Hydrogen Storage in Untreated/Ammonia-Treated and Transition Metal-Decorated (Pt, Pd, Ni, Rh, Ir and Ru) Activated Carbons

applied sciences

Article Hydrogen Storage in Untreated/Ammonia-Treated and Transition Metal-Decorated (Pt, Pd, Ni, Rh, Ir and Ru) Activated Carbons

Mohamed F. Aly Aboud 1,2,* , Zeid A. ALOthman 3 and Abdulaziz A. Bagabas 4

1 Sustainable Energy Technologies Center, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia 2 Mining, Metallurgical and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City, Cairo 11371, Egypt 3 Advanced Materials Research Chair Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia; [email protected] 4 National Petrochemical Technology Center (NPTC), Materials Science Research Institute (MSRI), King Abdulaziz City for Science and Technology (KACST), P.O. Box 6086, Riyadh 11442, Saudi Arabia; [email protected] * Correspondence: [email protected]; Tel.: +966-11-467-6832; Fax: +966-11-469-7122

Abstract: Hydrogen storage may be the bottle neck in hydrogen economy, where hydrogen spillover is in dispute as an effective mechanism. In this context, activated carbon (AC) was doped with nitrogen by using ammonia gas, and was further decorated with platinum, palladium, nickel, rhodium, iridium and ruthenium, via an ultrasound-assisted impregnation method, with average particle sizes of around 74, 60, 78, 61, 67 and 38 nm, respectively. The hydrogen storage was compared, before and after modiﬁcation at both ambient and cryogenic temperatures, for exploring the spillover Citation: Aboud, M.F.A.; ALOthman, effect, induced by the decorating transition metals. Ammonia treatment improved hydrogen storage Z.A.; Bagabas, A.A. Hydrogen at both 298 K and 77 K, for the samples, where this enhancement was more remarkable at 298 K. Storage in Untreated/Ammonia- Nevertheless, metal decoration reduced the hydrogen uptake of AC for all of the decorated samples Treated and Transition Metal-Decorated other than palladium at cryogenic temperature, but improved it remarkably, especially for iridium (Pt, Pd, Ni, Rh, Ir and Ru) Activated Carbons. Appl. Sci. 2021, 11, 6604. and palladium, at room temperature. This observation suggested that metal decoration’s counter https://doi.org/10.3390/app11146604 effect overcomes hydrogen spillover at cryogenic temperatures, while the opposite takes place at ambient temperature. Academic Editor: Konstantinos Spyrou Keywords: activated carbon; metal decoration; hydrogen storage and spillover

Received: 24 May 2021 Accepted: 7 July 2021 Published: 18 July 2021 1. Introduction Hydrogen possesses a gravimetric energy density of ~120 MJ kg−1, is abundant, and Publisher’s Note: MDPI stays neutral has an environmentally friendly nature, as it is derived from renewable resources, making with regard to jurisdictional claims in it more economically convenient in comparison to many other fuels [1,2]. It can also replace published maps and institutional afﬁl- conventional fuels in industrial and home applications, either to offer high temperatures iations. upon burning and/or to catalyze hydro-cracking in petrochemical industry [3,4]. Using hydrogen as an energy source in automobiles releases no harmful emissions and greatly reduces carbon dioxide emissions [5]. The reported travel distance range for a fuel cell electric bus (FCEB) is between 100–225 miles for fueling events, depending on Copyright: © 2021 by the authors. fulﬁlling of the set-point pressure of 350 bars of hydrogen storage tanks; these are still Licensee MDPI, Basel, Switzerland. lower than the ultimate target of DOE of 300 miles [6]. Hydrogen has the drawback of a This article is an open access article low density (0.08 g L−1) at STP, and thus a large volume for high-pressure tanks is required distributed under the terms and in the abovementioned application [7,8]. In addition, “some serious safety problems” are conditions of the Creative Commons faced when it is stored under high pressure, in case of automobile accidents [9]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Appl. Sci. 2021, 11, 6604. https://doi.org/10.3390/app11146604 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 6604 2 of 18

