«A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Chemical Engineering) in the University ...»
Two novel case studies of the crucial role of heterogeneous catalyst
supports: core@shell nanostructure and photocatalysis
Kevin A. Dahlberg
A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
in the University of Michigan
Professor Johannes W. Schwank, Chair
Adjunct Professor Galen B. Fisher
Professor Nicholas Kotov
Professor Richard M. Laine
Assistant Professor Dominika K. Zgid Acknowledgements Many individuals and organizations deserve acknowledgement for their irreplaceable contributions to this thesis. Financial support was provided by the U.S. Army Tank-Automotive Research, Development & Engineering Center (TARDEC) and the University of Michigan MCubed. Kai Sun and Haiping Sun provided transmission electron microscopy (TEM) training and expertise at the Electron Microbeam Analysis Laboratory (EMAL). United Silica and the University of Michigan Auto Lab custom fabricated components for my photocatalytic reactor.
I thank Keegan Cisowski and Jingyi Li, who during their undergraduate research projects aided in the preparation of catalyst materials and shared in my excitement for my project. I owe many thanks to my fellow Schwank Group members from over the years, especially Steve Edmund, Sameer Parvathikar, Joe Mayne, Tom Westrich, Andy Tadd, and Xiaoyin Chen, for their mentorship, encouragement, collaboration, invaluable ideas at crossroads, help in the lab, and senses of humor. Above all I thank my Ph.D. advisor, Johannes Schwank, for his constant support and encouragement of new (and even risky) ideas.
I owe the most thanks to my family: my father, Todd Dahlberg, my mother, Sandy Woodford, and my soon-to-be wife, Carmen Allen. Without their unconditional love and investments in my life, I would be capable of far less.
And last and certainly most importantly, I thank my Creator, who created all the fascinating chemistry I have been privileged to study, and gave me the curious and meticulous mind to appreciate it.
ii Table of Contents Acknowledgements............................................................... ii List of tables..................................................................... v List of figures.................................................................... vi
Figure 1.1 Diagrams and TEM images representing core@shell, yolk-shell, and impregnated morphologies of a Ni-based catalyst with SiO2 support.
(a,d) Ni@SiO2 core@shell nanospheres, (b,e) Ni@SiO2 yolk-shell nanotubes, and (c,f) impregnated Ni/SiO2
Figure 2.2 TEM images of calcined Ni@SiO2 samples synthesized with varying N2H4 concentration: (a) N2H4/Ni = 3, (b) N2H4/Ni = 12, (c) N2H4/Ni = 24, (d) N2H4/Ni = 45.
(e) Nanotube morphology fraction, (f) nanotube cavity diameter distribution, (g) nanosphere diameter distribution, and (h) nanotube cavity length distribution with varying N2H4 concentration. 3 hr aging before TEOS addition; 50°C synthesis temperature................................ 32
Figure 2.3 TEM images of calcined Ni@SiO2 samples synthesized with varying synthesis temperature:
(a) 46°C, (b) 50°C, and (c) 54°C. (d) Nanotube morphology fraction, (e) nanotube cavity diameter distribution, and (f) nanotube cavity length distribution with varying synthesis temperature. 3 hr aging before TEOS addition; N2H4/Ni = 45........................ 34
Figure 2.5 (a) TEM image of calcined SiO2 sample from base case synthesis except with Ni(NO3)2 replaced with equivalent amount of H2O.
(b) TEM image of calcined Ni@SiO2 sample from base case synthesis except with addition of N2H4 following addition of NH3. 3 hr aging before TEOS addition; N2H4/Ni = 45 (or equivalent N2H4 concentration); 50°C synthesis temperature.................................. 37 Figure 2.6 Schematic illustration of gas-induced elongation profile of reverse-micelles over time. (a) Initial spherical reverse-micelles containing Ni2+ and N2H4, (b) larger spherical micelles containing small amounts of entrained gas, (c) elongated micelles with distinct gas phases and reduced Ni nanoparticles, and (d) further elongated micelles with re-formed spherical micelles.......................... 42 Figure 2.7 TEM images of Ni@SiO2 nanotubes (a) after synthesis and drying at ca. 100°C, (b) after calcination at 500°C in air, and (c) after reduction at 700°C in H2. 3 hr aging before TEOS addition; N2H4/Ni = 45; 50°C synthesis temperature............... 43
Figure 4.3 Photocatalytic CO2 formation rates on TiO2 between 50°C and 500°C under conditions of (a) thermal-only (no UV irradiation) with 2.
