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Significant Promotion Effect of Mo Additive on Novel Ce-Zr Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx with NH3 Shipeng Ding, Fudong Liu, Xiaoyan Shi, Kuo Liu, Zhihua Lian, Lijuan Xie, and Hong He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b00636 • Publication Date (Web): 20 Apr 2015 Downloaded from http://pubs.acs.org on April 22, 2015

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ACS Applied Materials & Interfaces

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Significant Promotion Effect of Mo Additive on

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Novel Ce-Zr Mixed Oxide Catalyst for the Selective

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Catalytic Reduction of NOx with NH3

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Shipeng Ding, Fudong Liu*†, Xiaoyan Shi, Kuo Liu, Zhihua Lian, Lijuan Xie, Hong He*

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State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center

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for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, Haidian

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District, Beijing 100085, P.R. China.

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Fax: +86 10 62849123; Tel: +86 10 62849123

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* Corresponding authors: [email protected], [email protected] (F. Liu); [email protected]

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(H. He)

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† Present address: Materials Sciences Division, Lawrence Berkeley National Laboratory, 1

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Cyclotron Road, Berkeley, CA 94720

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KEYWORDS: selective catalytic reduction, nitrogen oxides, diesel engine exhaust, thermal

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stability, Ce-Zr mixed oxide.

ACS Paragon Plus Environment

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ABSTRACT: A novel Mo promoted Ce-Zr mixed oxide catalyst prepared by homogeneous

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precipitation method was used for the selective catalytic reduction (SCR) of NOx with NH3. The

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optimal catalyst showed high NH3-SCR activity, SO2/H2O durability and thermal stability under

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test conditions. The addition of Mo inhibited the growth of CeO2 particle size, improved the

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redox ability and increased the amount of surface acidity, especially the Lewis acidity, all of

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which were favorable for the excellent NH3-SCR performance. It is believed that the catalyst is

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promising for the removal of NOx from diesel engine exhaust.

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1. Introduction

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Nitrogen oxides (NOx) emitted from stationary and mobile sources have been major

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atmospheric pollutants. The selective catalytic reduction (SCR) of NOx with NH3 or urea has

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been extensively studied as one of the most effective methods to remove NOx, and the most

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widely used commercial catalyst is V2O5-WO3/TiO2 catalyst.1-4 However, there are still some

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inevitable disadvantages with this catalyst system, such as the toxicity of vanadium pentoxide,

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narrow operation temperature window and high conversion of SO2 to SO3 at high temperature.5

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Therefore, it is desirable to develop novel NH3-SCR catalyst which will substitute the

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conventional vanadium based catalyst in recent years.

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Cerium oxide (CeO2) as one of the most abundant rare earth oxides has attracted much

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interest over the past decades in many catalytic reactions such as carbon monoxide oxidation,

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water-gas shift, nitric oxide reduction and reforming reactions.6-8 However, pure CeO2 shows

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poor thermal stability and is susceptible to be sintered at high temperature.9 It has been reported

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that the addition of ZrO2 to CeO2 leads to the improvement in oxygen storage capacity and

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thermal stability.10-11 Ce-Zr mixed oxide combines the highly refractory property of ZrO2 with


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the oxygen storage capacity of CeO2, which can be used as catalyst or support for NH3-SCR of

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NOx. It was found that the nickel and sulfate modification increased the strength of Lewis acid

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sites and enhanced the NH3 adsorption capacity, all of which were beneficial for the

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improvement of activity and selectivity of Ce-Zr catalyst.12 It was also believed that NiO on the

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CeO2 nanorods played an important role for the enhancement of NH3-SCR activity due to the

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higher concentration of Ce3+, larger amount of active Oα, lower amount of energy required for

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oxygen vacancy distortion and strong interaction between NO and NH3.13 The experimental and

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kinetic model results indicated that the MnOx(0.6)/Ce0.5Zr0.5O2 catalyst showed high NO

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conversion and N2 selectivity at low temperatures; and the apparent activation energy of NH3-

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SCR reaction on MnOx(0.6)/Ce0.5Zr0.5O2 (18 kJ/mol) was lower than that on MnOx/TiO2 (38

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kJ/mol).14 A WO3/CeO2-ZrO2 catalyst annealed in air at 800 °C for 1 h still showed high NH3-

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SCR activity.15 The three dimensional ordered macroporous (3DOM) Ce0.75Zr0.2M0.05O2-δ (M =

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Fe, Mn, Co) synthesized by a colloidal template method also exhibited good activity for NH3-

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SCR of NOx; the characterization results showed that the dopants were effectively doped into the

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Ce-Zr oxide solid solution and strong synergistic effect existed between the dopants and Ce-Zr

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oxide, all of which were beneficial for the improvement of NH3-SCR activity.16 It was also found

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that the morphology of CeZrOx had a remarkable effect on the performance of MnOx/CeO2-ZrO2

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catalyst for the NH3-SCR reaction, and MnOx/CeO2-ZrO2 nanorods showed superior activity

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than nanotubes and nanopolyhedra.17 Ce0.75Zr0.25O2-PO43- catalyst prepared by impregnating

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phosphates on Ce0.75Zr0.25O2 still presented high SCR activity at 300-400 °C after hydrothermal-

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aged at 760 °C for 48 h, which might result from the fact that phosphates improved NH3

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adsorption and suppressed the unselective oxidation of NH3 at high temperatures.18 Although

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catalysts containing CeO2-ZrO2 oxide, such as WO3/CeO2-ZrO215, MnOx/Ce0.5Zr0.5O214,


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Ce0.75Zr0.25O2-PO43- and Ni-modified Ce-Zr oxide18, have be used towards NH3-SCR of NO in

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recent years. However, the promotional effect of Mo addition on the NH3-SCR activity over

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CeO2-ZrO2 and the thermal stability of CeMo0.5Zr2Ox catalyst have never been investigated in

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recent publications. In addition, MoO3 is widely used as stabilizer and promoter to improve the

