Generic models for the mean carbon number n̄, H/C ratio β and chain growth probability α, as a function of C5+ selectivity for Fischer–Tropsch reactions, have been developed based on gas analysis data and product mass. These models allow predicting the parameters when detailed compositions of oil and wax are unavailable. Meanwhile, the distribution of individual 2–5 methyl paraffins of iron and cobalt catalysts containing K, Zr and Pt promoters or poisoned by COS and HBr were studied. The chain-length-dependent formation of methyl paraffins was demonstrated on both iron and cobalt catalysts. The fraction of the methyl paraffins produced on an active iron catalyst (15–22%) was 5–7 times higher than that of a cobalt catalyst (1–3%). Ethylidene (CH2CH–M) was proposed as one possible iso-chain growth monomer, based on the higher fraction of 3-methyl product. The effects of the promoters and the poisons on the formation of iso-paraffins and 1-olefin were complicated. K (Fe), Zr and S (Co) improved the formation of 1-olefins and suppressed the formation of iso-paraffins, partly due to the electronic effect (back-donation) of the promoters. Pt inhibited the formation of both 1-olefins and iso-paraffins on a cobalt catalyst. Adding HBr was found to promote the formation of iso-paraffins on an iron catalyst.

As our societal need for materials and energy has grown, so has our need for catalyst processes in hydrogen production. A major function in these applications, for both homogenous and heterogeneous catalysis processes, is the synthesis of an active metal catalyst. It must first be soluble to control the physical properties of the metal being used. Recent work in metal precursors has begun to turn toward these metal carboxylate types of material. Structural models are proposed for bismuth 2-ethylhexanoate and 2,2-dimethyloctanoate and cerium 2-ethylhex-anoate. The bismuth compounds have been characterized at different ratios of bismuth to carboxylate as solutions of the free acids. Their structures are most consistent with a Bi-4(RCO2)(12) motif where the Bi ions are arranged in a flattened tetrahedron with Bi - Bi distances of about 4.3 & Aring;. There is evidence for Bi - O - Bi linkages at low free acid concentrations. The cerium compound is most consistent with a linear tetracerium molecule where the Ce - Ce distances repeat at about 4.3 & Aring; out to 16.4 & Aring;. The models were generated by analogy with known crystal structures and compared to high-energy x-ray scattering data. To further evaluate the models, DFT calculations were made, and the equilibrium geometries were compared. The vibrational spectra calculated from those geometries are presented and compared to the experimental results. Magnetization vs. temperature data was collected on the cerium compound, and its behavior was consistent with the proposed model. A geometrical approach to determining the dimensionality and relative positions of the metal ions in these structures is presented.

Iron-based catalysts are the most suitable candidates for converting CO2 or CO2-rich syngas to hydrocarbons. However, several issues about the mechanism of CO2 hydrogenation are still unclear. In this work, we investigated the performance of an iron-based catalyst with H2/CO2, H2/CO/N2 and H2/CO/13CO2/N2 gas mixtures at the same process conditions (T = 270°C, P = 175 psi and SV = 3 NL/h/gcat). The CO2 hydrogenation rate was much lower than that observed for CO hydrogenation. 13CO2 tracer experiments indicated that CO2 is hydrogenated to hydrocarbons via the reverse water-gas shift even when present in small concentration (1.8 vol%). 13C enrichment was observed in both CO and C1-C4 hydrocarbons.

Since the turn of the last century when the field of catalysis was born, iron and cobalt have been key players in numerous catalysis processes. These metals, due to their ability to activate CO and CH, haev a major economic impact worldwide. Several industrial processes and synthetic routes use these metals: biomass-to-liquids (BTL), coal-to-liquids (CTL), natural gas-to-liquids (GTL), water-gas-shift, alcohol synthesis, alcohol steam reforming, polymerization processes, cross-coupling reactions, and photocatalyst activated reactions. A vast number of materials are produced from these processes, including oil, lubricants, waxes, diesel and jet fuels, hydrogen (e.g., fuel cell applications), gasoline, rubbers, plastics, alcohols, pharmaceuticals, agrochemicals, feed-stock chemicals, and other alternative materials. However, given the true complexities of the variables involved in these processes, many key mechanistic issues are still not fully defined or understood. This Special Issue of Catalysis will be a collaborative effort to combine current catalysis research on these metals from experimental and theoretical perspectives on both heterogeneous and homogeneous catalysts. We welcome contributions from the catalysis community on catalyst characterization, kinetics, reaction mechanism, reactor development, theoretical modeling, and surface science.

