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Papers by Gary Jacobs

Industrial & Engineering Chemistry Research, 2014

The kinetics and the effect of indigenous and externally added water on methane formation during ... more The kinetics and the effect of indigenous and externally added water on methane formation during Fischer− Tropsch synthesis (FTS) was studied over Co based catalysts using a 1 L continuously stirred tank reactor (CSTR). The water cofeeding study (10% water) was conducted over a 0.27%Ru−25%Co/Al 2 O 3 catalyst at a low CO conversion level of 19% at 220°C in order to lessen the effect of catalyst aging during the addition of water, while the kinetic experiment was conducted over 25%Co/γ-Al 2 O 3 at the conditions of 205−230°C, 1.4−2.5 MPa, H 2 /CO = 1.0−2.5, and 3−16 (NL/g cat )/h (X CO < 60%). Indigenous and externally added water decreases methane formation by a kinetic effect. The addition of 10% water led to a decrease in the CH 4 rate by 12% (3.5 → 3.0 (mmol/g cat )/h), while little catalyst deactivation was observed during water addition. Increases in indigenous water partial pressure also lowered the CH 4 rate and its selectivity. Kinetic analysis was performed using a group of 220°C data collected between 365 and 918 h when the deactivation rate was very low. An empirical CH 4 kinetic model, with a water effect term (P H 2 O /P H 2 ), (r CH 4 = kP CO a P H 2 b /(1 + mP H 2 O /P H 2 )) was used to fit kinetic data. The CH 4 kinetic results suggest a negative water effect on CH 4 formation during FTS on the unpromoted cobalt catalyst, consistent with the water effect results. The final methane kinetics (r CH 4 ) equation obtained at 220°C over 25%Co/γ-Al 2 O 3 is as follows: r CH 4 /[(mol/g cat )/h] = 0.001053{P CO −0.86 P H 2 1.32 /[1 + 0.46(P H 2 O /P H 2 )]}. Meanwhile, a methane selectivity model at 220°C for the 25%Co/Al 2 O 3 catalyst was also developed: S CH 4 = 0.0792P CO −0.55 P H 2 0.44 [(1 − 0.24P H 2 O /P H 2 )/(1 + 0.46P H 2 O /P H 2 )]. The CH 4 selectivity model provided a good prediction of CH 4 selectivities under the experimental conditions used. Furthermore, our empirical CH 4 kinetic results on the cobalt catalyst are consistent with literature kinetic models that were derived from carbide mechanisms; high CH 4 selectivity from the cobalt catalyst is found to be mainly due to a high CH 4 reaction rate constant.

Journal of Catalysis, Jan 1, 2011

The phrases ''product accumulation '' or ''accumulated products'' or ''product holdup'' have appe... more The phrases ''product accumulation '' or ''accumulated products'' or ''product holdup'' have appeared in literature during the past several decades to qualitatively explain the experimental results for Fischer-Tropsch synthesis (FTS). This study develops an experimental method for a slurry reactor to evaluate the product accumulation inside the FT reactor by measuring the average residence time of products as a function of carbon number. The effect of accumulation of products on vapor-liquid equilibrium (VLE) in the reactor is also investigated. The results show that VLE is reached inside the FT reactor for components up to C 17 . Furthermore, the relationship between the mole fractions of components in the vapor and the liquid phase for lighter hydrocarbons, up to around C 17 , is adequately described by Raoult's law. These results suggests that chain length-dependent solubility in the liquid phase is the predominant cause for chain length dependencies of secondary olefin reactions in FTS, and diffusion-limited removal of products is only significant for products with carbon number greater than 17.

Catalysis Letters, 2005

Page 1. Fischer–Tropsch synthesis: Deactivation of promoted and unpromoted cobalt–alumina catalys... more Page 1. Fischer–Tropsch synthesis: Deactivation of promoted and unpromoted cobalt–alumina catalysts Tapan K. Das*, Gary Jacobs, and Burtron H. Davis Center for Applied Energy Research, 2540 Research Park Drive Lexington, KY 40511 ...