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activity, mechanical and structural properties of V2O5/TiO2 catalyst for NH3-SCR prcocess.19-21

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It was also found that the addition of MoO3 promoted the adsorption and activation of NH3,

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which was conducive to the improvement of NH3-SCR activity of MoO3-doped Ce/TiO2

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catalyst.22

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In order to take full advantage of the high NH3-SCR activity and thermal stability of Ce-Zr

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mixed oxide and promoting effect of MoO3, in the present work, a series of novel Mo promoted

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Ce-Zr catalysts were prepared by homogeneous precipitation method and were applied to NH3-

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SCR of NOx. The catalyst showed high SCR activity, SO2/H2O durability and thermal stability

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under test conditions. The structure, redox ability and acidity of the catalyst were characterized

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using different techniques. The promotional effect of Mo species and the NH3-SCR reaction

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mechanism over the catalyst were fully discussed. The catalyst is quite promising for the NOx

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emission control from diesel engine exhaust.

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2. Experimental

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2.1 Catalyst preparation and activity test

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The catalysts were prepared by a novel and facile homogeneous precipitation method using

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urea as precipitator.23 All chemicals used were of analytical grade. The aqueous solution of

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Ce(NO3)3·6H2O, Zr(NO3)4·5H2O and (NH4)6Mo7O24·4H2O were mixed with required molar

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ratio. Excessive urea aqueous solution was then added to the mixed solution. The solution was


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heated to 90 °C and continuously stirred for 12 h. The pH variation of the mixed solution during

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the homogeneous precipitation process was shown in Figure S1. The initial pH of the mixed

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solution which was measured by pH meter (Sartorius PB-10) was about 1.0 due to the hydrolysis

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of precursors. However, as the preparation time increased from 1.0 h to 4.0 h, substantial amount

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of precipitate was formed and the pH was greatly increased from 1.0 to 7.0 probably because of

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the decomposition of urea. After filtration and washing with deionized water, the resulting

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precipitate was dried at 100 °C overnight and subsequently calcined at 500 °C for 3 h in air. The

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bulk molar ratios of Ce:Mo:Zr in CeMoaZr2Ox series catalysts were analyzed by an inductively

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coupled plasma instrument (OPTMIA 2000DV). As shown in Table S1, the actual proportions of

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Ce:Mo:Zr in the bulk were not in consistence with the designed ones, indicating that not all the

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ions were precipitated in the process of homogeneous precipitation. In addition, the distribution

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of Zr, Ce and Mo derived from SEM-EDX (Hitachi, S-3000N) over CeMo0.5Zr2Ox catalyst

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prepared by a homogeneous precipitation was presented in Figure S2. The Zr exhibited a slight

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poor dispersion; however, the Ce and Mo species were homogeneously dispersed on the surface

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of the catalyst. The catalysts were also calcined at desired temperatures (500, 550, 600, 650, 700,

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750 °C) for 8 h to test the thermal stability. In addition, the CeMo0.5Zr2Ox catalysts were also

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hydrothermal aged at 700 and 760 °C for 48 h to evaluate the hydrothermal stability. The

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calcined samples were crushed and sieved to 40-60 mush for activity test. The catalysts were

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denoted as CeMoaZrbOx where a (a = 0, 0.1, 0.5, 1.0, 1.5) represented the molar ratio of Mo/Ce,

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and b (b = 0.5, 1.0, 2.0, 4.0) indicated the molar ratio of Zr/Ce. The CeMo0.5Zr2Ox catalysts

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calcined at different temperatures for 8 h were denoted as CeMo0.5Zr2Ox-t, where t represented

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the calcination temperature in °C. For comparison, conventional V2O5-WO3/TiO2 catalyst with 3

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wt.% (or 1 wt.%)V2O5 and 10 wt.% WO3 was also prepared using the conventional impregnation


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method. NH4VO3 and (NH4)10W12O41 were used as the source of vanadium and tungsten,

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respectively. After impregnation, the excess water was removed in a rotary evaporator at 60 °C.

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The sample was dried at 100 °C overnight and subsequently calcined at desired temperatures for

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8 h in air.

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The NH3-SCR activity tests were carried out in a fixed-bed quartz tube reactor at

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atmospheric pressure. The weights of catalysts employed were as follows: 1.10 g CeZr2Ox, 0.95

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g CeMo0.1Zr2Ox, 0.75 g CeMo0.5Zr2Ox, 0.67 g CeMo1.0Zr2Ox, 0.56 g CeMo1.5Zr2Ox to maintain

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the GHSV at 50,000 h-1 over various catalysts. The reaction gas contained 500 ppm NO (or [NO]

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= 250 ppm, [NO2] = 250 ppm), 500 ppm NH3, 5 vol.% O2, N2 balance and 500 ml/min flow rate.

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Furthermore, 5 vol.% H2O or 100 ppm SO2 (or 500 ppm SO2) was introduced to the reaction gas

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to test the poisoning effect of H2O/SO2 on NOx conversion over CeMo0.5Zr2Ox catalyst. The

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effluent gas was continuously analyzed by an FTIR spectrometer (Nicolet Nexus 670) equipped

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with a heated, low volume multiple-path gas cell (2 m). The FTIR spectra were collected after 1

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h when the SCR reaction reached a steady state. NOx conversion and N2 selectivity were

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calculated as follows:

NO‫ = ݊݋݅ݏݎ݁ݒ݊݋ܿ ݔ‬൬1 −

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17 18

N2 ‫= ݕݐ݅ݒ݅ݐ݈ܿ݁݁ݏ‬

[NO]out + [NO2 ]out ൰ × 100% [NO]in + [NO2 ]in

[NO]in + [NH3 ]in − [NO2 ]out − 2[N2 O]out × 100% [NO]in + [NH3 ]in

2.2 Characterization

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The N2 adsorption-desorption isotherms over catalysts were obtained at 77 K using a

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Quantachrome Autosorb-1C instrument. Prior to the N2 physisorption, all samples were degassed


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in vacuum at 300 °C for 5 h. The surface areas were determined by BET equation in 0.05-0.35

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partial pressure range. Pore volumes and average pore diameters were determined by the Barrett-

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Joyner-Halenda (BJH) method from the desorption branches of the isotherms.

