CeNiXAl0.5HZOY nano-oxyhydrides for H2 production by oxidative dry reforming of CH4 without carbon formation Yaqian Wei, Xiu Liu, Noura Haidar, Hervé Jobic, Sébastien Paul, Louise Jalowiecki-Duhamel
To cite this version:
Yaqian Wei, Xiu Liu, Noura Haidar, Hervé Jobic, Sébastien Paul, et al.. CeNiXAl0.5HZOY nano- oxyhydrides for H2 production by oxidative dry reforming of CH4 without carbon formation. Applied Catalysis A : General, Elsevier, 2020, 594, pp.117439. 10.1016/j.apcata.2020.117439. hal-03022420
HAL Id: hal-03022420 https://hal.archives-ouvertes.fr/hal-03022420 Submitted on 26 Nov 2020
HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. CeNiXAl0.5HZOY nano-oxyhydrides for H2 production by oxidative dry reforming of CH4 without carbon formation
Yaqian Weia, Xiu Liua, Noura Haidar a, Hervé Jobicb, Sébastien Paula, and Louise Jalowiecki-
Duhamela,* a Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 – UCCS – Unité de Catalyse et Chimie du Solide,
F-59000 Lille, France. b Institut de Recherches sur la Catalyse et I’Environnement de Lyon (IRCELyon), 69626 Villeurbanne Cedex, France.
Corresponding author. Tel.: +33 (0)3 20 33 77 35; fax: +33 (0)3 20 33 65 61. E-mail address: [email protected] (L. Jalowiecki-Duhamel)
Highlights:
Mixed oxides based hydride reservoirs with Ni2+ cations are performant catalysts.
CO2 conversion overcomes the predicted value at 500-600°C.
H2/CO ratio of 1.3 is obtained at 600°C.
No carbon formation at 600°C in O2/CH4 ratio of 0.3.
Keywords: Hydrogen; Oxyhydride; CH4; CO2; Ceria, Nickel
ABSTRACT
The oxidative dry reforming of CH4 (ODRM) was studied at 500-800°C in harsh conditions
-1 -1 (CH4/CO2/O2/N2 = 1:0.7::N2 with 20% of CH4, 96,000 mLh gcat and varying from 0 up to 0.5) on
CeNiXAl0.5HZOY (0.5 ≤ x ≤ 5) nano-oxyhydride catalysts. At low temperature (500-600°C), the conversion
of CH4 reaches the thermodynamic limit while those of CO2 overcomes the predicted value in particular
at 500°C. At 600°C with an O2/CH4 ratio of 0.3, the CeNi2Al0.5HZOY catalyst allows getting CH4 and CO2
conversions of about 63% and 50 %, respectively, and a H2/CO ratio of about 1.3, without carbon 1 formation. The physico-chemical characterizations performed before and after test show the existence of strong interactions existing between Ni2+ cations with other cations and the presence of nano- oxyhydride compound. The mixed oxide based nano-materials corresponding to hydride reservoirs with the presence of Ni2+ cations are shown to be highly active, selective and stable catalysts.
1. Introduction
Access to clean, affordable and reliable energy has been a cornerstone of the world’s increasing prosperity and economic growth since the beginning of the industrial revolution. Our use of energy in the 21th century must also be sustainable [1]. Hydrogen is seen as the “green” energy of the future while it is already largely used in chemical industry as an indispensable raw material. However up to date, hydrogen is mainly produced from fossil fuels and the main hydrogen production in industrial scale is the steam reforming of methane (SRM), an endothermic process, requiring high energy consumption.
It is urgently desirable to produce hydrogen from renewable energy sources, such as biomass-derived
materials and/or biogas. Biogas, a mixture of gases mainly containing CH4 and CO2, can be obtained by the anaerobic digestion of various bio-resources and the transformation of biogas has received growing interest [2-8]. Although operating at low temperature is of enormous importance because of
environmental, economic and maintenance reasons; it remains a great challenge for CH4 and CO2
transformations in the same time [9,10]. The CO2 reforming with CH4 is an endothermic process, so called dry reforming (DRM, Eq.1). This type of reaction is attractive from an environmental point of view, since it consumes two major greenhouse gases [5-7,9-14]. However, the main drawback of DRM is the deactivation of the catalyst due to heavy carbon deposition (particularly at low temperature) [2-6,15]
and the sintering of the active phase [5,6,11]. Co-feeding O2 with CH4 and CO2 (ODRM, Eq. 2) provides several advantages such as reducing the global energy requirement and enhancing catalyst stability,
2 by increasing deactivation resistance and inhibiting the carbon deposition rate by gasifying carbon
species, however it decreases the CO2 conversion [10,11,16-27]. Avoiding carbon formation (Eqs. 3-6) at low temperature (500-600°C) and sintering of the active phase are still challenges. In the meantime, as for DRM, another drawback is the reverse water gas shift reaction (RWGS) that can decrease the
selectivity in H2 by consuming it while it allows to transform CO2 (Eq. 7). Moreover, maintaining stability of the catalyst in presence of water that can be formed during reaction is also important, while water
gas shift reaction (WGS) can increase H2 formation (Eq. 8).
0 1 CH4 + CO2 → 2CO + 2H2 ΔH 298K = + 247 kJ mol (1)
0 1 CH4 + aCO2 + (1-a)/2O2 → (1+a)CO + 2H2 ΔH 298K = (285a-41) kJ mol 0≤ a ≤1 (2)
0 1 2CO CO2 + C ΔH 298K = -171 kJ mol (3)
0 1 CH4 2H2 + C ΔH 298K = +76 kJ mol (4)
0 -1 CO2 + 2H2 C +2H2O ΔH 298K = 90 kJ mol (5)
0 -1 CO + H2 C + H2O ΔH 298K = 131 kJ mol (6)
0 1 CO2 + H2 → CO + H2O ΔH 298K = + 41 kJ mol (7)
0 1 CO + H2O CO2 + H2 ΔH 298K = - 41 kJ mol (8)
Because of its high performance, Ni is one of the most studied metals for DRM of CH4 [5, 28-30], in particular in association with ceria [12-14] or when the catalyst is promoted with cerium allowing improving stability [31]. On some performant catalysts for dry reforming it has been shown that they are
also effective in presence of O2 and even better performance could be obtained [ 32 ]. Coking phenomenon was found to be limited in case of highly dispersed metal species at the surface of the
support. Moreover, among the different supports allowing high dispersion, CeO2 is widely known and largely studied due also to its excellent oxygen storage capacity, which might be favoring the oxidation
3 of the carbon deposits formed upon direct methane decomposition [5]. In general, better catalytic
performance and lower carbon content are obtained. Besides, it has been shown that CO2 can be directly dissociated to CO and active oxygen species by the metal-ceria catalyst, donating oxygen to a
lattice vacancy and producing CO [33] and that CO2 is activated in the defect sites of ceria, without noble metals, if the concentration of Ce3+ is high enough [34]. In the meantime, many studies have shown that the redox properties can be considerably enhanced if additional elements are introduced
into the CeOY lattice by forming a solid solution [13,35]. It is known that CeO2-MOx solid solution has
good thermal stability and better oxygen storage capacity than CeO2 alone [36]. Furthermore, it is often reported that the preparation method is of high importance [5], and usually, the coprecipitation method is used to increase the interactions between the cations [37] that can favor C-H bond activation at low
temperature [12,13,38 , 39 ] . Besides, CO2 activation requires high temperature/pressure conditions and/or active reductants, such as hydrogen or can be activated under ambient conditions with the help of a solid state catalyst [40,41]. In particular, it has been shown that the presence of hydride species
can be of great help for CO2 activation [42-45].
