Synergistic Effects of Alloying and Thiolate ... - ACS Publications

Nov 11, 2014 - Hydrogenation over Cu-Based Catalysts. Simon H. Pang, Nicole E. Love, and J. Will Medlin*. Department of Chemical and Biological Engine...
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Synergistic Effects of Alloying and Thiolate Modification in Furfural Hydrogenation over Cu-Based Catalysts Simon H. Pang, Nicole E. Love, and J. Will Medlin* Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: Control of bimetallic surface composition and surface modification with selfassembled monolayers (SAMs) represent two methods for modifying catalyst activity and selectivity. However, possible synergistic effects of employing these strategies in concert have not been previously explored. We investigated the effects of modifying Cu/Al2O3 catalysts by alloying with Ni and modifying with octadecanethiol (C18) SAMs, using furfural hydrogenation as a probe reaction. Incorporation of small amounts of Ni (Cu4Ni) improved catalytic activity while slightly reducing hydrogenation selectivity. Further incorporation of Ni resulted in high rates for decarbonylation and ring-opening. Modification of the Cu4Ni catalyst with C18-SAMs resulted in improvement in both the activity and hydrogenation selectivity. X-ray photoelectron spectroscopy experiments on bimetallic thin films and density functional theory calculations revealed that the C18-SAM kinetically stabilized Cu at the surface under hydrogenation conditions. These results indicate that thiolate monolayers can be used to control surface bimetallic composition to improve catalytic performance. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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assembled monolayers (SAMs) of alkanethiolates on Pd catalysts for this purpose.16,17 Here, we report on investigations focused on improving the performance of Cu catalysts using a combination of alloying with Ni and modification with octadecanethiol (C18) SAMs. We found that Cu-rich (Cu4Ni) catalysts improved activity for furfural hydrogenation relative to Cu with a relatively small loss in selectivity. Surprisingly, modification of these catalysts with C18 SAMs enhanced both catalyst activity and selectivity. A combination of vibrational spectroscopy, X-ray photoelectron spectroscopy on thin CuNi films, and density functional theory calculations were used to identify possible explanations for the improved performance. Cu, Ni, and CuxNiy catalysts supported on Al2O3 (SaintGobain S.A.) were prepared via incipient wetness impregnation. After calcination and reduction, bulk metal composition was analyzed using inductively coupled plasma atomic emission spectroscopy. Active metal surface area was determined via CO pulse chemisorption and average particle size was estimated via transmission electron microscopy (Supporting Information Figures S1 and S2). The catalysts were also characterized using temperature-programmed reduction (TPR) in Supporting Information Figure S3. Physical properties of the supported catalysts are summarized in Table 1. Gas-phase hydrogenation of furfural was conducted in a tubular packed bed reactor at 190 °C and near atmospheric

recious metal catalysts, such as Pt and Pd, have previously been used for selective hydrogenations of oxygenated molecules. However, investigations have shown that bimetallic catalysts made from cheaper base metals can be effective for these reactions as well. One bimetallic in particular, CuNi, is of interest due to its low cost and potential for higher activity than the monometallic catalysts. Recently, it has been investigated for hydrodeoxygenation of various biomass-derivatives such as pyrolysis oil,1 anisole,2 guaiacol,3 and glycerol.4 However, a number of studies have observed that the surface composition of bimetallic catalysts depends on the reaction environment5−8 with implications for catalytic activity; for example, Niterminated NiPt particles are more active for aqueous-phase reforming.9 Furfural is another biomass-derived molecule that, like anisole and guaiacol, has attracted attention due to its potential use as a fuel precursor, and has been used as a probe for understanding the reactivity of complex oxygenates. Furfural hydrogenation and hydrodeoxygenation over precious metal catalysts has been investigated in a number of studies.10−13 However, furfural hydrogenation over CuNi has been only been investigated in a limited number of publications. Lukes and Wilson examined various compositions of CuNi catalysts and reaction temperatures, finding that increased Ni content and higher temperatures up to about 250 °C resulted in greater furan production.14 Much more recently, Xu et al. investigated the aqueous-phase conversion of furfural into cyclopentanone under hydrogen.15 Although composition control of bimetallics has been shown to be effective in many cases for modifying selectivity, we have additionally demonstrated the use of self© XXXX American Chemical Society

