Fully Dehydroxylated Silica Generated from Hydrosilane: Surface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Fully Dehydroxylated Silica Generated from Hydrosilane: Surface Defects and Reactivity Petr Šot, Christophe Copéret, and Jeroen Anton van Bokhoven J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05196 • Publication Date (Web): 05 Sep 2019 Downloaded from pubs.acs.org on September 5, 2019

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The Journal of Physical Chemistry

Fully Dehydroxylated Silica Generated from Hydrosilane: Surface Defects and Reactivity Petr Šot†, Christophe Copéret†,*, Jeroen A. van Bokhoven†,‡,* †

ETH Zurich, Vladimir Prelog Weg 1−5, CH-8093 Zurich, Switzerland ‡

Paul Scherrer Institute, 5232 Villigen, Switzerland

ABSTRACT

A silanol-free silica with high surface area was prepared by post-functionalization of silica by means of sol-gel reaction with triethoxysilane followed by a high-temperature dehydroxylation treatment. Besides the absence of surface silanols, paramagnetic and diamagnetic defect sites are generated according to UV-Vis, EPR and reactivity studies. The diamagnetic defects are assigned to oxygen-deficient Si centers (silylenes), which exhibit a distinct UV-Vis signature and reactivity. The paramagnetic defects are attributed to siliconcentered radicals that display particularly high g values in EPR compared to reports of commonly described silicon radicals in silica (E’ sites), suggesting the presence of a silicon atom in its proximity. These defect sites are highly reactive and generate dihydrosilane, a silane

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radical, a peroxyl radical and methylsiloxane upon reaction with dihydrogen, dioxygen and methane, respectively.

INTRODUCTION Silica is used in a broad range of applications such as phase separation technology, heterogeneous catalysis, in the glass and ceramic industry, construction engineering, and tire manufacture. It has been studied in great details during the past 100 years. Intense research is devoted to a better understanding of its structure, morphology and properties upon postfunctionalization under a broad range of conditions and temperatures. Amorphous silica consists of tetrahedrally coordinated silicon linked through a network of siloxane bridges, making 4- to 12-membered rings, referred to as Q4 sites. The surface termination of the network is achieved through formation of silanol groups, which are differentiated by their local structure: silicon may be terminated by one OH group (Q3 sites), present as isolated, vicinal or interacting silanols or by two OH groups (Q2 sites) in the form of geminal silanols.1,2,3,4,5 Each of these species is associated with a specific infrared and NMR signature (Scheme 1). The number of silanol groups on the surface can be adjusted by an appropriate thermal treatment, leading to dehydroxylation. A thermal treatment under high vacuum or neutral gas is a well-established procedure to reduce the surface hydroxylation through the formation of water and new siloxane bridges, which is used for example in surface organometallic chemistry.4,6,7,8 This process takes place without a major loss of surface area up to about 700 °C, generating an increasing proportion of isolated silanols, while at higher temperature strained two-membered siloxane rings, are generated as evidenced by the appearance of two additional IR bands at 888 cm-1 and 908 cm-1.6,7,9 The


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textural properties depend strongly on the synthetic procedure and can, to a certain extent, be changed at later stages if required. Processes such as irradiation, mechanical processing (milling) and chemical treatment can introduce additional types of (surface) defects, i.e. silicon atoms with dangling bond(s). For instance, methoxylation of the silica followed by pyrolysis at 900 °C under high vacuum yields so-called “highly reactive” silica, which is capable of activating numerous small molecules such as dihydrogen, dioxygen, methane, and ammonia.10 The silica surface contains silanes and silanols, but the reactive sites have been suggested to be two dangling-bond silicon centers in close proximity. Later studies based on EPR and optical spectroscopy have refined the structures as three-coordinate silicon-centers (paramagnetic defects, so-called E’ sites) and oxygendeficient centers of the second type (diamagnetic defect), usually associated with silylenes (Scheme 1).11,12,13,14 Over the years, a variety of defects have been identified, but the fundamental difference in the preparation methods limits the possible transfer of findings from extensive studies of silicas to milled or chemically treated silicas, due to different localization of the defects (bulk/surface) and, thus, different transformation pathways. Scheme 1. A representation of common motifs in silica, including surface silane moieties and multiple defects15,16,17,35


