Reduction of Carbon Formation From Nickel Catalysts Using Nickel-Gold Surface Alloys

Reduction of Carbon Formation Reduction of Carbon Formation From Nickel Catalysts Using From Nickel Catalysts Using NickelNickel--Gold Surface AlloysGold Surface Alloys

David King, Yong Wang, Bob Rozmiarek, Ya-Huei (Cathy) Chin, John Hu

Pacific Northwest National LaboratorySECA Annual Workshop and

Core Technology Program Peer ReviewMay 12, 2004

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Project ObjectivesProject ObjectivesProject Objectives

Evaluate ability of gold modification of nickel catalysts to minimize carbon formation during hydrocarbon reformation Quantify effect of gold addition to nickel catalyst surface on catalyst activityDevelop characterization techniques to clarify the effect of gold on nickel surface properties Extend catalyst modification concepts to Ni anodes, enabling partial on-anode reforming of natural gas

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What is KnownWhat is KnownWhat is KnownCarbon can form during reforming with nickel catalysts even under conditions not predicted by thermodynamicsSmall crystallite nickel particles (<10nm) have been shown to be more resistant to carbon formation

Maintenance of small crystallites under reforming conditions is challenging due to sintering and compound formationPlausible approach for pre-reforming

Addition of gold to nickel catalysts retards deposition of carbon and consequent deactivation

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Relevant TheoriesRelevant TheoriesRelevant Theories

Step site modelThe most active catalytic sites on nickel (step and edge sites) are also nucleation points for carbon formationDeactivation of these most active sites (by sulfur, gold, other) results in reducing carbon formation at some loss of catalytic activity

Ensemble modelGroups of contiguous nickel atoms (ensembles) have high tendency to deposit carbon during reforming An impurity atom that breaks up ensemble size may retard carbon formation more than reforming activityElectronic properties of nickel atoms can also be altered by an ad-atom (ligand effect)

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Project TasksProject TasksProject Tasks

Baseline supported nickel catalyst (Ni/MgAl2O4) performance in reforming methane and butane Quantify effect of gold addition on carbon formation and reforming activity with supported Ni catalystDevelop analytical methodology to characterize nickel surfaceVerify methane reforming kinetics with Ni/YSZ anode

Provide to modeling effort

Quantify effect of gold addition on carbon formation and reforming activity with nickel anode catalyst

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TGA Studies Show Gold Addition Reduces Carbon Deposition From Supported Ni Catalysts

TGA Studies Show Gold Addition Reduces Carbon TGA Studies Show Gold Addition Reduces Carbon Deposition From Supported Ni CatalystsDeposition From Supported Ni Catalysts

Total Carbon Deposition n-Butane Reforming S/C = 0.258 hours Isothermal Operation

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Temperature (C)

% M

ass

Ni 15.8%

Ni 15.8% Au 0.5%

Ni 8%

Ni 8% Au 0.5%

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Particle Counting by TEM Shows Difference in Crystallite Size

Particle Counting by TEM Shows Particle Counting by TEM Shows Difference in Crystallite SizeDifference in Crystallite Size

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8.8 % Ni/MgO-Al2O3

15.8 % Ni/MgO-Al2O3

700oC Reduction

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Catalyst Preparation and PretreatmentCatalyst Preparation and PretreatmentCatalyst Preparation and Pretreatment

Ni/MgAl2O4Preparation

MgAl2O4 acquired from supplier, surface area 271 m2/gIncipient wetness impregnation using Ni(NO3)2Calcination: 500oC

TestingReduction: 700oC in H2 (2h)Begin flow H2 and steam, then butane or methane

Ni-Au/MgAl2O4Preparation

Reduce (700oC) and passivate (1%O2/He, ambient, 12h) Ni/MgAl2O4 catalyst Add Au by incipient wetness impregnation of HAuCl4Dry at 200oC under inert atmosphere

TestingSame as above

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Test ReactorTest ReactorTest Reactor

Catalyst

Zone

TC Ports

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Supported Nickel Catalyst Shows Rapid Deactivation in Butane Reforming