Applied Catalysis B Environmental, Feb 1, 2011

Hydrogen production CO2 reforming of ethanol Steam reforming of ethanol Rh/CeO2 catalyst Carbon f... more Hydrogen production CO2 reforming of ethanol Steam reforming of ethanol Rh/CeO2 catalyst Carbon formation a b s t r a c t

Journal of Physical Chemistry C, 2011

Applied Catalysis a General, 2007

A series of Pt promoted ceria-zirconia mixed oxides was prepared, characterized, and tested for t... more A series of Pt promoted ceria-zirconia mixed oxides was prepared, characterized, and tested for the low temperature water-gas shift reaction. An enhancement in the water-gas shift rate was observed by doping zirconium atoms into ceria to form a binary oxide for Pt promoted catalysts. By characterization using TPR and XANES, doping zirconia to ceria decreased the temperature for the surface reduction step. However, the total number of bridging OH group defect sites decreased, as Zr remained to a great extent in the Zr 4+ oxidation state. This was confirmed by CO adsorption, whereby the density of total surface formates was found to decline with increased Zr concentrations. However, the formate forward turnover rate in steam was increased by zirconia addition, and was found to be higher than either Pt/ceria or Pt/zirconia alone. Both the overall rate of the formate decomposition and the water-gas shift rate, as measured by the CO conversion, passed through a maximum with increasing Zr content.

ABSTRACT Temperature-programmed reduction combined with X-ray absorption fine-structure (XAFS) sp... more ABSTRACT Temperature-programmed reduction combined with X-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promote the carburization rate relative to the undoped catalyst. Since XAFS provides information about local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the type of carbides present in catalysts, it may be, as this example shows, a useful complement to verify results from Mössbauer spectroscopy and other temperature programmed methods. The EXAFS results suggest that either the Hagg or -carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the -value of the product distribution and the carburization rate.

J Catal, 2005

Implied in the proposed water-gas shift (WGS) mechanisms for Pt/ceria and Pt/thoria catalysts is ... more Implied in the proposed water-gas shift (WGS) mechanisms for Pt/ceria and Pt/thoria catalysts is the presumption that reduced defect centers are formed on the surface. This X-ray absorption near-edge spectroscopy study provides direct results indicating that Pt facilitates reduction in the surface shell of thoria. Mechanistic arguments from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are provided suggesting that the active sites for WGS, water-assisted formic acid decomposition, and methanol steam reforming are associated with oxygen-deficient centers. In all cases, a high H 2 O/reactant (i.e., carbon monoxide, formic acid, or methanol) ratio was used. For WGS, CO reacted with type II bridging OH groups at reduced centers to generate surface formate intermediates, the decomposition of which is suggested to be the rate-limiting step by the observation of a normal kinetic isotope effect (NKIE) associated with the formate coverage as monitored by DRIFTS under steady-state conditions using CO + H 2 O and CO + D 2 O feeds. The same NKIE was observed in steady-state reaction tests. Formic acid dissociated on the surface of thoria to yield the same surface formate species as observed when CO adsorbs. An identical NKIE associated with formate decomposition was observed when switching from a feed containing HCOOH + H 2 O and DCOOH + H 2 O, establishing two important commonalities: (1) similarity in the mechanistic pathway and (2) importance of the role of type II bridging OH groups at reduced centers in the catalysis. Methanol steam reforming likely proceeded through a mechanism involving adsorption at reduced centers to generate type II methoxy species, with subsequent conversion to formate, unidentate carbonate, and finally CO 2 . The higher NKIE when switching between H-labeled and D-labeled feeds suggests that conversion of methoxy species to formate may be the rate-limiting step. The methanol steam reforming reaction was selective to CO 2 at low conversion, but CO selectivity increased at higher conversions, suggesting competition with the secondary reaction of reverse WGS at higher temperature. Pt/thoria was more selective at higher conversion for CO 2 than a similarly loaded Pt/ceria catalyst. These results suggest that from a mechanistic standpoint, the two materials are virtually analogs of one another.

Applied Catalysis a General, Jun 20, 2009

Co/Al2O3 Fischer–Tropsch synthesis catalysts promoted with different quantities of Group 11 metal... more Co/Al2O3 Fischer–Tropsch synthesis catalysts promoted with different quantities of Group 11 metals (Cu, Ag, Au) were characterized and tested. The presence of relatively small quantities of such metals enhanced Co reducibility and, in the cases of Ag and Au, ...

