Nanosized transition metals in controlled environments of phyllosilicate-mesoporous silica composites as highly thermostable and active catalysts

Article abstract of DOI:10.1039/c3cc43197e

Stabilization of transition metals in nano-phyllosilicate phases generated by digestion of mesoporous silica is reported as an efficient route for the formation of highly dispersed metallic nanoparticles with outstanding catalytic activity. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc43197e

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Nanosized transition metals in controlled
environments of phyllosilicate–mesoporous silica
Cite this: DOI: 10.1039/c3cc43197e
composites as highly thermostable and active catalysts†
Received 30th April 2013,
Accepted 28th June 2013
Carmen Ciotonea,ab Brindusa Dragoi,a Adrian Ungureanu,*a Alexandru Chirieac,a
b
b
a
´
Sabine Petit, Sebastien Royer* and Emil Dumitriu
DOI: 10.1039/c3cc43197e
www.rsc.org/chemcomm
Stabilization of transition metals in nano-phyllosilicate phases generated species can be simultaneously/sequentially incorporated to yield multi-
by digestion of mesoporous silica is reported as an efficient route for the functional properties; (iii) capability to functionalize their surface by
formation of highly dispersed metallic nanoparticles with outstanding various inorganic or organic components which induce specific local
catalytic activity.
environments for nucleating and growing metal NPs and (iv) versatility
of the synthetic procedures to provide an enhanced control over the
Highly dispersed supported metal (oxide) nanoparticles (NPs) have types of guest inorganic species and, more importantly, their distribu-
attracted much interest in the last decade because of their excellent tions within the micro-mesostructured hosts. In this work, copper,
activity and/or selectivity in important oxidation and (de)hydrogenation nickel and cobalt phyllosilicate-SBA-15 mesoporous silica composites
reactions underpinning most of the catalytic applications in environ- were proposed to generate highly dispersed and thermally stable
mental pollution control, energy and fine chemistry.1–3 The outstanding metallic active phases. The potential of these materials for hetero-
catalytic performances of the supported NPs are related to their geneous catalysis was evaluated for the hydrogenation of cinnamalde-
morphostructural properties (i.e., size and shape),4 dispersion (i.e., hyde as test reaction to characterize the accessibility and dispersion of
surface-to-volume ratio and number of defect sites, steps, edges...),5 supported transition metals. PS-derived materials obtained by the
concentration and electronic properties of the metals.6 Nevertheless, deposition precipitation (DP) route were compared with materials
the low stability of NPs upon high-temperature treatments required for prepared by classical wet impregnation (WI) and incipient wetness
catalyst activation or high-temperature catalytic reactions largely limits impregnation (IWI), respectively. The versatility of the approach was
their practical applications. Particle sintering and restructuring4,5 demonstrated for catalytic systems based on different TMs such as
directly impact catalytic activity, especially when silica is used as a cobalt (Co_DP), nickel (Ni_DP) and copper (Cu_DP). The materials were
support. Therefore, the design and development of supported metal characterized in oxidized and reduced forms of transition metals. The
(oxide) NPs with high and thermally stable dispersion represent experimental details for synthesis and physico-chemical characteriza-
ongoing and challenging research tasks. A valuable approach, pre- tion of solids are given in the ESI† file.
sented here for the first time, to ensure both dispersion and stability of
Differences between the materials are directly observed by XRD. The
transition metal (TM) NPs consists of the stabilization of the NPs in patterns recorded for Co_WI and Co_IWI materials (Fig. 1(A)) evidenced
controlled environments of phyllosilicate (PS)–mesoporous silica com- the formation of the crystalline Co3O4 phase (ICDD 42-1467) with
posites. The ordered mesostructured materials have been chosen to average crystallite sizes of 13.1 and 12.6 nm, respectively, as calculated
have an accurate control over the local environment in which the NPs using the Scherrer equation. Only weak and broad reflections of the
will be sequentially stabilized. Particularly interesting for our approach highly dispersed Co–PS phase (ICDD 21-0871) are observed at 2y of
are the large-pore mesoporous silica materials with SBA-15 topology, B351 and 601 in the XRD pattern of the Co_DP sample.9 Similar
prepared using non-ionic amphiphilic poloxamers as supramolecular observation can be made for the Ni- and Cu-containing materials.
templates,7 due to several advantages:8 (i) high surface areas, well Thus, NiO and CuO phases are formed by IWI, whereas only PS phases
defined pore structures and thick pore walls; (ii) multiple inorganic are obtained for DP preparation (Fig. S1, ESI†).