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Powder X-ray diffraction (XRD) measurements of the catalysts were carried out on a

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computerized PANalytical X’Pert Pro diffractometer with Cu Kα (λ = 0.15406 nm) radiation.

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The data of 2θ from 20 to 80° were collected at 8°/min with the step size of 0.07°.

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Visible Raman spectra of the catalysts were collected at room temperature on a Spex 1877

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D Triplemate spectrograph with the spectral resolution of 2 cm-1. A 532 nm DPSS diode-pumped

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solid-state semiconductor laser was used as the excitation source with power output of 30 mW.

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The time for recording each spectrum was 10 s.

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The H2-TPR experiments were carried out on a Micromeritics AutoChem 2920

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chemisorption analyzer. In a typical measurement, 100 mg of the catalyst was firstly pretreated

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in a flow of 20 vol.% O2/N2 (50 ml/min) at 400 °C for 0.5 h and then cooled down to the room

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temperature (30 °C) followed by Ar purging for 0.5 h. Then the temperature was linearly raised

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at 10 °C/min from 30 to 700 °C in a flow of 10 vol.% H2/Ar (30 ml/min). The H2 consumption

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was monitored by a thermal conductivity detector (TCD).

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NH3-TPD together with NO-TPD were also conducted on Micromeritics AutoChem 2920

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chemisorption analyzer, equipped with a quadrupole mass spectrometer (MKS Cirrus) to monitor

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the signals of NH3(m/z = 17) and NO(m/z = 30). Prior to TPD experiments, the 100 mg samples

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were pretreated at 400 °C in a flow of 20 vol.% O2/N2 for 0.5h and cooled down to room

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temperature. Then the samples were exposed to a flow of NH3 or NO + O2 for 1.0 h. Finally the

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temperature was raised to 600 °C in Ar flow at the rate of 10 °C/min.


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The in situ DRIFTS experiments were carried out on an FTIR spectrometer (Nicolet Nexus

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670) equipped with a smart collector and an MCT/A detector cooled by liquid nitrogen. Prior to

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each experiment, the sample was pretreated at 400 °C for 0.5 h in 20 vol.% O2/N2 and then

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cooled down to 200 °C. The background spectrum was collected in flowing N2 and automatically

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subtracted from the sample spectrum. The reaction conditions were as follows: 500 ppm NH3,

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500 ppm NO, 5 vol.% O2, N2 balance and 200 ml/min flow rate. All spectra were recorded by

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accumulating 100 scans with a resolution of 4 cm-1.

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3. Results and discussion

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3.1 NH3-SCR activity

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Prior to the investigation of CeMoaZrbOx catalyst, the effect of Ce/Zr molar ratio on the

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SCR activity over CeZrOx series catalysts was systematically investigated and the results are

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illustrated in Figure S3. It was found that CeZr2Ox catalyst, with the Ce/Zr molar ratio of 1:2,

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showed the best NOx conversion. Therefore, in the later work, the Ce/Zr molar ratio was fixed at

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1:2. Figure 1 shows the NOx conversion and N2 selectivity as a function of temperature in the

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NH3-SCR reaction over pure MoO3 and CeMoaZr2Ox catalysts under a GHSV of 50,000 h-1.

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CeZr2Ox exhibited narrow operation temperature window and the maximum NOx conversion was

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only 90% at 350 °C. However, the addition of a small amount of Mo (Mo:Ce = 0.1:1) to CeZr2Ox

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led to an obvious enhancement of NOx conversion implying that the coexistence of Mo and

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CeZr2Ox species was very important for the promotion of SCR activity. CeMo0.5Zr2Ox with the

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molar ratio of Mo:Ce = 0.5:1 showed the best NH3-SCR activity and the widest operation

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temperature window, over which 100% of NOx conversion was obtained in a wide temperature

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range (250-400 °C). Further increasing the Mo/Ce molar ratios to 1:1 and 1.5:1 resulted in a


8

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decline of NOx conversion at both low and high temperatures, which was probably due to the

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decrease of active cerium sites on the catalyst surface. Similar results were also proposed by

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Shan et al.24 and Li et al.25 when investigating the CeO2-WO3 and CeO2-MoO3 catalysts. Pure

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MoO3 showed negligible activity in the whole temperature range, indicating that the Mo species

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played only a role as catalyst promoter rather than the active center for the NH3-SCR reaction.

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Although the addition of MoO3 to Ce-Zr catalyst could decrease the N2 selectivity slightly

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owning to the production of N2O, the N2 selectivity over CeMo0.5Zr2Ox was still more than 80%

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below 400 °C. The NH3-SCR results show that some synergistic effect possibly exists between

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Mo and Ce-Zr species, which will be discussed later in this work.

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In addition, the catalytic activity over CeMo0.5ZrbOx with various Zr/Ce molar ratios is

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presented in Figure S4. It was obvious that the NOx conversion over CeMo0.5Zr1Ox and

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CeMo0.5Zr2Ox was similar and was higher than that over CeMo0.5Zr0.5Ox and CeMo0.5Zr4Ox

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below 300 °C. However, CeMo0.5Zr2Ox showed better NH3-SCR activity than CeMo0.5Zr1Ox

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above 350 °C. Therefore, in the following studies, CeMo0.5Zr2Ox was chose to investigate the

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“fast SCR” effect, SO2/H2O resistance together with thermal durability of Mo-containing

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catalyst.

