Highly dispersed Pt-based catalysts were prepared by Strong Electrostatic Adsorption (SEA) for the PROX reaction. The effect of the catalyst synthesis pH was investigated to control the metal-support interactions for the system: Pt supported onto cobalt oxide, previously supported on TiO2. The Pt/CoOx/TiO2 catalysts were prepared in a pH range of 3.0-9.0, obtaining Pt dispersion values above 90% with Pt crystallite sizes of 1-2 nm. The effect of the synthesis pH was correlated with the concentration of (Pt-CoOx)i interfacial sites, and this, in turn, with the catalytic activity. The concentration of (Pt-CoOx)i interfacial sites was higher for the most active catalyst, which was prepared at a condition that ensures a higher selective-SEA of Pt over the co-support (pH = 6.0). This study shows that the synthesis pH can determine the concentration and type of active sites and paves the way to use SEA to control the activity of dual-supported catalysts.& COPY; 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

varying the pH of synthesis and the calcination temperature. By measuring the point of zero charge (PZC) of the support components, it was possible to determine the pH in which Pt can be selectively deposited onto one of the support components, obtaining Pt dispersion values above 90%. The selective SEA of a Pt precursor onto the co-support (FeOx) was achieved at a synthesis pH between the PZCs of the support components (i.e., TiO2 PZC = 5.2 and Fe2O3 PZC = 6.9) by using a Pt anionic complex. The catalytic activity for the PROX reaction, expressed in terms of the CO conversion, O2 selectivity to CO2, apparent activation energy, and turnover frequency, confirmed that the SEA prepared catalysts were active and selective for the PROX reaction. XPS and TPR results of the Pt/FeOx/TiO2 catalysts showed the formation of Pt-FeOx interfaces, called as (Pt-FeOx)i interfacial sites, which enhanced the stability and catalytic activity for the PROX reaction. The concentration of these sites can be controlled by the synthesis conditions used, mainly pH and to a lower (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The objective of this study was to determine the combustion mechanism of cellulose paper impregnated with Mg(NO3)(2), Ni(NO3)(2), glycine solutions, and their different combinations. It was established that the combustion mechanism changes as a function of the impregnated media composition. In the Mg(NO3)(2)-cellulose system, Mg2+ ions strongly catalyze cellulose pyrolysis; hence no atmospheric oxygen is needed for the self-sustained combustion reaction. In the Ni(NO3)(2)-cellulose system, Ni2+ ions catalyze pyrolysis at a lower rate, and atmospheric oxygen assists the combustion wave propagation. In glycine containing systems, glycine blocks the ability of metal cations to catalyze the early cellulose pyrolysis, and the combustion front propagates due to the exothermic reactions between metal nitrates and glycine. The above mechanisms influence the microstructure of the combustion products. Due to the near-complete degradation of cellulose fibers during the Mg2+ catalyzed pyrolytic combustion, the resulting materials had a highly porous, sponge-like microstructure with a BET surface area of up to 152 m(2)/g. After the reduction in hydrogen, Ni segregates to the surface of NiO-MgO solid solution, and the resulting catalysts exhibited near-equilibrium methane conversion during the dry reforming of methane reaction at 600 degrees C with low carbon formation and no deactivation for 24 h of time on stream. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Ni supported on solid solution (NiOMgO) catalysts with different Ni concentration (10, 20 and 30 wt.%) were prepared by a novel paper assisted combustion synthesis (PACS) method, followed by a reduction stage. All as-synthesized materials formed NiOMgO solid solutions, which under optimum PACS conditions exhibited up to about 140 m(2)/g BET surface area, which is one of the highest reported so far for this type of materials. Solid solutions were not active unless reduced at higher temperatures, causing a fraction of the Ni to segregate to the surface to become the active sites. The activity during the dry reforming of methane was studied as a function of temperature, and time on stream (TOS). The PACS solid solution with 10 wt.% of Ni had the highest surface area and, upon reduction, it was the most active and stable catalyst exhibiting low carbon formation at 600 degrees C, and no carbon deposition at 700 degrees C during 24 h TOS. The activity results correlated well with the higher surface area of the starting solid solutions, the smaller Ni crystallite sizes, and the number of Ni2+ and Ni3+ sites on the surface.