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A) Siloxane bridges Si O

Si

O Si

O

Si

Si

Si

O

O Si Si Si O

O

O

O

Si O

Si

Si

O

Si

O

Si

O O

B) Surface silanols O

H

H

H

O Si O O O

O Si O O

O

O

H

H

O Si O O

H O Si O O

O

H O

O Si O Si O O O

Q3 site Q2 site Q3 site Q3 site interacting silanols geminal silanols isolated silanols vicinal silanols (O-H) = 3720, 3530 cm-1 (O-H) = 3750-3740 cm-1 (O-H) = 3747 cm-1 (O-H) = 3750-3740 cm-1 C) Surface silanes H O Si O O isolated silanes (Si-H) = 2300 cm-1

D) Paramagnetic defects detected in chemically activated silicas

O Si O O

O

dihydrosilanes (Si-H) = 2230 cm-1 E) Diamagnetic defects O

Si

g1 = 2.0003 g2 = 2.0003 g3 = 2.0018 a(29Si) = 479 G

g1 = 2.0003 g2 = 2.0016 g3 = 2.0022 a(1H) = 76 G a(29Si) = 311 G

ODC(I) max = 163 nm

O Si O O

g1 = 2.0010 g2 = 2.0095 g3 = 2.08

O

O

O Si O O

g1 = 2.0018 g2 = 2.0078 g3 = 2.067

F) E' sites detected in irradiated silicas

O

O O O Si Si O O O

O

H(I) center

O

silylene, ODC(II) max = 248 nm

Si

E' site

H O Si H O

O

H

O Si O

O

O Si O

O

H

O Si O O

O Si O O

O

O Si O

O Si O O

structure unknown, 4 different models are proposed

E' site

E' site

E' site

E' site

g1 = 2.0018 g2 = 2.0013 g3 = 1.9998

g1 = 2.0018 g2 = 2.004 g3 = 2.004

g1 = 2.0018 g2 = 2.0006 g3 = 2.0003

g1 = 2.0018 g2 = 2.0021 g3 = 2.0021

Similar surface species were also observed upon chemisorption of trichlorosilane on silica, followed by a vacuum treatment at 650 °C.18 Later work on the water-vapor treatment of Si-Cl species and pyrolysis of trimethylsiloxylated silica also describes the generation of surfaces,


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which differ by the structure of the reactive groups.19,20,21 These species activate some small molecules, e.g. dioxygen, methane and carbon monoxide, but also initiate polymerization of olefins10,22,23 In addition to surface chemistry studies, the role of reactive species was recognized within the field of microelectronics: The formation of defects in a silica gate within a metaloxide-semiconductor causes dielectric breakdown and 1/f noise.24 The aforementioned treatments with trichlorosilane or trimethylchlorosilane allow the decrease of surface hydroxylation, for which triethoxysilane has also been used as an alternative reagent. For instance, the adsorption of triethoxysilane on silica (and other oxides) followed by a treatment at temperature up to 500 °C under vacuum removes most of the silanols without generating significant amount of residual surface silanes.25,26 However, this approach leads to a rehydroxylation of the surface, presumably because of the decomposition of surface ethoxy groups

upon

treatment

at

high

temperatures.26,27

While

the

use

of

aqueous

hydrolysis/condensation of triethoxysilane avoids the presence of residual ethoxy groups, this approach leads to a significant drop of surface area from 192 to 12 m2/g upon thermal treatment at 400 °C.27 In this study, we describe an alternative approach to generate a silanol-free defective silica with high-surface area by means of a two-step process that involves the formation of surface hydrosilanes via a sol gel deposition process on silica using triethoxysilane28 followed by a posttreatment of the material above 700 °C under vacuum. This approach provides high surface area silica, free of surface OH group and exposing highly reactive oxygen-deficient defects and radicals that activate small molecules, such as dihydrogen, dioxygen as well as methane.


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RESULTS The silica (Aerosil® 200, SBET = 206 m2/g) was post-functionalized by sol-gel using triethoxysilane – (EtO)3SiH, followed by thermal treatment at different final temperatures.28 Table 1 lists the reaction conditions, the sample color, possible additional treatment, the surface area of each sample and the sample names. The samples 1-150 and 1-500 have a similar surface areas, 202 m2/g and 200 m2/g. The surface areas of 1-700 and 1-900 decreased to 180 m2/g and 168 m2/g, respectively. These results indicate that the particle sintering is limited, despite a prolonged high-temperature treatment. This was also confirmed by electron microscopy, which did not reveal a major difference in morphology of the various samples (Figure S1). Samples treated at higher temperature changed color: 1-700 and 1-900 are orange and ochre, respectively, while 1-150 and 1-500 remained white (Figure S2-S4). Table 1. List of samples prepared from sol-gel modified silica with their basic characteristics. Heat treatment took place at high vacuum (10-5 mbar). Samples, which underwent further treatment with a reactive gas are referred to as 1-T-gas. Temperature ramp