ABSTRACT Abstract The kinetics of Fischer-Tropsch synthesis (FTS) for 12 and 25% Co/Al2O3 and Co/... more ABSTRACT Abstract The kinetics of Fischer-Tropsch synthesis (FTS) for 12 and 25% Co/Al2O3 and Co/SiO2 catalysts were studied in a 1-L continuously stirred tank reactor (CSTR) in order to define the impact of water on the FTS rate as a function of cobalt crystallite size, as measured by H2 chemisorption/pulse reoxidation experiments. An empirical kinetic model, rCO = k PCOaPH2b/ (1 + m PH2O/PH2), was used in this study. Intrinsic kinetic parameters were identified under a wide range of experimental conditions of 3.3 - 22 Nl-gcat/h, H2/CO ratio of 1.0-2.5, 205-220 oC and a fixed CO pressure of 0.51 MPa and a fixed hydrogen pressure of 0.81 MPa. CO conversion during entire kinetic testing is below 48%. Interestingly, the support type governed whether the impact of H2O on the rate was positive or negative, with less interacting Co/silica providing a positive kinetic effect, while the strongly interacting Co/alumina resulted in a negative impact on the CO conversion rate.

Studies in Surface Science and Catalysis, Dec 31, 2001

Ind Eng Chem Res, 2008

A 2% Ru-promoted 15% Co/Al 2 O 3 catalyst was tested after reduction and after being subjected to... more A 2% Ru-promoted 15% Co/Al 2 O 3 catalyst was tested after reduction and after being subjected to oxidationreduction cycles. The catalysts were characterized over four oxidation-reduction cycles by XANES/EXAFS, TPR, HRTEM, and EDS elemental mapping. The oxidation-reduction treatments were found to assist in sintering the metallic clusters to a larger size, and to promote mixing on at least the order of the nanoscale. The larger crystallites in closer proximity to the Ru promoter led to a more facile reduction of the cobalt crystallites. In addition, a catalyst exposed to two oxidation-reduction cycles resulted in slightly higher conversion, higher R-value product, slightly lower methane selectivity, and greater stability over a reduced freshly calcined catalyst.

Catal. Sci. Technol., 2014

ABSTRACT In this review, five different applications of isotopic tracers to the Fischer–Tropsch s... more ABSTRACT In this review, five different applications of isotopic tracers to the Fischer–Tropsch synthesis reaction on cobalt, iron, and ruthenium catalysts are reviewed. By co-feeding molecules containing a radioactive 14C-label and monitoring the fate of the species in the product distribution, researchers have identified molecules that are candidates for chain initiation or reincorporation into the growing chain. Kinetic isotope effects resulting from H2/D2 switching have provided insights into the rate limiting steps involved in chain initiation and propagation. H2/D2 switching has also been employed in order to account for the holdup of heavier products in the reactor, thus allowing one to correct for the product accumulation effect on the ASF plot so that the true impact of olefin reincorporation on the product distribution can be assessed. The competitive adsorption of H2 and D2 on the catalyst surface has also been explored in order to identify if H–D partitioning effects could influence the probability for one isotope to be favored for reaction during FTS and thus influence KIE results. Finally, steady state isotopic transient kinetic analysis (SSITKA) has been utilized to obtain a number of important kinetically relevant parameters (e.g., number of active sites, rate constant, TOF, etc.), to quantify the pool of reactive intermediates on the catalyst surface, and explore how co-adsorbed molecules may influence the size or reactivity of the reactive pool.

MARTINEZ:NANO.ENERGY E2 O-BK, 2013

A comprehensive kinetic model for Fischer-Tropsch synthesis (FTS) on promoted cobalt-alumina cata... more A comprehensive kinetic model for Fischer-Tropsch synthesis (FTS) on promoted cobalt-alumina catalyst was developed. Rate equations were derived based on the Langmuir-Hinshelwood-Hougen-Watson approach using well known carbide mechanism for FTS. To explain non-Anderson-Schulz-Flory (ASF) behavior, it was assumed that the desorption rate of an olefin precursor is a function of carbon number. Kinetic parameters were estimated by minimizing multiple response objective function using genetic algorithm and Levenberg-Marquardt method, as a global and local optimization tool, respectively. Model evaluation was based on physical meaningfulness of estimated parameters as well as on statistical relevance of the fit. Model yielded physically meaningful values of parameters. It was also able to correctly predict deviations from the ASF distribution and olefin to paraffin ratio dependence on carbon number. This is an abstract of a paper presented at the 2012 AIChE Spring Meeting and 8th Global C...