Our previous studies showed that CeNiXOY mixed oxides once transformed into oxyhydrides (solids
storing hydride species, after pretreatment in H2 at particular temperature) are efficient catalysts in CH4
partial oxidation in presence of O2 [38,39], while beside adding another cation to the system allows increasing the hydrogen storage of the catalyst [46]. The current challenge is to tune and maintain nano- oxyhydrides containing cations in strong interaction to favor C-H bond activation at low temperature,
with hydride species stored in the solid to facilitate CO2 activation. Herein we report the successful
development of CeNiXAl0.5HZOY nano-oxyhydrides as highly active, selective and stable catalysts that
are applied to low temperature CO2 and CH4 transformations for H2 production. The effect of different
4 parameters is studied such as the Ni content in the catalyst and the O2/CH4 ratio in the reaction mixture.
Different physico-chemical characterizations are performed before and after test. In particular, Inelastic
Neutron Scattering (INS) proves the presence of hydride species (H–). Furthermore, correlations between the catalyst properties and the catalytic performances are discussed, allowing us to participate to the open debate on this catalytic process and to propose an active site and a possible mechanism.
2. Experimental
2.1. Catalyst preparation
CeNiXAl0.5Oy catalysts where x is the Ni/Ce atomic ratio, were prepared by coprecipitation of cerium, nickel and aluminum hydroxides from a mixture of their nitrate solutions. At first, the nitrate precursors
(nickel nitrate hexahydrate 99%) Ni(NO3)2·6H2O, Ce(NO3)3·6H2O, and Al(NO3)3·6H2O were dissolved in distilled water, respectively, to get 0.5M solutions. After mixing the appropriate volume of each nitrate solution in order to get the adequate molar ratio of each element, the mixture was added drop-wise into a triethylamine (1.5 M) diluted in methanol solution while stirring. After filtration, the hydroxide mixture was washed, dried at 100°C for 24h, and calcined in air at 500°C for 4h.
2.2. Catalytic reaction
Reactions were conducted in a fixed-bed up-flow straight quartz reactor (Ø : 8 mm) mounted in a tubular electric furnace connected to a temperature controller. Prior to reactions, the catalyst (50 mg
-1 deposit on a frit located in the middle of the reactor) was pretreated in pure H2 (30 mL min ) at 250°C for 10 h followed by a purge in pure He during 30 min at the same temperature. The oven temperature was increased up to the reaction temperature (10°C min-1), maintaining the catalyst in He flow. The gas mixture controlled by mass flow controllers was introduced into the reactor at the reaction temperature,
5 introducing N2 first. A model biogas CH4/CO2 =1:0.7 mixture was used with a CH4 concentration of 20%.
-1 Whatever the O2/CH4 ratio, the total flow rate of the introduced gases was fixed at 80 mL min by using
-1 -1 N2 as the balanced carrier (leading to 96,000 mL h gcat ). In each test, O2 was introduced at last to avoid immediate reoxidation of the catalyst surface. The reported reaction temperature was measured by a thermocouple located near the catalytic bed.
All the products were analyzed on-line by gas chromatography (TRACE GC ULTRA) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Catalytic results are
reported after 5 h (at the steady state) by CH4 (XCH4) and CO2 (XCO2) conversions, products molar
composition and H2/CO ratio, based on the following equations (Eqs. 9-11). The carbon balance was closed +-5%. It has to be noted that some water is formed, but not reported as there is a high uncertainty on this value. The thermodynamic equilibrium values were calculated by using HSC chemistry 6 for providing a basis of comparison.
(CH in CH out) X 4 4 100% CH 4 CH 4 （9）
(CO in CO out) X 2 2 100% CO2 CO 2 （10）
H / CO ratio F out / F out 2 H 2 CO （11）
F out F out Where H 2 and CO correspond to the flow rates of each component in the effluent.
2.3. Catalyst and carbon characterizations
The catalysts were designated as CeNiXAl0.5OY with x = 0.5, 1, 2 and 5. The Ni, Ce, and Al metal
contents were analyzed by ICP technique (Agilent 720-ES ICP-OES). The Ni/MT molar ratio is the nickel
proportion in all the metals. (Ni/MT = x/(x + 1.5)).
A N2 adsorption TriStar II 3020 analyzer was used to determine the specific surface areas (BET).
6
Prior to each measurement, the sample was degassed for 40 min at 150°C.
A Bruker D8 Advance X-ray diffractometer, equipped with a fast detector type LynxEye and a copper anticathode, was used for X-ray powder diffraction (XRD) measurements. The XRD patterns were registered in the 2θ domain (10-90°) with a measured step of 0.02° and the time integration of 0.3 s. The average crystallites size was calculated based on the width of the main diffraction peak using the Scherrer equation.
A Labram Infinity HORIBA JOBIN YVON Raman spectrometer using a visible laser with an output laser power of λ = 532 nm at room temperature was used for Raman spectra.
A KRATOS Axis Ultra spectrometer under ultrahigh vacuum condition, using a twin Al X-ray source
(1486.6 eV) at a pass energy of 40 eV was used for the X-ray photoelectron spectroscopy (XPS) analysis of the samples. The solid in the form of pellet was fixed on a copper holder with copper tape.
The binding energy values were estimated by using as reference the C 1s peak of contaminant carbon at 285.0 eV. To report after test carbon, correct correction was further checked by adjusting the characteristic Al2p. The Casa XPS software package was used for data analysis.
A Micromeritics Autochem II Chemisorption analyzer equipped with a thermal conductivity detector
(TCD) was used for temperature programmed reduction (TPR) analyses of the catalysts. Treatment was performed by heating 50 mg of catalyst sample from room temperature to 1000°C (10°C min-1) in
a 5% H2-95% Ar gas mixture.