Target temperature

[°C/h]

[°C] 150 500 700

60

Sample color

white orange

900 900 900 900

ochre

Reactive gas Surface area post-treatment (3 [m2/g] h)

Sample name

-

202

1-150

-

200

1-500

-

180

1-700

-

168

1-900

H2

N/A

1-900-H2

O2

N/A

1-900-O2

CH4

N/A

1-900-CH4


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Figure 1a shows the infrared spectra of the various samples in the region between 1300 and 4000 cm-1; they illustrate the temperature-induced structural changes in the silanol and silane regions. Figure 1b focuses on the silanol region between 3200 and 4000 cm-1. The spectrum of 1-150 shows a range of silanol stretching vibrations between 3300 and 3800 cm-1. There is a continuous decrease in the intensity and a slight blue shift (up to 7 cm-1) of the band of the isolated silanols with increasing temperature (Figure 1a), with a total loss of the silanol band in the spectrum of 1-900. A similar effect was found for the band of interacting silanols at about 3640 cm-1 and 3530 cm-1, which shifts about 20 cm-1, before disappearing. Similarly, the intensity of the Si-H stretching band at 2256 cm-1 is lower in the spectra after the high temperature treatment. Its intensity in the spectrum of 1-900 is 6% that of 1-150 and blueshifted to 2296 cm-1 (Figure 1c). At the same time, a shoulder emerges at 2210 cm-1 in the spectrum of 1-500, indicating that partially condensed =Si(OH)H species may be present.29


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Figure 1. Effect of thermal treatment on the absorbance of Si-OH and Si-H bands. Plot a) spectra of all dehydroxylated samples, b) O-H region, c) Si-H region. All spectra were normalized to 1863 cm-1. Figures 2a shows the UV-Vis spectrum of 1–900, which exhibits two distinct maxima at 220 and 248 nm, consistent with π − π* or n − π* transitions (detail in Figure 2b).30,31,32 Exposure to for 15 seconds air decreases: a) the intensity of the two peaks likely due to the reaction of the respective centers with oxygen and water, b) the signal intensity in the region between 400 nm and 600 nm. The first site with an absorption maximum at 251 nm resembles oxygen-deficient centers of the second type (ODC II), which are typically associated with silylene sites.12,33,34,35


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The second peak has parameters with values between those typical of E’ sites and ODC(II), and can, therefore, be associated either with paramagnetic silicon defects or with distorted silylenes.34,36,37 Part of the broad shoulder might originate from another transition belonging to ODC(II), expected at 393 nm; however, the formation of a similar shoulder is also observed in the spectra of irradiated silicas.33,38 The spectrum of 1-700 looks very similar to 1-900, suggesting the presence of similar defects, while spectra of 1-150 and 1-500 do not show any absorption bands (Figures S5).

Figure 2. a) DR UV-Vis spectrum of 1-900 under argon and after exposure to air; b) magnification of the region between 200 and 350 nm. The room temperature X-band cw-EPR spectrum of 1-900 reveals three signals (Figure 3a): a dominant signal at 336 mT with a weak doublet around it and a very weak shoulder in the lowfield region of the doublet signal (Figure 3b, site 3). Fitting of the spectrum reveals a giso value of the dominant signal of 2.00751 (i.e. gxx = 2.00728, gyy = 2.00722, gzz = 2.00803 when the anisotropy is allowed). The doublet signal exhibits a g-value of 2.00367 with corresponding


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hyperfine coupling tensor values of A1 = 76.43 G, A2 = 76.72 G and A3 = 82.18 G (isotropic hyperfine coupling constant bears value of 78.44 G). There is a ratio of 1:0.055 between the areas of the dominant signal and the doublet. The dominant signal can be tentatively assigned to a three-coordinate silicon radical, ≡Si● (Figure 3b, site 1) and the weak doublet signal to silane radical, =Si●(H) (Figure 3b, site 2).11

Figure 3. a) X-band EPR spectrum of 1-900 at 25 °C. The dominant feature at 336 mT is surrounded by two smaller features. b) Magnification of the region of a smaller region visualizing signals 1, 2 and 3. Reactivity of 1-900 towards probe molecules In view of the reactivity of the silica materials with similar reactive centers, we investigated the reactivity of 1-900 towards dihydrogen, dioxygen and methane, yielding the corresponding materials 1-900-H2, 1-900-O2 and 1-900-CH4, respectively. All of the 1-900-H2 and 1-900-CH4 samples were prepared at 300 °C with 250 mbar of the gas, while sample 1-900-O2 was prepared at room temperature.