Received: October 9, 2014 Accepted: November 11, 2014

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shown in Supporting Information Figure S6. Interestingly, the effect of the SAM on catalyst performance depended strongly on catalyst composition (Figure 1). The C18-SAM had little effect on the Cu catalyst, which retained high selectivity to furfuryl alcohol with a 30% loss in activity. Surprisingly, the C18 coating increased both the selectivity and activity for the Cu4Ni catalyst, resulting in the highest aldehyde hydrogenation activity of all catalysts tested, approximately 2.3 times the rate of aldehyde hydrogenation on uncoated Cu. In sharp contrast, reaction rates using C18-CuNi dropped almost 2 orders of magnitude, and rates on the C18-coated Ni were reduced by roughly 1 order of magnitude compared to uncoated Ni. Thus, on more Ni-rich catalysts, the C18 modifier appeared to lower activity in a manner similar to its effects on Pt group metals, which has been attributed to site-blocking by the alkanethiolate SAM. In particular, the rates of decarbonylation and ring opening were significantly reduced (Supporting Information Figure S5), resulting in a small increase in furfuryl alcohol and methylfuran selectivity. Overall, it appears that there are synergistic effects between the modification of catalysts with C18 monolayers and inclusion of small amounts of Ni into Cu catalysts, resulting in increased aldehyde hydrogenation activity and retention of high selectivity. To examine the availability of surface sites on supported bimetallic catalysts, diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments were performed using CO as a probe, Figure 2a. Experiments performed at lower CO

Table 1. Summary of Physical Characterization for Catalysts Studieda

Cu Cu4Ni CuNi Ni

wt % metal

Ni:Cu atomic ratio

D (%)

active metal surface area (m2/g cat)

12.6(2) 12.2(3) 11.9(2) 12.4(2)

3.84(7) 0.89(2)

2.2 0.7 1.2 3.1

1.7 0.5 1.0 2.6

d (nm)

TPR Tpeak (°C)

17 ± 6 42 ± 38 43 ± 25 6±2

220, 320 250 280, 400 580

a

The numbers in parentheses represent the uncertainty in the least significant digit.

Figure 1. Combined selectivity to furfuryl alcohol and methylfuran and overall rate of furfural consumption from furfural hydrogenation (yF = 0.01, yH2 = 0.50, Treactor = 190 °C) over uncoated and C18-coated CuxNiy catalysts, reported at 4 ± 2% conversion of furfural. Error bars indicate the standard deviation of five repeated measurements of product mole fractions over 1 h.

pressure; selectivity to desired products furfuryl alcohol and methylfuran and overall rate of furfural consumption are reported in Figure 1. The pure Cu catalyst was over 90% selective to furfuryl alcohol, consistent with similarly high selectivities reported in the literature.18 The Cu-rich Cu4Ni catalyst was 1.3 times more active than pure Cu and showed higher activity for all reactions; however, the rates for decarbonylation and ring-opening reaction increased more than the hydrogenation rate (Supporting Information Figure S4), resulting in somewhat lower selectivity. Interestingly, further Ni enrichment to form a CuNi catalyst strongly decreased the rates of all reactions to below those of Cu and also decreased selectivity. Pure Ni catalysts were the most active for furfural reaction, with furan and butane as the major products; these results are consistent with previous studies showing that surface Ni is active for decarbonylation and ringopening reactions.10 Overall, it appears that there can be some beneficial activity effect from alloying Cu with smaller amounts of Ni, at some cost to selectivity. The Cu, Ni, and CuxNiy catalysts were modified by immersing the catalysts in a 5 mmol L−1 solution of octadecanethiol (C18) for at least 12 h, as done previously for modification of other hydrogenation catalysts, Supporting Information Figure S5.16,19−22 The SAM is stable on the catalyst even after exposure to hydrogenation conditions for 2 h and does not decompose until much higher temperatures, as

