Co-catalytic enhancement of H2 production by SiO2 nanoparticles

Catalysis Today 242 (2015) 146–152

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Co-catalytic enhancement of H2 production by SiO2 nanoparticles Panagiota Stathi a , Yiannis Deligiannakis b , Maria Louloudi a,∗ a b

Laboratory of Inorganic Chemistry, Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece Laboratory of Physical Chemistry, Department of Environmental and Natural Resources Management, University of Patras, Seferi 2, 30100 Agrinio, Greece

a r t i c l e

i n f o

Article history: Received 4 March 2014 Received in revised form 3 July 2014 Accepted 16 July 2014 Available online 20 August 2014 Keywords: Hydrogen Formic Iron Phosphine Silica Thermal treatment

a b s t r a c t SiO2 nanoparticles, of varying size 9–55 nm, show significant co-catalytic activity for H2 -production from HCOOH by the homogenous FeII /P(CH2 CH2 PPh2 )3 catalyst. Particle size and specific surface area are shown to play key-role in this phenomenon. Larger nanoparticles have better co-catalytic effect. Arrhenius analysis reveals that the significant co-catalytic effect can is attributed to a lowering of the activation energy of the rate-limiting step by Ea = 16–36 kJ/mol. This phenomenon is attributed to a thermodynamic promotion of HCOOH deprotonation by the SiO2 nanoparticles, accelerating in this way coordination of HCOO− on the FeII atom of active catalyst, during catalysis. The surface Si–O–Si bridges are shown to be responsible for the promotion of this HCOOH deprotonation, while surface-adsorbed H2 O retards the co-catalytic efficiency. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Catalytic H2 production is currently receiving considerable attention, reflecting the thirst for sustainable renewable energy. Formic acid (FA) is considered one of the most promising liquid sources for H2 production or as H2 -storage material [1]. FA can be catalytically decomposed to H2 and CO2 via dehydrogenation [2–4]. HCOOH (l) → H2 (g) + CO2 (g) G = −32.9 kJ/mol

(1)

However, CO, which is a fatal poison for catalysts and fuel cells, can be also generated by dehydration of FA [4]. HCOOH (l) → CO (g) + H2 O G = −12.4 kJ/mol

(2)

Currently, among the challenges in catalytic H2 production by FA are: [i] the type of catalyst, i.e. metal complex vs. nanoparticle, [ii] temperature of reaction, i.e. high temperatures typically >300 ◦ C vs. near-ambient temperatures T < 100 ◦ C, [iii] nature of metal, i.e. noble-metal vs. low-cost transition metals [1,2]. Noble-metal based homogeneous catalysts [2] have been intensively studied for H2 generation from FA. A binuclear Ru-phosphine complex [Ru2 (␮-CO)(CO)4 (␮-dppm)2 ], presented by Puddephat et al. in 1998, was the most active complex for this reaction, at that time, achieving TOF ∼ 500 h−1 at room temperature [5].

∗ Corresponding author. Tel.: +30 2651 0 08418; fax: +30 2651 0 08786. E-mail address: [email protected] (M. Louloudi). http://dx.doi.org/10.1016/j.cattod.2014.07.012 0920-5861/© 2014 Elsevier B.V. All rights reserved.

Thus, Ru-phosphine catalysts have been shown to be efficient in H2 generation from FA, at near-ambient temperatures [6,7]. Other noble-metals, i.e. Rh, Ir, have also been shown to give promising results [5,8–10], however, their use poses cost-efficiency and environmental-acceptance issues. To this front, the use of Fe-based catalysts has been demonstrated by Beller and coworkers as efficient/simple and low-cost system, e.g. exemplified by a FeII -tris[(2-diphenylphosphino)ethyl] phosphine complex, [FeII P(CH2 CH2 PPh2 )3 ], which affords TOF = 9425 h−1 at 80 ◦ C in propylene carbonate solvent [11]. From the mechanistic point of view, it has been proposed that a key-catalytic step for H2 -production by FA is the coordination of one deprotonated HCOO− molecule on the metal centre [2–4,6]. This has been confirmed also by our recent studies using advanced Pulsed-EPR spectroscopy [12]. The involvement of the formate anion has also been postulated for gas phase FA decomposition to CO2 and H2 by metallic and metal oxide particles [13–15]. This gasphase reaction was proposed to involve chemisorption of FA on surface acid sites [13–15]. Theoretical studies show that the proton migration from FA on the surface is the rate limiting step of the H2 -generating reaction in the gas-phase [15]. More recently, we have discovered that H2 -production from FA catalyzed by homogeneous catalyst FeII /P(CH2 CH2 PPh2 )3 can be improved by basic functional groups grafted on SiO2 particles [16]. The use of SiO2 –NH2 and SiO2 -imidazole as co-catalysts, gave remarkably high TOFs 14,942 and 16,432 h−1 , respectively [16]. This effect was attributed to the assistance of –NH2 or –imidazole