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Upon reaction with dihydrogen (1-900-H2) several new bands appeared in the silane and silanol region (Figures 4a, 4b). Specifically, a dihydrosilane band at 2223 cm-1 and a series of weak bands associated with surface silanols at 3743 cm-1, 3714 cm-1 and 3687 cm-1, appear.11 Figure 5c shows the room temperature EPR spectrum of the 1-900-H2, which exhibits two sets of intense signals: a singlet and a doublet. The former displays an isotropic g-tensor (gxx = 2.00631, gyy = 2.00535 and gzz = 2.00653) with giso = 2.00606. The doublet signal shows giso = 2.00264 and aiso = 78.40 G with the elements A1 = 77.05 G, A2 = 76.28 G and A3 = 81.88 G. Both g-values and hyperfine coupling tensors are nearly identical with the corresponding signals observed in 1-900, hence these signals are tentatively assigned to the three-coordinate silicon radical ≡Si● and silane radical =SiH●.11 There is a striking difference in the ratios of the signals with 1:0.325 for the spectrum of 1-900-H2 and 1:0.055 for the spectrum of 1-900 (Figure 5a). When reacted with oxygen, the infrared spectrum of 1-900-O2 shows two new features in the hydroxyl region (Figure 4a): a very weak isolated silanol band at 3742 cm-1 and an interacting silanol at 3588 cm-1. The room-temperature EPR spectrum in Figure 3d reveals the presence of paramagnetic species with two highly convoluted components centered around a field value of 336 mT. At -173 °C, the g tensor can be resolved and subsequently fitted with axial and perpendicular components. The assigned values are gǁ = 2.07037 and g⊥ = 2.00503, which fit well with a peroxyl radical.39 The region between gǁ and g⊥ contains weak unresolved features a. Comparison with the spectrum of 1-900 (Figure 5a) reveals the absence of the giso = 2.004 doublet signal. The reaction with methane (1-900-CH4) leads to the formation of five bands at 2182 cm-1, 2211 cm-1, 2223 cm-1, 2234 cm-1 and 2268 cm-1 (Figure 4c) along with bands at 2978 cm-1 and at 3745 cm-1 in the IR spectrum (Figure 4a). The latter two bands are assigned to C-H and silanol, respectively, while the bands appearing in the silane region (2100-2300 cm-1) likely correspond to dihydrosilane, methylsilane and


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condensed species =SiH(OH), although the assignment of bands at 2223 cm-1 and 2268 cm-1 is less clear.11,22,29 The band at 2978 cm-1 probably originates either from C-H stretching of methylsilane or methysiloxane group; it further supports that 1-900 can activate methane under mild conditions and illustrates the high reactivity of the surface defects. The room-temperature EPR spectrum of 1-900-

CH4 indicates the presence of a paramagnetic species with signal at the giso value, around 2.0078 (Figure 5b), but most of the paramagnetic species present in 1-900 disappeared (Figure 5a). The asymmetry and broadness of the signal suggests the presence of other unresolved components, but they are not further resolved upon cooling to -173 °C. The presence of a three-coordinate silicon radical was deduced from the EPR signal, although an exact assignment is hindered by the weak and asymmetric character of the signal.


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Figure 4. a) Comparison of IR spectra of 1-900-H2 with those of 1-900-H2, 1-900-O2, 1-900-CH4; Magnification of Si-H region of 1-900-H2 (b) and 1-900-CH4 (c). All spectra were normalized to 1863 cm-1.