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It was clear that when NO2 (NO2: NO = 1:1) was introduced to the reaction gas, the NOx

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conversion over CeMo0.5Zr2Ox at low temperatures was greatly improved due to the “fast SCR”

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effect, and the NOx conversion was as high as 100% in a wide temperature range from 150 to

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400 °C, see Figure S5 in Supporting Information. Therefore, in practical applications, the diesel

21

oxidation catalyst (DOC) usually is placed in the upstream of the SCR catalyst to convert some

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NO to NO2, which has a significant advantage on the NOx removal. To better evaluate the

23

performance of CeMo0.5Zr2Ox catalyst, the NH3-SCR activity over commercial V2O5(1)-


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WO3(10)/TiO2 catalyst was also conducted and the results are shown in Figure S6. The NOx

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conversion over CeMo0.5Zr2Ox catalyst was higher than that over V2O5(1)-WO3(10)/TiO2, even

3

though CeMo0.5Zr2Ox sample exhibited a relative low N2 selectivity at 400 and 450 °C.

4 5

Figure 1. The NOx conversion and N2 selectivity (inserted) in NH3-SCR reaction as a function of

6

temperature over pure MoO3 and CeMoaZr2Ox (a = 0, 0.1, 0.5, 1.0, 1.5) catalysts. Reaction

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conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol.%, GHSV = 50,000 h-1.

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3.2 Effect of SO2 and H2O

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The combustion exhaust often contains some SO2 and water vapor in the practical

10

application, which may lead to the deactivation of the SCR catalyst.26 Therefore, it is important

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to investigate the effect of SO2 and H2O on SCR activity. In this work, we studied the effect of

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100 ppm SO2 and 5 vol.% H2O on the NOx conversion over CeMo0.5Zr2Ox at 250 °C under the

13

GHSV of 50,000 h-1. As shown in Figure 2A, the introduction of 100 ppm SO2 into the reaction

14

atmosphere did not result in any decrease of NOx conversion for the first 14 h. With the time

15

increasing, the NOx conversion showed a slow decrease which might be associated with the

16

deposition of ammonium sulfate/bisulfate on the surface and the blockage of the active sites.

17

However, the conversion was still more than 90% during the measured period. The result in


10

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Figure 2B showed that, when 5 vol.% H2O was added to the feed stream, the NOx conversion

2

dramatically decreased from the initial 100% to 40% in the first 2 h and then recovered to 100%.

3

It was believed that the deactivation effect of H2O on the activity over catalysts required a long

4

time to achieve steady state. Therefore, when water vapor was first added into the reaction

5

atmosphere, due to the competitive adsorption between H2O and NH3/NO over the catalyst, large

6

amount of H2O was adsorbed on the active sites where the NH3/NO adsorbed and activated,

7

leading to the decline of NOx conversion. However, when the reaction reached steady state 2 h

8

later, the desorption of excess H2O adsorbed on the catalyst made it possible for the adsorption

9

and activation of NH3/NO on the active sites, which was responsible for the increase of NOx

10

conversion. As presented in Figure 2C, when both 100 ppm SO2 and 5 vol.% H2O were

11

introduced, the NOx conversion also showed a relatively significant decrease to 35% and then

12

maintained nearly 50%; after the cutting-off of SO2 and H2O inlet, the activity was rapidly

13

restored to 90%. In short summary, SO2 or H2O alone just showed a negligible influence on the

14

SCR activity; the coexistence of SO2 and H2O decreased the conversion significantly, indicating

15

some synergistic inhibition effect between SO2 and H2O was present on the SCR activity,

16

however, the effect was reversible to a certain extent. It was believed that when both SO2 and

17

H2O were introduced to the reaction atmosphere, the deactivation effect might not only be

18

related to the competitive adsorption between H2O and NH3/NOx, but also associated with the

19

formation of ammonium sulfate together with high thermally stable Ce(SO4)2/Ce2(SO4)3 on the

20

active sites due to SO2, which suggested that both H2O and SO2 could inhibit the adsorption and

21

activation of NH3/NOx on the active sites, leading to an obvious decline of NOx conversion.

22

The reaction temperature had a great impact on the performance of SCR catalyst. Therefore,

23

the SO2/H2O resistance of CeMo0.5Zr2Ox catalyst was also conducted at 400 °C. As shown in


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Page 12 of 35

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Figure S7A, the NOx conversion still remained 100% after the introduction of 100 ppm SO2 at

2

400 °C for 48 h, which was higher than that at 250 °C in Figure 2A. The results indicated that the

3

SO2 tolerance was improved as the reaction temperature increased from 250 to 400 °C. As

4

presented in Figure S7B, 5% H2O alone has an insignificant effect on the SCR activity over

5

CeMo0.5Zr2Ox catalyst at 400 °C and NOx conversion was almost 100%. The results in Figure

6

S7C demonstrated that when both SO2 and H2O were introduced to the reaction gas, the NOx

7

conversion exhibited similar trend to that in Figure 2C.

8

In addition, as shown in Figure S8, 500 ppm SO2 was added to the reaction gas at 250 °C to

9

evaluate the SO2 resistance of CeMo0.5Zr2Ox catalyst. The results showed that the NOx

10

conversion was greatly declined from initial 100% to 30% which was greatly different from the

11

effect of 100 ppm on NOx conversion in Figure 2B, indicating that a higher concentration of SO2

12

would lead to a serious deactivation of the catalyst. However, in real applications the

13

concentration of SO2 from diesel engine exhaust depends on the sulfur content of the diesel fuel.

14

In recent years, the SO2 in diesel engine exhaust is only tens of ppm, which is much lower than

15

500 ppm.


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Page 13 of 35

100 (A) 75 50 on 25 0 0 4

NOx Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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12 16 20 24 28 32 36 40 44 48 52

Time (h) 100 75 50

(B)

25 on 0 0 4

off

250°C,5% H2O 8

12 16 20 24 28 32 36 40 44 48 52

Time (h) 100 75 50

(C)

25 on 0

0

1

off

250°C,100ppm SO2

250°C,100ppm SO2 + 5% H2O

4

8

off

12 16 20 24 28 32 36 40 44 48 52 Time (h)

2

Figure 2. The effect of SO2 (A), H2O (B) and SO2 + H2O (C) on the SCR activity over

3

CeMo0.5Zr2Ox catalyst at 250 °C. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol.%,

4

[SO2] = 100 ppm, [H2O] = 5 vol.%, GHSV = 50,000 h-1.