Liquefaction techniques can be used at ambient pressure to store hydrogen as a liquid at 21.2 K, but hydrogen boiling creates technical hitches in addition to the complexity and large charge of the cryogenic tanks [9–11]. An alternative storing method is to store hydrogen as hydrides, whose chemical bonds stabilize hydrogen thermodynamically, and hence a large amount of energy is needed for its release during desorption processes, creating serious technical issues regarding the heat exchange management and its complex coupling to the storage system [12]. As an example, a temperature of 561 K is needed to dissociate MgH2 to release hydrogen. Furthermore, its formation kinetics are very slow because of the difficult dissociation of hydrogen molecules over the surface of magnesium metal to form a MgH2 compound [13,14]. Moreover, stress can be accumulated within the storage reactors due to densification and pulverization of hydrides upon hydrogenation cycles [15]. Furthermore, safety and hazard has to be considered when dealing with some hydrides such as NaAlH4 [16], while cost has to be considered for the heat transfer management and safety requirements [14,17]. Physical adsorption, in porous materials, occurs when the pores generate weak interaction energy between the adsorbate gas molecules and their walls, offering several benefits to overcome shortcomings of the aforementioned methods [18]. Various porous adsorbents such as zeolites [19], metal-organic frameworks (MOFs) [20], and activated carbons (ACs) [21] are good candidates for physical adsorption due to their high surface area. ACs may be the most suitable porous materials due to their abundance, intoxication, stability, cheapness, low density and their high surface area [22–27]. In addition, they can be synthesized from waste materials and from natural products which would be beneficial for the environment [28,29]. During the last decades, several theoretical and experimental works were performed to evaluate H2 uptake in different carbon materials such as nanotubes, nanofibers, activated carbons, etc. for selecting the most adequate receptor for hydrogen storage applications. Starting with carbon nanotubes (CNTs), it was reported that hydrogen uptake was favored at the end sites of the CNTs and on their outer surface sites in comparison to the inner surface (endohedral) sites within the tubes [30,31]. The researchers were attracted toward the use of CNTs for hydrogen storage when Dillon et al. [32] reported a high storage percentage of hydrogen (~10 wt.%) on SWCNTs at room temperature. Later, Liu et al. [33] claimed a value of up to 5.0 wt.% with pretreated SWCNTs. For MWCNTs, the situation did not vary too much in comparison to the SWCNTs, where the highest value of 2.0, 3.7, 4.0, and 4.6 weight percentages were reported [34,35]. Many scientists tried to reproduce these claims, but unfortunately, they were not reproducible [36–40]. A pessimistic conclusion about the storage limit in CNTs was reported to be 1.0 wt.% or even lower, while any claims above this limit were a form of experimental error [41–44]. Although the activated microporous carbon was reported to meet the DOE’s target [45–48], and the hydrogen storage claim of 23.76 wt.% was reported by Zhan et al. [49], others disputed these claims [34,50,51]. The biggest excitement was caused by Chambers et al. [52], when he claimed a room- temperature hydrogen storage capacity of 67 wt.% on graphite nanofibers (GNF). Cham- ber’s results were not reproducible, but some others claimed some values up to 10 wt.% [53] and 6.5 wt.% [54]. Such discrepancies, which were found in these reports, could be at- tributed to the problems of assessing hydrogen uptake and non-standardization of the techniques used for the evaluation, besides the interpretation of the experimental data. The main factors limiting the storage of hydrogen on the surface of carbonaceous materials are the low adsorption energy (~4–8 kJ mol−1) for the majority of the porous carbons and the weak interactions among hydrogen molecules, which decrease the possibility formation of more than one layer of hydrogen [55–59]. Appl. Sci. 2021, 11, 6604 3 of 18

Decoration of the graphitic surface with metals may improve the hydrogen adsorption by creating strong interaction sites. Furthermore, these metal centers can facilitate the dissociation of the hydrogen molecules to atomic hydrogen, and, subsequently, making them diffuse onto the graphitic skeleton through the spillover mechanism [28,48,60–66]. However, the spillover effect was a dilemma for long time between theoretical and experimental studies due to hydrides formation [67–70], in addition to the inactivity of the metal surface due to clustering [71], hampering the spillover effect. Many carbonaceous systems were studied to explore the spillover effect in hydrogen storage and hydrogenation [72–78]. Modifying carbonaceous structures by hetero-atoms, such as boron or nitrogen, before metal decoration, was suggested to enhance their catalytic effect in comparison to the unmodiﬁed ones [71,79–89]. Previously, we tested platinum-decorated, ammonia-treated activated carbons for carbon dioxide and methane gas storage [90]. They showed high storage capacities for both of these, with slight improvement upon ammonia treatment and more preference towards carbon dioxide gas storage. The ammonia treatment was used as a doping source for nitrogen, and acted as an erosion agent for enhancing the surface area of the activated carbons in addition to its role in removing impurities and amorphous carbon before the platinum metal decoration step [88,90,91]. In this work, we further tested other transition metal-decorated (Pd, Ni, Rh, Ir and Ru) AC samples for hydrogen storage, at cryogenic and ambient temperatures, before and after modiﬁcation. Furthermore, we aimed to investigate the hydrogen spillover phenomenon and its dependence on the identity of the metal at the studied two temperatures.

2. Experimental Section Norit RX 1.5 Extra activated carbon, a product of CABOT, was used as carbonaceous support for transition metal decoration. The metal precursors were: hexachloroplatenic acid (H2PtCl6 H2O, 99%, WinLab, Pt 39.96% min.), palladium acetate [Pd(O2CCH3)2, 99%, Aldrich], nickel acetate tetrahydrate [Ni(O2CCH3)2 4H2O, 99% Merck], rhodium(III) 2,4-pentanedionate (C15H21O6Rh, 99.99%, Alfa Aesar, Rh 25.2% min.), iridium(III) 2,4- pentanedionate (C15H21IrO6, 99.99%, Alfa Aesar, Ir 37.5% min.), and ruthenium(III) 2,4- pentanedionate (C15H21O6Ru, 99.99%, Alfa Aesar, Ru 24% min.). The ultrasound-assisted impregnation method was adopted because of its simplicity and large-scale applicabil- ity [45,46,92–94], where the NH3-treated or untreated activated carbons were dispersed within the dissolved metal ions to form 5.0 wt.% decorated metal/AC. De-ionized water was used as the dissolving medium for Pt and Ni salts, while the precursors of Pd, Ru, Rh and Ir were dissolved by using ultra-high purity benzene as a solvent. The synthesis method, structure, morphology, textural, elemental analysis (CHNS), and experimental details were reported by Aboud et al. [90]. Additionally, JEOL (JSX-3202M) with built–in energy-dispersive ﬂuorescence X-ray spectrometer (EDS) and high-resolution ultra-thin window (UTW) detector, for C-peak detection, was used for elemental analysis. The Debye– Scherrer formula, L (2θ) = (K × λ)/(β × cosθ)[95], was used to estimate the crystallite size of the decorating metals, where: L is the crystallite size, λ is the X-ray wavelength (Cu Kα radiation = 0.15405 nm), K is constant and equals to 0.94, θ is the angle of diffraction, and β is the full width at half maximum (FWHM) at 2θ multiplied by π/180 after considering the broadening contributions from instrument and other factors, which was around 29%. If we did not consider such a broadening effect, the uncorrected crystallite size, calculated from uncorrected β, would be smaller by a factor of 3.4. We used at least 5 diffraction peaks for calculating the average crystallite sizes, and we compared this average with the average particle sizes, obtained from scanning electron microscope (SEM) images. Appl. Sci. 2021, 11, 6604 4 of 18

