Characterization and Evaluation of Carbon-Supported Noble Metals

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Characterization and Evaluation of Carbon-Supported Noble Metals for the Hydrodeoxygenation of Acetic Acid Jose Contreras-Mora, Ritubarna Banerjee, Brandon K Bolton, John Valentin, John R. Monnier, and Christopher T Williams Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00288 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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Organic Process Research & Development

Characterization and Evaluation of Carbon-Supported Noble Metals for the Hydrodeoxygenation of Acetic Acid

Jose Contreras-Mora, Ritubarna Banerjee, Brandon Bolton, John Valentin, John R. Monnier, Christopher T. Williams* Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208

* To whom correspondence should be addressed: Christopher T. Williams Department of Chemical Engineering Swearingen Engineering Center University of South Carolina Columbia, SC 29208 Phone: (803)777-0143 Fax: (803)777-8265 E-mail: [email protected]

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Table of Contents:

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Abstract The gas-phase catalytic hydrodeoxygenation (HDO) of acetic acid (AA) over carbon supported noble metals (5%Pt/Cp97, 5%Pd/Cp-97, 5%Ru/Cp-97 and 5%Rh/Cp-97) were studied. Temperature-dependent conversion and selectivity were studied at 1 atm from 200-400 °C. For Pt, Pd and Rh the main pathway from 200-300 °C was decarbonylation and from 350-400 °C the main pathways were decarbonylation/decarboxylation and ketonization. However, for Ru from 200-250 °C the main pathway was decarboxylation, and from 300-400 °C, the main pathway was decarbonylation. The activity trend based on TOF at 200 °C was found to follow Ru > Rh~Pt > Pd. The activities of all catalysts at 200 °C were found to decrease after reaction at 400 °C and returning to 200 °C. This is attributed to sintering and coking. The reaction orders in AA and H2 measured at 200 °C for all catalysts are generally well below ~0.5, suggesting relatively strong adsorption of both reactants on all metal surfaces. The temperature-dependence of the reaction rates were examined over the range 200-240 °C having apparent activation energy values of around 21 kcal/mol for all the catalyst since decarbonylation/decarboxylation is the main pathway with similar product distribution.

Keywords: biomass; hydrodeoxygenation; decarbonylation; decarboxylation; hydrogenation;

acetic acid,

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1 Introduction Acetic acid (AA, CH3COOH) is the second simplest carboxylic acid, with many different industrial applications. For example, it is used for the production of cellulose acetate 1, and when diluted, it becomes vinegar and is used in a variety of food products. Another important industrial application for this carboxylic acid is the synthesis of the vinyl acetate monomer

2–4.

Based on these and other chemical processes, the global demand of acetic acid is around ~6.5 million tons per year (Mt/a) 5,6. Substantial research has been focused on replacing fossil fuels and chemicals with those derived from biomass, which is an alternative and renewable resource 7. With fast pyrolysis of biomass 8,

bio-oils can be obtained that contain many components. These include lignin, sugars, furans

and carboxylic acids, many of which contain high oxygen content

8,9.

From carboxylic acid

feeds, which can contain as much as ~9 wt% acetic acid 10, desirable products such as alcohols and hydrocarbons can be obtained with hydrogenation. For example 1,4-butanediol can be produced via the hydrogenation of maleic acid

11,

while hydrogenation of aqueous-phase

levulinic acid over a variety of supported catalysts (e.g., Ru/C 12–14, Ru/Al2O3 13,14, AuPd/TiO2 9, Ru/TiO2 13) can produce γ-valerolactone . One method to convert acetic acid into fuels and chemicals is through hydrodeoxygenation (HDO)

8,9,11,15,16.

The HDO process consists of the reactant (e.g. carboxylic acids) being co-fed

with hydrogen to facilitate hydrogenolysis and hydrogenation reactions required for the removal of oxygen. This results in the production of some liquids with high energy density that are similar to those of conventional fossil fuels, but also some gaseous products such as ethane and methane. The HDO pathways (Equations 1-3) consist of hydrogenation, decarbonylation and

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decarboxylation, and have been discussed in detail elsewhere 17–21. Hydrogenation:

R − COOH + 3H2 ↔ R − CH3 + 2H2O

(1)

Decarbonylation:

R − COOH + H2 ↔ R − H + CO + H2O

(2)

Decarboxylation:

R − COOH ↔ R − H + CO2

(3)

Rachmady and Vannice were among the first to investigate hydrogenation of AA over catalysts consisting of Pt supported on different types of materials

22.