P. Stathi et al. / Catalysis Today 242 (2015) 146–152 Table 1 Physical properties of SiO2 nanoparticles. Sample Ox-50 A90 A300

SSA (m2 /g)a 50 ± 15 90 ± 15 300 ± 30

Particle diameter(nm)a

[OH] (mmol/g)b

55 30 9

0.28 0.51 1.74

a Detailed characterization of size and SSA of the Aerosil NPs is presented in reference [17]. b Detailed characterization of surface OH groups of Aerosil NPs is presented in reference [18].

HCOO−

functionalities on FA deprotonation, facilitating coordination on the FeII atom of catalyst [16]. Herein, we show that nanosized SiO2 particles have a significant co-catalytic effect on H2 -production from FA by the homogeneous catalytic system FeII /P(CH2 CH2 PPh2 )3 . A systematic study has been carried out using well characterized SiO2 nanoparticles of varying size or specific surface area (SSA), which underwent through various protocols of controlled thermal treatments that varied their surface groups. This systematic study, together with thermodynamic data, allows a physicochemical understanding of the observed co-catalytic phenomena. 2. Materials and methods 2.1. Chemicals and SiO2 NPs All solvents were of commercial grade, purchased from Aldrich. Formic Acid (97.5% with 2.5% H2 O [v/v]), Fe(BF4 )2 and tris-2diphenylphosphino-ethyl-phosphine, P(CH2 CH2 PPh2 )3 , have been purchased from Aldrich and stored under argon. Commercial hydrophilic fumed silica nanoparticles with different specific surface area (SSA) were obtained from Evonic with code name Aerosil 90, Aerosil 300 and less aggregated Aerosil Ox-50. Their physical properties, particle size, specific surface area and surface OH groups are listed in Table 1. 2.2. Thermal treatment of SiO2 NPs Thermal treatment of the SiO2 NPs was performed in an electrically heated furnace under air for 6 h. The furnace temperature was controlled digitally and stabilized within ±1 ◦ C. The heating protocols are specified in Table S1 of Supporting Information. After heating, the NPs were stored in air-tight vials and used within 6 h for catalytic experiments. 2.2.1. FT-IR characterization of thermally treated NPs FT-IR of silica nanoparticles were recorded on a Spectrum GX Perkin Elmer FT-IR System. The KBr pellets were prepared by mixing 2 mg on NPs with KBr. It is stressed that low-percentage of SiO2 :KBr has to be used in order to avoid distortion of certain bands, e.g. such as flattening of the Si–O–Si band at 1100 cm−1 . 2.3. Catalytic experiments Catalytic experiments were carried out under argon atmosphere in double-walled thermostated reaction vessels under stirring. The reaction temperature was kept stable at 80 ◦ C. The volume of evolved gasses was measured with an automatic gas burette. For a typical catalytic experiment 1.8 mg (5.3 ␮mol) of Fe(BF4 )2 ·6H2 O were dispersed in 5 ml propylene carbonate, followed by addition of 7.1 mg of P(CH2 CH2 PPh2 )3 to achieve [Fe:ligand] ratio of 1:2. Then, 1 ml of formic acid was added to this solution, as well as the appropriate amount of SiO2 nanoparticles. The gas production started immediately after addition of FA. Reaction products’

