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Abstract: Thin films of Carbon-supported Co–B nanoparticles were synthesized by using Pulsed Laser Deposition (PLD) and used as catalysts in the hydrolysis of Ammonia Borane (AB) to produce molecular hydrogen. Amorphous Co–B-based catalyst powders, produced by chemical reduction of cobalt salts, were used as target material for nanoparticles-assembled Co–B film catalysts preparation through PLD. Various Ar pressures (10–50 Pa) were used during deposition of carbon films to obtain extremely irregular and porous carbon support with high surface area prior to Co–B film deposition. Surface morphology of the catalyst films was studied using Scanning Electron Microscopy, while structural characterization was carried out using X-Ray diffraction. The hydrogen generation rate attained by carbon-supported Co–B catalyst film is significantly higher as compared to unsupported Co–B film and conventional Co–B powder. Almost complete conversion (95%) of AB was obtained at room temperature by using present film catalyst. Morphological analysis showed that the Co–B nanoparticles produced after the laser ablation process act as active catalytic centers for hydrolysis while the carbon support provides high initial surface area for the Co–B nanoparticles with better dispersion and tolerance against aggregation. The efficient nature of our carbon-supported Co–B film is well supported by the obtained very low activation energy (∼29 kJ (mol)−1) and exceptionally high H2 generation rate (13.5 L H2 min−1 (g of Co)−1) by the hydrolysis of AB. [Copyright &y& Elsevier]

Abstract: In addition to high catalytic efficiency the catalyst must also comprise important features like high stability in severe conditions, ability to be recycled several times and should have high tolerance against deactivation. This work is oriented specifically to study these properties of already developed efficient transition-metal doped Co–B alloy catalyst. Various transition metals, namely Ni, Fe, Cu, Cr, Mo, and W, were singly added as dopants in Co–B catalyst by chemical reduction of the corresponding metal salts. These alloy catalysts were calcinated, in Ar atmosphere, at 673, 773, and 873 K in order to investigate the stability of the powders at elevated temperatures. The catalytic performances of these treated catalyst powders were tested for H2 generation by catalytic hydrolysis of sodium borohydride (NaBH4). The alloy powders were exposed to ambient condition for several days to test their tolerance against deactivation and self life. After separation from the reaction course and after rinsing, the catalyst powders were tested for several cycles to evaluate the reusability property. The observed changes in the catalytic activity were discussed on the basis of structural and morphological variations. The Co–B catalyst, when doped with Ni, Mo, and W metals showed high stability and resistance against deterioration, as function of both time and use, as compared to Cr- and Fe-doped alloy powders. A much lower performance with respect to calcination temperature, holding time, and number of cycles was established for Cu doped Co–B catalyst powder. [Copyright &y& Elsevier]

Abstract: Co-B nanoparticles supported over carbon films were synthesized by using pulsed laser deposition (PLD) and used as catalysts in the hydrolysis of sodium borohydride (NaBH4) to produce molecular hydrogen. Amorphous Co-B-based catalyst powders, produced by chemical reduction of cobalt salts, were used as target material for nanoparticles-assembled Co-B film catalysts preparation through PLD. Various Ar pressures (10–50Pa) were used during deposition of carbon films to obtain extremely irregular and porous-carbon support with high surface area prior to Co-B film deposition. Surface morphology of the catalyst films was studied using scanning and transmission electron microscopy, while structural characterizations were carried out using X-ray diffraction. The hydrogen generation rate attained by carbon-supported Co-B catalyst film is significantly higher as compared to unsupported Co-B film and to conventional Co-B powder. Morphological analysis along with NaBH4 hydrolysis tests showed that the Co-B nanoparticles produced with PLD act as active catalytic centers for hydrolysis while the carbon support provides high initial surface area for the Co-B nanoparticles with better dispersion and tolerance against aggregation. The hydrogen generation rate obtained by the present catalyst film was also investigated as a function of Co-B loading, carbon morphology, and solution temperature. The high performance of our carbon-supported Co-B film is well supported by the obtained very low activation energy (∼31kJ (mol)−1) and exceptionally high H2 generation rate (8.1LH2 min−1 (gofcatalyst)−1) in the hydrolysis of NaBH4. [Copyright &y& Elsevier]

Abstract: A systematic and comparative study is conducted on Co–B-based ternary alloy catalysts for H2 generation by hydrolysis of NaBH4. Various transition metals, namely Ni, Fe, Cu, Cr, Mo, and W, were added to Co–B catalyst by chemical reduction of the corresponding metal salts. All the transition metals in the Co–B compounds behave in a dissimilar manner while influencing the catalytic activity. Cr, W, Mo, and Cu impose significant promoting effects on the Co–B catalyst by increasing the H2 generation rate by 3–4 times when compared to the undoped catalyst. On the contrary, Ni and Fe are only able to create a marginal increment in the catalytic performance of the Co–B catalyst. The metal/(Co+metal) molar ratio was varied in the catalyst in order to study the effect of metal doping on surface morphology, electronic interaction, and catalytic efficiency of the alloy catalyst. This systematic variation also contributed to clarify the role of each dopant metal in the electron exchange mechanism involved in NaBH4 hydrolysis. The promoting effects of the dopant metals are mainly discussed in terms of large active surface area, ability to act as Lewis acid sites for better absorption of OH− group, electronic interaction with Co active metal, and amorphous nature of the alloy catalyst. [Copyright &y& Elsevier]

