Surface nanometric sulphur and carbon moieties in Ni-catalyzed steam reforming of hydrocarbons

1. Université de Sherbrooke Surface nanometric sulphur and carbon moieties in Ni-catalyzed steam reforming of hydrocarbons N. Abatzoglou, Kandaiyan Shanmuga Priya S. Rakass, H. Oudghiri-Hassani and P. Rowntree Department of Chemical & Biotechnological Engineering

2. Outline • Introduction • Rationale • Actual knowledge • Materials and methods • Results • Conclusions • Acknowledgments

3. Introduction Rationale • Previous published work by the authors proved the efficiency of pristine micrometric Ni powders as steam reforming catalysts • Sulfur contamination of the Ni surface is known to cause catalyst partial or total deactivation • Commercial natural gas is artificially contaminated with alkanethiols and sulfides (i.e tert-butyl-mercaptan and di-methyl-sulfide) This work tries to elucidate the role of the sulfur at the surface of Ni-based catalysts

4. Introduction Scientific background (1) Conventional supported Ni catalysts are known to deactivate by sintering, sulfur passivation and carbon deposition • The sulfur compounds in gasoline and H2S produced from these sulfur compounds in the hydrocarbon reforming process are poisonous to the Reforming and WGS catalysts • Deactivation of supported metal catalysts by carbon formation is another serious problem in steam reforming due to: • fouling of the metal surface • blockage of catalyst pores • loss of the structural integrity of the catalyst support material

5. Scientific background (2) • Sulfur passivated reforming process (SPARG) : Trace amount (2ppm) of H2S with the feed gas. • S selectively poisons active sites of Ni catalyst - Small loss in the reforming activity. Rationale: Trace amounts of S affect the deactivation rate much more than the reforming rate. • Adsorbed S deactivate the occupied Ni site, thus changing the “Number/Surface unit” of the catalytically active ensembles. • Size of these ensembles is critical in allowing SR with minimal formation of coke. • SR is thought to involve ensembles of 3-4 Ni atoms, while C formation requires 6-7 Ni atoms. • Complete coverage of catalyst with S results in total deactivation; however, at S coverage of around 70% of saturation, C deposition could effectively be eliminated while SR still proceeds. J.R. Rostrup-Nielsen, J. Catal. 85 (1984) 31

6. Scientific background (3) • Interfacial reactions between H2S and Ni surface leads to rapid adsorption of monolayer of S atoms on Ni surface. • These observations are consistent with predictions from first-principles calculations : H2S dissociation on transition-metal surfaces has small dissociation barriers (weak H-S bonds), and high exothermicities (strong S-metal bonds). • Self-assembled monolayers (SAM) are formed from adsorption of organothiols on metal surfaces such as Au and Ni. • G.A. Sargent, G.B. Freeman, J.L.Chao, Surf. Sci 100 (1980) 342. • B. McAllister, P. Hu, J. Chem. Phys. 122 (2005) 84709. • S. Rakass, H. Oudghiri-Hassani, N. Abatzoglou & P. Rowntree, J. Power Sources 162 (2006) 579.

7. Conclusions based on TPD & XPS Adsorbed CH3S on Ga sites exhibits greater thermal stability than CH3SH because surface hydrogen is absent. Comparison between the adsorptions of CH3SH and CH3SSCH3: dialkyl disulfides can produce a thiolate layer; the resulting monolayer survives to a greater temperature than that obtained from alkanethiols because surface hydrogen is not produced during adsorption. Stable thiolate self assembled monolayer is suggested to be prepared by adsorption of diakyl disulfides, rather than alkanethiols. Scientific background (4) T.P Huang, T.H. Lin, T.F. Teng, Y.H. Lai, W.H.Hung, Surf. Sci. 603(2009)1244-1252.

