γ-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution

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Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution Zhong Li a,b, Lina Wang b, Yue Zhang b, Guangwen Xie b,* a

Institute of Materials Science, Shanghai University, No. 149 Yanchang Road, Shanghai 200072, PR China Key Laboratory of Nanomaterials, Qingdao University of Science and Technology, No. 53 Zhengzhou Road, Qingdao 266042, PR China

b

article info

abstract

Article history:

The catalytic properties of CueCoeP/g-Al2O3 in hydrogen production via sodium borohy-

Received 24 September 2016

dride hydrolysis were investigated on aspects of the surface morphology, structure and

Received in revised form

chemical composition. The characterization methods employed are X-ray diffraction

22 November 2016

(XRD), scanning electron microscope (SEM), inductively coupled plasma-atomic emission

Accepted 29 November 2016

spectrometer (ICP-AES) and energy dispersive spectroscopy (EDS). The amorphous CueCoe

Available online xxx

P/g-Al2O3 alloy catalyst was synthesized by electroless deposition. It was found that the asprepared catalysts exhibited low activation energy (Ea ¼ 47.8 kJ mol1) and good cycle

Keywords:

properties in catalytic hydrolysis of NaBH4 solution. The influences of NaOH and NaBH4

Electroless deposition

concentration and temperature on hydrogen generation rate (HRG) were also investigated

NaBH4

in this paper.

Catalysts

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Hydrolysis

Introduction The utilization of fossil fuel has been considered as the major reason for global climate and environmental changes. For example, the greenhouse effect from excessive CO2 release may lead to serious global temperature increase [1]. The rapid growth in population and industrial production has been expected to increase the consumption rate of fossil fuel [2]. The fossil fuel is going to be used up in about eight decades, even if the climate problem turns out to be less important than expected. Thus, it is an urgent task to solve the problems of future energy supply without hurting the environments by searching clean and sustainable energy resources [3]. Solar power is the earliest renewable energy used by human, which is also the most abundant energy in the world. However, the

efficiency of solar energy utilization is very low due to the limitation of techniques and some other conditions. Therefore, other alternative energies such as hydrogen, wind power, methane or other form of renewable resources are also crucial in releasing the dependence on fossil resources. As one of the most ideal energy sources, hydrogen has attracted the attention of the researchers in the field of fossil fuel due to the abundance, high energy density and the environmentally benign of its oxidation product. However, the technological barriers on hydrogen storage and transportation still need to overcome. Borohydrides, with their super hydrogen capacities, have attracted much interest as potential materials for hydrogen storage [4,5]. Among them, aqueous sodium borohydride (NaBH4) has a high hydrogen storage capacity of 10.8 wt.%. Moreover, it also has the easy-

* Corresponding author. Fax: þ86 532 8402 2814. E-mail address: [email protected] (G. Xie). http://dx.doi.org/10.1016/j.ijhydene.2016.11.194 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194

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to-handle advantages with the stable, nonflammable and non-toxic chemical properties. Sodium hydroxide has been added into the solution to compress the self-hydrolysis of NaBH4. So the rate of alkaline NaBH4 solution hydrolysis needs to be controlled by proper catalysts. Catalysts in forms of inorganic and organic acids are effective to increase the rate of hydrolysis reaction, but the reaction is hard to control by using these catalysts [6]. The catalysts with noble metal as active components like Pt [7], Pd [8], and PdeBi, Pt-Bi [9] supported on carbon, Rh [10] supported on Al2O3 or TiO2, Ru-promoted sulphated zirconia [11], Ru nanoclusters [12] have demonstrated excellent hydrogen generation performance. However, these catalysts are not economically feasible for application in the industry because of their high cost and poor availability. Transition metals and the alloys, such as Raney Ni [13], cryogel supported Co and Ni [14], transition metals (Cu, Fe, Ni, Mo, W, and Cr) doped CoeB [15], FeeB [16], CoeP [17], CoePeB [18], CoeNieP [19,20], Coe NiePeB [21] and cobalt nanoparticles [22], were discovered to be efficient in promoting the hydrolysis reaction. The transition metal catalysts were mainly synthesized by the super critical method [24], the pulsed laser deposition [23] and the impregnation-chemical reduction method [25]. Recently, there are increasing numbers of researchers [26e29] synthesizing catalysts by electroless deposition because of its advantages of low processing temperature, easy control and wide range of compositions. Moreover, a uniform metal layer with complex shape can be obtained by electroless deposition on plastic and ceramics matrix. In this study, the catalyst of CueCoeP alloy supported on gAl2O3 were synthesized. Its catalytic performances on hydrolysis of alkaline NaBH4 solution were investigated by varying the electroless deposition time, NaOH and NaBH4 concentrations. Moreover, based on the hydrogen generation rates of the CueCoeP/g-Al2O3 catalysts at different temperature, the Ea of this hydrolysis reaction was calculated.