Appl. Sci. 2021, 11, x FOR PEER REVIEW The hydrogen storage measurements were conducted via the volumetric technique4 of 19

(Sievert’s method) by using a high-pressure volumetric analyzer (HPVA-II, Micromeritics), which could reach up to 200 bars. The hydrogen uptake was assessed by measuring the sorption isotherms (pressure–composition-isotherms (PCIs) at 77 K and 298 K, and within where the resulting mass loss was determined, and the degassed mass was considered to a pressure range from ambient to 100 bars. A 3-litre, well-insulated Dewar flask was used investigate the hydrogen uptake. The adsorbent specimen volume was corrected on the for performing the measurements at 77 K, where one filling of the flask was sufficient basis of skeletal density measurements, corresponding to the excess values obtained by during the experiment. The mass of the adsorbent specimen was not less than 2.0 g for helium gas. The specimen holder was dipped in liquid nitrogen at 77 K and was kept for reducing the experimental errors as much as possible. Before any sorption measurement, at least 45 min before performing the measurements at cryogenic temperature. Ultra-high- the adsorbent specimens were degassed for 12 h, at 200 ◦C, under vacuum, where the purity hydrogen gas grade (6N) was used in all adsorption experiments. The PCI curves resulting mass loss was determined, and the degassed mass was considered to investigate were measured several times to verify the hysteresis and the repeatability. All of the ad- the hydrogen uptake. The adsorbent specimen volume was corrected on the basis of sorbent specimens exhibited good repeatability. All of the uptakes were evaluated as ex- skeletal density measurements, corresponding to the excess values obtained by helium cess gas adsorption quantities, which were the change in the amounts of the hydrogen gas gas. The specimen holder was dipped in liquid nitrogen at 77 K and was kept for at least 45existing min before in the performingequal volume the o measurementsf adsorbate, and at the cryogenic appearance temperature. and nonappearance Ultra-high-purity of ad- hydrogensorbent, w gashile gradeconsidering (6N) was the surface used in (Gibbs) all adsorption excess as experiments. the difference The between PCI curves the density were measuredof the compressed several times gas and to verify the bulk the hysteresis gas [96,97 and]. At the high repeatability. pressure, the All ofexcess the adsorbentadsorbed specimenshydrogen was exhibited the most good common repeatability. for reporting All of hydrogen the uptakes storage were evaluated[98]. as excess gas adsorption quantities, which were the change in the amounts of the hydrogen gas existing in3. Results the equal and volume Discussion of adsorbate, and the appearance and nonappearance of adsorbent, while3.1. Crystallinity considering and the Morphological surface (Gibbs) Characterization excess as the difference between the density of the compressedFigure 1 gas shows and thethe bulkXRD gas pattern [96,97 of]. Atthe high ammonia pressure,-treated the excess AC. No adsorbed sharp diffraction hydrogen waspeaks the were most detected common for for the reporting AC due to hydrogen its poor crystal storagelinity. [98]. However, broad, low intensity peaks, at 2θ° around 21.9° (002), 43.2° (101), and 78.9° (110), might correspond to the 3. Results and Discussion planes of graphite, indicating some alignment of carbon layer planes [99,100]. 3.1. Crystallinity and Morphological Characterization Figure 2 shows the X-ray fluorescence (XRF) spectrum of the ammonia-treated AC, whereFigure carbon1 shows and nitrogen the XRD were pattern detected of the with ammonia-treated the convolution AC. of the No nitrogen sharp diffraction signal un- peaksder the were carbon detected signal. for As the per AC the due result to its of poorCHNS crystallinity. analysis, a However,value of 2.0 broad, wt.% low reflected intensity the ◦ ◦ ◦ ◦ peaks,nitrogen at 2content.θ around The 21.9 XRF(002), and the 43.2 micro(101),-elemental and 78.9 analysis(110), might indicated correspond the successfulness to the planes of graphite,our procedure indicating for doping some alignmentAC with nitrogen. of carbon layer planes [99,100].

600

500 (002)

400

300

Intensity/counts Arb-Unit (101)

200

100 (110)

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

 (degree)

Figure 1. X-rayX-ray diffraction (XRD) pattern of the ammonia-treatedammonia-treated AC.AC. Figure2 shows the X-ray ﬂuorescence (XRF) spectrum of the ammonia-treated AC, where carbon and nitrogen were detected with the convolution of the nitrogen signal under the carbon signal. As per the result of CHNS analysis, a value of 2.0 wt.% reﬂected the nitrogen content. The XRF and the micro-elemental analysis indicated the successfulness of our procedure for doping AC with nitrogen. Appl. Sci. 2021, 11, 6604 5 of 18

Figure3 shows the XRD diffraction patterns for the transition metal-decorated (Pd, Ni, Rh, Ir and Ru), ammonia-treated AC samples. Their corresponding backscattered scanning electron micrographs and EDX analyses are shown in Figures4–8. The ACs were ﬁbrous, layered structures, while the transition metal nanoparticles were spherical and homogenously distributed, as supported by the close match between the average crystallite size, calculated from XRD, and the particle size, estimated from the SEM, conﬁrming no agglomeration of the decorated metal particles. The presence of the transition metals was also supported by the EDX analysis, where platinum signals were detected in all of the samples around 2.1 keV, as platinum plating was used for removing the surface charge

Appl. Sci. 2021, 11, x FOR PEER REVIEW accumulation. The results of platinum-decorated samples were reported earlier5 by of Aboud19 et al. [90]. All of the decorated metals were found in their zero state, as a result of complete reduction. The data dragged from the XRD and their corresponding SEM is summarized in Table1.