Figure 2. (a) CO stretch region for uncoated Cu/Al2O3, Cu4Ni/ Al2O3, CuNi/Al2O3, and Ni/Al2O3; (b) comparison between coated and uncoated catalysts. DRIFTS experiments performed under 2000 mTorr CO.

exposure can be found in Supporting Information Figure S7. On uncoated Cu catalysts, the main absorption feature was at 2112 cm−1, associated with CO linearly bound to top sites on Cu(111) and defect sites.23,24 Cu4Ni retained the adsorption feature near 2120 cm−1 at reduced intensity and additionally showed a small amount of CO linearly bound on Ni top sites at 2005 cm−1.25 For the bimetallic CuNi catalyst, the peak associated with linearly bound CO increased in intensity with a concurrent decrease in intensity for CO bound to Cu. Interestingly, there was no peak associated with CO bound in a 3-fold site on Ni, suggesting that the Ni sites were still isolated from each other. Lastly, on pure Ni, CO adsorbed in both 3-fold hollow sites near 1863 cm−1 and linearly on Ni top sites at 2021 cm−1. The difference in overall absorbance for CO adsorption on the different catalysts can be attributed to the difference in dispersion for the various catalysts; the bimetallic catalysts had a consistently lower dispersion than the monometallic catalysts (Table 1), resulting in an overall 4111

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Upon introduction of 0.5 Torr H2 and heating to 190 °C, Cu was reduced from a partially oxidized layer (in vacuum, Supporting Information Figure S8) to purely metallic Cu at 75.07 ± 0.04 eV, as evidenced by the disappearance of the Cu 3p3/2 peak at 77.05 eV. Because the as-prepared surface was terminated with 5 nm of Cu, a Ni 3p peak was not observed but was also not expected, Figure 3a. However, after exposure to reaction conditions for 1 h, Figure 3b, the Cu 3p peak intensity was significantly attenuated and a peak corresponding to Ni 3p grew around 66.5 eV. This suggests that under hydrogenation conditions, Ni tends to be near the surface of CuNi catalysts. Similar behavior was observed when the as-prepared thin films were exposed to both furfural and H2. The behavior seen here is expected based on density functional theory (DFT) calculations of hydrogen adsorbed on bimetallic surfaces. On pure Ni, we calculated a hydrogen adsorption energy of approximately 60 kJ/mol at coverages ranging from one-fourth of a monolayer to a full monolayer. However, on pure Cu, the hydrogen adsorption energy dropped roughly linearly from 60 to 24 kJ/mol over the same range. The tendency for Ni to adsorb hydrogen more strongly at higher coverages resulted in a switch in the predicted surface termination of bimetallic skin alloys, as shown in Figure 4. When the surface had no adsorbate, Cu was more

lower absorbance in CO DRIFTS experiments. Others have also observed particles larger than 50 nm when preparing CuNi bimetallic catalysts via incipient wetness impregnation.1,26 CO DRIFTS experiments were performed on the C18coated Cu4Ni and Ni catalysts to examine the effect of the thiolate monolayer on available surface sites, Figure 2b. The spectrum for C18-Cu4Ni retained the feature associated with CO bound to Cu top sites, but the peak near 2005 cm−1 was nearly indistinguishable from noise, suggesting that the concentration of Ni near the surface had decreased and the Ni at the surface was blocked significantly by the thiolate. In contrast, the C18-Ni catalyst did not bind CO in 3-fold sites, likely due to the presence of the C18-monolayer. This behavior has also been seen for C18-coated Pd catalysts21 and is in agreement with the activity of furfural hydrogenation on C18coated Ni/Al2O3; the rates of decarbonylation and ring opening were reduced significantly, likely due to this reduction in ensemble size. The DRIFTS experiments suggested that modification of Cu4Ni catalysts with alkanethiolates decreased the surface concentration of Ni. To investigate this phenomenon under more well-defined conditions, thin films of CuNi were characterized using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) both before and during exposure to reaction conditions. Spectra collected under reaction conditions are shown in Figure 3 and spectra under vacuum both before and after reaction can be found in Supporting Information Figure S8.