C6H5Na3O7$2H2O and EDTA-2Na are complexing agents, and NH4F is the buffering additive. The electroless deposition process last for 180 s at 90 ± 0.5  C.

Characterization of catalysts The scanning electron microscope (SEM-FEG, JSM-6700F, JEOL) was employed to characterize the morphologies of the surface of CueCoeP/g-Al2O3. For catalyst compositions, inductively coupled plasma-atomic emission spectrometer (ICP-AES, PROFILE SPEC, LEEMAN) and energy dispersive spectroscopy (EDS) were used for analyzing. The conventional X-ray diffraction (XRD, D/max-500, JEOL) was used to characterize the structures of the CueCoeP/g-Al2O3 catalysts. The nitrogen adsorption-deposition using the BrunauereEmmetteTeller method (BET, ASAP 2020, MICROMERITICS) was employed to measure the specific surface areas of the catalysts. The pore size distributions were calculated by the nonlocal density functional theory calculations.

Measurement of catalytic performance The following procedure was employed for testing the performances of the CueCoeP/g-Al2O3 catalysts in hydrolysis of NaBH4. Firstly, 20 ml 5 wt.% NaBH4 with 5 wt.% NaOH was put into a 3-neck round bottom flash with a thermometer controlling the temperature. Then, 1.5 g as-prepared CueCoeP/gAl2O3 catalysts with 0.765 wt.% loading were completely dipped into the NaBH4 solution without stirring. The water replacement method was used to monitor the volume of produced hydrogen. The hydrogen production rates (ml min1 g1) of the prepared catalysts were calculated based on the reaction time, the volume of hydrogen and the weight of CueCoeP excluding the support g-Al2O3.

Results and discussion Experimental methods

Microstructure of the CueCoeP/g-Al2O3 catalysts

Preparation of catalysts

Fig. 1 (a) illustrates the morphology of the CueCoeP/g-Al2O3 alloy surface under a magnification of ten thousand times. As shown in the figure, the catalyst presents a rough surface, and there are plenty of punctate and spherical convex parts on the catalyst surface. Fig. 1 (b) shows the N2 adsorptionedesorption isotherm of the CueCoeP/g-Al2O3 catalyst. According to the IUPAC classification, the CueCoeP/g-Al2O3 catalyst shows a nature of type IV isotherm [30], which indicates monolayer adsorption at relatively low pressure because of the existed micropores and multilayer adsorption at higher pressure. There is a small H2-type hysteresis loop at relatively high pressure between 0.45 and 0.95, which indicates the existence of ink-bottle type pores with narrow necks in the CueCoeP/gAl2O3 catalyst. Fig. 1 (c) shows the incremental pore volume by using DFT calculations of the N2 adsorption isotherm. It depicts that the material is predominantly mesoporous with a diameter in the scope of 2e20 nm. The calculated BET surface area of the CueCoeP/g-Al2O3 catalyst was 246 m2 g1. Herein, large pore volume and specific surface area of the catalyst can favor the full contact of active sites with the BH4  anions and

g-Al2O3 spheres in spherical shape were used as supports. gAl2O3 samples were treated by washing in anhydrous ethanol and distilled water to remove the greasy dirt, sensitizing in the solution of 10 g L1 SnCl2 and 50 g L1 HCl for 3 min, and then rinsing in distilled water for another two times. Before the process of electroless deposition, the sensitized g-Al2O3 samples were activated in the aqueous solution of 0.5 g L1 PdCl2 and 20 g L1 HCl for four minutes. The activated g-Al2O3 samples were then washed with distilled water for another 2 times, and dried in vacuum for 8 h at temperature of 60  C. After pre-electroless deposition, thoseg-Al2O3 samples were immersed in the electroless deposition solution. The solution compositions were 20 g L1 CoSO4$7H2O, 0.7 g L1 CuSO4$5H2O, 40 g L1 NaH2PO2$H2O, 20 g L1 C6H5Na3O7$2H2O, 10 g L1 EDTA-2Na, and 25 g L1 NH4F. The CueCoeP bath pH was adjusted to 9 by the addition of ammonia. In the solution, CoSO4$7H2O and CuSO4$5H2O are the sources of Co and Cu ions; NaH2PO2$H2O is used as reducing agent;

Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194

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Fig. 1 e (a) SEM image, (b) N2 adsorption/desorption isotherm, (c) incremental pore volume obtained from N2 sorption isotherm, and (d) XRD diffraction spectrums of the CueCoeP/g-Al2O3 catalysts andg-Al2O3 support.

