Use of Hydrogen Chemisorption and Ethylene Hydrogenation as

Subscriber access provided by Northern Illinois University

Article

Use of Hydrogen Chemisorption and Ethylene Hydrogenation as Predictors for Aqueous Phase Reforming of Lactose over Ni@Pt and Co@Pt Bimetallic Overlayer Catalysts Qinghua Lai, Michael D Skoglund, Chen Zhang, Allen R Morris, and Joseph H. Holles Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01405 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Use of Hydrogen Chemisorption and Ethylene Hydrogenation as Predictors for Aqueous Phase Reforming of Lactose over Ni@Pt and Co@Pt Bimetallic Overlayer Catalysts Qinghua Lai, Michael D. Skoglund, Chen Zhang, Allen R. Morris and Joseph H. Holles* Department of Chemical and Petroleum Engineering, University of Wyoming, Dept. 3295, 1000 E. University Ave, Laramie, WY 82071, United States.

ABSTRACT: Overlayer Pt on Ni (Ni@Pt) or Co (Co@Pt) were synthesized and tested for H2 generation from APR of lactose. H2 chemisorption descriptor showed that Ni@Pt and Co@Pt overlayer catalysts had reduced H2 adsorption strength compared to a Pt only catalyst, which agree with computational predictions. The overlayer catalysts also demonstrated lower activity for ethylene hydrogenation than the Pt only catalyst, which likely resulted from decreased H2 binding strength decreasing the surface coverage of H2. XAS results showed that overlayer catalysts exhibited higher white line intensity than Pt catalyst, which indicates a negative d-band shift for the Pt overlayer, further providing evidence for overlayer formation. Lactose APR studies showed that lactose can be used as feedstock to produce H2 and CO under desirable reaction condition. The Pt active sites of Ni@Pt and Co@Pt overlayer catalysts showed

1 Environment ACS Paragon Plus

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

significantly enhanced H2 production selectivity and activity when compared with that of a Pt only catalyst. The single deposition overlayer with the largest d-band shift showed the highest H2 activity. The results suggest that overlayer formation using directed deposition technique could modify the behavior of the surface metal and ultimately modify the APR activity.

1. INTRODUCTION There is increasing demand for H2 for heavy oil upgrading, desulfurization and upgrading of conventional petroleum. Moreover, hydrogen is a potentially desirable alternative energy source as a non-polluting, efficient, and cost-attractive energy carrier, especially in fuel cells. However, commercial hydrogen production processes usually use non-renewable resources resulting in high carbon footprints.1 Recently, one approach that has attracted much attention is aqueous phase reforming (APR) for the production of H2 or alkanes from renewable resource, such as biomass derived oxygenated compound (e.g. ethylene glycol, glycerol, sorbitol, glucose).2-6 However, research about APR has focused mainly on simple biomass derived oxygenated molecules. In order to demonstrate the full potential of the process, further research using a pure biomass feed needs to be performed. Lactose (C12H22O11) is the primary ingredient in cheese whey, a main by-product of cheese production, and represents a significant disposal issue.7 In 2014, the U.S. cheese industry produced 11.5 billion pounds cheese.8 Up to 9 lbs. of wastewater, usually known as whey, are created for each pound of cheese.9 The whey typically contains about 5 wt% lactose in water.7 However, more than two-thirds of the main ingredient in whey is not recovered. It is instead treated as waste and sent to a wastewater treatment plant or a simply field spread for disposal, causing environmental problems.10,

11

Some studies were done to investigate conversion of


Page 2 of 38

Page 3 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

lactose to a valued-added product.10, 12 However, these value added products are in small volume, which will leave significant lactose for disposal. Thus, more studies and novel lactose processing technologies are needed. This research is to investigate converting lactose to useful products via aqueous phase reforming and thus minimizing the lactose waste disposal issues. The full reforming of lactose produced by the U.S. cheese industry could generate a maximum H2 of 0.33 million ton, which is approximately 9.3% of total hydrogen production capacity of U.S. refineries in 2014.13 Since a waste stream is being converted to fuel, use of lactose for fuel production has the advantage of not competing with the use of the biomass as food. In addition, significant quantities of anhydrosugars, sugars, and phenolic compounds are produced by pyrolysis, depending on the biomass source.14 For example, bio oils from pyrolysis comprise 5 to 13 wt% sugars and anhydrosugars, and 6 to 36 wt% phenolic compounds.15 When examining multiple different biomass derived feedstocks for APR, a similar step of removing CO or H2 products from the catalyst surface was shown to limit the reaction.4, 16 Thus, lactose can serve as a model molecule for bio oil compounds and polyols such as sucrose, glucose, sorbitol, glycol and ethylene glycol. Therefore, lactose was selected as a sustainable source of biomass for APR process and a model for other carbohydrates in this contribution. Among various supported metal catalysts investigated in the aqueous phase reforming of biomass, bimetallic catalysts have attracted attention due to the promotional effect resulting from the cooperative interactions between two metals.16-29 Studies have demonstrated that Pt/Ni, Pt/Co and Pt/Re bimetallic alloys enhance the catalytic activity for the APR compared with Pt monometallic catalysts.16, 19, 20 These results have indicated that the step of removing CO product from the catalyst surface limited the reaction rate. Huber et al. proposed that the d-band center of the platinum based bimetallic alloy could be decreased below that of pure platinum, resulting in


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

lower heats of adsorption of CO and H2 products.16 This reduction in molecular binding strength of CO and H2 will decrease the surface coverage and result in more available surface active sites for APR reaction. A similar observation that surface binding strength was shown to affect reactivity has been reported for HDO of phenolic compounds. Resasco et al. observed that strong binding of model phenolic compounds (guaiacol or catechol) by the catalyst could block surface sites and concluded that controlled, decreased binding strength was required for improved catalytic performance.30 Thus efforts to reduce binding strength of catalysts are broadly applicable to upgrading multiple bio-oil compounds resulting from pyrolysis of biomass. A pseudomorphic overlayer bimetallic catalyst is a specific type of bimetallic catalyst that consists of surface structure of a monolayer of one metal on the top of another support metal. The benefits of overlayer catalysts for a variety of reactions have been demonstrated by computational and single crystal studies.22,

31-39

These studies have shown that a bimetallic

overlayer sample may possess different surface properties than that of either pure metal component would suggest. In particular, adsorbate binding strengths are generally altered in overlayer systems due to the shift of the d-band center of the overlaying metal.35-40 Therefore, stronger or weaker binding strength can be achieved through various combinations of different overlayers and bulk metals. In previous work, our group has employed the directed deposition method to synthesize Re@Pd, Co@Pt and Ni@Pt bimetallic overlayer samples to modify the properties of the Pd or Pt overlayer. These samples were characterized using H2 and CO adsorption and ethylene hydrogenation to investigate the surface adsorption properties of the catalyst and compare these properties to computational predictions. XAS was also used to characterize the geometric and electronic properties of the overlayer metal. These overlayer catalysts have shown reduced H2 4 Environment ACS Paragon Plus

Page 4 of 38

Page 5 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

and CO binding energy when compared to monometallic metal and bimetallic alloy samples.41-44 This reduction in binding strength was in accordance with computational predictions. For an APR reaction, this decrease in H2 and CO adsorption strength is hypothesized to result in an activity increase because strong H2 and CO bonding have been shown to inhibit the reaction.4, 16, 45