This review is divided into three main sections. The first section of this overview examines in detail recent technoeconomic analyses on the utilization of biomass as a source of syngas for Fischer-Tropsch synthesis (FTS). FTS is a highly localized industry, and depends on the sources of syngas available, the products desired, and the demand for coproduction of electricity. Where coal or natural gas coexist in the region where biomass is available, economics may favor producing or blending syngas from multiple resources, making biomass-to-liquids a highly complex scenario. Land use management is essential for sustainable biomass production, if used as the carbon source for FTS. The second section discusses FTS mechanisms, reactions and catalysts. Supposing the catalyst surface energy favors CO hydrogenation to FTS products, the sweet spot of FTS, selectivity will depend on the degree of back donation from the metal. The degree of electron back-donation from the catalyst surface to both adsorbed CO and vinylic intermediates in part controls the FT synthesis product distribution. When the back-donation is sufficient, chain growth favors longer chained linear hydrocarbons (paraffins, alpha olefins, linear alcohols) suitable for upgrading to diesel, jet fuels, lubricants, and waxes. When back-donation is less significant, monomethyl branched hydrocarbons, internal olefins, and oxygenates (esters, acids and ketones) are elevated. Lastly, active supports are discussed for increasing the termination rate to oxygenates and olefins, as are hybrid catalysts that simultaneously carry out FTS and upgrading, often through secondary reactions. The last section provides an overview of the commercial scale reactor designs including the fixed bed, fluidized bed, and slurry bubble column reactor currently used in the FTS process. Economically, FTS is favored at larger scales; however, new compact reactor designs are coming into play aimed at monetizing highly localized (e.g. biomass) and stranded resources (e.g. natural gas). These types of reactors, involving microchannel and heat exchanger designs, must be highly efficient at managing heat, cost-effective, and mobile in order to exploit non-traditional sources of syngas.

Cobalt and iron have long history of importance in the field of catalysis that continues to this day [...]

Photocatalytic water splitting was performed on TiO2 and metal-containing TiO2 (MTiO2) catalysts under UV light irradiation. TiO2 was added with various metals of different work functions, namely, Pt (5.93 eV), Pd (5.60 eV), Cu (5.10 eV), Ru (4.71 eV), and Ag (4.26 eV). The hydrogen production was found to increase linearly with increasing metal work function. The rate of charge recombination increases with decreasing work function difference between the metal and TiO2. Pt and Pd were highly efficient co-catalysts to TiO2 for hydrogen generation from water due to their larger work function and upward-bent band causing the photoelectrons to be trapped in those metal sites that are eventually consumed during water reduction. Co-catalysts such as Ag, Ru, and Cu were less effective towards water splitting due to downward-bent band gaps that allow the photoelectrons to flow back to TiO2 from the metal which leads to faster charge recombination.

The effect of cobalt substitution with nickel was investigated for the Fischer–Tropsch synthesis reaction. Catalysts having different Ni/Co ratios were prepared by aqueous incipient wetness co-impregnation, characterized, and tested using a continuously stirred tank reactor (CSTR) for more than 200 h. The addition of nickel did not significantly modify the morphological properties measured. XRD, STEM, and TPR-XANES results showed intimate contact between nickel and cobalt, strongly suggesting the formation of a Co-Ni solid oxide solution in each case. Moreover, TPR-XANES indicated that nickel addition improves the cobalt reducibility. This may be due to H2 dissociation and spillover, but is more likely the results of a chemical effect of intimate contact between Co and Ni resulting in Co-Ni alloying after activation. FTS testing revealed a lower initial activity when nickel was added. However, CO conversion continuously increased with time on-stream until a steady-state value (34%–37% depending on Ni/Co ratio) was achieved, which was very close to the value observed for undoped Co/Al2O3. This trend suggests nickel can stabilize cobalt nanoparticles even at a lower weight percentage of Co. Currently, the cobalt price is 2.13 times the price of nickel. Thus, comparing the activity/price, the catalyst with a Ni/Co ratio of 25/75 has better performance than the unpromoted catalyst. Finally, nickel-promoted catalysts exhibited slightly higher initial selectivity for light hydrocarbons, but this difference typically diminished with time on-stream; once leveling off in conversion was achieved, the C5+ selectivities were similar (≈ 80%) for Ni/Co ratios up to 10/90, and only slightly lower (≈ 77%) at Ni/Co of 25/75.