H2O molecules, and this accelerates the reaction of hydrolysis of sodium borohydride solution. Fig. 1 (d) shows the XRD spectrums of CueCoeP catalysts. The catalyst was found to mainly possess amorphous structure. However, in the XRD pattern, the presence of the weak reflection peak at Co peaks indicated that heat treatment at 360  C under vacuum conditions for 1 h induces more crystallization. Therefore, Cue CoeP catalyst without heat treatment was employed in all other experiments.

Effect of NaBH4 concentration Effect of NaBH4 concentration on the rate of hydrogen generation was investigated with different initial NaBH4 concentration (2.5 wt.% to 20 wt.%), and the results are shown in Fig. 3. The initial NaOH concentrations were identical (5 wt.%) in all experiments. Results show that the hydrogen generation rates increase from 4634 to 5477 ml min1 g1 by increasing the concentrations of NaBH4 from 2.5 wt.% to 5 wt.%.

Effect of NaOH concentration In order to inhibit the self-hydrolysis of sodium borohydride, some alkaline substances such as NaOH are usually employed as stabilizing agent [31e33]. The dependence of hydrogen generation volume from alkaline NaBH4 solution on NaOH concentration was studied in 20 ml solution of 5 wt.% NaBH4 with 1.5 g CueCoeP/g-Al2O3 catalysts at 45  C. Fig. 2 shows the effect of NaOH concentration on hydrogen generation. It is clear that the rate of hydrogen generation with only g-Al2O3 is very slow, so the effects of Pd particles and self-hydrolysis of NaBH4 on hydrogen generation can be neglected. As shown in Fig. 2, the hydrogen generation rates increase by increasing the NaOH concentration from 5 to 20 wt.%. The inset of ln[HGR] versus ln[NaOH] shows that the slope of the line is 0.22, which proves that increasing the concentration of NaOH from 2.5% to 20 wt.% is favorable for the NaBH4 hydrolysis for hydrogen over the CueCoeP/g-Al2O3 catalysts. A similar phenomenon was also reported by Ai et al. [34] using Co@AHs catalyst.

Fig. 2 e Effect of NaOH concentration on hydrogen generation (5 wt.% NaBH4, 1.5 g catalysts, 45  C).

Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194

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Fig. 3 e Effect of NaBH4 concentration on H2 generation (5 wt.% NaOH, 1.5 g catalysts, 45  C).

However, the hydrogen generation rates decrease to 4556 ml min1 g1 at NaBH4 concentration up to 20 wt.%. Therefore, the highest hydrogen generation rate can be obtained when sufficient BH4  ions and H2O molecules contact the active sites on the CoeCueP/g-Al2O3 catalysts surface. However, the production of hydrogen was accompanied by the byproduct of NaBO2 simultaneously, and the higher NaBH4 concentration leads to the more excessive NaBO2 accumulation on the surfaces, which blocks the catalytic active sites and thereafter hinders the catalyzed sodium borohydride hydrolysis [35].

Effect of electroless deposition time CueCoeP/g-Al2O3 catalysts were prepared with different electroless deposition time. The activities of these catalysts were evaluated in NaBH4 solution (5 wt.% NaOH and 5 wt.% NaBH4) at 45  C, according to above experimental results. The volume change of hydrogen which is plotted against electroless deposition time is shown in Fig. 4. Results demonstrate that the longer electroless deposition time in the range of 60e180 s, the higher rate of hydrogen production would be obtained, and then it remains nearly constant at electroless deposition time longer than 180 s. Therefore, CueCoeP/gAl2O3 catalyst electroless deposited for 180 s was used for the hydrogen generation from NaBH4 hydrolysis in subsequent studies. For the purpose of analyzing the effect of electroless deposition time on catalyst surface morphologies, SEM studies of CueCoeP/g-Al2O3 catalysts electroless deposited for 60, 120, 180, and 300 s were performed and the results are shown in Fig. 5. We observed obvious differences between the four CueCoeP/g-Al2O3 catalysts with various electroless deposition time. It was found that the surface roughness increases with the increasing electroless deposition time from 60 to 180 s, then decreases at longer electroless deposition time. It is obvious that the CueCoeP/g-Al2O3 catalyst electroless deposited for 180 s has the largest surface roughness. It has been proposed that the larger surface roughness can provide more surface defects in forms of edge, angle or step,

