Orest Skoplyak

Experimental and theoretical study of reactivity trends for methanol on Co∕Pt(111) and Ni∕Pt(111) bimetallic surfaces

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on NiPt(111) and CoPt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various NiPt(111) and CoPt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni-Pt-Pt(111), as compared to the subsurface monolayer Pt-Ni-Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on NiPt(111) and CoPt(111) bimetallic surfaces. On Ni-Pt-Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300 K; this species reacted at approximately 300 K to form CO and H2. On Co-Pt-Pt(111), methoxy was stable up to approximately 350 K and decomposed to form CO and H2. Overall, trends in methanol reactivity on NiPt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.

Enhancing H2 and CO production from glycerol using bimetallic surfaces.

The development of alternative energy sources to fossil fuels is attracting considerable interest both for increasing energy supply and for decreasing greenhouse gas emissions. It has been argued that biomass is widely available, carbon-neutral, and renewable, and can therefore make important contributions to energy supply without increasing CO2 emissions. The carbohydrates in plant matter can be broken down to smaller oxygenates, such as glycerol, glucose, sorbitol, ethylene glycol, and others. Through aqueous phase reforming, the oxygenates can be further reacted to yield H2 for use in fuel cells. Alternatively, the reforming process can be tuned to produce H2/ CO mixtures that can be coupled with Fischer–Tropsch synthesis to yield liquid alkanes compatible with the existing petroleum transportation and storage infrastructure. Glycerol has attracted recent attention as a feed molecule for these processes and is particularly interesting because as a by-product of the transesterification of plant oils and animal fats its availability could potentially increase with the expected increase in biodiesel production. Fundamental surface science studies of glycerol conversion have not been performed previously, in part because of the difficulty of introducing compounds with low vapor pressures into ultrahigh vacuum (UHV) systems. Instead, smaller oxygenates such as ethanol and ethylene glycol have been used as surrogates for glycerol. Herein we report the first surface science studies of glycerol on PtACHTUNGTRENNUNG(111) and Ni/Pt ACHTUNGTRENNUNG(111) bimetallic surfaces. The objective of these experiments was to determine whether the chemistry of glycerol is sufficiently similar to that of smaller oxygenates to validate the use of smaller molecules as models in catalysis and surface science studies. We have previously investigated the reactivity of ethylene glycol, ethanol, and methanol on Ni/Pt ACHTUNGTRENNUNG(111) bimetallic surfaces. These studies demonstrated that surfaces with a Ni monolayer on top of PtACHTUNGTRENNUNG(111), designated as Ni-Pt-Pt ACHTUNGTRENNUNG(111), display increased reforming activity as compared to either an unmodified PtACHTUNGTRENNUNG(111) surface or a Pt ACHTUNGTRENNUNG(111) surface containing a subsurface Ni monolayer, designated as Pt-Ni-Pt ACHTUNGTRENNUNG(111). These results on smaller oxygenate molecules demonstrated the possibility of combining UHV surface science studies with density functional theory (DFT) calculations to understand the reaction mechanisms of reforming reactions. However, it is unclear to what extent these results can be extended to larger and more practically important oxygenates such as glycerol. Results from the current study provide the first confirmation that the same trends in reforming activity are observed between glycerol and smaller oxygenate molecules in fundamental UHV studies, validating the use of smaller molecule models. Deposition of Ni onto PtACHTUNGTRENNUNG(111) at room temperature leads to the Ni atoms remaining on the surface to produce an overlayer structure, Ni-Pt-Pt ACHTUNGTRENNUNG(111). Increasing the deposition temperature to 600 K allows the Ni atoms to diffuse to the subsurface, producing a sandwich structure, Pt-Ni-Pt ACHTUNGTRENNUNG(111). Both experiments and DFT calculations have shown that the different locations of the Ni atoms lead to very different chemical properties of Ni-Pt-Pt ACHTUNGTRENNUNG(111) and Pt-Ni-Pt ACHTUNGTRENNUNG(111) surfaces. Adsorbates such as H2, ethylene, cyclohexene, and other unsaturated hydrocarbons and oxygenates tend to interact more weakly with Pt-NiPtACHTUNGTRENNUNG(111) than with PtACHTUNGTRENNUNG(111), leading to novel low-temperature hydrogenation activity. In contrast, Ni-Pt-Pt ACHTUNGTRENNUNG(111) surfaces exhibit stronger interactions with adsorbates as compared to Pt ACHTUNGTRENNUNG(111), increasing the activity for C H and O H scission reactions of interest in processes such as oxygenate reforming. Glycerol was found to react selectively on these surfaces in temperature-programmed desorption (TPD) experiments to form H2 and CO; no other gas-phase products were detected. The absence of CO2 as a product indicated that the water gas shift reaction did not take place in these TPD experiments under ultrahigh vacuum. Figure 1 displays TPD spectra of H2, CO, and unconverted glycerol following exposure of 0.2 L glycerol on Pt ACHTUNGTRENNUNG(111), Pt-Ni-Pt ACHTUNGTRENNUNG(111), and Ni-Pt-Pt ACHTUNGTRENNUNG(111) surfaces. Glycerol desorbed in a single peak centered at 274 K from all surfaces, as is evident in the spectra for fragments with atomic mass units (amu) of 31 (major) and 28 (minor; Figure 1a and c, respectively). On Pt ACHTUNGTRENNUNG(111), H2 desorption occurred at 330 K, with a small peak at 403 K (Figure 1b, lower trace). The first peak was desorption-limited as the peak temperature corresponds to that following H2 exposure on PtACHTUNGTRENNUNG(111), while the higher temperature peak was reaction-limited. CO desorption occurred from a desorption-limited peak at 399 K. From Pt-Ni-Pt ACHTUNGTRENNUNG(111), H2 desorbed from overlapping broad and weak peaks centered at 389 K and 477 K. Both these peaks were reaction-limited as the desorption temperature of H2 on Pt-Ni-Pt ACHTUNGTRENNUNG(111) is lower than that on Pt ACHTUNGTRENNUNG(111). CO desorption occurred from a desorption-limited peak at 367 K. Overall, PtNi-Pt ACHTUNGTRENNUNG(111) displayed decreased H2 and CO peak areas as compared to PtACHTUNGTRENNUNG(111). In contrast, increased H2 and CO peak areas were observed on Ni-Pt-Pt ACHTUNGTRENNUNG(111) as compared to PtACHTUNGTRENNUNG(111). H2 desorption occurred from two peaks at 377 K and 425 K, while CO desorbed from a single peak at 429 K. The H2 evolution peak at 425 K was reaction-limited, while the other H2 and CO peaks were desorption-limited. There were also several weak peaks and shoulder peaks for the desorption of CO at higher temperatures in Figure 1, suggesting multiple pathways for the dissociation of glycerol. The TPD results obtained for glycerol were similar to previous experiments using ethylene glycol and ethanol. [a] O. Skoplyak, Prof. M. A. Barteau, Prof. J. G. Chen Department of Chemical Engineering University of Delaware, Newark, DE 19716 (USA) Fax: (+1)302-831-1048 E-mail : barteau@udel.edu