MWCNT nanoparticles deposited at moderate pressure and temperature supercritical carbon dioxide conditions

J. of Supercritical Fluids 157 (2020) 104706

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Thermal effects on hydrogen storage capacity of Pd/MWCNT nanoparticles deposited at moderate pressure and temperature supercritical carbon dioxide conditions Ebru Erünal a,∗ , Fatma Ulusal b a b

Chemical Engineering Department, C¸ukurova University, Adana, Turkey Chemistry Department, C¸ukurova University, Adana, Turkey

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• scCO2 deposition was performed • • • •

at 2500 psi and 80 ◦ C without cosolvent. The atmosphere change in heat treatment doubles the hydrogen uptake. Hydrogen uptake at room temperature is higher for samples heated under oxygen. Hydrogen uptake at elevated temperatures is higher for samples heated under nitrogen. Pd(111) and Pd(200) sites of heated samples trigger hydrogen adsorption.

a r t i c l e

i n f o

Article history: Received 25 August 2019 Received in revised form 25 November 2019 Accepted 26 November 2019 Available online 28 November 2019 Keywords: Hydrogen storage Multi-walled carbon nanotubes Thermal effect Supercritical carbondioxide deposition Temperature programmed reduction/desorption

a b s t r a c t Pd nanoparticles were deposited on MWCNT support for the first time at 80 ◦ and under 2500 psi scCO2 conditions via a bipyridyl precursor in the absence co-solvent. After deposition, the samples were heated under different atmospheres. Then their morphological and hydrogen uptake capacities were investigated. An average particle size between 7−9 nm was estimated via HR-TEM images while intense Pd(111) and Pd(200) sites were detected for all heated samples except no thermally treated one which exhibits PdHx sites. The highest hydrogen adsorption at room temperature was recorded for the samples heated under oxygen. At room temperature, Pd loading amount was found to be effective in means of captured hydrogen. Hence, at higher temperatures, hydrogen uptake of samples treated under nitrogen atmosphere was higher. Additionally, in case of the sample which was not thermally treated, a signal referring to hydrogen release between 200−300 ◦ C during reduction was detected. © 2019 Published by Elsevier B.V.

1. Introduction Supercritical carbon dioxide (scCO2 ) deposition method is a good and green alternative method to obtain supported metal

∗ Corresponding author. E-mail address: [email protected] (E. Erünal). https://doi.org/10.1016/j.supflu.2019.104706 0896-8446/© 2019 Published by Elsevier B.V.

nanoparticles with a much more controlled way in which neither hazardous solvents nor disposals are used. The low viscosity, zero surface tension and easier controllable density make scCO2 an important solution media [1–3]. In this method, the deposition is carried out in scCO2 medium to dissolve the metal precursor so that it can easily adsorb onto the support. For this reason, very high pressures (3000−4000 psi (20.7–27.6 MPa)) and temperatures (80−200 ◦ C) are employed to dissolve the metal precursor in scCO2 .

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After this step, reduction of the precursor to its metallic form is carried out thermally or by using reduction agents. The pressure, temperature, type of precursor and the reduction method have a strong effect on the metal amount, particle size and the distribution of nanoparticles on the support [3–8]. This provides to obtain effective catalysts in terms of dispersed nanoparticles on the support material [9]. Even though one can achieve dispersed nanoparticles at these high pressure and temperature conditions, in terms of energy saving during production processes, it is important to have desired materials at moderate pressure and temperature conditions. At this point, the nature and solubility of the organometallic precursor is crucial since not only playing deposition conditions but also changing the precursor influences the particle shape, size and final content of deposited nanoparticles [10]. Nearly none of the studies focus on to develop alternative precursors that provide to be used at moderate conditions for scCO2 deposition and reduction. Heretofore, acetylacetonate, cyclooctadiene, dithiocarbamate, hexamethylene glycol dimethyl ether, hexamethyltriethylene ligands, and their complexes are widely used and may be referred as conventional precursors for scCO2 deposition [1–16]. On the other hand, in a few studies, it was shown that the usage of so-called non-conventional precursors derived from vic-dioxime, bipyridyl complexes and 1,10-phenanthroline enable to get well dispersed supported Pd nanoparticles scCO2 deposition at moderate conditions [17–21]. Therefore, still less is known with the capacity of these precursors. In one of those works, deposition of Pd on multiwalled carbon nanotube (MWCNT) support through a bipyridyl derivative precursor was reported [21]. In this study, to see different insights of this precursor, scCO2 deposition pressure was lowered from 3000 psi (20.7 MPa) to 2500 psi (17.2 MPa) which is applied for the first time in similar deposition studies without the usage of co-solvent. Moreover, Pd loading amount and heat treatment effects on hydrogen storage capacity of the obtained materials were investigated in detail.

