The Medford Group
Computational Catalysis Research
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Publications

Medford group publications


 The proposed carbon-mediated catalytic cycle for photocatalytic nitrogen fixation. Surface-bound carbon radical species are generated by photo-oxidation of adventitous hydrocarbons on the surface of TiO2. These reactive carbon species efficiently bind dinitrogen, and subsequent reduction occurs through photo-generated electrons.

The proposed carbon-mediated catalytic cycle for photocatalytic nitrogen fixation. Surface-bound carbon radical species are generated by photo-oxidation of adventitous hydrocarbons on the surface of TiO2. These reactive carbon species efficiently bind dinitrogen, and subsequent reduction occurs through photo-generated electrons.

Benjamin M. Comer, Yu-Hsuan Liu, Marm B. Dixit, Kelsey B. Hatzell , Yifan Ye, Ethan J. Crumlin , Marta C. Hatzell, and Andrew J. Medford, JACS (2018) doi:10.1021/jacs.8b08464

Photo-catalytic fixation of nitrogen by titania catalysts at ambient conditions has been reported for decades, yet the active site capable of adsorbing an inert N2 molecule at ambient pressure and the mechanism of dissociating the strong dinitrogen triple bond at room temperature remain unknown. In this work in situ near-ambient-pressure X-ray photo-electron spectroscopy and density functional theory calculations are used to probe the active state of the rutile (110) surface. The experimental results indicate that photon-driven interaction of N2 and TiO2 is observed only if adventitious surface carbon is present, and computational results show a remarkably strong interaction between N2 and carbon substitution (C*) sites that act as surface-bound carbon radicals. A carbon-assisted nitrogen reduction mechanism is proposed and shown to be thermodynamically feasible. The findings provide a molecular-scale explanation for the long-standing mystery of photo-catalytic nitrogen fixation on titania. The results suggest that controlling and characterizing carbon-based active sites may provide a route to engineering more efficient photo(electro)-catalysts and improving experimental reproducibility.

 
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Andrew J. Medford,  M. Ross Kunz, Sarah M. Ewing, Tammie Borders, and Rebecca Fushimi, ACS Catalysis (2018) doi:10.1021/acscatal.8b01708

Catalysis informatics is a distinct subfield that lies at the intersection of cheminformatics and materials informatics but with distinctive challenges arising from the dynamic, surface-sensitive, and multiscale nature of heterogeneous catalysis. The ideas behind catalysis informatics can be traced back decades, but the field is only recently emerging due to advances in data infrastructure, statistics, machine learning, and computational methods. In this work, we review the field from early works on expert systems and knowledge engines to more recent approaches utilizing machine-learning and uncertainty quantification. The data–information–knowledge hierarchy is introduced and used to classify various developments. The chemical master equation and microkinetic models are proposed as a quantitative representation of catalysis knowledge, which can be used to generate explanative and predictive hypotheses for the understanding and discovery of catalytic materials. We discuss future prospects for the field, including improved quantitative coupling of experiment/theory, advanced microkinetic models, and the development of open-source software tools. Ultimately, integration of existing chemical and physical models with emerging statistical and computational tools presents a promising route toward the automated design, discovery, and optimization of heterogeneous catalytic processes.

 
 Free energies of nitrogen reduction intermediates for the associative mechanism on the rutile TiO2(110) surface calculated using density functional theory. The pristine surface, an oxygen vacancy defect, and iron substitution defect are investigated.

Free energies of nitrogen reduction intermediates for the associative mechanism on the rutile TiO2(110) surface calculated using density functional theory. The pristine surface, an oxygen vacancy defect, and iron substitution defect are investigated.

Benjamin C. Comer and Andrew J. Medford, ACS Sustainable Chemistry & Engineering (2018) doi:10.1021/acssuschemeng.7b03652

Photocatalytic nitrogen fixation provides a promising route to produce reactive nitrogen compounds at benign conditions. Titania has been reported as an active photocatalyst for reduction of dinitrogen to ammonia; however there is little fundamental understanding of how this process occurs. In this work the rutile (110) model surface is hypothesized to be the active site, and a computational model based on the Bayesian error estimation functional (BEEF-vdW) and computational hydrogen electrode is applied in order to analyze the expected dinitrogen coverage at the surface as well as the overpotentials for electrochemical reduction and oxidation. This is the first application of computational techniques to photocatalytic nitrogen fixation, and the results indicate that the thermodynamic limiting potential for nitrogen reduction on rutile (110) is considerably higher than the conduction band edge of rutile TiO2, even at oxygen vacancies and iron substitutions. This work provides strong evidence against the most commonly reported experimental hypotheses, and indicates that rutile (110) is unlikely to be the relevant surface for nitrogen reduction. However, the limiting potential for nitrogen oxidation on rutile (110) is significantly lower, indicating that oxidative pathways may be relevant on rutile (110). These findings suggest that photocatalytic dinitrogen fixation may occur via a complex balance of oxidative and reductive processes.

