5518 Phys. Chem. Chem. Phys., 2012, 14, 5518–5526 This journal is c the Owner Societies 2012

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 5518–5526

CO2 hydrogenation on a metal hydride surface

Shunsuke Kato,aAndreas Borgschulte,

aDavide Ferri,

cMichael Bielmann,

a

Jean-Claude Crivello,dDaniel Wiedenmann,

bMagdalena Parlinska-Wojtan,

e

Peggy Rossbach,fYe Lu,

cArndt Remhof

aand Andreas Zuttel*

a

Received 17th October 2011, Accepted 23rd February 2012

DOI: 10.1039/c2cp23264b

The catalytic hydrogenation of CO2 at the surface of a metal hydride and the corresponding

surface segregation were investigated. The surface processes on Mg2NiH4 were analyzed by in situ

X-ray photoelectron spectroscopy (XPS) combined with thermal desorption spectroscopy (TDS)

and mass spectrometry (MS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS).

CO2 hydrogenation on the hydride surface during hydrogen desorption was analyzed by catalytic

activity measurement with a flow reactor, a gas chromatograph (GC) and MS. We conclude that

for the CO2 methanation reaction, the dissociation of H2 molecules at the surface is not the rate

controlling step but the dissociative adsorption of CO2 molecules on the hydride surface.

Introduction

The way towards a chemical energy carrier from renewable

energy may be taken via two strategies: the first one develops

technologies to make hydrogen from water the novel energy

carrier; the second is the implementation of processes for the

production of established fuels such as hydrocarbons from

CO2, water and renewable energy. Both strategies have their

strengths and weaknesses: while the public acceptance of

hydrocarbons as synthetic fuels can be taken as granted, there

is concern about the safety of hydrogen. However, hydrogen is

the most efficient synthetic chemical energy carrier, with respect to

both production and conversion, if compared to the equivalent

processes involving hydrocarbons. The availability of a safe and

effective way to reversibly store hydrogen is one of the major

issues for its large scale use as an energy carrier. Solid state

hydrogen storage offers a safe alternative to compressed gas or

liquefaction.1,2 Currently, however, materials do not fulfil all

safety, volumetric, energy density and economic requirements.

The surface reactions on hydrogen storage alloys might play a

role for the effective hydrogen deliver in hydrogenation reactions,

for example, in hydrocarbon productions from CO2. Interestingly,

many scientific questions in hydrogen storage have their counter-

part in the catalytic processes during the synthesis of hydrocarbons

from hydrogen and CO2. Furthermore, there are analogies

between water electrolysis and the electrochemical reduction

of CO2, eventually forming hydrocarbons. Apart from hydro-

carbons, synthetic fuels may be methanol, DME (dimethyl

ether) or formic acid.1,3,4 The eventual market penetration of

the different fuels will depend on various parameters, such as

the efficiency of the whole cycle, abundance of the involved

materials, both parameters defining the price, the environmental

friendliness, and the consumers’ acceptance.Within this parameter

space, methane as a synthetic fuel has large potential, because its

production is highly efficient (see later), most materials (e.g. Ni

catalysts) are relatively abundant, it is non-poisonous although a

green-house gas, and consumers do use it in large quantity as

natural gas already. The use of the existing natural gas and LNG

infrastructures available around the world is the great advantage

of synthetic methane as an energy carrier.5 For example, in Japan

90% of city gas is from LNG.Moreover, currently the natural gas

conversion into liquid fuels via the Fischer–Tropsch process is

being realized on an industrial scale.6 The economic issue for the

synthetic methane is the hydrogen production cost.5

This study focuses on the catalytic property of the hydride

surface of Mg2NiH4 for hydrogenation of CO2, the Sabatier

reaction:

CO2 + 4H2 - CH4 + 2H2O DG298K = �110 kJ mol�1

at the hydride surface. The CO2 reduction process allows CO2

recycling in the form of hydrocarbons, e.g. methane, closing

the carbon cycle as mentioned above.5 The Sabatier reaction

is highly exothermic (DH298K = �165 kJ mol�1) and the

heat recovery is a key element of an industrial process.7,8

a Laboratory for Hydrogen & Energy, Dubendorf, Empa,Swiss Federal Laboratories for Materials Science and Technology,Uberlandstrasse 129, CH-8600 Dubendorf, Switzerland.E-mail: andreas.zuettel@empa.ch; Tel: +41 58 765 4038

bDepartment of Geosciences, University of Fribourg, Fribourg,Switzerland

c Laboratory for Solid State Chemistry and Catalysis, Empa,Swiss Federal Laboratories for Materials Science and Technology,Dubendorf, Switzerland

d ICMPE-CMTR, CNRS, Thiais, Francee Electron Microscopy Center, Empa, Swiss Federal Laboratories forMaterials Science and Technology, Dubendorf, Switzerland

f Laboratory for Nanoscale Materials Science, Empa,Swiss Federal Laboratories for Materials Science and Technology,Dubendorf, Switzerland

PCCP Dynamic Article Links

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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 5518–5526 5519

The process delivers methane as a gaseous product and the

compression or liquefaction of methane is necessary for the

energy dense storage and transportation. The product H2O is

separated by condensation (or bubbling the gasses into water

at room temperature). The effect of the water removal is

reported by Habazaki et al.9 and the CO2 conversion rate into

CH4 thereby increases. For example, when a series of two

reactors are used, by avoiding the equilibration of the reaction,

the water removal between two reactors is effective. Habazaki

and his co-workers achieved the 99% conversion of CO2 to

CH4 at a significantly high flow rate by using double reactors

with a Ni–Zr–Sm catalyst.9,10 A concrete application of the

Sabatier reaction is the ‘‘upgrade’’ of biogas: biogas produced

by fermentation of crops or organic waste contains around

60% CH4 and 40% CO2. Before the biogas can be released to

the natural gas grid, this CO2 must be removed using a

laborious process or converted into methane using renewable

hydrogen.11 The conventional heterogeneous catalysts for CO2

methanation are supported Ni, Fe, Co, Cu, Ru, Pd and Rh.12

On the search for novel methanation catalysts, hydrogen

storage alloys might be an alternative to classical catalysts.

Hydrogen storage alloys, such as RNi5 (R: rare earth metals,

e.g. La, Ce, Mm), readily dissociate hydrogen molecules at

ambient temperature and pressure. Hydrides of intermetallic

compounds, e.g. LaNi5, Mg2Ni, TiFe, exhibit practical properties

for compact and safe hydrogen storage as an energy storage

media, especially for stationary applications.1,13,14 Surface

properties of hydrogen storage alloys have been studied with

respect to poisoning and catalytic effect on the hydrogen sorption

processes.15 Surface processes determine the hydrogen absorp-

tion rate in the initial hydrogenation of hydrogen storage alloys,

i.e. initial activation.15–17 The metal surfaces are covered with

passivating oxide layers which retard dissociation of hydrogen

molecules and permeation of hydrogen in the surface layers. In

the hydrogen desorption process, various hydrides can be

stabilized by deactivating the surfaces.18–20 The state of the

surface, i.e. surface composition and oxidation state, is crucial

for the hydrogen sorption kinetics and controls the stability of

the hydride. The intermetallic compounds catalyze the hydro-

genation of CO, CO2 and ethylene.21–25 The disproportionated

surfaces of RNi5 act as a Ni catalyst supported on R oxides.

The intermetallic compound Mg2Ni readily reacts with

hydrogen at elevated temperature and forms the stoichio-

metric hydride Mg2NiH4. In this complex hydride, the tetra-

hedral molecular anion [NiH4]4� is surrounded by the cations

Mg2+.26,27 The hydrogen dissociation pressure is 3 bar at

570 K.28 The high temperature type hydrogen storage alloy

Mg2Ni exhibits cycle life of over 1500 cycles without change in

the storage capacity if purified hydrogen is used.29 Mg2NiH4

possesses high hydrogen storage capacity with a volumetric

and gravimetric density of 98 kgH2 m�3 and 3.6 mass%,

respectively. Compared to MgH2 and other Mg based alloys,

e.g. Mg–Al or Mg2Cu, Mg2Ni exhibits a fast activation under

reducing conditions for the surface layers.15,30–32 No dispro-

portionation reaction, e.g. into MgH2, accompanies the fast

hydrogen sorption process. Surface poisoning of the hydrogen

storage alloy Mg–Mg2Ni was investigated by Ono et al.33 The

authors found the formation of CH4 after exposing the

hydride to air due to CO2 uptake.