Fig. 1. ScCO2 deposition reactor system:(1) CO2 syringe pump, (2) H2 /CO2 mixing unit, (3) inlet valve, (4) water circulator for cooling, (5) vessel, (6) heating jacket, (7) MWCNT, (8) 2,2 -bipyridyl Pd precursor, (9) Deposited Pd nanoparticles, (10) magnetic stirrer, (11) safety disc, (12) vent.

(17.2 MPa) (2,3) and sent to the reactor (5) for chemical reduction of precursor on MWCNT. The system was stirred for 24 h under these conditions. Then, the gaseous mixture was slowly released (12). The obtained materials (7,9) were washed and dried. One set of the samples were calcined at 400 ◦ C (under O2 atmosphere) for 2 h, one set was treated at the same temperature under N2 atmosphere for 2 h while one set was not exposed to heat treatment at all. The whole processing sequence is given in Fig. 2. 2.3. Material characterization and hydrogen storage experiments

2. Experimental 2.1. Materials MWCNT, with a purity of ≥ 95 % and a mean diameter between 6–9 nm, was purchased from Sigma-Aldrich. 2,2 bipyridyl (C10 H8 N2 ) and Palladium chloride (PdCl2 ) with 99 % purities were purchased from ABCR GmbH, separately. No further purification was applied on these substances. Ultrahigh pure grade (99.999 %) of hydrogen (H2 ), argon (Ar) and helium (He) gases (purchased from Linde) were used for adsorption and desorption experiments. 2.2. Process conditions The details of [Pd(Pyr)2 ]Cl2 precursor synthesis are given in Erünal et al. [21]. The deposition conditions were 2500 psi (17.2 MPa) CO2 and 80 ◦ C. After deposition, the reduction was carried out under H2 pressure. The samples were prepared according to 1 % and 3 % of Pd loading amounts on 300 mg of MWCNT separately with the appropriate amounts of [Pd(Pyr)2 ]Cl2 precursor. The experimental setup can be seen in Fig. 1. The detailed deposition procedure is described according to the equipment numbers shown on Fig. 1. The reactor vessel (5) was heated with a circulating heater/cooler (4,6) gradually up to 80 ◦ C. Then, 2500 psi (17.2 MPa) CO2 gas was charged to the system with an Isco 260D syringe pump (1) under continuous stirring (10) at 1100 rpm. The reactor pressure was decreased to 2000 psi (13.8 MPa) after one hour. Next, a 10 ml mixture of H2 (23 mmol) and CO2 gas was prepared in a high pressure vessel of 2500 psi

The quantitative analysis of palladium was done by ICP-OES (Perkin Elmer 2100 DV). Particle size and crystal structure were analyzed with a Rigaku Miniflex X-Ray Diffraction (XRD) instrument (CuK˛, ␭ = 0.154 nm). High resolution transmission electron microscope (HR-TEM) images were taken via a Jeol 2100 F (200 kV) HR-TEM instrument. The surface area (BET) and average pore sizes (BJH) were determined through N2 adsorption–desorption isotherms recorded at 77.4 K with a Micromeritics Tristar II 3020 instrument. Before the analysis, the materials were degassed under vacuum at 300 K for 3 h. A home-built high-vacuum glass manifold equipped with an MKS Baratron Gauge (up to 100 Torr pressure), ACE Glass high vacuum Teflon valves and a Pfeiffer vacuum pump were used for hydrogen chemisorption experiments at room temperature while Temperature Programmed Reduction (TPR) and Temperature Programmed Desorption (TPD) analysis were carried out on a Micromeritics 2720 Instrument. The chemisorption tests were conducted between 0.5−100 Torr (0.066−13 kPa) range pressures. For TPR tests, 25 ml/min of a gas mixture composed of 10 % H2 -90 % Ar was used low between room temperature and 900 ◦ C while only He gas was used in the same temperature range for TPD measurements with a temperature ramp of 5 ◦ C/min for each case. 3. Results and discussions 3.1. Material characterization Palladium loading amounts were determined around 2.94 % and 0.79 % for 3 % and 1 % Pd/MWCNT, respectively, via ICP-OES analysis. XRD results are given in Fig. 3. Since the heated samples showed similar spectra, only samples treated under inert atmosphere are

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Fig. 2. Process diagram of Pd/MWCNT preparation.