Recently other researchers have

been using noble metals such as Pt 9, Ru, Pd, Ir and non-noble metals such MoOx

23

to obtain

high activity for AA hydrogenation. In this work, several different commercial group VIII noble metal catalysts have been studied to explore their catalytic chemistry towards AA HDO. These catalysts include Pt, Pd, Ru and Rh supported on an activated carbon. The various features of the reaction kinetics, including activities, selectivities, reactant reaction orders, and apparent activation energies, are compared with a view towards elucidating the mechanism that governs these reactions. 2 Experimental 2.1 Catalysts Four different commercial (BASF) carbon-supported metal catalysts were used for the HDO of AA: 5%Pt/Cp-97, 5%Pd/Cp-97, 5%Ru/Cp-97 and 5%Rh/Cp-97. The carbon support was an activated carbon from BASF (Cp-97 SBET=615m2/g).

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2.2 Catalyst Characterization 2.2.1 Pulse H2 Chemisorption The metal dispersions of the catalysts were determined by pulsed hydrogen titration of oxygen pre-covered sites using a Micromeritics 2920 AutoChem II Analyzer equipment. The catalysts were reduced at 200 °C for Pt, Ru and Rh and 250 °C for Pd in 10% H2 for two hours. Then, the gas was changed to argon to flush the hydrogen out for 1 hour, followed by cooling to 40 °C. For all the catalysts a flow of 50 sccm of 10% O2/Ar was added and then 30 minutes of Ar to remove any O2 (at 40 °C). Then, 10% H2/Ar at 40 °C for Pt, Pd, Rh and at 250 °C for Ru (since there is no H2 adsorption at 40 °C

24)

was added until there were no more uptake of H2. This was

repeated at least three times for reproducibility for each catalyst. 2.2.2 Electron Microscopy Imaging of catalysts for determining particle size distributions was performed using a JEOL 2100F 200 kV scanning transmission electron microscope (STEM) equipped with a CEOS Cs corrector on the illumination system. The geometrical aberrations were measured and controlled to provide less than a π/4 phase shift of the incoming electron wave over the probe-defining aperture of 17.5 mrad, which at 200kV provides a nominal probe size of Ru~Rh. In addition, Table 1 shows ∑𝑗

surface average diameters (Ds) calculated from STEM (𝐷𝑠 =

𝑛 𝐷3 𝑖=1 𝑖 𝑖

∑𝑗

𝑛 𝐷2 𝑖=1 𝑖 𝑖

) were Di is the diameter

and ni is the number of particles 26,27. The dispersion and particle size (Ds) taken from the H2-pulse chemisorption are the only measurements performed in a reduced environment, thus ensuring the reduction of the catalyst (or at least most of the sample) to its metal form. Figure S2 shows the XPS data for the fresh catalyst sample (already reduced ex-situ) and in-situ reduced (in the XPS catalyst cell) sample for: a) Pt (Pt0-71.3 eV

28,

PtO2-74.5 eV

29),

b) Pd (Pd0-335.3 eV

8


30,

PdO-337.0 eV

31)