In this study the catalytic interactions of the hydride surface

of Mg2NiH4 with CO2 in the course of the hydrogen desorption

was investigated. The surface processes were studied in great

detail in view of a novel class of methanation catalysts on one

hand, and to understand the poisoning processes on surfaces of

Mg2Ni when exposed to contaminated hydrogen on the other.

Experimental

Synthesis of Mg2Ni

Mg2Ni compound was prepared by powder metallurgy. From

the nominal 2 : 1 composition of Mg and Ni elemental

powders of 100 mm size, the mixture was compressed into

pellets and annealed at 500 1C for 24 hours under an argon

atmosphere. The phase purity was confirmed by X-ray diffrac-

tion. The crystalline phases were quantified by an analysis

based on Rietveld refinement, which indicates the presence of

99 wt% of hexagonal Mg2Ni (P6222 space group) and 1 wt% of

cubic Ni (Fm%3m). Mg2NiD4 was prepared by using a volumetric

Sievert’s type apparatus and deuterium (purity 99.8%).

Phase identification by X-ray powder diffraction (XRD)

XRD was performed using Cu Ka radiation with a wavelength

of lCuKa = 1.5418 A (weighted average of Cu Ka1 and Cu

Ka2 radiation) from a Bruker D8 diffractometer equipped with

a Goebel mirror. The samples were measured in glass capillaries

(diameter: 0.7 mm, wall thickness: 0.01 mm). The crystalline

phases were identified with the help of the pdf-database and

quantified by the Rietveld method using the TOPAS software.

Hydrogen absorption and cyclic measurements

Mg2NiH4 was activated during several hydrogen sorption

(absorption/desorption) cycles. The amount of hydrogen

absorption was determined volumetrically with a Sievert’s

type apparatus. Absorption measurements were carried out

at 623 K and an initial H2 pressure of 25 bar. The cyclic

hydrogen sorption measurement was performed by combining the

hydrogen desorption in CO2 of 1.0 bar up to 773 K (1 K min�1).

Subsequently, the sample was fully degassed under vacuum

below 773 K prior to each hydrogen absorption measurement.

The hydrogen desorption in the CO2 atmosphere was accom-

panied by CO2 methanation. During hydrogen desorption, the

gasses (H2, CO2, CH4, H2O) were analysed with MS connected

to the reactor in the volumetric Sievert’s type apparatus via

differential pumping.

Surface analysis by X-ray photoelectron spectroscopy (XPS)

and time-of-flight secondary ion mass spectroscopy (ToF-SIMS)

After five hydrogen sorption cycles, the pellet sample was

prepared from the Mg2NiH4 powder inside an Ar glove box

(O2 o 0.1 ppm, H2O o 0.1 ppm). The sample was transferred

directly from the Ar glove box connected to the XPS spectro-

meter without contact with air. In situ-XPS surface analysis

combined with thermal desorption spectroscopy (TDS) was

performed in a modified VG EscaLab spectrometer with the

base pressure o1 � 10�10 mbar. A mass spectrometer (MS),

Pfeiffer Vacuum QME 200, was aligned to the sample holder.