6 kPa H2O and 10 kPa O2, (b) photothermal (with UV irradiation) with 2.6 kPa H2O and 10 kPa O2, (c) thermal-only with no H2O and 10 kPa O2, (d) photothermal with no H2O and 10 kPa O2, (e) thermal-only with 2.6 kPa H2O and no O2, and (f) photothermal with 2.6 kPa H2O and no O2. Differences between thermal-only and photothermal CO2 formation rates on TiO2 between 50°C and 500°C under conditions of (g) 2.6 kPa H2O and 10 kPa O2, (h) no H2O and 10 kPa O2, and (i) 2.6 kPa H2O and no O2..... 82 Figure 4.4 Rates of photocatalytic ethylene oxidation on TiO2 between 50°C and 500°C with and without H2O to (a) CO2, (b) CO, and (c) H2CO. Selectivities between 50°C and 500°C on TiO2 to partial oxidation products (d) CO and (e) H2CO................ 83 Figure 4.5 Total photocatalytic ethylene oxidation rate on TiO2 between 50°C and 500°C with and without H2O, showing the contributions of each product.................. 85
Figure 4.9 In-situ DRIFTS spectra for TiO2 and Au/TiO2 under thermal-only and photothermal ethylene oxidation conditions.
Spectra for TiO2 without H2O under conditions of (a) thermal-only at 120°C, (b) photothermal at 120°C, (c) thermal-only at 390°C, and (d) photothermal at 390°C. Differences between thermal-only and photothermal spectra on TiO2 at (e) 120°C and (f) 390°C. Spectra for Au/TiO2 without H2O under conditions of (g) thermal-only at 120°C, (h) photothermal at 120°C, (i) thermal-only at 390°C, and (j) photothermal at 390°C. Differences between thermal-only and photothermal spectra on Au/TiO2 at (k) 120°C and (l) 390°C................... 94 Figure 4.10 Photocatalytic CO2 formation rates on SiC and Au/SiC without H2O between 50°C and 500°C. (a) SiC under thermal-only conditions, (b) SiC under UV irradiation, (c) Au/SiC under thermal-only conditions, and (d) Au/SiC under UV irradiation.
Differences between thermal-only and photothermal CO2 formation rates without H2O between 50°C and 500°C on (e) SiC and (f) Au/SiC........................ 97
Two catalysis technologies that are promising in the pursuit of more sustainable energy use, particularly reduced use of carbon-based fuels, are fuel reforming and photocatalysis.
Unfortunately these technologies each have well-known limitations in their performance, but novel engineering strategies of their catalyst supports could offer effective solutions.
Autothermal reforming of hydrocarbons can produce H2 for fuel cells, yielding greater overall process efficiency than the use of those hydrocarbons in a combustion engine. However, reforming catalysts suffer from metal particle growth, carbon deposition, and sulfur poisoning.
The benefits of core@shell nanostructured catalysts over conventional, impregnated catalysts are widely recognized and include increased activity and stability. Less recognized is the potential of the diffusion barrier for reactants and products imposed by the shell material to engender size selectivity characteristics, which may enhance the selectivity of equilibriumlimited reforming reactions. Yolk-shell nanostructured catalysts with encapsulated “egg white” cavities may retain the advantages of core@shell catalysts, and also have more available metal surface area and enhanced size selectivity characteristics due to the accumulation of reaction intermediates in cavities that contain the catalytically active surface.