8. Based on DFT calculations A new S-Ni phase diagram Existence of an intermediate state between pure Ni and nickel sulfide Ni3S2-S atoms adsorbed on Ni surfaces due to rapid reaction of H2S with Ni(100) and Ni(111) surfaces. Clear distinction between Ni surfaces partially covered with adsorbed S atoms and bulk Ni3S2. Accurate prediction of this adsorption phase is vital to a fundamental understanding of the sulfur poisoning mechanism of Ni-based anodes. Scientific background (5) J.H. Wang, M. Liu, Electrochem.Commun., 9 (2007) 2212-2217

9. Materials and methods The unsupported Ni powder • Inco Ni 255 • BET Surface = 0.44 m2/g • Particle size distribution: 1-20µm • Open filamentary structure and irregular spiky surface • Produced by the thermal decomposition of Ni(CO)4

10. Powder I (1-20µm) Volume (%) Number (%) Materials and methods SEM of the Ni Powder

11. Thiols/Disulfides as S-source • Thiols : H-(CH2) n -SH, with n = 4, 5, 6 and 10 • All liquids at room temperature and used as received: • n-decanethiol (Aldrich, 98%) • n-hexanethiol (Aldrich, 98%) • n-pentanethiol (Aldrich, 99%) • n-butanethiol (Aldrich, 99%) • Disulfides : All liquids at room temperature and used as received from Aldrich. • Ethyl disulfide - C4H10S2 • Propyl disulfide - C6H14S2 • Iso pentyl disulfide – C10H22S2 • Hexyl disulfide – C12H26S2 • Methanol (Aldrich, 99%) used as solvent.

12. Materials and methods Ni Impregnation • Pristine Ni powder in 10-3 M sol. of alkanethiols/methanol • 5g of Ni in 100 ml of solution: several orders of magnitude excess thiol as compared to the monolayer quantities • Immersion time under stirring: 20 h • Rinsed thoroughly with fresh methanol • Samples dried for 12 hours at ambient temperature

13. Materials and methods Experimental set-up A multi-differential isothermal reactor set-up equipped with a gas humidification system, a programmable furnace and coupled to a Quadrupole Mass Spectrometer

14. b:Four a: b a c C: Materials and methods The differential reactor set-up

15. Materials and methods The differential reactor set-up: details

16. Materials and methods Basic experimentalprotocol • The reactant gas is composed of Ultra high purity CH4 andsteam • Ar was used as inert diluent • The partial pressure of water in the gas is used to regulate the CH4/H2O • The gas compositions and flow rates are controlled by rotameters • The flow rate used was 25 ml/min per tube • 0.25 g of catalyst packed into the quartz tubes and retained by quartz wool • The inner tubes include porous fused quartz disks (coarse porosity of 40-90 mm, 1.5 cm diameter) supporting the Ni catalyst bed • No entrainment of catalyst particles occurs downstream • The reforming tests were conducted at a CH4/H2O molar ratio of 1:2 and at sufficiently low GHSV

17. Materials and methods Experimental campaigns • Q1: What happens to the Ni ? • Steam Reforming with pristine and alkanethiols- impregnated Ni • Q2: What if the surfaces are thermally pretreated? • Steam reforming with thermally pretreated pristine and alkanethiols impregnated Ni • Q3: Which is the source of the aromatic carbon? • CH4 vs Alkanethiols

18. Results 0: Analyses before steam reforming DRIFTS spectra of the as-prepared thiol- contaminated Ni catalysts

19. Results 0: Analyses before steam reforming XPS spectra of the as-prepared thiol- contaminated Ni • carbon C(1s) • (b) sulfur S(2p)

20. Results 0: Analyses before steam reforming S/Ni Evaluation through XPS • The coverage ratio of the Ni by the sulfur increases with the chain length of the alkanethiol molecule • The longer chain species lead to a higher number density of adsorbates (alkanethiol molecules) on the Ni powder surfaces.

21. Results 1: Steam Reforming Gas composition and T profile over time-on-stream for steam reforming with Pristine Ni catalyst

22. Results 1: Steam Reforming Methane Conversion for Ni and Ni-C5S

23. Results 1: Steam Reforming Gas composition and T profiles over time-on-stream for steam reforming with impregnated Ni

24. Results 1: Steam Reforming Observations (1) • The high catalytic activity and stability of Ni-C4S and Ni-C5S catalysts were similar to that of pristine Ni catalysts • The activity of Ni-C6S catalysts decreased for temperatures above 580oC • No activity was obtained over the Ni-C10S at any temperature

25. Results 1: Steam Reforming XPS spectra after steam reforming • carbon C(1s) • (b) sulfur S(2p)

26. Results 1: Steam Reforming Carom/Ni and S/Ni after steam reforming

27. Results 1: Steam Reforming S/Ni before and after steam reforming

28. Results 1: Steam Reforming Observations (2) • In all cases, the total sulfur content (S/Ni) decreased following use in steam reforming • The quantity of aromatic carbon for the thiol contaminated Ni catalysts measured after their use in steam reforming test increased with the length of the alkyl chain. • The observed deactivation of Ni-C6S and Ni-C10S during the steam reforming of methane may be due to: • the deposition of aromatic carbon on the catalyst surface • a permanent poisoning of the surface caused by the high level of chemisorbed sulfur species