147

analysis was carried out with a gas chromatograph (Shimadzu GC2014) equipped with a Carboxen-1000 column and TC-detector. CO2 and H2 were the only reaction products. The detection limit for CO was 6ppb. It is noticed that in our system, we have carried out the experiments for a [ligand:Fe ratio] = [2:1]. Under these conditions, all Fe atoms are coordinated by the P(CH2 CH2 PPh2 )3 ligands, as shown previously by Beller and co-workers [11]. Accordingly, it is considered [11] that all Fe-P(CH2 CH2 PPh2 )3 complexes are catalytically active. 2.3.1. Arrhenius study 5.3 ␮mol Fe(BF4 )2 ·6H2 O, were dispersed in 5 ml propylene carbonate, followed by addition of 10.6 ␮mol of P(CH2 CH2 PPh2 )3 , then 1 ml of FA plus 10 mg of SiO2 NPs were added. The Reaction temperature adjusts from 60 to 80 ◦ C. 3. Results and discussion 3.1. Catalytic results The co-catalytic performance of SiO2 NPs was investigated by monitoring the total gas volume evolved from formic acid by the FeII /P(CH2 CH2 PPh2 )3 catalyst. Fig. 1A shows the volume generated at 80 ◦ C in the presence of 10 mg SiO2 NPs as co-catalyst. Reference data, with no SiO2 nanoparticle addition, is also presented for comparison, Fig. 1A. Accordingly, the gas volume generated by FeII /P(CH2 CH2 PPh2 )3 with no SiO2 added was V(H2 +CO2 ) = 1042 ml within 200 min corresponding to 83.4% conversion of FA vs. the max theoretical volume (see Fig. 1A). The remaining amount of FA was quantified by Ion Chromatography (Dionex). Addition of SiO2 NPs increases the gas production rate: for example, V(H2 +CO2 ) = 1100 ml were produced within 46 min after addition of 10 mg of A300 SiO2. This corresponds to 88% conversion of FA. Addition of 10 mg of A90 SiO2 NPs produced V(H2 +CO2 ) = 1081 ml in 31 min (86.5% conversion). The effect of Ox-50-SiO2 NPs was impressive: 10 mg of Ox-50-SiO2 NPs produced V(H2 +CO2 ) = 1017 ml, within only 17 min corresponding to 81.3% conversion. It is stressed that in all case, GC-TCD analysis of the produced gases showed that the evolved gas mixture consisted of H2 (50% ± 0.1%) and CO2 (50% ± 0.1%). No CO was detected in the gas mixture (CO detection limit of our GC-TCD/Carboxen-1000 column, was 6 ppb). Table 2 summarizes the catalytic data. The initial gas production rates, calculated from the linear slope of the initial part of gas evolution curves, are presented in Fig. 1B. The rate with no co-catalyst addition was 10 ml/min. 10 mg of A300, A90 or Ox-50 NPs SiO2 increased progressively the gas production rates to 27.6, 47.4 and 60.0 ml/min, respectively. Thus, SiO2 nanoparticles exert a significant co-catalytic effect on H2 production from HCOOH by the Fe-phosphine catalyst. Based on the present catalytic data, we have calculated the Turn Over Number (TON) and Turn Over Frequencies (TOF) using the following Eqs. (3a) and (3b), TON = TOF =

V(H2 +CO2 ) /(Vm

H2 ,25 ◦ C

nFe TON t

+ Vm

CO2 ,25 ◦ C )

(3a) (3b)

where Patm is the pressure (101,325 Pa), R the ideal gas constant (8.314 m3 Pa mol−1 K−1 ). T = 298 K is the temperature where the gas volume was measured. As shown, [11], under the conditions of [lignad:Fe] = [2:1], i.e. used herein, it is considered that 100% of the Fe atoms are complexed by the P(CH2 CH2 PPh2 )3 ligand and all complexes are catalytically active. Thus, nFe in (3a) is the used Fe moles. We underline that the catalytic experiments were run at 80 ◦ C = 353 K, however the gas-volume measurements were done

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80

Ox-50

70 60 50

A300

40 30

A90

20 10 0

(A)

No Co-catalyst addition 0

5

10

15

20

Amount of SiO2 NPs (mg)

25

Gas Prod. Rate (ml/min) /SSA

Fig. 1. (A) Volumes of gas evolution by the catalytic systems [FeII /P(CH2 CH2 PPh2 )3 /SiO2 NPs]. (B) Gas production rate and calculated TOFs. Reaction conditions: 5.3 ␮mol Fe(BF4 )2 ·6H2 O, 10.6 ␮mol phosphine-ligand in 5 ml propylene carbonate, 1 ml of FA plus 10 mg of SiO2 NPs were added. Reaction temperature 80 ◦ C. Dotted line: maximum theoretical volume produced from complete dehydrogenation of 1 ml of FA. Error bars were derived based on three repetitions of each experiment.