5

In order to further investigate the influence of SO2/H2O treatment on the CeMo0.5Zr2Ox

6

catalyst, the NH3-SCR activity as a function of reaction temperature of CeMo0.5Zr2Ox catalysts

7

treated in SO2/H2O for 48 h in Figure 2 was retested and the results are shown in Figure 3. Prior

8

to activity measurements of CeMo0.5Zr2Ox catalysts treated in SO2/H2O for 48 h, the catalysts

9

were calcined at 400 ºC for 1 h to eliminate the ammonium sulfate deposited on the surface of

10

the catalysts. Compared to the fresh CeMo0.5Zr2Ox catalyst, the SCR activity of catalysts treated

11

in SO2/H2O showed a slight decrease between 150 and 250 °C. However, the NOx conversion

12

was still kept at more than 90% in the temperature range from 250 to 400 °C. Therefore, the

13

CeMo0.5Zr2Ox catalyst exhibits strong resistance to SO2 and H2O poisoning, which can be used to

14

remove NOx from diesel engine exhaust containing a certain amount of SO2 and water vapor.


13


100

80

NOx Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 35

60

40

CeMo0.5Zr2Ox-fresh CeMo0.5Zr2Ox-treated in SO2

20

CeMo0.5Zr2Ox-treated in H2O CeMo0.5Zr2Ox-treated in SO2 and H2O

0 150

1

200

250

300

350

400

450

Temperature (°C)

2

Figure 3. The NOx conversion in NH3-SCR reaction as a function of temperature over

3

CeMo0.5Zr2Ox catalysts after treatment in SO2/H2O for 48 h. Reaction conditions: [NO] = [NH3]

4

= 500 ppm, [O2] = 5 vol.%, GHSV = 50,000 h-1.

5

3.3 Effect of GHSV

6

The NOx conversion over CeMo0.5Zr2Ox under different GHSVs is shown in Figure 4. It can

7

be seen that the NOx conversion below 250 °C decreased slightly when the GHSV was increased

8

from 50,000 to 200,000 h-1. However, even at the highest GHSV of 200,000 h-1, the NOx

9

conversion was still more than 90% in the temperature range from 250 to 400 °C. The effect of

10

GHSV on NOx conversion at high temperature was weaker than that at low temperatures. These

11

results indicated that the CeMo0.5Zr2Ox catalyst is highly effective for the remove of NOx in a

12

wide temperature range from 250 to 400 °C even when the reaction condition is harsh.


14

Page 15 of 35

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1 2

Figure 4. The NOx conversion in NH3-SCR reaction as a function of temperature over

3

CeMo0.5Zr2Ox catalyst under different GHSVs. Reaction conditions: [NO] = [NH3] = 500 ppm,

4

[O2] = 5 vol.%.

5

3.4 Thermal stability test

6

In practical use, the NH3-SCR catalyst should also have good thermal stability in the long

7

term deNOx process. At the same time, to meet tougher emission standards of diesel engine

8

exhaust, the diesel particulate filter (DPF) has to be used. The diesel exhaust gas temperature can

9

reach above 700 °C during the regeneration process of DPF system.27 Therefore, the

10

CeMo0.5Zr2Ox catalysts were calcined at different temperatures (500, 550, 600, 650, 700, 750 °C)

11

for 8 h to test the thermal stability and the results are illustrated in Figure 5. As the calcination

12

temperature increased from 500 to 700 °C, the NOx conversion below 250 °C showed a slight

13

and monotonic decrease, while the conversion above 250 °C was quite similar. Further

14

increasing the calcination temperature to 750 °C resulted in an obvious decrease of NH3-SCR

15

activity at both low and high temperatures, which might be caused by the decrease of surface

16

area and sintering of the active component. For comparison purpose, the SCR activity of

17

commercial V2O5-WO3/TiO2 catalysts with various V2O5 contents calcined at 700 or 750 ° C


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

1

was also studied. As shown in Figure 5, these catalysts show poor activity in the whole

2

temperature range, which might be due to the decrease of surface area, the anatase-to-rutile

3

transition of TiO2 and the loss of vanadia.15 Compared to the commercial V2O5-WO3/TiO2

4

catalyst, CeMo0.5Zr2Ox catalyst exhibits much better high temperature stability.

5

Moreover, as shown in Figure S9, CeMo0.5Zr2Ox catalyst hydrothermal treated at 700 °C for

6

48 h still exhibited high NH3-SCR activity, over which more than 80% NOx conversion was

7

obtained between 300-400 °C. Further increasing the aged temperature to 760 °C would resulted

8

in an obvious diminution of the SCR activity, indicating that the CeMo0.5Zr2Ox could only be

9

used below 700°C. The hydrothermal stability of CeMo0.5Zr2Ox catalyst needs to be improved

10

further in future study.


16

Page 17 of 35

NOx Conversion (%)

100

(A)

80 CeMo0.5Zr2Ox-500 °C

60

CeMo0.5Zr2Ox-550 °C

40


20


CeMo0.5Zr2Ox-650 °C CeMo0.5Zr2Ox-750 °C

0 150

200

250

300

350

400

450

Temperature (°C) 100

NOx Conversion(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(B)

80 60 V2O5(3)-WO3(10)/TiO2-700 °C

40

V2O5(3)-WO3(10)/TiO2-750 °C V2O5(1)-WO3(10)/TiO2-750 °C

20 0 150

200

250

300

350

400

450

Temperature (°C)

1 2

Figure 5. NOx conversion in NH3-SCR reaction as a function of temperature over CeMo0.5Zr2Ox

3

(A) and V2O5-WO3/TiO2 (B) catalysts calcined at different temperatures for 8 h. Reaction

4

conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol.%, GHSV = 50,000 h-1.

