Article pubs.acs.org/JPCC
Conversion of 1,2-Propylene Glycol on Rutile TiO2(110) Long Chen, Zhenjun Li,† R. Scott Smith, Bruce D. Kay, and Zdenek Dohnálek* Fundamental and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: We have studied the reactions of 1,2-propylene glycol (1,2-PG), DOCH(CH3)CH2OD, on partially reduced, hydroxylated, and oxidized TiO2(110) surfaces using temperature-programmed desorption. On reduced TiO2(110), propylene, propanal, and acetone are identified as primary carboncontaining products. While the propylene formation channel dominates at low 1,2-PG coverages, all of the above-mentioned products are observed at high coverages. The carbon-containing products are accompanied by the formation of D2O and D2. The observation of only deuterated products shows that the source of hydrogen (D) is from the 1,2-PG hydroxyls. The role of bridging oxygen vacancy (VO) sites was further investigated by titrating them via hydroxylation and oxidation. The results show that hydroxylation does not change the reactivity because the VO sites are regenerated at 500 K, which is a temperature lower than the 1,2-PG product formation temperature. In contrast, surface oxidation causes significant changes in the product distribution, with increased acetone and propanal formation and decreased propylene formation. Additionally, D2 is completely eliminated as an observed product at the expense of D2O formation.
1. INTRODUCTION The conversion of renewable biomass resources to biofuels and value-added chemicals has been receiving increased interest due to the limited availability of fossil fuels.1,2 Because of the complexity of biomass composition and a vast number of simultaneous reaction pathways, a molecular level understanding of biomass conversion on metal and/or metal oxide catalysts is extremely difficult. Typically, the simplest analogues, such as ethylene glycol (EG), propylene glycols (PGs), and glycerol, have been used as biomass surrogates to probe catalytic activity and selectivity for larger polyols and sugars.3−9 The surface chemistry of EG has been extensively studied on several well-defined single crystal metal surfaces.5,6,10−15 On most of these surfaces, the bidentate ethylenedioxy species is believed to be the initial intermediate formed by O−H bond scission. This species further decomposes either by C−H bond cleavage to form glyoxal (CHO)2 and H2 or by C−C bond cleavage, yielding CO and H2. However, different behaviors have been observed on Ag(110),12 where EG adsorbs reversibly, and on Mo(110),13 which favors C−O bond scission to evolve ethylene. Compared to EG, previous studies examining the reaction of 1,2-PG on transition metal surfaces are limited to Pd(111)6 and Ag(110)14 surfaces. On both of these surfaces, the behavior of 1,2-PG parallels that of EG. In contrast to the extensive efforts on single-crystal metal surfaces, there has been very limited activity on the surface chemistry of glycols on single-crystal metal oxides.4,7−9,16 Currently, the most detailed studies have been carried out on TiO2(110), which is one of the most extensively studied model oxide surfaces.17−19 Using high resolution scanning tunneling microscopy (STM) and density functional theory (DFT), our © 2014 American Chemical Society
group has recently investigated the adsorption and reaction of EG and 1,3-PG on partially reduced TiO2(110) (designated as r-TiO2(110)).7,8 Both glycols were found to initially adsorb on 5-fold coordinated surface titanium (Ti5c) sites, where a dynamic equilibrium between molecularly bound and deprotonated species was observed (HO(CH2)nOH + Ob ⇄ HO− (CH2)n−OTi5c + HOb, where Ob and HOb denote bridging oxygen and bridging hydroxyl, respectively). As the glycols started to diffuse along the Ti5c rows above 230 K, they were found to dissociate irreversibly upon encountering bridge bonding oxygen vacancy (VO ) sites. Interestingly, the dissociation was shown to proceed via two competitive channels involving O−H or C−O bond scission (HO(CH2)nOH + VO + Ob → Ob−(CH2)n−OH + HOb) that yield in both cases pairs of Ob−(CH2)nOH and HOb species. The comparison of EG and 1,3-PG provided molecular level insight into how steric effects introduced by the second Ti5c bound OH can weaken the C−O bond and thereby facilitate its cleavage. The resulting Ob−(CH2)nOH species were observed to rotate around their Ob anchor, switching the position of the Ti5c bound OH between the two neighboring Ti5c rows. The OH group of the monoalkoxide species was further observed to dissociate on a Ti5c site, yielding a bidentate type dioxo Ob(CH2)nOTi5c species and an additional HOb. The Ob− (CH2)n−OTi5c species can recombine with HOb to reversibly convert back to Ob−(CH2)nOH, thus forming a dynamic Received: May 14, 2014 Revised: June 23, 2014 Published: June 23, 2014 15339
