Hughes, Matt (2014) Silica supported transition metal phosphides- Alternative materials for the water-gas shift reaction. PhD thesis. https://theses.gla.ac.uk/5548/ Copyright and moral rights for this work are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This work cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Enlighten: Theses https://theses.gla.ac.uk/ [email protected] Silica supported transition metal phosphides- Alternative materials for the water-gas shift reaction Matt Hughes School of Chemistry University of Glasgow Thesis presented for fulfilment of the degree Doctor of philosophy June 2014 Abstract Transition metal phosphides remain relatively unexplored as water-gas shift catalysts; however the little that has been done has produced promising results in terms of performance due to the presence of oxygen rich phases. They can be produced through a number of approaches, the most common being reduction of the phosphate precursor using hydrogen. Bimetallic phosphides have also generated interest recently, e.g. with the addition of cerium there are improvements to the phosphides‟ activity and with the addition of palladium a decrease in reduction temperature is evident in the precursor material. Nickel phosphide and a range of bimetallic derivatives on a silica support were investigated for their activity in both high and low temperature water-gas shift reactions. All of the catalysts were tested using a continuous flow, fixed bed reactor. Characterization of the catalysts was carried out using XRD, EXAFS, XPS, FTIR, SEM and TEM. The comparison of supported nickel phosphide and an iron based industrial catalyst shows supported nickel phosphide to have good mass normalized activity in comparison to the industrial counterpart under high temperature water-gas shift conditions. The results complement previous studies with regards to potential of nickel phosphide as a water-gas shift catalyst. In view of the results obtained so far for other transition metal phosphides, Ni2P appears to be superior for the water-gas shift reaction. One of the most promising aspects of using nickel phosphide in the water-gas shift reaction is its potential resistance to sulfur. Catalyst poisoning caused by impurities such as sulfur are known to deactivate current commercial catalysts for both high and low temperature shift conditions, therefore a catalyst which is resistant to sulfur is an attractive prospect. The sulfur resistant properties of the nickel phosphide catalysts were highlighted by testing them under high temperature water-gas shift conditions in the presence of sulfur. Whilst the catalysts deactivated under the conditions, they showed some residual catalytic activity still remained after sulfur was removed from the feed. Compared with the industrial catalyst, nickel phosphide displayed greater resistance to deactivation. 2 Acknowledgements I would like to take this opportunity to acknowledge the many people that help me throughout the duration of my PhD, without their help I would not have be able get to this point. I would first like to thank both of my supervisors, Dr Justin Hargreaves and Prof. David Jackson for their support and guidance throughout. I would also like to thank my industrial supervisors, Dr Martin Fowles, and Dr Norman MacLeod for all of their support as well as taking the time to come to Glasgow and accommodate me down in Billingham. I would like to say a big thank you to Andy Monaghan for all his help with technical aspects of day to day life in lab and for being there when things go wrong especially with the XRD. I would also like to say thanks to everyone in the Hargreaves and Jackson groups for their help in various aspects of the project over the last 4 years and for generally being a great bunch to spend the last few years working with. Finally, thanks to my friends and family for always being there for me and helping me see light at the end of the tunnel, especially in the last few months. Finally I would like to say thanks to Catriona Smith my wonderful girlfriend who has been there through all the ups and downs of the PhD you truly have the patience of saint! 3 Declaration The work contained in this thesis, submitted for the degree of Doctor of Philosophy, is my original work, except where due reference is made to other authors. No material within has been previously submitted for a degree at this or any other university. …………………………………………………………………………………………………………… Matt Hughes 4 Contents Chapter 1: Introduction .................................................................. 20 1.1 Background ........................................................................... 20 1.2 The water-gas shift reaction ................................................... 21 1.3 Thermodynamics of the water-gas shift reaction ....................... 23 1.4 Kinetics and proposed reaction mechanisms for the water -gas shift reaction .............................................................................. 25 1.4.2 The regenerative mechanism ............................................ 25 1.4.3 The associative mechanism .............................................. 27 1.5 Water-gas shift process catalysts ........................................... 30 1.6 Sulfur poisoning of water-gas shift catalysts ............................ 32 1.7 Transition metal phosphides ................................................... 33 1.7.1 General properties of transition metal phosphides .............. 34 1.7.2 Review of recent publications in the synthesis of transition metal phosphides. .................................................................... 37 1.7.4 Review of recent publications in the catalytic application of transition metal phosphides. ...................................................... 43 1.8 References ........................................................................... 50 Chapter 2: Experimental procedure ................................................. 53 2.1 Catalyst Synthesis ................................................................. 53 2.1.1 Micro reactor for material synthesis ................................... 54 2.1.2 Bulk phase synthesis ........................................................ 54 2.1.3 Synthesis of supported materials ....................................... 57 2.2 List of all nickel phosphide catalysts ....................................... 58 2.3 Testing Material Stability ....................................................... 59 2.4 Summary of bulk phase synthesis of nickel phosphide .............. 59 2.5 Water-Gas Shift Reaction ....................................................... 60 2.5.1 Reactor procedure ........................................................... 62 5 2.5.2 Gas chromatographic analysis of gases ............................. 63 2.5.3 Mass flow controllers (MFCs) ............................................ 64 2.5.4 GC gas standards ............................................................ 65 2.5.5 Sulfur poisoning ............................................................... 66 2.7 Characterization of materials .................................................. 66 2.7.1 Thermo-gravimetric analysis (TGA) .................................... 67 2.7.2 Scanning electron microscope ........................................... 67 2.7.3 Powder X-ray diffraction ................................................... 68 2.7.4 X-ray photoelectron spectroscopy ...................................... 69 2.7.5 Diffuse Reflectance Infrared Spectroscopy (DRIFTS) .......... 70 2.7.6 Transmission Electron Microscopy (TEM) ........................... 72 2.7.7 Extended X-ray absorption fine structure spectroscopy (EXAFS) .................................................................................. 74 2.7.8 Raman Spectroscopy ........................................................ 76 2.8 Calculations .......................................................................... 78 2.8.1 CO conversion ................................................................. 78 2.8.2 Carbon balance ................................................................ 78 2.8.3 Selectivity of carbon based products ................................. 78 2.8.4 Water-gas shift reaction equilibrium ................................... 79 Chapter 3: Nickel phosphide, a novel material fo r application in the water-gas shift reaction .................................................................. 80 3.1 Introduction .......................................................................... 80 3.2 Catalytic activity of Nickel Phosphide ...................................... 80 3.3 Development and characterisation of bulk phase nickel phosphide ................................................................................................. 83 3.4 Preparation and characterisation of silica supported nickel phosphide ................................................................................ 101 3.5 Catalytic testing of silica supported nickel phosphide ............. 117 6 3.6 Characterisation of post-reaction silica supported nickel phosphide