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Catalysis Today 242 (2015) 146–152

Contents lists available at ScienceDirect

Catalysis Today

j o ur na l ho me page: www.elsev ier .com/ locate /ca t tod

o-catalytic enhancement of H2 production by SiO2 nanoparticles

anagiota Stathia, Yiannis Deligiannakisb, Maria Louloudia,∗

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

r t i c l e i n f o

rticle history:eceived 4 March 2014eceived in revised form 3 July 2014ccepted 16 July 2014vailable online 20 August 2014

a b s t r a c t

SiO2 nanoparticles, of varying size 9–55 nm, show significant co-catalytic activity for H2-production fromHCOOH by the homogenous FeII/P(CH2CH2PPh2)3 catalyst. Particle size and specific surface area are shownto play key-role in this phenomenon. Larger nanoparticles have better co-catalytic effect. Arrhenius anal-ysis reveals that the significant co-catalytic effect can is attributed to a lowering of the activation energy ofthe rate-limiting step by �Ea = 16–36 kJ/mol. This phenomenon is attributed to a thermodynamic promo-

−

eywords:ydrogenormicronhosphineilicahermal treatment

tion of HCOOH deprotonation by the SiO2 nanoparticles, accelerating in this way coordination of HCOOon the FeII atom of active catalyst, during catalysis. The surface Si–O–Si bridges are shown to be responsi-ble for the promotion of this HCOOH deprotonation, while surface-adsorbed H2O retards the co-catalyticefficiency.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Catalytic H2 production is currently receiving considerablettention, reflecting the thirst for sustainable renewable energy.ormic acid (FA) is considered one of the most promising liquidources for H2 production or as H2-storage material [1]. FA can beatalytically decomposed to H2 and CO2 via dehydrogenation [2–4].

COOH (l) → H2 (g) + CO2 (g) �G = −32.9 kJ/mol (1)

However, CO, which is a fatal poison for catalysts and fuel cells,an be also generated by dehydration of FA [4].

COOH (l) → CO (g) + H2O �G = −12.4 kJ/mol (2)

Currently, among the challenges in catalytic H2 production byA are: [i] the type of catalyst, i.e. metal complex vs. nanoparticle,ii] temperature of reaction, i.e. high temperatures typically >300 ◦Cs. near-ambient temperatures T < 100 ◦C, [iii] nature of metal, i.e.oble-metal vs. low-cost transition metals [1,2].

Noble-metal based homogeneous catalysts [2] have been inten-ively studied for H2 generation from FA. A binuclear Ru-phosphine

omplex [Ru2(�-CO)(CO)4(�-dppm)2], presented by Puddephatt al. in 1998, was the most active complex for this reaction,t 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: mlouloud@uoi.gr (M. Louloudi).

ttp://dx.doi.org/10.1016/j.cattod.2014.07.012920-5861/© 2014 Elsevier B.V. All rights reserved.

Thus, Ru-phosphine catalysts have been shown to be efficientin H2 generation from FA, at near-ambient temperatures [6,7].Other noble-metals, i.e. Rh, Ir, have also been shown to givepromising results [5,8–10], however, their use poses cost-efficiencyand environmental-acceptance issues. To this front, the use ofFe-based catalysts has been demonstrated by Beller and co-workers as efficient/simple and low-cost system, e.g. exemplifiedby a FeII-tris[(2-diphenylphosphino)ethyl] phosphine complex,[FeIIP(CH2CH2PPh2)3], which affords TOF = 9425 h−1 at 80 ◦C inpropylene carbonate solvent [11].

From the mechanistic point of view, it has been proposed thata key-catalytic step for H2-production by FA is the coordination ofone deprotonated HCOO− molecule on the metal centre [2–4,6].This has been confirmed also by our recent studies using advancedPulsed-EPR spectroscopy [12]. The involvement of the formateanion has also been postulated for gas phase FA decomposition toCO2 and H2 by metallic and metal oxide particles [13–15]. This gas-phase reaction was proposed to involve chemisorption of FA onsurface acid sites [13–15]. Theoretical studies show that the protonmigration from FA on the surface is the rate limiting step of theH2-generating reaction in the gas-phase [15].