5

3.5 N2 physisorption and XRD results

6

The specific surface area and BJH desorption pore volume of MoO3, CeMoaZr2Ox and

7

CeMo0.5Zr2Ox calcined at different temperatures are shown in Table 1 and 2, respectively. As

8

shown in Table 1, the addition of small amount of Mo (Mo:Ce = 0.1:1) to CeZr2Ox showed a

9

negligible effect on the surface area. However, when the molar ratio of Mo:Ce was more than

10

0.5:1, the surface area of the catalysts decreased obviously as the Mo loading increased due to

11

the quite small surface area of MoO3. It was also reported that the addition of Mo could reduce

12

the surface area of MoO3-CeO2 catalysts, which were prepared by a co-precipitation using urea


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

28

Page 18 of 35

1

as the precipitant

2

volume.

3

Table 1. BET surface area, pore volume and CeO2 Crystallite size of MoO3 and CeMoaZr2Ox

4 5 6

. In the meanwhile, the doping of Mo into CeZr2Ox decreased the pore

Catalysts

SBETa (m2/g)

Pore volumeb (cm3/g)

CeO2 Crystallite sizec (nm)

MoO3

1.4

0.006

-

CeZr2Ox

107.3

0.097

11.8

CeMo0.1Zr2Ox

109.1

0.079

11.4

CeMo0.5Zr2Ox

82.6

0.037

11.2

CeMo1.0Zr2Ox

61.0

0.028

9.6

CeMo1.5Zr2Ox

51.5

0.027

9.5

a

BET surface area.

b

BJH desorption pore volume.

c

CeO2 Crystallite size calculated by Scherrer equation from XRD results.

7

Table 2 shows the surface area and pore volume of CeMo0.5Zr2Ox calcined at different

8

temperatures. It could be seen that increasing the calcination temperature from 500 to 750 °C

9

resulted in a decrease of the surface area from 77.8 to 21.1 m2/g while the pore volume increased

10

gradually. The decrease of surface area and the sintering of the active component at high

11

calcination temperatures might be responsible for the decrease of NOx conversion at low

12

temperatures.29


18

Page 19 of 35

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1

Table 2. BET surface area, pore volume and CeO2 Crystallite size of CeMo0.5Zr2Ox calcined at

2

different temperatures for 8 h Catalysts

SBET (m2/g)

Pore volume (cm3/g)

CeO2 Crystallite size (nm)


77.8

0.038

12.1


75.8

0.040

12.1


71.3

0.058

14.8


47.7

0.070

15.9


33.0

0.070

21.2


21.1

0.063

22.4

3

The XRD results of the catalysts with different Mo loadings are summarized in Figure 6A.

4

The reflections provided typical diffraction patterns for the CeO2 cubic phase (JCPDS 34-0394)

5

and the ZrO2 tetragonal phase (JCPDS 50-1089). No diffraction peaks attributed to Mo species

6

were detected in the XRD patterns, implying that Mo species were finely dispersed on the

7

surface of catalysts or existed as amorphous species. Furthermore, as shown in Table 1, the

8

average crystallite size of CeO2 calculated by Scherrer equation decreased slightly as Mo content

9

increased. As presented in Figure 6B, the XRD patterns of CeMo0.5Zr2Ox samples calcined at

10

various temperatures from 500 to 750 °C for 8 h showed typical diffraction peaks for cubic CeO2

11

phase and tetragonal ZrO2, and no diffraction peaks for Mo species were detected. With the

12

increasing of calcination temperature, the intensity of all the diffraction peaks increased slightly

13

and the CeO2 average crystallite size increased from 12.1 to 22.4 nm (Table 2). However, after

14

high temperature calcination, the position of diffraction peaks stayed the same, indicating that

15

the change of crystallite phases of CeO2 and ZrO2 did not occur in this process.


19


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Page 20 of 35

1 2

Figure 6. Powder XRD of CeMoaZr2Ox catalysts with different Mo loadings (A) and

3

CeMo0.5Zr2Ox catalysts calcined at different temperatures for 8 h (B)

4

3.6 Raman spectra

5

The visible Raman spectra were also applied to characterize the CeMoaZr2Ox catalysts, and

6

the results are presented in Figure 7 CeZr2Ox showed an obvious band at 463 cm-1 and three

7

weak peaks at about 630, 315 and 270 cm-1. The band at 463 cm-1 was due to the Raman active

8

F2g mode of the CeO2 fluorite structure.30 The band at 630 cm-1 could be a consequence of the

9

lattice contraction brought about by zirconium ion insertion.9 The band at 315 and 270 cm-1

10

could be assigned to the zirconium phase (t-ZrO2).31-33 With the increase of Mo content, the

11

Raman spectra intensity of CeO2 decreased significantly, which proved that the introduction of

12

Mo inhibited the growth of CeO2 particle size.25 These analytical results were in good

13

accordance with the conclusion drawn from XRD results. In combination with the XRD and

14

Raman results, it could be concluded that Mo species were mainly present in a highly dispersed


20

Page 21 of 35

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1

state and the addition of Mo resulted in the decrease of CeO2 crystallite size. However, CeO2

2

crystallite size is not the only factor leading to the discrepancy of catalytic activity between

3

different catalysts.

4 5

6

Figure 7. Visible Raman spectra of CeMoaZr2Ox catalysts

3.7 H2-TPR

7

H2-TPR experiments were conducted to investigate the redox ability of the catalysts and the

8

results are shown in Figure 8. For the CeZr2Ox catalyst, the reduction peak at about 510 °C was

9

attributed to the reduction of surface Ce4+ to Ce3+.25, 34 After the introduction of Mo to CeZr2Ox,

10

the reduction peak of surface Ce4+ moved to lower temperatures which indicated that the

11

mobility of surface oxygen was greatly improved due to the strong synergetic effect among Zr,

12

Ce and Mo species. It was believed that the synergetic effect gave rise to sever structural

13

distortion and abundant oxygen defects.16, 35 The oxygen defects facilitated oxygen diffusion

14

from the subsurface layers and might progressively proceed deeper into the bulk.36-37 All the

15

above features were beneficial for the SCR activity. In addition, it was also reported that the

16

mobility of surface oxygen on CeMoAlOx catalyst was enhanced by the Mo addition.38

17

CeMo0.5Zr2Ox exhibited the lowest reduction temperature at 448 °C, which was in harmony with


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

1

its highest SCR activity. However, further increasing the Mo/Ce molar ratios from 0.5 to 1.5, the

2

reduction peaks attributed to overlap of Ce and Mo species shifted to the higher temperatures,

3

which might be caused by the coverage of Mo species. These results indicate that the stronger

4

oxidation reduction ability of CeMo0.5Zr2Ox benefits the excellent SCR reaction Performance.