Group 11 (i.e., Cu, Ag, and Au) promoted 15%Co/alumina catalysts were prepared by an aqueous impr... more Group 11 (i.e., Cu, Ag, and Au) promoted 15%Co/alumina catalysts were prepared by an aqueous impregnation method. Catalysts were characterized by temperature programmed reduction, in-situ EXAFS/XANES spectroscopies, and by adsorption methods (e.g., BET surface area and hydrogen chemisorption/pulse reoxidation). All of the Group 11 promoters clearly facilitated the reduction of cobalt oxide species interacting with the support. However, the promoters were most effective only after an initial reduction step, and should be transferred directly to the reactor and not passivated prior to use. EXAFS/XANES results indicate that the promoting effect is significantly damaged due to structural changes in the catalyst promoter-cobalt interaction at the atomic level if the catalysts are first passivated prior to re-activation and use. The catalysts were tested in a CSTR slurry reactor, and significant improvements in CO conversion rates on a per g catalyst basis were observed with the addition ...

Catalysts, 2014

This focused review article underscores how metal reduction promoters can impact deactivation phe... more This focused review article underscores how metal reduction promoters can impact deactivation phenomena associated with cobalt Fischer-Tropsch synthesis catalysts. Promoters can exacerbate sintering if the additional cobalt metal clusters, formed as a result of the promoting effect, are in close proximity at the nanoscale to other cobalt particles on the surface. Recent efforts have shown that when promoters are used to facilitate the reduction of small crystallites with the aim of increasing surface Co 0 site densities (e.g., in research catalysts), ultra-small crystallites (e.g., <2-4.4 nm) formed are more susceptible to oxidation at high conversion relative to larger ones. The choice of promoter is important, as certain metals (e.g., Au) that promote cobalt oxide reduction can separate from cobalt during oxidation-reduction (regeneration) cycles. Finally, some elements have been identified to promote reduction but either poison the surface of Co 0 (e.g., Cu), or produce excessive light gas selectivity (e.g., Cu and Pd, or Au at high loading). Computational studies indicate that certain promoters may inhibit polymeric C formation by hindering C-C coupling.

Journal of Catalysis, 2015

The effect of ammonia in syngas on the Fischer-Tropsch synthesis (FTS) reaction over 100Fe/5.1Si/... more The effect of ammonia in syngas on the Fischer-Tropsch synthesis (FTS) reaction over 100Fe/5.1Si/2.0Cu/ 3.0K catalyst was studied at 220-270°C and 1.3 MPa using a 1-L slurry phase reactor. The ammonia added in syngas originated from adding ammonia gas, ammonium hydroxide solution, or ammonium nitrate (AN) solution. A wide range of ammonia concentrations (i.e., 0.1-400 ppm) was examined for several hundred hours. The Fe catalysts withdrawn at different times (i.e., after activation by carburization in CO, before and after co-feeding contaminants, and at the end of run) were characterized by ICP-OES, XRD, Mössbauer spectroscopy, and synchrotron methods (e.g., XANES, EXAFS) in order to explore possible changes in the chemical structure and phases of the Fe catalyst with time; in this way, the deactivation mechanism of the Fe catalyst by poisoning could be assessed. Adding up to 200 ppmw (wt. NH 3 /av. Wt. feed) ammonia in syngas did not significantly deactivate the Fe catalyst or alter selectivities toward CH 4 , C 5+ , CO 2 , C 4 -olefin, and 1-C 4 olefin, but increasing the ammonia level (in the AN form) to 400 ppm rapidly deactivated the Fe catalyst and simultaneously changed the product selectivities. The results of ICP-OES, XRD, and Mössbauer spectroscopy did not display any evidence for the retention of a nitrogen-containing compound on the used catalyst that could explain the deactivation (e.g., adsorption, site blocking). Instead, Mössbauer spectroscopy results revealed that a significant fraction of iron carbides transformed into iron magnetite during co-feeding high concentrations of AN, suggesting that oxidation of iron carbides occurred and served as a major deactivation path in that case. Oxidation of v-Fe 5 C 2 to magnetite during co-feeding high concentrations of AN was further confirmed by XRD analysis and by the application of synchrotron methods (e.g., XANES, EXAFS). It is postulated that AN oxidized v-Fe 5 C 2 during FTS via its thermal dissociation product, HNO 3 . This conclusion is further supported by reaction tests with co-feeding of similar concentrations of HNO 3 . Additional oxidation routes of iron carbide to magnetite by HNO 3 and/or by its thermal decomposition products are also considered: Fe 5 C 2 + NO x (and/or HNO 3 ) ? Fe 3 O 4 . In this study, ion chromatography detected that 50-80% HNO 3 directly added or dissociated from AN eventually converted to ammonia during or after its oxidation of iron carbide, resulting from the reduction of NO x (NO x + H 2 + CO ? NH 3 + CO 2 + N 2 + H 2 O) by H 2 and/or CO.

