In This Issue
The Bridge: 50th Anniversary Issue
January 7, 2021 Volume 50 Issue S
This special issue celebrates the 50th year of publication of the NAE’s flagship quarterly with 50 essays looking forward to the next 50 years of innovation in engineering. How will engineering contribute in areas as diverse as space travel, fashion, lasers, solar energy, peace, vaccine development, and equity? The diverse authors and topics give readers much to think about! We are posting selected articles each week to give readers time to savor the array of thoughtful and thought-provoking essays in this very special issue. Check the website every Monday!

Catalysis and the Future of Transportation Fuels

Monday, February 15, 2021

Author: José G. Santiesteban and Thomas F. Degnan Jr.

Transportation is a large and diverse sector that encompasses road (passenger and freight vehicles), aviation, marine, and rail transport. In 2018 this sector accounted for nearly a quarter of global anthropogenic carbon dioxide emissions,[1] so efforts to decarbonize it are critical to achieving the goals of the Paris Agreement.[2]

But the transition to a net-zero-emission transportation sector will take decades, cost hundreds of billions of dollars, and may never be complete (Ogden et al. 2016). Decarbonization of the sector will depend on advances in technology, policies, incentives, investment in infrastructure, and the manufacture of low-carbon and zero-emission vehicles.

There is no single energy carrier that, in the foreseeable future, can satisfy requirements across all aspects of the transportation sector. For example, the prospects for battery-powered supersonic aircraft remain quite distant (Epstein 2020; Langford and Hall 2020). In the interim, a “bridging” low-carbon energy transition strategy will rely on the combined increased use of energy carriers such as electrons, hydrogen, and lower-emission liquid fuels (advanced biofuels and synthetic liquid fuels).

Catalytic Challenges

Catalyst technologies have played an essential role in the economic and energy-efficient conversion of crude oil into liquid energy carriers that meet the demands of the current transportation sector (Rostrup-Nielsen 2004). Catalytic breakthroughs have also played a crucial role in onboard abatement systems to eliminate emissions of environmental pollutants such as SOx and NOx (Farrauto et al. 2019). Novel catalyst technologies are needed to enable the low-carbon energy transition for the transportation sector.

The fuel and vehicle industries must focus their catalytic expertise on improving the sustainability of and reducing the nonrenewable carbon footprint of liquid fuels. Significant advances are needed in three areas:

  1. improving the yield and quality of biofuels,
  2. increasing the amount of both “green” and “blue” hydrogen[3] produced and incorporated in hydro­carbon fuels, and
  3. reducing the cost and improving the robustness of fuel cells.

Biofuels

A major obstacle in the direct substitution of bio­fuels (e.g., biodiesel) for conventional hydrocarbons is the ubiquitous presence of chemically bonded oxygen in biomass. The substitution of biomass-derived diesel and gasoline for conventional liquid fuels requires the near-complete removal of oxygen (oxygenates can lead to the formation of gum and engine deposits and they lessen the energy content per unit volume or mass). Oxygen can be catalytically removed through the selective addition of hydrogen to biomass in a process known as hydrodeoxygenation; the process is analogous to the methods used in the petroleum industry for sulfur and nitrogen removal. However, for deoxygenated fuels to be economical, more selective catalysts must be identified and developed.

Biodiesel is composed of long-chain hydrocarbons, whose fluidity often has to be improved by modifying the chemical structure. Improved catalysts that both optimally rearrange the hydrocarbon structure and simultaneously remove oxygen would constitute a significant advance.

Hydrogen

The concept of using hydrogen as a transportation fuel has always been environmentally attractive, but economically challenged—and likely to remain so. The high pressures (and low temperatures) required for onboard H2 storage are daunting, as are the prospects for setting up a national—or even statewide—­distribution system.

Liquid fuels with higher hydrogen content derive more of their energy from the production of H2O than from CO2, thereby creating more energy per unit mass of CO2 produced. Cheaper hydrogen would enable a more hydrogen-rich fuel supply.

About 95 percent of all hydrogen is produced by steam methane reforming (SMR). SMR (CH4 + H2O ® CO + 3H2), when coupled with the water gas shift process (CO + H2O ® CO2 + H2), produces 5.5 tonnes of CO2 for every tonne of hydrogen (not counting the CO2 generated by the heat required for the net endothermic process). Thus, CO2 needs to be captured and sequestered to enable low-carbon (blue) hydrogen (van Hulst 2019). New catalytic systems may make it possible to use biomass (e.g., cellulose and/or municipal solid waste gasification) rather than natural gas–derived methane as the hydrogen source.

Electrolysis is less economically attractive than SMR. However, some companies are constructing green hydrogen plants based on large-scale electrolysis using wind and solar (Parnell 2020). Catalysts used either in electrolysis or for the purification of SMR hydrogen are both expensive and susceptible to poisoning by a number of contaminants in the feed streams. Identifying improved catalysts that allow the design and economical manufacture of small modular SMR, with carbon capture, or electrolysis units would open up many new possibilities for further reducing greenhouse gases.

