The impact of residual adsorbate on the characterization of microporous carbons with small angle scattering

CARBON

5 0 ( 2 0 1 2 ) 3 0 7 4 –3 0 8 2

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The impact of residual adsorbate on the characterization of microporous carbons with small angle scattering C. Scherdel *, G. Reichenauer Bavarian Center for Applied Energy Research, Am Hubland, 97074 Wu¨rzburg, Germany

A R T I C L E I N F O

A B S T R A C T

Article history:

Microporous carbon materials were investigated with small angle X-ray scattering to deter-

Received 18 January 2012

mine whether a pre-treatment with heat and vacuum of the samples is necessarily

Accepted 29 February 2012

required to obtain accurate and reliable data. The findings of this study indicate strongly

Available online 8 March 2012

erroneous data in terms of micropore volume and apparent sample density, if the samples are not carefully degassed prior to the measurement, even when the measurement is performed in high vacuum. The observed scattering curves were compared to a systematically wetted sample. The results further show an ageing of the samples that causes predominant cluster adsorption most likely of water at oxygen surface groups. Some recommendations are given for an adequate sample preparation prior to characterizing (micro)porous materials with small angle scattering. Ó 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

For the characterization of (micro)porous materials such as activated carbons, metal organic frameworks (MOFs), zeolites or porous silicas, usually gas sorption measurements are performed to determine the morphological properties at the nanoscale. The sorption technique represents a well established and widespread characterization method. However, complementary techniques like small angle scattering (SAS) with X-rays (SAXS) or neutrons (SANS) are known for a long time [1]. Especially in recent years, availability and thus the importance of SAS techniques for the characterization of (micro)porous materials increased. This shows for example when searching for the keywords ‘‘carbon’’, ‘‘SAXS’’ and ‘‘porous’’ in combination with the year in a scientific database. The number of related publications increased by more than a factor of two in the last 10 years. More and more materials scientists make use of this method as it is a non-invasive, and compared to sorption experiments, a fast method suitable e.g. for the screening of sample series [2] or applications, that probe special heterogeneities [3]. Furthermore, also pores, that are

inaccessible for the analysis gas upon gas sorption, can be detected [4]. The aim of the present study is to analyze the impact of remaining adsorbate in (micro)porous carbons on the data deduced and to further provide some recommendations for performing SAS experiments on (micro)porous materials.

2.

Experimental

2.1.

Samples

The porous samples used for this study were synthesized via a sol–gel process using phenolic resins as precursors and ambient pressure drying. The resulting organic xerogels were subsequently pyrolysed under inert atmosphere at 800 °C and converted to carbon xerogels, i.e. monolithic amorphous porous carbons. For convenience, the six samples investigated are just labelled with letters from A to F. Details of the sample synthesis can be found elsewhere: samples A–C [5]; sample D [6]; samples E and F [7]. A through D are based on phenol– formaldehyde solutions with (A through C) water as solvent

* Corresponding author: Fax: +49 931 70564 60. E-mail address: [email protected] (C. Scherdel). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.02.093

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and NaOH as catalyst or (D) n-propanol as solvent and HCl as catalyst. E and F are based on aqueous resorcinol–formaldehyde solutions with Na2CO3 as catalyst and subsequent solvent exchange with ethanol prior to ambient pressure drying. Each sample was measured repeatedly with SAXS with several months in between the individual measurements to detect a possible time-dependent change of the scattering curve. Furthermore, experiments with explicit degassing of the samples before the measurement at temperatures up to 250 °C under vacuum (<1 mbar) for at least 6 h prior to the measurement were performed. In contrast thereto, measurements were preformed with one of the samples being exposed to different relative humidity. For this experiment four slices were cut from one parent sample. One slice was heat and vacuum treated (HVT) as stated above. The other three slices were first placed in an atmosphere of 100% relative humidity (100% RH) for 4 h. Then each of these three slices were placed in an airtight vessel (with polyimide foil as windows) above a reservoir of (i) water (100% RH), saturated NaBr salt solution (57% RH) and saturated K2CO3 salt solution (43% RH) in order to generate well defined desorption conditions. The relative humidity established was measured with a moisture sensor. The three slices were kept at least 24 h in the airtight vessels prior to the SAXS experiment.

2.2.