Fig. 3. XRD spectra of (a) 2.94 % (higher) Pd loaded MWCNT with heat treatment (b) 2.94 % (higher) Pd loaded MWCNT without heat treatment (c) 0.79 % (lower) Pd loaded MWCNT after heat treatment (d) Pristine MWCNT.

shown. As Pd ratio increases, peaks corresponding to 40.3◦ , 46.9◦ , and 68.4◦ become more distinguishable. The related diffraction peaks correspond to Pd(111), Pd(200), and Pd(220), respectively [22] according to JCPDS card number 46-1043. Plus, regardless of heat treatment, for higher (2.94 %) Pd loaded samples, Pd(311) and Pd(222) centers can be observed clearly. The peaks around 26◦ and 43◦ are due to pristine MWCNT material C(002) and C(100) centers [3] as can given in Fig. 3. The nanoparticle sizes estimated via Scherrer equation according to Pd(111) peaks were found around 14.4 nm with an error of 0.96 % for higher Pd loaded material and 13.6 nm with an error of 1.02 % for lower Pd loaded materials. The error was calculated considering the microstrain broadening with the help of full width at half maximum (FWHM) values. The effect of heat treatment is seen strongly due to the peaks exhibit

at 38.56◦ , 44.8◦ , 65.34◦ and 78.2◦ which are assigned to PdHx (111), PdHx (200), PdHx (220) and PdHx (311) centers, respectively [23–26] (JCPDS card number 84-0448) for the non-heated samples. Hence, by thermal treatment, the formation of metallic Pd peaks become sharper while the peaks related to PHx centers -that can be identified on non-heated material- vanish. It shows that even under inert atmosphere, heat treatment helps to get rid of adsorbed hydrogen on Pd centers. HR-TEM images at 20 nm of lower and higher Pd loaded MWCNT are given in Figs. 4(a) and 4(b) together with their calculated average size distributions. Detailed images at 10 and 5 nm of Pd loaded MWCNT are also given in Fig. 4(c) and (d). A homogenous distribution without agglomeration on MWCNT surface can be seen in both figures. According to 146 counted particles, the average size was calculated as 6.96 ± 1.82 nm for lower Pd loaded material while it was estimated approximately 7.09 ± 1.98 nm for higher Pd loaded material due to 113 counted particles. Some of the identified particles were marked on the images. As found on Fig. 4(d), the distances between Pd lattice fringes were calculated around 0.234 nm which corresponds to Pd (111) geometry when compared with literature data [27]. The nitrogen adsorption–desorption isotherms recorded at 77.4 K are shown in Fig. 5 (a) and (b) for higher and lower Pd loaded MWCNT. According to these analyses, the Brunauer-Emmet-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore size distributions were evaluated as given in Fig. 6. Decorating MWCNT with Pd ions seems to increase the surface area of pristine MWCNT. A slight increase of surface area is observed when Pd loading increases. This shows that Pd loading on MWCNT would enhance the surface morphology in favor of catalytical activities. Thus, there have been various discussions about synergistic effect between Pd and MWCNT in terms of catalyzing the migration of hydrogen atoms from the surface through metal catalyst particles onto the catalyst support [21,28–30]. Also, at low Pd loading increase of surface area seems advantageous since an important issue in many applications is to keep the total catalyst weight not so high due to the loaded metal particle

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Fig. 4. HR-TEM images of 0.79 % Pd/MWCNT at (a) 20 nm (c) 10 nm (d) 5 nm (b) HR-TEM image of 2.94 % Pd/MWCNT at 20 nm, (a) and (b) includes calculated average size distribution of particles.

Fig. 5. N2 adsorption–desorption isotherms of (a) 2.94 % Pd/MWCNT and (b) 0.79 % Pd/MWCNT at 77.4 K.

amount. Hence, the contact area between catalytically active centers and support must be maximized by increasing the dispersion of metal particles [31]. Accordingly, the increase in surface area with increasing Pd amount reveals the dispersion of Pd nanoparticles on MWCNT surface. Consequently, a decrease in pore size

diameter which is also affected by dispersion as a function of increased Pd load amount is inevitable. All in all, this feature doesnot seem to have a direct effect on the adsorption performance of higher Pd loaded material as discussed in the next section.