More recently, we have discovered that H2-production fromFA catalyzed by homogeneous catalyst FeII/P(CH2CH2PPh2)3 can

be improved by basic functional groups grafted on SiO2 particles[16]. The use of SiO2–NH2 and SiO2-imidazole as co-catalysts, gaveremarkably 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 Toda

Table 1Physical properties of SiO2 nanoparticles.

Sample SSA (m2/g)a Particlediameter(nm)a

[OH] (mmol/g)b

Ox-50 50 ± 15 55 0.28A90 90 ± 15 30 0.51A300 300 ± 30 9 1.74

a Detailed characterization of size and SSA of the Aerosil NPs is presented inr

r

ft

cccivtmo

2

2

Fdphf9pa

2

twphf

2

PiSe

2

ireawoTtt

eference [17].b Detailed characterization of surface OH groups of Aerosil NPs is presented in

eference [18].

unctionalities on FA deprotonation, facilitating HCOO− coordina-ion on the FeII atom of catalyst [16].

Herein, we show that nanosized SiO2 particles have a significanto-catalytic effect on H2-production from FA by the homogeneousatalytic system FeII/P(CH2CH2PPh2)3. A systematic study has beenarried out using well characterized SiO2 nanoparticles of vary-ng size or specific surface area (SSA), which underwent througharious protocols of controlled thermal treatments that variedheir surface groups. This systematic study, together with ther-

odynamic data, allows a physicochemical understanding of thebserved co-catalytic phenomena.

. Materials and methods

.1. Chemicals and SiO2 NPs

All solvents were of commercial grade, purchased from Aldrich.ormic Acid (97.5% with 2.5% H2O [v/v]), Fe(BF4)2 and tris-2-iphenylphosphino-ethyl-phosphine, P(CH2CH2PPh2)3, have beenurchased from Aldrich and stored under argon. Commercialydrophilic fumed silica nanoparticles with different specific sur-

ace area (SSA) were obtained from Evonic with code name Aerosil0, Aerosil 300 and less aggregated Aerosil Ox-50. Their physicalroperties, particle size, specific surface area and surface OH groupsre listed in Table 1.

.2. Thermal treatment of SiO2 NPs

Thermal treatment of the SiO2 NPs was performed in an elec-rically heated furnace under air for 6 h. The furnace temperatureas controlled digitally and stabilized within ±1 ◦C. The heatingrotocols are specified in Table S1 of Supporting Information. Aftereating, the NPs were stored in air-tight vials and used within 6 h

or catalytic experiments.

.2.1. FT-IR characterization of thermally treated NPsFT-IR of silica nanoparticles were recorded on a Spectrum GX

erkin Elmer FT-IR System. The KBr pellets were prepared by mix-ng 2 mg on NPs with KBr. It is stressed that low-percentage ofiO2:KBr has to be used in order to avoid distortion of certain bands,.g. such as flattening of the Si–O–Si band at 1100 cm−1.

.3. Catalytic experiments

Catalytic experiments were carried out under argon atmospheren double-walled thermostated reaction vessels under stirring. Theeaction temperature was kept stable at 80 ◦C. The volume ofvolved gasses was measured with an automatic gas burette. For

typical catalytic experiment 1.8 mg (5.3 �mol) of Fe(BF4)2·6H2Oere dispersed in 5 ml propylene carbonate, followed by addition

f 7.1 mg of P(CH2CH2PPh2)3 to achieve [Fe:ligand] ratio of 1:2.hen, 1 ml of formic acid was added to this solution, as well ashe appropriate amount of SiO2 nanoparticles. The gas produc-ion started immediately after addition of FA. Reaction products’

y 242 (2015) 146–152 147

analysis was carried out with a gas chromatograph (Shimadzu GC-2014) equipped with a Carboxen-1000 column and TC-detector.CO2 and H2 were the only reaction products. The detection limitfor CO was 6ppb. It is noticed that in our system, we have car-ried out the experiments for a [ligand:Fe ratio] = [2:1]. Under theseconditions, all Fe atoms are coordinated by the P(CH2CH2PPh2)3ligands, as shown previously by Beller and co-workers [11]. Accord-ingly, it is considered [11] that all Fe-P(CH2CH2PPh2)3 complexesare catalytically active.