We report on the high activity and stability towards deactivation during ethanol decomposition of mono (Ni, Cu) and bimetallic Ni-Cu catalysts supported on ceria and silica that were prepared by impregnated support solution combustion synthesis (ISCS). Ni supported on ceria, behaves much like bulk Ni, as it deactivates within a few hours of time on stream (TOS). Addition of Cu improves the stability of ceria supported Ni-Cu bimetallic catalysts but it still deactivates during 10 h of TOS. Ni prepared by dry impregnation on silica deactivates completely during a few h of TOS. Ni supported on silica prepared by ISCS, show a remarkable high stability for over 30 h TOS and Ni-Cu is stable over 100 h of TOS operating at 100% conversion. XRD, XPS, HRTEM, in-situ XAS and textural characterization of these catalysts show that the surface Ni concentration is small on silica-supported catalysts due to partial encapsulation of Ni by the silica support, which protects the metal particles from carbon formation, thus hindering deactivation. Addition of Cu to silica increases the number of active sites, but also decreases CO chemisorption on Ni indicating the formation of a Cu-Ni alloy, which had been reported to inhibit carbon formation on Ni. The combined effect of encapsulation and formation of Ni-Cu surface alloys result in the remarkable stability of the Cu-Ni silica supported catalyst.

In this work, the catalytic properties of Pt/MOx/TiO2 (M = Fe, Co) catalysts prepared by strong electrostatic adsorption (SEA), were investigated for the preferential oxidation (PROX) of CO in H-2. The SEA method was chosen to achieve a preferential adsorption of Pt onto supported MOx nanoparticles rather than onto the TiO2 surface. XPS, TPR and TEM analysis indicate that the enhancement in the catalytic activity of Pt/MOx/TiO2 is related to a synergetic effect originated by a close contact between Pt and MOx (M = Fe, Co) surface. The results clearly show that Pt supported on CoOx/TiO2 calcined at 300 degrees C, and then reduced at 100 degrees C, exhibits the highest activity, e.g. higher CO conversion and CO2 selectivity, among the supported catalysts studied in this work, that was attributed to a higher concentration of Pt-CoOx moieties over the catalytic surface. This study reports a simple and efficient way to synthesize Pt-promoted catalysts where strong metal-support interactions are highly desired to improve the catalytic performance of selective oxidation reactions.

In this chapter, we summarize work accomplished primarily by the authors on the use of solution combustion synthesis (SCS) in catalysis. Research in combustion synthesis at University of Notre Dame started with the group of Prof. A. Varma, now at Purdue University, in collaboration with Prof. A. Mukasyan and its application to catalysis was pursued jointly with Prof. E. Wolf. Prof. A. Kumar worked on the subject during his graduate studies at Notre Dame and now he is continuing work on the application of combustion synthesis to catalysis at Qatar University. After an introduction to combustion synthesis, we describe reaction pathways involved in the preparation of unsupported and supported catalysts using SCS. The catalytic applications focus on preparation and performance of active and stable catalysts for the hydrogen generation from methanol and ethanol, followed by application to electrocatalysis for fuels cell utilization.