MARTINEZ:NANO.ENERGY E2 O-BK, 2013

Industrial & Engineering Chemistry Research, 2014

The kinetics and the effect of indigenous and externally added water on methane formation during ... more The kinetics and the effect of indigenous and externally added water on methane formation during Fischer− Tropsch synthesis (FTS) was studied over Co based catalysts using a 1 L continuously stirred tank reactor (CSTR). The water cofeeding study (10% water) was conducted over a 0.27%Ru−25%Co/Al 2 O 3 catalyst at a low CO conversion level of 19% at 220°C in order to lessen the effect of catalyst aging during the addition of water, while the kinetic experiment was conducted over 25%Co/γ-Al 2 O 3 at the conditions of 205−230°C, 1.4−2.5 MPa, H 2 /CO = 1.0−2.5, and 3−16 (NL/g cat )/h (X CO < 60%). Indigenous and externally added water decreases methane formation by a kinetic effect. The addition of 10% water led to a decrease in the CH 4 rate by 12% (3.5 → 3.0 (mmol/g cat )/h), while little catalyst deactivation was observed during water addition. Increases in indigenous water partial pressure also lowered the CH 4 rate and its selectivity. Kinetic analysis was performed using a group of 220°C data collected between 365 and 918 h when the deactivation rate was very low. An empirical CH 4 kinetic model, with a water effect term (P H 2 O /P H 2 ), (r CH 4 = kP CO a P H 2 b /(1 + mP H 2 O /P H 2 )) was used to fit kinetic data. The CH 4 kinetic results suggest a negative water effect on CH 4 formation during FTS on the unpromoted cobalt catalyst, consistent with the water effect results. The final methane kinetics (r CH 4 ) equation obtained at 220°C over 25%Co/γ-Al 2 O 3 is as follows: r CH 4 /[(mol/g cat )/h] = 0.001053{P CO −0.86 P H 2 1.32 /[1 + 0.46(P H 2 O /P H 2 )]}. Meanwhile, a methane selectivity model at 220°C for the 25%Co/Al 2 O 3 catalyst was also developed: S CH 4 = 0.0792P CO −0.55 P H 2 0.44 [(1 − 0.24P H 2 O /P H 2 )/(1 + 0.46P H 2 O /P H 2 )]. The CH 4 selectivity model provided a good prediction of CH 4 selectivities under the experimental conditions used. Furthermore, our empirical CH 4 kinetic results on the cobalt catalyst are consistent with literature kinetic models that were derived from carbide mechanisms; high CH 4 selectivity from the cobalt catalyst is found to be mainly due to a high CH 4 reaction rate constant.

Journal of Catalysis, Jan 1, 2011

The phrases ''product accumulation '' or ''accumulated products'' or ''product holdup'' have appe... more The phrases ''product accumulation '' or ''accumulated products'' or ''product holdup'' have appeared in literature during the past several decades to qualitatively explain the experimental results for Fischer-Tropsch synthesis (FTS). This study develops an experimental method for a slurry reactor to evaluate the product accumulation inside the FT reactor by measuring the average residence time of products as a function of carbon number. The effect of accumulation of products on vapor-liquid equilibrium (VLE) in the reactor is also investigated. The results show that VLE is reached inside the FT reactor for components up to C 17 . Furthermore, the relationship between the mole fractions of components in the vapor and the liquid phase for lighter hydrocarbons, up to around C 17 , is adequately described by Raoult's law. These results suggests that chain length-dependent solubility in the liquid phase is the predominant cause for chain length dependencies of secondary olefin reactions in FTS, and diffusion-limited removal of products is only significant for products with carbon number greater than 17.

Catalysis Letters, 2005

Page 1. Fischer–Tropsch synthesis: Deactivation of promoted and unpromoted cobalt–alumina catalys... more Page 1. Fischer–Tropsch synthesis: Deactivation of promoted and unpromoted cobalt–alumina catalysts Tapan K. Das*, Gary Jacobs, and Burtron H. Davis Center for Applied Energy Research, 2540 Research Park Drive Lexington, KY 40511 ...