Figure 4. Preferred surface termination of CuNi alloys as a function of coverage of atomic hydrogen or sulfur. The surface energy difference is the calculated energy of the Cu-terminated surface subtracted from the energy of the Ni-terminated surface. Thus, positive energies indicate that the energetically favorable structure is terminated with Cu, and negative energies indicate termination with Ni. Inset shows the example surface structure used in the DFT calculation. Orange denotes Cu atoms and green denotes Ni atoms.

stable on the surface. However, hydrogen coverage induced a flip in the preferred surface termination at approximately onethird of a monolayer. Though Figure 4 shows results only for skin alloys, it provides a general indication that the presence of surface hydrogen will favor accumulation of some Ni at the surface. The thin film was held in vacuum and then cooled to produce a surface that was Cu-terminated, then coated with a C18 monolayer. The attenuation of photoelectron flux by the C18 monolayer has been studied previously;27 not only is the overall intensity of the photoelectron signal reduced by the presence of the monolayer, but the number of atomic layers sampled is also reduced. In fact, we calculated that 75% of the photoelectron signal for the C18-coated thin films originates from the top monolayer of metal atoms. Additional details can be found in the Supporting Information.

Figure 3. AP-XP spectra (open circles) in the Cu and Ni 3p region taken in situ under reaction conditions for: (a) and (b) uncoated CuNi thin films in H2 and (c) and (d) C18-modified CuNi thin films in H2 and furfural. Spectra (a) and (c) were taken upon introduction of gases and heating to 190 °C. Spectra (b) and (d) were taken after exposure to reaction conditions for 1 h. Cu 3p3/2/3p1/2 doublet separation = 2.50 ± 0.06 eV (solid/dashed colored lines). 4112

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uncoated catalysts, leading to high selectivity for furfuryl alcohol. Because high activity also requires an optimal surface Ni content, the decrease in surface Ni can improve activity as well. We propose that the retention of small amounts of Ni resulted in enhanced hydrogen dissociative adsorption compared to Cu, whereas the C18 modifier prevented the accumulation of large densities of Ni atoms that lead to reduced activity through strong adsorption of furfural-derived intermediates. The more optimal surface Ni content led to higher aldehyde hydrogenation activity for the C18−Cu4Ni catalyst than uncoated Cu4Ni and C18−Cu. Remarkably, the high coverage of thiolates, which are normally considered poisons, still improved catalytic activity, perhaps because the thiolates left the most active sites on the Cu-rich surfaces available. It has previously been observed that C18-modified Pd catalysts retain available corner and edge sites while losing availability of terrace sites.16,17 We have demonstrated that inclusion of small amounts of Ni can increase the activity of Cu catalysts for aldehyde hydrogenation, at the expense of selectivity due to the tendency for Ni to be at the surface under hydrogenation conditions. However, further modification by a C18-monolayer resulted in kinetic stabilization of Cu at the surface, allowing retention of high selectivity to furfuryl alcohol while likely retaining small amounts of surface Ni to aid in hydrogen splitting. This method of kinetic stabilization via surface modifiers may be applicable in other systems as well, where the reaction conditions cause the desired surface structure to become less thermodynamically favorable, allowing for the potential for higher activity and selectivity.