Supported Nickel Catalyst Shows Rapid Supported Nickel Catalyst Shows Rapid Deactivation in Butane ReformingDeactivation in Butane Reforming

n-C4 Reforming Ni Small Particle 247,500 GHSV; 485C; Varying Steam to Carbon

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Time (hrs)

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and

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ctiv

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C4 ConversionCO SelectivityCH4 SelectivityChange to 3:1 Steam to CarbonChange to 6:1 Steam to Carbon

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0.4% Gold Addition Eliminates Nickel Catalyst Deactivation

0.4% Gold Addition Eliminates 0.4% Gold Addition Eliminates Nickel Catalyst DeactivationNickel Catalyst Deactivation

Ni/Au Small Particle n-C4 Reforming 247,500 GHSV; 485C; Varying Steam to Carbon

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Time (hours)

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and

Sele

ctiv

ity (%

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n-C4 ConversionCH4 SelectivityCO SelectivitySteam to Carbon to 3:1Steam to Carbon 1.5:1

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Effect of Gold Addition on n-Butane Reforming with Supported Nickel Catalyst

Effect of Gold Addition on nEffect of Gold Addition on n--Butane Reforming Butane Reforming with Supported Nickel Catalystwith Supported Nickel Catalyst

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0 0.5 1 1.5 2 2.5 3 3.5Time on Stream (hours)

C4H

10 C

onve

rsio

n (%

8.8% Ni

8.8% Ni 0.2%Au

8.8% Ni 0.4% Au

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Gold Addition to Achieve Theoretical Monolayer Coverage

Gold Addition to Achieve Gold Addition to Achieve Theoretical Monolayer CoverageTheoretical Monolayer Coverage

0.0110.0250%Ni-50%YSZ anode

0.3648.8% Ni/MgAl2O4

0.61415.8% Ni/MgAl2O4

Wt.% Au for monolayer coverage (1Au/3Ni)% Ni dispersionCatalyst

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XPS Spectra of Nickel and Nickel-Gold Catalysts Supported on Mg-Al2O4

XPS Spectra of Nickel and NickelXPS Spectra of Nickel and Nickel--Gold Gold Catalysts Supported on MgCatalysts Supported on Mg--AlAl22OO44

840850860870880890900

Binding Energy (eV)

7580859095

Binding Energy (eV)

0.3% Au/MgO-Al2O3

15.8% Ni/MgO-Al2O3

15.8% Ni-0.5% Au/MgO-Al2O3

15.8% Ni-0.5% Au/MgO-Al2O3

15.8% Ni-0.3% Au/MgO-Al2O3

0.3% Au/MgO-Al2O3

15.8% Ni/MgO-Al2O3

15.8% Ni-0.3% Au/MgO-Al2O3

Suppression of Ni 2p peak observed with increasing Au loadings.

Ni 2p

Au4f

Mg 2s

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Comparison of XPS and Hydrogen Chemisorption on Au Promoted 15.8%Ni/MgAl2O4 Catalyst

Comparison of XPS and Hydrogen Chemisorption on Au Comparison of XPS and Hydrogen Chemisorption on Au Promoted 15.8%Ni/MgAl2O4 CatalystPromoted 15.8%Ni/MgAl2O4 Catalyst

XPS represents possible method to determine the fraction of Ni covered by Au

Attenuation of Ni signal consistent with Au adsorbed on Ni surfaceAddition of 0.5% Au estimated to provide approximately 8-9% of a monolayerNot all Au is associated with Ni

Decrease in H2 chemisorption upon Au addition is far greater than estimated monolayer coverage by XPS

Consistent with effect of Au addition extending beyond simple site blocking

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1.01

0.989 0.991 0.993 0.995 0.997 0.999 1.001

Ni/(Ni+Au) Atomic Ratio

XPS

Ni/(

Ni+

Au)

ato

mic

co

ncen

trat

ion

0.15

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0.35

0.45

0.55

0.65

0.75

0.85

0.95

H 2 c

hem

isor

bed

(cc/

g, S

TP)

0.3%Au0.5% Au

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Summary of H2 Chemisorption DataSummary of HSummary of H22 Chemisorption DataChemisorption Data