The IN1 BeF spectrometer at ILL (Institut Laue Langevin, Grenoble) was used to perform Inelastic
Neutron Scattering (INS) experiments. For each analysis, 36 g of solid was placed in a stainless steel container. The solid has been analysed after treatment in vacuum at 200°C (2h) and after treatment in
H2 (10 h) at 250°C using high purity gas. After each treatment the container has been sealed. INS-
7 experiment was carried out at 10 K using a Cu (200) monochromator for energy transfers between 80 cm-1 and 380 cm-1 and a Cu (220) monochromator for energy transfers between 380 cm-1 and 3000 cm-
1. The scattering cross-section is much greater for hydrogen (80 barns) than for other elements (5 barns), therefore, INS emphasizes motions of hydrogen species.
The amount of solid carbon formed was determined by measuring the mass variation of the catalyst after test. The production of carbon was reported in gram of carbon produced per gram of catalyst per hour taking into account the duration of the catalytic test. A Micromeritics Autochem 2920 analyzer was
1 used for O2-TPO performed in 5% O2 diluted in He mixture with a 50 mL min flow rate. The temperature was increased up to 1000C with a heating rate of 5C min1. The desorption species from the sample were detected by using a OmniStar GSD 300 O mass spectrometer.
3. Results and discussion
3.1. Catalytic transformations of CH4 and CO2
Fig. 1 presents the oxidative dry reforming (ODRM) catalytic results obtained in CH4/CO2/O2/N2 =
1:0.7:0.3:N2 (with CH4 = 20%) mixture on the CeNiXAl0.5OY compounds in situ pretreated in H2 at 250°C.
Fig. 1A shows on CeNi2Al0.5OY compound that both conversions of CH4 and CO2 become more efficient
as the temperature increases, as expected. O2 is completely converted and high CH4 and CO2 conversions are observed in the temperature range of 500-800°C. The results reported at the steady-
state (5h) show that at low temperature (500-600°C) CH4 conversion reaches the thermodynamic limit
while, very interestingly, CO2 conversion is higher than those predicted by thermodynamics (using HSC chemistry 6). However, it has to be noted that the thermodynamic limits are calculated taking into account reported chemical reactions. With the increase of the temperature up to 800°C, both
conversions of CH4 and CO2 increase, up to about 90% and 74%, respectively. In the meantime, H2/CO
8 ratio decreases from a value of 1.4 at 500°C to about 1.0 at 800°C. The obtained results reach the
thermodynamic limits and theoretical H2/CO value expected in DRM (Eq. 1), in agreement with little
effect of O2 reported when DRM becomes the dominant reaction [17]. The CeNi2Al0.5OY compound presents a good stability in reaction during at least 24h at 700°C (Fig. 1B). In the first 5h, the conversions
of both CH4 and CO2 decrease slightly from 81% to 77%, and from 68% to 63%, respectively, and then
they remain globally constant. The H2/CO ratio obtained is close to 1.2 with negligible fluctuations. On
the CeNiXAl0.5OY compounds, conversions globally increase with Ni content (Figs. 1C and D) and the
conversion of CH4 is always higher than the conversion of CO2 in such conditions. At 500°C the
CeNi2Al0.5OY compound allows to get the highest CO2 conversion. The highest Ni content CeNi5Al0.5OY
compound leads to a lower CO2 conversion, may be due to the occurrence of WGS reaction (Eq. 8) as
H2 increases, and/or, more probably, can be related to the higher carbon formation on this compound
(Table 1) that can lead to CO2 formation (Eq. 3). At 500°C, when Ni/MT is of about 0.6 (CeNi2Al0.5OY),
the methane conversion reaches 42% and the CO2 conversion is of 22%, with a H2/CO ratio at about
1.4. At 600°C, even if the CeNi2Al0.5OY compound leads to slightly lower values compared to the high
Ni content compound which provides best performance with CH4 and CO2 conversions of 68% and 43%,
respectively, (and H2/CO ratio at 1.4), it avoids carbon formation (Table 1). The carbon formation is negligible for the compounds with lower Ni contents (x = 0.5 - 2) (Table 1) but becomes effective at 0.24
-1 -1 g·gcat ·h when the Ni content is increased up to 49 wt% (CeNi5Al0.5OY).
In dry reforming (DRM) conditions (CH4/CO2/N2 = 1:0.7:N2) at 600°C, in absence of O2, CH4
conversion of 52% is observed, while CO2 conversion is higher at about 65%, and the H2/CO molar
ratio obtained is at 1 (Fig. 2). The conversion of CH4 increases when adding and increasing O2
concentration while the conversion of CO2 decreases. Adding O2 leads to higher H2/CO ratios in all
9 studied cases (CH4/CO2/O2/N2 = 1:0.7::N2, with CH4 = 20%) with O2/CH4 ratio varying from 0.1 up to
0.5. More precisely, H2 formation slightly increases in presence of O2 while CO formation decreases,
so an increase in the H2/CO ratio is obtained (1.3 - 1.4) compared to the results observed in DRM
conditions using similar CH4/CO2 ratio. With an O2/CH4 ratio of 0.1 the conversions of CH4 and CO2 are
relatively high at about 60% and leading to a relatively high H2/CO ratio of about 1.4, however carbon
is formed in such conditions (Table 2). O2 addition has a largely beneficial effect on carbon formation,
and carbon can be avoided when O2/CH4 ratio is of 0.3. A high fraction of O2 in reactants (O2/CH4 = 0.5)
results in severe decrease in CO2 conversion and higher formation of H2O. Compared to the classical
dry reforming, numerous reactions could be involved when adding O2 into the system, as reported in
Eqs 12-15, for example, that can explain higher CH4 transformation and carbon removal [18].
0 1 CH4 + 1/2O2 → CO + 2H2 ΔH 298K = - 36 kJ mol (12)
0 1 CH4 + O2 → CO2 + 2H2 ΔH 298K = - 322 kJ mol (13)
0 1 CH4 + 2O2 → CO2 + 2H2O ΔH 298K = - 803.03 kJ mol (14)
0 1 C + 1/2O2 → CO ΔH 298K = - 111 kJ mol (15)
In agreement with several studies the main effect of the addition of oxygen in CH4 reforming
process is to increase the conversion of CH4 [22], while avoiding carbon formation, however also to
reduce the conversion of CO2, as the conversion of CO2 could be suppressed in excess of O2 or air [19].