In line with the XRD results, major differences between
a Technical University ‘Gheorghe Asachi’ of Iasi, Faculty of Chemical Engineering
and Environmental Protection, 73 D. Mangeron Bvd., 700050 Iasi, Romania.
materials can also be observed in the (HR)TEM images. Thus,
Co_WI shows NPs of calibrated size, i.e. close to the pore size of the
support and confined in the mesopores (Fig. 2(a)). These elementary
particles grow in the form of large aggregates separated by silica
walls (Fig. 2(a), inset). Mesopore-confined NPs are also observed for
Co_IWI without the formation of large aggregates. In this case the
aggregates are limited to few particles, as shown in the inset of
E-mail: aungureanu@tuiasi.ro; Tel: +40 232 278683
b
´
Universite de Poitiers, UMR 7285 CNRS, IC2MP, 4 Rue Michel Brunet, Poitiers,
86022 Poitiers Cedex, France. E-mail: sebastien.royer@univ-poitiers.fr;
Fax: +33 549453479
† Electronic supplementary information (ESI) available: Materials and methods;
characterization results. See DOI: 10.1039/c3cc43197e
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Table 1 Textural properties of derived TM-containing materials
Sample M/wt% SBET/m2 gÀ1 Sm/m2 gÀ1 Vp/cm3 gÀ1 Vm/cm3 gÀ1 Dp/nm
SBA-15
—
729
516
476
363
354
327
139
59
47
5
33
28
1.28
0.98
0.89
1.11
1.20
1.09
0.061
0.023
0.017
0.001
0.011
0.012
9
9
9
11
11
12
Co_WI 4.5
Co_IWI 5.1
Co_DP 5.1
Ni_DP 4.4
Cu_DP 6.1
M, metal loading by ICP; SBET and Sm, total and micropore surface area;
Vp and Vm, total and micropore volume; Dp, mean mesopore size.
host (Fig. 2).13 For Co_DP, the synthesis conditions (slightly alkaline
pH) induced modification of the SBA-15 pore network architecture,
originating from silica digestion necessary for phyllosilicate crystal-
lisation, and consequently the surface area decreased two times
(Table 1). Likewise, the isotherms show less defined hysteresis loops
which are shifted to higher P/P0 values, as compared with SBA-15. This
indicates a broader pore size distribution as well as larger mesopores.
An increase in mesopore size as well as change in mesopore shape are
obvious in the TEM images, where large pores (>10 nm) are observed
(Fig. 2(c)). Similar results were obtained for Ni_DP and Cu_DP, with loss
of surface area and an increase in pore size because of the partial
shrinkage of SBA-15 mesostructure. However, while Co_DP presents
very limited microporosity, the two other materials present significant
fraction of micropores, suggesting that the disappearance of the
micropores for Co_DP is due to their transformation into mesopores.9
The reducibility of the calcined materials was monitored by TPR
(Fig. 1(B) and Fig. S3, ESI†), while the crystal phase evolution
during TPR was followed by in situ XRD. The TPR profiles recorded
for Co_WI and Co_IWI (Fig. 1(B)) exhibit three distinct reduction
stages: (i) o350 1C, reduction of large Co3O4 particles, according to a
two-step process (Co3+ - Co2+ - Co0);14 (ii) 420–450 1C, reduction of
small Co3O4 particles, in interaction with the support; (iii) >600 1C,
reduction of cobalt in strong interaction with the support or in the PS
phase.14 These reduction steps were further validated by in situ XRD.
Thus, the Co3O4 phase still observed after reduction at 300 1C
Fig. 1 (A) XRD patterns recorded for Co-containing materials calcined at 500 1C.
Bottom: ICDD 42-1467 (Co3O4); *: ICDD 21-0871 (Co3(Si2O5)2(OH)2). (B) TPR
profiles of cobalt-containing materials.
Fig. 2(b). In contrast, the Co_DP sample shows completely different
morphostructural properties: (i) alteration of the mesopore long-
range ordering; (ii) absence of mesopore-confined Co3O4 NPs; (iii)
appearance of highly dispersed filamentous particles (Fig. 2(c)).
Exactly similar observations can be made for Ni_DP (Fig. 2(d)) and
Cu_DP (Fig. S4, ESI†). This filamentous morphology is characteristic
of metal phyllosilicates10,11 and confirms that the DP route leads to
the stabilization of TMs in the phyllosilicate phases.
The nitrogen adsorption–desorption isotherms of the materials are
shown in Fig. S2 (ESI†) and the corresponding textural properties are
gathered in Table 1. Co_WI and Co_IWI samples exhibit IUPAC type IV
isotherms with H1 hysteresis loops, which are typical for mesoporous
materials with cylindrical pores, and usually obtained for ordered SBA-
15 materials.12 As compared with the parent SBA-15, the values of
surface areas and pore volumes are found to decrease for the samples
obtained by impregnation. Also, the decrease in micropore surface area
and volume suggests partial plugging of intrawall micropores by oxide
clusters which are confined in the primary mesopores of the SBA-15
Fig. 2 Representative TEM images recorded for: Co_WI oxidized (a) and reduced (e); Co_IWI oxidized (b) and reduced (f); Co_DP oxidized (c) and reduced (g), and
Ni_DP oxidized (d) and reduced (h).