2&

500

Ox-50

450

(B)

400 350 300

A90

250 200 150

A300

100 50 0

0

5

10

15

20

Amount of SiO2 NPs (mg)

Fig. 2. (A) Gas production rate vs. amount of SiO2 NPs. (B) Gas production rate normalized per SSA for various amounts of SiO2 NPs Reaction Conditions as in Fig. 1.

at room temperature 298 K, thus the molar volume is 24.49 l/mol for H2 , and 24.42 l/mol for CO2 [11]. The calculated TOFs, see Fig. 1B and Table 2, for FeII /P(CH2 CH2 PPh2 )3 , FeII /P(CH2 CH2 PPh2 )3 /A300, II II Fe /P(CH2 CH2 PPh2 )3 /A90 and Fe /P(CH2 CH2 PPh2 )3 /Ox-50 are TOF = 1206, 5571, 8337 and 13,882 h−1 , respectively. Thus, the present data reveal that: [i] there is a significant enhancement of TOFs by the SiO2 NPs, [ii] the enhancement factor is correlated with the type of NPs. Since in all cases in Fig. 1, the same amount of NPs was used (10 mg), lower SSA NPs appear to have better effect. Ox-50-SiO2 with an SSA that is 15% of A300 achieves a remarkable 10-fold enhancement of TOFs. This finding reveals that the co-catalytic effect is controlled by surface morphology also, e.g. since Ox-50-SiO2 particles are spherical with low aggregation degree [17–19]. To further investigate the mechanism of this co-catalytic effect of SiO2 NPs, experiments with different amounts of co-catalyst were also performed. Fig. 2 shows the gas production rate vs. the amount of added SiO2 NPs. In all cases, the gas production rate increased gradually by addition of SiO2 NPs up to an amount of 15 mg and then it reached a plateau, due to saturation of the reaction solution with

NPs. Detailed catalytic data are provided in Tables S2–S4 of Supporting Information. Upon increasing the SiO2 amount, the gas-production rate increased from 10 ml/min for FeII /P(CH2 CH2 PPh2 )3 , to 66.6 ml/min for FeII /P(CH2 CH2 PPh2 )3 /A300 system by addition of 20 mg of particles. For FeII /P(CH2 CH2 PPh2 )3 /A90, addition of 20 mg NPs achieved a reaction rate of 71.0 ml/min. Even better, the addition of the Ox50-SiO2 NPs increased the gas production rate to a remarkable rate 74.3 ml/min. When normalized per SSA, the gas production rates, Fig. 2B, clearly reveal the superiority of lower SSA (larger particle size) nanoparticles. This intriguing finding is nicely exemplified by comparing the maximum [rate/SSA] for A300-SiO2 vs. Ox-50SiO2 in Fig. 2B that gives a 7-fold higher [rate/SSA] achieved by Ox-50-SiO2 vs. A300-SiO2 . 3.1.1. Arrhenius study To better understand the physicochemical mechanism of the NPs in conjunction with the fact that FA dehydrogenation is a thermally activated process [11], we have carried out a temperature-dependent analysis of the catalytic reaction, in the presence of SiO2 NPs. Fig. 3 shows the Arrhenius plot derived using data from Table 3. The solid lines are best-linear-fits of the relation

Table 2 H2 + CO2 production by FeII /P(CH2 CH2 PPh2 )3 in the presence of 10 mg of SiO2 NPs as co-catalyst (T = 80 ◦ C). Co-catalyst

V(H2 +CO2 ) (ml)

Reaction time (min)

Gas flow rate (ml/min)

TON

TOF (h−1 )

– A300 A90 Ox-50

1042 1100 1081 1017

200.0 45.7 30.8 17.0

10.0 27.6 47.4 60.0

4019 4271 4168 3923

1206 5571 8377 13,882

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Fig. 3. (A) Arrhenius plot for H2 production reactions by () FeII /P(CH2 CH2 PPh2 )3 /A300, ( ) FeII /P(CH2 CH2 PPh2 )3 /A90 and ( ) FeII /P(CH2 CH2 PPh2 )3 /Ox-50. (B) Activation energies vs. particle SSA. The activation energy for the homogeneous system without addition of co-catalyst also is also presented for comparison.