Increasing O2/CH4 ratio favors an increase in available O2 for partial oxidation (Eqs 12 and 13) or
combustion (Eq 14). Besides, when the formation of CO2 occurs, the CO2 conversion can even appear
negative. It has been shown that an appropriate percentage of O2 in reactants improved the activity and
- stability of the catalyst. At 800°C on 10Ni15Ce/illite in CH4/CO2/O2 = 1:0.8:0.2 mixture (with 60,000 mLh
1 -1 g ), CH4 conversion was around 84% and the conversion of CO2 was at 78%, with a H2/CO ratio at
10
0.9 [20]. At 750C in the oxidative reforming of a model biogas (1.5CH4 + 1CO2 + 0.25O2) on
NiO/Y2O3/ZrO2 (100 mg), the conversions of CH4 and CO2 were at 65% and 70%, respectively, with a
H2/CO ratio around 1 [18]. At 700°C on Ni/5ZrO2-SiO2 catalyst with large Ni-ZrO2 boundary CH4 and
CO2 conversions at 68% and 52%, respectively, were reported after 8 h in CH4/CO2/O2 = 1:0.4:0.3 (with
-1 a GHSV = 90,000 h ) [21]. The effects of various O2 concentrations in biogas on initial conversions and
stability were investigated at various temperatures on a Ni/SiO2 catalyst [22]. At 700°C, on 100 mg of
-1 -1 catalyst (30,000 mLh gcat ) in the CH4/CO2/O2 = 1:1:0.3 condition, CH4 conversion at 73% and CO2
conversion at 45% were obtained with a H2/CO ratio of 1.0. While besides, low cerium loading was
reported to ameliorate coke resistance of Ni/SiO2 catalysts [23]. Therefore, it appears that the present
-1 -1 catalyst shows high performances at low temperature and in harsh conditions (with 96,000 mLh gcat ).
3.2. Materials properties
2 - The CeNiXAl0.5OY (0.5 x 5) compounds have large surface areas ranging from 73 to 141 m g
1 depending on Ni content (Table 3) with an average crystallites size at about 4 nm for CeO2 phase and
at about 7 nm for NiO phase when present (Fig. 3). The CeO2 like phase (34-0394 JCPDS file) is always present, while no structure related to aluminum could be observed, which can be due to the presence of an amorphous phase, the high dispersion, and/or the insertion of aluminum species into ceria phase.
NiO phase (4-0835 JCPDS file) appears for a value of x above 0.5. It has been shown that aluminum doping ameliorates dispersion, delaying the formation of NiO crystallites large enough to be detected
-1 by XRD [47]. By Raman, the first-order F2g ceria peak (Fig. 4) located near 460 cm related to fluorite
-1 nano-crystalline CeO2 is shifted to lower frequencies of 449 cm as reported previously on Ce-Ni mixed
oxides [48,49] and shows a broadening effect. The Raman-active mode in CeO2 corresponding to the
-1 frequency of R = 466 cm , attributed to a symmetrical stretching mode of the Ce-O8 vibrational unit,
11 is very sensitive to any disorder in the oxygen sublattice and/or grain size induced non-stoichiometry and it has been shown that the line shifts and broadens with decreasing grain size (increasing lattice defects) [50]. Therefore, the obtained results are in agreement with the presence of small nanoparticles
2+ 3+ and the solubility of Ni (and Al ) into CeO2 creating anionic vacancies. As the shift increases when adding Ni and is the highest when x = 2, while the average crystallites size obtained for ceria by XRD is always at 4 nm (Table 3), it can be concluded that the insertion of Ni in ceria phase increases with Ni
content (up to x = 2) and is maximum for the CeNi2Al0.5OY compound. Moreover, bands between 500 cm-1 and 650 cm-1 were observed for all samples, attributed to oxygen vacancies created by the incorporation of the dopant in ceria [51], in agreement with the presence of a solid solution [37,47]. The
XPS analyses (Fig. 5 and Table 4) show the presence of Ni2+ cations with the characteristic satellite line
(Fig. 5A). As summarized in Table 4, the BE of the main emission Ni 2p3/2 peak is varying between 855.5
and 855.7 eV on CeNixAl0.5OY compounds when x varying between 0.5 and 2, while for higher Ni content
the main Ni 2p3/2 peak is at 854.6 eV with a shoulder at 856 eV. When x 2, the values are higher than the BE reported for bulk NiO (853.7-854.6 eV) in the literature, slightly higher than the values reported previously for binary cerium nickel mixed oxides (854.5-854.8 eV), but very close to that observed for
2+ NiAl2O4 (856.0 eV) [47]. Therefore, beside a contribution of Ni cations in NiO, and even if a contribution
2+ peak at 855.7 eV can be also attributed to Ni species related to the presence of a Ni(OH)2 hydroxide
[51], the Ni2+ cations in strong interaction with other cations are probably in the highest amount when x
= 2, in agreement with Raman results showing the highest insertion of Ni species in ceria phase, for this compound. For the compound with the highest Ni content the values become close to those obtained on binary cerium nickel mixed oxides. Cerium compounds have XPS spectra with rather complex features due to numerous initial and final 4f electronic configurations. The Ce3d spectrum,
12 registered on pure ceria, can be resolved into three spin-orbit doublets, 3d3/2-3d5/2, denoted (u, v), (u’’, v’’) and (u’’’, v’’’) [47]. These characteristics are readily visualized in the Ce 3d spectra (Fig 5B) and one
4+ can unambiguously ascribe the 3d envelope to Ce cations in CeO2-like species. The characteristic peak of Ce4+ is observed at about 917 eV, and only when x = 1 a slightly lower value (916.1 eV) is
obtained (Table 4). Besides, it has been proposed that a deconvolution labelled as u (u-u’’’) for 3d3/2 and
v (v-v’’’) for 3d5/2 can be done taking into consideration that the doublet v’/u’ is the fingerprint of the presence of Ce3+ [52], however, this is a practical simplification since it corresponds only to the center
of most intense peak among the set of peaks observed for Ce2O3 [53]. According to this, the relative contribution of the Ce3+ species to the spectrum can be determined at 6%. The Al3+ species are present at the surface as shown by the Al 2p peak (Fig 5C) obtained at about 74 eV (Table 4). Nevertheless, the uncertainty of the Al 2p photopeak position due to close proximity of Al 2p and Ni 3p spectra does not allow an interpretation of the environment of the Al species [47]. The O 1s core level presents two major oxygen species at 529.4 eV and 531.3 eV (Fig. 5D). The first peak is assigned to typical O2- lattice species, while the second peak is usually ascribed to oxygen species of hydroxyl groups (OH-) [47].
However, a contribution of O2- lattice species at 531.5 eV could be also obtained when Ce3+ cations are present, as a shift to higher binding energies has been reported for Ce(III) compounds [52]. Therefore,
XPS results show the presence of Ni2+ cations presenting strong interactions with other cations (Ce4+,
Ce3+, Al3+), together with the presence of O2- species and hydroxyl groups at the surface. Moreover,
there is a homogeneous distribution of the nickel species in the CeNiXAl0.5OY solids as illustrated in Fig.
