A look at the use of perovskites as subsitutes of noble metals in heterogeneous catalysis is presented. Topics discussed include the synthesis of nanoscaled perovskite and the use of perovskites to ...replace platinum-group metals.
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•Manganese nitride materials were prepared using a molten salt technique.•Lattice nitrogen reactivity was studied using the ammonia synthesis reaction.•Ammonia synthesis occurs via a ...mechanism akin to the Mars − van Krevelen mechanism.•Adding lithium to the system improved the reactivity of lattice nitrogen.
A systematic study was carried out to investigate the potential of manganese nitride related materials for ammonia production. A-Mn-N (A=Fe, Co, K, Li) materials were synthesised by nitriding their oxide counterparts at low temperature using NaNH2 as a source of reactive nitrogen. The reactivity of lattice nitrogen was assessed using ammonia synthesis as a model reaction. In the case of Mn3N2, limited reactivity was observed and only 3.1% of the available lattice nitrogen was found to be reactive towards hydrogen to yield ammonia while most of the lattice nitrogen was lost as N2. However, the presence of a co-metal played a key role in shaping the nitrogen transfer properties of manganese nitride and impacted strongly upon its reactivity. In particular, doping manganese nitride with low levels of lithium resulted in enhanced reactivity at low temperature. In the case of the Li-Mn-N system, the fraction of ammonia formed at 400°C corresponded to the reaction of 15% of the total available lattice nitrogen towards hydrogen. Li-Mn-N presented high thermochemical stability after reduction with hydrogen which limited the regeneration step using N2 from the gas phase. However, the results presented herein demonstrate the Li-Mn-N system to be worthy of further attention.
The performance of Co3Mo3N, Co3Mo3C, and Co6Mo6C for ammonia synthesis has been compared. In contrast to Co3Mo3N, which is active at 400 °C, a reaction temperature of 500 °C, which was preceded by an ...induction period, was necessary for the establishment of steady state activity for Co3Mo3C. Co6Mo6C was found to be inactive under the conditions tested. During the induction period, nitridation of the Co3Mo3C lattice was found to occur, and this continued throughout the period of steady state reaction with the material transforming in composition toward Co3Mo3N. Taken together, these observations demonstrate that ammonia synthesis activity in ternary cobalt molybdenum systems is associated with the presence of N in the 16c Wyckoff lattice site.
In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and ...nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a
dissociative mechanism
. We have recently found, using DFT techniques, that on Co
3
Mo
3
N (111) surfaces, an
associative mechanism
in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases.
A cobalt rhenium catalyst active for ammonia synthesis at 400 °C and ambient pressure was studied using in situ XAS to elucidate the reducibility and local environment of the two metals during ...reaction conditions. The ammonia reactivity is greatly affected by the gas mixture used in the pre-treatment step. Following H
2
/Ar pre-treatment, a subsequent 20 min induction period is also observed before ammonia production occurs whereas ammonia production commences immediately following comparable H
2
/N
2
pre-treatment. In situ XAS at the Co K-edge and Re L
III
-edge show that cobalt initiates reduction, undergoing reduction between 225 and 300 °C, whereas reduction of rhenium starts at 300 °C. The reduction of rhenium is near complete below 400 °C, as also confirmed by H
2
-TPR measurements. A synergistic co-metal effect is observed for the cobalt rhenium system, as complete reduction of both cobalt and rhenium independently requires higher temperatures. The phases present in the cobalt rhenium catalyst during ammonia production following both pre-treatments are largely bimetallic Co–Re phases, and also monometallic Co and Re phases. The presence of nitrogen during the reduction step strongly promotes mixing of the two metals, and the bimetallic Co–Re phase is believed to be a pre-requisite for activity.
The treatment of volatile organic compounds (VOC) emissions is a necessity of today. The catalytic treatment has already proven to be environmentally and economically sound technology for the total ...oxidation of the VOCs. However, in certain cases, it may also become economical to utilize these emissions in some profitable way. Currently, the most common way to utilize the VOC emissions is their use in energy production. However, interesting possibilities are arising from the usage of VOCs in hydrogen and syngas production. Production of chemicals from VOC emissions is still mainly at the research stage. However, few commercial examples exist. This review will summarize the commercially existing VOC utilization possibilities, present the utilization applications that are in the research stage and introduce some novel ideas related to the catalytic utilization possibilities of the VOC emissions. In general, there exist a vast number of possibilities for VOC utilization via different catalytic processes, which creates also a good research potential for the future.