Therefore, Co@Pt and Ni@Pt bimetallic overlayer catalysts with reduced binding strength of

CO and H2 should increase availability of surface active sites for the APR reaction and thus result in improved APR activity. In this work, the previously synthesized and characterized Co@Pt and Ni@Pt bimetallic overlayer catalysts are further investigated for aqueous phase reforming of lactose with a fixed bed reactor. Lactose conversion, hydrogen formation rates and H2 selectivity for pure Pt and Pt overlayer catalysts are reported. The use of the previous hydrogen chemisorption and ethylene hydrogenation data as predictors for lactose APR activity is also demonstrated. 2. EXPERIMENTAL SECTION 2.1 Materials and catalyst preparation. The monometallic host catalysts used for this paper were created using standard techniques. All catalysts were supported with a γ-alumina (Alfa Aesar, 3 µm particle size, 99.97%). 5 wt% Ni/Al2O3 and Co/Al2O3 parent catalysts were made in large batches through incipient wetness impregnation of alumina with solutions of nickel or cobalt nitrate hexahydrate (Aldrich 99.999%). 5 wt% and 0.5 wt% Pt/Al2O3 control catalysts were created with a slurry of toluene, γ-alumina (Alfa Aesar, 99.97%), and platinum(II) acetylacetonate (Alfa Aesar). Non-structured bimetallic Ni–Pt and Co–Pt/Al2O3 catalysts were also made with the similar loadings as overlayer samples using incipient wetness coimpregnation. The measured maximum amounts of water and toluene absorbed by the catalyst support without wetting it are 0.76 and 1.30 cm3 per gram, respectively. These numbers were


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

used for wetness impregnation. After impregnation all samples were dried in an oven overnight and then calcined in near stagnant air (450°C for Ni and Co, and 400°C for any sample having Pt) for 4 h using a 3°C min−1 ramp rate. The catalysts were then reduced in 60 sccm H2 flow at calcination temperature for 4 h using a 3°C·min−1 ramp rate. The overlayer catalysts were synthesized by the directed deposition method described in our previous work.42, 43 Detailed synthesis procedures are described in the supporting information. 2.2. Catalyst characterization. For measuring the metal loadings of each sample, elemental analysis was performed on inductively coupled plasma optical emission spectrometry (ICP-OES) at Galbraith Laboratories Inc. Nitrogen physisorption at 77 K was performed with a Micromeritics APAP 2020 with surface area and porosimetry analyzer to measure the surface areas and pore volume of samples. H2 chemisorption experiments were performed in a Micromeritics APAP 2020 using a static volumetric technique. The instrument doses a catalyst sample of known mass with a known amount of hydrogen. Prior to analysis, each sample was reduced at 400°C in H2 flow. Metal dispersions were determined assuming chemisorption stoichiometry of H/Msurf = 1. Ethylene hydrogenation descriptor reaction was conducted as described in our previous work.42, 43 The ethylene hydrogenation experiment detail is provided in the supporting information. XAS studies were performed to characterize catalyst samples with Pt content using the Materials Research Collaborative Access Team (MRCAT) beam line at the Advanced Photon Source (7.0 GeV ring energy), Argonne National Laboratory. Platinum data was collected in fluorescence mode according to the method described in literature.46 Samples were reduced under 4% H2/He flow at 400°C before analysis. The XAS samples were prepared under a He


Page 6 of 38

Page 7 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

atmosphere. Fluorescence studies were performed at a 90° angle to the incident X-ray beam using a cryogenically cooled 13-element Germanium detector with continuous scanning and a duration of 2 s per 0.05 eV. 2.3. Catalytic reaction. The lactose APR tests were performed in a 9.5 mm diameter stainless steel up-flow reactor followed by a condenser and a gas-liquid separator, as reported in the literature.45 The reaction temperatures were measured via a K-type thermocouple attached to wall of the stainless steel reactor. 250 mg of catalyst (< 400 mesh) was mixed with 1 g of silica (Alfa Aesar, SiO2, 70-230 mesh) and placed in the reactor. 1 g of additional silica (Alfa Aesar, SiO2, 28-200 mesh) and quartz wool on both sides were used to retain the catalyst in the reactor. The catalyst was reduced in H2 flow (60 sccm) at 400°C for 1 h using a 10°C·min−1 ramp rate, and then cooled to room temperature. After reduction, the system was purged with flowing nitrogen until no H2 was detected via an online GC. The system was pressurized to 750 psi with nitrogen and then a 3.0 wt % aqueous solution of lactose was introduced into the reactor by a HPLC pump. Once liquid was observed in the separator, the reactor was heated to reaction temperature using a 10°C·min−1 ramp rate. The liquid flow-rate was set as 1.00 mL·min−1. Reaction temperatures were varied from 150°C to 300°C. The effluent liquid was drained periodically for detection of the primary carbonaceous species using an Agilent 1260 infinity HPLC with RID detector and ZORBAX Carbohydrate column. The effluent gas stream mixed with make-up N2 was analyzed by an online gas chromatograph (Thermo-Fisher Trace GC Ultra). Conversion of lactose was defined by using lactose fed and lactose collected in the liquid, and hydrogen selectivity was defined according to the previous work by Dumesic et al.2-4 Lactose conversion = (Lactose fed − Lactose collected)⁄Lactose fed × 100%


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Hydrogen selectivity = (Molecules H Produced⁄C atoms in gas phase) × (1⁄2) × 100% 3. RESULTS AND DISCUSSION 3.1. Physicochemical properties. The loadings, dispersions, calculated percent overlayer coverage, surface areas and pore volumes for each sample are summarized in Table 1. For Ni@Pt overlayer catalysts, the Pt metal loading increased with additional depositions. As shown in Table 1, co-impregnated bimetallic Ni–Pt and Co–Pt catalysts with metal loadings similar to overlayer samples were also synthesized for comparison. The percent surface coverage of Pt on overlayer samples was calculated assuming 100% dispersion of loaded Pt (from elemental analysis) on Ni or Co. The number of Pt atoms was then divided by the number of Ni or Co (from chemisorption) to calculate a percentage of the host metal atoms covered by Pt. The overlayer coverage value provides an idea about the maximum percentage of surface Ni or Co atoms covered by Pt atoms. Although the amount of platinum precursor added in the overlayer synthesis was enough for 100% coverage of parent sample, calculated Pt loadings were less than 13%. Ni@Pt single deposition (SD) and triple deposition (TD) overlayer had higher overlayer coverage than Co@Pt, and this phenomenon has been consistently observed in our reported works.43,

44

Nitrogen physisorption results showed that all samples had similar surface areas

(about 60 m2g−1) and pore volumes (about 0.40 cm3g−1). This suggested that the metal loading did not significantly affect the surface area and pore volume of the final catalyst. 3.2. Hydrogen chemisorption and ethylene hydrogenation results. We characterized the catalysts by hydrogen chemisorption and ethylene hydrogenation. These two studies have been described in detail in the prior work.43 The Clausius-Claperyon equation was used for calculation of heat of H2 adsorption and values at 0.3 cm3g−1 adsorbed H2 volume condition (representing


Page 8 of 38

Page 9 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

intermediate coverages) are presented in Table 2 (see reference for detailed plots).42,

47, 48

As

shown in Table 2, the heat of H2 adsorption of the Ni catalyst and the Pt catalyst were 51 kJ·mol−1 and 12 kJ·mol−1, respectively. The Ni−Pt bimetallic sample showed a heat of H2 adsorption of 51 kJ·mol−1, which is very similar to value of monometallic Ni sample and indicates that the Ni-Pt bimetallic catalyst is predominantly Ni. Unlike the co-impregnated Ni−Pt bimetallic catalyst, both the Ni@Pt SD and Ni@Pt TD overlayer samples had a lower heat of H2 adsorption value than the Ni and Pt samples, respectively −2.6 and −8.0 kJ·mol−1. This result is in accordance with the computational prediction that Ni@Pt overlayer sample would show a negative d-band shift and a reduced hydrogen adsorption strength compared to a monometallic Pt sample.