, c) Ru

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(Ru0-280.3 eV 32, RuO2-281.3 33) and e) Rh (Rh0-307.3 eV 34, Rh2O3-308.9 eV 35) . While every fresh catalyst sample showed metal oxides, the in-situ reduced samples are completely reduced except for Pt, which is 91 % reduced (our reactor system reduces the catalyst with 20% H2 which might reduce Pt entirely). Usually, particle sizes derived from STEM and chemisorption should be consistent. However, in Table 1 it is observed that when the chemisorption particle size is compared with the STEM surface mean diameter (Ds), both measurements are comparable for Pt/Cp-97 and Pd/Cp-97 but not for Ru/Cp-97 and Rh/Cp-97. Carbon decoration phenomena may explain these discrepancies between characterization techniques. When catalysts are supported on activated carbon such as Cp-97, a carbon decoration from the support to the metal surface can occur 27. This results in the blocking of some of the active sites of the metals, effectively reducing the metal dispersion (and thus increasing the estimated particle size from chemisorption). The carbon decoration mechanism is still unknown, and one of the hypothesis is that the metal salt counterion (such as metal-chloride and metal-nitrate) is involved in the carbon decoration mechanism. This might explain why the particle size of both techniques agree with Pt and Pd, but not with Ru and Rh supported on Cp-97. For the purposes of calculating the intrinsic rates of reaction using turnover frequencies (i.e., molecules site-1 min-1), the metal dispersions based on chemisorption were used to estimate the active sites in the reactor. This was chosen since the chemisorption measurement if made on a reduced catalyst surface such as that found within the reactor at the start of the HDO reaction. In addition, XRD measurements were performed for all the fresh and after reaction catalysts. In all cases, oxides were observed, which is likely a result of their exposure to air during the measurement

36.

The analysis of these complex data will be the subject of a separate

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forthcoming publication. 3.2 Catalytic Evaluation of Carbon-supported Noble Metals Temperature-dependent conversion and selectivity plots for each catalyst are shown in Figure 2. For all the plots, selectivity is on the left Y-axis and is represented by bars, CO ratio is on the right Y-axis and is represented by open circles, and conversion on the top X-axis. To ensure that the water-gas shift (WGS) reaction was not taking place at any temperature and catalyst, the maximum amounts of CO2 that could be produced by the WGS reaction at different temperatures were calculated using the equilibrium constants Keq

37.

These results show that the WGS

reaction does not account for any of the CO2 observed in the present studies and can be seen in Table S1. Overall, the highest conversion for all temperatures was obtained over Ru, followed by Pt, Rh and Pd. For Pt/Cp-97 (Fig. 2a) methane is the most selective product since it is relatively easy for the C-C bond of AA to cleave over Pt 16. For instance, at 200 °C the reaction of AA results in the production of methane (96% selectivity), and ethane. As the temperature increases from 250400 °C, acetone is produced as well, including trace amounts of ethanol (< 2% selectivity) at around 300 °C. Decarbonylation is the main pathway, therefore, CO ratio is greater than 0.5 (right y-axis) from 200-300 °C. The second pathway is ketonization with the production of acetone (CO2 and water), decarboxylation with the CO ratio less than 0.5 (CO2 production from 350-400 °C) and then hydrogenation with the production of ethane. For Pd/Cp-97 (Fig.2b), the selectivity is mostly towards methane over the selected temperature range. At 200 °C there is methane (93% selectivity) and acetone, with no ethane, in contrast to Pt. As the temperature increases, the selectivity remains high towards methane, with the

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remainder acetone and some traces of ethane. When the temperature reaches 300 °C and higher, a trace amount (1-3% selectivity) of acetaldehyde begins to be produced. The main pathway for this reaction from 200 °C to 300 °C on Pd/Cp-97 is decarbonylation (CO ratio is greater than 0.5), with a shift to ketonization, some decarboxylation and hydrogenation (traces amount of acetaldehyde) from 350-400 °C. For Ru (Fig. 2c) at 200 °C the main product was methane (94% selective), followed by ethane (5% selective) and acetone (1% selective). As the temperature increases, the selectivity goes completely to methane, and this can be due to Ru promoting C-C bond cleavage

8,38.

The main

pathway is decarbonylation, decarboxylation (production of CO2), ketonization (trace amount of acetone) and hydrogenation to form ethane from 200-250 °C, and then completely towards decarbonylation, since the CO ratio is greater than 0.5 at higher temperature. For Rh (Fig. 2d) at 200 °C the only product is methane and as the temperature increases there is some acetone and ethane formed. The main pathway at lower temperatures is decarbonylation, with some ketonization and decarboxylation occurring at elevated temperatures. The activity of the catalysts at 200 °C measured before, and after the reaction was carried out at 400 °C, are shown in Table 2 as turn over frequencies (TOF, 1/min) based on the chemisorption site estimate. The comparison is made at 200 °C since at this temperature the measured conversions are differential (i.e., < 10%). The activity trend goes as follows: Ru>Rh~Pt>Pd and this trend is in agreement with aqueous-phase hydrogenation of acetic acid for a continuous-flow packed-bed reactor from 110-290 °C and 5.17 MPa total pressure

39.