ln(TOF) = (−Ea /R) × (1/T) + C which allows an estimate of the activation energy Ea . The linearity of the Arrhenius plots indicates that a single rate limiting step exists for this catalytic reaction in the presence of the NPs, as in the absence of the NPs [11]. According to Fig. 3, the derived activation energies are 61 ± 3, 54 ± 3 and 41 ± 3 kJ/mol for the catalytic systems FeII /P(CH2 CH2 PPh2 )3 /A300, FeII /P(CH2 CH2 PPh2 )3 /A90 and FeII /P(CH2 CH2 PPh2 )3 /Ox-50, respectively, see Table 3. Comparing these Ea values with that for the FeII /P(CH2 CH2 PPh2 )3 , i.e. without co-catalyst Ea = 77 kJ/mol [11], we see that all NPs lower the activation energy by at least 16 kJ/mol

(that is 20.7% reduction for A300-SiO2 ) up to a Ea = −36 kJ/mol (i.e., 46.75% reduction) achieved by Ox-50-SiO2. Overall, the present data reveal that: [i] both the amount and type of SiO2 NPs is important for the enhancement of H2 production. [ii] SSA alone is not the determining factor that could explain the observed superiority of the larger particles, i.e. Ox-50. Thus, we conclude that molecular surface interactions should differentiate between the three SiO2 NPs studied herein. As shown previously, the dynamics of these SiO2 NPs in solution can be severely influenced by the mobility, diffusion, aggregation and particle–particle

Fig. 4. Co-catalytic activity of thermally treated SiO2 NPs: (A) A300-SiO2 , (B) A90-SiO2 and (C) Ox-50-SiO2 . (D) Gas production rate vs. temperature of NPs treatment under addition of 10 mg of NPs. Reaction conditions: 5.3 ␮mol Fe(BF4 )2 ·6H2 O, 10.6 ␮mol phosphine-ligand in 5 ml propylene carbonate, 1 ml of FA, then the appropriate amount of SiO2 NPs were added. Reaction temperature kept constant at 80 ◦ C.

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Fig. 5. FTIR spectra of the SiO2 NPs, before (black lines) and after (colored lines) thermal treatments under air. (A) Ox-50-SiO2 , (B) A90-SiO2 and (C) A300-SiO2 . In all figures (a) untreated NPs, (b) heated at 120 ◦ C for 6 h, (c) heated at 200 ◦ C for 6 h, (d) heated at 300 ◦ C for 6 h, (e) heated at 600 ◦ C for 6 h and (f) heated at 900 ◦ C for 6 h.

interactions themselves [19]. In this context, the differences in cocatalytic activity between the three types of NPS can be attributed to the availability/accessibility of particle-surface that interacts with FA. This seems to be easier for the less-aggregated particle of Ox-50 than the more aggregated particles of A90 and A300 Table 3 Catalytic results vs. reaction temperature. Reaction temperature (◦ C)

TOF (h−1 )

Activation energy, Ea (kJ/mol)

FeII /P(CH2 CH2 PPh2 )3 /A300 60 ◦ C 80 ◦ C 100 ◦ C

2070 5571 19,930

61 ± 3

FeII /P(CH2 CH2 PPh2 )3 /A90 60 ◦ C 80 ◦ C 100 ◦ C

2921 8377 20,952

54 ± 3

FeII /P(CH2 CH2 PPh2 )3 /Ox-50 60 ◦ C 80 ◦ C 100 ◦ C

5400 13,882 24,195

41 ± 3

[18,19]. An analogous trend was documented by detailed Dynamic Light Scattering data for the hydrogen-atom-transfer activity of gallic acid-functionalized Ox-50, A90 and A300 SiO2 NPs [19]. To further probe the role of SiO2 surface groups, e.g. chemisorbed H2 O, concentration of Si–OH and Si–O–Si [20–22], we have used a simple thermal treatment protocol that is known to modify progressively the amount of chemisorbed H2 O, concentration of Si–OH and Si–O–Si [20–22].

3.2. Thermal treatment of SiO2 NPs 3.2.1. Co-catalytic effect of thermal treated NPs Fig. 4 presents the effect of thermal treatment on co-catalytic efficiency of NPs. Fig. 4A–C demonstrates that the gas production rate was increased after thermal treatment of the NPs, up to T = 600 ◦ C. This is better viewed in Fig. 4D: thermal treatment, up to 200 ◦ C, increased the co-catalytic activity of all SiO2 NPs. 10 mg of untreated A300-SiO2 NPs achieved a reaction rate 28 ml/min, while treatment at 200 ◦ C increased the gas production rate to 66 ml/min. This is attributed to H2 O removal [20–22], as we show in the following by FT-IR data. Further heating of A300-SiO2 at 900 ◦ C did not increase the rate, but had an inhibitory effect, e.g. rate decreased to