5 6

Figure 8. H2-TPR profiles of CeMoaZr2Ox catalysts

7

3.8 In situ DRIFTS studies

8

3.8.1

NH3 adsorption

9

The in situ DRIFTS of NH3 adsorption at 200 °C were measured to examine the change of

10

acidity on the catalysts after Mo addition and the results are shown in Figure 9A. After exposure

11

to NH3, the catalysts were covered by several kinds of NH3 species. The bands at 1668 cm-1 and

12

1425, 1440, 1414 cm-1 were assigned to symmetric and asymmetric bending vibrations of NH4+

13

species on Brønsted acid sites.5, 22, 39-41 The bands at 1605, 1595 cm-1 and 1232, 1190 cm-1 were

14

attributed to asymmetric and symmetric bending vibrations of the N-H bonds in coordinated NH3

15

linked to Lewis acid sites respectively.42-43 The bands at 3354 and 3264 cm-1 ascribed to N-H

16

stretching modes of coordinated NH3 were also observed.23 Some negative bands around 3700


22

Page 23 of 35

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1

cm-1 were also found, which could be assigned to the hydroxyl consumption due to the

2

interaction between hydroxyl groups and NH3 to form NH4+.5, 35

3

Compared to CeZr2Ox, the addition of Mo enhanced the band intensities of adsorbed NH4+

4

greatly. This meant that the introduction of Mo resulted in more Brønsted acid sites on the

5

catalysts surface similar results were also found on CeO2-MoO3 catalysts in the published

6

literatures.25, 28 For coordinated NH3 bound to Lewis acid sites, the band intensity firstly had an

7

obvious increase and then showed a decrease when the molar ratios of Mo:Ce was higher than

8

0.5:1. In addition, the NH3-TPD results indicated that the total amount of NH3 adsorbed on

9

CeMo0.5Zr2Ox was higher than that on CeZr2Ox catalyst, see Figure S10A in Supporting

10

Information. The band intensity due to Lewis acid sites over CeMo0.5Zr2Ox were the highest

11

among the series catalysts, which was in good accordance with the results of SCR activity.

12

Although the intensity of adsorbed NH4+ over CeMo1.0Zr2Ox were stronger than that over

13

CeMo0.5Zr2Ox, CeMo0.5Zr2Ox exhibited higher band intensity attributed to coordinated NH3

14

adsorption. It can be concluded that the Lewis acid sites over CeMo0.5Zr2Ox catalyst are

15

responsible for its highest SCR activity. Peng et al. investigated the structure activity relationship

16

of MoO3-CeO2 catalyst for NH3-SCR of NOx and found that the Lewis acid sites could be

17

involved in the reaction even more rapidly than Brønsted acid sites.28

18

3.8.2

NO +O2 adsorption

19

Figure 9B presents the in situ DRIFTS results of NOx adsorption at 200 °C. After NO + O2

20

adsorption and N2 purge, several distinct bands attributed to monodentate nitrate (1506 and 1276

21

cm-1), bidentate nitrate (1593 and 1568 cm-1) and bridging nitrate (1615, 1620, 1241 and 1217

22

cm-1) appeared.39, 44-46 With the increasing of Mo addition content, the bands due to adsorbed


23


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Page 24 of 35

1

nitrate species showed an obvious decrease in intensity, and only bridging nitrate species were

2

observed on the CeMo1.5Zr2Ox catalyst. It is concluded that the addition of Mo significantly

3

limits the adsorption of nitrate species, which could also be confirmed by the results drawn from

4

NO-TPD in Figure S10B. It was believed that the introduction of MoO3 resulted in the formation

5

of more acid sites accompanied by the reduction of basic sites where nitrate adsorbed.22 The

6

inhibitory effect of Mo on the adsorption of nitrate species has also been reported by other

7

researchers on the MoO3-CeO2/TiO222 and CeMo0.5AlOx38 catalysts for NH3-SCR process.

8 9

Figure 9. In situ DRIFTS of NH3 adsorption (A) and NO + O2 adsorption (B) at 200 °C on

10

CeMoaZr2Ox series catalysts

11

3.8.3

Reaction between NO + O2 and adsorbed NH3 species


24

Page 25 of 35

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1

To investigate the reactivity of adsorbed NH3 species in the SCR reaction on the

2

CeMo0.5Zr2Ox catalyst, the in situ DRIFTS of reaction between pre-adsorbed NH3 and NO + O2

3

at 200 °C were recorded as a function of time (Figure 10A). After the introduction of NO + O2,

4

the bands ascribed to ionic NH4+ (1668 and 1425 cm-1) and coordinated NH3 (3354, 3264, 1605

5

and 1232 cm-1) showed an apparent decrease in intensity, and all the bands were replaced by

6

nitrate species after 5 min. This indicates that both coordinated NH3 and ionic NH4+ on

7

CeMo0.5Zr2Ox catalyst could react as reducing agents to reduce NOx. Though the coordinated

8

NH3 over CeMo0.5Zr2Ox catalyst was responsible for its highest SCR activity, the ionic NH4+

9

could also be involved in the SCR reaction. It could be conclude that the addition of Mo to

10

CeZr2Ox resulted in more coordinated NH3 and ionic NH4+, both of which could participate in the

11

NH3-SCR reaction.