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completely disappears at 500 1C along with the appearance of is obtained for Cu_DP (Fig. S4, ESI†), TEM images displaying only
characteristic reflections of the Co0 phase (Fig. S5, ESI†). Co_DP traces of filamentous particles besides highly dispersed Cu0 NPs. Again,
shows a completely different reduction profile (Fig. 1(B)), with a the limited size of the generated metallic NPs could be at the origin of
depressed reduction at low temperature, and main reduction occur- the weak and broad reflections observed in Fig. S5 (ESI†).
ring at T > 750 1C (incomplete at 900 1C). Such high reduction
Catalytic activity evolutions (hydrogenation of cinnamaldehyde,
temperature is unambiguously attributed to the reduction of Co2+ in Scheme S1, ESI†) highlight the important benefit obtained over the
the PS phase,9,14,15 in agreement with vide supra XRD and TEM DP-materials compared with the WI and IWI derived materials (Fig. 3,
results. In situ XRD patterns recorded for the Co_DP sample show catalytic data gathered in Table S1, ESI†). Thus, a 2.5-fold higher initial
weak diffraction peaks assigned to the PS phase, highly thermostable activity is calculated for Co_DP as compared with Co_WI and Co_IWI,
and not reducible even after treatment under hydrogen at 500 1C. despite the incomplete reduction of cobalt under the selected condi-
Albeit a fraction of cobalt oxide was observed to be reduced below tions. This result evidenced the remarkable benefit in the accessibility
500 1C (Fig. 1(B)), no reflection ascribed to Co0 can be detected by of metallic sites over DP-derived materials. Ni- and Cu-containing
XRD. For Cu_DP, Cu2+ reduction was observed to occur at 295 1C, DP-materials were also compared with their homologues obtained
indicating the reduction of dispersed particles in strong interactions by the IWI route (Fig. 3), confirming the outstanding activity of the
with the support.16 The weak reflections corresponding to the PS DP-materials. Nevertheless, the chemoselectivity was not significantly
phase observed below 300 1C by in situ XRD disappeared (Fig. S6, affected by the preparation route. The small differences in selectivity
ESI†), while Cu0 appeared at temperatures higher than 300 1C. (Table S1, ESI†) can be related to the modification of adsorption site
However, the intensity and FWHM of Cu0 reflection peaks indicate density with metal particle size. For example, large particles are
no subsequent sintering of the metallic phase up to 500 1C. Finally, favourable to the CNA adsorption via CQO bonds due to a higher
Ni_DP presents a reduction maximum at 664 1C (Fig. S3, ESI†), in proportion of dense Co(111) faces,18 which can explain the slightly
agreement with the reduction of Ni2+ in PS phases (1 : 1 PS, B550 1C; higher selectivity to CNOL obtained for Co_WI and Co_IWI catalysts.
2:1 PS, B650 1C).10,11 For the Ni_DP sample, in situ XRD results
confirm the stability of the PS phase up to 500 1C, with no sintering of phase is hence evidenced as a powerful route to produce highly
the Ni0 phase.
active metallic catalysts. The strong metal–support interactions
The stabilization of TM in a highly dispersed state in the PS
Complementary TEM analysis was performed to evaluate the result in limited particle sintering during reduction, given highly
dispersion of the metallic phase (Fig. 2). Fibrous Co–PS particles are dispersed metallic NPs. Due to the small size of NPs, catalytic activity
always observed for Co_DP samples after reduction at 500 1C (Fig. 2(g)). is remarkably enhanced for PS-mediated metallic catalysts.
Very small metallic NPs were also observed, issued from partial
This work was partially supported by two grants of the Romanian
reduction of TM cations in the PS phase. However, no large metallic National Authority for Scientific Research, CNCS – UEFISCDI (project
particles can be observed throughout the analyzed grains, confirming numbers PN-II-ID-PCE-2011-3-0868 and PN-II-RU-TE-2012-3-0403).
the limited sintering of NPs in this material. In contrast, large metallic
Notes and references
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and f)). The reduction of the Ni_DP sample (Fig. 2(h)) leads to the
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particles are still observed. Due to the lower thermal stability of the 1 : 1
PS phase compared with 2 : 1 PS,11,17 Ni0 NPs could originate from the
reduction of the 1 : 1 PS phase, while the reminiscent filamentous
phase may correspond to the 2 : 1 PS phase. The small size of the
generated NPs can explain the absence of Ni0 reflection in the
corresponding XRD pattern (Fig. S5, ESI†). Finally, similar morphology
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Fig. 3 Catalytic activity in cinnamaldehyde hydrogenation measured for:
(a) Co-containing materials; (b) Cu- and Ni-containing materials.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.