(699f) Aqueous Phase Reforming of Lactose with Pseudomorphic Overlayer Catalysts

A great deal of today’s research seeks to produce sustainable fuels to meet our world’s ever growing energy demands.  Aqueous phase reforming (APR) has emerged as a process capable of turning renewable feedstocks into hydrogen gas or alkanes.  Much of our current hydrogen supply is derived from light hydrocarbons (e.g. natural gas) and a sustainable method of production will prove imperative as demand increases.  The majority of past APR studies have used feedstocks that are derived from biomass (methanol, ethylene glycol, glycerol, sorbitol, and glucose), but in order to prove the full potential of the process, further studies need to be done using a pure biomass feed.  Lactose, a sugar waste product of the cheese and dairy industry, has the potential to provide a sustainable source of biomass for APR as well as serve as a model for other carbohydrate studies.

Past APR studies have looked at a variety of different supported metal catalysts for the reaction.  These studies have shown that supported bimetallic Pt/Ni and Pt/Co make ideal catalysts for production of hydrogen through APR.  Computational work has also predicted that a Pt overlayer on Ni or Co will decrease the heat of adsorption of the H₂and CO products.  Hydrogen and CO are inhibitory to the APR reaction and it is believed that this decrease in adsorption strength will lead to a better APR catalyst.  Thus, we have synthesized pseudomorphic overlayer type bimetallic catalysts which aim to increase the activity of the catalyst while reducing the necessary loading of costly platinum. 

We have previously synthesized Re@Pd (host@overlayer) supported catalysts.  A decrease in heat of hydrogen adsorption was seen for Re@Pd catalysts when compared to both the Re and Pd parents; these results were consistent with computational predictions found in literature.  Re@Pd overlayer catalysts also demonstrated a decrease in activity for ethylene hydrogenation when compared to the activity of Pd [1, 2].  The correlation between heats of adsorption (computational) and activity (experimental) were attributed to the decrease in hydrogen adsorption measured for Re@Pd overlayer catalysts.

We have built upon these findings and have extended the directed deposition synthesis technique toward creating Ni@Pt and Co@Pt catalysts for APR.  Ni/Al₂O₃, Co/Al₂O₃, and Pt/Al₂O₃ monometallic baseline catalysts were prepared along with Ni/SiO₂-Al₂O₃, Co/SiO₂-Al₂O₃ and Pt/SiO₂-Al₂O₃ catalysts using standard techniques.  Previous studies have shown that alumina supported catalysts favor hydrogen production while silica-alumina supported catalysts favor alkane production; each support is to be examined in the APR of lactose.  The samples were then calcined and reduced to deposit the host metal on the support surface.  The directed deposition technique was used to selectively deposit the overlayer (platinum) metal on the host metal without it depositing onto the support surface.  Multiple depositions were done (single, double, triple) on each catalyst to increase platinum overlayer coverage.  Elemental analysis has been done to show that platinum has been deposited with the directed deposition and to calculate the percent coverage of the overlayers.

Hydrogen and carbon monoxide chemisorption studies for isotherm analysis and heats of adsorption determination were done using a Micromeritics ASAP 2020.  Isotherms for each sample were measured from 35 to 300°C and pressures from 1 mtorr to 900 torr.  The activity of each sample was studied through the ethylene hydrogenation reaction.  The ethylene hydrogenation reaction was performed using a plug flow reactor at temperatures from -10 to 300°C at atmospheric pressure with a typical gas flow rate of 750 mL/min.  The reactor effluent was monitored using a GCMS. 

Chemisorption results showed that hydrogen and carbon monoxide adsorption isotherms were distinct in shape and temperature dependence for each of the Ni/Al₂O₃ and Co/Al₂O₃ parent catalysts.  After each subsequent overlayer deposition, hydrogen adsorption further decreases and isotherm behavior becomes increasingly dissimilar to both the parent and platinum catalysts.  Deposition of the overlayer results in decreased hydrogen adsorption; with subsequent overlayer depositions resulting in furthered decreases in H₂adsorption.  This indicates that the Pt overlayer increasingly becomes the controlling surface species with each deposition.

Adsorption measurements done at multiple temperatures allowed for the determination of isosteric heat of hydrogen adsorption.  As predicted computationally, the heat of adsorption for hydrogen decreased for the overlayer compared to platinum and the parent metal.  After multiple overlayer depositions, nickel begins to show behavior similar to that of platinum, potentially indicating the formation of isolated platinum particles; whereas cobalt shows further decrease away from platinum heats of adsorption.  The weaker hydrogen adsorption for Ni@Pt when compared to monometallic Pt should aid in increasing APR reaction rates because the hydrogen product will not be as tightly bound to the metal surface; thus freeing up active sites for adsorption of reactants.

Pt/Al₂O₃ is highly active for ethylene hydrogenation and both of the Ni/Al₂O₃ and Co/Al₂O₃baseline catalysts show lower activities.  Addition of a platinum overlayer to make Ni@Pt and Co@Pt overlayer catalysts increased the activity of each catalyst, compared to the host, but still showed reduced activity when compared to a pure platinum catalyst.  These results agree with predicted d-band shifts that would cause weaker hydrogen adsorption on the metal surface and ultimately reduced activity for ethylene hydrogenation when compared to platinum metal alone. 

Through using this ethylene hydrogenation reaction as a characterization tool we have been able to directly correlate reaction activity with hydrogen heat of adsorption.  We have previously shown that as hydrogen heat of adsorption decreases, the TOF for ethylene hydrogenation decreases in the Re@Pd system.  This activity decrease is consistent with literature results that show ethylene hydrogenation activity is directly related to the amount of hydrogen adsorbed on the metal surface.  Thus, the relationship between hydrogen heat of adsorption and ethylene hydrogenation activity has been extended to multiple systems. 

Initial work has focused on optimizing the synthesis procedure and using ethylene hydrogenation studies along with hydrogen and carbon monoxide adsorption studies to identify promising catalysts for APR studies.  Ni@Pt and Co@Pt overlayer catalysts have demonstrated behavior in line with computational predictions and shown qualities desirable for an APR system.  Further APR studies are to be conducted using these promising catalysts.

[1] Latusek, M. P.; Heimerl, R. M.; Spigarelli, B. P.; Holles, J. H., Synthesis and Characterization of Supported Bimetallic Overlayer Catalysts. Applied Catalysis A: General 2009, 358, 79-87.

[2] Latusek, M. P.; Spigarelli, B. P.; Heimerl, R. M.; Holles, J. H., Correlation of H2 Heat of Adsorption and Ethylene Hydrogenation Activity for Supported Re@Pd Overlayer Catalysts. Journal of Catalysis 2009, 263, 306-314.