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Figure 5. X-band EPR spectra of 1-900-H2 (a), 1-900-CH4 (b), 1-900-H2 (c), 1-900-O2 at 25 °C and -173 °C. DISCUSSION The parameters of a thermal treatment dramatically influence the surface structure of silane-functionalized silica. 1-150 and 1-500 contain surface silanes and silanols; 1-900 is free of silanols and contains diamagnetic or paramagnetic defect sites. The diamagnetic defect has UVVis characteristics of a so-called oxygen-deficient site of the second type (ODC II).34 It readily reacts with gas probes such as dihydrogen, dioxygen and methane, yielding surface species as


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shown in Scheme 2. Specifically, the defect activates dihydrogen to form surface dihydrosilane =SiH2 (a), while methane forms methylsilanes =SiHCH3 (c), thus confirming the assignment of the defect to silylene site =Si: (h).22 The first paramagnetic defect in 1-900 shows EPR parameters characteristic of a silicon-centered radical with a hydrogen atom in its vicinity, such as =Si●-(H), and exhibits the expected reactivity towards gas probes. For example, upon exposure to methane we observed the formation of dihydrosilane =SiH2 (a), while the reaction with oxygen results in the formation of a peroxyl radical Si-O-O● (d).40 Reaction with dihydrogen yields dihydrosilane =SiH2 (a) but also leads to a significant increase of the concentration in the paramagnetic silicon defect. The former is consistent with the presence of the three-coordinate radical silane =Si●-(H), commonly referred as the H(I) site (g), while the latter suggest the presence of another site, able to react with dihydrogen to yield H(I). Such a site may be closely associated with another one, similarly to proposed structures of E’ defects in the Scheme 1. For example, a divalent silicon may be associated with the silicon radical, facilitating dihydrogen splitting while generating monohydrosilane ≡Si-H (e) and the H(I) site =Si●-(H) (g). A similar model of ODC(II), based on the divalent silicon in the close proximity of two silicon atoms, was already discussed by Griscom et al.14 The second paramagnetic site, identified by a different EPR signature, also reacts with oxygen, dihydrogen and methane and yields peroxyl ≡Si-O-O● radical (d), monohydrosilane ≡Si-H (e), methylsiloxane ≡Si-CH3 (b) and monohydrosilane ≡Si-H (e), respectively. Both spectroscopy and reactivity studies show that the structure of the second site is also based on the silicon-centered radical as well, but its g-value is remarkably high compared to the common silica defects, such as E’ sites. The unusual parameters of the site are probably due to the


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presence of one or more silicon atoms in its vicinity (f), as suggested by the comparison with similar organosilyl radicals and defects on the interface of Si/SiO2.41,42,43 Potentially, such a site could originate from the oxygen-deficient sites, which can easily trap electron form paramagnetic defects.14 They can also agglomerate and form further moieties with a siliconsilicon bond.44 We propose that the formation of the defect sites (Scheme 2) is directly linked to the high dehydroxylation temperature of the surface, which is, for the most part, composed of surface hydrosilanes. For example, 1-900 completely dehydroxylates, as shown in Figures 1a-b. This unusual characteristic is typically observed only for silicas heated above 1200 °C and which also show a very low surface area.4 Since the condensation of two adjacent silanols (Scheme 2, eq. 1.1) cannot explain complete dehydroxylation at 900 °C, condensation of adjacent surface silanes and silanols (eq. 1.2) can be proposed.4,45 However, the rapidly decreasing amount of surface silanes observed in 1-700 and 1-900 requires an alternative explanation, such as the dehydrogenative coupling of silanes, leading again to the formation of dihydrogen and oxygendeficient centers (eq. 1.3).46 These defects are known to relax the strain while rearranging to ODC(II), though the precise mechanism is debatable.34 Alternatively, surface hydrosilanes disproportionate yielding dihydrosilanes (eq. 1.4), which are susceptible to dehydrogenation, forming directly silylene sites (eq. 1.5).10,47 Scheme 2. Overview of sites in the sample 1-900 as well as new sites formed during the reaction with the gas probes and proposed mechanism of silica dehydroxylation at 900 °C.


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The surface sites for 1-900, 1-900-H2, 1-900-O2, 1-900-CH4 CH3

H

CH3

X Si O O

O Si H O a. dihydrosilane (Si-H) = 2224 cm-1

b. methylsiloxane X = O, Si (C-H) = 2978 cm-1

Si Si O O OO

Si

O

f. E'-like site

O Si O O

O Si O O

c. methylsilane

d. peroxyl radical

e. monohydrosilane

(Si-H) = 2211 cm-1

gII = 2.07037 g = 2.00503

(Si-H) = 2296 cm-1

O

O

g. H(I) site giso = 2.00367 aiso(H) = 78.4 G

giso = 2.00751

H

O Si H O

H

O

O

O

Si

O

OH O Si H O

O

O

X Si O

h. silylene

i. condensed silane/silanol

(Rmax) = 248 nm

(Si-H) = 2234 cm-1

Si

O

j. association of f and h

The proposed mechanism behind the formation of the defects in 1-900: silanol condensation O O

