Deposition of palladium nanoparticles in SBA-15 templated silica using supercritical carbon dioxide Andrew J. Hunt a,n , Vitaliy L. Budarin a , James W. Comerford a , Helen L. Parker a , Vlado K. Lazarov b , Simon W. Breeden a , Duncan J. Macquarrie a , James H. Clark a a Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK b Department of Physics, University of York, Heslington, York, YO10 5DD, UK article info Article history: Received 29 July 2013 Accepted 19 November 2013 Available online 27 November 2013 Keywords: Porous materials Nanoparticles Impregnation Textural XPS abstract Preparation of supported nanoparticles by palladium deposition onto silica (SBA-15) from a solution of tris(dibenzylideneacetone) dipalladium(0) (Pd 2 (dba) 3 ) in supercritical carbon dioxide (scCO 2 ) was successfully achieved. Pd 2 (dba) 3 avoided agglomeration of nanoparticles, which is typically associated with secondary processes including reduction of metal salts. scCO 2 impregnation was superior at depositing palladium nanoparticles within the porous channels of SBA-15, as compared to organic solvents. The resulting material demonstrated significant potential for use in catalytic applications including hydrogenations and showed specific selectivity towards alkene hydrogenation. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Over the last 10 years investigation of supported metal nanopar- ticle formation has received significant attention [1–3]. Considerable focus has been directed towards the nature of nanoparticle support; these frequently include mesoporous silicas which possess large surface areas [1,4]. Addition of metal precursors to these supports can lead to nanoparticle formation with narrow pore size distribution; such materials demonstrate promise as selective catalysts [5, 6]. In particular, as a support the templated mesoporous silica, SBA-15 (also known as Santa Barbara-15) has been demonstrated to possess high stability and catalytic activity [7, 8]. Formation of nanoparticles within mesoporous networks has received significant literature attention [8–10]. However, a major drawback preventing widespread success of these materials is a lack of uniform distribution and blockage of the porous network due to limited diffusion of precursors [9]. Some organic solvents such as ethanol have been reported to give larger disordered external nanoparticles, however, supercritical carbon dioxide (scCO 2 ) has been demonstrated to allow increased dispersion into pores [11]. scCO 2 has enhanced diffusivity and reduced viscosity, ideal for impregnation as compared to traditional solvents [12]. Work by Tenorio et al. has demonstrated that palladium hexa- fluoroacetylacetonate (Pd(hfac) 2 ) and scCO 2 have significant potential in formation of nanoparticles within the internal struc- ture of templated silicas, however use of Pd(hfac) 2 requires further processing (reduction) after impregnation [13]. This results in palladium agglomeration and thereby reduced the potential activ- ity of these materials as catalysts. Herein, we describe a unique approach utilising the properties of scCO 2 and tris(dibenzylide- neacetone) dipalladium(0) (Pd 2 (dba) 3 ) for preparation of sup- ported palladium nanoparticles by deposition into the porous network of SBA-15. Pd 2 (dba) 3 has a zero oxidation state required for efficient catalytic activity, thus eliminating the need for secondary processing. 2. Methods Pd 2 (dba) 3 (54.3 mg) and SBA-15 (250 mg) were mixed and pressurised with carbon dioxide to 41.4 MPa and 85 1C for 4 h in a Supercritical Fluid Technologies SFT-150. Depressurisation took place over 8 h and the resulting sample was washed with acetone (20 ml) and dried in vacuo. Further experimental procedures are included in Supporting information. 3. Results and discussion Impregnation of Pd 2 (dba) 3 into SBA-15 was achieved with acetone (known as Ac-Pd) and scCO 2 (known as sc-Pd). Both SBA-15 catalysts were prepared for identical durations and temperatures. X-ray photo- electron spectroscopy (XPS) data showed four species of palladium Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.075 n Corresponding author. Tel. þ44 1904 322546; fax: þ44 1904 322705. E-mail address: andrew.hunt@york.ac.uk (A.J. Hunt). Materials Letters 116 (2014) 408–411