In this work, we report a liquid-phase reduction method to prepare porous non-supported amorphous nickel catalysts with high surface areas (65-250 m(2)/g). A highly crystalline face center cubic Ni (fcc-Ni) catalyst with 110 m(2)/g surface area was also prepared by frontal crystallization of the amorphous nickel catalyst. The catalytic activity and stability of these catalysts for ethanol decomposition was investigated at different time on stream (TOS) to understand structural transformations occurring at the early stages of catalyst activation deactivation. Activity vs. TOS results obtained at 473 K show that on the amorphous catalysts the conversion increases from about 50% to 60-75% reaching a steady value at similar to 30 h TOS, which remains constant during the observed 96 h of TOS. The fcc-Ni catalyst initially exhibits a higher conversion (-85%), however, it quickly deactivates to a conversion in the similar range as the amorphous catalysts. It is also shown that BET surface areas of amorphous catalysts decreases during hydrogen pretreatment at 473 K due to crystallization, grain growth, and sintering. The structure of amorphous catalysts continuously refines to form a combination of fcc-Ni and hexagonal close-packed nickel (hpc-Ni) phases, as well as nickel carbide (Ni3C) and carbon layers that stabilize catalytic activity. The structure of the fcc-Ni catalyst remains unchanged during the 96 h TOS experiment indicating that carbon deposition might cause its initial deactivation. At 523 K, the amorphous catalyst shows 100% conversion, which remains constant during 96 h of TOS, while the fcc-Ni crystalline catalyst initially exhibits 95% conversion and then slowly deactivates to similar to 80% at 96 h TOS. Thus at 523K the stabilized amorphous catalyst does not deactivate under the same TOS compared to the crystalline fcc-Ni catalyst, showing that the active sites on these catalysts are different. The findings of this work suggest that the liquid-phase reduction method can be used to prepare active and stable catalysts for reactions involving decomposition of alcohols and hydrocarbons to produce hydrogen. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

A novel method of catalyst preparation, using pressure dilution, which increased the dispersion and stability of a Ni/fumed SiO(2 )catalyst is described. We first studied the effect of carbon formation on the activity and stability of Ni/fumed SiO2 catalysts at different times on stream (TOS) during the dry reforming of methane (DRM). The catalysts were characterized by SEM/TEM and BET, XRD, XPS, TGA/DSC, and CO chemisorption. Catalysts prepared by impregnation of Ni(NO3)(2)center dot 6H(2)O onto fumed SiO2, were subjected to various pretreatments which yielded high initial activities at 600 degrees C. The activation energy was measured to be 91 kJ/mol and the initial rate was of 8.5 [mol of CH4 /g(Ni )h] comparable with Pt-Ni alloy catalysts. Depending on the pretreatment, however, the catalysts deactivated at different rates due to carbon formation. The carbon structure was studied by SEM/ TEM and its amount measured by a carbon analyzer. We found that carbon accumulates mostly in the form of carbon nanotubes (C-NT) with Ni crystallites at the top, where the reaction takes place without affecting the activity during first hours of TOS. We also found formation of clumps of entangled C-NT with Ni crystallites encapsulated by the entangled C-NT, leading to loss of active area and deactivation. A newly discovered method of catalyst preparation, referred as pressure dilution, resulted in a significant increase in Ni dispersion from 19% to 61%, when supported on fumed SiO2. Moreover, Ni dispersion can be controlled by the pressure applied during catalysts' preparation. The higher dispersion resulted in a higher catalyst activity and increased stability towards carbon formation. A hypothesis of the re dispersion process occurring during p-dilution is proposed.

