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The microstructure and stability of a Ni-nano-CaO/

Al2O3 reforming catalyst under

carbonationecalcination cycling conditions

Xiaochong Xue a, Sufang Wu a,b,*

a College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, Chinab Key Laboratory of Biomass Chemical Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China

a r t i c l e i n f o

Article history:

Received 27 May 2014

Received in revised form

14 November 2014

Accepted 5 February 2015

Available online 20 March 2015

Keywords:

Hydrogen

Steam methane reforming

CatalystCarbonationecalcination cycles

Calcium aluminate

Stability

a b s t r a c t

The paper investigated the effect of carbonationecalcination cycling conditions on the

microstructure and CO2 sorption property of a sorption complex catalyst. The carbonation

operation condition consisted of temperature of 600 C with 20% CO2e80% N2 and 20% CO2

e80% steam atmosphere to simulate the methane reforming reaction conditions; the

calcination condition was 800 C with 100% N2. The BrunauereEmmereTeller (BET) surface

area and thermogravimetric analysis (TGA) were measured to investigate the microstruc-

ture and variation in sorption property of the catalyst after multiple cycles under each

condition. Results showed that the microstructure and CO2 sorption capacity of the sorp-

tion complex catalyst decayed significantly in the initial carbonationecalcination cycles,

especially under a steam atmosphere. X-ray diffraction analysis revealed that a stable

compound Ca12Al14O33 formed gradually during the initial carbonatione

calcination cycleseven at a temperature of 800 C. A model is proposed to explain the observed effect of

carbonationecalcination cycling on Ca12Al14O33 formation. Furthermore, based on our

findings, a new sorption complex catalyst was prepared by pretreating at a high temper-

ature of 900 C. Evaluation of the catalyst prepared by the ReSER hydrogen production

process through 10 circulations revealed significant improvement instability.

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Hydrogen is an environmentally friendly energy source with a

wide array of applications in the chemical and petroleum in-

dustries. Among various feedstock, natural gas is the major

source for production of hydrogen via steam methane

reforming (referred to as SMR) [1,2]. In recent years, an

improved SMR-based method for hydrogen production called

sorption enhanced reaction process (referred to as SERP) hasattracted strong attention. SERP involves the use of a CO2

adsorbent to enhance the methane reforming process by in

situ removal of CO2. It has advantages of low reaction tem-

perature, high concentration hydrogen outlet and controllable

emission of the greenhouse gas CO2 [3e5].

In a traditional SERP process, the CO2 adsorbent and nickel

reforming catalyst are mixed together mechanically, and CaO

wastypicallyselected as the CO2 adsorbent because of its high

* Corresponding author. College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China.

Tel.: þ86 571 87953138; fax: þ86 571 87953735.

E-mail address: [email protected] (S. Wu).

Available online at www.sciencedirect.com

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http://dx.doi.org/10.1016/j.ijhydene.2015.02.032

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CO2 sorption capacity and natural abundance [6,7]. Recently,

Wu etal. [8] proposed a concept of reactive sorption-enhanced

methane reforming (referred to as ReSER)process based on the

use of a sorption complex catalyst that is a composite of a

nano-particulate CaO adsorbent and a Ni-based catalyst in a

single micro-spherical particle. Compared with mechanically

mixed adsorbent and catalyst, this sorption complex catalyst

can reduce resistance to heat and CO2 diffusion transfer dur-ing the reforming process and also offers other advantages

such as increased efficiency for enhanced reforming and

convenient handling in a fluidized bed reactor [9e11].

Despite the advantages of the sorption complex catalyst,

previous studies showed that its catalytic activity decreased

after several ReSER cycles [8]. Various methods have been

proposed to improve its stability and activity. Wang and Feng

[12,13] used ZrO2 and La2O3 as additives to improve the sta-

bility of the catalyst by increasing dispersion of nickel and

inhibiting the formation of NiAl2O4 spinel. Zhang and Tang

[14] used polyethylene glycol (PEG-6000) as a template to

expand the pore size and thereby enhance the dispersion of

nickel to increase the activity. All of the aforementioned so-lutions mainly focused on improving the stability and activity

of nickel catalyst, but ignored the influence of nano-

particulate CaO on the sorption catalyst. Since the

embedded CaO needs undergo multiple carbonationecalci-

nation cycles to keep its sorption activity, this process may

have a negative influence on the sorption complex catalyst.

