Molecular sieving platinum nanoparticle catalysts kinetically frozen in nanoporous carbon{ Ramakrishnan Rajagopalan, a Ayyappan Ponnaiyan, a Pratik J. Mankidy, a Anthony W. Brooks, a Bo Yi a and Henry C. Foley* a,b a 158, Fenske Laboratory, Department of Chemical Engineering, Penn State University, University Park, PA 16802, USA. E-mail: hcf2@psu.edu; Fax: 11 814 865 7846; Tel: 11 814 865 2574 b Department of Chemistry, Penn State University, University Park, PA 16801, USA Received (in Cambridge, UK) 27th May 2004, Accepted 2nd August 2004 First published as an Advance Article on the web 9th September 2004 Highly active shape selective catalysts with excellent thermal stability are synthesized by entrapping well dispersed platinum nanoparticles in a polyfurfuryl alcohol derived nanoporous carbon matrix; these nanocomposites are excellent candidates for new catalytic applications including fuel cells, pharmaceutical synthesis and biomass conversion. There has been considerable interest in developing well dispersed platinum nanoparticles on carbon due to their applications in fuel cells, catalytic converters and catalytic burners. 1–5 There are excellent reviews in the literature on this topic. 6–8 Carbon- supported platinum catalysts are usually synthesized by one of a few methods: (a) impregnation of an activated carbon support with a platinum salt precursor solution such as chloroplatinic acid, (b) adsorption of platinum oxide, platinum metal or colloids on the carbon surface 9,10 or (c) by ion exchange at the surface with a platinum amine complex. These steps are followed by reduction of platinum species to Ptu by heating in the presence of hydrogen at 300–500 uC. Each method can result in the formation of initially well dispersed platinum nanoparticles typically ranging in size from 2–4 nm. However, these nanometer-sized particles on such a surface are unstable and as a result seek to minimize their surface chemical potential through agglomeration and sintering at higher temperatures. In this communication, we report a novel synthesis{ for the creation of a highly active and thermally stable platinum–carbon catalyst that also shows shape selective properties. We find that platinum nanoparticles of dimensions ranging from 2–4 nm can be dispersed well with high stability towards agglomeration in a glassy nanoporous carbon matrix. The essential feature underlying this stability is that the diameter of the platinum nanoparticles are six to ten times larger than the pore size of the carbon (0.4–0.5 nm). Hence, it is impossible for the particles to move through the matrix, thereby preventing their agglomeration and sintering even under forcing conditions (i.e. at 800 uC in flowing hydrogen). However, since the matrix of carbon is nanoporous, reactants and products are able to diffuse to and from the surfaces of the embedded platinum nanoparticles. Furthermore, since the pore dimensions in the carbon are comparable to the molecular dimension of alkenes, we find that the global reactivities of these alkenes are strongly dependent upon their size. This molecular sieving effect arises from restricted or configurational diffusion in the nanopores. It shows that the nanoporosity extends from the carbon matrix to the platinum nanoparticle surfaces. Platinum nanoparticles were prepared by reduction of platinum(II) acetylacetonate in the presence of furfuryl alcohol under reflux. There are several reports on the use of alcohols as reducing agents to prepare platinum nanoparticles 11,12 and our investigation shows that furfuryl alcohol is a very good reducing agent for this purpose. The reduced platinum nanoparticles are dispersed in excess furfuryl alcohol using TritonX-100 as a surfactant. The monomer–nanoparticle mixture is polymerized using p-toluenesulfonic acid monohydrate at 15–20 uC for 24 hours. The resultant polymer composite was redissolved in acetone, ultrasonicated for 1 hour and pyrolyzed at 800 uC for 8 hours under an argon atmosphere. All the results discussed in this paper are based on 6.4 wt% platinum loaded nanoporous carbon. HRTEM and STEM images were used to study the size and distribution of platinum nanoparticles. The particle sizes range between 2–4 nm and were very well dispersed in the nanoporous carbon matrix.§ Closer inspection shows that the platinum particles are clearly embedded in the carbon and thus are accessible only through the pores of the nanoporous carbon (Fig. 1). The total pore volume and pore size distribution of the sample were measured using methyl chloride porosimetry described elsewhere in detail. 13 The pore size distribution of 6.4 wt% platinum loaded carbon is essentially of nanopore dimensions centered at 0.4– 0.5 nm.} These micropores are characteristic of nanoporous carbon derived from polyfurfuryl alcohol, however pores larger than 1.5 nm were also evident. The presence of pores larger than 1.5 nm is not typical of nanoporous carbon derived from polyfurfuryl alcohol alone and may be a consequence of the presence of platinum during pyrolysis. These pores also aid in the transport of reactants through the carbon and undoubtedly increase catalyst effectiveness. Alkene hydrogenation test reactions were carried out in a tubular quartz reactor. The catalyst amount for each run was kept at 0.2 g and the mole ratio of the carrier gas (argon) to hydrogen to alkene was 2 : 1 : 1 respectively. The total flow rate of the gas mixture was kept at y60 ml min 21 . Pretreatment of the catalyst involved heating the sample in argon and hydrogen to 800 uC over a 30 minute period and maintaining at 800 uC for 1 hour. Reaction products were analyzed by gas chromatography using a flame ionization detector. Ethylene hydrogenation was tested using a 10 m Porapakt column maintained at 50 uC, while hydrogenations of propene, 1-butene and isobutene were tested using a 30 m Chromosorb column kept at 30 uC (propene) and 80 uC (1-butene and isobutene) respectively. Hydrogenation reactions were studied at reaction temperatures ranging from 25–200 uC using a 6.4 wt% Pt/C sample (Fig. 2). The results indicate that the catalyst was active for ethylene and propylene hydrogenation even at 25 uC. Conversion at equivalent conditions was strongly dependent { Electronic supplementary information (ESI) available: Appendix A, B, C and D. See http://www.rsc.org/suppdata/cc/b4/b407854c/ Fig. 1 HRTEM image of a platinum nanoparticle embedded in the nanoporous carbon matrix. DOI: 10.1039/b407854c 2498 Chem. Commun. , 2004, 2498–2499 This journal is ß The Royal Society of Chemistry 2004 Published on 09 September 2004. Downloaded by York University on 23/10/2014 09:39:23. View Article Online / Journal Homepage / Table of Contents for this issue