Characterization

The SAXS measurements of the porous carbons were performed at the German synchrotron laboratory HASYLAB in Hamburg at the SAXS-beamline B1.1 The samples were placed in a vacuum sample chamber with a gas pressure of about 6 Æ 105 mbar (high vacuum, HV) during the experiment. The measurements were performed in two detector–sample distances using X-rays with energy of 12 keV and 14 keV, corre˚ and sponding to wave lengths of the X-rays of 1.035 A ˚ , respectively. The transmission T of the samples was 0.887 A measured with a diode comparing the intensity of the empty beam with the beam intensity attenuated by the samples. If the thickness d and the molar composition of the sample are known (here pure non porous carbon with a density of qcarbon = 2.2 g/cm3 is assumed), the density q of the porous degassed sample can be evaluated using the total attenuation coefficient for coherent scattering (l/qcarbon) by: q¼

 1 ln T l  d qcarbon

ð1Þ

The total attenuation coefficient for coherent scattering of carbon is 1.43 cm2/g for 12 keV and 0.955 cm2/g for 14 keV, respectively.2 Using a glassy carbon reference, the X-ray scattering curves were calibrated on an absolute scale providing the volume and the mass specific scattering differential crosssections for the samples in units of [cm1sr1] and [cm2g1sr1], respectively. For the volume specific scattering cross section (V1[dr/dX]), the scattering intensity I(q) is 1 2

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normalized to the respective sample thickness d and transmission T [8,9], whereas q represents the scattering vector.   1 dr IðqÞ / : ð2Þ V dX dT For the quantitative evaluation of external surface area Sext or micropore volume Vmic, the mass specific scattering cross section (m1[dr/dX]) is used, which can be obtained by dividing the volume specific scattering cross section by the density q as derived from Eq. (1).   1 dr ½  1 dr ¼ V dX ð3Þ m dX q For microporous materials a characteristic shoulder appears in the scattering curve at large scattering vectors q that can be evaluated by the two phase model (TPM) according to Debye’s formula [10]. The TPM is based on the assumption of two statistically homogeneously distributed phases with different electron densities. Within each phase the electron density is assumed to be constant. Thus the volume specific differential scattering cross section V1[dr/dX] is proportional to the squared electron density difference of the two phases [8].   1 dr / ðqe1  qe2 Þ2 ð4Þ V dX If also an adsorbate phase is present (in addition to the carbon phase and the vacuum), the TPM is strictly speaking not applicable. However, as first approximation, an ‘‘average electron density’’ by combining two phases can be assumed, i.e. the (micro)pore filling (vacuum plus adsorbate) or the microporous primary particle (carbon matrix plus adsorbate). For the micropore contribution to the scattering curve (see Fig. 1), the relationship   1 dr a ¼ ð5Þ m dX mic ð1 þ ‘2  q2 Þ2 is used, where the parameter a is proportional to the number of scattering entities and the micropore volume. ‘ is the Debye characteristic length (correlation length). To calculate the micropore volume Vmic, the carbon density qcarbon and the average density of the microporous particles qparticle are required [11]; the latter can be determined from the scattering data according to 1 Vmic ¼ q1 particle  qcarbon

with;

ð6Þ

Q mic qparticle ¼ qcarbon  ð7Þ 2p2  C2 using the invariant of the micropore scattering Qmic defined as Z 1   Z 1 1 dr a  q2 pa  q2 dq ¼ dq ¼ : ð8Þ Q mic ¼ m dX mic 4  ‘3 ð1 þ ‘2  q2 Þ2 0 0 The parameter C = 8.5 Æ 1011 m/kg in Eq. (7) connects the mass density of the scattering entities to the electron density governing the scattering cross section and is a constant for light elements (except of hydrogen) [12]; qcarbon, the skeletal density of nonporous carbon is assumed to be 2.2 g/cm3 [13].Towards smaller scattering vectors q, the scattering curve typically shows a straight line in the double-logarithmic plot, which is called the Porod-range (see Fig. 1). If the surface is smooth, i.e. the exponent a in the power law is 4, the envelope

http://hasylab.desy.de/facilities/doris_iii/beamlines/b1/index_eng.html (24. July 2010) http://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html (access date: 25. November 2010)

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in airtight vessels were decoupled from the high vacuum environment. To provide an overview over the measurements, only one representative sample for each subitem is shown. However, the data derived for q, Vmic, a and ‘ are listed in Table 1 for all samples investigated. To make differences between scattering curves better visible, the Kratky-plot (V1[dr/dX](q) Æ q2 vs. q) is chosen for presenting the micropore section of the scattering curves, whereas the Porod-plot (V1[dr/dX](q) Æ q4 vs. q) is used to analyse the power-law dependence at q-values below the micropore regime.