H Si O

O

H

+

O Si

O

10 mbar

O

Si

O

-5

O

O

O

T

Si

O

O

O

O +

H 2O

(1.1)

+

H2

(1.2)

+

H2

(1.3)

silanol-silane condensation O

O O

+

Si O

H Si

O

O

H

O

O

T 10-5 mbar

O

T

O

Si

O

O

Si O

O O

dehydrogenative coupling of silanes O O

Si H O

O + H Si

O

10-5 mbar

O

silane disproportionation H T O Si O 2 -5 10 mbar O dehydrogenation of dihydrosilanes H T Si H -5 O 10 mbar O

O

O

Si

O

Si O

H H Si O O

O Si

O

O

+

O Si O O O

(1.4)

+

H2

(1.5)


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CONCLUSION A highly defective surface of silica free of hydroxyls was created by combining a sol-gel reaction and high-temperature dehydroxylation without requiring the formation of alkoxy or chloride surface species. This implies the occurrence of surface reorganization. The mechanism of such a process involves dehydroxylation, specifically condensation of two silanols, but possibly also silane-silanol condensation and dehydrogenative coupling of silanes. Further treatment leads to the complete removal of O-H groups, the formation of diamagnetic defects, i.e. silylene groups (detected by UV-Vis), and two different types of paramagnetic defects (detected by EPR), namely the three-coordinate silicon radical and the silane radical. The former radical species is dissimilar to the expected E’ sites, which are usually present in activated silicas and contain a silicon-silicon bond. The latter paramagnetic site is the three-coordinate silicon radical bearing a hydrogen atom. These findings shed new light on the reactivity of surface hydrosilane with possible implications for multiple fields, including heterogeneous catalysis, the semiconductor industry or manufacture of optic fibers. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The description of analytic methods (IR, UV-Vis, EPR, TEM), standard procedures and reaction protocols (PDF) AUTHOR INFORMATION Corresponding Author


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*

E-mail

for

J.A.v.B.:

[email protected],

Email

for

C.C.:

[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT P.S. thanks to Shell Global Solutions International B.V. for financial support. We acknowledge A. P. van Bavel and A. D. Horton for very helpful discussions. REFERENCES (1) Ulrich, G. D. Theory of particle formation and growth in oxide synthesis flames. Combust. Sci. Technol. 1971, 4, 47-57. (2) Rahman, I. A.; Padavettan, V. Synthesis of silica nanoparticles by sol-gel: size-dependent properties, surface modification, and applications in silica-polymer nanocomposites — a review. J. Nanomater. 2012, 2012, 132424. (3) Buckley, A. M.; Greenblatt, M. The sol-gel preparation of silica gels. J. Chem. Educ. 1994, 71, 599. (4) Zhuravlev, L. T. The surface chemistry of amorphous silica. Zhuravlev model. Colloids Surf. A 2000, 173, 1-38. (5) Comas-Vives, A. Amorphous SiO2 surface models: Energetics of the dehydroxylation process, strain, ab initio atomistic thermodynamics and IR spectroscopic signatures. Phys. Chem. Chem. Phys. 2016, 18, 7475-7482.


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Page 20 of 26

(6) Morrow, B. A.; Cody, I. A. Infrared studies of reactions on oxide surfaces. 5. Lewis acid sites on dehydroxylated silica. J. Phys. Chem. 1976, 80, 1995-1998. (7) Bunker, B. C.; Haaland, D. M.; Michalske, T. A.; Smith, W. L. Kinetics of dissociative chemisorption on strained edge-shared surface defects on dehydroxylated silica. Surf. Sci. 1989, 222, 95-118. (8) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.-M. Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry. Angew. Chem., Int. Ed. 2003, 42, 156-181. (9) Scott, S. L.; Basset, J.-M. Coordination chemistry on surfaces: a new method to graft rhenium(VII) oxide on highly dehydroxylated oxides. J. Am. Chem. Soc. 1994, 116, 1206912070. (10) Morterra, C.; Low, M. J. D. Reactive silica: novel aspects of the chemistry of silica surfaces. Ann. N. Y. Acad. Sci. 1973, 220, 133-244. (11) Radzig, V. A. Reactive silica—a new concept of the structure of active sites. Colloids Surf. A 1993, 74, 91-100. (12) Radzig, V. A. Point defects in disordered solids: differences in structure and reactivity of the (≡Si–O)2Si: groups on silica surface. J. Non-Cryst. Solids 1998, 239, 49-56. (13) Hochstrasser, G.; Antonini, J. F. Surface states of pristine silica surfaces: I. ESR studies of Es′ dangling bonds and of CO2− adsorbed radicals. Surf. Sci. 1972, 32, 644-664. (14) Griscom, D. L. Trapped-electron centers in pure and doped glassy silica: a review and synthesis. J. Non-Cryst. Solids 2011, 357, 1945-1962.


