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On the Kinetic Interpretation of DFT-Derived Energy Profiles: Cu-Catalyzed Methanol Synthesis

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Abstract

A mean field microkinetic evaluation of previously reported DFT-derived Gibbs free energy profiles for CO and CO2 hydrogenation to methanol on Cu(111), Cu(211) and Zn-modified Cu(211) is presented. It is demonstrated that explicit consideration of the effect of surface coverages of reaction intermediates on rates is needed in order to arrive at a realistic evaluation of the activity and selectivity. In particular, both the methanol formation rate and the CO/CO2 selectivity for methanol production are demonstrated to be highly sensitive to the saturation coverage of formate at steady state. In general, the study emphasises the importance of including explicit kinetic analyses when mechanistic DFT-derived energy profiles are interpreted for catalytic processes.

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Notes

  1. See Supplementary Material for more information.

  2. Given that experimental Cu-catalyzed MeOH synthesis selectivity is >99%, it is reasonable to assume that selectivity towards formic acid as product will not exceed 1 %. Therefore, the formic acid pressures associated with 1 % formic acid production in the kinetic runs were determined and found to be of the order of 0.28 mbar for all CO2 hydrogenation cases considered. Analysis 2 runs thus treat this formic acid pressure as a fixed kinetic model input parameter essential in ensuring the reincorporation of formic acid into the mechanism for methanol production. Furthermore, the formic acid pressure is required to be relatively low to ensure that both formic acid and methanol are produced from CO2 hydrogenation, rather than the production of CO2 and methanol from the consumption of externally introduced formic acid. From an experimental perspective this approach is regarded as a more accurate kinetic treatment of CO2 conversion via formic acid to methanol compared to Analysis 1 where no formic acid pressure build-up is allowed, artificially inhibiting the formation of methanol. Further validity of the low formic acid pressure is reflected in the similarity of kinetic data obtained in Table 2 (formally gas phase HCOOH treatment) and Table 3 (formally surface intermediate HCOOH* treatment) for the Cu(211) surface

  3. See footnote 2

  4. See Footnote 2

  5. See details as discussed in the Supplementary Material of both Refs. [9] and [10]

  6. Analysis 2 data in Table 2 is considered to be more appropriate than the data corresponding to Analysis 1 for this comparison to the results in Table 3, because of the artificial omission of any formic acid pressure in Analysis 1. See Footnote 2 for more details

  7. Note that for these energy profiles and microkinetic analyses no gas phase formation of formic acid is considered, but rather the formation of surface intermediate HCOOH*, effectively eliminating the possibility for formic acid to form as a product.

  8. See Footnote 7

  9. The 0.31 ML formate saturation coverage was calculated from the 3.4 × 1014 formates/cm2 coverage [reported by Henn FC, Rodriguez JA, Campbell CT (1990) Surf. Sci. 236:282] based on a Cu(110) (1 × 1) surface unit cell with dimensions a = 3.6147 Å and b = 2.5560 Å.

  10. The formate saturation coverage limit is defined as the sum of the saturation coverage limit of HCOO* and HCOOH* on the surface. It is found that HCOOH* constitutes at most only 0.02 ML of the combined total coverage, effectively implying that the steady state coverage for these two species is mainly due to HCOO* on the surface. For the RPBE functional a saturation coverage limit range of 0.1–0.8 ML was considered, because it is evident from the RPBE steady state formate coverage for combined CO and CO2 hydrogenation in Table 3 that the highest coverage of 0.86 ML obtained represents an upper limit for formate coverage. Similarly, for the BEEF-vdW functional a saturation coverage limit of 0.1–0.9 ML was considered because the upper limit for formate saturation coverage approaches 0.97 ML (Table 3). See Supplementary Material for more details

  11. The procedure, based on the methods described by (a) Saltelli A, Ratto M, Tarantola S, Campolongo F (2012) Chem. Rev. 112:PR1 and (b) Stegelmann C, Anfreasen A, Campbell CT (2009) J. Am. Chem. Soc. 131:8077, involved isolated increase of each of the seven barriers in the CO2 hydrogenation reaction sequence by 0.1 eV in each case determining the methanol formation rate at steady state. Four sets of data were obtained for each of the RPBE and BEEF-vdW determined DFT data sets at 0.5 and 1.0 ML formate saturation coverage limit, respectively

  12. See Footnote 9

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Acknowledgement

Prof Eric van Steen (University of Cape Town) is acknowledged for his inputs and proof reading of the manuscript.

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Authors and Affiliations

  1. R&D Division, Sasol Technology (Pty) Ltd, 1 Klasie Havenga Road, Sasolburg, 1947, South Africa

    Werner Janse van Rensburg, Melissa A. Petersen, Michael S. Datt, Jan-Albert van den Berg & Pieter van Helden

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  1. Werner Janse van Rensburg
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  2. Melissa A. Petersen
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  3. Michael S. Datt
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  4. Jan-Albert van den Berg
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  5. Pieter van Helden
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Correspondence to Werner Janse van Rensburg.

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Janse van Rensburg, W., Petersen, M.A., Datt, M.S. et al. On the Kinetic Interpretation of DFT-Derived Energy Profiles: Cu-Catalyzed Methanol Synthesis. Catal Lett 145, 559–568 (2015). https://doi.org/10.1007/s10562-014-1407-1

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