2.3.1. Arrhenius study5.3 �mol Fe(BF4)2·6H2O, were dispersed in 5 ml propylene car-

bonate, followed by addition of 10.6 �mol of P(CH2CH2PPh2)3, then1 ml of FA plus 10 mg of SiO2 NPs were added. The Reaction tem-perature adjusts from 60 to 80 ◦C.

3. Results and discussion

3.1. Catalytic results

The co-catalytic performance of SiO2 NPs was investigated bymonitoring the total gas volume evolved from formic acid by theFeII/P(CH2CH2PPh2)3 catalyst. Fig. 1A shows the volume generatedat 80 ◦C in the presence of 10 mg SiO2 NPs as co-catalyst. Refer-ence data, with no SiO2 nanoparticle addition, is also presented forcomparison, Fig. 1A.

Accordingly, the gas volume generated by FeII/P(CH2CH2PPh2)3with no SiO2 added was V(H2+CO2) = 1042 ml within 200 min corre-sponding to 83.4% conversion of FA vs. the max theoretical volume(see Fig. 1A). The remaining amount of FA was quantified by IonChromatography (Dionex). Addition of SiO2 NPs increases the gasproduction rate: for example, V(H2+CO2) = 1100 ml were producedwithin 46 min after addition of 10 mg of A300 SiO2. This corre-sponds to 88% conversion of FA. Addition of 10 mg of A90 SiO2 NPsproduced V(H2+CO2) = 1081 ml in 31 min (86.5% conversion). Theeffect of Ox-50-SiO2 NPs was impressive: 10 mg of Ox-50-SiO2 NPsproduced V(H2+CO2) = 1017 ml, within only 17 min correspondingto 81.3% conversion. It is stressed that in all case, GC-TCD analysis ofthe produced gases showed that the evolved gas mixture consistedof H2 (50% ± 0.1%) and CO2 (50% ± 0.1%). No CO was detected inthe gas mixture (CO detection limit of our GC-TCD/Carboxen-1000column, was 6 ppb). Table 2 summarizes the catalytic data.

The initial gas production rates, calculated from the linear slopeof 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 ofA300, A90 or Ox-50 NPs SiO2 increased progressively the gas pro-duction rates to 27.6, 47.4 and 60.0 ml/min, respectively. Thus,SiO2 nanoparticles exert a significant co-catalytic effect on H2 pro-duction from HCOOH by the Fe-phosphine catalyst. Based on thepresent catalytic data, we have calculated the Turn Over Number(TON) and Turn Over Frequencies (TOF) using the following Eqs.(3a) and (3b),

TON = V(H2+CO2)/(Vm H2,25 ◦C + Vm CO2,25 ◦C)

nFe(3a)

TOF = TONt

(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 thegas volume was measured. As shown, [11], under the conditionsof [lignad:Fe] = [2:1], i.e. used herein, it is considered that 100% of

the Fe atoms are complexed by the P(CH2CH2PPh2)3 ligand and allcomplexes are catalytically active. Thus, nFe in (3a) is the used Femoles. We underline that the catalytic experiments were run at80 ◦C = 353 K, however the gas-volume measurements were done

148 P. Stathi et al. / Catalysis Today 242 (2015) 146–152

Fig. 1. (A) Volumes of gas evolution by the catalytic systems [FeII/P(CH2CH2PPh2)3/SiO2 NPs]. (B) Gas production rate and calculated TOFs. Reaction conditions: 5.3 �molFe(BF4)2·6H2O, 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: maximumtheoretical volume produced from complete dehydrogenation of 1 ml of FA. Error bars were derived based on three repetitions of each experiment.

2&0 5 10 15 20

050

100150200250300350400450500

A90

A300

Ox-50

Gas

Pro

d. R

ate

(ml/m

in) /

SSA

Amoun t of SiO2 NPs (mg)

(B)

0 5 10 15 20 250

10

20

30

40

50

60

70

80

Amoun t of SiO2 NPs (mg)

No Co-ca talyst add ition

A30 0

A90

Ox-50

(A)

norm

af

fFTpTwoerted

ow

Sai

TH

Fig. 2. (A) Gas production rate vs. amount of SiO2 NPs. (B) Gas production rate

t room temperature 298 K, thus the molar volume is 24.49 l/molor H2, and 24.42 l/mol for CO2 [11].