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52 ml/min. This is attributed to partial aggregation of these small NPs after heating at 900 ◦ C [18]. Importantly, thermal treatment was found to further boost the co-catalytic action of Ox-50-SiO2 NPs: when heated at 600 or 900 ◦ C this nanomaterial prompted gas production rate >150 and 130 ml/min, respectively, vs. 60 ml/min of the untreated Ox50-SiO2 . The data in Fig. 4 shows that thermally induced surface modifications of the nanoparticles exert a decisive effect on their co-catalytic efficiency that is mostly beneficial. 3.2.2. FTIR characterization of thermally treated NPs Representative FTIR spectra for the SiO2 NPs, after thermaltreatments are displayed in Fig. 5A–C. According to the literature [20,21], the sinanol groups on the SiO2 surface can be: [i] isolated (free) Si–OH, [ii] geminal sinanols Si(OH)2 or [iii] bridged through hydrogen bonds [20]. The bending vibration of Si–OH group is at 972 cm−1 and the stretching vibration at 812 cm−1 . The stretching vibration of the Si–O bond is detected at 625 cm−1 [20]. Siloxane bridges (Si–O–Si) present on the SiO2 surface, are detected by the intense characteristic peak at 1110 cm−1 [21]. Finally, surface absorbed water, is identified by the bands at 3365 and 1633 cm−1 [20,21]. Although FT-IR spectroscopy cannot provide a quantitative analysis of the surface groups, a comparative survey of the data in Fig. 5A–C provides some useful information: upon heating the 3365 and 1633 cm−1 bands of H2 O decreased rapidly and disappeared at higher temperatures. The bending vibration of Si-OH group at 972 cm−1 is also sensitive to thermal treatment. However, this band is not eliminated even at 900 ◦ C indicating the presence of bulk Si–OH groups that are not affected by this thermal treatment.

Fig. 6. Left Y-axis: gas production rate vs. heating temperature of NPs treatment () data upon addition 10 mg of OX-50 NPs. Right Y-axis: % formation of the surface species Si–OH ( ), Si–O–Si ( ) vs. thermal treatment temperature (data from Refs. [21], [22]).

3.3. A physicochemical mechanism During last decade, a variety of techniques -such as TGA and 27 Si NMR- have been applied for the quantification of the silanol concentration [22] on SiO2 . Based upon more than 100 samples, with SSA from 5 to 1000 m2 /g, Zhuravlev found that the average silanol number of fully hydroxylated silica is 4.6 ± 0.5 OH/nm2 [22–24]. This number is claimed to be independent of the origin and structural characteristics of the sample. However, detailed study of the Aerosil samples used herein shows an average value of 2–3 OH/nm2 [18]. According to previous works [20–24], there is a gradual loss of surface Si–OH groups at the temperatures used herein and a progressive increase of the Si–O–Si bridges. Here, assuming an initial concentration of 3 OH/nm2 [18], their surface concentration has been calculated based on the SSA, see Table 1. Then, the thermal evolution of the surface concentration of the Si–OH and Si–O–Si was modeled using the speciation vs. temperature as provided by Zhuravlev [20–24]. In brief, this OH-speciation profile vs. T, entails that thermal treatments of SiO2 NPs can induce two types of surface effects: [i] H2 O desorption, [ii] structural modification of the Si–OH groups and formation of Si–O–Si bridges [20–24]. It is well known that thermal treatment causes structural modification of the SiO2 NPs by gradually modification of water content absorbed on particle surface [16–18]. Here it is important to distinguish between the two main processes – dehydration/dehydroxylation – taken place during the thermal treatment of NPs. Dehydration is the loss of physisorbed water while dehydroxylation stands for the condensation of hydroxyl groups to form siloxane bridges. It is generally agreed that heating of SiO2 at 120 ◦ C removes most of physisorbed water [20]. Then, thermal treatment of SiO2 particles up to 900 ◦ C, involves three main stages (I II, III) as exemplified by Zhuravlev [22–24]: (I) dehydration occurs during stage I (25–190 ◦ C), where the SiO2 surface shows the maximum OH-concentration due to gradual removal of physically adsorbed water [22]. During stage II (190–400 ◦ C), the total