 
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Andrew J. Medford and Marta C Hatzell, ACS Catalysis (2017) doi:10.1021/acscatal.7b00439

Over the last century the industrialization of agriculture and the consumption of fossil fuels have resulted in a significant imbalance and redistribution in nitrogen containing resources. This has sparked an interest in developing more sustainable and resilient approaches for producing nitrogen-containing commodities such as fertilizers and fuels. One largely neglected but emerging approach is photocatalytic nitrogen fixation. There is significant evidence that this process occurs spontaneously in terrestrial settings, and it has been demonstrated in numerous engineered systems. Yet many questions still remain unanswered regarding the rates, mechanisms and impacts of photocatalytically producing fixed nitrogen "out of thin air". This work reviews the fascinating history of the reaction and examines current progress toward understanding and improving photo-fixation of nitrogen. This is supplemented by a quantitative review of the thermodynamic considerations and limitations for various reaction mechanisms. Finally, future prospects and preliminary performance targets for photocatalytic nitrogen fixation are discussed.


Affiliated Publications


31) “Vision for Data and Informatics in the Future Materials Innovation Ecosystem”
S. R. Kalidindi, A. J. Medford, D. L. McDowell
JOM (2016) 68 pp. 2126-2137

30) “Framework for Scalable Adsorbate-Adsorbate Interaction Models”
M. J. Hoffmann, A. J. Medford, T. Bligaard
Journal of Physical Chemistry C (2016) 120 pp. 13087-13094

29) “Analyzing the Case for Bifunctional Catalysis”
M. Andersen, A. J. Medford, J. K. Nørskov, K. Reuter
Angewandte Chemie (2016) 55 pp. 5210-5214

28) “On the Intrinsic Selectivity and Structure Sensitivity of Rhodium Catalysts for C2+ Oxygenate Production”
N. Yang, A. J. Medford, X. Liu, F. Studt, T. Bligaard, S. Bent, J. K. Nørskov
Journal of the American Chemical Society (2016) 138 pp. 3705-3714

27) “Degree of rate control approach to computational catalyst screening”
C. A. Wolcott, A. J. Medford, F. Studt, C. T. Campbell
Journal of Catalysis (2015) 330 pp. 197-207

26) “From the Sabatier principle to a predictive theory of transition-metal heterogeneous catalysis”
A. J. Medford, A. Vojvodic, J. S. Hummelshøj, J. Voss, F. Abild-Pedersen, F. Studt, T. Bligaard, A. Nilsson, J. K. Nørskov
Journal of Catalysis (2015) 328 pp. 36-42

25) “CatMAP: A software package for descriptor-based micro-kinetic mapping of catalytic trends”
A. J. Medford, C. Shi, M. J. Hoffmann, A. C. Lausche, S. Fitzgibbon, T. Bligaard, J. K. Nørskov
Catalysis Letters (2105) 145 pp. 794-807

24) “Assessing the reliability of calculated catalytic ammonia synthesis rates”
A. J. Medford, J. Wellendorff, A. Vojvodic, F. Studt, F. Abild-Pedersen, K. W. Jacobsen, T. Bligaard, J. K. Nørskov
Science 345 (2014) pp. 197-200

23) “Departures from the adsorption energy scaling relations for metal carbide catalysts”
R. Michalsky, Y. Zhang, A. J. Medford, A. A. Peterson
Journal of Physical Chemistry C 118 (2014) pp. 13026-13034

22) “Exploring the limits: A low-pressure, low-temperature Haber-Bosch process”
A. Vojvodic, A. J. Medford, F. Studt, F. Abild-Pedersen, T. S. Khan, T. Bligaard, J. K. Nørskov
Chemical Physics Letters 598 (2014) pp. 108-112

21) “High pressure CO hydrogenation over bimetallic Pt-Co catalysts”
J. M. Christensen, A. J. Medford, F. Studt, A. D. Jensen
Catalysis Letters 144 (2014) pp. 777-782

20)  “Methanol-to-hydrocarbons conversion: The alkene methylation pathway”
R. Y. Brogaard, H. R. Schuurman, A. J. Medford, P. G. Moses, P. Beato, S. Svelle, J. K. Nørskov, U. Olsbye
Journal of Catalysis 314 (2014) pp. 159-169

19) “Activity and selectivity trends in synthesis gas conversion to higher alcohols”
A. J. Medford, A. C. Lausche, F. Abild-Pedersen, B. Temel, N. C. Schjødt, J. K. Nørskov, F. Studt
Topics in Catalysis 57 (2014) pp. 135-142

18) “Thermochemistry and micro-kinetic analysis of methanol synthesis on ZnO(0001)”
A. J. Medford, J. Sehested, J. Rossmeisl, I. Chorkendorff, F. Studt, J. K. Nørskov, P. G. Moses
Journal of Catalysis 309 (2014) pp. 397-407