The calculated TOFs, see Fig. 1B and Table 2,or FeII/P(CH2CH2PPh2)3, FeII/P(CH2CH2PPh2)3/A300,eII/P(CH2CH2PPh2)3/A90 and FeII/P(CH2CH2PPh2)3/Ox-50 areOF = 1206, 5571, 8337 and 13,882 h−1, respectively. Thus, theresent data reveal that: [i] there is a significant enhancement ofOFs by the SiO2 NPs, [ii] the enhancement factor is correlatedith the type of NPs. Since in all cases in Fig. 1, the same amount

f NPs was used (10 mg), lower SSA NPs appear to have betterffect. Ox-50-SiO2 with an SSA that is 15% of A300 achieves aemarkable 10-fold enhancement of TOFs. This finding reveals thathe co-catalytic effect is controlled by surface morphology also,.g. since Ox-50-SiO2 particles are spherical with low aggregationegree [17–19].

To further investigate the mechanism of this co-catalytic effectf SiO2 NPs, experiments with different amounts of co-catalystere also performed.

Fig. 2 shows the gas production rate vs. the amount of addediO2 NPs. In all cases, the gas production rate increased gradu-lly by addition of SiO2 NPs up to an amount of 15 mg and thent reached a plateau, due to saturation of the reaction solution with

able 22 + CO2 production by FeII/P(CH2CH2PPh2)3 in the presence of 10 mg of SiO2 NPs as co-c

Co-catalyst V(H2+CO2) (ml) Reaction time (min)

– 1042 200.0

A300 1100 45.7

A90 1081 30.8

Ox-50 1017 17.0

alized per SSA for various amounts of SiO2 NPs Reaction Conditions as in Fig. 1.

NPs. Detailed catalytic data are provided in Tables S2–S4 of Sup-porting Information.

Upon increasing the SiO2 amount, the gas-production rateincreased from 10 ml/min for FeII/P(CH2CH2PPh2)3, to 66.6 ml/minfor FeII/P(CH2CH2PPh2)3/A300 system by addition of 20 mg of parti-cles. For FeII/P(CH2CH2PPh2)3/A90, addition of 20 mg NPs achieveda reaction rate of 71.0 ml/min. Even better, the addition of the Ox-50-SiO2 NPs increased the gas production rate to a remarkable rate74.3 ml/min. When normalized per SSA, the gas production rates,Fig. 2B, clearly reveal the superiority of lower SSA (larger parti-cle size) nanoparticles. This intriguing finding is nicely exemplifiedby comparing the maximum [rate/SSA] for A300-SiO2 vs. Ox-50-SiO2 in Fig. 2B that gives a 7-fold higher [rate/SSA] achieved byOx-50-SiO2 vs. A300-SiO2.

3.1.1. Arrhenius studyTo 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 atemperature-dependent analysis of the catalytic reaction, in thepresence of SiO2 NPs. Fig. 3 shows the Arrhenius plot derived usingdata from Table 3. The solid lines are best-linear-fits of the relation

atalyst (T = 80 ◦C).

Gas flow rate (ml/min) TON TOF (h−1)

10.0 4019 120627.6 4271 557147.4 4168 837760.0 3923 13,882

P. Stathi et al. / Catalysis Today 242 (2015) 146–152 149

F A300,e ut add

lvapt4FtFw

FaS

ig. 3. (A) Arrhenius plot for H2 production reactions by (�) FeII/P(CH2CH2PPh2)3/nergies vs. particle SSA. The activation energy for the homogeneous system witho

n(TOF) = (−Ea/R) × (1/T) + C which allows an estimate of the acti-ation energy Ea. The linearity of the Arrhenius plots indicates that

single rate limiting step exists for this catalytic reaction in theresence of the NPs, as in the absence of the NPs [11]. Accordingo Fig. 3, the derived activation energies are 61 ± 3, 54 ± 3 and1 ± 3 kJ/mol for the catalytic systems FeII/P(CH2CH2PPh2)3/A300,

eII/P(CH2CH2PPh2)3/A90 and FeII/P(CH2CH2PPh2)3/Ox-50, respec-ively, see Table 3. Comparing these Ea values with that for theeII/P(CH2CH2PPh2)3, i.e. without co-catalyst Ea = 77 kJ/mol [11],e see that all NPs lower the activation energy by at least 16 kJ/mol

ig. 4. Co-catalytic activity of thermally treated SiO2 NPs: (A) A300-SiO2, (B) A90-SiO2 anddition of 10 mg of NPs. Reaction conditions: 5.3 �mol Fe(BF4)2·6H2O, 10.6 �mol phosphiO2 NPs were added. Reaction temperature kept constant at 80 ◦C.