3.1.

Fig. 1 – Representative scattering curve (sample A). The fit ranges of the Porod and TPM range are shown with their respective fit functions. surface area of the carbon particles Sext can be determined by evaluating the Porod constant K.     1 dr 1 dr  qa ) ¼ K  qa : ð9Þ K ¼ lim q!1 m dX m dX q!1 If a 5 4, an evaluation of Sext is not possible due to fractal behaviour [14] and thus a length scale dependent surface area of the sample.

2.3.

Data evaluation procedure

For the evaluation of the scattering data, in principle, different strategies can be applied. The following fit-function includes all structural sections of the scattering curve detected, i.e. the Porod regime, the micropore shoulder and the incoherent scattering background B at large scattering vectors (q  8 to 10 nm1):   1 dr a ¼ K  qa þ þ B: ð10Þ m dX ð1 þ ‘2  q2 Þ2 As the scattering intensity as well as the scattering vector vary over several orders of magnitude, a weighting of Eq. (10) is mandatory upon fitting. Therefore, e.g. the statistical error of each q-value can be applied. However, the visible congruence between fit and experimental data strongly depends on the shape of the scattering curve. Furthermore, the inflexion point between Porod-regime and micropore shoulder is seldomly hit, which strongly affects the variables a and ‘ and thus the quantitative values of Vmic, qparticle and Sext. Therefore, to fit the data, the structural sections of the scattering curve were split into the range of power law dependence (Porod-regime) on the one hand and sum of micropore contribution with incoherent background on the other hand (Fig. 1). To minimize the effect of the experimenter, in both sections, Porod-regime and micropore contribution as well, the applied q-range was kept constant for each measurement of one individual sample.

3.

Results

All measurements were performed in high vacuum (p  6 Æ 105 mbar), independent of the history of the sample. Only the measurements with systematic exposure to water vapour

Time dependence of sample storage

In Fig. 2 the volume specific scattering cross sections of sample A measured at different periods of time after synthesis are shown. The first measurement was performed 3 months after the synthesis of the sample (Apr09), the second 7 months later (Nov09) and the third again with a time delay of another 7 months (Jun10). In the micropore region, the scattering intensity of the first measurement (Apr09, Fig. 2) shows the highest intensity, whereas the intensities of the later measurements reveal a continuous decrease with time passed since synthesis. The respective values derived for the sample density q, the micropore volume Vmic and the fractal exponent a are listed in Table 1.

3.2.

Heat and vacuum treatment

To make sure, that no adsorbate is left in the sample, in Jun10 as well as in Oct10, the samples were heat treated at 250 °C under vacuum (<1 mbar) for at least 6 h and then measured again. Fig. 3a shows the resulting changes in scattering intensity. In the Kratky-plot, the height of the plateau is significantly increased for the sample exposed to heat and vacuum treatment (HVT) directly before the measurement. A second effect is observed in the Porod regime (Fig. 3b), where the height of the scattering intensity after HVT treatment is slightly decreased compared to the intensity in the run without prior sample treatment, hence, a trend opposite to the one observed in the micropore regime (Fig. 3a). After HVT sample A was stored again for 24 h at ambient conditions in the laboratory and then measured again; Fig. 3c shows the related scattering curves. Here, the Kratkyplot shows no changes in the micropore range at q-values between 2.5 and 10 nm1 for sample A after conditioning in lab atmosphere. However, a difference is visible in the transition zone between Porod and micropore region at about 1 nm1.

3.3.