Applied Catalysis B Environmental, Feb 1, 2011

Hydrogen production CO2 reforming of ethanol Steam reforming of ethanol Rh/CeO2 catalyst Carbon f... more Hydrogen production CO2 reforming of ethanol Steam reforming of ethanol Rh/CeO2 catalyst Carbon formation a b s t r a c t

Journal of Physical Chemistry C, 2011

Applied Catalysis a General, 2007

A series of Pt promoted ceria-zirconia mixed oxides was prepared, characterized, and tested for t... more A series of Pt promoted ceria-zirconia mixed oxides was prepared, characterized, and tested for the low temperature water-gas shift reaction. An enhancement in the water-gas shift rate was observed by doping zirconium atoms into ceria to form a binary oxide for Pt promoted catalysts. By characterization using TPR and XANES, doping zirconia to ceria decreased the temperature for the surface reduction step. However, the total number of bridging OH group defect sites decreased, as Zr remained to a great extent in the Zr 4+ oxidation state. This was confirmed by CO adsorption, whereby the density of total surface formates was found to decline with increased Zr concentrations. However, the formate forward turnover rate in steam was increased by zirconia addition, and was found to be higher than either Pt/ceria or Pt/zirconia alone. Both the overall rate of the formate decomposition and the water-gas shift rate, as measured by the CO conversion, passed through a maximum with increasing Zr content.

ABSTRACT Temperature-programmed reduction combined with X-ray absorption fine-structure (XAFS) sp... more ABSTRACT Temperature-programmed reduction combined with X-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promote the carburization rate relative to the undoped catalyst. Since XAFS provides information about local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the type of carbides present in catalysts, it may be, as this example shows, a useful complement to verify results from Mössbauer spectroscopy and other temperature programmed methods. The EXAFS results suggest that either the Hagg or -carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the -value of the product distribution and the carburization rate.

J Catal, 2005

Implied in the proposed water-gas shift (WGS) mechanisms for Pt/ceria and Pt/thoria catalysts is ... more Implied in the proposed water-gas shift (WGS) mechanisms for Pt/ceria and Pt/thoria catalysts is the presumption that reduced defect centers are formed on the surface. This X-ray absorption near-edge spectroscopy study provides direct results indicating that Pt facilitates reduction in the surface shell of thoria. Mechanistic arguments from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are provided suggesting that the active sites for WGS, water-assisted formic acid decomposition, and methanol steam reforming are associated with oxygen-deficient centers. In all cases, a high H 2 O/reactant (i.e., carbon monoxide, formic acid, or methanol) ratio was used. For WGS, CO reacted with type II bridging OH groups at reduced centers to generate surface formate intermediates, the decomposition of which is suggested to be the rate-limiting step by the observation of a normal kinetic isotope effect (NKIE) associated with the formate coverage as monitored by DRIFTS under steady-state conditions using CO + H 2 O and CO + D 2 O feeds. The same NKIE was observed in steady-state reaction tests. Formic acid dissociated on the surface of thoria to yield the same surface formate species as observed when CO adsorbs. An identical NKIE associated with formate decomposition was observed when switching from a feed containing HCOOH + H 2 O and DCOOH + H 2 O, establishing two important commonalities: (1) similarity in the mechanistic pathway and (2) importance of the role of type II bridging OH groups at reduced centers in the catalysis. Methanol steam reforming likely proceeded through a mechanism involving adsorption at reduced centers to generate type II methoxy species, with subsequent conversion to formate, unidentate carbonate, and finally CO 2 . The higher NKIE when switching between H-labeled and D-labeled feeds suggests that conversion of methoxy species to formate may be the rate-limiting step. The methanol steam reforming reaction was selective to CO 2 at low conversion, but CO selectivity increased at higher conversions, suggesting competition with the secondary reaction of reverse WGS at higher temperature. Pt/thoria was more selective at higher conversion for CO 2 than a similarly loaded Pt/ceria catalyst. These results suggest that from a mechanistic standpoint, the two materials are virtually analogs of one another.

Applied Catalysis a General, Jun 20, 2009

Co/Al2O3 Fischer–Tropsch synthesis catalysts promoted with different quantities of Group 11 metal... more Co/Al2O3 Fischer–Tropsch synthesis catalysts promoted with different quantities of Group 11 metals (Cu, Ag, Au) were characterized and tested. The presence of relatively small quantities of such metals enhanced Co reducibility and, in the cases of Ag and Au, ...