( ) FeII/P(CH2CH2PPh2)3/A90 and ( ) FeII/P(CH2CH2PPh2)3/Ox-50. (B) Activationition of co-catalyst also is also presented for comparison.

(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 andtype of SiO2 NPs is important for the enhancement of H2 produc-tion. [ii] SSA alone is not the determining factor that could explainthe observed superiority of the larger particles, i.e. Ox-50. Thus,

we conclude that molecular surface interactions should differentiatebetween the three SiO2 NPs studied herein. As shown previously,the dynamics of these SiO2 NPs in solution can be severely influ-enced by the mobility, diffusion, aggregation and particle–particle

d (C) Ox-50-SiO2. (D) Gas production rate vs. temperature of NPs treatment underine-ligand in 5 ml propylene carbonate, 1 ml of FA, then the appropriate amount of

150 P. Stathi et al. / Catalysis Today 242 (2015) 146–152

F rmal

( at 30

ictwc

TC

ig. 5. FTIR spectra of the SiO2 NPs, before (black lines) and after (colored lines) thea) untreated NPs, (b) heated at 120 ◦C for 6 h, (c) heated at 200 ◦C for 6 h, (d) heated

nteractions themselves [19]. In this context, the differences in co-

atalytic activity between the three types of NPS can be attributedo the availability/accessibility of particle-surface that interactsith FA. This seems to be easier for the less-aggregated parti-

le of Ox-50 than the more aggregated particles of A90 and A300

able 3atalytic results vs. reaction temperature.

Reaction temperature (◦C) TOF (h−1) Activationenergy, Ea

(kJ/mol)

FeII/P(CH2CH2PPh2)3/A30060 ◦C 2070

61 ± 380 ◦C 5571100 ◦C 19,930

FeII/P(CH2CH2PPh2)3/A9060 ◦C 2921

54 ± 380 ◦C 8377100 ◦C 20,952

FeII/P(CH2CH2PPh2)3/Ox-5060 ◦C 5400

41 ± 380 ◦C 13,882100 ◦C 24,195

treatments under air. (A) Ox-50-SiO2, (B) A90-SiO2 and (C) A300-SiO2. In all figures0 ◦C for 6 h, (e) heated at 600 ◦C for 6 h and (f) heated at 900 ◦C for 6 h.

[18,19]. An analogous trend was documented by detailed DynamicLight Scattering data for the hydrogen-atom-transfer activity ofgallic acid-functionalized Ox-50, A90 and A300 SiO2 NPs [19]. Tofurther probe the role of SiO2 surface groups, e.g. chemisorbedH2O, concentration of Si–OH and Si–O–Si [20–22], we have useda simple thermal treatment protocol that is known to modify pro-gressively the amount of chemisorbed H2O, concentration of Si–OHand Si–O–Si [20–22].

3.2. Thermal treatment of SiO2 NPs

3.2.1. Co-catalytic effect of thermal treated NPsFig. 4 presents the effect of thermal treatment on co-catalytic

efficiency of NPs. Fig. 4A–C demonstrates that the gas produc-tion rate was increased after thermal treatment of the NPs, up toT = 600 ◦C. This is better viewed in Fig. 4D: thermal treatment, up to200 ◦C, increased the co-catalytic activity of all SiO2 NPs. 10 mg ofuntreated 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 H2O removal [20–22], as we show in the fol-lowing by FT-IR data. Further heating of A300-SiO2 at 900 ◦C did notincrease the rate, but had an inhibitory effect, e.g. rate decreased to

s Today 242 (2015) 146–152 151

5N

toa5mc

3

t[(h9vata[

yFaa9iS

3

NcSnTtA[sgcbewZtegtNctd

ysrm(oti

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

P. Stathi et al. / Catalysi

2 ml/min. This is attributed to partial aggregation of these smallPs after heating at 900 ◦C [18].

