Elsevier

Journal of Catalysis

Volume 344, December 2016, Pages 806-816
Journal of Catalysis

Catalytic routes and oxidation mechanisms in photoreforming of polyols

https://doi.org/10.1016/j.jcat.2016.08.009Get rights and content

Highlights

  • Photoreforming and oxidation mechanisms of polyols on Rh/TiO2 for H2-generation.

  • Anodic reaction network for glycerol photoreforming was quantitatively elucidated.

  • Oxidative C–C cleavage is attributed to direct transfer of photogenerated holes.

  • Formation of carbonyl moieties and dehydration result from indirect hole transfer.

  • C–C bond rupture is favored with increasing polyol carbon number.

Abstract

Photocatalytic reforming of biomass-derived oxygenates leads to H2 generation and evolution of CO2 via parallel formation of organic intermediates through anodic oxidations on a Rh/TiO2 photocatalyst. The reaction pathways and kinetics in the photoreforming of C3–C6 polyols were explored. Polyols are converted via direct and indirect hole transfer pathways resulting in (i) oxidative rupture of C–C bonds, (ii) oxidation to α-oxygen functionalized aldoses and ketoses (carbonyl group formation) and (iii) light-driven dehydration. Direct hole transfer to chemisorbed oxygenates on terminal Ti(IV)-OH groups, generating alkoxy-radicals that undergo ß-C–C-cleavage, is proposed for the oxidative C–C rupture. Carbonyl group formation and dehydration are attributed to indirect hole transfer at surface lattice oxygen sites [Ti⋯O⋯Ti] followed by the generation of carbon-centered radicals. Polyol chain length impacts the contribution of the oxidation mechanisms favoring the C–C bond cleavage (internal preferred over terminal) as the dominant pathway with higher polyol carbon number.

Introduction

Photocatalytic H2 generation from bio-derived oxygenates (‘photoreforming’) is a desired pathway for the production of a chemical energy carrier utilizing solar energy as it lowers the energy requirements compared to water cleavage [1], [2], [3], [4], [5]. Aqueous glycerol, an abundant by-product from triglyceride transesterification, and polyol-containing wastewaters, e.g. from industry or catalytic upgrading conceptually, could be suitable feedstocks for photoreforming [1], [2], [3], [4], [6]. Thereby valorization and/or purification of those resources are coupled to the production of H2. Photoreforming benefits from a narrow energetic separation of the two redox half-reactions (E0(H+/H2) = 0 V; e.g. for glycerol photoreforming E0(CO2/C3H8O3) = −0.004 V vs. NHE [7]), which provides a large overpotential at the anode facilitating cathodic H2 evolution. Moreover, compared to overall water splitting, substitution of the oxygen evolution reaction for anodic oxygenate oxidation to CO2 eliminates the need of separation and the back-reaction of H2 and O2 [8].

While the electron-hole recombination and charge carrier transport to the surface and their relation to physicochemical properties of the photocatalyst have been explored with great depth [9], [10], [11], [12], the role of the chemically coupled reactions have been hardly explored mechanistically and kinetically. In such reactions, the co-catalyst decorated semiconductor acts as coupled micro-electrochemical cell [13], [14], [15]. Anodic half-reactions are thought to occur on the semiconductor surface as a consequence of interfacial transfer of photogenerated holes through either direct transfer to the oxygenate or via an indirect mechanism, e.g. mediated by radical dotO(H)-radicals [6], [16], [17]. The co-catalyst serves as cathode, electron trap, and, thus, H2 evolution site and does not participate in the anodic half-reactions [18], [19]. Yet, H2-evolution and thus oxygenate degradation rates (due to charge balance) are influenced by co-catalyst nature [7], loading and particle size [7], [20] as well as its composition and morphology [21], [22]. The co-catalyst could even aid the suppression of surface back-reactions at the cathode [8], [23]. These factors provide essential means for optimization of the efficiency of electron transfer at the semiconductor/co-catalyst interface.

It is established that H2-evolution rates over TiO2-based photocatalysts depend on oxygenate nature and coverage [3], [7], [24], [25], [26], [27], [28]. However, there is ambiguity in the anodic pathways and mechanisms toward full oxidation to CO2. Initial anodic transformations of polyols over metal/metal oxide loaded TiO2 were proposed to result in the formation of the corresponding aldehydes, which undergo either decarbonylation followed by water-gas shift [17] or cleavage of formic acid [19]. Oxidation to carboxylic acids that decarboxylate [6], [16] or initial polyol dehydration to compounds with non-functionalized carbon atoms [17] was also suggested prior to C–C-cleavage. On the other hand, photoreforming of linear aldehydes was reported to involve sequential cleavage of formic acid to form the C1-deficient aldehyde in a single reaction pathway [19], [29].