ABSTRACT Abstract The kinetics of Fischer-Tropsch synthesis (FTS) for 12 and 25% Co/Al2O3 and Co/... more ABSTRACT Abstract The kinetics of Fischer-Tropsch synthesis (FTS) for 12 and 25% Co/Al2O3 and Co/SiO2 catalysts were studied in a 1-L continuously stirred tank reactor (CSTR) in order to define the impact of water on the FTS rate as a function of cobalt crystallite size, as measured by H2 chemisorption/pulse reoxidation experiments. An empirical kinetic model, rCO = k PCOaPH2b/ (1 + m PH2O/PH2), was used in this study. Intrinsic kinetic parameters were identified under a wide range of experimental conditions of 3.3 - 22 Nl-gcat/h, H2/CO ratio of 1.0-2.5, 205-220 oC and a fixed CO pressure of 0.51 MPa and a fixed hydrogen pressure of 0.81 MPa. CO conversion during entire kinetic testing is below 48%. Interestingly, the support type governed whether the impact of H2O on the rate was positive or negative, with less interacting Co/silica providing a positive kinetic effect, while the strongly interacting Co/alumina resulted in a negative impact on the CO conversion rate.

Studies in Surface Science and Catalysis, Dec 31, 2001

Ind Eng Chem Res, 2008

A 2% Ru-promoted 15% Co/Al 2 O 3 catalyst was tested after reduction and after being subjected to... more A 2% Ru-promoted 15% Co/Al 2 O 3 catalyst was tested after reduction and after being subjected to oxidationreduction cycles. The catalysts were characterized over four oxidation-reduction cycles by XANES/EXAFS, TPR, HRTEM, and EDS elemental mapping. The oxidation-reduction treatments were found to assist in sintering the metallic clusters to a larger size, and to promote mixing on at least the order of the nanoscale. The larger crystallites in closer proximity to the Ru promoter led to a more facile reduction of the cobalt crystallites. In addition, a catalyst exposed to two oxidation-reduction cycles resulted in slightly higher conversion, higher R-value product, slightly lower methane selectivity, and greater stability over a reduced freshly calcined catalyst.

Catal. Sci. Technol., 2014

ABSTRACT In this review, five different applications of isotopic tracers to the Fischer–Tropsch s... more ABSTRACT In this review, five different applications of isotopic tracers to the Fischer–Tropsch synthesis reaction on cobalt, iron, and ruthenium catalysts are reviewed. By co-feeding molecules containing a radioactive 14C-label and monitoring the fate of the species in the product distribution, researchers have identified molecules that are candidates for chain initiation or reincorporation into the growing chain. Kinetic isotope effects resulting from H2/D2 switching have provided insights into the rate limiting steps involved in chain initiation and propagation. H2/D2 switching has also been employed in order to account for the holdup of heavier products in the reactor, thus allowing one to correct for the product accumulation effect on the ASF plot so that the true impact of olefin reincorporation on the product distribution can be assessed. The competitive adsorption of H2 and D2 on the catalyst surface has also been explored in order to identify if H–D partitioning effects could influence the probability for one isotope to be favored for reaction during FTS and thus influence KIE results. Finally, steady state isotopic transient kinetic analysis (SSITKA) has been utilized to obtain a number of important kinetically relevant parameters (e.g., number of active sites, rate constant, TOF, etc.), to quantify the pool of reactive intermediates on the catalyst surface, and explore how co-adsorbed molecules may influence the size or reactivity of the reactive pool.

MARTINEZ:NANO.ENERGY E2 O-BK, 2013

A comprehensive kinetic model for Fischer-Tropsch synthesis (FTS) on promoted cobalt-alumina cata... more A comprehensive kinetic model for Fischer-Tropsch synthesis (FTS) on promoted cobalt-alumina catalyst was developed. Rate equations were derived based on the Langmuir-Hinshelwood-Hougen-Watson approach using well known carbide mechanism for FTS. To explain non-Anderson-Schulz-Flory (ASF) behavior, it was assumed that the desorption rate of an olefin precursor is a function of carbon number. Kinetic parameters were estimated by minimizing multiple response objective function using genetic algorithm and Levenberg-Marquardt method, as a global and local optimization tool, respectively. Model evaluation was based on physical meaningfulness of estimated parameters as well as on statistical relevance of the fit. Model yielded physically meaningful values of parameters. It was also able to correctly predict deviations from the ASF distribution and olefin to paraffin ratio dependence on carbon number. This is an abstract of a paper presented at the 2012 AIChE Spring Meeting and 8th Global C...

