmꢁ3). The loading amount of Pt was adjusted
to 1 wt %. After drying overnight at 373 K, the catalysts were
calcined in air at 523 K for 3 h and then were treated under a
flow of H2 at 723 K for 2 h.
even at low reaction temperature (353 K) when Pt/TiO2 was
treated with H2 in the reactor just before the reaction and a
deoxygenated reaction solution was used.
Using GC analysis of the gas at the reactor outlet, it was
confirmed that the main gas product was N2 (>95%), although
a small amount of N2O (<5%) formed. NO and NO2 were not
detected by using GC analysis. In addition, the conversion of
ꢁ
NH4þ was almost equal to that of NO2ꢁ (data not shown). NO3
The dispersion of Pt was estimated from the adsorbed
amount of CO at 323 K by using an automatic apparatus
in the aqueous phase was not detected by using FIA. These
results indicate that the reaction proceeds basically according
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.10 (2008)
1025
þ
ꢁ
þ
Figure 1. Time courses of the oxidation of NH4 with NO2
Figuꢁre 2. Time cꢁourses of the oxidation of NH4 with
(NH4þ:NO2 = 1.0:1.0) over 1.0 wt % Pt/TiO2. ( ) The cata-
lyst was reduced with H2 in the reactor just before the reaction,
and a deoxygenated reaction solution was used; ( ) the catalyst
was reduced with H2 in the reactor just before the reaction, and
the reaction solution was not deoxygenated; ( ) the deoxygenat-
ed reaction solution was used, but the catalyst was not reduced
with H2 just before the reactiꢁon. Reaction conditions: tempera-
ꢁ
NO2 (NH4þ:NO2 = 1.0:1.0) over various Pt catalysts. (
1.0 wt % Pt/TiO2, ( ) 1.0 wt % Pt/Al2O3, and ( ) 1.0 wt %
)
Pt/SꢁiO2. Reaction conditions: temperature, 353 K; concn of
þ
NO2 from NaNO2, 4.3 mmol dmꢁ3; concn of NH4 from
(NH4)2CO3, 4.3 mmol dmꢁ3; WHSV = 15 hꢁ1
.
oxidation of the Pt surface or adsorption of oxygen onto it.
However, the activity was fully recovered by reduction of the
Pt/TiO2 with H2 in the reactor.
ture, 353 K; cþoncn of NO2 from NaNO2, 4.3 mmol dmꢁ3
;
concn of NH4 from (NH4)2CO3, 4.3 mmol dmꢁ3; WHSV =
15 hꢁ1. Regeneration of the catalyst was carried out by treating
Pt/TiO2 with H2 in the reactor at 353 K for 20 h.
References and Notes
1
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to eq 2. Since the turnover number (TON) from 0 to 13 h for
the reaction over Pt/TiO2 was 17, it is obvious that the reaction
is catalytic.
2
3
The conversion gradually decreased over time; however, no
Pt was detected in the aqueous phase at the reactor outlet by
using ICP-AES analysis. When the catalyst was reduced under
the H2 flow again in the reactor at 353 K for 20 h (Figure 1, at
75 h), the catalytic activity was completely recovered; 100%
conversion was obtained. The catalyst could be regenerated at
least three times. The XRD pattern of the deactivated catalyst
was the same as that of the initial one (data not shown). From
these results, it was concluded that metallic Pt showed a high ac-
tivity for the reaction, and the Pt surface was deactivated owing
to oxidation or adsorption of oxygen during the reaction. Actual-
ly, when the reaction was performedꢁusing a reaction solution
4
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containing excess NH4 (NH4þ:NO2 = 1.5:1.0), deactivation
´
62, 77; J. Sa, S. Gross, H. Vinek, Appl. Catal., A 2005,
was remarkably suppressed, and 100% conversion was main-
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enceþof H2, NO2 was reacted with H2, only low conversion of
NH4 was obtained.
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Supporting Information is available electronically on the
CSJ-Journal Web site; http://www.csj.jp/journals/chem-lett/.
Figure 2 shows time courses for the oxidation of NH4þ with
ꢁ
NO2 over Pt/TiO2, Pt/Al2O3, and Pt/SiO2. Contrary to Pt/
TiO2, Pt/Al2O3, and Pt/SiO2 showed only low activity. The
dispersions of Pt, estimated from the amount of CO adsorbed
on the catalyst, were 12%, 25%, and 10% for Pt/TiO2, Pt/
Al2O3, and Pt/SiO2, respectively, which do not correlate with
the activities, suggesting that the electronic state of Pt controls
the catalytic activity, but further investigations are needed.
In conclusion, Pt/TiO2, which was reduced with H2 in a re-
actor just before the ꢁreaction, effectively catalyzed the oxidation
6
7
8
þ
of NH4 with NO2 in water at a mild reaction temperature
(353 K). The activity of Pt/TiO2 gradually decreased because of
Published on the web (Advance View) August 30, 2008; doi:10.1246/cl.2008.1024