Relative difference in scattering

Depending on the representation chosen (V1[dr/dX](q); V1[dr/dX](q) Æ q2; V1[dr/dX](q) Æ q4 vs. q), differences between individual measurements become better visible. However, to show the differences of the whole scattering curve – depending on the q-scale and where they appear – it is more advantageous to plot the relative difference D

 1 dr    ½  ðqÞ  1 ½ dr  ðqÞ 1 dr ðqÞ ¼ V dX 1 1 dr V dX 2 V dX ½  ðqÞ V dX 1

ð11Þ

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Table 1 – Carbon samples investigated with information on the date and pre-treatment of the scattering experiment. Pretreatments performed prior to the measurement: (HVT) = heat and vacuum treatment at 250 °C and <1 mbar for at least 6 h. HVT+1d = Sample storage at ambient conditions in the laboratory for one day after HVT. Evaluated data are given for the apparent sample density q (from transmission), the apparent micropore volume Vmic, the fractal exponent a and the correlation length ‘. In fact, the data are not accurately enough to warrant three significant digits (q, Vmic, a, and ‘). However, they are provided to make variations better traceable. Date Sample A Apr09 Nov09 Jun10 Jun10_HVT Jun10_HVT+1d Oct10 Oct10_HVT Sample B Apr09 Nov09 Jun10 Jun10_HVT Jun10_HVT+1d Oct10 Oct10_HVT Sample C Nov09 Jun10 Jun10_HVT Jun10_HVT+1d Oct10 Oct10_HVT Sample D Mar07 Jun10 Jun10_HVT Jun10_HVT+1d Oct10 Oct10_HVT Sample E Mar10 Oct10 Oct10_HVT a b

q [g/cm3]

Vmic [cm3/g]

a

‘ [nm]

0.456 0.493 0.509 0.467 0.466 0.493 0.467

0.194 0.136 0.104 0.173 0.189 0.129 0.179

n.d.a 3.79 3.81 3.82 3.83 3.83 3.81

0.284 0.309 0.330 0.293 0.280 0.317 0.288

0.359 0.380 0.396 0.382 0.382 0.394 0.384

0.154 0.170 0.087 0.113 0.116 0.095 0.121

3.66a 3.59 3.52 3.51 3.52 3.55 3.54

0.362 0.392 0.441 0.394 0.385 0.419 0.384

0.450 0.468 0.425 0.431 0.438 0.429

0.089 0.058 0.107 0.106 0.081 0.104

3.78 3.81 3.78 3.82 3.82 3.78

0.337 0.384 0.338 0.329 0.365 0.336

0.409 0.460 0.436 0.435 0.453 0.435

0.089 0.057 0.074 0.074 0.067 0.074

3.65 3.69 3.69 3.70 3.71 3.71

0.455 0.463 0.414 0.415 0.426 0.406

0.285 0.293 0.281

0.376 0.283 0.410

3.81b 3.97 3.94

0.272 0.271 0.245

Smaller fit-range (only short sample-detector distance available). Reason for discrepancy to later measurements is unknown.

Fig. 2 – Kratky-plot of the scattering curves measured for sample A as a function of time passed since its synthesis in Jan09.

of the two curves to be compared after different conditioning. Hereby, the indices ‘‘1’’ and ‘‘2’’ represent two individual measurements of the same sample. For the run of sample A with HVT, deviation between the two scattering patterns occur over the whole scattering range (Fig. 4), taking positive as well as negative values, as already indicated in Fig. 3. When the experiment of heat and vacuum treatment is performed a second time about 4 months later (Oct10), the deviation shows the same trends in terms of the q-dependence, however, with a smaller amplitude. Comparing the HVT measurement with data of the same sample subsequently stored for 1 day at ambient conditions (HVT+1d) in the laboratory, a broad peak at the transition zone between Porod and micropore region at about 1 nm1 is clearly visible in Fig. 4. For the other samples, i.e. B–E, the trends observed for sample A also hold. However, the deviations are quantitatively different and

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Fig. 3 – Scattering curves for the untreated and the heat and vacuum treated (HVT) sample A in Jun10. (a) Kratky plot, (b) Porod-plot and (c) Kratky-plot directly after HVT and measured again 1 day later, after storage of the sample at ambient conditions (HVT + 1d).

3.5.

Fig. 4 – Relative difference of the scattering intensities measured of sample A for (j) (Jun10 – Jun10_HVT)/Jun10; (d) (Jun10_HVT – Jun10_HVT+1d)/Jun10_HVT and ( ) (Oct10 – Oct10_HVT)/Oct10.

furthermore a shift of the broad peaks on the q-scale may appear (e.g. for HVT+1d run).