After reaction at elevated

temperature (200-400 °C), the activities for all the catalysts were reduced significantly when measured again at 200 °C (Table 2). The decrease in activity goes as follows: Rh>Pd>Ru~Pt.

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Therefore, the Pt and Ru supported on carbon are most stable catalyst under HDO of acetic acid at the corresponding conditions. Particle sizes estimated from STEM images after reaction at elevated temperature are shown in Table 1. The particle sizes after reaction are larger compared to the ones at 200 °C, suggesting sintering. Figure S3 shows typical STEM images of Pt, Pd, Ru and Rh, respectively, after reaction at high temperature, clearly showing sintering. Given the differences in activity after elevated temperature reaction, TPO was used to look for evidence of coking. Figure 3 shows the fresh catalyst (black curve), the support (dashed black curve) and the after reaction (red curve) catalyst curves. CO2 peaks are seen at 160 °C and 180 °C for Pt, 220 °C for Pd and 260 °C for Rh for the after reaction samples. For Ru, CO2 is generated beginning at around 240°C and continues to higher temperatures. While more is produced on the post reaction sample, the evolution is still spread out over a wide temperature range. This convolution of the oxidation of support carbon and deposited carbon is a function of small particle size of Ru, which enhances its ability to oxidize the support carbon to a greater extent than the other metals. Essentially, the activity for 5% Pt and 5%Pd reduces mostly because of coking and for 5%Ru and 5%Rh the activity reduces due to sintering. Reactant reaction order studies were conducted at 200 °C by varying the concentration of AA or hydrogen, while holding all other parameters constant. The lower temperature was chosen for two reasons. First, it allowed the reaction to be carried out at very low differential conversion, ensuring that true kinetics could be measured. Second, this temperature essentially eliminates the above issue of deactivation that occurs at higher temperatures. External mass transfer limitations were eliminated for each catalyst by choosing an appropriate flow rate (Fig. S1). Also, the possible presence of internal mass transfer limitations was investigated using the Weisz-Prater 12


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criterion (Eq. S1

40,41),

with calculations indicating that there were no pore diffusion limitations

(details provided in the Supporting Material section). The kinetics were modeled by a power rate law ( rA  k[ PAA ] [ PH 2 ] ), with Figures 4 and 5 showing ln(rA) plotted versus the mole fraction of AA and H2, respectively, for each of the catalysts. The slopes of the lines therefore indicate the reaction orders. The AA reaction order (α) was found to be 0.21 ± 0.08, 0.09 ± 0.08, 0.36 ± 0.04 and 0.20 ± 0.04 for Pt, Ru, Rh and Pd, respectively. In the case of H2, the reaction order (β) was found to be 0.61 ± 0.12, 0.40 ± 0.08, 0.05 ± 0.04 and 0.23 ± 0.07 for Pt, Ru, Rh and Pd, respectively. There has not been much discussion about reaction orders for AA HDO in the literature. Rachmady and coworkers

22

determined reaction orders for AA (from 0.2-0.4) and H2 (0.4-0.6)

for Pt/TiO2 at different conditions, which is similar to the reaction orders found here for Pt/Cp97. Also, for a single crystal Pt (111) hydrogenation at 97 °C, the reaction order for H2 was found to be 0.5

42.