The effect of carbonationecalcination cycles on the sta-

bility of the catalyst is not clear, and the deactivation mech-

anisms of the catalyst are not well understood. Numerous

studies have found structural destruction and decline in

sorption activity of CaO-based absorbent after multiple car-

bonationecalcination cycles [15e18]. In addition, steam in the

carbonation process has been reported to accelerate the CaOsintering rate, resulting in a rapid decline in its sorption

property [19e21]. However, no study has been undertaken to

understand structural destruction of CaO in the complex

sorption catalyst and its influence on catalyst activity during

carbonationecalcination cycles.

In this study, we prepared Ni-nano-CaO/Al2O3 sorption

complex catalyst and subjected it to three different carbo-

nationecalcination cycling conditions to determine the

mechanism of micro-structural changes and activity decline.

Then, a Ni-nano-CaO/Ca12Al14O33 sorption complex catalyst

was prepared based on our findings in order to improve the

stability of the complex catalyst.

Experiment

Preparation of catalyst

Preparation of Ni-nano-CaO/Al2O3 sorption complex catalyst

The Ni-nano-CaO/Al2O3complex catalyst was prepared by

impregnating Ni(NO3)2 solution on the nano-CaO/Al2O3 sup-

port as described in the literature [13]. The support was pre-

pared by mixing nano-CaCO3 (70 nm, >95% purity, Hu Zhou

LingHua Co., Ltd., China) with alumina sol (10%, Zibo Longao

Co. Ltd., China). The mixed slurry was dried and extruded into

a cylinder with a diameter of approximately 2 mm. The final

Ca/Al molar ratio in the slurry was 1:1.4. The support was

calcined at 550 C f o r 2e4 h and groundto 1.3e1.5 mm. A 0.2 M

Ni(NO3)2 solution (98% purity, Shanghai HengXin Chemical

Reagent Co., Ltd., China) was prepared and infused into the

nano-CaCO3 /Al2O3 support, and the catalyst was dried at

100e150 C and calcined at 500e800 C. The final Ni content

was about 15%, and the catalyst was named “CA-cat1”.

Preparation of Ni-nano-CaO/Ca12Al14O33 sorption complex

catalyst

Preparation of the sorption complex catalyst was similar to

that of the catalyst described in section Preparation of Ni-

nano-CaO/Al2O3 sorption complex catalyst. The nano-

CaCO3 /Al2O3 support was obtained in the same way and

calcined at 900 C for 2 h to form nano-CaO/Ca12Al14O33. Then

impregnated the same Ni(NO3)2 solution as described above.

The catalyst was named ‘CA-cat2’.

Characterization of the sorption complex catalyst

Surface area and pore structures of the sorption complex cata-

lyst were characterized by nitrogen physical sorption in liquid

N2 at 77 K using an apparatus (BELII-mini Japan). Surface area

wascalculatedaccordingtotheBrunauereEmmetteTeller(BET)

formula, and pore size distribution was calculated using the

Barrette JoynereHalenda (BJH) model. Micro-crystallinity of the

samples wasmeasured by X-raydiffraction (XRD) on a Rigaku D/

MAX-RA X-ray diffractometer (Japan) equipped with a copper

anode. The measurement conditions were: voltage 40 kV, cur-

rent 40 mA, and diffraction angle 2q with a range of 10 to 80.

Thermogravimetric analysis (Pyris1, PerkineElmer, USA)

was used to measure the sorption property of the samples.

The sorption process was conducted at 600 C under 0.02 MPa

ofCO2 in N2. Regeneration was performed at 750 C in pure N2.

The sorption capacity was calculated according to Equa-

tion (1):

Sorption capacity ¼CO2 sorption mol amount

Mass of catalyst

1000ðmol=kg Þ (1)

Evaluation of catalyst

Evaluation of ‘CA-cat1’ by carbonationecalcination cycling

A fixed-bed reactor was used for the carbonationecalcination

cycling test. Further details of the fixed-bed are provided

elsewhere [12,13]. Mass flow controllers and water pump wereutilized to achieve the desired inlet gas concentrations. Two

cyclic carbonation conditions were employed: 600 C for 1 h

with 20% CO2e80%N2, and 20% CO2e80% H2O respectively. All

calcinations were carried out at 800 C with 100%N2 for1h.To

determine the effect of temperature variation, a cyclic con-

dition in which the temperature was varied between 600 C

and 800 C with100%N2 for the sametime was used. A total of

30 carbonationecalcination cycles were performed and 10 g of

catalyst was used to obtain different samples that were tested

after the 1st, 3rd, 7th, 15th, and 30th cycles. The samples were

designated as ‘Sn-m’ where ‘n’ represents the carbonation

condition and ‘m’ represents the number of cycle. The con-

dition with pure N2 without carbonation was defined as ‘S1-m’






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for comparison. The carbonation condition with 20%

CO2e80%N2 was defined as ‘S2-m’. The carbonation condition

with 20% CO2e80% H2O was defined as ‘S3-m’.