Fig. 4 e Effect of electroless deposition time on H2 generation (5 wt.% NaBH4, 5 wt.% NaOH, 1.5 g catalysts, 45  C).

which were considered as active sites on the catalyst surfaces [36,37]. The EDS spectra of the CueCoeP/g-Al2O3 catalysts with the electroless deposition time ranging from 60 s to 300 s were shown in Fig. 6. As can be seen in Fig. 6, the composition of the chemical elements in the CueCoeP/g-Al2O3 is very sensitive to deposition time. Table 1 shows average results of the elements percentages at three randomly selected regions on Cue CoeP surface coatings deposited for different time by EDS. It can be seen that the Co content in the surface of the CueCoeP/ g-Al2O3 catalysts decreases with the increasing deposition time, but the total contents of Cu and P increase. Moreover, the Cu content and the weight ratio of Cu: P are the highest when the electroless deposition time is 180 s. Furthermore, the composition of the CueCoeP/g-Al2O3 catalyst electroless depositing for 300 s is similar with that of electroless depositing for180 s. This may explain why the rate of hydrogen generation is almost the same in Fig. 4, although the surface roughness is relatively smoother than that of electroless depositing for 180 s in Fig. 5.

Kinetics of NaBH4 hydrolysis Fig. 7 (a) shows the effect of reaction temperature on performance of the CueCoeP/g-Al2O3 in hydrogen generation in the solution of 5 wt.% NaBH4 and 5 wt.% NaOH with 1.5 g catalysts. It was found that the rate of alkaline NaBH4 solution hydrolysis increases exponentially at increased temperature. A maximum rate (8218 ml min1 g1) is observed at 55  C. The hydrogen generation rate measured at 55  C is obviously larger than 2327.7 ml min1 g1 on NieRu nanocomposite catalyst measured at 75  C reported by Liu et al. [38]. According to the rates of hydrogen generation at different temperatures in Fig. 7 (b), the activation energy (Ea) of the reaction catalyzed by CueCoeP/g-Al2O3 is 47.8 kJ mol1. For comparison, the experimental results of other Co-based catalysts in literature are listed in Table 2. The Ea of CueCoeP/gAl2O3 catalyst (47.8 kJ mol1) is close to the result of Wang et al. [39] and Zhang et al. [17] on CoeP/Cu sheet catalyst, which is

Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194

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Fig. 5 e SEM images of CueCoeP/g-Al2O3 catalysts with different electroless deposition time of (aed) 60, 120, 180 and 300 s.

Fig. 6 e The EDS spectra of CueCoeP/g-Al2O3 catalysts with different electroless deposition time of (aee) 60, 120, 180, 300s and 180 s, after one cycle.

Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194

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Reusability of CueCoeP/g-Al2O3 catalysts in NaBH4 solution hydrolysis

Table 1 e Compositions of the CueCoeP coatings by EDS. Co/wt.% Cu/wt.% P/wt.% Cu:Co (Cu þ P)/wt.% 60 s 120 s 180 s 300 s 180 s, after one cycle

18.8 16.4 8.8 8.7 9.7

62.8 76.4 79.9 77.0 79.5

18.4 7.2 11.3 14.3 10.8

3.34 4.66 9.08 8.85 8.20

81.2 83.6 91.2 91.3 90.3

lower than the other values in literature. The CueCoeP/gAl2O3 catalysts is very likely preferable in industrial application due to its low energy barrier and the low prices of raw materials.

The reusability of the catalyst is a crucial aspect in the industrial application. In this work, the reusability of the Cue CoeP/g-Al2O3 was tested in 20 ml 5 wt.% NaOH and 5 wt.% NaBH4 solution at 45  C. Fig. 8 (a) shows the catalytic activity of the CueCoeP/g-Al2O3 amorphous alloy catalysts as a function of cycle numbers. After four cycles, the H2 production rate gradually decreases by 12.2%, and remains 66% of its initial activity after 6 cycles. As shown in Fig. 8 (b), the channel of the used catalyst may have been blocked by the byproduct NaBO2, compared to the fresh catalyst in Fig. 5 (c). In addition, the composition of the catalyst has changed after one cycle, which can be seen from Fig. 6 (e) and Table 1. Therefore, the

Fig. 7 e Effect of reaction temperature on the performance of the catalysts in hydrogen generation in the solution of 5 wt.% NaBH4 and 5 wt.% NaOH with 1.5 g catalysts (a) and the plot (b).