Importantly, thermal treatment was found to further boosthe co-catalytic action of Ox-50-SiO2 NPs: when heated at 600r 900 ◦C this nanomaterial prompted gas production rate >150nd 130 ml/min, respectively, vs. 60 ml/min of the untreated Ox-0-SiO2. The data in Fig. 4 shows that thermally induced surfaceodifications of the nanoparticles exert a decisive effect on their

o-catalytic efficiency that is mostly beneficial.

.2.2. FTIR characterization of thermally treated NPsRepresentative FTIR spectra for the SiO2 NPs, after thermal-

reatments are displayed in Fig. 5A–C. According to the literature20,21], the sinanol groups on the SiO2 surface can be: [i] isolatedfree) Si–OH, [ii] geminal sinanols Si(OH)2 or [iii] bridged throughydrogen bonds [20]. The bending vibration of Si–OH group is at72 cm−1 and the stretching vibration at 812 cm−1. The stretchingibration of the Si–O bond is detected at 625 cm−1 [20]. Silox-ne bridges (Si–O–Si) present on the SiO2 surface, are detected byhe intense characteristic peak at 1110 cm−1 [21]. Finally, surfacebsorbed water, is identified by the bands at 3365 and 1633 cm−1

20,21].Although FT-IR spectroscopy cannot provide a quantitative anal-

sis of the surface groups, a comparative survey of the data inig. 5A–C provides some useful information: upon heating the 3365nd 1633 cm−1 bands of H2O decreased rapidly and disappearedt higher temperatures. The bending vibration of Si-OH group at72 cm−1 is also sensitive to thermal treatment. However, this band

s not eliminated even at 900 ◦C indicating the presence of bulki–OH groups that are not affected by this thermal treatment.

.3. A physicochemical mechanism

During last decade, a variety of techniques -such as TGA and 27SiMR- have been applied for the quantification of the silanol con-entration [22] on SiO2. Based upon more than 100 samples, withSA from 5 to 1000 m2/g, Zhuravlev found that the average silanolumber of fully hydroxylated silica is 4.6 ± 0.5 OH/nm2 [22–24].his number is claimed to be independent of the origin and struc-ural characteristics of the sample. However, detailed study of theerosil samples used herein shows an average value of 2–3 OH/nm2

18]. According to previous works [20–24], there is a gradual loss ofurface Si–OH groups at the temperatures used herein and a pro-ressive increase of the Si–O–Si bridges. Here, assuming an initialoncentration of 3 OH/nm2 [18], their surface concentration haseen calculated based on the SSA, see Table 1. Then, the thermalvolution of the surface concentration of the Si–OH and Si–O–Sias modeled using the speciation vs. temperature as provided by

huravlev [20–24]. In brief, this OH-speciation profile vs. T, entailshat thermal treatments of SiO2 NPs can induce two types of surfaceffects: [i] H2O desorption, [ii] structural modification of the Si–OHroups and formation of Si–O–Si bridges [20–24]. It is well knownhat thermal treatment causes structural modification of the SiO2Ps by gradually modification of water content absorbed on parti-le surface [16–18]. Here it is important to distinguish between thewo main processes – dehydration/dehydroxylation – taken placeuring the thermal treatment of NPs.

Dehydration is the loss of physisorbed water while dehydrox-lation stands for the condensation of hydroxyl groups to formiloxane bridges. It is generally agreed that heating of SiO2 at 120 ◦Cemoves most of physisorbed water [20]. Then, thermal treat-ent of SiO2 particles up to 900 ◦C, involves three main stages

I II, III) as exemplified by Zhuravlev [22–24]: (I) dehydrationccurs during stage I (25–190 ◦C), where the SiO2 surface showshe maximum OH-concentration due to gradual removal of phys-cally adsorbed water [22]. During stage II (190–400 ◦C), the total

species Si–OH ( ), Si–O–Si ( ) vs. thermal treatment temperature (data from Refs.[21], [22]).

number of silanol groups decreases gradually [21]. Finally, duringstage III (400–900 ◦C), the overall hydroxylation degree continu-ously decreases with concomitant increment of the concentrationof siloxane-bridges [22–24].