Evaluation of ‘CA-cat2’ by the ReSER hydrogen production

process

Evaluation of the ‘CA-cat2’ complex catalysts was carried out

in the same fixed-bed as described in section 2.2.1. The con-

dition for ReSER hydrogen production was as follows: 5 g

sorption complex catalyst, 45 mL/min CH4, 180 mL/min H2O,

reaction at 600 C and calcination at 800 C with 100% N2.

Before the reaction, the sorption catalyst was calcined at

800 C with 100% N2 and then reduced under 20%H2, 80%N2

with total gas flow of 100 mL/min. GC analysis was used to

detect the outlet production mixtures. The conversion of

methane was calculated according to Equation (2), where

XCH4 ð%Þ is the conversion of methane, FCH4 ðmL=minÞ is theflow rate of methane, FoutðmL=minÞ is the effluent flow rate of

the product gas, and yCH4 (%) is the methane content.

XCH4 ¼FCH4

Fout yCH4

FCH4

100% (2)

Multiple reactions and regeneration runs were conducted

to evaluate the stability of the sorption complex catalyst.

Results and discussions

Effect of carbonationecalcination cycles on microstructure

and sorption property of the catalyst

The specific surface area of all carbonationecalcination

cycling test samples was shown in Fig. 1. The results showed

that the ‘S1’ condition of use N2 as the environmental gas has

a little change of surface area. Temperature variation between

600 C and 800 C had little effect on the catalyst microstruc-

ture with a slightly specific surface area fluctuation at

20 m2 g 1. Alternatively, conditions of ‘S2’ and ‘S3’ which all

contain CO2 as the environmental gas have an obvious

Fig. 1 e Variation of specific surface area of CA-cat1 under

S1, S2, S3 conditions.

Fig. 2 e

Pore size distribution of CA-cat1 under S1, S2, S3 conditions. (a) 1st cycle, (b) 7th cycle, (c) 15th cycle, (d) 30th cycle.

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decrease of surface area. And contain the steam and CO2 both

of ‘S3’ condition has a big decrease of surface area. The rapiddecay of ‘S2’ and‘S3’ in the initial three cycles indicates that

the catalyst microstructure changes strongly during the initial

carbonationecalcination cycles, while the presence of steam

during carbonation appeared to accelerate the decay rate. No

significant variation was observed in the later cycles, which

implies that the catalyst structure was stable.

Fig. 2(a), (b), (c), (d) show a comparison of the evolution of

pore size distribution of the catalyst under S1, S2, S3 three

different test conditions after 1, 7, 15, 30 cycles respectively.

Two types of meso-pores were found with the main part

around 8e10 nm and the subordination part around 35 nm,

and we defined them as ‘SPs’ and ‘LPs’, respectively. The ‘LPs’

appeared stable during the 30 carbonatione

calcination cyclesat all cycling conditions while the ‘SPs’varied significantly

with changes in the cycling conditions. The maximum in the

curve of dV p /drp vs. rp shifted slightly from 8 nm to 13 nm in

the comparison cycles,which indicated expansion of the pore.

The structure of ‘SPs’ decayed strongly during the carbo-

nationecalcination cycles in the initial 15 cycles with the

presence of steam accelerating the decay rate, while a stable

trend was observed in subsequent 15 cycles. The pore vol-

umes of pores with a diameter of about 13 nm appeared

relatively stable at all cyclic conditions after 30 cycles. These

results show that apart from the influence of high tempera-

ture variation, the carbonationecalcination process has a

major effect on pore evolution, with the small pores decaying during the initial cycles.

To investigate the influence of cycles on CO2 sorption ca-

pacity, each sample obtained under S1, S2, S3 conditions was

tested by thermogravimetric analysis, and the results are

shown in Fig. 3. The declining trend in sorption capacity was

similar to the decrease in the specific surface area, suggesting

that they are related; higher specific surface area could pro-

vide more sorption reactive sites and thereby increase the CO2

sorption capacity. In the cyclic condition of ‘S1’, the CO2

sorption capacity declined from 3.3 mol/kg to 2.5 mol/kg

during 30 cycles, and the decline of ‘S2’, ‘S3’ was significant in

the initial 15 cycles, but only modest in the subsequent 15

cycles. Sintering of CaO during the cycles may have caused

decay of the catalyst's microstructure and sorption capacity,

but further detailed investigation should be conducted to

determine the exact relationship between decay rate and

carbonationecalcination cycles.