A study on HDO of a microalgae-derived palmitic acid compound over 1%

Pt/γ-Al2O3 in a microreactor reported reaction orders of ~1 for both α and β

43

. The low reaction

orders for both reactants in the present case suggest relatively strong adsorption of acetic acid and H2 on the metal surfaces based on a Langmuir-Hinshelwood adsorption assumption 44. The temperature-dependence of the reaction rates was examined over the range 200 – 240 °C. Apparent activation energy values were extracted through an Arrhenius analysis, as shown in Figure 6. The apparent activation energy values were found to be 23.6 ± 2.8, 21.8 ± 2.0, 20.4 ± 1.5 and 20.3 ± 0.8 kcal/mol for Pt, Ru, Rh and Pd respectively. Rachmady et al. found an apparent activation energy for Pt/TiO2 of 11 kcal/mol, which not only depended on the product distribution but also on the AA and H2 partial pressure 22. The apparent activation energies for

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all catalyst in this study are similar, likely due to the fact that the main pathways at the temperature range are decarbonylation/decarboxylation reactions. Similar activation energies were observed for such pathways for the HDO of propanoic acid (PA) using Pd/C, with an activation energy of 16.7 ± 0.6, for a decarbonylation/decarboxylation pathway 17. The HDO of acetic acid at 300 psig using NiMo/Al2O3 45 exhibited an activation energy of 20.9 kcal/mol for a combination of hydrogenation, ketonization and decarboxylation/decarbonylation pathways. Activation energies based on the rates of product formation were also calculated, and are shown in Table 3. For Pt, the highest activation energy was from acetone and methane production (~23 kcal/mol), while for Pd and Ru, acetone was produced with the highest activation energy, and methane for Rh. Other studies have suggested that ketonization of AA have a greater activation energy (~38 kcal/mol, Ru/TiO2)

46

than that of C-C cleavage (~ 16 kcal/mol, Ru (0001))

47,

which provides an explanation for the present observation. 4 Conclusion In conclusion, it has been found that the main pathway for the gas-phase HDO of AA on carbon supported noble metals is decarbonylation, with decarboxylation only becoming significant at elevated tempratures (350-400°C) for Pt, Rh and Pd. Over the entire temperature range, the main product was methane, although small quantities of ethane (from C-O bond cleavage and hydrogenation) and acetone (from ketonization) were detected. Kinetic measurements revealed AA reaction orders < 0.4 and H2 reaction orders < 0.6, suggesting relatively strong adsorption of both reactants on these metals. This may suggest that there are different sites for adsorption, since two species cannot both be strongly adsorbed on the same site. In addition, the activation energies were very similar on all catalysts, ranging from 20-24 kcal/mol. Overall, the results

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suggest that these monometallic catalysts are too active for C-C bond cleavage, which results in the formation of undesired C1 products rather than C2 oxygenates. A strategy that we are pursuing is to add Sn as a second metal to these catalysts, with a view towards increasing selectivity towards oxygenate products through geometric and bifunctional bimetallic effects. Acknowledgments We gratefully acknowledge NSF IGERT (DGE-1250052) for our funding. The authors acknowledge John Tengco and Qiuli Liu for their help in TPO experiments. Supplemental Information Table S1 shows that the CO2 amount produced from the HDO reaction is significantly less than the theoretical CO2 amount that could be produced from the Water Gas Shift (WGS) reaction. Figure S1 shows the external mass transfer limitation experiment for the different catalysts. Different flows were chosen for the experiments to ensure that there was no external mass transfer limitation. Figure S2 shows the XPS data for the fresh catalyst sample (already reduced ex-situ) and in-situ reduced (in the XPS catalyst cell) sample for Pt, Pd, Ru and Rh. Figure S3 shows representative STEM images with the particle size distribution for Pt/Cp-97, Pd/Cp-97, Ru/Cp-97 and Rh/Cp-97 for the after reaction catalysts showing sintering. Finally, the evaluation of internal mass transfer limitation is explained in detail and it is confirmed that there are no internal mass transfer limitations for any catalysts.

