X-ray diffraction and H-storage in ultra-small palladium particles D.G. Narehood a , S. Kishore a,1 , H. Goto c , J.H. Adair b , J.A. Nelson b,2 , H.R. Gutie ´rrez a , P.C. Eklund a,b, * a Department of Physics, Pennsylvania State University, University Park, PA 16802 USA b Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802 USA c Honda Fundamental Research Laboratories, Tokyo, Japan article info Article history: Received 9 July 2007 Received in revised form 21 October 2008 Accepted 21 October 2008 Available online 17 December 2008 Keywords: Hydrogen storage Palladium Nanoparticle X-ray Palladium hydride abstract In situ X-ray diffraction (XRD) and gravimetric hydrogen uptake measurements of d w 2– 3 nm spherical PdH x particles have been studied in the temperature and pressure range of 323 < T < 428 K and 0 < P < 10 bar. The Pd particles were protected from sintering with a hydrogen-permeable carbon coating. While only containing w300–1000 atoms, the Pd particles were found to exhibit the same fcc structure and lattice constant as the bulk. Our isothermal studies show that, with increasing x, these highly crystalline PdH x nano- particles also exhibit a complete transformation from the dilute a solid solution phase to the more concentrated b hydride phase. However, we observed that the character of the a– b phase transition in these nanoparticles is very different from that in the bulk. Indeed, the hydrogen uptake isotherm exhibits a noticeable positive slope in the a þ b co-existence region. Furthermore, we also observed a noticeable narrowing of the a þ b co-existence region (dx) in the nanoparticles. Also, a significant suppression of the critical temperature T c for the phase boundary was observed: T c (nano) z 430 K while T c (bulk) z 570 K. These results signal a significant change in the thermodynamic behavior of very small hydride nanoparticles that may be common to many other nano-scale metal hydride systems as well. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction The transformation of bulk palladium into a hydride was first reported almost 140 years ago [1]. Since that time, PdH x has been widely studied in bulk form as a classic hydride system. The interest in PdH x over the years stems, in part, due to advantageous materials issues, such as a relatively high hydrogen concentration for an elemental metal, the noble character of palladium, the availability of pure samples, and the fact that no special surface treatments of the bulk sample are usually required. As a result, bulk PdH x is possibly one of the best-understood chemical hydrides [1,2]. Recently, there * Corresponding author. Pennsylvania State University, Department of Physics, 104 Davey Lab, University Park, PA 16802, USA. Tel.: þ1 814 865 5233; fax: þ1 814 865 9851. E-mail address: pce3@psu.edu (P.C. Eklund). 1 General Electric Research Center, Bangalore 560066, INDIA 2 Gillette Technical Center, 37 A Street, Needham, MA 02494 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.10.080 international journal of hydrogen energy 34 (2009) 952–960