Mechanism of the influence of carbonationecalcination on

catalysts

The experiments were designed use CA-cat1 under S1, S2, S3

conditions for 30 cyclic carbonationecalcination runs to study

the relation between the conditions and the formation of

Ca12Al14O33.

XRD was used to measure phase transformation of thesamples after different cycling operations. Data on the sample

from the 30th cycle under each cyclic condition is presented in

Fig. 4. From Fig. 4, there are obvious Ca12Al14O33 peak under S3

condition, and a few Ca12Al14O33 peak is under S2 condition,

and few Ca12Al14O33 peak appeared under S1 condition. The

result of the Ca12Al14O33 peak was the most intense of sample

S3-30 indicates the sample contained the largest amount of

Ca12Al14O33.

Fig. 3 e Sorption capacity of CA-cat1 samples treated under

S1, S2, S3 conditions.Fig. 4 e XRD test result of the 30th sample of CA-cat1 under

S1, S2, S3 conditions.

Fig. 5 e

TEM image of the sample CA-cat1.






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The cyclic reaction between CaO and CO2 may push thecatalyst to an unstable state, and the Al2O3 in the catalyst

support may react with CaO [22,23] according to Equation (3):

12CaO þ 7Al2O3/Ca12Al14O33 (3)

Normally, the reaction shown in Equation (3) occurs only at

temperatures greater than 900 C [22]. However, in this study,

Ca12Al14O33 was found to be formed at temperatures as low as

800 C. A possible reason for this behavior is the use of nano-

sized CaO as CO2 sorbent, and in most previous studies,

micro-sized CaO was used as the CO2 sorbent.

Fig. 5 showed the TEM image of sample of CA-cat1. The

dark point presents the NiO before reduced to Ni as a size of

5e

20 nm. And the other is the support of a mixture of CaO/Al2O3. The size is also under 100 nm. The results proved the

both NiO and CaO/Al2O3 are under a nanometer scale.

To determine the relationship between formation of

Ca12Al14O33 and cycling conditions, we characterized the

samples cycled under the S3 condition in detail s as shown in

Fig. 6. From Fig. 6, Ca12Al14O33 was not detected during the

original three cycles, but was easily detected after the 7th

cycle. This indicates that the quantity of Ca12Al14O33 formed

accumulated during cycling as the as the carbonationecalci-

nation cycles running on. The catalyst we prepared was a

complex of alumina sol and nano-CaCO3, which has a rela-

tively low decomposition temperature [24]. The nano-CaCO3

in the catalyst likely decomposed into nano-CaO at 800 C.

Hence, the reaction shown in Equation (3) must have occurred

during each calcination cycle under a temperature of 800 C.

A possible mechanism of Ca12Al14O33 formation under

different cycling conditions is proposed in Fig. 7. First, after

initial calcination at 800 C, nano CaCO3 decomposed into

nano-CaO, which immediately reacted with Al2O3 to form

Ca12Al14O33. Owing to the smaller molar volume of CaO

compared to that of CaCO3, a pore channel was simulta-

neously formed. If the channel was not filled-in during sub-

sequent cycles, it would prevent continued reaction between

CaO and Al2O3, as shown case 1 in Fig. 7. Therefore, cycling

performed under N2 atmosphere did not lead to accumulationthe quantity of Ca12Al14O33. However, if the channel became

blocked during carbonation, as shown in cases 2 and 3 in

Fig. 7, a new phase interface between CaCO3 and Al2O3 would

have formed and led to the formation of Ca12Al14O33 during

subsequent calcination. This may explain why Ca12Al14O33

accumulated as the number of cycles increased. The effect of

steam is explained as follows: since steam may accelerate

carbonation rate by forming the intermediate Ca(OH)2, the

amount of CaCO3 may have increased during carbonation in

our experiments, thereby resulting in increased amounts of

Ca12Al14O33 after calcination.

A comparison of the sintering behavior of the sorption

complex catalyst under different conditions revealed thatformation of Ca12Al14O33, which consumed the porous Al2O3

carrier, played the largest role in the evolution of micro-

structure of the catalysts. As a result, the specific surface area

of catalysts cycled under the S2 and S3 conditions decreased

Fig. 6 e XRD results of samples under the S3 condition.

Fig. 7 e Mechanism of Ca12Al14 O33formation during the carbonationecalcination cycle.