number of silanol groups decreases gradually [21]. Finally, during stage III (400–900 ◦ C), the overall hydroxylation degree continuously decreases with concomitant increment of the concentration of siloxane-bridges [22–24]. In this context, in Fig. 6, we present the gas production rate in the presence of 10 mg of thermal treated Ox-50 NPs vs. temperature of treatment. In the same graph, the percentage of surface silanol- and siloxane-groups vs. temperature is presented based on Zhuravlev’s model [22,23]. According to Fig. 6, heating of Ox-50 NPs at 200 ◦ C increase the gas production rate from 60 and 96 ml/min respectively; heating of Ox-50 NPs at 200 ◦ C removes most of adsorbed H2 O and decrease the SiOH-groups by 15%. Further heating of the Ox-50-SiO2 at 300 ◦ C decreases further the SiOH-groups by 20% and increases the Si–O–Si up to 25%. Treatment of NPs at higher temperatures, i.e. 600 ◦ C, further affects the surface groups: 39% of the Si–OH remain on the surface, accompanied by formation of more Si–O–Si bridges. These Ox-50 NPs treated at 600 ◦ C increase further the H2 production rate to 155 ml/min. Finally at 900 ◦ C, 98% of siloxane-bridges have been formed. The co-catalytic activity of 900 ◦ C heated NPs shows reaction rate 130 ml/min. Thus the present catalytic and thermal treatment data as encoded in Fig. 6, demonstrate a correlation between the co-catalytic efficiency of the thermally treated SiO2 NPs and the surfacial H2 O/Si–OH/Si–O–Si: [i] surface-adsorbed H2 O retards co-catalytic efficiency, thus thermal treatment eliminates this retarding effect of H2 O and [ii] there is a beneficial effect of surface Si–O–Si bridges. Thus, H2 O should be desorbed and surfacial Si–O–Si bridges should be maximized in order to optimize the co-catalytic efficiency of SiO2 NPs. We suggest that the surface Si–O–Si bridges are responsible for the co-catalytic activity of SiO2 NPs according to the reaction Scheme 1: first, Si–O–Si bridges promote FA deprotonation towards HCOO− anion formation. Then, the next catalytic step involves HCOO− coordination on FeII atom of active catalyst following by its decomposition to H2 and CO2 .

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deprotonation towards HCOO− anion formation that coordinates on FeII atom of active catalyst during catalysis. Surface Si–O–Si formation can be maximized by thermal treatment at high temperatures up to 400 ◦ C. The present findings show that unprecedented highly efficient H2 production from FA can be achieved by a low-cost catalytic system consisting of FeII /P(CH2 CH2 PPh2 )3 plus thermally treated Ox-50-SiO2 NPs. Acknowledgments This research has been co-financed by the European Union (European Social Fund—ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF)-Research Funding Program: THALIS. Investing in knowledge society through the European Social Fund. Scheme 1. Schematic representation of co-catalytic mechanism of SiO2 NPs.

In this model, Ox-50-SiO2 nanoparticles show superior performance due to their morphology and agglomeration state [17,18]. It is well known that Ox-50-SiO2 are spherical particles with lowaggregation degree [18] thus their surface is highly available for surface reactions [18,19]. This explains the present – counterintuitive at first glance – finding. Co-catalytic effect : Ox-50 > A90 > A300 Previous TEM and BET data show that our higher SSA Aerosil SiO2 such as A300 are much more aggregated than Ox-50 [18,19] thus, despite the higher number of Si-O-Si surface groups, they are not accessible by HCOOH and this is the origin of their inferior cocatalytic activity towards H2 production by HCOOH. 4. Conclusion Here, we reveal an unprecedented co-catalytic activity of SiO2 nanoparticles on H2 -production from FA, catalyzed by homogeneous catalyst FeII /P(CH2 CH2 PPh2 )3 . The ‘net’ homogeneous system for this reaction provided TOF = 1205 h−1 at 80 ◦ C, while addition of 10 mg of SiO2 nano-particles accelerated gas production showing TOF = 13,882 h−1 in the case of the less aggregated Ox-50-SiO2 NPs. The co-catalytic efficiency is modulated by the surfacial H2 O, Si–OH and Si–O–Si. Surfacially adsorbed H2 O retards co-catalytic efficiency. This effect can be easily overcommed by desorbing H2 O by heating of the NPs. In addition, the co-catalytic activity of SiO2 NPs is benefiting from the surface Si–O–Si bridges. They promote FA

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod. 2014.07.012. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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