The effect of alkali promoter (K, Rb and Cs) on the performance of precipitated iron-based catalysts was investigated for carbon dioxide (CO2) hydrogenation. Characterization by temperature-programmed reduction with CO, Mossbauer spectroscopy, and transmission electron microscopy were used to study the effect of alkali promoter interactions on the carburization and phase transformation behavior of the catalysts. Under similar reaction conditions, cesium (Cs) and rubidium (Rb) promoted catalysts exhibited the highest initial CO2 conversions to higher hydrocarbons. CO2 conversions then decreased to reach steady state conversions around 170 h on stream. At steady state conversion, all three catalysts exhibited similar CO2 conversions and selectivities. For comparison, a lower loaded Cs (1.5 Cs) promoted iron-based catalyst was prepared. It exhibited slightly lower initial conversion than the higher loaded Cs catalyst, but remained very stable. Among all the catalysts at steady state conversion, the 1.5 Cs promoted catalyst exhibited the highest stability. Results indicate a synergistic effect brought on by these promoters that, if balanced, could potentially yield superior CO2 hydrogenation catalysts. (C) 2018 Elsevier Inc. All rights reserved.

Zn addition was found to affect both activity and selectivity of ZrO2 for dehydration of 1,5-pentanediol. ZrO2 tends to produce more or less equimolar mixture of tetrahydropyran (THP) derivatives and 4-penten-1-ol from 1,5-pentanediol. The conversion of 1,5-pentanediol on ZrO2 increases with increasing Zn content up to 30-50 mole percent; however, catalyst containing Zn beyond 50 mole percent had an adverse effect on both conversion of diol and the selectivity for unsaturated alcohol (i.e., 4-penten-1-ol). XRD and Raman analysis infer that the presence of tetragonal ZrO2, the amorphous phase (ZrO2, ZrZnOx, ZnO), and hexagonal wurtzite structure of ZnO in the catalysts. The interplanar spacing of ZrO2 (111) and ZnO (100) planes for catalysts indicate that Zn incorporates into ZrO2 lattice and vice-versa. Basicity assessed from CO2-TPD and acidity from FTIR-pyridine adsorption techniques indicate that both basicity and Lewis acid sites density increases with increasing Zn proportion on ZrO2 up to 50:50 molar ratios of Zn to Zr. An optimum Zn:Zr mole ratio is required to achieve higher density of oxygen vacant metal sites (i.e., Lewis acidity) and balanced acid-base strength which improves the diol conversion.

The bulk of the products that were synthesized from Fischer–Tropsch synthesis (FTS) is a wide range (C1–C70+) of hydrocarbons, primarily straight-chained paraffins. Additional hydrocarbon products, which can also be a majority, are linear olefins, specifically: 1-olefin, trans-2-olefin, and cis-2-olefin. Minor hydrocarbon products can include isomerized hydrocarbons, predominantly methyl-branched paraffin, cyclic hydrocarbons mainly derived from high-temperature FTS and internal olefins. Combined, these products provide 80–95% of the total products (excluding CO2) generated from syngas. A vast number of different oxygenated species, such as aldehydes, ketones, acids, and alcohols, are also embedded in this product range. These materials can be used to probe the FTS mechanism or to produce alternative chemicals. The purpose of this article is to compare the product selectivity over several FTS catalysts. Discussions center on typical product selectivity of commonly used catalysts, as well as some uncommon formulations that display selectivity anomalies. Reaction tests were conducted while using an isothermal continuously stirred tank reactor. Carbon mole percentages of CO that are converted to specific materials for Co, Fe, and Ru catalysts vary, but they depend on support type (especially with cobalt and ruthenium) and promoters (especially with iron). All three active metals produced linear alcohols as the major oxygenated product. In addition, only iron produced significant selectivities to acids, aldehydes, and ketones. Iron catalysts consistently produced the most isomerized products of the catalysts that were tested. Not only does product selectivity provide a fingerprint of the catalyst formulation, but it also points to a viable proposed mechanistic route.