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substantially, whereas that of the catalyst cycled under the S1

conditions decreased only slightly. The reasons for these be-

haviors are similar in that the sorption capacity changes with

consumption of CaO during the carbonationecalcination

cycling process. Since Ca12Al14O33 is thermally stable at tem-

peratures less than 1000 C [25], it serves as an inert compo-

nent to maintain the skeleton of the complex catalyst and to

prevent further sintering during cycling. This may explain

why the microstructure and sorption capacity stabilized after

several cycles.

Results of ‘CA-cat2’ evaluation by the ReSER hydrogen production process

Based on our finding that formation of Ca12Al14O33led to a

more stable microstructure and sorption properties, a new

sorption catalyst Ni-nano-CaO/Ca12Al14O33 was obtained to

improve the stability of sorption complex catalyst as illus-

trated above (section Preparation of Ni-nano-CaO/Ca12Al14O33

sorption complex catalyst).

Results of stability evaluation of ‘CA-cat2’ based on ReSER

hydrogen production are presented in Fig. 8. Ten runs of the

ReSER hydrogen production were conducted. The first

reformingreaction showedthe highestactivitywith 96.2%(v/v)

hydrogen content and 86.1% methane conversion. The sorp-

tion catalyst showedan improvement in stabilityin theinitial8

reaction cycles with >90.0% (v/v) hydrogen content in theoutlet. Comparison of our results with the previous study [13]

that investigated the stability of Ni-nano CaO/Al2O3 is shown

in Fig. 9. The catalyst Ni-nano CaO/Ca12Al14O33 shows greater

stability than ‘NCA’ with improvement of six reaction cycles.

The sorption complex catalysts were characterized before

and after ReSER hydrogen production by N2 nitrogen physic-

sorption to investigate variation in the microstructure, and

by TGA testing to define the reason of sorption capacity sta-

bility. As is evident from Table 1, the surface area and pore

volume of CA-cat2-0 was much smaller than that of NCA-0,

and the average pore diameter was much larger than that of

NCA-0. However, the microstructure of NCA changed signifi-

cantly after 7 ReSER cycles while the CA-cat2 remained almostthe same after 10 ReSER cycles. This improvement in micro-

structure stability may have contributed to stability of sorp-

tion capacity, making the catalyst highly suitable for ReSER

hydrogen production.

Conclusions

In this paper, the influence of carbonationecalcination con-

ditions on microstructure of the sorption catalyst was studied.

The results showed that microstructure and CO2 sorption

capacity of the sorption complex catalyst decayed signifi-

cantly during the carbonatione

calcination cycle, especiallyunder a steam atmosphere during the initial cycles. This was

caused not only by sintering of CaO to effectively decrease the

surface area, but also because the reaction between Al2O3 and

CaO to form Ca12Al14O33, which decreased the Al2O3 and

reactive CaO content, occurred even at 800 C. A model for

formation of Ca12Al14O33 is proposed. Meanwhile, our results

suggest that formation of Ca12Al14O33 is the main factor

contributing to stability of the subsequently cyclic process.

Based on these results, a new catalyst was prepared by

pretreatment of the nano-CaCO3 /Al2O3 support at a high

temperature to improve the stability of the sorption complex

catalyst. Ten cycles of ReSER process of hydrogen production

has proved the more stable of conversion of methane, while

Fig. 8 e Evaluation of ‘CA-cat2’ catalyst by the ReSER

hydrogen production process. (reaction temperature of

600 C, calcination temperature of 800 C, steam methane

molar ratio of 4, reaction pressure of 1 atm).

Fig. 9 e Evaluation the stability of ‘CA-cat2’ and ‘NCA’

catalysts by the ReSER hydrogen production process.

(reaction temperature of 600 C, calcination temperature of

800 C, steam methane molar ratio of 4, reaction pressure

of 1 atm).

Table 1 e Microstructure variation of CA-cat2 and NCA.

Name Surfacearea

(m2$g 1)

Average poresize(nm)

Porevolume

(cm3$g 1)

Adsorptioncapacity(mol/kg)

NCA-0 33.4 17.2 0.198 2.1

NCA-7 10.7 19.8 0.055 0.9

CA-cat2-0 5.2 49.3 0.064 1.4

CA-cat2-10 5.4 41.1 0.055 1.1






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the catalyst without pretreatment yielded only four cycles of

stable of conversion of methane.

Acknowledgment

We gratefully acknowledge financial supports from the Na-

tional Natural Science Foundation of China (NSFC,Grant No.

21276234).

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