0.10H2, 900C8.8%Ni-

0.4%Au/MgAl2O4

0.196H2, 900C8.8%Ni-

0.2%Au/MgAl2O4

206.00.856H2, 700C8.8%Ni/MgAl2O4

6 (SEM); 10 (TEM)254.50.642H2, 900C8.8%Ni/MgAl2O4

0.206H2, 900C15.8%Ni-

0.5%Au/MgAl2O4

0.771H2, 900C15.8%Ni-

0.3%Au/MgAl2O4

14 (TEM)403.10.967H2, 900C15.8%Ni/MgAl2O4

Xtal size (other)Xtal size (H2)Dispersion,%H2 Uptake, cc/gPretreatmentCatalyst

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Effect of Gold on Steam Reforming of Methane Over Supported Nickel Catalyst

Effect of Gold on Steam Reforming Effect of Gold on Steam Reforming of Methane Over Supported Nickel Catalystof Methane Over Supported Nickel Catalyst

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0 0.5 1 1.5 2 2.5 3 3.5 4Time on Stream (hours)

CH

4 co

nver

sion

(%

8.8% Ni 8.8%Ni 0.4%Au

247,500 GHSV; 485C; 3:1 Steam to Carbon

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Effect of Gold Addition on Hydrocarbon Conversion

Effect of Gold Addition Effect of Gold Addition on Hydrocarbon Conversionon Hydrocarbon Conversion

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Butane Initial Conversion Methane Initial Conversion Hydrogen Uptake

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mal

ized

Con

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ion,

8.8% Ni8.8%Ni-0.4%Au

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Temperature (oC)

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Temperature (oC)

8.8 wt% Ni 8.8 % Ni

0.2% Au/8.8 % Ni0.2% Au/8.8 % Ni

N2O Temperature Programmed DesorptionNN22O Temperature Programmed DesorptionO Temperature Programmed Desorption

Mass 28 N2 Mass 46 N2O

N2O N2

Ni 9583 3500Ni-Au 3430 366

Area under the peak

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Summary of Probe Molecule ResultsSummary of Probe Molecule ResultsSummary of Probe Molecule ResultsH2 chemisorption

Best method to quantify available surface sites, dispersionH2 uptake less than expected from XRD line broadening or TEM Uptake significantly decreased with Au additionH2 uptake correlates with observed activity

N2 chemisorptionProposed (in literature) as method to titrate step sites (“B5” sites)Does not correlate with catalyst activity, carbon formation, or gold effects

H2S titration of Ni surface sitesAmbient temperature H2S adsorption is significantly greater than H2 chemisorption—suggests formation of bulk plus surface sulfides

N2O TPDTwo types of sites observed:

“A” sites--desorb N2 (form NiO) at low temperature“B” sites--desorb N2O at higher temperature

Addition of Au reduces “A” sites more than “B” sitesEvaluating approach as method to distinguish step from planar sites

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Nickel Anode Steam Methane Reforming Nickel Anode Steam Methane Reforming Nickel Anode Steam Methane Reforming

Potential benefitsUse endothermic reforming reaction to consume heat generated in fuel cell operationReduce cathode cooling by excess airReduce or eliminate external reformer

ChallengesCarbon formation catalyzed by NiTemperature gradients near fuel inlet due to strong endotherm may lead to failureNeed to spread out conversion over broader area of anode to achieve thermal balance

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Nickel Anode Steam Methane ReformingNickel Anode Steam Methane ReformingNickel Anode Steam Methane Reforming

ApproachObtain kinetic data on methane reforming to support modeling effortIdentify operating parameters leading to deactivation by carbon formationQuantify effect of Au addition on activity and activity maintenance under carbon forming conditionsDevelop methods for reliable, reproducible introduction of Au to anode

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Ni YSZ AnodeNi YSZ AnodeNi YSZ Anode

Composition: 60%ZrO2-40% NiO2 by volume; 50% NiO2 by weightParticle size: 0.5-5 µm agglomeratesBET: 0.43 m2/gH2 adsorption: 0.0221 cc/g; 0.023% Ni dispersion Testing method—anode substrate

Two test strips 0.5x1” against opposite walls of channelStrips have YSZ on back side—only one active surface