Here, we report a simple and scalable synthesis strategy of metal (Ni or Cu) nanoparticles uniformly distributed inside a mesoporous silica matrix. This method involves incorporation of metal nitrates and citric acid in a stable silica gel through controlled hydrolysis of tetraethyl orthosilicate. Combustion of dried gels with ammonium nitrate in an inert gas atmosphere enables preparation of highly porous Ni/SiO2 and Cu/SiO2 nanomaterials with tunable metal content (-5-30 wt.%). This approach also allows for independent tuning of the metal nanoparticle size (from 2 to 50 nm) and textural parameters, such as surface area (50-600 m(2)/g), average pore size (3-8 nm), and pore volume (0.05-0.6 cm(3)/g) of the materials during the one-step combustion. This new approach also enables uniform incorporation of metal nanoparticles within a porous silica matrix. This feature allows for synthesis of encapsulated stable ultra-small metallic nanoparticles with unusual properties. We tested Ni/SiO2 nanoscale materials with different textural parameters as catalysts in the ethanol decomposition reaction. The catalysts exhibited high activity toward hydrogen generation for 100 h. The relevant links between the textural parameters and stability of the catalysts are revealed. Characterization of the spent catalysts showed no structural changes, indicating superior stability over long periods of time. We also used spark plasma sintering (SPS) of Ni/SiO2 and Cu/SiO2 nanoscale materials to fabricate porous (70-80%) compact samples. These materials exhibited significantly low thermal diffusivity, which makes them attractive for thermal management applications. We also showed that the simple preparation method allows for production of large batches of final product, such as 10-50 g, in laboratory conditions. (C) 2017 Elsevier Inc. All rights reserved.

The temperature effect and basic structural features of a potassium catalyst supported on hydrothermal TiO2 were studied. The calcination of a 20% K/TiHT catalyst at temperatures higher than 600 degrees C, led to a change of the crystalline phase of potassium oxide into a potassium titanate. Despite the low surface area of the catalyst calcined at high temperature total conversion to methyl esters was achieved at 2 h of reaction. These results are explained by the presence of highly strong basic sites associated with the potassium titanate structure formed on these catalysts. (C) 2015 Elsevier Ltd. All rights reserved.

The preparation, characterization, activity, and stability of a Ni-Al2O3 catalyst derived from reduction of a Ni-Al layered double hydroxide precursor (LDH, Ni6Al2(OH)(16)(CO3)(0.75)(OH)(0.25)center dot 4H(2)O) are reported in this paper. In-situ X-ray adsorption spectroscopy shows that reduction of Ni from the LDH precursor to form a highly loaded 80% Ni-Al2O3 catalyst (Ni-Al2O3-LDH) is faster than reduction of a 10% impregnated Ni-Al2O3 alumina (Ni-Al2O3-I) catalyst. The reduced Ni-Al2O3-LDH catalyst exhibits highly dispersed Ni nanoparticles (3-5 nm) distributed on top, partially embedded nanoparticles, and some encapsulated in the Al2O3 matrix. The nanoparticles impregnated on alumina (Ni-Al2O3-I) are larger (similar to 7-15 nm) and appear on top of the alumina support. Conversion vs time on stream (TOS) results during ethanol decomposition at 250 degrees C on Ni-Al2O3-LDH exhibits only a slight deactivation during 100 h TOS, while the Ni-Al2O3-I catalyst shows rapid deactivation with no conversion after 2h TOS. X-ray photoelectron spectroscopy shows that the carbon content increases up to 48% after 100 h TOS on the Ni-Al2O3-LDH catalyst, while a similar increase occurs after 2 h TOS on the Ni-Al2O3-I catalyst. TEM shows that after 100 h TOS either a thin layer of amorphous carbon or carbon nanotubes forms on Ni on top of the alumina matrix and on partially embedded Ni nanoparticles on the Ni-Al2O3-LDH catalyst. Total surface area of the Ni-Al2O3-LDH catalyst increased during TOS, which may be suplying fresh surface Ni from the encapsulated Ni nanoparticles that sustain the high activity. (C) 2015 Elsevier B.V. All rights reserved.

Cu and Ni based catalysts were synthesized using solution combustion synthesis method. The catalytic activity and hydrogen selectivity were investigated for ethanol decomposition reaction. The amount of fuel content in the combustion solution was found to greatly affect the phase and the microstructure of the synthesized catalyst. In situ X-ray absorption spectroscopy (XAS) studies were carried out to study the reduction of the catalyst containing mixed oxides of copper and nickel. The reduced catalyst was further subjected to an oxidizing environment to collect the in situ XAS data during the oxidation of the catalyst. These investigations show that the catalyst oxidation state changes rapidly in the first few minutes of the pretreatment process and then gradually slows downs.