Fuel Cells

New catalysts and catalytic systems are needed to improve the economics of fuel cells suitable for ­vehicles. Fuel cells can use a range of sources for hydrogen, including methanol (direct methanol fuel cells), ­ethanol, and even gasoline.

Proton-exchange membrane fuel cells (PEMFCs) are approximately three times more efficient than internal combustion engines in converting chemical energy to power, but they require an expensive noble metal catalyst, platinum (Pt), that is particularly sensitive to impurities in the hydrogen-rich fuel. Identification of a substitute for the Pt catalyst would reduce the cost of fuel cells by as much as 25 percent (Mitchem 2020).

A potentially more economically attractive alternative to the PEMFC is the anion exchange membrane fuel cell, which does not require Pt and uses less expensive metal catalysts thanks to the high pH of the electrolyte. The performance and durability of anion exchange membrane fuel cells have recently been significantly improved through the development of new catalytic materials, improved systems design, and refinement of operating conditions (Gottesfeld et al. 2017).

Finally, several academic research groups around the world are advancing the science and technology to fabricate systems that combine solar energy–gathering semiconductors and photocatalytic materials to drive chemical reactions to produce sustainable liquid fuels (Wadsworth et al. 2019). Of particular interest is the use of semiconductor photoelectrodes for water splitting. This area of photoelectrochemistry for solar energy conversion dates back at least 40 years, but has garnered a tremendous amount of attention in the last decade (e.g., Lee et al. 2019).

Summary

The pathway to a net-zero-emission transportation sector must capitalize both on liquid fuels that produce less CO2 from nonrenewable sources and on CO2 capture and sequestration. This strategy translates into ­greater reliance on biomass, green and blue hydrogen, and improved fuel cells. The identification and development of improved catalysts is the critical enabler for advances in all three of these areas.

The road to a more sustainable transportation ­sector involves research on more product-selective catalysts incorporating earth-abundant elements (e.g., iron, ­copper, nickel, molybdenum). Particularly attractive will be catalysts that are designed around the use of ­photons or electrons rather than heat to drive the desired chemical transformations.

References

Epstein AH. 2020. Aeropropulsion: Advances, opportunities, and challenges. The Bridge 50(2):8–14.

Farrauto RJ, Deeba M, Alerasool S. 2019. Gasoline automobile catalysis and its historical journey to cleaner air. Nature Catalysis 2:603–13.

Gottesfeld S, Dekel DR, Page M, Bae C, Yan Y, Zelenay P, Kim YS. 2017. Anion exchange membrane fuel cells: ­Current status and remaining challenges. Journal of Power Sources 375:170–84.

Langford JS, Hall DK. 2020. Electrified aircraft propulsion. The Bridge 50(2):21–27.

Lee DK, Lee D, Lumley MA, Choi KS. 2019. Progress on ternary oxide-based photoanodes for use in photoelectro­chemical cells for solar water splitting. Chemical Society Reviews 48(7):2126–57.

Mitchem S. 2020. Platinum-free catalysts could make ­cheaper hydrogen fuel cells. Press release, May 20. Lemont IL: Argonne National Laboratory.

Ogden J, Fulton L, Sperling D. 2016. Making the transition to light-duty electric-drive vehicles in the US: Costs in perspective to 2035. UC Davis Institute of Transportation Studies Program.

Parnell J. 2020. World’s largest green hydrogen project unveiled in Saudi Arabia. Greentech Media, Jul 7.

Rostrup-Nielsen JR. 2004. Fuels and energy for the future: The role of catalysis. Catalysis Reviews 46(3-4):247–70.

van Hulst N. 2019. The clean hydrogen future has already begun. Commentary, Apr 23. Paris: International Energy Agency.

Wadsworth BL, Beiler AM, Khusnutdinova D, Reyes Cruz EA, Moore GF. 2019. The interplay between light flux, quantum efficiency, and turnover frequency in molecular-modified photoelectrosynthetic assemblies. Journal of the American Chemical Society 141(40):15932–41.

 

 

 

Improved catalysts that both optimally rearrange the hydrocarbon structure and simultaneously remove oxygen would constitute a significant advance.

 

 

[1]  Of the total 8 billion tons of CO2 emitted by transportation in 2018, 45% came from passenger vehicles, 29% from road freight vehicles, 12% from aviation, 11% from shipping, 1% from rail, and the remaining 2% from other sources. (International Energy Agency, https://www.iea.org/topics/transport).

[2]  https://ec.europa.eu/clima/policies/international/ negotiatio ns/paris_en

[3]  Blue hydrogen is made from natural gas through the process of steam methane reforming coupled with carbon capture and storage; green hydrogen is produced from water using renewable power.

About the Author:José Santiesteban (NAE) is strategy manager at ExxonMobil Research and Engineering Co. Thomas Degnan (NAE) is the Anthony Earley Professor of Energy (emeritus) in the Department of Chemical and Biomolecular Engineering at the University of Notre Dame.