38

This is desirable as a reduced heat of adsorption seen in the overlayer sample is

postulated to improve the catalyst activity for eventual APR applications. For the cobalt catalysts shown in Table 2, the single deposition Co@Pt sample also had a reduced heat of hydrogen adsorption value of −3.3 kJ·mol−1. This decreased heat of adsorption result is also in accordance with the computational prediction. The negative heat of hydrogen adsorption values at adsorbed H2 volume of 0.3 cm3g−1 for some of catalysts might be ascribed to formation of multiple layers of hydrogen adsorption. Abaza et al. have reported similar phenomena and found that multiple layers of adsorption and exchanges of molecules between multiple layers of adsorption could cause negative heats of adsorption.49 Turnover frequency (TOF) of ethylene hydrogenation at 110°C for each sample is also presented in Table 2. The ethylene hydrogenation turnover frequencies for all overlayer samples were calculated using measured Pt loading with an assumed Pt dispersion of 100%, while turnover frequencies for all other samples were calculated based on total active sites (from H2 chemisorption). See supporting information for a discussion of TOF calculations. The ethylene


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 38

hydrogenation turnover frequency for the monometallic Pt catalyst was 28 s−1, orders of magnitude higher than 0.52 s−1 of the Ni or 0.081 s−1 of the Co parent catalysts. Compared to the Ni or Co sample, the Ni@Pt SD and Co@Pt SD samples with single platinum deposition showed an increased TOF of 9.0 and 12 s−1, respectively, suggesting platinum was added to the catalysts via directed deposition synthesis, thus shifting activity away from that of the parent metal toward that of the monometallic platinum catalyst. The Ni@Pt TD with triple deposition showed a TOF of 3.7 s−1, lower than value of the Pt and Ni@Pt SD samples. The reduction in reactivity from monometallic Pt to Ni@Pt SD is consistent with hydrogen chemisorption results that showed decreases in adsorption strength from Pt to Ni@Pt SD. Like Ni@Pt overlayer catalysts, Co@Pt catalyst showed a similar trend. Co-impregnated Ni−Pt had a decreased TOF compared with Ni, and the Co−Pt sample showed a slightly increased TOF compared with the Co sample. In sum, compared to the pure platinum catalyst, all overlayer catalysts showed reduced activity, which is in accordance with computational predictions in literature.38 Hydrogen chemisorption and ethylene hydrogenation served as descriptors for examining binding strength changes and electronic changes, and indicated that the Ni@Pt and Co@Pt catalysts can be strong candidates for subsequent APR studies. For these overlayer catalysts, overlayer stability may be a concern. According to theoretical computational predictions, the Pt overlayer on the Ni or Co metal is stable.50 The computational results showed that the Pt overlayer on the Ni or Co metal has negative segregation energies suggesting that the Pt atoms prefer to stay on the surface of the host metal. In addition for the hydrogen adsorption isotherms, data was collected at random temperatures (i.e., not sequentially increasing or decreasing) and low temperature runs were repeated after high temperature runs. These repeated temperature runs showed repeatable isotherms. This indicates catalyst stability,


Page 11 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

or at a minimum, the ability for the desired overlayer structure to reform when going from high to low temperature. 3.3. XAS results. X-ray adsorption spectroscopy (XAS) studies were performed to measure the dependence of the electronic structure of the catalysts on the catalyst synthesis conditions. Figure 1 shows a comparison of white line intensity of the Pt LIII edge of Pt, Ni@Pt SD, Ni@Pt TD, and bimetallic Ni-Pt catalysts. It was observed that the monometallic Pt sample showed the lowest white line intensity and the bimetallic Ni-Pt and the Ni@Pt SD overlayer catalyst had the highest white line intensity. From the literature, the intensity of Pt LIII edge correlates directly with the Pt d-band vacancy.51-54 The sample with an increased white line intensity should have an increased d-band vacancy.52 Figure 1 indicates that the Ni@Pt overlayer catalysts have an increased d-band vacancy compared with Pt only catalyst. This agrees with the computational prediction that synthesis of Pt monolayer on the top of a Ni base will give rise to a negative shift of the d-band center of the Pt overlayer and then an increased d-band vacancy.38 The Co@Pt catalyst also showed similar results in the d-band center and the d-band vacancy (see Figure 2). Additional Pt depositions cause the white line intensity of Ni@Pt TD to reduce toward the intensity of the monometallic Pt sample, which suggests the existence of Pt clusters in the Ni@Pt TD sample. According to the previous EXAFS results, the Pt-metal coordination number increased for both Ni@Pt (from 5.6 to 8.2) and Co@Pt samples as additional Pt was deposited, which indicated Pt ensemble formation.55 This conclusion was supported by the increase of the Pt-metal bond length from 2.67 Å for Ni@Pt SD to 2.72 Å for Ni@Pt TD. The formation of isolated clusters of Pt also could explain why the Ni@Pt TD with lower white line intensity showed lower ethylene hydrogenation TOF than Ni@Pt SD. Platinum dispersion of 100% was assumed for the overlayer samples when calculating ethylene hydrogenation turnover frequency.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 38

However, the XAS result demonstrating Pt cluster formation indicates that an overestimated number of Pt active site was used for calculation of ethylene hydrogenation TOF over the Ni@Pt TD. The longer Pt-metal bond length for Ni@Pt TD indicates that all 3 of the additional Pt atoms are associated with other Pt and not Ni. It is likely that the Pt active site of Ni@Pt TD would show higher ethylene hydrogenation TOF than Ni@Pt SD if the actual Pt active site number was used for calculation. Although the bimetallic catalyst showed similar white line intensity as the overlayer samples the co-impregnated Ni–Pt and Co–Pt samples have a unique shoulder feature behind the white line peak.55 This behaviour was not observed in Ni@Pt or Co@Pt overlayer samples or the pure Pt sample. Additionally, as shown previously, bimetallic samples showed dissimilar EXAFS wave behaviour to both the Pt and overlayer sample.55 These differences indicate some structure differences between the bimetallic sample and the overlayer sample. Previously reported EXAFS results also showed that the Pt-metal distances of bimetallic catalysts (Ni-Pt and Co-Pt) were much shorter than that for overlayers and the Pt-metal coordination numbers of bimetallic catalysts were much higher than that for overlayers, indicating that the Pt of the bimetallic catalysts was more likely in a Ni (or Co) matrix and the Pt of overlayer was more likely in an overlayer.55 As a result, the ethylene hydrogenation TOF for Ni-Pt and Co-Pt was calculated based on total active sites. The XAS results tend to suggest that the simple bimetallic alloys don’t create the desired structure or catalytic effect. Thus, the XAS results indicate a bimetallic structure consistent with random, co-impregnated bimetallic catalysts and inconsistent with desired electronic changes. 3.4 Aqueous phase reforming of lactose. Lactose conversion, the overall rates of gas phase production of H2 and CO2, and H2 selectivity based on gas phase products from APR of lactose