Both Ni@SiO2 nanospheres and SiO2 nanotubes containing Ni nanoparticles were synthesized in a template nonionic surfactant water-in-oil microemulsion, and were characterized by bright field TEM. The highly defined cylindrical cavities in Ni@SiO2 nanotubes had a uniform diameter of 12-13 nm; however cavity lengths were shown to be strongly dependent on aging time before silica precursor addition, hydrazine concentration, and synthesis temperature.
x Depending on synthesis conditions, Ni@SiO2 nanotubes attained lengths up to 2 microns. A hypothesis is advanced that gas phases form and remain entrained in reverse-micelles, effecting both their elongation and also the exclusion of aggregating silica species to outer micellar regions, thereby causing cavity formation. SiO2 shell thicknesses could be controlled in the range of 5.1 to 12.4 nm by simple variation of the amount of silica precursor used.
Furthermore, H2 chemisorption measurements demonstrated 14-15% Ni dispersions, and N2 physisorption analyses showed intraparticle pores on the order of 1 nm, confirming the accessibility of active metal sites via diffusion through the porous silica shells.
Ni@SiO2 nanotubes, Ni@SiO2 nanospheres, and impregnated Ni/SiO2 were comparatively evaluated for propane autothermal reforming performance at 700°C. Results showed that Ni@SiO2 nanotubes and Ni@SiO2 nanospheres both had stable activity and H2 selectivity during 20 hours on stream, whereas impregnated Ni/SiO2 deactivated continuously over the same time. TGA and TEM analyses of samples following reaction experiments showed that Ni@SiO2 materials resisted particle growth and carbon deposition, but also experienced SiO 2 shell sintering with unknown consequences for long-term performance. Interestingly, size selectivity characteristics were indicated in the higher H2, higher CO2, and lower CO selectivities observed with Ni@SiO2 catalysts compared to impregnated Ni/SiO2. These results suggested that H2, formed in the SiO2-encapsulated space in contact with Ni nanoparticles, was selectively removed from this space due to its relatively fast diffusion through the SiO2 shell. These results demonstrate that core@shell nanostructured catalysts hold major promise for reforming applications, and the performance of Ni@SiO2 may be further improved by utilizing other shell materials less prone to sintering under reforming conditions, such as Al2O3 or ZrO2.
The primary limitation in photocatalysis is low photo efficiency caused by high rates of photogenerated charge carrier recombination, and improvements are needed for photocatalysis to become a practical and economically viable technology. In principle, photocatalysis can offset some of the demand on fossil fuels compared to conventional catalysis by using solar energy to provide the activation energy for catalysts rather than heat. Our group has previously shown for xi the first time that photoactivity occurs and even increases at elevated temperatures. There are two beneficial photocatalysis phenomena that potentially occur at elevated temperature with the deposition of Au nanoparticles on the surface of TiO2. Both phenomena involve the migration of photo-generated electrons to deposited Au nanoparticles according to a wellknown mechanism. First, this decreases the rate of electron-hole recombination in TiO2, which could increase its photocatalytic activity at elevated temperature. Second, this could cause an electrochemical promotion of the Au catalytic activity.
The photoactivities of TiO2 and Au/TiO2 catalysts prepared by flame-spray pyrolysis and photodeposition were evaluated by C2H4 oxidation experiments in a novel quartz plate reactor and in in-situ DRIFTS experiments. STEM images showed that Au particles had an average size of 11.6 nm. Results showed no photochemical promotion of Au, likely due to a combination of loss of surface hydroxyl species and Au particle growth at elevated temperature. However, a new photocatalytic reaction pathway was discovered on TiO2 alone which had a maximum rate near 400°C, and which was enhanced by Au nanoparticles in the presence of H2O. This new pathway may represent an exciting new type of photocatalysis, accessible at high temperature, and inspires future research to understand what other reactions may be catalyzed by its chemistry. Moreover, photocatalytic promotion of catalysis may be achieved by stabilizing a high metal dispersion in close contact with the semiconductor, and a Au@TiO2 nanostructured photocatalyst is in view.
Introduction It was reported that the global catalyst market totaled approximately $15.1 billion US dollars in 2012, and is projected to reach $20.1 billion in 2015 . In addition to aiding in the manufacture of an estimated two-thirds of products in the chemical industry , catalysts are utilized in transportation fuel refining and in environmental applications. Catalysts are a vital technology for the world’s economy and the environment, and there is an ongoing need for innovations and developments in catalysis to meet major current and future challenges in sustainable industrial practice, energy efficiency and production, and environmental protection.