The role of metrology in the industrial and manufacturing sectors is of paramount importance to ensure informed decision-making whether for product quality control, process monitoring and R&D ...activities. However, to guarantee the quality and reliability of analytical measurements, the development and use of appropriate reference materials (CRMs) is essential. In particular, certified reference materials (CRMs) are extensively used to validate analytical methods in a multitude of applications, measure uncertainty, improve the accuracy of measurement data, as well as to establish the meteorological traceability of analytical results. In this paper, we report the improvement of the characterization uncertainty of an in-house matrix reference material by direct determination of the concentration of fluorosilicic acid recovered from the fertilizers production industry. The certified reference material was characterized by the potentiometric method as a novel and direct approach for the determination of H
2
SiF
6
concentration and the results were compared against a reference measurement procedure based on molecular absorption spectrophotometry (UV–VIS). The approach adopted in the work resulted in the improvement of the uncertainty of the CRM by decreasing the characterization uncertainty, which constitutes the major contribution to the overall uncertainty. The newly obtained characterization combined standard uncertainty was 2.0 g.kg
−1
, which gives an expanded uncertainty (
k
= 2 with a confidence interval of 95%) of the CRM of 6.3 g.kg
−1
instead of 11.7 g.kg
−1
reported in previous works. This improved CRM can be used to improve the uncertainty of the analytical methods used for the determination of H
2
SiF
6
mass fraction and, therefore, to improve the accuracy of measurement data.
Graphical abstract
Ammonia, which is one of the most important chemicals for the synthesis of dyes, pharmaceuticals, and fertilizers, is produced by the reaction of molecular hydrogen with nitrogen, over an iron-based ...catalyst at 400-500 °C under pressure of over 100 bar. Decreasing the operating temperature and pressure of this highly energy-intensive process, developed by Haber and Bosch over 100 years ago, would decrease energy consumption in the world. In this work, we used two-dimensional Mo2CTx MXene as a support for a cobalt-based catalyst. The MXene functionalized by Co showed catalytic activity for ammonia synthesis from H2 and N2 at temperatures as low as 250 °C, without any pretreatment. The developed catalyst was highly active for ammonia synthesis, demonstrating a high rate of up to 9500 μmol g-1active phase h-1 at 400 °C under ambient pressure in steady-state conditions, and did not suffer from any deactivation after 15 days of reaction. The apparent activation energy (Ea) was found to be in the range of 68-74 kJ mol-1, which is in line with values reported for highly active catalysts. This improved catalyst may decrease the energy consumption in the synthesis of ammonia and its derivatives, as well as facilitate the use of ammonia as a hydrogen carrier for renewable energy storage.Ammonia, which is one of the most important chemicals for the synthesis of dyes, pharmaceuticals, and fertilizers, is produced by the reaction of molecular hydrogen with nitrogen, over an iron-based catalyst at 400-500 °C under pressure of over 100 bar. Decreasing the operating temperature and pressure of this highly energy-intensive process, developed by Haber and Bosch over 100 years ago, would decrease energy consumption in the world. In this work, we used two-dimensional Mo2CTx MXene as a support for a cobalt-based catalyst. The MXene functionalized by Co showed catalytic activity for ammonia synthesis from H2 and N2 at temperatures as low as 250 °C, without any pretreatment. The developed catalyst was highly active for ammonia synthesis, demonstrating a high rate of up to 9500 μmol g-1active phase h-1 at 400 °C under ambient pressure in steady-state conditions, and did not suffer from any deactivation after 15 days of reaction. The apparent activation energy (Ea) was found to be in the range of 68-74 kJ mol-1, which is in line with values reported for highly active catalysts. This improved catalyst may decrease the energy consumption in the synthesis of ammonia and its derivatives, as well as facilitate the use of ammonia as a hydrogen carrier for renewable energy storage.
Modulating the interaction between Mo nanoparticles and their support is an elegant approach to finely tune the structural, physico-chemical, redox and electronic properties of the active site. In ...this work, a series of molybdenum nitride catalysts supported on TiO
2
, and SBA-15 has been prepared and fully characterized. The results of characterization confirmed the high dispersion of Mo and the formation of small molybdenum nanoparticles in both the 10-Mo-N/SBA-15 and 10-Mo-N/TiO
2
catalysts. In this context, we have shown that the catalytic activity of Mo species was strongly impacted by the nature of the catalytic support. Amongst the studied supports, SBA-15 was found to be the most appropriate for Mo dispersion. In comparison, when supported on a reducible oxide (TiO
2
), Mo species showed poor catalytic activity in both ammonia synthesis and decomposition and were prone to quick deactivation in the ammonia synthesis reaction. Evidence of charge transfer from the reducible support to the active phase, indicative of possible SMSI behaviour, has been observed by XPS and EPR. Differences in the oxidation states, redox behaviours, and electronic properties have been further studied by means of EPR, H
2
-TPR and H
2
-TPD.
An elegant approach to finely tune the structural, physico-chemical, redox and electronic properties of the active site by modulating the interaction between Mo nanoparticles and their support.
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