XPS spectra were collected with a SPECS PHOIBOS 100 analyzer

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5520 Phys. Chem. Chem. Phys., 2012, 14, 5518–5526 This journal is c the Owner Societies 2012

using a non-monochromated X-ray source (Al Ka: 1486.6 eV,

twin anode Mg Ka/Al Ka). Charge-up of +1 to 2 eV was

observed on the surface ofMg2NiH4 covered with the insulating

Mg oxide layer. The charge-up correction of Mg 1s binding

energies is not made in the figure (Fig. 3). The oxidation state of

Mg was assessed by the Wagner plot.34 The FWHM values for

the deconvoluted components were 2.2–2.3 eV for the metallic

component and 2.6–2.7 eV for the oxide, respectively. The

simultaneous TDS measurement was carried out by resistive

heating with a temperature ramp of 0.5 K min�1. During the

hydrogen desorption, impurity gasses H2O, CO, CO2, O2 and

CH4 were o1 � 10�9 mbar. XPS depth profiles on powder

samples were made by Ar ion sputtering with the discharge

potential 2 kV and the ion current of 10 mA cm�2. The powder

samples were pressed on an In sheet. Atomic concentrations

were calculated from Mg(2s), O(1s), C(1s) and Ni(3p) peaks.

CasaXPS software was used for data processing.

The powder sample of Mg2NiH4 from a hydrogenation

reactor was pressed on an In sheet and transferred into the

ToF-SIMS instrument (ToFSIMS 5, Iontof, Germany) under

an Ar atmosphere. The analysis was done at room temperature

and a vacuum of 2 � 10�8 mbar. The images were measured in

the burst alignment mode using Bi+ ions at an energy of

25 kV as primary ions after pre-sputtering of the sample with

O2+ ions for 90 s (2 kV, 400 nA, 300 � 300 mm). Under the

sputtering conditions the sputter equilibrium was reached after

35 s and the organics are removed after 20 s as checked by

doing depth profiling and surface spectroscopy with high

mass resolution. Impurities Cl, Al, Na, and K were detected

in ToF-SIMS, which were below the XPS detection limit,

i.e. o1 at.%.

Surface area measurement

The BET surface area of the sample was extracted from nitrogen

adsorption isotherms at 77 K measured with BELSORP-

max, BEL.

Microstructure analysis

Scanning electron microscopy (SEM) was performed for

microstructural characterization. For investigation of the

surface structure on the mm-scale, secondary electron SEM

imaging was carried out with a FEI XL30 Sirion FEG at an

acceleration voltage of 5 kV. Prior to the analysis, the powder

sample was covered with a 7 nm thick Au-film and a 7 nm

thick C-film in order to make the surface electrically conduc-

tive. The structure and chemical composition at the nano-scale

was examined by scanning transmission electron microscopy

(STEM) with a JEOL 2200FS TEM equipped with a FEG

cathode (an omega energy filter to perform EELS, JEOL EDX

detector, STEM-HAADF). STEM investigations were per-

formed on a powder sample at an acceleration voltage of

200 kV. The energy-dispersive X-ray spectroscopy (EDX) line

profile was made with the electron beam diameter of 0.7 nm

and step size of 1 nm. Electron energy loss spectroscopy

(EELS) spectra on different Ni particles were collected in the

STEM mode with a beam diameter of 1 nm. An energy

dispersion of 8.8 meV per pixel was chosen to cover both

L-edges of Ni.

Catalytic activity

Prior to the investigation of the catalytic activity tests, the

samples were exposed to air for 2 hours. The powder samples

(47 mg, sieve fraction 150–200 mm, diluted with sea sand) were

firmly fixed with two quartz wool plugs inside a fixed bed

quartz reactor tube (di = 6 mm) placed vertically in an

electrically heated furnace. Prior to reaction, the sample cycled

in CO2 methanation was reduced in 10 mol% H2/He at 673 K

for 2 hours. After cooling to 318 K, H2 and CO2 (mole fraction

H2/CO2 = 8) with He (50 ml min�1) were admitted to the reactor

at a total flow rate of 100mlmin�1 andGHSV=6� 104 h�1 and

the activity was monitored from 373 K to 773 K at 5 K min�1.

The reactants ratio H2/CO2 = 8 was chosen based on ref. 35

and on the increased conversion rates observed on Ni/MgO

with this feed ratio. Product analysis was performed with a gas

chromatograph (Agilent Technologies 3000 A micro, equipped

with a Poraplot U and MolSieve 5 A columns) and a mass

spectrometer (Pfeiffer Vacuum, GSD 301 O2).