Nearly a century ago, Fischer and Tropsch discovered a means of synthesizing organic compounds ranging from C1 to C70 by reacting carbon monoxide and hydrogen on a catalyst. Fischer–Tropsch synthesis (FTS) is now known as a pseudo-polymerization process taking a mixture of CO as H2 (also known as syngas) to produce a vast array of hydrocarbons, along with various small amounts of oxygenated materials. Despite the decades spent studying this process, it is still considered a black-box reaction with a mechanism that is still under debate. This investigation sought to improve our understanding by taking data from a series of experimental Fischer–Tropsch synthesis runs to build a computational model. The experimental runs were completed in an isothermal continuous stirred-tank reactor, allowing for comparison across a series of completed catalyst tests. Similar catalytic recipes were chosen so that conditional comparisons of pressure, temperature, SV, and CO/H2 could be made. Further, results from the output of the reactor that included the deviations in product selectivity, especially that of methane and CO2, were considered. Cobalt was chosen for these exams for its industrial relevance and respectfully clean process as it does not intrinsically undergo the water–gas shift (WGS). The primary focus of this manuscript was to compare runs using cobalt-based catalysts that varied in two oxide catalyst supports. The results were obtained by creating two differential equations, one for H2 and one for CO, in terms of products or groups of products. These were analyzed using sensitivity analysis (SA) to determine the products or groups that impact the model the most. The results revealed a significant difference in sensitivity between the two catalyst–support combinations. When the model equations for H2 and CO were split, the results indicated that the CO equation was significantly more sensitive to CO2 production than the H2 equation.

The effects of 1% of Cd, In and Sn additives on the physicochemical properties and Fischer-Tropsch synthesis (FTS) performance of a 15% Co/Al2O3 catalyst were investigated. The fresh and spent catalysts were characterized by BET, temperature programmed reduction (TPR), H2-chemisorption, NH3 temperature programmed desorption (TPD), X-ray absorption near edge spectroscopy (XANES), and X ray diffraction (XRD). The catalysts were tested in a 1 L continuously stirred tank reactor (CSTR) at 220 °C, 2.2 MPa, H2/CO = 2.1 and 20–55% CO conversion. Addition of 1% of Cd or In enhanced the reduction degree of 15%Co/Al2O3 by ~20%, while addition of 1% Sn slightly hindered it. All three additives adversely impacted Co dispersion by 22–32% by increasing apparent Co cluster size based on the H2-chemisorption measurements. However, the decreased Co active site density resulting from the additives did not result in a corresponding activity loss; instead, the additives decreased the activity of the Co catalysts to a much greater extent than expected, i.e., 82–93%. The additional detrimental effect on catalyst activity likely indicates that the Cd, In and Sn additives migrated to and covered active sites during reaction and/or provided an electronic effect. XANES results showed that oxides of the additives were present during the reaction, but that a fraction of metal was also likely present based on the TPR and reaction testing results. This is in contrast to typical promoters that become metallic at or below ~350 °C, such as noble metal promoters (e.g., Pt, Ru) and Group 11 promoters (e.g., Ag, Au) on Co catalysts in earlier studies. In the current work, all three additives remarkably increased CH4 and CO2 selectivities and decreased C5+ selectivity, with the Sn and In additives having a greater effect. Interestingly, the Cd, In, or Sn additives were found to influence hydrogenation and isomerization activities. At a similar conversion level (i.e., in the range of 40–50%), the additives significantly increased 2-C4 olefin content from 3.8 to 10.6% and n-C4 paraffin from 50 to 61% accompanied by decreases in 1-C4 olefin content from 48 to 30%. The Sn contributed the greatest impact on the secondary reactions of 1-olefins, followed by the In and Cd. NH3-TPD results suggest enhanced acid sites on cobalt catalysts resulting from the additives, which likely explains the change in selectivities for the different catalysts.

Deuterium tracer studies coupled with kinetic approach were developed to accurately determine the kinetic isotopic effects (KIE) for the CO hydrogenation and the formations of CO2, CH4 and light hydrocarbons (C-2 -C-4) over iron, cobalt and ruthenium catalysts. Pronounced inverse kinetic isotope effects (IKIE) = 0.64-0.66) over the Fe and Co catalysts and a moderate IKIE (k(CH4)/k(CD4) = similar to 0.8) on the Ru catalysts for the CO hydrogenation were obtained under typical FTS conditions. The IKIE was also found in CH4 formation, but the Co and Ru catalysts showed stronger IKIE than the Fe catalysts k(CH4)/k(CD4) = 0.6 vs 0.87). Regardless of catalyst type, only a minor normal KIE for the CO2 formation was obtained (kCO(2), H H/kCO2 D(similar to)1.1). The IKIE was found on various light 1-olefins and n-paraffin hydrocarbons as well. All the IKIEs can be explained by thermodynamic and kinetic isotope effects. The results suggested that deuterium was involved in quasi-equilibrated and hydrogenation steps that determined KIE. However, the Fe, Co and Ru catalysts showed different deuterium isotopic effect for the hydrocarbon selectivity. Deuterium promoted chain growth, thus lowered CH4 and light hydrocarbons selectivities and enhanced C5+, selectivity on both the Fe and Co catalysts; while it enhanced the hydrogenation rate to the low hydrocarbons and suppressed the formation of heavier hydrocarbons on the Ru catalysts. Deuterium isotopic effect on the formation of 1-olefins, n-paraffins and 2-olefins in C-10 -C-18 range (in liquid phase) was also studied in detail. Deuterium was found to enhance 1-olefins formation, but suppressed or only slightly changed n-paraffins formation over the Fe and Co catalysts. However, deuterium led to greater amount of n-paraffins and less 1-olefins on the Ru catalysts. The results suggested that the IKIE originated from different hybridization changes of carbon in C-H (sp(l) - > sp(2) and/or sp(2) - > sp(3)) on the Fe, Co and Ru catalysts, which implied different reaction pathways on the Fe, Co and Ru catalysts.