Gold doping procedureIncipient wetness impregnation not viable methodPore fill anode material with solution of appropriate concentration HAuCl4Procedure tends to give poorer dispersion of metals onto substrate

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SEM of Ni/YSZ Anode (Reduced)SEM of Ni/YSZ Anode (Reduced)SEM of Ni/YSZ Anode (Reduced)

5000 X back-scattered image5000 X

10,000 X

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Ni/YSZ Anode Shows Significant Deactivation in Butane ReformingNi/YSZ Anode Shows Significant Ni/YSZ Anode Shows Significant

Deactivation in Butane ReformingDeactivation in Butane Reforming

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n-C

4H10

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vers

ion

(%),

S/C = 6S/C = 3

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Anode Catalyst Post Reaction with n-Butane

Anode Catalyst Anode Catalyst Post Reaction with nPost Reaction with n--ButaneButane

Carbon readily wipes off surface of anode

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SEM of Deactivated Ni-Anode Following Butane Steam Reforming

SEM of Deactivated NiSEM of Deactivated Ni--Anode Anode Following Butane Steam ReformingFollowing Butane Steam Reforming

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SEM of Ni/YSZ Anode (Spent)SEM of Ni/YSZ Anode (Spent)SEM of Ni/YSZ Anode (Spent)

30,000 X 10,000 X

Back Scattered Image + SE image 50,000 X

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Ni Anode vs. Supported Nickel Catalyst ActivityNi Anode vs. Supported Nickel Catalyst ActivityNi Anode vs. Supported Nickel Catalyst Activity

1.40.0113490.015499Mols C4 converted/ (cc H2 uptake-min)

176.90.0004630.08190Mols C4 converted/g Ni-min

31.10.0002320.00721Mols C4 converted/g catalyst-min

RatioNi/YSZ Anode8.8%Ni/MgAl2O4

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Ni/YSZ Shows Small Effect of Au AdditionNi/YSZ Shows Small Effect of Au AdditionNi/YSZ Shows Small Effect of Au Addition3:1 Steam to Carbon; 10:1 CH4 to H2, 700oC

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Ni Anode

Ni Anode + 0.02% Au

0.02%Au on anode is equivalent to 0.8%Au on supported Ni catalystNo deactivation observed with either material

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Anode SEM After CH4 ReformingAnode SEM After CHAnode SEM After CH44 ReformingReforming

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Summary and ConclusionsSummary and ConclusionsSummary and Conclusions

Addition of gold to supported nickel catalyst at sub-monolayer coverage significantly retards carbon formationSufficient gold to retard carbon formation nickel results in decrease in catalyst activity by factor ~65-85%

Methane conversion more affected by Au addition than butane conversion

H2 chemisorption provides best method to correlate Ni availabilitywith catalyst activityN2O chemisorption may provide method to measure step sites

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Summary and ConclusionsSummary and ConclusionsSummary and Conclusions

Step site poisoning model does not fully explain resultsAddition of gold affects many nearest neighbor Ni sitesAnode activity on surface Ni basis (H2 chemisorption) comparable to supported Ni despite expected differences in step sites between two catalysts

Reforming studies have been initiated over Ni/YSZ anodeButane reforming at even 6:1 S:C observed

Filamentous carbon identifiedCarbon filament diameter smaller than Ni/YSZ crystal size (by XRD)

Carbonizing effect not observed with CH4 feed at 3:1 S:C and short reaction timesAddition of gold to Ni/YSZ shows small activity decrease compared to supported Ni case, based on current preparation method

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Future WorkFuture WorkFuture Work

Obtain kinetic data for on-anode reforming to support model developmentEvaluate effect of gold addition to Ni/YSZ for methane steam reforming

Carbon tolerance at reduced S/C ratiosAu concentration-reforming activity correlationIdentify best methods for Au introduction onto Ni/YSZ

Evaluate effect of other additives to improve nickel anode performance (alkaline earth, Sn, Ce)Evaluate efficacy of natural gas pre-reforming with modified Ni catalysts Initiate studies of doped strontium titanate as sulfur-tolerant pre-reforming catalyst