Laboratory Experiment pubs.acs.org/jchemeduc
Volcano Plot for Bimetallic Catalysts in Hydrogen Generation by Hydrolysis of Sodium Borohydride Anais Koska, Nikola Toshikj, Sandra Hoett, Laurent Bernaud, and Umit B. Demirci* Universite de Montpellier, Faculte des Sciences, Departement de Chimie, 34090 Montpellier, France S Supporting Information *
ABSTRACT: In the field of “hydrogen energy”, sodium borohydride (NaBH4) is a potential hydrogen carrier able to release H2 by hydrolysis in the presence of a metal catalyst. Our laboratory experiment focuses on this. It is intended for thirdyear undergraduate students in order to have hands-on laboratory experience through the synthesis of Co−Cu catalysts, X-ray characterization, and catalytic tests to qualitatively illustrate the Sabatier principle (volcano plot). The experiments require minimal preparation and are simple and instructive. Students gain experience in handling H2 and a better understanding of the current issues hindering the development of the “hydrogen economy”.
KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Applications of Chemistry, Catalysis, Materials Science, Rate Law, Transition Elements
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with copper4,10−12 as copper modifies the electronic properties of cobalt.13 The adsorption of B(OH)4− is then mitigated. To find the optimal composition of M and M′ in MM′, the metal contents are varied. Several combinations of MM′ are prepared. Then, the catalytic tests are performed and the reaction rates plotted as a function of the content of one metal. A volcano shape is often obtained.14 This is consistent with the Sabatier principle where an optimal catalyst surface offers optimal sorption properties, that is, intermediate strength of adsorption/desorption of the reaction species allowing reaction turnover and preventing surface blocking.15 For example, platinum has an intermediate binding energy in comparison to nickel (lower energy) and silver (higher energy), and is therefore more active in electrochemical H2 evolution and H2O2 decomposition.16,17 The above-mentioned experimental approach is simple and feasible in a third-year undergraduate (inorganic) chemistry laboratory. Herein, one group of three students worked on cobalt−copper catalysts and hydrolysis of NaBH4 (see SI: sections 1, 2, and 3a−c). They first had a practical introduction to heterogeneous catalysis, H2 generation and safety rules with H2.
INTRODUCTION Catalysis is important in the development of new energy sources and carriers. Hydrogen, H2, is one of these energy solutions. It has to be produced from different feedstocks (water, formic acid, etc.) through (photo)catalytic reactions,1 which thus require transition-metal-based heterogeneous catalysts. A catalyst accelerates a reaction, and is not consumed or transformed; when the catalyst is in a phase different from the reaction mixture, it is considered to be heterogeneous.2 The catalytic performance of a transition metal, M, in a chemical reaction is assessed in terms of activity, conversion, selectivity, and/or stability. It can be improved by combining M with another metal M′ (generally inactive in the targeted reaction). The bimetallic catalyst MM′ is expected to lead to improved performance.3 For instance, cobalt−copper is more active and stable than pure cobalt for H2 generation by hydrolysis of sodium borohydride NaBH4 (eq 1):4 BH4 −(aq) + 4H 2O(l) → B(OH)4 − (aq) + 4H 2(g)
(1)
Though NaBH4 spontaneously reacts with H2O, the kinetics is sluggish and the conversion low.5 The use of a heterogeneous catalyst is necessary to totally convert BH4−, tune the kinetics, and enable on-demand generation of H2.6 A number of catalysts have been examined for this reaction.7 Cobalt is more attractive than noble metals owing to similar or even better activity, and also a lower cost.8 However, it suffers from deactivation over cycles because of strong adsorption of byproducts B(OH)4− on the catalytic surface.9 On the other hand, improved stability can be obtained by combining cobalt © XXXX American Chemical Society and Division of Chemical Education, Inc.
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EXPERIMENTAL PROCEDURES The catalysts were synthesized by chemical reduction of Co2+ and Cu2+ (see SI: Figure S1). Six CoxCuy (with x the weight Received: February 17, 2017 Revised: June 7, 2017
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DOI: 10.1021/acs.jchemed.7b00134 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Table 1. Details for the Syntheses of ∼40 mg of CoxCuy with Information about the Target Weights of the Metal Salts CoCl2· 6H2O and CuCl2·2H2O, Target Weight Ratio w = m(Co)/m(Cu), and EDX Results for the Measured Weight Ratio w′ = m′(Co)/m′(Cu) weight ratio m(Co)/m(Cu)
target weights of the metal salts CoxCuy
m(CoCl2·6H2O)/mg
Cu Co25Cu75 Co50Cu50 Co75Cu25 Co90Cu10 Co
0 40 81 121 145 162
a
m(CuCl2·2H2O)/mg
a
target weight ratio w = m(Co)/m(Cu)
EDX results: measured weight ratio w′ = m′(Co)/m′(Cu)
0 0.33 1 3 9
pure copperb 0.35 1.28 2.95 7.33 pure cobaltb
107 81 54 27 11 0
a Mw(CoCl2·6H2O) = 237.9 g mol−1; Mw(CuCl2·2H2O) = 170.5 g mol−1; Mw(Co) = 58.9 g mol−1; Mw(Cu) = 63.5 g mol−1. bNo trace of cobalt in Cu; no trace of copper in Co.
percentage of Co and y the weight percentage of Cu; x + y = 100%) catalysts were targeted. They are denoted Cu, Co25Cu75, Co50Cu50, Co75Cu25, Co90Cu10, and Co (Table 1). Syntheses of these catalysts must be performed under a hood because of H2 generation during the reduction step. Hereafter is detailed the synthesis of Co90Cu10. A 0.1 M alkaline solution was prepared with distilled water and sodium hydroxide (NaOH), and kept in an ice bath (