As shown in Figure 3c, upon initial exposure to reaction conditions, Cu was observed as the only metal at the surface, similarly to the uncoated film in Figure 3a. A small shift in the Cu 3p3/2 binding energy to 75.68 ± 0.02 eV was observed, which we attribute to the formation of Cu−S bonds. Spectra in the S 2p region indicated the presence of thiolate, thiol, and disulfide species28−30 that were not significantly affected over the course of the reaction (Supporting Information Figure S9). Interestingly, after exposure to reaction conditions for 1 h, Figure 3d, there was little change in the metallic 3p spectrum the intensity of the Cu 3p peak did not change and there was no evidence of Ni near the surface. This behavior was also observed on the C18-coated CuNi thin films that were not thermally treated prior to C18-SAM deposition. Considering the differences in affinity between Ni and Cu for sulfur, this behavior is not predicted by thermodynamics. Our calculations (Figure 4) indicate that the Ni-terminated bimetallic surface should become favorable at approximately one-fourth of a monolayer sulfur coverage. Therefore, one would expect that formation of the thiolate monolayer would cause Ni to surface segregate to achieve a lower surface energy. We therefore propose that formation of the monolayer kinetically trapped Cu at the surface, creating a surface that was not thermodynamically favored. This resulted in the observation that Cu remained as the surface metallic species, even during exposure to high temperature and reaction conditions. This result is also consistent with DRIFTS experiments comparing uncoated and coated Cu4Ni catalysts. From the results presented here, two major effects of the bimetallic can be identified. First, Cu4Ni/Al2O3 catalysts exhibited higher activity than their monometallic counterparts due to the presence of both Cu and Ni at the surface, as observed in CO DRIFTS and AP-XPS experiments. DFT calculations (see Supporting Information for details) indicated that the higher selectivity for hydrogenation on Cu was due to the low affinity of Cu-rich surfaces for the furyl ring (Supporting Information Figure S10 and ref 18). However, hydrogenation activity on Cu is expected to be limited by the much lower affinity of the surface for H2 dissociation. The increase in activity for Cu4Ni can be attributed to a hydrogen spillover effect from Ni to Cu sites where the aldehyde hydrogenation reaction occurs.31 Hydrogen spillover has been studied in a number of bimetallic systems where a more active metal is used to activate molecular hydrogen; the atomic hydrogen then diffuses to the less active metal sites.32 This less active site can then be the site of reaction. This effect has been observed in, for example, benzene hydrogenation on PtCo bimetallic catalysts.33 Thus, inclusion of small amounts of Ni can increase activity at the expense of selectivity. Larger amounts of Ni further reduce selectivity and, in the case of NiCu, activity. DFT calculation results presented in the Supporting Information suggest that the latter effect may be due to strong adsorption of the aldehyde function. In any case, achieving optimal activity at high selectivity clearly involves maintaining a low surface Ni content. The second major effect observed was the surprising observation of increased aldehyde hydrogenation activity on the C18-coated Cu4Ni catalysts. We hypothesize that the presence of the C18-modifer on the Cu4Ni catalysts kinetically stabilized Cu at the surface, as indicated by AP-XPS experiments on the C18-coated bimetallic thin films. This kinetic stabilization would produce catalyst particles that were more Cu-rich on the surface under reaction conditions than

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ASSOCIATED CONTENT

S Supporting Information *

Catalyst preparation, characterization, and computational methods; transmission electron micrographs and particle size distributions; temperature-programmed reduction spectra; diffuse-reflectance Fourier transform spectra of C18 coating and adsorbed CO as a function of pressure; complete catalytic data; X-ray photoelectron spectra under vacuum; DFT calculations of furfural adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.H.P. acknowledges the Colorado Center for Biorefining and Biofuels (C2B2) Fellowship Grant and the Engineering Excellence Fund of the University of Colorado. N.E.L. acknowledges support from the C2B2 research experience for undergrads program. The work was supported by the U.S. National Science Foundation (CHE-1149752). We thank ChihHeng Lien for his assistance running some of the selectivity and rate experiments, and Lucas Ellis and Allison Robinson for running the TPR of C18-coated Cu4Ni. We also thank Samuel Tenney and Peter Sutter at Brookhaven National Laboratory Center for Functional Nanomaterials for beamtime at the National Synchrotron Light Source, beamline X1A1, and assistance performing ambient-pressure XPS experiments. We 4113

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also acknowledge computing time at the Extreme Science and Engineering Discovery Environment (XSEDE) under grant TG-CHE040023N, which is supported by the National Science Foundation grant OCI-1053575.

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