Results and discussion

Surface change on Mg2NiH4 during the hydrogen desorption

The XPS depth profile of MgNi2H4 powder after 5 hydrogen

absorption/desorption cycles is shown in Fig. 1. The surface of

MgNi2H4 is covered with Mg oxide layers. The segregation of

Mg oxides results from the surface oxidation by impurity gases

of H2, e.g. O2, H2O, CO, CO2.15,30,31 The surface segregation

phenomena have been investigated in various families of solid

hydrogen storage material, e.g. LaNiH6,36–38 TiFeH2,

39,40

Mg2NiH4,30,31 LiBH4,

41 with respect to catalytic and poisoning

effects on the hydrogen sorption process. The driving force for

surface segregation is the difference in binding energies between

constituents. In addition, surface oxidation, i.e. interaction

between the substrate and adsorbates, further lowers the surface

energy. Selective oxidation of a constituent has been observed

Fig. 1 The XPS depth profile of Mg2NiH4 powder after 5 hydrogen

absorption/desorption cycles. Atomic concentrations vs. sputtering

time.

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on the surfaces of the hydrides. Fig. 1 shows that the surface

region of Mg2NiH4 possesses a gradient of Ni in the Mg oxide

layers. The decomposed Ni lies below the surface layers of

Mg oxide, which prevent Ni from oxidizing.30,31 In Fig. 2, the

ToF-SIMS image shows the distribution of Ni at the topmost

layers on Mg2NiH4 covered by Mg oxide layers. The active Ni

site is responsible for readily dissociating hydrogen molecules

and leads to the fast initial activation of the host intermetallic

compound Mg2Ni.30,31

Fig. 3 shows the change in the XPS spectra of Mg 1s on

Mg2NiH4 covered with theMg oxide layers during the hydrogen

desorption. The electron effective attenuation length lEAL is

B0.6 nm and the sampling depth (3 lEAL) B1.8 nm.43 The

Mg 1s core level spectra consist of two main components, i.e. the

oxidized and metallic states, judged from the Wagener plot.34

Fig. 4(a) shows the change of hydrogen desorption rate from

Mg2NiH4�X (TDS). The hydrogen desorption rate significantly

increases around 450 K. A structural transition from monoclinic

to cubic takes place at 507 K.44 Fig. 4(b) indicates that at elevated

temperature the metallic state of Mg increases on the surface

of Mg2NiH4 during the hydrogen desorption. The absence of

any evident correlation between the changes in Mg 1s spectra

and the hydrogen desorption rate suggests no contribution of

MgH2 to the Mg 1s spectra. The vaporization of Mg might

increase the metallic Mg at the surface and lower the surface

energy. However, no detectable change in the ion-current

m/z = 24 (Mg+) was observed in MS. Mg oxide layers may

fracture at the surface due to the difference in thermal expansion

between metallic Mg and the oxides. The coefficients of linear

thermal expansion are 24.8 � 10�6 K�1 for Mg45 and 10.2 �10�6 K�1 for MgO46 at 300 K, respectively. A similar tendency

was observed on the surfaces of MgH2 and AlH3 during the

hydrogen desorption processes.20,42

CO2 hydrogenation on the surface of Mg2NiH4 during hydrogen

desorption. The reaction mechanism was investigated by using

Mg2NiD4 in place of the hydride. The decomposition of

Mg2NiD4 in the flow of H2 and CO2 was probed by labeling

hydrogen. The deuterium desorption from Mg2NiD4 covered

with surface oxide layers occurs above 500 K (Fig. 5(a)).

Above the decomposition temperature, the recombination of

deuterium takes place and the deuterium-rich surface of

Mg2NiD4�X is exposed to the CO2 atmosphere. The CO2

conversion commences at 600 K and CO2 molecules dissociate

significantly. Simultaneously, the CO2 methanation proceeds

and desorption rates of CH4 andH2O increase (Fig. 5(b) and (c)).

