Article pubs.acs.org/JPCC
Modeling the Surface Chemistry of Sugars: Glycolaldehyde on Rhodium (100) Basar Caglar,†,§ M. Olus Ozbek,‡,∥ J. W. (Hans) Niemantsverdriet,†,‡ and C. J. (Kees-Jan) Weststrate*,†,‡ †
Laboratory for Physical Chemistry of Surfaces, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands Syngaschem BV, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands
‡
S Supporting Information *
ABSTRACT: It is important to understand the interaction of C−OH and CO functional groups of sugar with a catalytically active metal surface for selectively converting of biomass-derived molecules into useful chemicals. Glycolaldehyde (HOCH2CHO), with its C−OH and CO functional groups, is the smallest molecule to model aspects of the chemistry of sugars on metal surfaces. Rhodium catalysts are candidates for activation of biomass-derived molecules. We have investigated the decomposition of glycolaldehyde on the Rh(100) surface using a combination of experimental surface science techniques (temperature-programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS)) and a computational method (density functional theory (DFT)). At low coverage, glycolaldehyde decomposition commences with O−H bond breaking upon adsorption at 100 K and proceeds via dehydrogenation and C−C bond breaking below room temperature, ultimately producing CO and hydrogen (synthesis gas). At high coverage a side reaction becomes apparent, involving C−O bond breaking. As a result, some methane and carbon formation are observed as well. Our findings on the decomposition of glycolaldehyde on Rh(100) suggest that sugars can be converted into synthesis gas on Rh surfaces, and, depending on the surface coverage, small hydrocarbons can be produced from sugar molecules, leaving the surface covered by surface carbon.
1. INTRODUCTION Biomass-derived molecules can be an alternative for fossil resources to produce fuel, chemicals, and hydrogen.1−4 The current scientific interest is to convert these highly functionalized molecules catalytically to desirable products via selective bond activation. The building blocks of biomass mainly consist of C5 and C6 sugars, polyols with oxygen atoms attached to each carbon atom. The complexity and low vapor pressure of biomass-derived molecules complicate experimental surface science studies, which can provide insights into the interaction between these sugars and the catalytically active metal surface.5,6 Ethylene glycol serves as a probe molecule for polyols to understand the interaction of C−OH group with the surface; however, sugars have two functional groups in their open chain forms: (i) a C−OH group at the alcohol end and (ii) a CO group at the aldehyde end. Because the interaction of the CO group with the surface can lead to a different chemistry, a glycolaldehyde molecule that has both functional groups was used to explore the chemistry of basic sugars on the Rh surface. The chemistry of ethylene glycol has been studied on many transition-metal surfaces, for example, Ni,7 Ag,8,9 Cu,10 Rh,11,12 Mo,13 Pt,14,15 and Pd,16 while the literature on glycolaldehyde is limited (Pt,5 Pd,17 Zn−Pt,18 Ni−Pt19) on Pd(111) and Pt(111) surfaces. Glycolaldehyde binds to the surface in the η1(O) configuration, in which the oxygen atom at the aldehyde end interacts with the surface (Figure 1a). On Pd(111), after heating to 185 K, the η1(O) configuration is transformed into a © 2015 American Chemical Society
Figure 1. Potential adsorbate configurations of adsorbed surface intermediates derived from glycolaldehyde.17,18
η2(C,O) configuration where both the carbon and oxygen of the carbonyl group interact with the surface (Figure 1b). The η2(C,O) glycolaldehyde configuration undergoes O−H bond breaking to produce η3(O,C,O) α-oxo-η2-glycolal, which binds to the surface via the carbonyl group at the aldehyde end and the oxygen atom at the alcohol end (Figure 1c). Conversely, on Pt(111) O−H bond breaking results in the formation of an η2(O,O) α-oxo-η1-glycolal intermediate in which the molecule binds to the surface via oxygen atoms (Figure 1d). On both surfaces, the decomposition of η3(O,C,O) at 200−250 K and η2(O,O) at >355 K results in synthesis gas production. A minor amount of formaldehyde formation was also observed on Pd(111). Studies on bimetallic surfaces showed that alloying Pt with Zn increases the barrier for C−H bond scission and Received: June 20, 2015 Revised: September 17, 2015 Published: September 17, 2015 22915
DOI: 10.1021/acs.jpcc.5b05916 J. Phys. Chem. C 2015, 119, 22915−22923
Article
The Journal of Physical Chemistry C
the quantification of products TPD peak areas were used. The TPD area for the saturation coverage of CO and H2 was determined and linked to the absolute coverage to determine CO and H2 coverages, which were obtained from LEED studies.20−24 When the coverage of CO was known, the quantities of CH4 and H2O produced were determined from the TPD peak areas using a correction factor to account for the mass spectrometer sensitivities to CH4 and H2O, respectively. Mass spectrometer sensitivities for each gas previously indicated were determined by leaking a sample of gas into the UHV chamber at a measured pressure rise of 1 × 10−8 mbar and collecting the appropriate mass signals. Also, the relative ionization efficiencies of the pressure gauge were used to correct the pressure reading for each gas. RAIRS spectra were performed using a Fourier-transform infrared spectrometer equipped with a KRS-5 wire grid polarizer allowing only the p-polarized component of the light to be detected. The infrared beam undergoes a single reflection from the crystal surface near the grazing angle (85°). An MCT detector was used with a spectral range of 4000−650 cm−1. RAIRS spectra consist of 512 scans taken at 4 cm−1 spectral resolution and subtracted by a stored background spectrum of a clean surface. The crystal was heated with a rate of 1 K/s and kept at a constant temperature for 5 min to record 512 scans for each temperature. 2.2. Computational Details. Periodic DFT computations were performed using the VASP package25,26 with plane-wave (PW) basis sets and an RPBE functional.27−29 The cutoff energy used was 400 eV. Necessary dipole corrections due to the asymmetric usage of slabs were included in the computations. The Rh(100) surface was modeled with a five-layer slab to describe the p(2 × 2) unit cell, corresponding to a surface coverage of 0.25 ML for all species, where the repeating slabs were separated with a 12 Å vacuum in the Z direction. For the computations on p(2 × 2) slabs, the k-points sampling mesh was automatically generated by the Monkhorst−Pack procedure with a (5 × 5 × 1) mesh. During the simulations, the bottom three layers were kept frozen. The remaining top two layers and the adsorbed molecules were relaxed until the net force acting on the ions was