17) “On the effect of coverage-dependent adsorbate-adsorbate interactions for CO methanation on transition metal surfaces”
A. C. Lausche, A. J. Medford, T. S. Khan, Y. Xu, T. Bligaard, F. Abild-Pedersen, J. K. Nørskov, F. Studt
Journal of Catalysis 307 (2013) pp. 275-282

16) “Finite-size effects in O and CO adsorption for the late transition metals”
A. A. Peterson, L. C. Grabow, T. P. Brennan, B. Shong, C. Ooi , D. M. Wu, C. W. Li, A. Kushwaha, A. J. Medford, F. Mbuga, L. Li, J. K. Nørskov
Topics in Catalysis 55 (2012) pp. 1276-1282

15) “Elementary steps of syngas reactions on Mo2C(001): Adsorption thermochemistry and bond dissociation”
A. J. Medford, A. Vojvodic, F. Studt, F. Abild-Pedersen, J. K. Nørskov
Journal of Catalysis 290 (2012) pp. 108-117

14) “Electrocatalytic interaction of nano-engineered palladium on carbon nanofibers with hydrogen peroxide and β-NADH”
Z. Lin, L. Ji, A. J. Medford, Q. Shi, W. E. Krause, X. Zhang
Journal of Solid State Electrochemistry 15 (2011) pp. 1287-1294

13) “An inter-laboratory stability study of roll-to-roll coated flexible polymer solar modules”
S. A. Gevorgyan, A. J. Medford, E. Bundgaard, F. C. Krebs et. al.
Solar Energy Materials and Solar Cells 95 (2011) pp. 1398-1416

12) “Ultra-fast and parsimonious materials screening for polymer solar cells using differentially pumped slot-die coating”
J. Alstrup, M. Jørgensen, A. J. Medford, F. C. Krebs
ACS Applied Materials and Interfaces 2 (2011) pp. 2819-2827

11) “The effect of post-processing treatments on inflection points in current-voltage curves of roll-to-roll processed polymer photovoltaics”
M. R. Lilliedal, A. J. Medford, M. V. Madsen, K. Norrman and F. C. Krebs
Solar Energy Materials and Solar Cells 94 (2011) pp. 2018-2031

10) “Grid-connected polymer solar panels: initial considerations of cost, lifetime, and practicality”
A. J. Medford, M. R. Lilliedal, M. Jørgensen, D. Aarø, H. Pakalski, J. Feynbo, and F. C. Kreb
Energy Express 18 (2010) pp. A272-A285

9) “Assembly of carbon-SnO2 core-sheath composite nanofibers for superior lithium storage”
L. Ji, Z. Lin, B. Guo, A. J. Medford, X. Zhang
Chemistry-A European Journal 16 (2010) pp. 11543-11548

8) “Formation and electrochemical performance of copper/carbon composite nanofibers”
L. Ji, Z. Lin, R. Zhou, Q. Shi, O. Toprakci, A. J. Medford, C. R. Millns, and X. Zhang
Electrochimica Acta 55 (2010) pp. 1605-1611

7) “Fabrication of carbon nanofiber-driven electrodes from electrospun polyacrylonitrile/polypyrrole bicomponents for high-performance rechargeable lithium-ion batteries”
L. Ji, Y. F. Yao, O. Toprakci, Z. Lin, Y. Z. Liang, Q. Shi, A. J. Medford, C. R. Millns, and X. Zhang
Journal of Power Sources 195 (2010) pp. 2050-2056

6) “In-situ encapsulation of nickel particles in electrospun carbon nanofibers and the resultant electrochemical performance”
L. Ji, Z. Lin, A. J. Medford, and X. Zhang
Chemistry-A European Journal 15 (2009) pp. 10718-10722

5) “Porous carbon nanofibers from electrospun polyacrylonitrile/SiO2 composites as an energy storage material”
L. Ji, Z. Lin, A. J. Medford, and X. Zhang
Carbon 47 (2009) pp. 3346-3354

4) “Electrospun polyacrylonitrile micro- and nanofibers with dispersed Si nanoparticles and their electrochemical behaviors after carbonization”
L. Ji, K. H. Jung, A. J. Medford, and X. Zhang
Journal of Materials Chemistry 19 (2009) pp. 4992-4997

3) “Porous carbon nanofibers loaded with manganese oxide particles: formation mechanism and electrochemical performance as energy-storage materials”
L. Ji, A. J. Medford, and X. Zhang
Journal of Materials Chemistry 19 (2009) pp. 5593-5601

2) “Fabrication of carbon fibers with nanoporous morphologies from electrospun polyacrylonitrile/poly(L-lactide) blends”
L. Ji, A. J. Medford and X. Zhang
Journal of Polymer Science B 47 (2009) pp. 493-503

1). “Electrospun polyacrylonitrile/zinc chloride composite nanofibers and their response to hydrogen sulfide”
L. Ji, A. J. Medford, and X. Zhang
Polymer 50 (2009) pp. 605-612


 
Essentially, all models are wrong, but some are useful.
— George Box