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Fig. 6. BET surface area and BJH pore sizes of materials as a function of Pd loading amount.

3.2. Hydrogen storage capacity The adsorption-desorption hysteresis at room temperature of pristine and Pd loaded MWCNT prepared with different heating

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atmospheres are given in Figs. 7(a) and (b). In general, the aim of chemisorption studies is to find the dispersion degree of active metals on supports. However, to achieve it, different types of gases like CO, O2 , and H2 should be employed [32,33]. Nevertheless, in this work, hydrogen chemisorption measurements were used to interpret the hydrogen storage capacity of Pd/MWCNT materials at room temperature. In order to conduct these tests, the materials were exposed to very low hydrogen pressures (up to 100 Torr =0.0133 MPa) at room temperature. The calculated cumulative hydrogen uptake according to these results can be found in Table 1. The drastic effect of heat treatment regardless of Pd loading amount can easily be seen there. For both 2.94 % and 0.79 % Pd loaded MWCNT materials, hydrogen uptake doubles if heating atmosphere is changed from N2 to O2 . Therefore, when only room temperature uptake performances are considered, among heat treatments, heating under O2 atmosphere (calcined) seems to be more effective than heating under inert (N2 ) atmosphere. For 2.94 % Pd loaded materials, the cumulative hydrogen adsorption per g sample was calculated as 380 ␮mole H2 for the calcined sample and 196 ␮mole H2 for the sample heated under inert atmosphere. Similarly, for 0.79 % Pd loaded materials, the cumulative hydrogen

Fig. 7. Room temperature adsorption and desorption behavior of (a) 2.94 % (higher) Pd loaded MWCNT heated under O2 or N2 atmospheres (b) 0.79 % Pd loaded MWCNT heated under O2 or N2 atmospheres and pristine MWCNT as a function of heat treatment. The samples heated under O2 atmosphere are shown with filled symbols according to the loading amounts while samples heated under N2 atmosphere are shown with unfilled symbols. Solid lines designate adsorption curves, dashed lines are given for desorption curves.

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Table 1 H2 uptake amount of MWCNT as a function of Pd loading and heat treatment. Pd load (%)*

2.94 0.79 0 * **

ICP-OES measurements. 400 ◦ C for 2 h.

Heat Treatment Atmosphere**

H2 Uptake (␮mole H2 /g sample)

O2 N2 O2 N2 –

380 196 110 67 44

adsorption of the calcined sample was around 110 ␮mole H2 /g sample while the sample treated under inert atmosphere was around 67 ␮mole H2 /g. Moreover, pristine MWCNT was recorded only 44 ␮mole H2 /g. It was already reported that MWCNT materials mostly exhibit physisorption of hydrogen which disables higher uptake of hydrogen due to weak van der Waals interactions at ambient temperature and moderate pressure [34–37]. Besides, it was later shown that further modifications of MWCNT with transition metals may enhance the hydrogen uptake capacity [28–30]. However, as seen from the measurements, hydrogen uptake amount was similar to pristine MWCNT hydrogen uptake amount if Pd loading amount is below 1 % and the heat treatment is conducted under inert atmosphere. Thus, as soon as the heat treatment atmosphere

Fig. 8. Heating cycle of temperature programmed reduction (TPR) of pristine MWCNT, 0.79 % (lower) Pd loaded and 2.94 % (higher) Pd loaded MWCNT that were prepared with diferent heating atmospheres (a) Initial hydrogen desorption around 60 ◦ C due to Pd loading regardless of heat treatments (b) Increase in hydrogen uptake amount as a function of increasing temperature.