Page 13 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

at 150°C on different catalysts are summarized in Table 3. It can be seen from Table 3 that the hydrogen formation rate decreased in the following order: Ni@Pt SD > Pt > Ni@Pt TD > Co@Pt SD > Co–Pt > Ni–Pt ~ Co > Ni. Both Ni and Co parent catalysts showed low H2 formation rates and H2 selectivity. The pure Pt catalyst showed a much higher H2 formation rate compared with Ni and Co catalysts, agreeing with the previous glycerol APR study using similar γ-alumina supported catalysts.56 The non-structured Ni–Pt and Co–Pt bimetallic samples exhibited slightly higher H2 formation rates and H2 selectivity than Ni and Co only catalysts likely due to the presence of active Pt surface atoms. Unlike the non-structured Ni–Pt and Co–Pt bimetallic samples, the Ni@Pt SD and Co@Pt SD overlayer sample (with similar Pt loadings as the bimetallic catalysts) showed significantly increased H2 formation rates and H2 selectivity than Ni and Co parent catalysts or their corresponding bimetallic counterpart. This result is in accordance with the hydrogen chemisorption and ethylene hydrogenation results that overlayer samples showed different behavior (i.e. weaker binding, which should allow for fewer blocked sites) than that of non-structured bimetallic samples. Selectivity of lactose to gas phase products over all catalysts was below 1% at 150°C. The H2 selectivity (as defined in the experimental section and in the literature2-4) was low at this reaction condition due to the low reactant conversion and the use of lactose as the APR feedstock. The only other detected gas phase products was CO2. The major products remained in the liquid phase and were not further defined. Previous studies have shown that the selectivity for hydrogen production decreased with decreasing glycerol conversion57 or larger feed molecule4, 58. The ‘H2 yield’ on different catalysts calculated by multiplying lactose conversion by H2 selectivity is also shown in Table 3. When combining conversion and selectivity to determine yield, the advantage of the overlayer


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 38

compared to pure Pt or bimetallic catalysts becomes more apparent with Ni@Pt SD > Co@Pt SD > Ni@Pt TD ~ Pt. Since the catalysts have different amounts of active sites, it is incomplete to compare activity by only using overall H2 formation rate. In order to illustrate more clearly the difference in H2 production activity of active sites on catalysts, the turnover frequencies (TOF) for H2 production was calculated and presented in Figure 3 and Figure 4. For monometallic Ni, Co, Pt and nonstructured Ni–Pt/Co–Pt bimetallic samples, active sites for TOF calculation were calculated by using total surface metal atoms (with no distinction between Pt and Ni or Co) measured from hydrogen chemisorption. Since the previous XAS study suggested that the single deposition platinum overlayer sample was highly dispersed on surface,55 all measured platinum of overlayer catalysts was assumed to be 100% on the surface, and it was assumed that Ni (or Co) active sites on overlayer catalysts have the same activity for H2 production as that of active sites on monometallic Ni (or Co) catalyst. Therefore, the production of H2 by Pt active sites could be calculated via subtracting the H2 produced by Ni (or Co) active sites from the total H2 production. Then the activity of the Pt active sites on the overlayer catalysts was calculated. While the 100% dispersion assumption is not valid of the Ni@Pt TD sample based on XAS results, the same procedure was used to allow comparison (see supporting information for a discussion of TOF calculations). As shown in Figure 3, the active sites of the Ni parent catalyst showed a low TOF for H2 production, about one-tenth of that of the Pt only catalyst. Although the non-structured Ni–Pt bimetallic catalyst had slightly higher overall H2 formation rate than the Ni parent catalyst, its TOF for H2 production was slightly lower than that of the Ni sample. Accordingly, adding Pt increased the dispersion of the Ni–Pt bimetallic catalyst, but didn’t increase H2 production of the


Page 15 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

active site of the bimetallic catalyst. This result is contrary to the report by Huber et al. that Ni– Pt bimetallic catalyst showed increased TOF of H2 production.16 This discrepancy is likely because the ratio of Pt atom to Ni atom of the Ni–Pt catalyst used in this paper is approximately 1:370, much smaller than the 1:8 which is the lowest Pt to Ni ratio used by Huber et al.16 Since TOF of H2 production was observed to decrease with the decrease of Pt loading for the Ni–Pt catalyst,16 it is reasonable to speculate that the low Pt loading make the surface of the Ni–Pt catalyst mainly covered by Ni, resulting in a low TOF for H2 production. This conclusion is supported by the heat of adsorption for Ni-Pt being alsomst identical to pure Ni (Table 2). Unlike the Ni–Pt bimetallic catalyst, the Pt active site of the Ni@Pt SD overlayer catalyst with similar Pt loading as the Ni-Pt showed greatly improved TOF for H2 production, about 7 times higher than the Pt only catalyst. This result is in accordance with the prediction that Ni@Pt overlayer catalysts with reduced strength of hydrogen adsorption (that has been previously demonstrated in H2 adsorption and ethylene hydrogenation result), can increase the availability of surface sites for the APR reaction and then make these Pt sites on overlayers more active than that on monometallic Pt.16 TOF of H2 production for the Pt active sites of the Ni@Pt TD overlayer sample was lower than that of the Ni@Pt SD, but still higher than that of the Pt only sample. This is in accordance with EXAFS result that Ni@Pt TD with additional Pt depositions showed a lower white line intensity and more pure Pt like behaviour than the Ni@Pt SD. The activity difference among Ni@Pt SD and Ni@Pt TD overlayer samples may be due to formation of small, isolated Pt particles after additional depositions, giving rise to the H2 production activity of Ni@Pt TD moving back toward that of the monometallic Pt sample. Formation of isolated Pt clusters was previously observed by XAS for multiple depositions of Pt on alumina supported Ni catalysts


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

with the Pt-metal coordination number and interatomic distance increasing as additional Pt was deposited.55 Since the amount of actual Pt surface active sites is not available, the calculated TOF in Ni@Pt TD might deviate from the true value. The correlation between the TOF for H2 production and d-band vacancy indicates that the increased d-band vacancy of Ni@Pt overlayer is beneficial for H2 production. For Pt, Ni@Pt SD and Ni@Pt TD catalysts, TOF of H2 production increased in the following sequence: Pt < Ni@Pt TD < Ni@Pt SD, which follows the trend of the d-band vacancy increase based on XAS white line intensities. In summary, for the lactose APR reaction, the Ni@Pt overlayer catalysts, prepared with controlled synthesis approach to deposit Pt atop on Ni, showed a desired catalytic effect when compared with bimetallic Ni–Pt catalyst prepared with the impregnation method. For the cobalt-based catalysts shown in Figure 4, active sites of the Co only sample and the non-structured Co–Pt bimetallic samples displayed similar TOF for H2 production, about onesixth of that of the monometallic Pt sample. Hence, the Co–Pt bimetallic catalyst showing higher overall H2 formation rate than the Co catalyst (Table 3) is due to the higher dispersion of catalyst. Like the Ni@Pt overlayer catalyst, a single deposition of Pt on Co resulted in the Pt active site having a much high TOF for H2 production than that of the monometallic Pt sample. This result further supports the proposal that Co@Pt overlayer catalyst can decrease heats of H2 adsorption and then make these Pt sites in an overlayer more active than that of monometallic Pt.16 Figure 5 (a) shows the correlation between the hydrogen turnover frequency production from APR of lactose as a function of ethylene hydrogenation turnover frequency for the studied overlayer platinum samples and monometallic platinum sample. Ni@Pt SD, Ni@Pt TD, Co@Pt SD, and pure Pt in Figure 5 (a) were chosen to compare the two hosts (Ni and Co) and the


Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

different synthesis (Ni@Pt SD and Ni@Pt TD). Since the goal was to modify the behavior of Pt, Ni and Co are not included in this comparison. For Ni@Pt SD, Co@Pt SD, and pure Pt, the trend is clear as the APR H2 productivity of these samples increases as the TOF of ethylene hydrogenation decreases. However, the Ni@Pt TD with a lower TOF of ethylene hydrogenation didn’t show a further increased TOF of H2 production. This may be due to the inaccurate assumption of 100% Pt atoms on the surface for the Ni@Pt TD, as the Ni@Pt TD has shown Pt particle formation by XAS.55 Figure 5 (b) shows the relationship between the hydrogen TOF from APR of lactose and normalized XAS white line intensity. The quantitative white line intensity of each sample was determined by integrating areas under the white line from 10 eV below the edge to 13 eV above the edge.53 As shown in Figure 5 (b), there is a non-linear relation between the TOF for H2 from APR of lactose and white line intensity. The Pt only catalyst showed the lowest TOF for H2 from APR of lactose and the lowest white line intensity when compared to the Ni@Pt SD, Ni@Pt TD and Co@Pt SD catalysts. H2 production trended up as a function of white line intensity from Pt to Ni@Pt TD with the Ni@Pt SD having the highest TOF for H2 from APR of lactose. However, the Co@Pt SD sample showed a decrease in H2 production even as the white line intensity further increased. This non-linear relation might be due to serval factors. First, the role of binding strength in the APR reaction is complex. The hydrogen binding strength is not the only factor affecting the H2 production activity, as the binding energy of the reactant on the active site also governs the final APR H2 activity. If the reactant binding is too weak, it will be hard for any any reaction occurs. Therefore, the lactose APR H2 activity appears to have the maximum value at an intermediate binding strength (meaning an intermediate white line intensity). In this way, the data may demonstrate the peak behavior commonly seen in volcano plots. Similar


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

phenomena was observed by Humbert et al. in that the Pt-Ni-Pt with an intermediate hydrogen and cyclohexene binding energy showed the highest cyclohexene hydrogenation activity compared to the Ni-Pt-Pt(111), Pt(111), Pt-Co-Pt(111), and Pt-Fe-Pt(111).59 Second, the nonlinear relationship between TOF for H2 and white line intensity may also result from Ni having the same number of electrons in its valence shell as Pt while Co has one less. Since Co has one more bonding molecular orbital that is available and at a lower energy, compared to Ni, this causes the d-band width to increase even more for the Co@Pt and therefore the Pt ed shifts further downward than for the Ni@Pt system.55 H2 chemisorption, ethylene hydrogenation and XANES have demonstrated the ability to act as descriptors for the overlayer catalysts. All three techniques have strengths and weaknesses. The strength of H2 chemisorption and ethylene hydrogenation is that they can be performed in the laboratory in a strait forward and inexpensive manner as a reasonably trip to a synchrotron is not required. Thus, they can serve as first lever characterization tools. However, if the overlayer metal starts to self agglomerate (as in the TD sample), ethylene hydrogenation result can become difficult to interpret due to the TOF normalization issue. Chemisorption results may become difficult to interpret when multi-layers form and the values become negative. XANES seems to more directly reflect electronic structure of the Pt overlayer metal in the catalysts, but it is not surface sensitive and the XAS white line intensity reflects an average electronic property of surface and bulk agglomerated Pt atoms. So, XANES becomes an effective descriptor when more overlayers are deposited on the catalyst. As with many techniques, the combination of data form all three descriptors (H2 chemisorption, ethylene hydrogenation and XANES) provides the most complete description of the overlayer system. However each individual technique can serve


Page 19 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

as a descriptor (under certain limitations) while also providing the researcher the flexibility depending on the capabilities of their laboratory. When compared to pure Pt, both Ni@Pt SD and Co@Pt SD showed decreases in both heat of hydrogen adsorption and ethylene hydrogenation activity. If, as predicted in the literature,16 APR activity can be increased by a decrease in active site binding strength, two promising catalysts were identified. The near edge white line provides quantitative evidence for the desired electronic change (to decrease binding strength) and demonstrates that this effect is observed for the overlayer catalyst. Finally, the lactose APR demonstrates improved TOF for the most promising overlayer catalysts (Ni@Pt SD and Co@Pt SD) compared to pure Pt. Thus hydrogen heat of adsorption and ethylene hydrogenation both serve as descriptors or predictors for lactose APR. With increasing temperature, the conversions of the lactose exceeded 90% over Ni based catalysts at 300°C. However, gas products yields from lactose conversion over all these catalysts were still below 5%, which means that most products were in liquid form. It is likely that converting lactose into intermediate products is a relatively fast step, and the lactose and its intermediate products are slowly reformed to hydrogen via APR reaction. Figure 6 shows the TOF of H2 production over Ni based catalysts as a function of reaction temperature. Clearly, the H2 TOF of all Ni based catalysts increased with the increasing reaction temperature from 200°C to 300°C. The Pt active sites on the Ni@Pt overlayer catalysts showed a higher TOF for H2 formation than Ni–Pt, and monometallic Ni or Pt samples at these reaction temperatures. The triple deposition Ni@Pt TD overlayer catalyst showed lower H2 production activity than the single deposition Ni@Pt SD overlayer catalysts likely due to Pt agglomeration


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

after several depositions. Arrhenius representations of the TOF data for Ni-based catalysts are shown in Figure 7. The Weisz-Prater ΦWP criterion was evaluated at 40% lactose conversion. The ΦWP was 0.0350, which was much less than 1, indicating a lack of significant intra-phase diffusion effects.60 The detailed calculation process is described in the supporting information. The apparent activation energies for H2 formation calculated from Figure 7 are 68, 66, 55 and 63 kJ·mol−1 for Pt, Ni@Pt TD, Ni@Pt SD and Ni, respectively. It can been seen that the Pt active sites of the single deposition Ni@Pt SD showed a lower activation barrier compared with the pure Pt catalyst, while the Ni@Pt TD with triple deposition showed a similar activation energy to the Pt only catalyst. The Ni@Pt TD showing similar activation barrier to that of the Pt only catalyst could again be ascribed to Pt agglomeration. The phenomena that bimetallic catalysts showed lower H2 or CO production activation energy in APR has been reported by others as well. Huber et al. reported that, compared with Pt/Al2O3, Pt1Fe9/Al2O3 had a lower activation energy for H2 production from ethylene glycol aqueous phase reforming. They speculated that the PtFe catalyst had lower heats of H2 and CO adsorption than the monometallic Pt catalyst.16 Our work and Huber’s work are in accordance with the computational work of Christoffersen et al. which predicted that Pt on Co, Ni, or Fe would have lower heats of adsorption for H2 or CO compared to pure Pt. Kunkes et al. also observed lower apparent activation energies for CO production from APR of glycerol on PtRe and PtRu bimetallic catalysts when compared with monometallic Pt catalyst.19, 20 It was speculated that the binding energy of CO on the catalyst surface was decreased by addition of ruthenium or rhenium to platinum.20 When reaction temperature increased to 250°C, CO became one of major components in gas production. TOFs of CO production over Ni based catalyst at 250°C were calculated based on dispersion (see Figure 8). Unlike H2 production, the Pt only catalyst showed similar TOF for CO


Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

production as Ni parent and Ni–Pt bimetallic catalysts. Again, the Ni@Pt overlayer catalyst shows about four times higher CO TOF than Ni and Pt only catalysts. Since CO can be further converted into H2 and CO2 via water gas shift reaction, the high CO activity over the overlayer samples is beneficial for final H2 production. We speculate that the reduced CO adsorption strength of the Ni@Pt overlayer catalyst enhances its activity for CO production. Similar CO production activity enhancement on bimetallic catalysts was also reported by other researchers.19, 20

Lactose APR studies suggested that the overlayer bimetallic catalysts are capable of producing H2 and CO from the lactose under desirable reaction conditions. As shown in Figures 3 and 4, the Ni@Pt and Co@Pt overlayer samples showed better performance than Co, Ni, and Pt monometallic catalysts and Pt–Ni and Pt–Co co-impregnated bimetallic samples. These results are accordance with hydrogen chemisorption and ethylene hydrogenation results. 4. CONCLUSIONS APR of lactose using alumina supported Ni@Pt and Co@Pt overlayer bimetallic catalysts has been studied. It has been demonstrated that H2 and CO could be produced from the lactose under desirable reaction conditions. The Pt active sites of the Ni@Pt and Co@Pt overlayer samples showed significantly enhanced H2 production selectivity and activity compared with that of the Pt only catalyst. The results of H2 chemisorption, ethylene hydrogenation and XAS showed that the behaviour of the prepared Ni@Pt and Co@Pt overlayer samples was consistent with the computational predictions for overlayer catalysts with Pt atop a Ni or Co parent. Thus, heat of H2 adsorption and ethylene hydrogenation also served as descriptors or predictors for lactose APR activity. The increased XAS white line intensity for the Pt of overlayer samples indicated a


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

negative d-band shift of Pt. This electronic modification of the Pt sites caused different hydrogen bonding strength, ethylene hydrogenation activity and APR H2 production activity between Pt only and Pt overlayer catalysts. Additionally, single deposition Ni@Pt SD overlayer with higher APR H2 activity had a larger d-band shift than the triple deposition overlayer Ni@Pt TD. Coimpregnated bimetallic catalysts (Ni–Pt and Co–Pt) were investigated as well and showed notably different behaviour compared with overlayer catalysts. This result further confirms that prepared overlayer catalysts were not normal bimetallic alloy. This work indicates that when specific changes in catalyst bonding are desired to improve catalyst performance, the use of overlayer catalysts to predictably modify the electronic behaviour of the catalyst may prove an attractive approach to improving catalyst behaviour. ASSOCIATED CONTENT Supporting Information. The overlayer sample synthesis procedures, ethylene hydrogenation experiment detail, calculation process of Weisz-Prater criterion, and a discussion of alternative methods to calculated turnover frequency are provided in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Tel.:+1-307-766-6772; fax: +1-307-766-6772. E-mail address: [email protected] (J.H. Holles). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The authors would like to acknowledge funding support from National Science Foundation, Chemical, Bioengineering, Environmental and Transport Systems (Grant No. CBET-0933017). The authors thank financial support from School of Energy Resources of University of Wyoming through its Graduate Assistantship program. MRCAT experiments were supported by the Department of Energy and the MRCAT member institutions. The authors acknowledge Dr. Jeff Miller for providing XAS sampling facilities. The operation of the Advanced Photon Source, Argonne National Laboratory, was supported by the United States Departments of Energy (Grant No. DE-AC02-06CH11357). ABBREVIATIONS APR, aqueous phase reforming; EXAFS, extended X-ray absorption fine structure; SD, single deposition; TD, triple deposition; TOF, turnover frequency; XAS, X-ray adsorption spectroscopy. REFERENCES (1)

Navarro, R. M.; Pena, M. A.; Fierro, J. L. G., Hydrogen production reactions from carbon

feedstocks: Fossils fuels and biomass. Chem. Rev. 2007, 107, 3952-3991. (2)

Cortright, R. D.; Davda, R. R.; Dumesic, J. A., Hydrogen from catalytic reforming of

biomass-derived hydrocarbons in liquid water. Nature 2002, 418, 964-967. (3)

Davda, R. R.; Shabaker, J. W.; Huber, G. W., et al., A review of catalytic issues and

process conditions for renewable hydrogen and alkanes by aqueous-phase reforming of oxygenated hydrocarbons over supported metal catalysts. Appl. Catal., B 2005, 56, 171-186. (4)

Huber, G. W.; Dumesic, J. A., An overview of aqueous-phase catalytic processes for

production of hydrogen and alkanes in a biorefinery. Catal. Today 2006, 111, 119-132.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(5)

Page 24 of 38

James, O. O.; Maity, S.; Mesubi, M. A., et al., Towards reforming technologies for

production of hydrogen exclusively from renewable resources. Green Chem. 2011, 13, 22722284. (6)

Chen, D.; He, L., Towards an Efficient Hydrogen Production from Biomass: A Review of

Processes and Materials. ChemCatChem 2011, 3, 490-511. (7)

Burkhalter, G.; Steffen, C.; Puhan, Z., Cheese, Processed Cheese, and Whey. In

Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH: Weinheim: 2003. (8)

National Agriculture Statistics Service, Dairy Products 2014 Summary. In Agriculture

Statistics Board, U.S. Department of Agriculture: Washington, D.C., 2015. (9)

Parmentier, K., Determination of the Volume of Industrial Waste from Wisconsin's Dairy

Product Industry. University of Wisconsin-Green Bay: 2000. (10) Chia, Y. N.; Latusek, M. P.; Holles, J. H., Catalytic wet oxidation of lactose. Ind. Eng. Chem. Res 2008, 47, 4049-4055. (11) Prazeres, A. R.; Carvalho, F.; Rivas, J., Cheese whey management: A review. J. Environ. Manage. 2012, 110, 48-68. (12) Seki, N.; Saito, H., Lactose as a source for lactulose and other functional lactose derivatives. Int. Dairy J. 2012, 22, 110-115. (13) Worldwide Captive Hydrogen Production Capacity at Refineries; Hydrogen Analysis Resource Center: 2015. (14) Mohan, D.; Pittman, C. U.; Steele, P. H., Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy Fuels 2006, 20, 848-889. (15) Diebold, J. A Review of the Chemical and Physical Mechanisms of the Storage Stability of Fast Pyrolysis Bio-oils; National Renewable Energy Laboratory (NREL): Golden, CO, 2000.