5E by Ni/MT surface ratio obtained by XPS versus the bulk Ni/MT molar ratio (from elemental analysis).
This is also observed for the aluminum species as illustrated in Fig. 5F by the Ni/Al surface ratio obtained by XPS versus the bulk Ni/Al molar ratio for compounds with x ≤ 2, while a slight deviation is
13 obtained for the compound with the highest Ni content.
TPR analysis shows the reduction of nickel species in various environments (Fig. 6) as the total
hydrogen consumption follows a linear relationship with the Ni content of the CeNiXAl0.5OY compounds
[38]. The reduction peak at low temperature, between 210°C and 354°C, can be attributed to the nickel species in small NiO nanoparticles and/or in solid solution which are easily reducible but also easily reoxidized, as a matter of fact, redox processes between Ce4+, Ce3+, Ni0 and Ni2+ have been demonstrated [37]. Then, larger NiO nanoparticles (those observed by XRD) are reduced when increasing the temperature to about 500-600°C, as the peak becomes more intense and shifts to higher temperature as x increases. The temperature is higher compared to the one needed to reduce bulk NiO
(370C) showing strong interactions between Ni cations with other cations. The peak at 900°C can be associated to the reduction of bulk Ce4+ cations to Ce3+ [37]. The obtained average H/Ni ratio at about
2.5 (Fig. 6B) shows that a higher quantity of hydrogen is consumed by the solids compared to the H/Ni ratio of 2 that is theoretically required (Eq. 16), in agreement with a hydrogen “spillover” phenomenon.
Finally, for a treatment temperature in H2 of 250C only a partially reduced solid is obtained, as 250C corresponds to a temperature before the main TPR peak.
2+ 2- 0 "Ni O " + H2 Ni + H2O + with : anionic vacancy (16)
Inelastic Neutron Scattering (INS) evidences two new intense and large bands at about 500 cm-1
-1 and 900 cm on the solid treated at 250°C in H2 (Fig. 7), due to the insertion into the solid of new
hydrogen species [54-56]. The INS spectrum of the CeNi1Al0.5OY solid (treated in vacuum at 200°C) presents vibration bands at about 250 cm-1, 400 cm-1 and 630 cm-1, showing that the mixed oxide
contains already hydroxyl groups before the treatment in H2 [57]. The new hydrogen species created
during the activation treatment in H2 can be better observed when the INS spectrum of CeNi1Al0.5OY is
14 subtracted to that of CeNi1Al0.5HZOY (treated in H2 at 250°C) evidencing hydrogen species of hydride nature with a peak at about 500 cm-1. This is in good agreement with previous results obtained on
CeNiHZOY nano-oxyhydride (in absence of Al), for which the hydride species have been reported at about 460 cm-1 [48, 58].
On the CeNi2Al0.5OY compound after pretreatment in H2 at 250°C followed by ODRM
2+ (CH4/CO2/O2/N2 = 1:0.7:0.3:N2, CH4 = 20%) at 700°C, XPS reveals the main presence of Ni cations
on the surface (Fig. 8A, Table 5). A main Ni 2p3/2 peak at 856.0 eV is observed with a small shoulder at
852.7 eV due to a very small quantity of metallic Ni° estimated at 5% (Table 6). The BE of the main Ni
2p3/2 peak is varying between 855.7 and 855.9 eV (Table 5) demonstrating that the strong interactions between Ni2+ cations with other cations are well preserved after test at 700°C (24h), even if a contribution peak at 854.3 eV is better observed after test, with a value that becomes closer to the one obtained on bulk NiO [47]. The proportion of Ce3+ species increases after test up to 10% compared to
6% obtained before test (Table 6). As before test, the O 1s core level presents also two major oxygen species at 529.8 and 532.1 eV (Fig. 8B) showing the presence of O2- species and oxygen species in hydroxyl groups (OH-) [51]. The absence of carbon formation measured after test is confirmed by XPS and TPO results (Fig. 9 and Table 5). As a matter of fact, the C1s XPS peak near 285 eV corresponds
to adventitious carbon (Fig. 8C) and can be also attributed to CHX groups formed by dissociation of CH4
[9], while TPO evidences the soft matter of the scarce carbon [16] that can be recovered at the surface after test. As shown in Fig. 9, the TPO profile presents 2 peaks, one at 460°C and a very small broad
one at 580°C. Therefore, under test at 700°C in presence of O2, this type of carbon should be removed.
3.3. Active site modeling and reaction mechanism
There is a debate in literature on the exact nature of the active site on Ni based catalysts for
15 reforming processes, as well as on the mechanism [5-7]. Ni is known as the active species, but the exact state of the active Ni species (Ni°, Ni2+ or Ni+) is under debate, and more importantly, even if all the different Ni species can be active, the relation between the Ni species state and the selectivity is another fundamental question. It is often reported that metallic nickel Ni0 is beneficial to activate methane to syngas. However, some authors suggested that the nickel species with different oxidation states play different roles in surface reaction steps, and that the strong metal-oxide interaction produces
active sites. It has been suggested that the active phase in DRM on Ni/CeO2(111) catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria and that the strong metal-support interactions activated Ni for the dissociation of methane [12,59]. It has been even reported that a
2+ Ni /CeO2(111) system [12,59] and/or Ni cations in strong interaction with other cations [38,39] can
activate CH4 at room temperature. Concerning the mechanism, it is generally accepted that the C−H bond activation is the determining step [2-7]. It has been proposed that once the first hydrogen is removed from the reactant molecule, a quick transformation occurs on the surface and the deposited carbon could react with oxygen atoms to yield gaseous CO. The H atoms abstracted from the reactant
could transfer to H2 or H2O. In presence of O2, many other reactions are possible [2] not only limited to partial oxidation (Eqs. 12-13) or total oxidation/combustion (Eq. 14) and the gasification of carbon (Eq.