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List of Tables and Figures Table 1: Particle sizes for catalysts obtained using chemisorption and STEM. Table 2: Activity of catalyst at 200 °C before and after reaction at 400 °C Table 3: Activation energy for individual products on Pt, Pd, Ru and Rh supported on Cp-97 Figure 1: Representative STEM images for a) 5%Pt/Cp-97, b) 5%Pd/Cp-97, c) 5%Ru/Cp-97 and d) 5%Rh/Cp-97. Size distributions of particles are shown in the insets. Figure 2: Temperature-dependent conversion and selectivity for HDO of AA on: a) 5%Pt/Cp97, b) 5%Pd/Cp-97, c) 5%Ru/Cp-97 and d) 5%Rh/Cp-97. The black circles indicate the CO/(CO+CO2) ratio. Figure 3: Temperature program oxidation for fresh catalyst (gray), after reaction (black) showing coking and support (dashed curve) on: a) Pt/Cp-97, b) Pd/Cp-97, c) Ru/Cp-97 and d) Rh/Cp-97. Figure 4: Power rate law for the kinetic dependencies of AA for activity at 200 °C and 1 atm pressure for 20% H2. Total flow rates were 150 sccm for 5% Pt/Cp-97 (circles), 50 sccm for 5%Ru/Cp-97 (squares), 200 sccm for 5%Pd/Cp-97 (triangles) and 100 sccm for 5%Rh/Cp-97 (diamonds). Figure 5: Power rate law for the kinetic dependencies of H2 for activity at 200 °C and 1 atm pressure and 1.1% AA. Total flow rates were 150 sccm for 5% Pt/Cp-97 (circles), 50 sccm for 5%Ru/Cp-97 (squares), 200 sccm for 5%Pd/Cp-97 (triangles) and 100 sccm for 5%Rh/Cp-97 (diamonds). Figure 6: Arrhenius apparent activation energies for (circle) 5% Pt/Cp-97 (circles), 5%Ru/Cp-97 (squares), 5%Pd/Cp-97 (triangles), and 5%Rh/Cp-97 (diamonds).

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Table 1: Particle sizes for catalysts obtained using chemisorption and STEM.

a-Fresh sample b-After reaction sample Ds- Surface average diameter

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Table 2: Activity of catalyst at 200 °C before and after reaction at 400 °C

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Table 3: Activation energy for individual products on Pt, Pd, Ru and Rh supported on Cp-97

-- Not produced

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Figure 1: Representative STEM images for a) 5%Pt/Cp-97, b) 5%Pd/Cp-97, c) 5%Ru/Cp-97 and d) 5%Rh/Cp97. Size distributions of particles are shown in the insets.

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Figure 2: Temperature-dependent conversion and selectivity for HDO of AA on: a) 5%Pt/Cp-97, b) 5%Pd/Cp-97, c) 5%Ru/Cp-97 and d) 5%Rh/Cp-97. The black circles indicate the CO/(CO+CO2) ratio.

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Figure 3: Temperature programmed oxidation for fresh catalyst (black), after reaction (red) showing coking and support (dashed curve) on: a) Pt/Cp-97, b) Pd/Cp-97, c) Ru/Cp-97 and d) Rh/Cp-97.

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Figure 4: Power rate law for the kinetic dependencies of AA for activity at 200 °C and 1 atm pressure for 20% H2. Total flow rates were 150 sccm for 5% Pt/Cp-97 (circles), 50 sccm for 5%Ru/Cp-97 (squares), 200 sccm for 5%Pd/Cp-97 (triangles) and 100 sccm for 5%Rh/Cp-97 (diamonds).

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Figure 5: Power rate law for the kinetic dependencies of H2 for activity at 200 °C and 1 atm pressure and 1.1% AA. Total flow rates were 150 sccm for 5% Pt/Cp-97 (circles), 50 sccm for 5%Ru/Cp-97 (squares), 200 sccm for 5%Pd/Cp-97 (triangles) and 100 sccm for 5%Rh/Cp-97 (diamonds).

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1 Ea [=] kcal/mol

0 Ln(TOF (min-1))

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


21.8 ± 2.0 23.6 ± 2.8 20.4 ± 1.5 20.3 ± 0.8

-1 -2 -3 -4 -5 -6 0.0019

0.0020

0.0021

0.0022

1/T(1/K) Figure 6: Arrhenius apparent activation energies for (circle) 5% Pt/Cp-97 (circles), 5%Ru/Cp-97 (squares), 5%Pd/Cp-97 (triangles), and 5%Rh/Cp-97 (diamonds).

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Characterization and Evaluation of Carbon-Supported Noble Metals

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