dx.doi.org/10.1021/jp504770f | J. Phys. Chem. C 2014, 118, 15339−15347
The Journal of Physical Chemistry C
Article
equilibrium between the two species (Ob−(CH2)nOH + Ob ⇄ Ob(CH2)nOTi5c+ HOb) at 300 K. At elevated temperatures (>400 K), formation of a new Ob(CH2)nOb intermediate was observed and interpreted to be a result of the reaction of a diffusing VO with a Ti5c bound oxygen of the dioxo species, Ob(CH2)nOTi5c + VO → Ob(CH2)nOb. Further annealing was shown to result in a sequential C−Ob bond cleavage and alkene desorption above 500 K (Ob(CH2)nOb → CnH2n + 2Ob). While STM results provide an unprecedented level of insight into the reactivity and dynamics of glycols on TiO2(110), they are limited to low coverages (∼VO concentration) where isolated species can be observed. In order to probe the surface chemistry of glycols on TiO2(110) at higher coverages, other surface sensitive techniques were employed. Using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS), Farfan-Arribas and Madix followed products from the saturation coverage of EG on “defect free” and “defective” surfaces.16 On both types of surfaces, ethylene and acetaldehyde were observed as the primary carboncontaining products between 550 and 700 K with a small amount of ethanol. On the “defective” surface, an additional ethylene feature evolved between 450 and 550 K. Using a combination of TPD with molecular beam dosing, our group recently examined the coverage dependent product formation from EG on r-TiO2(110).9 Two reaction channels have been identified: one leading to ethylene and water and the other to acetaldehyde and hydrogen. The ethylene formation channel was found to dominate at low EG coverages and plateau as the coverage was increased to saturation, while acetaldehyde was primarily observed at high EG coverages. The hydrogen formation on TiO2(110) at high EG coverages is rather interesting,9 as it was not observed previously for water, alcohols, and other simple organic molecules.19−22 In our recent work,23 we have systematically studied hydrogen formation from glycols on TiO2(110) using TPD. We found that hydrogen originates exclusively from the glycol hydroxyl groups and that increasing the steric hindrance around the hydroxyls by adding methyl end-groups (from EG to 2,3butylene glycol and 2,3-dimethyl-2,3-butylene glycol) impedes and eventually eliminates hydrogen formation. Further, scavenging the available surface charge by oxidation also led to suppressed hydrogen formation. These results indicate that hydrogen formation results from proximal glycol pairs, and that the redox reaction is driven by defect electrons in TiO2(110). The subject of this study, 1,2-propylene glycol, allows us to further compare deoxygenation of both primary and secondary alcohol functionalities. Using TPD, we show that the reactions of 1,2-PG with r-TiO2(110) generally parallel those of EG. We find the following: (i) Acetone is formed in addition to propylene and propanal as carbon-containing products. (ii) D2 formation from 1,2-PG is significantly reduced, while D2O formation is dramatically enhanced as compared to EG. (iii) The evolution of propylene from 1,2-PG occurs at a slightly lower temperature (∼30 K lower) than that of ethylene from EG. We have also explored the effects of TiO2(110) preparation (hydroxylation, h-TiO2(110), and oxidation, oTiO2(110)) on the surface chemistry of 1,2-PG. We find that hTiO2(110) shows identical reactivity as r-TiO2(110), while on o-TiO2(110) propylene formation decreases and propanal and acetone formation increase.
2. EXPERIMENTAL SECTION The experiments were performed in an ultrahigh vacuum (UHV) molecular beam surface scattering apparatus (base pressure