12

3.8.4

Reaction between NH3 and adsorbed NOx species

13

The reactivity of absorbed NOx species in SCR reaction on the CeMo0.5Zr2Ox catalyst was

14

also investigated by the in situ DRIFTS of reaction between pre-adsorbed NOx and NH3 at 200

15

°C, and the results are shown in Figure 10B. After exposure to NO + O2, the catalyst was mainly

16

covered by bidentate nitrate (1568 cm-1) and bridging nitrate (1620 and 1217 cm-1). When NH3

17

was introduced, the bridging nitrate was greatly decreased and disappeared in 5 min, while the

18

bidentate nitrate did not show an obvious decrease. At the same time, the bands attributed to

19

ionic NH4+ (1668 and 1425 cm-1) and coordinated NH3 (3354, 3264, 1605 and 1232 cm-1) were

20

observed after 5 min. The results indicate that the bridging nitrate rather than bidentate nitrate

21

can react with NH3. Although the addition of Mo inhibited the adsorption of nitrate species on

22

the surface, the reaction between adsorbed nitrate species and NH3 still played a significant role

23

in the reduction of NOx. Judging from Figure 9B, the amount of nitrate species on CeMo0.1Zr2Ox


25


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Page 26 of 35

1

was higher than other Mo-containing catalysts, indicating that CeMo0.1Zr2Ox should be an

2

excellent catalyst for SCR reaction. However, the lack of enough NH3 species on the surface of

3

CeMo0.1Zr2Ox (Figure 9A) lowered SCR conversion, as seen in Figure 1. The adsorption

4

amounts of coordinated NH3 and nitrate species on CeMo0.5Zr2Ox were both higher than that on

5

CeMo1.0Zr2Ox or CeMo1.5Zr2Ox. It was believed that the adsorption of NH3 and NOx were

6

considered to be essential in the NH3-SCR reaction at low temperatures. Liu et al investigated

7

the promoting effect of MoO3 on the NH3-SCR activity over CeO2/TiO2 catalyst and believed

8

that the unsaturated Mo resulted in more acid sites, which was favorable for the adsorption of

9

NH3 thus improving the low-temperature activity.22 Nitro and nitrate groups were beneficial to

10

promote the SCR process at low temperatures over F-doped CeO2-TiO2 catalyst.47 In addition,

11

the results in Figure 10 showed that both adsorbed NH3 species (coordinated NH3, ionic NH4+)

12

and adsorbed bridging nitrate species were active in the NH3-SCR action. Therefore, it was

13

reasonable to conclude that the high adsorption amounts of NH3 and nitrate species were

14

favorable for the high activity of CeMo0.5Zr2Ox catalyst.


26

Page 27 of 35

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1 2

Figure 10. In situ DRIFTS of NO + O2 reacted with pre-adsorbed NH3 species (A) and NH3

3

reacted with pre-adsorbed NOx species (B) at 200 °C on CeMo0.5Zr2Ox catalyst

4

4. Conclusions

5

A novel Mo promoted Ce-Zr catalyst prepared by homogeneous precipitation method was

6

used for NH3-SCR of NOx. CeMo0.5Zr2Ox catalyst with the Mo/Ce molar ratio of 0.5:1 showed

7

high SCR activity, SO2/H2O durability and thermal stability under test conditions.

8

Characterization results indicated that the Mo addition inhibited the growth of CeO2 particle size,

9

improved the redox ability and increased the amount of surface acidity, especially the Lewis

10

acidity. Coordinated NH3 and ionic NH4+ species together with bridging nitrate were active over

11

the CeMo0.5Zr2Ox catalyst during the NH3-SCR reaction. All the features above are responsible

12

for the excellent NH3-SCR performance.


27


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Page 28 of 35

1

AUTHOR INFORMATION

2

Corresponding Author

3

Fudong Liu*, E-mail: [email protected], [email protected] (F. Liu)

4

Hong He*, E-mail: [email protected] (H. He)

5

Author Contributions

6

The manuscript was written through contributions of all authors. All authors have given approval

7

to the final version of the manuscript.

8

ACKNOWLEDGMENT

9

This work was supported by the National Natural Science Foundation of China (51221892) and

10

the Ministry of Science and Technology, China (2013AA065301).


28

Page 29 of 35

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1 2 3 4 5


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6

(3) Brandenberger, S.; Kröcher, O.; Tissler, A.; Althoff, R., The State of the Art in Selective

7

Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts. Catal.

8

Rev. 2008, 50, 492-531.

9

(4) Grossale, A.; Nova, I.; Tronconi, E., Ammonia Blocking of the “Fast SCR” Reactivity over

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a Commercial Fe-zeolite Catalyst for Diesel Exhaust Aftertreatment. J. Catal. 2009, 265,

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141-147.

12

(5) Liu, F. D.; He, H.; Ding, Y.; Zhang, C. B., Effect of Manganese Substitution on the Structure

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and Activity of Iron Titanate Catalyst for the Selective Catalytic Reduction of NO with NH3.

14

Appl. Catal., B 2009, 93, 194-204.

15 16

(6) Paier, J.; Penschke, C.; Sauer, J., Oxygen Defects and Surface Chemistry of Ceria: Quantum Chemical Studies Compared to Experiment. Chem. Rev. 2013, 113, 3949-3985.

17

(7) Ma, Z. R.; Weng, D.; Wu, X. D.; Si, Z. C., Effects of WOx Modification on the Activity,

18

Adsorption and Redox Properties of CeO2 Catalyst for NOx Reduction with Ammonia. J.

19

Environ. Sci.-China 2012, 24, 1305-1316.

20 21

(8) Zhang, D.; Du, X.; Shi, L.; Gao, R., Shape-Controlled Synthesis and Catalytic Application of Ceria Nanomaterials. Dalton Trans. 2012, 41, 14455-75.

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Table of Contents Graphic and Synopsis

The addition of Mo significantly improved the NH3-SCR performance of CeZr2Ox catalyst. Compared to the commercial V2O5-WO3/TiO2, Mo-containing CeMo0.5Zr2Ox catalyst exhibited much better high thermal stability.


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Significant Promotion Effect of Mo Additive on a ... - ACS Publications

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