C C 100 (b) N

100 80

N 60

80 (a.u.) Intensity 40

20 60 (a) 0

0.25 0.30 0.35 0.40 Intensity (a.u.) Intensity 40 Energy (keV)

0 0.5 1.0 1.5 2.0 Energy (keV)

Figure 2. X-ray ﬂuorescence spectroscopy of the ammonia-treated AC. ( a) X-ray ﬂuoresence spec- Figuretroscopy 2. X-ray of fluorescen the ammonia-treatedce spectroscopy AC andof the (b )ammon deconvolutedia-treated XRF AC. spectrum. (a) X-ray fluoresence spectroscopy of the ammonia-treated AC and (b) deconvoluted XRF spectrum. Table 1. XRD crystallite size and SEM particle size. Figure 3 shows the XRD diffraction patterns for the transition metal-decorated (Pd, Corresponding Ave. Crystallite Peak Position (2θ◦), and Its Ave Particle Size Sample Ni, Rh, Ir and Ru), ammoniaJCPDS-treated Card No. AC Crystallite samples. Size, Their correspondingSize from XRD, backscattered Miller Index (hkl) from SEM, nm scanning electron micrographs and EDX analysesnm are shown in Figuresnm 4–8. The ACs were Pd-decorated, 40.6◦ fibrous,(111), 47.2 ◦lay(200),ered 68.6 structure◦ (220), s, while the transition59.37, 54.36, metal 62.57, nanoparticles were spherical and ◦ ◦ 03-065-2867 59.87 61 NH3-treated AC 82.5homogenously(311), and 86.9 distributed,(222) as supported by59.72, the close and 63.37 match between the average crystal- Ni-decorated, 44.7◦ (111), 52.1◦ (200), 76.5◦ (220), 65.96, 65.37, 74.18, lite◦ size, calculated◦ from XRD,00-004-0850 and the particle size, estimated from78.14 the SEM, confirming 64 NH3-treated AC 93.1 (311), and 98.5 (222) 93.87, and 91.32 Rh-decorated, 41.1◦ no(111), agglomeration 47.8◦ (200), 69.9◦ of(220), the decorated metal particles.68.51, 62.59, The 70.07, presence of the transition metals ◦ ◦ 00-005-0685 69.46 59 NH3-treated AC 84.4was(311), also and supported 89.1 (222) by the EDX analysis, where73.14, andplatinum 72.99 signals were detected in all of Ir-decorated, 40.8◦ (111), 47.4◦ (200), 69.2◦ (220), 58.16, 58.52, 70.94, the◦ samples around◦ 2.1 keV, 03-065-1686as platinum plating was used for removing67.35 the surface charge 63 NH3-treated AC 83.5 (311), and 88.2 (222) 81.60, and 67.52 38.5◦ accumulation.(100), 42.2◦ (002), 44.1The◦ (101),results of platinum-decorated samples were reported earlier by Aboud 37.87, 33.33, 38.24, Ru-decorated, 58.3◦ (101), 69.4◦ (110), 78.4◦ (103), et al. [90]. All of the decorated01-089-3942 metals were found45.51, 51.16, in their 43.43, zero state,38.14 as a result of complete 35 NH -treated AC 84.5◦ (112), 85.99◦ (201), 91.98◦ 3 22.33, and 33.30 reduction.(004), and 96.94 The◦ (202) data dragged from the XRD and their corresponding SEM is summarized in Table 1.

Table 1. XRD crystallite size and SEM particle size.

Ave. Crystallite Ave Particle Peak Position (2θ), and Its JCPDS Corresponding Crystal- Sample Size from XRD, Size from Miller Index (hkl) Card No. lite Size, nm nm SEM, nm Pd-decorated, 40.6° (111), 47.2° (200), 68.6° (220), 03-065- 59.37, 54.36, 62.57, 59.72, NH3-treated 59.87 61 82.5° (311), and 86.9° (222) 2867 and 63.37 AC Ni-decorated, 44.7° (111), 52.1° (200), 76.5° (220), 00-004- 65.96, 65.37, 74.18, 93.87, NH3-treated 78.14 64 93.1° (311), and 98.5° (222) 0850 and 91.32 AC Rh-decorated, 41.1° (111), 47.8° (200), 69.9° (220), 00-005- 68.51, 62.59, 70.07, 73.14, NH3-treated 69.46 59 84.4° (311), and 89.1° (222) 0685 and 72.99 AC Ir-decorated, 40.8° (111), 47.4° (200), 69.2° (220), 03-065- 58.16, 58.52, 70.94, 81.60, NH3-treated 67.35 63 83.5° (311), and 88.2° (222) 1686 and 67.52 AC Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 19

38.5° (100), 42.2° (002), 44.1° (101), Ru-decorated, 37.87, 33.33, 38.24, 45.51, 58.3° (101), 69.4° (110), 78.4° (103), 01-089- NH3-treated 51.16, 43.43, 22.33, and 38.14 35 84.5° (112), 85.99° (201), 91.98° (004), 3942 Appl. Sci.AC2021 , 11, 6604 33.30 6 of 18 and 96.94° (202)

FigureFigure 3. 3.XRD XRD patterns patterns of of (A ()A Pd-,) Pd (-B, )(B Ni-,) Ni (C-,) ( Rh-,C) Rh (D-,) ( Ir-,D) andIr-, and (E) Ru- (E) decorated,Ru- decorated, ammonia-treated ammonia-treated AC. AC. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 19 Appl. Sci. 2021, 11, 6604 7 of 18 Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 19

Figure 4. Scanning electron micrograph of Pd-decorated, ammonia-treated AC and its corresponding EDX spectrum. FigureFigure 4. 4. ScanningScanning electronelectron micrograph of Pd--decorated,decorated, ammoniaammonia--treatedtreated AC AC and and its its corresponding corresponding EDX EDX spectrum. spectrum.