Intensity of CH4, and H2O was found by more than two

orders of magnitude higher than that of isotope D2O and CD4

(m/z = 20). The catalytic reaction is related to the surface

Fig. 2 The ToF-SIMS image of Mg2NiH4 after 5 hydrogen absorption/

desorption cycles.

Fig. 3 Change in the XPS core level spectrum of Mg 1s during the

hydrogen desorption with a temperature ramp of 0.5 K min�1.

Fig. 4 Surface change on Mg2NiH4 during hydrogen desorption with

a temperature ramp of 0.5 K min�1. (a) Mass to charge ratio m/z = 2,

and (b) the ratio of the metallic to oxide state of Mg 1s (Fig. 3) vs.

temperature.

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5522 Phys. Chem. Chem. Phys., 2012, 14, 5518–5526 This journal is c the Owner Societies 2012

structure covered with Mg oxide layers with inhomogeneous

distribution of Ni (Fig. 1 and 2). The result implies that the

reaction sites on the disproportionated surface are not adjacent

to deuterium-rich layers and the precipitated Ni leads to the

catalytic activity. Moreover, no correlation between rates of the

deuterium desorption and CO2 conversion was observed and

the deuterium desorption does not increase the conversion rate

dominantly. The dissociative adsorption of CO2 molecules

promotes the conversion into CH4 above 600 K and the

dissociation of hydrogen molecules is not a rate controlling

step. This observation is confirmed by the mechanistic studies

on supported Ni catalysts,47 and furthermore, on other metals.

Peterson et al. investigated CO2 conversion into hydrocarbons

on Cu by density functional theory.48 Their results suggest that

the key enabling step is the protonation of adsorbed CO(ad) to

form adsorbed CHO(ad). William et al. investigated CO2 hydro-

genation on a Rh foil covered with a submonolayer of titania.49

The methanation of CO2 was proposed to start with the

dissociation of CO2 into CO(ad) and O(ad). The interaction

between CO(ad) and titania, i.e. Ti3+ ions located at the edge of

TiOx islands on Rh, facilitates C–O bond cleavage and the

subsequent hydrogenation.

The formation of CO was observed to occur up to the

CO/CO2 mole fraction of 0.31 while the CO2 conversion rate

increased up to 0.67 (Fig. 5(d)). The molecules CO2, CO and

H2O produced in the reaction oxidize the increasing metallic

state of Mg at the surface during the deuterium desorption (in

Fig. 3 and 4). The oxidation induces a further segregation of

Mg oxides onto the surface. Because of the higher binding

energy of CO on the surface, CO becomes dominant at the

chemisorption sites.48 Compared to CO2 and H2O, the chemi-

sorbed CO can further poison metallic Ni and Mg2Ni.16,50,51

Fig. 5 CO2 methanation on Mg2NiD4 in the flow of H2 and

CO2 (H2/CO2 = 8). (a) Mass to charge ratio m/z = 3 for HD;

(b) m/z = 15 for CH4; (c) m/z = 18 for H2O; (d) relative CO2-

conversion (J), expressed as 1 � I/I0 where I is the GC area of the

CO2 peak and I0 is the background intensity of the CO2 flow without

any reaction, and mole fraction of CO/CO2 (K) vs. temperature

(5 K min�1). m/z = 20 for D2O and CD4 was o3 � 10�9 A. The

hydrogen dissociation pressure of Mg2NiH4 is 3 bars at 570 K.28

Fig. 6 The XRD pattern after 18 cycles of the hydrogen absorption/

desorption and the hydrogen desorption in CO2 (Fig. 7). The inset

shows the deconvolution into the reflections (Ni, MgO, MgNi2,

Mg2NiH4) and their contribution to the XRD pattern in the 2y range

between 421 and 481.

Fig. 7 Change in the amount of absorbed hydrogen by the dispro-

portionated Mg2NiH4 during cycling. The cycle number n vs. the

amount of the hydrogen absorbed after hydrogen desorption under

vacuum (J) or desorption in CO2 of 1.0 bar ( ). The absorption

measurements were carried out at 623 K and an initial H2 pressure of

25 bar. (J) The amount of the absorbed hydrogen after desorption

under vacuum was measured in the equilibrium. ( ) The amount of

the absorbed hydrogen after desorption in CO2 (1 K min�1, o773 K)

was measured in 4 hours and not in the equilibrium.