During aerobic photooxidation of oxygenates, multiple reaction pathways were accounted for by a two-site model for direct and indirect mechanisms [30], [31], [32]. Holes trapped at surface lattice oxygen sites [Ti⋯O⋯Ti] abstract a H-atom from a C–H bond whereas direct hole transfer yields alkoxy radicals chemisorbed on terminal OH-groups. We hypothesize that under photoreforming conditions (anaerobic environment and hydrogen evolving) similar mechanisms operate, which determine the routes toward complete oxidation. We have shown in a preceding contribution that the anodic transformations of ethylene glycol during photoreforming may be rationalized on this basis [18]. We further propose that identical anodic transformations are followed across families of compounds. However, the impact of molecular polyol structure and associated surface adsorption complexes on the contribution of the different reaction pathways to the complete oxidation is largely unknown.

Here, we explore the mechanisms and kinetics of photoreforming of C3–C6 polyols on benchmark TiO2 P 25 with Rh as co-catalyst. On the basis of quantitative analysis of gas- and liquid-phase species we establish general relationships between structural reactant functional groups, anodic reaction pathways and oxidation mechanisms. We provide rationalization of the impact of anodic surface chemistry on the photoreforming kinetics in terms of oxygenate conversion and associated H2-evolution rates.

Section snippets

Experimental

A comprehensive list of compounds used during the study, and detailed experimental procedures regarding the characterization of the photocatalyst are compiled in the Supporting Information. Briefly, experiments are conducted over AEROXIDE® TiO2 P 25 (referred to as TiO2 hereafter), commonly employed as benchmark semiconductor, decorated with nanoparticulate Rh as a co-catalyst (1 wt.% loading, particle size 1.9 nm (±0.7 nm), dispersion 57%). Further physicochemical properties of the materials are

Glycerol photoreforming

Photoreforming of glycerol (Scheme 1) proceeded with continuous evolution of H2 and CO2 (1405 μmol and 366 μmol, respectively after 12 h reaction time) reaching a conversion of 39% after 12 h. The H2-evolution rates declined over time from a maximum value of 143 μmol h−1 to about 95 μmol h−1 after 12 h (Fig. 1A). This decline is related to a first-order dependence on concentration (SI-Fig. 3, see kinetic model in Section 3.2 and Supporting Information). Chemical transformations were not observed in

Conclusions

Photocatalytic rates of linear C1–C3 oxygenates are primarily dependent on the substrate specific apparent adsorption constants and follow a Langmuir adsorption model. In the anodic half-reactions of photoreforming linear polyols are converted via (i) oxidative rupture of C–C bonds, (ii) oxidation to the corresponding aldoses or ketoses or (iii) light-driven dehydration, while evolving H2 at the cathode. The first pathway is proposed to result from direct hole transfer to the chemisorbed

Acknowledgments

We would like to thank Clariant for fruitful discussions within the framework of MuniCat and the iC4 PhotoCOO project. The authors are grateful to Donald M. Camaioni for critical reading of the manuscript. Additionally, we would like to thank the German Federal Ministry of Education and Research (BMBF) for financial support (project no. 01RC1106A). K.E.S. gratefully acknowledges financial support by the Fond der Chemischen Industrie (FCI). The authors thank Xaver Hecht for BET and H2

References (58)

  • Z.H.N. Al-Azri et al.

    J. Catal.

    (2015)
  • F. Dionigi et al.

    J. Catal.

    (2012)
  • A.J. Bard

    J. Photochem.

    (1979)
  • J. Chen et al.

    Water Res.

    (1999)
  • P. Panagiotopoulou et al.

    Catal. Today

    (2013)
  • T.F. Berto et al.

    J. Catal.

    (2016)
  • H. Bahruji et al.

    J. Photoch. Photobio. A

    (2010)
  • H. Bahruji et al.

    Appl. Catal. B

    (2011)
  • G.N. Nomikos et al.

    Appl. Catal. B

    (2014)
  • X. Fu et al.

    Int. J. Hydrogen Energy

    (2008)
  • V.M. Daskalaki et al.

    Catal. Today

    (2009)
  • R. Chong et al.

    J. Catal.

    (2014)
  • C. Minero et al.

    Appl. Catal. B

    (2012)
  • J. Schnaidt et al.

    J. Electroanal. Chem.

    (2011)
  • C.A. Martins et al.

    Electrochim. Acta

    (2011)
  • L. Roquet et al.

    Electrochim. Acta

    (1994)
  • X. Fu et al.

    Appl. Catal. B

    (2011)
  • T. Sakata et al.

    Chem. Phys. Lett.

    (1981)
  • X.-J. Zheng et al.

    Int. J. Hydrogen Energy

    (2009)
  • M. Bellardita et al.

    Int. J. Hydrogen Energy

    (2016)
  • W. van Bronswijk et al.

    Colloid. Surface A

    (1999)
  • K. Shimura et al.

    Energy Environ. Sci.

    (2011)
  • R.M. Navarro et al.

    Energy Environ. Sci.

    (2009)
  • D. Kondarides et al.

    Catal. Lett.

    (2008)
  • Y. Ma et al.

    Chem. Rev.

    (2014)
  • X. Chen et al.

    Chem. Rev.

    (2010)
  • M. Cargnello et al.

    Eur. J. Inorg. Chem.

    (2011)
  • W. Jiao et al.

    ACS Catal.

    (2012)
  • J. Zhang et al.

    Angew. Chem. Int. Ed.

    (2008)
  • Cited by (0)

    View full text