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Radzig, V. A. Point defects on the silica surface: Structure and reactivity. In Thin Films and Nanostructures, Trakhtenberg, L. I.; Lin, S. H.; Ilegbusi, O. J., Eds. Academic Press, 2007; pp 231-345. (16) Griscom, D. L.; Friebele, E. J. Fundamental radiation-induced defect centers in synthetic fused silicas: Atomic chlorine, delocalized E' centers, and a triplet state. Phys. Rev. B 1986, 34, 7524-7533. (17) Dondi, D.; Buttafava, A.; Zeffiro, A.; Bracco, S.; Sozzani, P.; Faucitano, A. Reaction mechanisms in irradiated, precipitated, and mesoporous silica. J. Phys. Chem. A 2013, 117, 3304-3318. (18) Low, M. J. D.; Severdia, A. G. Reactive silica: XIII. Activation of silica by pyrolizing chemisorbed HSiCl3. J. Catal. 1978, 54, 219-222. (19) Low, M. J. D.; Severdia, A. G.; Chan, J. Infrared study of the sorption of HSiCl3 on silica and the stability of the chemisorbed layers. J. Colloid Interface Sci. 1982, 86, 111-118. (20) Wovchko, E. A.; Camp, J. C.; Glass, J. A.; Yates, J. T. Active sites on SiO2: role in CH3OH decomposition. Langmuir 1995, 11, 2592-2599. (21) Burneau, A.; Lalevée, J.; Carteret, C. Infrared spectroscopic study of the formation of reactive silica by pyrolysis in vacuo of a trimethylsiloxylated sample. J. Phys. Chem. B 2000, 104, 990-996. (22) Permenov, D. G.; Radzig, V. A. Mechanisms of heterogeneous processes in the system SiO2 + CH4: II. Methylation of >Si=O Groups. Kinet. Catal. 2004, 45, 265-272.


21


Page 22 of 26

(23) Low, M. J. D.; Mark, H. Reactive silica: XII. The sorption and polymerization of several alkenes. J. Catal. 1977, 50, 373-378. (24) Fleetwood, D. M.; Xiong, H. D.; Lu, Z.; Nicklaw, C. J.; Felix, J. A.; Schrimpf, R. D.; Pantelides, S. T. Unified model of hole trapping, 1/f noise, and thermally stimulated current in MOS devices. IEEE Trans. Nucl. Sci. 2002, 49, 2674-2683. (25) Riccio, M.; Montanari, T.; Castellano, M.; Turturro, A.; Negroni, F. M.; Busca, G. An IR study of the chemistry of triethoxysilane at the surface of metal oxides. Colloids Surf. A 2007, 294, 181-190. (26) Marrone, M.; Montanari, T.; Busca, G.; Conzatti, L.; Costa, G.; Castellano, M.; Turturro, A. A Fourier transform infrared (FTIR) study of the reaction of triethoxysilane (TES) and bis[3triethoxysilylpropyl]tetrasulfane (TESPT) with the surface of amorphous silica. J. Phys. Chem. B 2004, 108, 3563-3572. (27) Larrubia Vargas, M. Á.; Busca, G.; Montanari, T.; Herrera Delgado, M. C.; Alemany, L. J. Preparation and characterization of silicon hydride oxide: a fully hydrophobic solid. J. Mat. Chem. 2005, 15, 910-915. (28) Chu, C. H.; Jonsson, E.; Auvinen, M.; Pesek, J. J.; Sandoval, J. E. A new approach for the preparation of a hydride-modified substrate used as an intermediate in the synthesis of surfacebonded materials. Anal. Chem. 1993, 65, 808-816. (29) Rascón, F.; Berthoud, R.; Wischert, R.; Lukens, W.; Coperet, C. Access to well-defined ruthenium mononuclear species grafted via a Si−Ru bond on silane functionalized silica. J. Phys. Chem. C 2011, 115, 1150-1155.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. The first isolable dialkylsilylene. J. Am. Chem. Soc. 1999, 121, 9722-9723. (31) Tsutsui, S.; Sakamoto, K.; Kira, M. Bis(diisopropylamino)silylene and its dimer. J. Am. Chem. Soc. 1998, 120, 9955-9956. (32) Leites, L. A.; Aysin, R. R.; Bukalov, S. S.; Yao, S.; Driess, M. The study of the structure of the six-membered unsaturated N-heterocyclic silylene LSi: and related compounds by the methods of optical (Raman, IR, UV–vis) spectroscopy. J. Mol. Struct. 2018, 1166, 311-314. (33) Girard, S.; Kuhnhenn, J.; Gusarov, A.; Brichard, B.; Uffelen, M. V.; Ouerdane, Y.; Boukenter, A.; Marcandella, C. Radiation effects on silica-based optical fibers: Recent advances and future challenges. IEEE Trans. Nucl. Sci. 2013, 60, 2015-2036. (34) Skuja, L. Optically active oxygen-deficiency-related centers in amorphous silicon dioxide. J. Non-Cryst. Solids 1998, 239, 16-48. (35) Zhou, Q.-L.; Liu, L.-Y.; Xu, L.; Wang, W.-C.; Zhu, C.-S.; Gan, F.-X. Near-infrared femtosecond laser induced defect formation in high purity silica below the optical breakdown threshold. Chin. Phys. Lett. 2003, 20, 1763. (36) Pacchioni, G. Ab initio theory of optical transitions of point defects in SiO2. Phys. Rev. B 1998, 2, 818-832. (37) Magruder, R. H., III; Stesmans, A.; Clémer, K.; Weeks, R. A.; Weller, R. A. Sources of optical absorption between 5.7 and 5.9 eV in silica implanted with Si or O. J. Appl. Phys. 2006, 100, 033517.


