This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol., 2013, 3, 2197--2198 2197 Cite this: Catal. Sci. Technol., 2013, 3, 2197 Reply to the ‘Comment on ‘‘Nickel nanoparticles catalyse reversible hydration of carbon dioxide for mineralization carbon capture and storage’’’ by D. Britt, Catal. Sci. Technol., 2013, 3, DOI: 10.1039/C3CY00142C† Gaurav A. Bhaduri, a Richard A. Henderson b and Lidija S ˇ iller* a This is a reply to the Comment made on the paper titled ‘‘Nickel nanoparticles catalyse reversible hydration of carbon dioxide for mineralization carbon capture and storage’’, Catal. Sci. Technol., 5(3), 2013, 1234–1239. The method suggested in the Comment on the kinetic rate analysis for the NiNPs cannot be used. The NiNPs help catalyse the reaction of hydration of CO 2 for a fluid–fluid–solid (catalysed) reaction scheme. The kinetic rate determination of this process is not straightforward. Introduction The authors would like to reply to the Comment 1 on the article by Bhaduri and S ˇ iller. 2 There is a misunderstanding in the interpretation of the information provided in the paper. It has been clearly mentioned in the conclusion of the article that there is a three times enhancement in the dissolution of CO 2 (i.e. saturated concentration of CO 2 after equilibrium) in water and not a three times increase in the rate of the reaction. The paper 2 reported that in the pH profile curves (Fig. S5a and c, ESI†) there is an enhancement in the rate of the reaction but the actual value of the enhancement is not so trivial, as explained below and in the ESI† of this reply. The catalytic activity determination method by Mirjafari et al. 3 for the hydration of CO 2 by carbonic anhydrase (CA) that has been cited in the comment is based on an entirely different method of analysis than the one observed in the article by Bhaduri and S ˇ iller. 2 This is because the role of CA is to maintain the pH of the blood by changing the equilibrium of the CO 2 concentration (i.e. in the lungs). 4–6 It is also known that the catalytic rate of CA is limited by the protonation of the water molecules, which takes place at a pH above 7. 7 Thus, buffer is needed for maintaining the pH of the solution otherwise no catalytic activity can be observed as the enzyme is inactive in low pH solutions. The importance of buffers in observing the activity of CA is shown by Vinoba et al. 8 (free and immobilized bovine CA). Further details of the discussion can be seen in the ESI.† Discussion of results The experimental section and results and discussion have been given in detail in the ESI† of this reply, an overview of the results has been presented here. As our apparatus did not have a quick response time to repeat the experiment as reported by Mirjafari et al., 3 we used stopped-flow spectrophotometry for the analysis of the catalytic activity, which is also used for the same purpose in the literature. 9 A few other validation experi- ments were also performed. From the results of the stopped-flow experiments no change in the equilibrium constants of the hydration reaction was observed (see Fig. S1 in the ESI† of this reply). Thus, another experiment similar to the one by Bhaduri and S ˇ iller 2 was performed in carbonate solutions (bubbling of CO 2 in sodium and potassium carbonate solutions (0.1 M) and the pH changes were monitored, see Fig. S2 in the ESI†) and an enhancement in the absorption rate was observed. It could be therefore argued that the observed enhancement was just a mass transfer enhancement rather than a catalytic enhancement as reported in the paper by Bhaduri and S ˇ iller. 2 However, in order to differentiate between the effect of the mass transfer enhance- ment and catalytic activity, Fe 2 O 3 nanoparticles were also used and the bubbling experiment was repeated. The choice of Fe 2 O 3 NPs was made due to the similar surface chemistry a School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK. E-mail: Lidija.Siller@ncl.ac.uk b School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cy00357d Received 23rd May 2013, Accepted 25th June 2013 DOI: 10.1039/c3cy00357d www.rsc.org/catalysis Catalysis Science & Technology COMMENT Published on 26 June 2013. Downloaded on 07/08/2013 11:41:01. View Article Online View Journal | View Issue