In this context, in Fig. 6, we present the gas production rate in thepresence of 10 mg of thermal treated Ox-50 NPs vs. temperature oftreatment. In the same graph, the percentage of surface silanol- andsiloxane-groups vs. temperature is presented based on Zhuravlev’smodel [22,23]. According to Fig. 6, heating of Ox-50 NPs at 200 ◦Cincrease the gas production rate from 60 and 96 ml/min respec-tively; heating of Ox-50 NPs at 200 ◦C removes most of adsorbedH2O and decrease the SiOH-groups by 15%. Further heating of theOx-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 highertemperatures, i.e. 600 ◦C, further affects the surface groups: 39%of the Si–OH remain on the surface, accompanied by formation ofmore Si–O–Si bridges. These Ox-50 NPs treated at 600 ◦C increasefurther the H2 production rate to 155 ml/min. Finally at 900 ◦C,98% of siloxane-bridges have been formed. The co-catalytic activ-ity of 900 ◦C heated NPs shows reaction rate 130 ml/min. Thus thepresent catalytic and thermal treatment data as encoded in Fig. 6,demonstrate a correlation between the co-catalytic efficiency of thethermally treated SiO2 NPs and the surfacial H2O/Si–OH/Si–O–Si:[i] surface-adsorbed H2O retards co-catalytic efficiency, thus ther-mal treatment eliminates this retarding effect of H2O and [ii] thereis a beneficial effect of surface Si–O–Si bridges. Thus, H2O shouldbe desorbed and surfacial Si–O–Si bridges should be maximized inorder to optimize the co-catalytic efficiency of SiO2 NPs. We suggestthat the surface Si–O–Si bridges are responsible for the co-catalyticactivity of SiO2 NPs according to the reaction Scheme 1: first,

−

Si–O–Si bridges promote FA deprotonation towards HCOO anionformation. Then, the next catalytic step involves HCOO− coordina-tion on FeII atom of active catalyst following by its decompositionto H2 and CO2.

152 P. Stathi et al. / Catalysis Toda

mIasi

C

Stnc

4

ngsatO

SebN

[[

[

[[[[[[[

[

Scheme 1. Schematic representation of co-catalytic mechanism of SiO2 NPs.

In this model, Ox-50-SiO2 nanoparticles show superior perfor-ance due to their morphology and agglomeration state [17,18].

t is well known that Ox-50-SiO2 are spherical particles with low-ggregation degree [18] thus their surface is highly available forurface reactions [18,19]. This explains the present – counterintu-tive at first glance – finding.

o-catalytic effect : Ox-50 > A90 > A300

Previous TEM and BET data show that our higher SSA AerosiliO2 such as A300 are much more aggregated than Ox-50 [18,19]hus, despite the higher number of Si-O-Si surface groups, they areot accessible by HCOOH and this is the origin of their inferior co-atalytic activity towards H2 production by HCOOH.

. Conclusion

Here, we reveal an unprecedented co-catalytic activity of SiO2anoparticles on H2-production from FA, catalyzed by homo-eneous catalyst FeII/P(CH2CH2PPh2)3. The ‘net’ homogeneousystem for this reaction provided TOF = 1205 h−1 at 80 ◦C, whileddition of 10 mg of SiO2 nano-particles accelerated gas produc-ion showing TOF = 13,882 h−1 in the case of the less aggregatedx-50-SiO2 NPs.

The co-catalytic efficiency is modulated by the surfacial H2O,

i–OH and Si–O–Si. Surfacially adsorbed H2O retards co-catalyticfficiency. This effect can be easily overcommed by desorbing H2Oy heating of the NPs. In addition, the co-catalytic activity of SiO2Ps is benefiting from the surface Si–O–Si bridges. They promote FA

[

[[[

y 242 (2015) 146–152

deprotonation towards HCOO− anion formation that coordinateson FeII atom of active catalyst during catalysis. Surface Si–O–Siformation can be maximized by thermal treatment at high tem-peratures up to 400 ◦C.

The present findings show that unprecedented highly efficientH2 production from FA can be achieved by a low-cost catalyticsystem consisting of FeII/P(CH2CH2PPh2)3 plus thermally treatedOx-50-SiO2 NPs.

Acknowledgments

This research has been co-financed by the European Union(European Social Fund—ESF) and Greek national funds throughthe Operational Program “Education and Lifelong Learning” of theNational Strategic Reference Framework (NSRF)-Research Fund-ing Program: THALIS. Investing in knowledge society through theEuropean Social Fund.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.cattod.2014.07.012.

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