FigureFigureFigure 5. 5. 5.Scanning ScanningScanning electron electronelectron micrograph micrograph ofof Ni-decorated,Ni--decorated,decorated, ammonia ammonia-treatedammonia--treatedtreated AC AC and and its its corresponding corresponding EDX EDX EDX spectrum. spectrum. spectrum. Appl. Sci. 2021, 11, 6604 8 of 18 Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 19

Figure 6. Scanning electron micrograph of Rh-decorated, ammonia-treated AC and its corresponding EDX spectrum. Figure 6. Scanning electron micrograph of Rh-decorated, ammonia-treated AC and its corresponding EDX spectrum.

Figure 7. Scanning electron micrograph of Ir Ir-decorated,-decorated, ammonia ammonia-treated-treated AC and and its its corresponding corresponding EDX EDX spectrum. spectrum. Appl. Sci. 2021, 11, 6604 9 of 18 Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 19

Figure 8. Scanning electron micrograph of Ru-decorated, ammonia-treated AC its corresponding EDX spectrum. Figure 8. Scanning electron micrograph of Ru-decorated, ammonia-treated AC its corresponding EDX spectrum.

3.2. Textural Textural Characterization The textural propertiesproperties were were reported reported before before and and after after ammonia ammonia treatment treatment for thefor pris-the pristinetine and and platinum-decorated platinum-decorated samples samp [90les]. These[90]. These samples samples had a had complex a complex structure structure of mixed of mixedmicropores micropores and mesopores, and mesopores with an, averagewith an micropore average micropore volume of ~0.4 volume cm3 ofg− ~0.41, which cm3 wasg−1, whichhalf of was the total half poreof the volume total pore of ~0.8 volume cm3 g −of1. ~0.8 Their cm nitrogen3 g−1. Their adsorption nitrogen isotherms, adsorption at 77 iso- K, therms,showed at that 77 ammonia K, showed treatment that ammonia improved treatment the BET improved surface area, the while BET notsurface much area, alteration while notwas much observed alteration for the was micropore observed volume for the or micropore diameter volume with an or average diameter micropore with an widthaverage of ~2micropore nm, as displayed width of ~ in2 Figurenm, as9 displayeda–d. The steadinessin Figure 9 ofa– thed. The micropore steadiness attributes, of the micropore while im- attributes,proving the while BET improving surface area the for BET the ammonia-treatedsurface area for the specimens, ammonia supports-treated specimens, the assumption sup- portsof the erosionthe assumption effect of ammonia of the erosion acting on effect mesopores of ammonia and macropores, acting on while mesopores the microp- and macropores,ores were not while affected the duemicropores to their inaccessibilitywere not affected by the due ammonia to their inaccessibility gas molecules [by90 ,the91]. am- On moniathe other gas hand, molecules platinum [90,91 decoration]. On the had other an hand, opposite platinum effect, owingdecoration to its had additional an opposite metal effectmass, and owing nanoparticle to its additional metal ﬁlling, metal asmass reported and nanoparticle for deterioration metal upon filling, metal as reported decorations for deteriorationof activated carbons upon metal [28,101 decorations]. In addition, of activated the average carbons size of[28, the101 decorated]. In addition, metal the particles aver- agewere size in theof the range decorated of meso/macropore metal particles sizes were [102 in ],the implying range of that meso/macropore pore blocking sizes would [102 not], implyingaffect the that micropores, pore blocking but might would block not onlyaffect the the mesopores micropores, and/or but might macropores, block only which the mesoporeswould decrease and/or the macropores overall porosity, which and would the surfacedecrease area, the overall as was porosity reported and [90]. the The surface same areaconclusions, as was canrepor beted applied [90]. The for allsame of theconclusions other decorated can be metalapplied samples. for all of the other decorated metal samples. Appl. Sci. Appl.2021 ,Sci.11, 20 660421, 11, x FOR PEER REVIEW 10 of 1019 of 18

(a) Non- treated AC (b) 1.0 Ammonia-treated AC

1.2 0.8

0.6

0.6 0.4

0.2

0.0

Differential Pore Differential Volume (cm³/g) Differential Pore Differential Volume (cm³/g)

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 110 Pore Width (Angstroms) Pore Width (Angstroms)

1.2

0.8

Pt-decorated, Non-treated AC Pt-decorated, ammonia-treated AC 0.6

0.6 0.4

0.2

0.0 Differential Pore Differential Volume (cm³/g)

0.0 Pore Differential Volume (cm³/g)

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 110

Pore Width (Angstroms) Pore Width (Angstroms)

Figure 9.FigureAdsorption 9. Adsorption average average pore pore widths widths using using the the BET BET for for pristine pristine activated activated carbon (a (a) )and and ammonia ammonia-treated-treated activated activated carbon (carbonb), and (b both), and were both decoratedwere decorated with with Pt ( cPt,d ().c,d).

3.3. Hydrogen3.3. Hydrogen Storage Storage Measurements Measurements

Appl. Sci. 2021, 11, x FOR PEER REVIEWFigures Figures 10– 1015– show15 show the the hydrogen hydrogen adsorption adsorption and and desorption desorption curves curves at both at both 77 K11 77 and of K 19 and 298 K298 for K pristinefor pristine and and ammonia-treated ammonia-treated activatedactivated carbon carbon and and their their decoration decoration with with Pt, Pd, Pt, Pd, Ni, Rh,Ni, IrRh, and Ir and Ru, Ru, respectively. respectively. Their Their excess excess hydrogen storage storage capacities capacities are aresummarized summarized in Tablein Table2. 2.