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Therefore, CO chemisorption retards the recombination of

deuterium and the H–D exchange with H2, resulting in

decreased HD desorption above 600 K (Fig. 5(a)). The

significant decomposition of Mg2NiD4 under the oxidative

atmosphere was observed above 700 K while Mg2NiH4

decomposes at 450 K under vacuum (Fig. 4(a)). The formation

of the chemisorption layers, i.e. surface oxide layers, hinders

the decomposition and stabilizes the deuteride (hydride).20

Disproportionation of Mg2NiH4. During the hydrogen

desorption in CO2 the metallic state of Mg is oxidized and

segregation of Mg oxides proceeds. Fig. 6 shows the XRD

patterns after 18 cycles of the hydrogen absorption/desorption

and hydrogen desorption in CO2 (in Fig. 7). Due to the

numerous reflections especially of Mg2NiH4 and due to finite

size broadening, many reflections overlap. The inset in Fig. 7

Fig. 8 CO2 hydrogenation in the flow of CO2 and H2 on the sample

after 18 cycles (Fig. 7) ( and ), compared to the precursor Mg2NiH4

(� andE). (a) The relative CO2 conversion, (b) the CO/CO2 mole

fraction vs. temperature (5 K min�1).

Fig. 9 The XRD pattern after 27 reaction cycles (after the methanation

rate became saturated). The broad amorphous feature observed around

2y = 221 originates from the glass capillary containing the sample.

Fig. 10 The XPS depth profile of the powder after 27 reaction cycles.

Atomic concentrations vs. sputtering time (compare to Fig. 1).

Fig. 11 (a) Secondary electron SEM image after 27 reaction cycles

(width of the image: 8 mm), and the BET surface area is 14 m2 g�1 (N2).

(b) STEM dark-field image of a Mg oxide whisker with the diameter

o50 nm (confirmed by EDX).

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5524 Phys. Chem. Chem. Phys., 2012, 14, 5518–5526 This journal is c the Owner Societies 2012

shows the deconvolution of the XRD pattern identifying

MgO, Ni and MgNi2 as products of the disproportionation

of Mg2NiH4 caused by the selective oxidation of Mg. The

segregated phases of MgNi2 and Ni do not form hydrides

below 25 bar H2 at 623 K.28 Fig. 7 shows the change in the

amount of absorbed hydrogen in the disproportionation process

of Mg2NiH4. The cyclic oxidation process (in Fig. 7, n= 5 to 8)

causes the sharp drop in the hydrogen absorption due to the

disproportionation reaction and the surface poisoning. The

hydrogen absorption process involves volume expansion by

28%.44 A large stress is built up at phase boundaries, and leads

to cracks and powdering of the hydride which increases the

surface areas of the hydride. The subsequent hydrogen

desorption in CO2 makes the increased surface further oxidized.

On the poisoned surface, the dissociation rate of hydrogen

molecules decreases due to the difficulty of hydrogen molecules

to share electrons with the surface metal atoms. Furthermore,

the thicker oxide layer lowers hydrogen diffusion in the surface

layers.17,52,53 In the subsequent hydrogen absorption (n = 9),

however, the hydrogen absorption rate recovered under the

reducing conditions for the Ni oxide. As the hydrogen absorption

capacity became lowered with increasing cycles, the CO2

methanation was further enhanced in the subsequent hydrogen

desorption in CO2.

In Fig. 8, the CO2 conversion and CO formation on the

resulting sample after the cycles shown in Fig. 7 are compared with

those of the precursor Mg2NiH4. The further disproportionated

surface exhibits a higher CO2 conversion rate. Desorption of CO

decreases above 710 Kwhile COmarkedly increases onMg2NiH4.