15). However, it has been also reported that strong metal support interaction between Ni species and
CeO2 nanocrystals was the main driving force for high activity of Ni-CeO2 catalysts in partial oxidation of methane (POM) [60]. Furthermore, it is often reported that the lattice oxygen atoms contribute to the gasification of CHx, which is helpful to the catalytic activity and the resistance to carbon deposition. The
support with high Lewis basicity, such as La2O3 and CeO2, can react with carbon to form CO, decreasing
carbon formation [2]. In particular, CeO2 phase has been largely reported to allow oxygen species
16 migration [51]. In its most stable phase, CeO2 adopts a fluorite-type crystal structure in which each metal cation is surrounded by eight oxygen atoms. The easy oxygen species migration allows creation of anionic vacancies. It has been already proposed that oxygen vacancies in Ce-Ni-(Al)-O based catalysts can be produced by the transformation between Ce4+ and Ce3+ [61] and literature showed that the higher Ce3+ percentage the more oxygen vacancies formed [62]. On the other hand, it has been reported that the existence of Ni particles weakens the bond energy between Ni particles and oxygen
atoms close to Ni particles in the CeO2 crystal lattice, which makes this kind of oxygen atoms easier to be reduced, so the number of oxygen vacancies can be increased [59,62].
The developed nano-materials provide high conversions for both CH4 and CO2, in agreement with the often reported relations between low size particles with high catalytic activity and decreased carbon deposition [5]. Depending on the catalytic results and characterizations, it is more probable that a high activity without carbon formation could be related to a partially reduced solid involving Ni cations. Ni species can be surrounded and influenced greatly by different neighbor atoms, as Ni species are strongly interacting with Ce, or Al species which is confirmed by XRD, XPS, Raman and TPR. Strong interactions between Ni2+ species with other cations can be obtained in the cerium and nickel (and
aluminum) solid solution and/or at the CeO2 (or solid solution) and NiO interfaces. After H2 treatment at
250°C a partially reduced solid is obtained with still the presence of cations in strong interactions
(Scheme 1). The reduction process of the present CeNiXAl0.5OY compounds can also facilitate the formation of anionic vacancies at low temperature due to the existence of a redox process between Ni and Ce cations as reported in Eqs. 16 and 17 [37,46,47]. This phenomenon allows still increasing the number of anionic vacancies.
Ni0 + 2Ce4+ ↔ Ni2+ + 2Ce3+ (17)
17
In a previous study on the relation between hydrogen content stored in the solid and treatment
temperature in H2 on similar formulation catalysts [57], it was clear that the treatment in H2 at an appropriate temperature induces the creation of a large catalytic hydrogen reservoir that was related to
the creation of anionic vacancies [61]. During the activation in H2 (Eq. 18), the anionic vacancy is filled
with hydride species by the heterolytic splitting of H2. And, it has been proposed that CH4 in presence
of O2 (POM) can also be activated on an anionic vacancy on the cerium nickel based mixed oxides as reported in Eq. 19 [38,39].
2− n+ − n+ −" H2 + "O M " → "H M OH (with : anionic vacancy). (18)
2− n+ − n+ −" CH4 + 2"O M "→ C + 2"H M OH (19)
Concerning CO2 activation, hydrogenation of CO2 is possible at low temperature in presence of hydrogen (Eq. 7) [40] and in particular in presence of hydride species [41-44]. Therefore, the high
conversion of CO2 (higher than the thermodynamic value calculated using HSC chemistry 6) can be explained by the participation of the solid (catalyst) to the reaction, not taken into account in the calculations. The presence of a hydride reservoir in the bulk and at the surface of the solid can explain
the high selectivity to H2 with a particularly high conversion of CO2, as it is available for CO2 activation,
avoiding the use/consumption of H2 in gas phase. Moreover, it has been shown on Ir/CeO2 catalyst [63], that the chemical state of Ir species, induced by a strong metal-support interaction, had a major impact on the reaction selectivity in carbon dioxide hydrogenation reaction (metallic Ir particles select for methane while partially oxidized Ir species select for CO production). Therefore, the presence of strong interactions between cations, anionic vacancies and hydrogen species stored in the solid can help to
activate CO2 [45]. The highly reactive hydride species can i) react with O2 permitting consumption of O2
while generating hydroxyl groups (Eq. 20) and then H2O (Eq. 21) which regenerates the anionic vacancy
18 and ii) help in activation of CO2 as oxyhydride catalysts have been reported as highly active catalysts for hydrogenation reactions [64].
– – 1/2O2 + H → OH (20)
– 2‒ 2OH → H2O + "O " (21)
2– In presence of CO2, an active site involving an anionic vacancy and an O species of the solid can
also be envisaged for the heterolytic dissociation of CH4 (Eq. 19) [13,37,38]. The dehydrogenation of
CH4 firstly requires the abstraction of hydrogen species from CH4, with the rupture of C-H bond. The
dissociation of CH4 leads to the formation of C species that could react with oxygen atoms of the solid
to yield gaseous CO. However, it is not necessarily required that the CH4 molecule fully dissociate to lead to C that can then be oxidized to CO on ceria. The complete transformation can be done on a complex site if all the functions are present together. An active site involving Ni2+ (or Ni+) cations in strong interaction with other cations can be modeled, taking into account the structure of ceria (solid solution) and the presence of anionic vacancies and hydride species (Scheme 1). During the reaction,
the hydride species are replaced and provided by CH4. This catalytic route smartly saves energy by combining a chemical reaction that makes full use of the specific property of the nano-material, the reactivity of hydride species stored in the highly active and stable nano-oxyhydride catalysts.
4. Conclusion
CeNiXAl0.5HZOY nano-oxyhydrides are shown highly active, selective and stable catalysts for
-1 -1 oxidative dry reforming of CH4 in relatively harsh conditions (500-600°C, 96,000 mLh gcat ,
CH4/CO2/O2/N2 = 1:0.7::N2, with CH4 = 20%). Both conversions of CH4 and CO2 become more efficient
as the temperature increases and high CH4 and CO2 conversions are observed in the temperature
range of 500-800°C. At low temperature (500-600°C), CH4 conversion is reaching the thermodynamic
19 limit while CO2 conversion is particularly high that can be explained by the presence of a nano- oxyhydride compound (solid storing hydride species) evidenced by INS. The nano-materials are easily prepared by co-precipitation method. In the oxide state they are composed of small nanoparticles of
NiO (6-7 nm), CeO2 and/or Ce-Ni-(Al)-0 solid solution (4 nm) depending on Ni content, with the
2+ existence of strong interactions between Ni species with other cations. The H2 treatment at 250°C leads to partially reduced solids with simultaneous generation of anionic vacancies and insertion of hydride species inside the solid. After test the oxide phase is largely maintained. Hence, it is reasonable to propose an active site involving cations in close interaction, O2- species, anionic vacancies, and hydride species.
Acknowledgments
Y. Wei and X. Liu thank China Scholarship Council for their grants and N. Haidar thanks for her grant from University of Lille. The authors thank L. Burylo (XRD), O. Gardoll (TPR, TPO), J. C. Morin
(Raman), M. Trentesaux and P. Simon (XPS) from UCCS, and Institut Laue Langevin (ILL) Grenoble
France for funding and helping for INS experiments. Chevreul Institute (FR 2638), Ministère de l’enseignement Supérieur et de la Recherche, Région Hauts de France, and FEDER are acknowledged for supporting and funding partially this work.