(a) 0.5 (b) Pt-decorated, ammonia treated AC Pt-decorated, ammonia treated AC 2.5 Pt-decorated AC Pt-decorated AC Ammonia treated AC 0.4 Ammonia treated AC AC AC

2.0 0.3

1.5 0.2 Excess HydrogenExcess wt.% Excess HydrogenExcess wt.% 0.1 1.0

0.0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Pressure(bar) Pressure (bar)

Figure 10. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Pt at (a) 77Figure K and 10. (b Pressure) 298 K.–composition isotherms for AC samples before and after ammonia treatment and decorated with Pt at (a) 77 K and (b) 298 K.

0.7 (b) (a) Pd-decorated, ammonia-treated AC Pd-decorated, ammonia-treated AC 2.5 Pd-decorated AC 0.6 Pd-decorated AC

2.0 0.5

1.5 0.4

1.0 0.3

0.5 0.2

Excess HydrogenExcess wt.% Excess HydrogenExcess wt.% 0.0 0.1

-0.5 0.0 0 5 10 15 20 25 30 35 40 45 50 55 0 20 40 60 80 100 120 140 Pressure(bar) pressure(bar)

Figure 11. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Pd at (a) 77 K and (b) 298 K. Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 19

(a) 0.5 (b) Pt-decorated, ammonia treated AC Pt-decorated, ammonia treated AC 2.5 Pt-decorated AC Pt-decorated AC Ammonia treated AC 0.4 Ammonia treated AC AC AC

2.0 0.3

1.5 0.2 Excess HydrogenExcess wt.% Excess HydrogenExcess wt.% 0.1 1.0

0.0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Pressure(bar) Pressure (bar)

Figure 10. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Pt at Appl. Sci.(2021a) 77, 11K ,and 6604 (b) 298 K. 11 of 18

0.7 (b) (a) Pd-decorated, ammonia-treated AC Pd-decorated, ammonia-treated AC 2.5 Pd-decorated AC 0.6 Pd-decorated AC

2.0 0.5

1.5 0.4

1.0 0.3

0.5 0.2

Excess HydrogenExcess wt.% Excess HydrogenExcess wt.% 0.0 0.1

Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 19 -0.5 0.0 0 5 10 15 20 25 30 35 40 45 50 55 0 20 40 60 80 100 120 140 Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 19 Pressure(bar) pressure(bar)

Figure 11. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Pd at (a)Figure 77 K3.0 and 11.(a) (Pressureb) 298 K.–composition isotherms for AC samples before0.5 and after ammonia treatment and decorated with Pd Ni-decorated, ammonia treated AC (b) Ni-decorated, ammonia treated AC at (a) 77 K and (b) 298 K. Ni-decorated AC Ni-decorated AC 3.02.5 0.5 (a) Ni-decorated, ammonia treated AC (b) 0.4 Ni-decorated, ammonia treated AC Ni-decorated AC Ni-decorated AC 2.52.0 0.4 0.3

2.01.5 0.3 0.2

1.51.0 Excess HydrogenExcess wt.% 0.2 Excess HydrogenExcess wt.% 0.1

1.00.5 Excess HydrogenExcess wt.%

Excess HydrogenExcess wt.% 0.1 0.50.0 0.0 0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Pressure (bar) Pressure (bar) 0.0 0.0 0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Pressure (bar) Pressure (bar) Figure 12. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Ni at

Figure(a) 77 12. K Pressure–compositionand (b) 298 K. isotherms for AC samples before and after ammonia treatment and decorated with Ni at (a)Figure 77 K and 12. (Pressureb) 298 K.–composition isotherms for AC samples before and after ammonia treatment and decorated with Ni at (a) 77 K and (b) 298 K.

2.5 0.5 (b) (a) Rh-decorated, ammonia-treated AC Rh-decorated AC Rh-decorated, ammonia-treated AC 2.5 0.5 Rh-decorated AC (a) Rh-decorated, ammonia-treated AC (b) 2.0 0.4

Rh-decorated AC Rh-decorated, ammonia-treated AC

Rh-decorated AC 2.0 0.4 1.5 0.3

1.5 0.3 1.0 0.2

1.0 0.2 Excess HydrogenExcess wt.%

Excess HydrogenExcess wt.% 0.5 0.1 Excess HydrogenExcess wt.% Excess HydrogenExcess wt.% 0.5 0.1 0.0 0.0 0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Pressure(bar) Pressure (bar) 0.0 0.0 0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Figure 13. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Rh at. Pressure(bar) Pressure (bar) (a)Figure 77 K and 13. (bPressure) 298 K.–composition isotherms for AC samples before and after ammonia treatment and decorated with Rh . at (a) 77 K and (b) 298 K. Figure 13. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Rh at (a) 77 K and (b) 298 K. Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 19

Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 19

Appl. Sci. 2021, 11, 6604 12 of 18

2.5 Ir-decorated, ammonia-treated AC 0.5 (b) (a) Ir-decorated, ammonia-treated AC 2.5 Ir-decoratedIr-decorated, AC ammonia-treated AC 0.5 (b) (a) Ir-decoratedIr-decorated, AC ammonia-treated AC Ir-decorated AC Ir-decorated AC 2.0 0.4

2.0 0.4

1.5 0.3 1.5 0.3

1.0 0.2

1.0 0.2 Excess HydrogenExcess wt.%

Excess HydrogenExcess wt.% 0.5 0.1 Excess HydrogenExcess wt.% Excess HydrogenExcess wt.% 0.5 0.1

0.0 0.0 0.0 0 5 10 15 20 25 30 35 40 45 50 55 600.0 0 20 40 60 80 100 0 5 10 15 20 Pressure(bar)25 30 35 40 45 50 55 60 0 20 40 Pressure(bar)60 80 100 Pressure(bar) Pressure(bar)

FigureFigure 14. 14.Pressure–composition Pressure–composition isotherms isotherms for for AC AC samples samples before before and and after after ammonia ammonia treatment treatment and and decorated decorated with with Ir at Ir (a at) 77(Figurea K) 77 and K 14. (andb) Pressure 298 (b) K.298 –K.composition isotherms for AC samples before and after ammonia treatment and decorated with Ir at (a) 77 K and (b) 298 K.