Multi layer enhanced infrared reflection spectroscopy (MEIRAS) experiments were performed on parallel Pt nanowires nanofabricated on TiO2 and SiO2 substrates, while being exposed to 1% CO at atmospheric pressure and 75 degrees C. We studied the effect of nanowire widths and IR polarization and incidence angle on peak wavenumbers and intensities of absorption bands of CO molecules adsorbed on Pt surfaces. The samples consist of 4 mm X 4 mm arrays of rough polycrystalline parallel Pt nanowires placed 200 nm apart, 15-90 nm wide, and approximately 8 nm high/thick. The nanowires have somewhat straight side-faces because of the directional nature of the Pt deposition process used. We also studied 5 nm thick polycrystalline Pt film samples. Spectra from unpolarized IR at small incidence angles (20 from surface normal) showed a decrease in linearly adsorbed CO peak wavenumbers with decreasing nanowire width, from about 2090 cm(-1) on Pt film to less than 2055 cm(-1) on thin nanowires, indicating differences in Pt-CO interactions of the CO populations being sensed on different samples. MEIRAS results at small incidence angles allows selective sampling of vibrational modes tangential to the multilayer structure, which are more abundant on the side faces of the nanowires than on their top surfaces. This expectation is confirmed by polarization dependent experiments, which show high intensity when IR polarization is perpendicular to the sides of nanowires, thus aligned with CO adsorbed on the sides, and little intensity when polarization is parallel to the sides of the wires. Lower wavenumber on thinner wires is consistent with increased fraction (in the sampled population) of side CO molecules that are closer to the metal-support interface and thus potentially affected by metal-support charge transfer effects well-known for Pt/TiO2 interfaces; these and other possible contributing effects such as the presence of impurities and low-coordination sites are discussed. Sensitivity to top CO relative to CO adsorbed on the edges and sides of nanowires is improved by increasing the incidence angle with p-polarization, and these top-CO vibrations appear at slightly higher wavenumbers that the side-CO on the same sample. These results demonstrate that polarized MEIRAS can be used to detect effects of adsorbate molecular orientation on metal catalysts on flat supports and selectively sense molecules with tangential dipole projections as opposed to surface normal only sensitivity in IRAS on metal surfaces or metal particles on ultrathin oxides.

A simple, surfactant-free liquid-phase reduction of Cu, Ni and Fe salts (e.g. nitrates, chlorides) was used to prepare nanostructured FeNi/Cu catalysts for hydrous hydrazine (N-2 H-4 center dot H2O) decomposition. The synthesis of nanomaterials includes reduction of copper salt using N2H4, followed by rapid reduction of iron and nickel salts by NaBH4. The catalysts were characterized by XRD, BET, TEM, XPS, XANES/EXAFS techniques and their activity and selectivity was studied during hydrous hydrazine decomposition at temperatures ranging from 300 to 345 K. The selectivity to hydrogen increases to similar to 100% with increasing temperature up to similar to 345 K. The catalytic performance of these materials depends on the structure of NiFe layer formed over a Cu core, which may be controlled by changing of NiFe/Cu mass ratio. Investigation of the catalytic performance for bi- and tri-metallic materials show that main active metal is nickel but a NiFe alloy could be responsible for the increased selectivity. Alloying of nickel with iron coupled with a favorable dispersion on copper nanoparticles remarkably enhances the catalytic conversion and selectivity of hydrogen evolution. (C) 2014 Elsevier B.V. All rights reserved.