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Figure captions
Fig. 1. CH4 (◆) and CO2 (◇) conversions, gas-phase products distribution (H2 (△), CO (●)) and H2/CO
ratio ( ) on in-situ pretreated in H2 at 250°C CeNiXAl0.5OY compounds (50 mg) A) versus reaction
temperature. B) Stability test at 700°C on CeNi2Al0.5OY catalyst. Versus Ni content at C) 500°C and (D)
600°C. O2 conversion is total. Reaction conditions: CH4/CO2/O2/N2 = 1:0.7:0.3:N2 (with CH4 = 20%).
Unless in B) points reported after 5h. CH4 (----) and CO2 (--) conversions thermodynamic limit.
Fig. 2. CH4 and CO2 conversions and products distribution (H2, CO) obtained at 600°C on pretreated
in H2 at 250°C CeNi2Al0.5OY compound versus O2/CH4 ratio. Reaction conditions: CH4/CO2/O2/N2=
1:0.7::N2 (with CH4 = 20%). Values reported after 5h.
Fig. 3. XRD patterns of CeNiXAl0.5OY compounds: x = a) 0.5, b) 1, c) 2, d) 5. CeO2 , NiO .
Fig. 4. Raman spectra of a) CeO2 and CeNiXAl0.5OY compounds with x = b) 0.5, c) 1, d) 2, e) 5. Line
shows position of first-order F2g ceria peak.
Fig. 5. XPS spectra of CeNixAl0.5OY compounds with x = a) 0.5, b) 1, c) 2 and d) 5. A) Ni2p3/2 B) Ce 3d,
C) Al2p-Ni3p, D) O1s, E) surface Ni content versus bulk and F) Ni/Al ratio on surface versus bulk.
Fig. 6. TPR of the CeNiXAl0.5OY compounds. A) TPR profiles: x = 0.5 (a), 1 (b), 2 (c) and 5 (d). B) H2
consumption () and H/Ni atomic ratio (△). Ni/MT=x/(1+x+0.5).
Fig. 7. INS spectra of CeNi1Al0.5OY compound treated in vacuum at 200°C for 2h (black) and after in
situ treatment in H2 at 250C for 10 h (blue). The spectrum evidencing hydride species (pink) =
compound in H2 - compound in vacuum.
Fig. 8. XPS spectra of CeNi2Al0.5OY a) calcined compound and b) after ODRM at 700°C during 24 h on
pretreated in H2 at 250°C compound (Figure 2B). A) Ni 2p3/2 B) Ce 3d, C) O 1s and D) C1s.
Fig. 9. TPO profile after ODRM at 700°C during 24 h on pretreated in H2 at 250°C CeNi2Al0.5OY
29 compound (Figure 2B).
Table 1. Carbon formation rate at 600°C in ODRM on CeNiXAl0.5OY compounds pretreated in H2 at
250°C. Reaction conditions: CH4/CO2/O2/N2 = 1:0.7:0.3:N2 (with CH4 = 20%).
Table 2. H2/CO ratio and carbon formation rate in different ODRM conditions on CeNi2Al0.5OY compound
(50 mg) pretreated in H2 at 250°C. Reaction conditions: CH4/CO2/O2/N2= 1:0.7::N2 (with CH4 = 20%).
Table 3. Ni content, specific surface area and average crystallites size of CeNiXAl0.5OY compounds.
Table 4. Binding energies of the CeNiXAl0.5OY compounds.
Table 5. XPS analysis before and after ODRM at 700°C during 24h on in-situ pretreated in H2 at 250°C
CeNi2Al0.5OY compound (reaction conditions: CH4/CO2/O2/N2 = 1:0.7:0.3:N2, CH4 = 20%).
Table 6. Data from XPS analysis before and after experiment (Table 5, Fig. 8).
Scheme 1. Active site and proposition of mechanism on CeNiXAl0.5HZOY nano-oxyhydride catalysts.
Nin+ =Ni2+,Niδ+, Cem+ = Ce4+, Ce3+, (Al3+ can replace a cerium cation in the solid solution), : anionic vacancy (position and number arbitrary).
30
Table 1 - Carbon formation rate at 600°C on CeNiXAl0.5OY compounds (50 mg) pretreated in H2 at 250°C.
CH4/CO2/O2/N2 = 1:0.7:0.3:N2 (with CH4 = 20%).
Ni/MT 0.25 0.4 0.57 0.77
Carbon formation 0 0 0 0.24 -1 -1 (g·gcat ·h )
Table 2 - H2/CO ratio and carbon formation rate at 600°C in different ODRM conditions on CeNi2Al0.5OY
compound (50 mg) pretreated in H2 at 250°C. CH4/CO2/O2/N2= 1:0.7::N2 (with CH4 = 20%).
O2/CH4 ratio 0 0.1 0.3 0.5
H2/CO ratio 1.0 1.4 1.3 1.3
-1 -1 Carbon formation (g·gcat ·h ) 0.60 0.24 0 0
Table 3 - Ni content, specific surface area and average crystallites size of CeNiXAl0.5OY compounds
Ni S. A. dCeO2 dNiO
Sample Ni/MT (wt.%) (m²/g) (nm) (nm)
CeNi0.5Al0.5OY 12.0 0.25 141 4 —
CeNi1Al0.5OY 19.5 0.4 115 4 7
CeNi2Al0.5OY 30.8 0.57 73 4 6
CeNi5Al0.5OY 49.2 0.77 106 4 6
Ni/MT = x/(x + 1.5); (—) = not observed.
31
Table 4 - Binding energies of the CeNiXAl0.5OY compounds
Ni2p3/2 O1s Ce3d Al2p Catalyst (eV) (eV) (eV) (eV)
CeNi0.5Al0.5OY 855.6 529.4/ 531.3 916.7/ 900.7/ 898.4/ 882.3 73.8
CeNi1Al0.5OY 855.5 529.3/ 531.3 916.1/ 900.4/ 897.8/ 882.0 73.8
CeNi2Al0.5OY 855.7 529.8/ 532.0 916.8/ 901.1/ 898.7/ 882.6 74.1
CeNi5Al0.5OY 854.6 529.8/ 531.3 917.1/ 901.0/ 898.5/ 882.5 73.9
Table 5 - XPS analysis before and after ODRM at 700°C during 24h on in-situ pretreated in H2 at 250°C
CeNi2Al0.5OY compound (reaction conditions: CH4/CO2/O2/N2 = 1:0.7:0.3:N2, CH4 = 20%).