Ru-decorated, ammonia-treated AC 2.5 (a) Ru-decoratedRu-decorated, AC ammonia-treated AC 0.5 (b) 2.5 Ru-decorated AC 0.5 (a) (b) Ru-decorated, ammonia-treated AC

Ru-decoratedRu-decorated, AC ammonia-treated AC 2.0 0.4 Ru-decorated AC 2.0 0.4

1.5 0.3 1.5 0.3

1.0 0.2

1.0 0.2 Excess HydrogenExcess wt.%

0.5 HydrogenExcess wt.% 0.1 Excess HydrogenExcess wt.% 0.5 HydrogenExcess wt.% 0.1

0.0 0.0 0.0 0 5 10 15 20 25 30 35 40 45 50 55 600.0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 0 5 10 15 20 Pressure(bar)25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35Pressure(bar)40 45 50 55 60 65 70 75 80 85 90 Pressure(bar) Pressure(bar) Figure 15. Pressure–composition isotherms for AC samples before and after ammonia treatment and decorated with Ru at

(aFigure) 77 K and15. Pressure (b) 298 K.–composition isotherms for AC samples before and after ammonia treatment and decorated with Ru atFigure (a) 77 15. K andPressure (b) 298–composition K. isotherms for AC samples before and after ammonia treatment and decorated with Ru at (a) 77 K and (b) 298 K. At cryogenic temperatures, the isotherm shape may be a hybrid of a Type I isotherm, indicatedAt cryogenic by the steep temperature uptake at lows, the pressure, isotherm and shape a Type may IV be isotherm, a hybrid indicated of a Type by I isotherm, gradual increase,indicatedAt cryogenic beyond by the thesteep temperature steep uptake region, ats, low the to the isothermpressure, saturation shapeand point a mayType [102 be IV ].a isotherm, Athybrid thelow-pressure of indicated a Type I isotherm, by region grad- aroundualindicated increase, the by ambient thebeyond steep pressure, the uptake steep anat region, low initial pressure, to sharp the saturation gasand uptake a Type point occurred,IV isotherm, [102]. owing At indicated the to low the- pressureby strong grad- interaction between hydrogen gas molecules and the narrow micropores, and this indicated regionual increase, around beyond the ambient the steep pressure, region, an to initial the saturation sharp gas point uptake [102 occurred]. At the, owinglow-pressure to the their complete filling. The adsorption curves were overlapped for all of the samples at low strongregion interactionaround the between ambient hydrogen pressure, gasan initialmolecules sharp and gas the uptake narrow occurred micropores, owing, and to thisthe pressure, indicating similar micropore fillings for all of the samples, which is consistent indicatedstrong interaction their complete between filling. hydrogen The adsorption gas molecules curves and were the overlapped narrow micropores for all of, theand sam- this with the stability of the micropore volume with ammonia treatment and metal decoration. plesindicated at low their pressure, complete indicating filling. Thesimilar adsorption micropore curves fillings were for overlapped all of the samples, for all of whichthe sam- is As the micropores are just a portion of the total porosity, the filling of mesopores and consistples at entlow with pressure, the stability indicating of the similar micropore micropore volume fillings with forammonia all of the treatment samples, and which metal is macropores will begin after filling of the micropores and is indicated by the gradual increase decoration.consistent with the stability of the micropore volume with ammonia treatment and metal in adsorption beyond the steep uptake. The fillings of the mesopores and macropores decoration.As the micropores are just a portion of the total porosity, the filling of mesopores and take place through overlapping between the monolayer/multilayer adsorption [102]. The macroporesAs the microporeswill begin afterare just filling a portion of the of micropores the total porosity, and is indicated the filling by of themesopores gradual and in- hydrogen storage uptake at 77 K maximized around 7.0 bar, reached saturation, and then creasemacropores in adsorption will begin beyondafter filling the of steep the micropores uptake. The and fillings is indicated of the by mesopores the gradual and in- decreased with increasing hydrogen gas pressure. The descending behavior after reaching macroporescrease in adsorption take place beyondthrough theoverlapping steep uptake. between The the fillings monolayer/multilayer of the mesopores adsorp- and the maximum was experimentally reported [34] as a supercritical adsorption and a typical macropores take place through overlapping between the monolayer/multilayer adsorp- behaviortion [102 for]. The the hydrogen excess isotherm storage model uptake reported at 77 K bymaximized Chilve et around al. [103 –7.0105 bar,]. It reached also follows satu- tion [102]. The hydrogen storage uptake at 77 K maximized around 7.0 bar, reached satu- theration, same and approach then decreased proposed by with Do increasinget al. [106], hydrogenwho described gas pressure.the decrease The in descending the excess ration, and then decreased with increasing hydrogen gas pressure. The descending adsorbed gas, defined by Gibbs excess. At the maximum, a complete filling of the adsorbed Appl. Sci. 2021, 11, 6604 13 of 18

gas was achieved, and then the excess adsorbed gas would decrease to zero, and a further increase in pressure would result in negative values [97,107]. At ambient temperature, the adsorption behavior was different in comparison to cryogenic temperature, as the porosity ﬁlling was increasing gradually for all samples over the studied pressure range, and the excess amount did not reach the maximum over the applied pressure range due to the very weak adsorption forces at this temperature [59]. These excess hydrogen storage capacities agree with the data reported for several carbonaceous materials with similar textural properties at these conditions [28,37,59,67,68,108,109], and are also reliable with the theoretical hydrogen storage capacity values, calculated by Ströbel et al. [110], for carbonaceous materials with similar textural properties.