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was changed from N2 to O2 , H2 uptake amount increased to 110 ␮mole H2 /g sample, nearly three times more than pristine MWCNT even though loading Pd amount was below 1 %. A similar trend, as a function of heat treatment can be seen for 2.94 % Pd loaded MWCNT. As Pd was loaded nearly up to 3 % and the sample was calcined (under O2 ), a drastic hydrogen uptake was recorded by nearly 9 times more than pristine MWCNT. These results are in accordance with literature which shows the enhancement of hydrogen uptake of MWCNT by depositing Pd on it but also shows the importance of heat treatment to achieve this. Moreover, in literature there have been various discussions about this synergy between Pd and MWCNT in terms of spillover effect which is described simply as the migration of hydrogen atoms from the surface through metal catalyst particles onto the catalyst support [21,28–30]. In terms of crystallographic information (Fig. 3), since the highest uptake amounts were obtained from higher Pd loaded MWCNT, it can be interpreted that the increase in Pd(111) sites might trigger the intense spillover effect when Pd loading, consequently, Pd(111) sites on MWCNT increase. On the other hand, unlike adsorption measurements, the heat treatment showed no effect on the desorption behavior at room temperature (Fig. 7(a) and (b)). After desorption takes place, 2.94 % and 0.79 % Pd loaded MWCNT materials still capture nearly 100 ␮mole H2 /g and 40 ␮mole H2 /g sample, respectively regardless of heating atmospheres. Therefore, the heat treatment effect can be thought to catalyze the adsorption process at room temperature but not effective for the desorption process. It was predicted that the uniform diffusion length through uniformly distributed Pd particles over the support would result in similar rates of recombinative hydrogen desorption and diffusion of hydrogen back to surface [21]. But, obviously differences in the heat treatment affect the rate of dissociation of hydrogen molecules to hydrogen atoms and the migration of these dissociated hydrogen atoms on and into the MWCNT. All in all, the highest hydrogen desorption capacity of calcined samples was calculated nearly 180 ␮mole/g sample for 2.94 % Pd and 70 ␮mole/g sample for 0.79 % Pd loaded MWCNT. The temperature dependent hydrogen storage capacities were estimated via temperature programmed reduction (TPR) and desorption (TPD) measurements. Because the TPR/TPD instrument gives an electrical signal intensity related to gas amount detected for each sample, all spectra were normalized according to each sample’s highest signal intensity and then compared with each other in Figs. 8 and 9. At higher temperatures, hydrogen adsorption and desorption follows a more interesting trend.. In general, during TPR experiments, as the reduction capacity of the material increases at relatively lower temperatures below 100 ◦ C, an increase of the area under the obtained TPR curve is formed. However, a reverse peak was recorded around 60 ◦ C for all Pd loaded MWCNT samples as shown on the normalized TPR heating cycle measurements given in Fig. 8(a) and (b). This peak presents an initial hydrogen gas release. It was attributed to two possibilities in literature as the desorption of weakly adsorbed hydrogen from the Pd surface, and decomposition of the ␤-PdH phase formed at room temperature by diffusion of hydrogen into the Pd crystallites [38,39]. In general, ␤ phase hydrogen is defined due to Van’t Hoff plots as the higher concentration phase that appears when a surface is exposed to hydrogen atmosphere and hydrogen starts to nucleate in the system. First, formation of a solid solution phase called as ␣-phase occurs because of slowly dissolved hydrogen atoms on metal particles. Then, the accumulation of hydrogen in the interstitial sites and increase in concentration will end up in ␤ phase hydrogen [31,40]. Therefore, this peak might rise both as a result of capturing hydrogen very fast due to catalytically active Pd sites on MWCNT support so that after dilution at this temperature some hydrogen gas was released and/or due to the decomposition of hydrogen on the PdHx sites. Even though in the XRD analysis (Fig. 3) only the sample

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Fig. 9. Heating cycle of temperature programmed desorption (TPD) of pristine MWCNT, 0.79 % (lower) Pd loaded and 2.94 % (higher) Pd loaded MWCNT prepared by heating under different gas atmospheres at 400 ◦ C for 2 h.