3.4. Apparent density from X-ray transmission and fractal behaviour In Table 1 the apparent sample density q is given, which was determined from the sample thickness d, the attenuation coefficient for pure carbon (l/qcarbon) and the X-ray transmission T. From one to the next experimental session, the apparent sample density q increases continuously, unless HVT was applied. For the measurements performed on a specific sample directly after HVT, the agreement of the values derived (‘‘dry’’ sample density) is excellent and shows a deviation of less than 1% between individual measurements of the same sample at different dates (e.g. sample A, Jun10_HVT and Oct10_HVT). However, the ‘‘dry’’ sample density never drops to the initial apparent sample density determined shortly after the pyrolysis of the respective sample (see e.g. sample A, Apr09 and sample D, Mar07, respectively). In the power law dependence at small scattering vectors q, the fractal exponent a for the individual samples varies only slightly with sample conditioning, except for sample E.

Systematic wetting of microporous carbons

In Fig. 5 the Kratky-plot of sample F at different, but well defined relative humidity is shown. Hereby, desorption was performed at room temperature, starting at 100% RH and decreasing the relative humidity to the dry state with two intermediate steps at 57% RH and 43% RH (sample exposed to saturated salt solutions for at least 24 h), respectively. Decreasing the relative humidity increases the scattering intensity in the micropore regime (Fig. 5). However, in the Porod-regime, the scattering intensity shows the opposite trend. The peak at 4 nm1 is not due to the sample itself, but an artefact of the polyimide foil used as windows in the sample holder. The relative changes of the scattering intensity compared to the dry state of the carbon sample are shown in Fig. 6. The relative difference reaches a value of up to 60% for the measurement performed at 100% RH. In Table 2 the data derived for the fractal exponent a, the apparent density q and the apparent micropore volume Vmic are given as a function of the relative humidity. For the data evaluation the adsorbed water was ignored and the data were treated as the ones of a dry porous carbon structure.

4.

Discussion

With the experiments performed, different questions were addressed:

Fig. 5 – Kratky-plot of carbon sample F at different well defined relative humidities.

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Table 2 – Data determined for the apparent sample density q, the apparent micropore volume Vmic and the fractal exponent a for sample F, ignoring the adsorbed water phase.

Fig. 6 – Relative difference in scattering intensity for sample F for different well defined relative humidities. Here the reference scattering curve is the one of the dry sample (0% RH).

1. The characterization of microporous materials with SAS becomes more and more important, as it is a fast and noninvasive method. In publications applying SAS experiments on (micro)porous materials, on the one hand, usually detailed information on the SAS setup is given. On the other hand, information about pre-treatment of the sample itself or the environmental conditions during characterization is usually missing. Only few publications explicitly mention a pre-treatment, mostly publications, where in situ adsorption experiments are investigated with SAS. The applied pre-treatment comprises degassing under vacuum [15], heat treatment at different temperatures [16–19] and combinations of heat and vacuum treatment [20–23]. For in situ adsorption experiments, it is of course self-evident, that adsorbate present in the sample has to be removed before starting the measurement and the adsorption process. But the first question arises, if it is really necessary for all SAS experiments with (micro)porous materials. As micropores in carbon are expected to be partly or even completely filled with water at relative humidity above 50% to 60% [24] (micro)porous carbon samples should of course not be characterized at ambient conditions with SAS to avoid artefacts. Therefore, at many SAS experiments, a vacuum sample chamber is part of the beamline. Bock et al. [12] believed that due to the fact that the measurements are performed under vacuum ‘‘It was not necessary to heat the samples before the measurement to prevent water from adsorbing in the pores.’’ and provided some estimations for this assumption. However, if the scattering curves of one sample before and after the HVT (8 h; 250 °C; <1 mbar) are compared (Fig. 3), a distinct difference of the scattering intensity is visible in the Porod-range and even more pronounced in the micropore regime. The differences in the scattering curve between ‘as derived’ and HVT run are comparable to the trends observed in the systematic wetting (Fig. 5) between the scattering curves of the dry and the partially wetted samples. These findings suggest that without HVT, a relevant amount of adsorbate is present in the microporous carbons even when

RH%

q [g/cm3]

Vmic [cm3/g]

a

100 57 43 0 (HVT)