The effects of CeO2 doped with Na on the dehydration of 1,5-pentanediol were studied by using a fixed-bed reactor at two different temperatures (350 and 400 degrees C) and atmospheric pressure. For characterization, BET surface area, hydrogen temperature-programmed reduction, CO2 temperature-programmed desorption, and diffuse reflectance infrared Fourier transform spectroscopy techniques were utilized. The conversion of the diol on CeO2 was found to depend on Na loading. The selectivity to the desired product (i.e., unsaturated alcohol) increased and the selectivity to undesired products (i.e., tetrahydropyran, tetrahydropyran-2-one, cyclopentanol and cylopentanone) decreased with increasing Na content on CeO2. The basicity of hydroxyl groups or surface oxygen on CeO2 was altered with the addition of Na, and controlled the dehydration reaction pathway.

Industrial catalyst processes, such as the Fischer-Tropsch (FT) process, produce a vast array of products from syngas (H2 /CO), such as jet fuel, gasoline, diesel, synthetic rubbers, monomers for plastics industries, and other oil/wax materials for specific purposes such as cosmetics. Multitudes of publications since the discovery of the FT process in 1925 have been composed, attempting to elucidate the mechanism. Many of these publications attempt to investigate the mechanism of FT by the utilization of specific deuterium experimentation through the switching of the syngas from H2 /CO to D2 /CO. Results from this switching indicated that hydrogen was involved in the rate-limited step; the overall process conversion produced an inverse kinetic isotope effect. To confirm that results were not hindered by the physical switch of the hydrogen isotopes, further experimentation was performed using equal molar of each isotope competitively (equal molar H2 /D2 )/CO. Complications arose from this competitive work as it generated fully exchanged products, i.e. all partially deuterated hydrocarbons that could not be separated by chromatography. These compounds could no longer be separated by the chromatography and required a further separation by mass. The overall scope for this work was to determine if a range of partially deuterated paraffin compounds, generated by FT, can be analyzed using an EI-MSD without interinstrumental H/D exchange. Results indicate that no real exchange occurs in the EI MSD for a carbon range from about C6 to C16 . Even though the materials cannot be separated chromatographically, they can be further separated and analyzed to determine the overall H/D content for these specific chain lengths.

The explanation for CH4 selectivity for iron based Fischer-Tropsch catalysts in the low conversion region (Le., < 50%) remains elusive. In this contribution, the CO conversion effect was carefully examined over four K promoted Fe catalysts (100 Fe/5.1Si/2Cu/ x K, where x = 1-5) over a wide range of CO conversion (i.e., 4-85%). Moreover, the effect of CO conversion on oxygenate selectivity of the Fe-K catalysts was carefully studied as well. The change in CH4 selectivity with CO conversion was found to resemble asymmetric "V" shaped curves, with the minimum values occurring at approximately 50% CO conversion. Adding greater than x = 2 K significantly alleviated the CO conversion effect which was attributed to the high K loading greatly decreasing the surface H coverage while improving CO adsorption. The unique CH4 selectivity trend suggests a complicated CH4 formation process that results from different aspects of the catalyst (i.e., chain growth and hydrogenation rates), and process conditions. Oxygenate selectivity was in the range of 0.7-2.8% and varied with the CO conversion and K loading. The addition of K up to x = 3 was found to promote oxygenate formation and chain growth. The overall oxygenate distribution up to C-17 follows an Anderson-Schulz Flory (ASF) distribution, with ethanol being the dominant oxygenate. Mechanisms of oxygenate formation different from that of hydrocarbon formation (e.g., CO insertion versus CO dissociation) were proposed to explain the experimental results.