29. Results 2: Thermal Pretreatment and Steam Reforming Gas composition and T profile over time-on-stream for steam reforming with thermally pretreated Ni at 700°C

30. Results 2: Thermal Pretreatment and Steam Reforming Gas composition and T profile over TOS for steam reforming with thermally pretreated at 700°C impregnated Ni

31. Results 2: Thermal Pretreatment and Steam Reforming XPS spectra • carbon C(1s) • (b) sulfur S(2p)

32. Results 2: Thermal Pretreatment and Steam Reforming Carom/Ni and S/Ni

33. Results 2: Thermal Pretreatment and Steam Reforming S/Ni without and with thermal pretreatment

34. Results 2: Thermal Pretreatment and Steam Reforming Car/Ni without and with thermal pretreatment

35. Results 2: Thermal Pretreatment and Steam Reforming Observations (2) • The catalytic activity of the Ni contaminated by the short chain thiols decreases over time following the Ar thermal pretreatment at 700oC • For Ni-C6S and Ni-C10S, no catalytic activity was observed • The S/Ni is lower in the case of the thermal pretreatment; but, the catalytic activity is worse ! • The Carom/Ni is higherin the case of the thermal pretreatment

36. Results 2: Thermal Pretreatment and Steam Reforming Conclusion Despite the reduced S content, the Ni-C4S and Ni-C5S samples exhibit reduced catalytic activity following the Ar thermal pretreatment These findings suggest that the loss of catalytic activity observed for the thiol-contaminated Ni samples is due to the accumulation of aromatic carbon on the Ni surface

37. Results 3: CH4 vs Alkanethiols Which molecule is responsible for the formation of aromatic carbon ? Are the pre-adsorbed alkanethiols or feed-gas CH4?

38. Results 3: CH4 vs Alkanethiols XPS spectra after thermal treatment under Ar at 700°C for 2h • carbon C(1s) • (b) sulfur S(2p)

39. Results 3: CH4 vs Alkanethiols Carom/Ni and S/Ni a) after thermal treatment and b) after steam reforming The area coverage by aromatic carbon and sulfur are similar to those reported for thiol contaminated Ni catalysts after their use in steam reforming test These results confirm that the formation of aromatic carbon is due to the degradation of the n-alkanethiols pre-adsorbed on the nickel surfaces

40. Results 3: CH4 vs Alkanethiols XPS C(1s) spectra of Ni-C6S catalyst obtained after its use in steam reforming up to a temperature of (A) 400°C, (B) 580°C and (C) 700°C

41. Results 3: CH4 vs Alkanethiols Observations (4) The Ni-C6S catalyst was deactivated as the temperature exceeded ~580oC and at this temperature the area coverage percentage of aromatic carbon was 4.6% Estimated threshold for significant surface deactivation

42. Conclusions • The longer alkyl chain species lead to increased surface coverage on the catalyst • The catalytic activity of the Ni-C4S, Ni-C5S, Ni-C6S and Ni-C10S catalysts depends on the alkyl chain lengths • The deactivation of the unsupported Ni catalysts is mainly due to the coverage of the catalyst surface by aromatic-aliphatic carbon

43. Conclusions (cont.) • The formation of aromatic-aliphatic carbon during steam reforming was found to be due to the pyrolysis of carbon from n-alkanethiols preadsorbed on the catalyst surface and not from the methane feed gas • A Ni surface area coverage by aromatic carbon of over 4.6% leads to complete deactivation of Ni catalyst surface

44. RecentExperimentalcampaigns • Q1 : Compare Ni-255 disulfide vs thiol impregnation • Q2 : What happens if the disulfide impregnated catalysts were thermally • treated (TT) followed by SR? • Q3 : Is there any change in the ratio of reforming to WGS reaction due to • the different chain length of disulfides? • Q4: What is the reason for the catalyst deactivation as chain length of • disulfide increases; surface C or S species?

45. Results & Discussion

46. Results & Discussion

47. Results & Discussion

48. Results & Discussion

49. TT-TOS-Ni 255 TT-TOS-C4S2 TT-TOS-C6S2 TT-TOS-C6S Results & Discussion

50. Results & Discussion XPS