that was not heated during processing gave PdHx centers, those centers might be hindered due to the intense metallic Pd peaks for the heated samples. Consequently, the absence of hydrogen release peak around 60 ◦ C during TPR for pristine MWCNT material supports the immediate hydrogen uptake on the Pd sites. The amount of the released hydrogen of calcined (heated under O2 ) sample was estimated around 118 ␮mole H2 /g sample for 2.94 % Pd/MWCNT and 8.38 ␮mole H2 /g sample for 0.79 % Pd/MWCNT. When the TPR of non-heated material is investigated, the extra peak around 250 ◦ C which corresponds to another gas release may be strongly attributed to the PdHx centers that were quite distinctive on XRD spectrum. Hence, the initial hydrogen gas release of the other heated materials might be due to the immediate hydrogen uptake rather than PdHx sites. A more detailed surface analysis is necessary to enlighten this phenomenon. Even though TPR measurements were done under different conditions and materials prepared with different techniques in literature [41–43], Yoo et al. [43] also pointed out the dependence of hydrogen uptake capacity with annealing temperature. In this sense, their outcomes are also in agreement with this study. Lastly, another important outcome of the heating period of TPR measurements is; as temperature increases during reduction, the hydrogen uptake is higher for the samples heated under nitrogen atmosphere. This situation is the contrariwise of the room temperature chemisorption measurements. This can be referred to the faster reduction than hydrogen dissociation at higher temperatures and pressures. The heating cycle of temperature programmed desorption (TPD) measurements under He flow are given in Fig. 9. A decreased peak around 40 ◦ C tailing until 100 ◦ C for pristine MWCNT can be tracked. This sharp decrease which is related to hydrogen desorption at moderate temperatures, cannot be detected for Pd/MWCNTs. Apart from that, similar desorption peaks at higher temperatures above 600 ◦ C were obtained for pristine MWCNT and Pd/MWCNT which were heated under either O2 or N2 atmosphere. Only the material without heat treatment lacks of this distinct peak around 600 ◦ C. Thus for all heat treated samples, strong peaks during TPD can be detected at higher temperatures. The hydrogen desorption at these high temperatures was associated with the aromatization of C C bonds saturated during the hydrogen storage period as discussed by Erünal et al. [21]. A strong desorption peak with 0.79 % Pd/MWCNT (heated under N2 atmosphere) is observed closer to 700 ◦ C. How-

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ever, if Pd loading amount is increased, much stronger peak can be observed. Besides, the desorption temperature seems decrease from 700 ◦ C to 600 ◦ C with the increasing Pd amount. Hence, the signal seems saturated and the peak point cut due to exceeding the detection limit of the detector for this sample when compared with others. Furthermore, similar to TPR case, at higher temperatures, regardless of Pd amount, samples treated under N2 atmosphere have stronger peaks when compared to samples treated under O2 atmosphere. Again unlike room temperature desorption measurements, TPD results show a significant change in terms of heat treatment and desorbed hydrogen amount. All in all, this result can be expected due to the exponentially increasing hydrogen uptake with increasing measurement temperatures. 4. Conclusions Pd was successfully loaded in different amounts on MWCNT supports through scCO2 deposition with non-conventional bipyridyl precursors that provide deposition at 80◦ and under 2500 psi (17.2 MPa) without using co-solvent. Actually Pd/MWCNT nanoparticles were prepared for the first time at these relatively moderate scCO2 conditions in the absence co-solvent. The obtained particle sizes were estimated between 7−9 nm via HR-TEM images. The enhancement of hydrogen uptake as a function of Pd loading amount was observed together with the enhancement of surface areas. XRD measurements verified the increased intensity of Pd(111) sites due to the increase in Pd amount. Those sites were attributed to trigger spillover effect leading to higher hydrogen uptakes. Apart from that, PdHx sites were detected on the nonheated samples. The room temperature adsorption-desorption and also TPD-TPR measurements confirmed that the heat treatment method has a strong influence in the hydrogen uptake capacity materials as much as Pd loading amount. Hence at room temperature, the calcined (heated under O2 ) samples showed a higher hydrogen uptake while at higher temperatures, the samples heated under N2 atmosphere had much more enhanced hydrogen uptake. Moreover, all materials exhibit an initial H2 decomposition regardless of the Pd loading amount and also heat treatment. Only the samples without heat treatment did not have any strong peak which was observed for all other heated materials including pristine MWCNT during TPD experiments at higher temperatures above 600 ◦ C. As a result, thermal treatment affects hydrogen storage and release behavior drastically at room and elevated temperatures for Pd/MWCNT nanoparticles prepared at moderate scCO2 conditions through bipyridyl Pd precursors. Authorship conformation form All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version. Declaration of Competing Interest None. Acknowledgements Prof. Dr. Ramazan Esen for XRD, Selda Odabas¸ı and Mustafa Y. Aslan for assisting in adsorption-desorption and Atalay C¸alıs¸an for BET measurements are greatly acknowledged. Authors are also deeply thankful for the endless supports of Prof. Dr. Bilgehan Güzel and Prof. Dr. Deniz Üner.

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