CeNi2Al0.5OY Ni2p3/2 (eV) O1s (eV) C1s (eV)
Calcined 854.3/ 855.7 529.8/ 532.0 284.8
Spent 854.3/ 855.9 529.8/532.1 284.3
Table 6 - Data from XPS analysis before and after experiment (Table 5, Fig. 8).
Sample Ni (at %) Ce (at %) O (at %)
Ni0 Ni2+ Ce3+ Ce4+ O2- OH O’
Calcined 0 100 6 94 35 54 11
Spent 5 95 10 90 41 51 8
32
100 2 100 (A) (B) 1.2
1.5
/CO 2
50 1 /CO 50
2 H
H 0.6
Conv. (%) Conv. 0.5 Conv., Prod. Prod. (%) Conv.,
0 0 0 0 400 500 600 700 800 900 0 8 16 24 Temperature (°C) Time (h)
100 2 100 2 (C) (D)
1.5 1.5
/CO /CO /CO /CO
50 1 2 50 1
2
H
H
Conv. (%) Conv. Conv. (%) Conv. 0.5 0.5
0 0 0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Ni/M Ni/M T T
Fig. 1 - CH4 (◆) and CO2 (◇) conversions, gas-phase products distribution (H2 (△), CO (●)) and H2/CO
ratio ( ) on in-situ pretreated in H2 at 250°C CeNiXAl0.5OY compounds (50 mg) A) versus reaction
temperature. B) Stability test at 700°C on CeNi2Al0.5OY catalyst. Versus Ni content at C) 500°C and (D)
600°C. O2 conversion is total. Reaction conditions: CH4/CO2/O2/N2 = 1:0.7:0.3:N2 (with CH4 = 20%).
Unless in B) points reported after 5h. CH4 (----) and CO2 (--) conversions thermodynamic limit.
33
100 Equlibrium values (CH4) ----
H2/CO ratio 71 63 62 1.4 52 1.3 1.3 1.0 50 CH₄%
CO₂% Conv., products Conv., H₂% CO% 0 0 0.1 0.3 0.5
O2/CH4 ratio
Fig. 2 - CH4 and CO2 conversions and products distribution (H2, CO) obtained at 600°C on pretreated
in H2 at 250°C CeNi2Al0.5OY compound versus O2/CH4 ratio. Reaction conditions: CH4/CO2/O2/N2=
1:0.7::N2 (with CH4 = 20%). Values reported after 5h.
34
d
c Intensity Intensity (a.u.) b a
20 30 40 50 60 70 80
2 θ (°)
Fig. 3 - XRD patterns of CeNiXAl0.5OY compounds: x = a ) 0.5, b) 1, c) 2, d) 5. CeO2 , NiO
35
e ) u. d c
Intensity (a. Intensity b
a 200 400 600 800 Raman shift (cm-1)
Fig. 4 - Raman spectra of a) CeO2 and CeNiXAl0.5OY compounds with x = b) 0.5, c) 1, d) 2, e) 5. Line
shows position of first-order F2g ceria peak.
36
2+ (A) (B) Ni2p3/2 Ni Ce3d v''' Al2p - Ni3p (C) 854.6 u''' u v 856.0 d u'' u' v' v'' c d 855.7 d b
c c
Intensity (a.u.) Intensity Intensity (a.u.)Intensity b (a.u.)Intensity a b a a
865 860 855 850 925 915 905 895 885 875 80 75 70 65 60 Binding energy (eV) Binding energy (eV) Binding energy (eV)
1 10 O1s (D) (E) (F) 8 d 6 c 0.5
onsurface 4
T
Intensity (a.u.)Intensity Ni/Al onsurface Ni/Al
b Ni/M 2 a 0 0 535 533 531 529 527 0 0.5 1 0 2 4 6 8 10 Binding energy (eV) Ni/MT in bulk Ni/Al in bulk
Fig. 5 - XPS spectra of CeNixAl0.5OY compounds with x = a) 0.5, b) 1, c) 2 and d) 5. A) Ni2p3/2 B) Ce 3d,
C) Al2p-Ni3p, D) O1s, E) surface Ni content versus bulk and F) Ni/Al ratio on surface versus bulk.
37
6 (A) 240 (B)
200 5
) 1 d - 160 4
c 120 3 H /H Ni
cons. (a.u.) b 2
2 2 80
H cons. cons. (mL.g
a 2 H 40 1
0 0 100 300 500 700 900 0 0.2 0.4 0.6 0.8 1 Ni/M Temperature in H2 (°C) T
Fig. 6 - TPR of the CeNiXAl0.5OY compounds A) TPR profiles: x = 0.5 (a), 1 (b), 2 (c) and 5 (d). B) H2
consumption () and H/Ni atomic ratio (△). Ni/MT=x/(1+x+0.5).
38
H-
10
cts) 3 3
5 Intensity (10 Intensity
0 0 500 1000 1500 -1 E (cm )
Fig. 7 - INS spectra of CeNi1Al0.5OY compound treated in vacuum at 200°C for 2h (black) and after in
situ treatment in H2 at 250 C for 10 h (blue). The spectrum evidencing hydride species (pink) =
compound in H2 - compound in vacuum.
39
2+ Ni 2p Ni (A) Ce 3d u''' (B) 3/2 u v''' 856.0 v u'' u' v' Ni0 v'' b 852.7 b
a a
Intensity (a.u.)
Intensity (a.u.)
870 865 860 855 850 930 920 910 900 890 880 Binding energy (eV) Binding energy (eV)
O 1s 529.8 (C) C 1s (D) 532.0
b b
a
Intensity (a.u.) Intensity (a.u.) a
538 536 534 532 530 528 526 290 288 286 284 282 Binding energy (eV) Binding energy (eV)
Fig. 8 - XPS spectra of CeNi2Al0.5OY a) calcined compound and b) after ODRM at 700°C during 24 h on
pretreated in H2 at 250°C compound (Figure 2B). A) Ni 2p3/2 B) Ce 3d, C) O 1s and D) C1s.
40
460 intensity intensity (a.u.)
2 2 580 CO
100 300 500 700 900 Temperature ( C)
Fig. 9 - TPO profile after ODRM at 700°C during 24 h on pretreated in H2 at 250°C CeNi2Al0.5OY compound (Figure 2B).
41
- - H O H O Cem+ Ni n+ HO O O O O H O O
Scheme 1 - Proposition of active site and mechanism on CeNiXAl0.5HZOY nano-oxyhydride catalysts.
Nin+ =Ni2+,Niδ+, Cem+ = Ce4+, Ce3+, (Al3+ can replace a cerium cation in the solid solution), : anionic vacancy (position and number arbitrary).
42
Graphical abstract
43