Table 2. The excess hydrogen storage capacities values at both ~7 bar & 77 K and ~100 bar & 298 K.

Hydrogen (H2) Excess Capacity (wt.%) Incremental Incremental wt.% after Incremental wt.% Sample wt.% after Metal ~7 bar & 77 K Ammonia ~100 bar & 298 K after Ammonia Decoration at Treatment at Treatment at 298 K 298 K 77 k AC 2.5 ± 0.1 0.21 ± 0.01 ~4 ~57 Ammonia-treated AC 2.6 ± 0.1 0.33 ± 0.01 Pt-decorated AC 2.3 ± 0.1 ~24 0.26 ± 0.01 Pt-decorated, ~4 ~42 2.4 ± 0.1 ~12 0.37 ± 0.01 ammonia-treated AC Pd-decorated AC 2.5 ± 0.1 ~95 0.41 ± 0.01 Pd-decorated, ~4 ~20 2.6 ± 0.1 ~48 0.49 ± 0.01 ammonia-treated AC Ni-decorated AC 2.4 ± 0.1 ~86 0.39 ± 0.01 Ni-decorated, ~4 ~10 2.5 ± 0.1 ~30 0.43 ± 0.01 ammonia-treated AC Rh-decorated AC 2.3 ± 0.1 ~71 0.36 ± 0.01 Rh-decorated, ~4 ~11 2.4 ± 0.1 ~21 0.40 ± 0.01 ammonia-treated AC Ir-decorated AC 2.3 ± 0.1 ~105 0.43 ± 0.01 Ir-decorated, ~4 ~12 2.4 ± 0.1 ~45 0.48 ± 0.01 ammonia-treated AC Ru-decorated AC 2.3 ± 0.1 ~86 0.39 ± 0.01 Ru-decorated, ~4 ~8 2.4 ± 0.1 ~27 0.42 ± 0.01 ammonia-treated AC

The incremental wt.% in the hydrogen storage was calculated by the following math- ematical equation: (Hydrogen storage after treatment or decoration—Hydrogen storage before treatment or decoration) × 100/(Hydrogen storage before treatment) At cryogenic temperature, ammonia treatment improved the maximum hydrogen storage by an average of ~4.0 wt.% for all treated samples. At the same temperature, the decoration correspondingly decreased the hydrogen storage by ~8.0 wt.% for most of the decorated metals other than palladium. The lower hydrogen uptake of the transition metal- decorated samples suggested that the spillover effect at this temperature was negligible in comparison to the effect of blocking the pore by metal nanoparticles and their additional mass. Such result is reliable with the reported negative effect of metal decoration at cryogenic temperatures [28,67]. Appl. Sci. 2021, 11, 6604 14 of 18

At room temperature, the small hydrogen storage capacities could be ascribed to the low enthalpy of the adsorption of hydrogen at this temperature [28,59]. At this temperature, ammonia treatment resulted in hydrogen storage enhancement of ~57 wt.% for the pristine AC, and between 8–42 wt.% for the metal-decorated ones. At the same temperature, metal decoration improved the hydrogen storage capacity of the pristine AC between 24–105 wt.% and between 12–48 wt.% for the ammonia-treated AC, suggesting that the spillover worked well at room temperature with the highest contribution belonging to iridium and palladium. No hysteresis, especially at cryogenic temperature below the experimental error, was observed, conﬁrming the total reversibility of the hydrogen storage when removing the applied pressure. This reversibility was free of any storage capacity loss after many cycles, and without performing any adsorbent treatment between the cycles. Minor hysteresis may be observed, with complete desorption, conﬁrming no hydride formations of the initially adsorbed hydrogen at ambient pressure due to the adsorption metastability and/or network effects at ambient temperatures [102].

4. Conclusions Ammonia treatment had no effect on the micropore properties of AC, but it improved the overall textural properties. It improved the hydrogen storage of AC at both ambient and cryogenic temperatures, where this enhancement was more obvious at room temperature with increments of around 57 wt.%. In addition, the hydrogen storage at cryogenic temperature was not clearly increased by transition metal decoration, but it rather was decreased for the metals other than palladium, due to the negligible effect of the spillover mechanism at such low temperatures in comparison to the counter effect by metal decoration. In contrast, the transition metal decoration had encouraged the hydrogen storage at ambient temperatures, especially for iridium and palladium, owing to the spillover mechanism working well at such a temperature. None of the samples showed hysteresis, at both cryogenic and ambient temperature, which conﬁrmed the total release of the hydrogen gas molecules once the applied pressure was released. Therefore, the results of this study demonstrated that ammonia treatment enhanced the hydrogen storage in AC-based adsorbents due to its erosion effect that can alter the AC’s textures, especially at ambient temperature. Furthermore, transition metal decoration motivated hydrogen storage only at ambient temperatures via the spillover mechanism, while it was of minor effect at cryogenic temperatures.

Author Contributions: M.F.A.A. conceived and designed the experiments, performed all experiments and all data analysis; M.F.A.A. wrote the paper; Z.A.A. participated in the hydrogen storage measurements, data analysis, paper writing and corrections; A.A.B. participated in materials synthesis, the textural characterization, paper writing and corrections. All authors examined and approved the final manuscript. All authors read and agreed to the publisher version of the manuscript. Funding: This research was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdul-Aziz City for Science and Technology, Kingdom of Saudi Arabia, grant Number (11-ENE1472-02). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No supporting information for this study. Thus, this section is excluded. Acknowledgments: We are grateful to the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdul-Aziz City for Science and Technology, Kingdom of Saudi Arabia, grant Number (11-ENE1472-02). Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2021, 11, 6604 15 of 18

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