0.588 0.577 0.344 0.290

0.006 0.009 0.034 0.059

3.67 3.66 3.57 3.61

the sample is at high vacuum (6 Æ 105 mbar) for several ten minutes. The conclusion is confirmed by the apparent sample densities derived from the X-ray transmission determined in the different measurements (Tables 1 and 2). On the one hand, the results from the different experiments where HVT had been applied to a certain individual sample are almost identical with a relative error less than 1%, whereas the apparent sample density derived from X-ray transmission for the measurements performed directly before HVT is always clearly higher (up to 10%, Table 1). Only adsorbate in the sample can be the reason for the difference in apparent sample density. Also the relative difference plots of sample A (Fig. 4) and the systematic wetting of sample F (Fig. 6) show the same trends (ignoring the algebraic signs, as it only depends on the choice of measurement ‘‘1’’ and ‘‘2’’); these results are in line with the findings of Laszlo and co-workers [17,18,24]. The shape of the relative difference plots can be interpreted – according to Laszlo and co-workers – rather as a predominant cluster adsorption of water at oxygen surface groups than a volume filling of the micropores. Monolayer coverage on the external surface of the carbon is also improbable, as the fractal exponent a remains constant within the relative error (Table 1). Due to a density gradient on the external surface a decrease in fractal dimension would be expected in case of a surface layer formation. To get an impression, how fast SAS-detectable uptake of adsorbate takes place, which can not be removed by high vacuum, the samples were measured again with one day time delay and storage at ambient condition after HVT. The relative difference plots in Fig. 4 as well as the apparent sample density (Table 1) reveal an almost negligible detectable adsorbate uptake; only at q-values around 1 nm1 (Fig. 3c and Fig. 4) a small increase of scattering intensity is observed. Laszlo et al. [18] showed, that the increase in height of V1[dr/dX] at values at about 1 nm1 is a signature of clusters. Indeed, water adsorption on carbon surfaces is expected to take place first as cluster growth at oxygen surface groups and are, due to their energetic stability, the last adsorbate to be removed during desorption [25]. However, one day at ambient conditions does not significantly affect the values determined for the apparent sample density q and apparent micropore volume Vmic (Table 1) compared to the measurement directly after HVT. Nevertheless, even after one day at ambient conditions, the scattering curves themselves change slightly. 2. The second question was, whether the microporous carbons are ageing with time. At first glance, Fig. 2 seems to confirm this assumption, as the scattering height

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decreases with time over about a year. However, the adsorbate in the sample has first to be removed in order to avoid artefacts thus increasing the height of the scattering curves again. Nevertheless, if the data of q and Vmic of the first measurement of the samples (Mar07, Apr09, respectively) are compared with the data after HVT, the initial values of q and Vmic can not be reached again (Table 1). This is an astonishing result. Although the experimental setup of beamline B1 at Hasylab was strongly modified after Mar07 (detector, software for data preparation and pre-evaluation), the setup did NOT change after Apr09. So, an artefact due to a modified experimental setup can be excluded. The remaining difference between initial measurement and HVT is assumed to be also due to a difference in oxygen surface groups and/or persistent adsorbate, which can not totally be eliminated by the HVT applied. This interpretation is supported by a degassing experiment of sample C (initial mass m0 = 204.5 mg; accuracy ± 0.2 mg) at the degas station of a sorption analyzer (ASAP 2020, Micromeritics). The experiment reveals that the applied HVT may be too weak to remove all adsorbate. Fig. 7 shows, that after a total of 6 h at 200 °C, the degassing is clearly not complete. The mass loss Dm/m0 increases still verifyable, when increasing the timescale or the temperature is shifted to 300 °C. But even at 300 °C, the sample still looses mass for a longer period of time. So, overall the results suggest that the investigated carbon xerogels age and that SAS is sensitive to it. Similar effects can be expected at least for other microporous carbons such as activated carbons. However, the ‘‘ageing’’ at room temperature is not taking place at a timescale of hours, days or weeks, but moreover is a function of several months or even years of storage at ambient conditions. 3. The third goal of this study was to estimate the possible (systematic) errors, their effect on the evaluated data, if adsorbate was not (completely) removed and furthermore to establish some experimental recommendations for the proper characterization of (micro)porous materials with SAS techniques. If some residual adsorbate is present in the microporous sample during the SAS measurement, the following system-

