Catalysis Final

Cellulose

linear polymer of glucopyranose units

• high MW n>1,000,000

• Crystalline, forms fibers

• cellulose fibers provide wood’s strength

hemicellulose

amorphous branched polymer

• low degree of polymerization n~200

• easier to hydrolyze than cellulose

Lignin

amorphous, three-dimensional, cross-linked resin

• no exact structure

• main binder for cellulosic fibers, chemical shield for cellulose

problems with lignin as fuel

requires more energy than it provides. requires more water

biofuels

bioethanol, biodiesel, green diesel, pyrolysis oils

bioethanol production from corn or lignocellulose

• enzymes (cellulases) convert cellulose to glucose

• yeasts convert glucose to ethanol

problems with bioethanol production

• using land to make ethanol from corn we can’t eat, making corn more expensive

• use a small fraction of biomass and compete with food production

vegetable oils

• main constituent is triglycerides

• length of chains in the diesel range

biodiesel production from vegetable oils

• transesterification of triglycerides. fatty esters are biodiesel

• produced at scale today

conversion of lignocellulose

want to remove oxygen functionality and form C-C bonds

gasification of biomass

• syngas can be produced from biomass gasification

• fischer-tropsch synthesis can be used if liquid fuels are desired

pyrolysis of biomass

heating biomass in the absence of oxygen to make bio oil

issues with bio oil

• high MW of oxygen

• cant be thermally fractioned unlike crude oil

• furanics polymerize, acids catalyze polymerization, phenolics consume excess hydrogen

• catalytic transformations are needed to convert to fuels

production of furanics: HMF and Furfural

furanics are formed from acid catalyzed dehydration of C5 and C6 sugars

• C5 sugars like xylose → furfural

• C6 sugars like glucose → HMF

furfural

• commercially produced today

• used as a selective solvent or converted into fuels and chemicals

• most common chemistry is to upgrade by hydrogenation: metal catalyst

• most commonly makes furfuryl alcohol

conversion of furfural over metal catalysts

• Cu catalysts adsorb over O site because it doesnt like C → more selective for furfuryl alcohol

• Pd and Ni catalysts adsorb over C and/or O → makes 3 different products

• bimetallic catalysts (Ni-Fe, Pd-Fe) increases selectivity to 2-methyl-furan (fuel additive)

  • bifunctional catalyst to cleave C-O bond

production of levulinic acid: the Biofine process

produces levulinic acid from C5 sugars in hemicellulose

• can be converted to a range of products

• DALA can be used as a benign herbicide and cancer treatment agent

• can be converted to GVL by hydrogenation on Ru catalysts

HMF as a platform chemical

• biobased polymers → furan dicarboxylic acid to replace terephthalic acid

  • PEF as a replacement for PET

issues with HMF compared to furfural

• HMF can decompose or polymerize in aqueous solution to make Humins (complex polymerized species, usually burned for heat

cathode

• where reduction happens

electrolyte

ionically conductive, electrically insulating

electrode

• electron conductor

• dictates the current produced at a given voltage

• catalyst

RHE

if we define our zero point in potential to be the point when the hydrogen rxn is in equilibrium

spontaneous rxn

• delta G < 0

• delta G^o=-nFE^o

delta E⁰

amount of voltage required to be added for rxn to happen

how do we assess the properties of a catalyst

• measure outlet concentration of reactant and product gases

• relate rates (steady state or transient) to driving force (temperature)

• charge passed is directly related to catalytic rate

• relate rates to driving force (potential)

reference electrode

• a reference point to compare the potential of working electrode to a fixed point

• always at equilibrium

overpotential

• n=E-E⁰

• defines electrocatalyst activity by how much driving force must be applied beyond the equilibrium potential to drive current

• rates depend exponentially on overpotential because Ea is lower as we apply a higher potential

normalizing current

• if we do not normalize current our results will depend on how much catalyst we have instead of intrinsic catalyst properties

• we must normalize by number of active sites → Turnover frequency

• measure TOF using chemical titrants or counting metal atoms

influence of mass transport in meausred rates

• want to be in a kinetically limited regime or else rates do not reflect solely properties of the catalyst

volmer step

• H⁺ + e^- + * → H*

• 1st step of HER

HER in acidic conditions

• has one adsorbed intermediate H* → H binding energy dictates reactivity

• peak of activity volcano at G=0

• Pt has overpotential close to 0V but is expensive

Heyrovsky step

• H⁺ + e^- + H* → H₂

• ER step

Tafel Step

• H* + H* → H₂

• _{LH step}

Potential replacements for Pt

• MoS2 → already used for hydrodesulfurization, DFT predicted high rate, research has been done to increase surface area and edges but still is not increasing the intrinsic activity

• metal phosphides → earth abundant hydrodesulfurization catalysts

HER in basic conditions

• water dissociation is important in base, reflected by delta G_OH

• Pt is much less active in base than in acid

• use bifunctional catalyst → add oxophilic alloy which allows Pt to dissociate H2O

what limits the efficiency of water splitting/activity of fuel cells

OER/ORR

pourbaix diagram

• shows the stability window of catalysts

• the OER potential window is incredibly oxidizing → all catalyss for the OER exist as oxides

• in acid only IrO2 is stable

why is operation in acid important

• we have a good proton exchange membrane

• anion exchange membranes are still being developed

• alkaline electrolyzers use porous diaphragms

OER catalyst challenges

• overpotentials for the OER rarely fall below 0.3V

• OER involves three O containing adsorbates → scaling relations limit catalysts

• best we can do is optimize O* binding energy at 1.6 eV

OER best catalysts allkaline

• layered NiOOH

  • very poor catalyst without Fe

• Persovskites are a tunable class of metal oxides

  • overly reactive perovskites lose crystallinity

• Ir and Ru oxides are stable for OER

methanol synthesis catalyst

Cu-Zn

propane dehydrogenation catalyst

Pt-Sn

fluid catalytic cracking catalyst

ZSM-5

hydrodesulfurization catalyst

Co-Mo-S

ORR catalyst

• Pt is the best catalyst but it is too strongly binding for optimal ORR activity

• weaken Pt binding by using more coordinated/close packed surfaces (111 facet), larger particle size

• ligand effect: subsurface metal shifting away from the fermi level

• compressive strain shift d states away from fermi level

• Pt3Ni alloys form Pt skin with strain

• core shell particles can reduce Pt dissolution

• Breaking Pt ensembles for optimal H2O2 generation