23


Page 24 of 26

(38) Sporea, D.; Mihai, L.; Sporea, A.; Lixandru, A.; Bräuer-Krisch, E. Investigation of UV optical fibers under synchrotron irradiation. Opt. Express 2014, 22, 31473-31485. (39) Stapelbroek, M.; Griscom, D. L.; Friebele, E. J.; Sigel, G. H. Oxygen-associated trappedhole centers in high-purity fused silicas. J. Non-Cryst. Solids 1979, 32, 313-326. (40) Pacchioni, G.; Vitiello, M. EPR and IR spectral properties of hydrogen-associated bulk and surface defects in SiO2: Ab initio calculations. Phys. Rev. B 1998, 58, 7745-7752. (41) Tumanskii, B.; Karni, M.; Apeloig, Y. Silicon-centered radicals. In Organosilicon Compounds; Lee, V. Y., Ed.; Academic Press, 2017; pp 231-294. (42) Lenahan, P. M.; Conley, J. F., Jr. What can electron paramagnetic resonance tell us about the Si/SiO2 system? J. Vac. Sci. Technol. B 1998, 16, 2134-2153. (43) Chatgilialoglu, C. Formation and structures of silyl radicals. In Organosilanes in Radical Chemistry, John Wiley & Sons, 2004; pp 1-17. (44) Manveer, S. M.; David, Z. G.; Alexander, L. S. Diffusion and aggregation of oxygen vacancies in amorphous silica. J. Phys. Condens. Matter 2017, 29, 245701. (45) Woolley, C. L.; Markides, K. E.; Lee, M. L. Deactivation of fused-silica capillary columns with polymethylhydrosiloxanes: Optimization of reaction conditions. J. Chromatogr. A 1986, 367, 9-22. (46) Kim, D.; Joo, J.; Pan, Y.; Boarino, A.; Jun, Y. W.; Ahn, K. H.; Arkles, B.; Sailor, M. J. Thermally induced silane dehydrocoupling on silicon nanostructures. Angew. Chem., Int. Ed. 2016, 55, 6423-6427.


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(47) Wells, A. S. On the perils of unexpected silane generation. Org. Process. Res. Dev. 2010, 14, 484-484.


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TABLE OF CONTENT STEP 1: sol-gel reaction H H O O Si O Si O O O O

OH Si O OO

H O

O Si O O O

SiO2 surface of amorphous silica

HSi(OEt)3 HCl dioxane

STEP 2: thermolysis

O

H H Si Si O O OO O

SiH/SiO2 surface with silane moieties

O 900 °C 10-5 mbar

H Si

O

Si

Si O Si O O OO

(SiH/SiO2)-900 silanol-free radicalized surface


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Fully Dehydroxylated Silica Generated from Hydrosilane: Surface

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