Table 2 e The comparison of different Co-based metal catalysts. Co-based catalysts CoeP/Cu sheet CoeCueB CoeP/Cu sheet CoeMneB Co(II)eCu(II) based complex catalyst CueCo Pt/LiCoO2 Ru/LiCoO2 CoeP/Cu sheet CoeP/Cu sheet CoeB/Cu sheet Co-aAl2O3/Cu plates CoeWeP/Cu sheet CoeNieMoeP/gAl2O3 CoeWeP/g-Al2O3 CoeCueB

CoeLaeZreB CoeCueP

Synthetic method

Activation Hydrogen Hydrolysis NaOH conc. NaBH4 conc. Year (wt.%) energy generation rate temperature (wt.%) (kJ mol1) (ml min1 g1) ( C)

Ref.

Electroless plating Chemical reduction Electroless plating Chemical coprecipitation Mechanical mixing method Precipitation Microwave-assisted rapid heating method Microwave-assisted rapid heating method Electroless deposition Electroplating Electroless plating Electrodeposited Electrodeposition Electroplating

48.1 49.6 47.0 52.1

1846 4800 1647.9 1.44

30 30 30 20

1 7.5 1 7

5 7.5 5 7

2010 2010 2015 2012

[17] [35] [39] [40]

e

188

30

5

2.5

2012 [41]

42.66 70.4

1212 2700

30 25

1 5

2 10

2015 [42] 2008 [43]

68.5

3000

25

5

10

2008 [43]

60.2 e 43.3 e 22.8 52.43

3300 954 7935.6 383 5000 3680

30 30 30 80 30 30

1 1 1 1 10 10

10 10 5 3 10 7

2008 2007 2016 2010 2012 2016

Electroless plating Chemical reduction þ plasma treatment In situ reduction Electroless deposition

49.58 17.38

4000 4972

30 30

10 5

4 2.5

2015 [50] 2016 [31]

53 47.8

1252 1115

50 25

2 5

5 5

2013 [51] e This work

[44] [45] [46] [47] [48] [49]

Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194

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Fig. 8 e (a) Reusability tests of the CueCoeP/g-Al2O3 catalysts in 20 ml 5 wt.% NaOH and 5 wt.% NaBH4 solution at 45  C and (b) SEM image of the catalysts after one cycle.

activity decrease is very likely from the change of morphology and composition of the CueCoeP/g-Al2O3 catalyst surface. Despite the activity loss, the CueCoeP/g-Al2O3 catalysts are reusable, durable and easily separable.

Possible mechanism of the reaction Based on the above results and the work of Holbrook et al. [52], Wu et al. [53], and Guella et al. [54], a possible mechanism for CueCoeP/g-Al2O3 catalyst used for hydrogen generation from hydrolysis of NaBH4 could be propounded. First of all hydroxide and borohydride ions are reversibly chemisorbed to the metal atoms (M). After that a proton on the M  BH 4 exchanges with a OH to form MH and intermediate MBH3OH, then it continues to generate M  BðOHÞ 4 , which can  further react with BH 4 to generate BðOHÞ4 . Finally the products MH can react with water to produce H2.

Conclusion In this study, CueCoeP/g-Al2O3 catalysts were synthesized by electroless deposition method. The component and structure of the catalysts, as well as the effect of NaOH and NaBH4 concentration, were investigated. Furthermore, the effect of electroless deposition time on the rate of hydrogen generation was carefully analyzed. And the rate of hydrogen generation is highest when the electroless depositon time is 180 s. The calculated activation energy of this reaction catalyzed by CueCoeP/g-Al2O3 alloy catalysts is 47.8 kJ mol1. CueCoeP/g-Al2O3 catalysts can be easily separated from the alkaline NaBH4 solution, which exhibit favorable cycle capability.

Acknowledgments The work was financially supported by the key project of the National Natural Science Foundation (Contracted No. 21136008).

references

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Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194

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Please cite this article in press as: Li Z, et al., Properties of CueCoeP/g-Al2O3 catalysts for efficient hydrogen generation by hydrolysis of alkaline NaBH4 solution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.194