Page 25 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(16) Huber, G. W.; Shabaker, J. W.; Evans, S. T., et al., Aqueous-phase reforming of ethylene glycol over supported Pt and Pd bimetallic catalysts. Appl. Catal., B 2006, 62, 226-235. (17) Alonso, D. M.; Wettstein, S. G.; Dumesic, J. A., Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chem. Soc. Rev. 2012, 41, 8075-8098. (18) Dal Santo, V.; Gallo, A.; Naldoni, A., et al., Bimetallic heterogeneous catalysts for hydrogen production. Catal. Today 2012, 197, 190-205. (19) Simonetti, D. A.; Kunkes, E. L.; Dumesic, J. A., Gas-phase conversion of glycerol to synthesis gas over carbon-supported platinum and platinum-rhenium catalysts. J. Catal. 2007, 247, 298-306. (20) Kunkes, E. L.; Soares, R. R.; Simonetti, D. A., et al., An integrated catalytic approach for the production of hydrogen by glycerol reforming coupled with water-gas shift. Appl. Catal., B 2009, 90, 693-698. (21) Wang, X.; Li, N.; Zhang, Z., et al., High-Yield Hydrogen Production from Aqueous Phase Reforming over Single-Walled Carbon Nanotube Supported Catalysts. ACS Catal. 2012, 2, 1480-1486. (22) Chen, J. G.; Menning, C. A.; Zellner, M. B., Monolayer bimetallic surfaces: Experimental and theoretical studies of trends in electronic and chemical properties. Surf. Sci. Rep. 2008, 63, 201-254. (23) Rahman, M. M.; Church, T. L.; Variava, M. F., et al., Bimetallic Pt-Ni composites on ceria-doped alumina supports as catalysts in the aqueous-phase reforming of glycerol. RSC Adv. 2014, 4, 18951-18960. (24) Boga, D. A.; Oord, R.; Beale, A. M., et al., Highly Selective Bimetallic Pt-Cu/Mg(Al)O Catalysts for the Aqueous-Phase Reforming of Glycerol. ChemCatChem 2013, 5, 529-537.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

(25) Xiong, H.; DeLaRiva, A.; Wang, Y., et al., Low-temperature aqueous-phase reforming of ethanol on bimetallic PdZn catalysts. Catal. Sci. Technol. 2015, 5, 254-263. (26) Ciftci, A.; Ligthart, D. A. J. M.; Hensen, E. J. M., Aqueous phase reforming of glycerol over Re-promoted Pt and Rh catalysts. Green Chem. 2014, 16, 853-863. (27) Ciftci, A.; Eren, S.; Ligthart, D. A. J. M., et al., Platinum–Rhenium Synergy on Reducible Oxide Supports in Aqueous-Phase Glycerol Reforming. ChemCatChem 2014, 6, 1260-1269. (28) Dietrich, P. J.; Lobo-Lapidus, R. J.; Wu, T. P., et al., Aqueous Phase Glycerol Reforming by PtMo Bimetallic Nano-Particle Catalyst: Product Selectivity and Structural Characterization. Top. Catal. 2012, 55, 53-69. (29) Dietrich, P. J.; Akatay, M. C.; Sollberger, F. G., et al., Effect of Co Loading on the Activity and Selectivity of PtCo Aqueous Phase Reforming Catalysts. ACS Catal. 2014, 4, 480491. (30) Gonzalez-Borja, M. A.; Resasco, D. E., Anisole and Guaiacol Hydrodeoxygenation over Monolithic Pt-Sn Catalysts. Energy Fuels 2011, 25, 4155-4162. (31) Hwu, H. H.; Eng, J.; Chen, J. G. G., Ni/Pt(111) bimetallic surfaces: Unique chemistry at monolayer Ni coverage. J. Am. Chem. Soc. 2002, 124, 702-709. (32) Skoplyak, O.; Barteau, M. A.; Chen, J. G. G., Reforming of oxygenates for H-2 production: Correlating reactivity of ethylene glycol and ethanol on Pt(111) and Ni/Pt(111) with surface d-band center. J. Phys. Chem. B 2006, 110, 1686-1694. (33) Zellner, M. B.; Chen, J. G., Supporting monolayer Pt on W(110) and C/W(110): Modification effects on the reaction pathways of cyclohexene. J. Catal. 2005, 235, 393-402.


Page 27 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(34) Zellner, M. B.; Goda, A. M.; Skoplyak, O., et al., Trends in the adsorption and decomposition of hydrogen and ethylene on monolayer metal films: A combined DFT and experimental study. Surf. Sci. 2005, 583, 281-296. (35) Pallassana, V.; Neurock, M.; Hansen, L. B., et al., Theoretical analysis of hydrogen chemisorption on Pd(111), Re(0001) and Pd-ML/Re(0001), Re-ML/Pd(111) pseudomorphic overlayers. Phys. Rev. B 1999, 60, 6146-6154. (36) Pallassana, V.; Neurock, M., Electronic factors governing ethylene hydrogenation and dehydrogenation activity of pseudomorphic Pd-ML/Re(0001), Pd-ML/Ru(0001), Pd(111), and Pd-ML/Au(111) surfaces. J. Catal. 2000, 191, 301-317. (37) Ruban, A.; Hammer, B.; Stoltze, P., et al., Surface electronic structure and reactivity of transition and noble metals. J. Mol. Catal. A: Chem. 1997, 115, 421-429. (38) Christoffersen, E.; Liu, P.; Ruban, A., et al., Anode materials for low-temperature fuel cells: A density functional theory study. J. Catal. 2001, 199, 123-131. (39) Greeley, J.; Mavrikakis, M., Alloy catalysts designed from first principles. Nat. Mater. 2004, 3, 810-815. (40) Zhang, C.; Lai, Q.; Holles, J. H., Ir@Pt bimetallic overlayer catalysts for aqueous phase glycerol hydrodeoxygenation. Appl. Catal., A 2016, 526, 113-125. (41) Latusek, M. P.; Heimerl, R. M.; Spigarelli, B. P., et al., Synthesis and characterization of supported bimetallic overlayer catalysts. Appl. Catal., A 2009, 358, 79-87. (42) Latusek, M. P.; Spigarelli, B. P.; Heimerl, R. M., et al., Correlation of H-2 heat of adsorption and ethylene hydrogenation activity for supported Re@Pd overlayer catalysts. J. Catal. 2009, 263, 306-314.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

(43) Skoglund, M. D.; Jackson, C. L.; McKim, K. J., et al., Characterization of Ni@Pt and Co@Pt overlayer catalysts using adsorption, reactivity descriptors, and X-ray absorption spectroscopy. Appl. Catal., A 2013, 467, 355-362. (44) Zhang, C.; Lai, Q.; Holles, J. H., Influence of adsorption strength in aqueous phase glycerol hydrodeoxygenation over Ni@Pt and Co@Pt overlayer catalysts. Catal. Sci. Technol. 2016, 6, 4632-4643. (45) Shabaker, J. W.; Davda, R. R.; Huber, G. W., et al., Aqueous-phase reforming of methanol and ethylene glycol over alumina-supported platinum catalysts. J. Catal. 2003, 215, 344-352. (46) Vijay, S.; Wolf, E. E.; Miller, J. T., et al., A highly active and stable platinum-modified sulfated zirconia catalyst - Part 2. EXAFS studies of the effect of pretreatment on the state of platinum. Appl. Catal., A 2004, 264, 125-130. (47) Chou, S. C.; Lin, S. H.; Yeh, C. T., Isosteric heat of sorption of dihydrogen on aluminasupported palladium. J. Chem. Soc., Faraday Trans. 1995, 91, 949-951. (48) Ranke, W.; Joseph, Y., Determination of adsorption energies and kinetic parameters by isosteric methods. PCCP 2002, 4, 2483-2498. (49) Abaza, S.; Aranovich, G. L.; Donohue, M. D., Adsorption compression in surface layers. Mol. Phys. 2012, 110, 1289-1298. (50) Ruban, A. V.; Skriver, H. L.; Nørskov, J. K., Surface segregation energies in transitionmetal alloys. Phys. Rev. B 1999, 59, 15990-16000. (51) Pearson, D. H.; Ahn, C. C.; Fultz, B., White lines and d-electron occupancies for the 3d and 4d transition metals. Phys. Rev. B 1993, 47, 8471-8478.