The catalytic generation by ethanol decomposition and partial oxidation over copper-chromite and copper-zinc catalyst supported on alumina has been investigated. The catalysts have been prepared by the method of combustion synthesis, characterized by a fast heating rate and a short reaction time, leading to increase catalyst porosity and total surface area. The catalytic activity and selectivity have been investigated without O2 and under various O2 and C2H5OH molar ratio in the temperature range up to 500°C. It was found that copper chromite supported on alumina shows the best activity and hydrogen selectivity during ethanol decomposition. The selectivity decreased during partial oxidation but with a low O2/EtOH=0.6 molar ratio at 300°C, a hydrogen rich mixture (35–40%) was obtained. The use of relatively low amount of oxygen is necessary to reduce coke formation, which causes catalyst deactivation. The catalysts were characterized by ex situ methods such as XRD, BET, XPS, and in situ EXAFS and FTIR with the aim to evaluate their physico-chemical properties and to correlate them with the catalysts performance.

Multicomponent catalysts containing Ni, Fe, Cu active for ethanol reforming reactions, prepared by solution combustion synthesis are characterized by multiple techniques such as ex situ XRD, XPS, and in situ XAFS and FTIR. XRD results indicate copper to be present in the reduced state as Cu-Ni bimetal while nickel and iron are observed to be partially in a spinel NiFe2O4 structure. In situ XANES and XAFS analysis show a change in Ni, Fe and Cu oxidation states during reaction. Cu, which was fully reduced before reaction, became partly oxidized upon exposure to ethanol and oxygen. Ni is mostly (75%) reduced and does not seem to change its oxidation state during the reaction. Fe is not present in metallic form after reduction and during the reaction, but some change in the oxidation state from Fe(II) to Fe(III) occurred during the reaction. XPS and SEM images indicate the formation of carbon filament on the spent catalyst. XPS results also indicate the enrichment of surface by Fe and Cu during the reduction of the catalyst. Based on the activity and characterization results obtained, and literature review, the role of predominant phases during ethanol decomposition reaction is proposed. (C) 2013 Elsevier B.V. All rights reserved.

The mechanism of structural transformation during combustion of nickel nitrate (oxidizer)-glycine (fuel) system is investigated by using different in situ techniques, including time-resolved X-ray diffraction (TRXRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) with dynamic mass spectrometry (MS), and high-speed infrared thermal imaging. It is shown that for initial compositions with a relatively large fuel-to-oxidizer ratio (phi), pure Ni phase forms directly in the combustion front. For fuel-lean conditions, only NiO phase can be detected. Analysis of the obtained data, including transmission and scanning electron microscopy (TEM-SEM) studies of the quenched reaction fronts, allows us to suggest the intrinsic mechanism of pure metal formation in the investigated system. It is shown that the combustion front propagates because of the reaction between N2O and NH3, which are the products of decomposition of the oxidizer and fuel. The excess of NH3 gas produced in fuel-rich conditions rapidly (<0.2 s) reduces nickel oxide to pure metal in the reaction front.

Combustion synthesis (CS) of graphene by a novel type of exothermic self-sustain reaction between a refractory ceramic compound (silicon carbide) and polymer (polytetrafluoroethylene, PTFE) under the inert gas (argon) environment is reported. The synthesis of graphene is confirmed by both transmission electron microscopy and Raman spectroscopy. It is important that the produced graphene has low (<1wt.%) oxygen content. The mechanism for CS of graphene is also discussed. It is experimentally shown that fluorocarbon gases (e.g. tetrafluoroethylene, C2F4) released due to PTFE decomposition in the combustion wave, reduces SiC to tetrafluorosilane (SiF4) gas and meso-porous carbon particles with folded “native” graphene layers on their surfaces. The continuous supply of carbon, in the form of fluorocarbon gases, and the high reaction temperature (∼1400K) enables further rapid growth of “free-standing” graphene sheets on the surface of those graphene-coated particles. The developed method for synthesis of graphene does not require an external energy source, since it occurs in a self-sustained synergetic manner. This approach is also flexible in terms of tuning the synthesis conditions, and allows easy scale-up.