The disproportionated surface becomes active in dissociating

CO2 and CO molecules, and promotes the methanation. The

observation is consistent with the disproportionated LaNi5and ThNi5 intermetallics.22,24

The methanation reaction was cycled and the reaction was

carried out in CO2 and H2 atmospheres (CO2: 1.0 bar, H2: 0.2 bar,

773 K, 1 K min�1) until the methanation rate became saturated.

Fig. 9 shows the XRD pattern after the 27 reaction cycles. The

Mg2NiH4 decomposes principally into MgO and Ni during

the cyclic process. The XPS depth profile (Fig.10) indicates the

oxidized surface further enriched with Mg and the increased

C concentration relative to the precursor Mg2NiH4 (Fig. 1).

The disproportionation process combined with the hydrogen

sorption cycles and oxidation induces the rough surface

structure, where Mg oxide whiskers were formed (Fig. 11).

The precipitation of Ni with the particle size in the range

o20 nm was found by means of TEM and EDX (Fig. 12). The

corresponding EESL spectrum (Fig. 12(e)) suggests that the

Ni particle core is in the metallic state, judged from the L2,3

Fig. 12 STEM images after 27 reaction cycles. (a) Bright-field image, (b) dark-field image, (c) the EDX mapping corresponding to (b), (d) the

EDX line profile in (b), (e) the EELS spectrum of Ni L2,3 edges measured on a Ni nano-particle compared with the reference spectrum of NiO.56

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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 5518–5526 5525

white-line shape.54–56 This is due to the selective oxidation of

Mg and the result is consistent with the macroscopic analysis by

XRD (Fig. 9). The metallic Ni nano-particles are surrounded

with Mg oxide, which might prevent Ni from further sintering

at high temperatures.35,57 Moreover, Akiba and Ono reported

the formation of the carbonate on surface oxide layers on Mg

by the uptake of CO2 from air: MgO + CO2 - MgCO3 +

118 kJ mol�1.58 Compared to the surface on Mg2NiH4, the

higher concentration of basic MgO layers and the rough

surface structure might be effective in the CO2 chemisorption.

As the reaction cycle increases (Fig. 7), the precipitation of

Ni, resulting from the surface segregation, enhances the CO2

methanation. Therefore, the precipitated Ni and the adlineation

sites are responsible for dissociative adsorption of CO2 molecules

and catalyse the subsequent hydrogenation.

Conclusions

Mg2NiH4 was investigated with respect to the catalytic property

of the hydride surface for CO2 methanation. The formation of

surface oxide layers hinders the decomposition of the hydride.

The surface oxidation is accompanied by segregation of Mg

oxides and Ni at the disproportionated surface on Mg2NiH4�X(Fig. 13). The active Ni sites at the surface are associated with

the ready dissociation of hydrogen molecules and subsequent

hydrogenation of the intermetallics Mg2Ni. In the CO2 hydro-

genation process on the hydride surface, the hydrogen desorption

does not determine the reaction rate. Rather dissociative adsorption

of CO2 molecules on the hydride facilitates the rate of methanation.

During the hydrogen desorption in a CO2 atmosphere, selective

oxidation of Mg takes place and the Mg oxides segregate. The

simultaneously precipitated Ni-clusters are attributed to the

catalytic activity of the hydrides surface, i.e. for dissociative

adsorption of CO2 molecules and their subsequent hydrogenation.

The continuing disproportionation reaction of the hydride

induces a rough surface structure. As the Ni particles are

formed during decomposition, the modified surface becomes

further active in the CO2 methanation.

The catalyst preparation from the hydride was demonstrated to

occur via disproportionation of the hydride precursor under

cycles of hydrogen sorption and oxidation. The hydrogen storage

alloys after thousands of hydrogen sorption cycles are fine powder

accompanied by surface segregation and consist of catalytic

elements, especially, Ni, Fe, Co, Cu, for the hydrogenation of

CO2 and CO. The disproportionation reaction allows re-utilization

of hydrogen storage alloys as a CO2 reduction catalyst.

Acknowledgements

This work was financially supported by Swiss Federal Office of

Energy (BFE) and the European Commission (contract number

MRTN-CT-2006-035366 (COSY)).

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