Page 29 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(52) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P., et al., Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction - An In Situ XANES and EXAFS Investigation. J. Electrochem. Soc. 1995, 142, 1409-1422. (53) Mansour, A. N.; Cook, J. W.; Sayers, D. E., Quantitative technique for the determination of the number of unoccupied d-electron states in a platinum catalyst using the L2,3 x-ray absorption edge spectra. J. Phys. Chem. 1984, 88, 2330-2334. (54) McBreen, J.; Ogrady, W. E.; Tourillon, G., et al., XANES study of underpotential deposited copper on carbon-supported platinum. J. Electroanal. Chem. 1991, 307, 229-240. (55) Morris, A. R.; Skoglund, M. D.; Holles, J. H., Characterization of Ni@Pt and Co@Pt overlayer catalysts using XAS studies. Appl. Catal., A 2015, 489, 98-110. (56) Wen, G. D.; Xu, Y. P.; Ma, H. J., et al., Production of hydrogen by aqueous-phase reforming of glycerol. Int. J. Hydrogen Energy 2008, 33, 6657-6666. (57) King, D. L.; Zhang, L. A.; Xia, G., et al., Aqueous phase reforming of glycerol for hydrogen production over Pt-Re supported on carbon. Appl. Catal., B 2010, 99, 206-213. (58) Shabaker, J. W.; Huber, G. W.; Dumesic, J. A., Aqueous-phase reforming of oxygenated hydrocarbons over Sn-modified Ni catalysts. J. Catal. 2004, 222, 180-191. (59) Humbert, M. P.; Chen, J. G., Correlating hydrogenation activity with binding energies of hydrogen and cyclohexene on M/Pt(111) (M = Fe, Co, Ni, Cu) bimetallic surfaces. J. Catal. 2008, 257, 297-306. (60) Froment, G. F.; Bischoff, K. B., Chemical Reactor Analysis and Design. Wiley: New York, 1990.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions Figure 1. XAS study results at Pt LIII-edge for Pt, Ni@Pt SD, Ni@Pt TD, and bimetallic Ni-Pt samples. Figure 2. XAS study results at Pt LIII-edge for Pt, Co@Pt SD, and bimetallic Co-Pt samples. Figure 3. TOFs for H2 production over Ni based catalyst at 150°C. Figure 4. TOFs for H2 production over Co based catalyst at 150°C. Figure 5. (a) Relation between TOF for hydrogen production from APR of lactose and TOF for ethylene hydrogenation, (b) relation between TOF for hydrogen production from APR of lactose and white line intensity over overlayer platinum samples and pure platinum sample. Figure 6. TOF of H2 production over Ni based catalysts as function of reaction temperature. Figure 7. Arrhenius-type plots and apparent activation energies for H2 production from lactose APR on selected samples. Figure 8. TOFs for CO production over Ni based catalyst at 250°C.


Page 30 of 38

Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 1. XAS study results at Pt LIII-edge for Pt, Ni@Pt SD, Ni@Pt TD, and bimetallic Ni-Pt samples.

Figure 2. XAS study results at Pt LIII-edge for Pt, Co@Pt SD, and bimetallic Co-Pt samples.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. TOFs for H2 production over Ni based catalyst at 150°C.

Figure 4. TOFs for H2 production over Co based catalyst at 150°C.


Page 32 of 38

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 5. (a) Relation between TOF for hydrogen production from APR of lactose and TOF for ethylene hydrogenation, (b) relation between TOF for hydrogen production from APR of lactose and white line intensity over overlayer platinum samples and pure platinum sample.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. TOF for H2 production over Ni based catalysts as function of reaction temperature.

Figure 7. Arrhenius-type plots and apparent activation energies for H2 production from lactose APR on selected samples.


Page 34 of 38

Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8. TOFs for CO production over Ni based catalyst at 250°C.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table Captions Table 1. Physicochemical properties of samples. Table 2. Heat of H2 adsorption and ethylene hydrogenation results. Table 3. APR activities of catalysts at 150°C.


Page 36 of 38

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1. Physicochemical properties of samples. Sample

Metal loading [wt%]

Dispersion Overlayer [%] coverage [%]

SBET Pore [m2g−1] volume [cm3g−1]

Ni/Al2O3

5.02 Ni

3.9

57

0.40

Ni@Pt SD

0.0363 Pt

2.1

10

53

0.41

Ni@Pt TD

0.0482 Pt

2.2

13

52

0.41

54

0.41

53

0.40

Ni-Pt Bimetallic

4.51Ni

6.4 0.0405Pt

Co/Al2O3

4.702 Co

Co@Pt SD Co-Pt Bimetallic

5.10

2.8 0.0093 Pt

2.6

2.3

Co

4.3

56

0.42

61

0.31

0.0139 Pt [a]

0.468 Pt

44.4

[b]

5 Nom. Pt

15.3

Pt Pt

[a] Used for ethylene hydrogenation and aqueous phase reforming of lactose activity studies [b] Used for hydrogen chemisorption isotherms

Table 2. Heat of H2 adsorption and ethylene hydrogenation results. Sample

Heat of H2 adsorption [kJ·mol−1]

TOF of ethylene hydrogenation [s−1]

Pt

12

28

Ni/Al2O3

51

0.52

Ni@Pt SD

-2.6

9.0

Ni@Pt TD

-8.0

3.7

Ni-Pt Bimetallic

51

0.092

Co/Al2O3

-6.5

0.081[a]

Co@Pt SD

-3.3

12

Co-Pt Bimetallic

49

0.33

[a] Measured at 130°C.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3. APR activities of catalysts at 150°C. Catalyst

Lactose conversion [%]

H2 formation CO2 formation H2 rate rate selectivity [mmol/h/gcat] [mmol/h/gcat] [%]

‘H2 yield’ [%]

Pt

21.1

0.0218

0.0472

23.2

4.90

Ni

13.4

0.0064

0.0325

9.8

1.31

Ni-Pt

14.2

0.0075

0.0278

13.5

1.91

Ni@Pt SD

20.3

0.0307

0.0301

50.9

10.3

Ni@Pt TD

10.9

0.0187

0.0208

45.0

4.91

Co

16.2

0.0075

0.0226

16.6

2.69

Co-Pt

26.8

0.0122

0.0272

23.4

6.27

Co@Pt SD

16.4

0.0156

0.0191

40.9

6.71

[a] Reaction condition: 0.25 g catalyst, 150°C, 750 psi, 3 wt% lactose, 1.00 ml/min.


Page 38 of 38

Use of Hydrogen Chemisorption and Ethylene Hydrogenation as

Recommend Documents