Abstract: Cr-modified Co–B (Co–Cr–B) catalyst alloy powders have been synthesized by chemical reduction of cobalt and chromium salt at room temperature to study the hydrogen production by catalytic hydrolysis of NaBH4. The Cr/Co molar ratio was varied in the catalyst in order to study the effect of Cr doping on surface modification and catalytic efficiency of Co–B catalyst. The resulting catalyst powders were characterized by scanning electron microscopy, X-ray diffraction, X-photoelectron spectroscopy, and BET surface area measurement. When the molar ratio χ Cr =Cr/(Cr+Co) exceeds 9% the BET surface area of the Co–Cr–B catalyst increases by one order of magnitude as compared to that of Co–B catalyst. The catalytic activity of the Co–Cr–B for hydrogen production depends on Cr concentration: specifically, the activity increases by increasing χ Cr up to about 4% and then it gradually decreases by further increasing χ Cr. We established that the increased catalytic activity is related to the formation of chromium oxide on the catalyst surface, with the oxide favoring the dispersion of Co–B particles resulting in high catalyst surface area. However as χ Cr exceeds 4%, Cr starts to cover the Co active sites and the corresponding catalytic activity decreases. The highest catalytic activity was obtained at the optimum Cr-content, χ Cr =4%, in Co–Cr–B catalyst, showing nearly 4 times higher H2 generation rate than that of pure Co–B catalyst. Kinetic studies on the hydrolysis reaction of NaBH4 with Co–Cr–B catalyst reveal that the concentrations of both NaBH4 and NaOH have essentially no effects on hydrogen generation rate. The promoting effect of Cr in Co–Cr–B catalyst results in lower activation energy for hydrogen production, which is 37kJmol−1 as compared to 45kJmol−1 obtained with pure Co–B powder. Finally, the possible role of Cr3+ species in the electron exchange mechanisms involved in NaBH4 hydrolysis with the Co–Cr–B catalyst has been discussed. [Copyright &y& Elsevier]

Abstract: The aim of the present work is to study the catalytic efficiency of amorphous Co–Ni–P–B catalyst powders in hydrogen generation by hydrolysis of alkaline sodium borohydride (NaBH4). These catalyst powders have been synthesized by chemical reduction of cobalt and nickel salt at room temperature. The Co–Ni–P–B amorphous powder showed the highest hydrogen generation rate as compared to Co–B, Co–Ni–B, and Co–P–B catalyst powders. To understand the enhanced efficiency, the role of each chemical element in Co–Ni–P–B catalyst has been investigated by varying the B/P and Co/Ni molar ratio in the analyzed powders. The highest activity of the Co–Ni–P–B powder catalyst is mostly attributed to synergic effects caused by each chemical element in the catalyst when mixed in well defined proportion (molar ratio of B/P=2.5 and of Co/(Co+Ni)=0.85). Heat-treatment at 573K in Ar atmosphere causes a decrease in hydrogen generation rate that we attributed to partial Co crystallization in the Co–Ni–P–B powder. The synergic effects previously observed with Co–Ni–B and Co–P–B, now act in a combined form in Co–Ni–P–B catalyst powder to lower the activation energy (29kJmol−1) for hydrolysis of NaBH4. [Copyright &y& Elsevier]

Abstract: Amorphous catalyst alloy powders in form of Co–P, Co–B, and Co–P–B have been synthesized by chemical reduction of cobalt salt at room temperature for catalytic hydrolysis of NaBH4. Co–P–B amorphous powder showed higher efficiency as a catalyst for hydrogen production as compared to Co–B and Co–P. The enhanced activity obtained with Co–P–B (B/P molar ratio=2.5) powder catalyst can be attributed to: large active surface area, amorphous short range structure, and synergic effects caused by B and P atoms in the catalyst. The roles of metalloids (B and P) in Co–P–B catalyst have been investigated by regulating the B/P molar ratio in the starting material. Heat-treatment at 773K in Ar atmosphere causes the decrease in hydrogen generation rate due to partial Co crystallization in Co–P–B powder. Kinetic studies on the hydrolysis reaction of NaBH4 with Co–P–B catalyst reveal that the concentrations of both NaOH and catalyst have positive effects on hydrogen generation rate. Zero order reaction kinetics is observed with respect to NaBH4 concentration with high hydride/catalyst molar ratio while first order reaction kinetics is observed at low hydride/catalyst molar ratio. Synergetic effects of B and P atoms in Co–P–B catalyst lowers the activation energy (32kJmol−1) for hydrolysis of NaBH4. The stability, reusability, and durability of Co–P–B catalyst have also been investigated and reported in this work. It has been found that by using B/P molar ratio of 2.5 in Co–P–B catalyst, highest H2 generation rate of about ∼4000mlmin−1 g−1 can be achieved. This can generate 720W for Proton Exchange Membrane Fuel Cells (0.7V): which is necessary for portable devices. [Copyright &y& Elsevier]