Group 11 (i.e., Cu, Ag, and Au) promoted 15%Co/alumina catalysts were prepared by an aqueous impr... more Group 11 (i.e., Cu, Ag, and Au) promoted 15%Co/alumina catalysts were prepared by an aqueous impregnation method. Catalysts were characterized by temperature programmed reduction, in-situ EXAFS/XANES spectroscopies, and by adsorption methods (e.g., BET surface area and hydrogen chemisorption/pulse reoxidation). All of the Group 11 promoters clearly facilitated the reduction of cobalt oxide species interacting with the support. However, the promoters were most effective only after an initial reduction step, and should be transferred directly to the reactor and not passivated prior to use. EXAFS/XANES results indicate that the promoting effect is significantly damaged due to structural changes in the catalyst promoter-cobalt interaction at the atomic level if the catalysts are first passivated prior to re-activation and use. The catalysts were tested in a CSTR slurry reactor, and significant improvements in CO conversion rates on a per g catalyst basis were observed with the addition ...

Catalysts, 2014

This focused review article underscores how metal reduction promoters can impact deactivation phe... more This focused review article underscores how metal reduction promoters can impact deactivation phenomena associated with cobalt Fischer-Tropsch synthesis catalysts. Promoters can exacerbate sintering if the additional cobalt metal clusters, formed as a result of the promoting effect, are in close proximity at the nanoscale to other cobalt particles on the surface. Recent efforts have shown that when promoters are used to facilitate the reduction of small crystallites with the aim of increasing surface Co 0 site densities (e.g., in research catalysts), ultra-small crystallites (e.g., <2-4.4 nm) formed are more susceptible to oxidation at high conversion relative to larger ones. The choice of promoter is important, as certain metals (e.g., Au) that promote cobalt oxide reduction can separate from cobalt during oxidation-reduction (regeneration) cycles. Finally, some elements have been identified to promote reduction but either poison the surface of Co 0 (e.g., Cu), or produce excessive light gas selectivity (e.g., Cu and Pd, or Au at high loading). Computational studies indicate that certain promoters may inhibit polymeric C formation by hindering C-C coupling.

Journal of Catalysis, 2015

The effect of ammonia in syngas on the Fischer-Tropsch synthesis (FTS) reaction over 100Fe/5.1Si/... more The effect of ammonia in syngas on the Fischer-Tropsch synthesis (FTS) reaction over 100Fe/5.1Si/2.0Cu/ 3.0K catalyst was studied at 220-270°C and 1.3 MPa using a 1-L slurry phase reactor. The ammonia added in syngas originated from adding ammonia gas, ammonium hydroxide solution, or ammonium nitrate (AN) solution. A wide range of ammonia concentrations (i.e., 0.1-400 ppm) was examined for several hundred hours. The Fe catalysts withdrawn at different times (i.e., after activation by carburization in CO, before and after co-feeding contaminants, and at the end of run) were characterized by ICP-OES, XRD, Mössbauer spectroscopy, and synchrotron methods (e.g., XANES, EXAFS) in order to explore possible changes in the chemical structure and phases of the Fe catalyst with time; in this way, the deactivation mechanism of the Fe catalyst by poisoning could be assessed. Adding up to 200 ppmw (wt. NH 3 /av. Wt. feed) ammonia in syngas did not significantly deactivate the Fe catalyst or alter selectivities toward CH 4 , C 5+ , CO 2 , C 4 -olefin, and 1-C 4 olefin, but increasing the ammonia level (in the AN form) to 400 ppm rapidly deactivated the Fe catalyst and simultaneously changed the product selectivities. The results of ICP-OES, XRD, and Mössbauer spectroscopy did not display any evidence for the retention of a nitrogen-containing compound on the used catalyst that could explain the deactivation (e.g., adsorption, site blocking). Instead, Mössbauer spectroscopy results revealed that a significant fraction of iron carbides transformed into iron magnetite during co-feeding high concentrations of AN, suggesting that oxidation of iron carbides occurred and served as a major deactivation path in that case. Oxidation of v-Fe 5 C 2 to magnetite during co-feeding high concentrations of AN was further confirmed by XRD analysis and by the application of synchrotron methods (e.g., XANES, EXAFS). It is postulated that AN oxidized v-Fe 5 C 2 during FTS via its thermal dissociation product, HNO 3 . This conclusion is further supported by reaction tests with co-feeding of similar concentrations of HNO 3 . Additional oxidation routes of iron carbide to magnetite by HNO 3 and/or by its thermal decomposition products are also considered: Fe 5 C 2 + NO x (and/or HNO 3 ) ? Fe 3 O 4 . In this study, ion chromatography detected that 50-80% HNO 3 directly added or dissociated from AN eventually converted to ammonia during or after its oxidation of iron carbide, resulting from the reduction of NO x (NO x + H 2 + CO ? NH 3 + CO 2 + N 2 + H 2 O) by H 2 and/or CO.

MARTINEZ:NANO.ENERGY E2 O-BK, 2013