Fig. 7 – Relative mass loss of sample C for increasing time and temperature used for degassing (m0 = 204.5 mg).

atic errors and their impact on the respective physical quantities are observed: i. transmission decreases ! V1[dr/dX] decreases ii. apparent sample density evaluated from transmission is too high ! m1[dr/dX] decreases iii. average density of primary particle WITH adsorbate is higher ! scattering contrast and thus V1[dr/dX] in the Porod region (power law dependence) increases iv. the incoherent scattering background B at large q-values increases v. when the micropores are partly filled with adsorbate, the correlation length ‘ (and thus the apparent micropore size) is changing; the net effect depends on the pores predominantly filled with adsorbate and their arrangement relative to larger micropores. Furthermore the value determined for Vmic decreases due to the smaller void fraction in the micropore regime. Indeed, there are opposite effects, that shift the scattering intensity to larger or smaller values (iii + iv vs. i + ii + v). Furthermore, the shape of the scattering curve is modified (iv and v). Also some of the listed effects partly compensate each other (i + ii vs. iii + iv), however, not improving the defective experimental data, if remaining adsorbate in the sample is ignored. The resulting error, e.g. in Vmic can take up to a factor of 2 (see sample C in Table 1).Therefore beamline responsibles and in particular materials scientists dealing with (micro)porous materials should follow some recommendations when performing SAS experiments, which are similar as for sorption experiments: – adsorbate in the sample must be removed by heating, preferably at temperatures that do not modify the sample itself, but >110 °C if possible (assuming, that at ambient conditions mainly water is adsorbed) – the applied temperature should be higher, if stable surface groups are present, which can act as nuclei for cluster adsorption – the combination of heating with vacuum surely improves the degassing – the sample must be placed in vacuum during the SAS experiment to avoid uptake of adsorptive during the measurement. The results showed, that adsorbate left in the sample can strongly influence shape and evaluated data of SAS experiments; this effect may also explain the differences between scattering curves derived from SAXS and SANS in the papers of Hall and coworkers [26,27]. They argued, that for SAXS, the electron density in the micropores is not zero and therefore responsible for a less distinct micropore shoulder. However, they give no information on sample treatment that might have removed residual adsorbate prior to the measurement, on the time delay between the two measurements or the conditions applied (ambient or vacuum) during the experiments at the different beamlines. So, the reason for the difference may be much simpler, especially as Hoell [28] observed a perfect correlation between SAXS and SANS with glassy carbon,

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a material with only closed micropores [29].In fact, the topic investigated in this work is very likely not limited to microporous carbon materials characterized with SAS. Indeed Hoinkis [20] stated for mesoporous solids (mainly silicas), that before applying SAS, ‘‘Degassing at elevated temperatures (  ) is essential’’, thus indicating that adsorptive will also be not negligible for mesoporous solids.

[6]

[7]

[8]

5.

Conclusions

The aim of this work was to investigate whether (micro)porous materials have necessarily to be subjected to a degassing step (vacuum and heat) to ensure proper data. The experimental findings within this study reveal, that heat and vacuum treatment (HVT) before the measurement must be applied to remove adsorbate from the sample; otherwise, the evaluated data can be highly erroneous and yield parameters that are deceptive up to a factor of 2 (e.g. the micropore volume). However, even after the HVT, apparent density q and apparent micropore volume Vmic of the microporous carbons investigated do not reach the initial values determined shortly after synthesis. The conclusion is that the investigated samples age over several months (or even years) by generating more and more stable oxygen surface groups, which can not be removed by the applied HVT. At these oxygen surface groups, predominant cluster adsorption of water is assumed to take place. Summarizing, the samples have to be degassed always before the SAS measurement – similar to sorption experiments. This conclusion is not limited to microporous carbons (xerogels, activated carbons) in combination with SAXS and SANS, but also evident for other porous materials like microporous silica.

[9]

[10]

[11] [12]

[13]

[14] [15]

[16]

[17]

[18]

Acknowledgements The authors thank Ulla Vainio, Ph.D. from the HASYLAB for her support, Dr. Matthias Wiener for providing sample E, Philipp Eitelwein for the SAXS measurements of sample F and Christian Balzer for executing the degassing experiment.

[19]

[20] R E F E R E N C E S

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