scholarly journals Using pH dependence to understand mechanisms in electrochemical CO reduction

2021 ◽  
Author(s):  
Georg Kastlunger ◽  
Lei Wang ◽  
Nitish Govindarajan ◽  
Hendrik H. Heenen ◽  
Stefan Ringe ◽  
...  
Keyword(s):  
Ph Dependence ◽  
Modern Technology ◽  
Catalyst Design ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Wide Range ◽  
Alkaline Conditions ◽  
Novel Catalyst ◽  
Co Reduction ◽  
Rate Limiting

Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the reaction mechanisms towards the plethora of products are disputed, complicating the search for novel catalyst materials. In order to conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant potential DFT kinetics and experiments at a wide range of pH values (3 - 13). We find that *CO dimerization is energetically favoured as the rate limiting step towards multi-carbon products. This finding is consistent with our experiments, where the reaction rate is nearly unchanged on an SHE potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step in acidic/neutral conditions to a later one in alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.

scholarly journals Using pH dependence to understand mechanisms in electrochemical CO reduction

2021 ◽  
Author(s):  
Georg Kastlunger ◽  
Lei Wang ◽  
Nitish Govindarajan ◽  
Hendrik H. Heenen ◽  
Stefan Ringe ◽  
...  
Keyword(s):  
Ph Dependence ◽  
Modern Technology ◽  
Catalyst Design ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Wide Range ◽  
Alkaline Conditions ◽  
Novel Catalyst ◽  
Co Reduction ◽  
Rate Limiting

Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the reaction mechanisms towards the plethora of products are disputed, complicating the search for novel catalyst materials. In order to conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant potential DFT calculations and experiments at a wide range of pH values (3 - 13). We find that *CO dimerization is energetically favoured as the rate limiting step towards multi-carbon products. This finding is consistent with experiments, where the reaction rate is nearly unchanged on an SHE potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step in acidic/neutral conditions to a later one in alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.

scholarly journals Using pH dependence for understanding mechanisms in electrochemical CO reduction

2021 ◽  
Author(s):  
Georg Kastlunger ◽  
Lei Wang ◽  
Nitish Govindarajan ◽  
Hendrik H. Heenen ◽  
Stefan Ringe ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Ph Dependence ◽  
The Other ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Other Hand ◽  
Alkaline Conditions ◽  
Co Reduction ◽  
Rate Limiting ◽  
Electrolyte Ph

Utilizing electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the exact reaction mechanisms towards the plethora of single and multi-carbon products on Cu electrodes are still disputed. This uncertainty even extends to the rate-limiting step of the respective reactions. Since multi-carbon products do not show a dependence on the electrolyte pH in neutral and alkaline media, CO dimerization on the Cu surface has been proposed as the rate-limiting step. However, other elementary steps would lead to the same pH dependence, namely the proton-electron transfer to *CO followed by subsequent coupling or the protonation of the *OCCO dimer. The pH dependence of methane production on the other hand suggests that the rate limiting step is located beyond the first proton-electron transfer to *CO. In order to conclusively identify the rate limiting steps in CO reduction, we analyzed the mechanisms on the basis of constant potential DFT calculations, CO reduction experiments on Cu at varying pH values (3 - 13) and fundamental rate theory. We find that, even in acidic media, the reaction rate towards multi-carbon products is nearly unchanged on an SHE potential scale, which indicates that its rate limiting step does not involve a proton donor. Hence, we deduce that the rate limiting step can indeed only consist of the coupling of two CO molecules on the surface, both in acidic and alkaline conditions. For methane, on the other hand, the rate-limiting step changes with the electrolyte pH from the first protonation step in acidic/neutral conditions to a later step in alkaline conditions. Finally, based on an in-depth kinetic analysis, we conclude that the pathway towards CH4 involving a surface combination of *CO and *H is unlikely, since it is unable to reproduce the measured current densities and Tafel slopes.

scholarly journals Glucose transport as rate-limiting step in the growth of Escherichia coli on glucose

1976 ◽  
Vol 156 (2) ◽  
pp. 477-480 ◽  
Author(s):  
D Herbert ◽  
H L Kornberg
Keyword(s):  
Escherichia Coli ◽  
Continuous Culture ◽  
Glucose Transport ◽  
Growth Rates ◽  
Glucose Limitation ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Wide Range ◽  
Rate Limiting

Over a wide range of growth rates, two strains of Escherichia coli growing aerobically in continuous culture under glucose limitation utilized glucose at rates identical with those at which cells harvested from the chemostats transported [14C]glucose.

B-lymphocyte depletion therapy in rheumatoid arthritis and other autoimmune disorders

2002 ◽  
Vol 30 (4) ◽  
pp. 824-828 ◽  
Author(s):  
J. C. W. Edwards ◽  
M.J. Leandro ◽  
G. Cambridge
Keyword(s):  
Rheumatoid Arthritis ◽  
B Lymphocyte ◽  
Lymphocyte Subsets ◽  
Autoimmune Disorders ◽  
Lymphocyte Depletion ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Early Evidence ◽  
Wide Range ◽  
Rate Limiting

B-lymphocyte depletion therapy is being explored in a wide range of autoimmune disorders. In many, there is early evidence for efficacy, and immunosuppression has not been a major problem. The mechanism of action is unclear, but appears to be consistent with the lowering of autoantibody levels, where relevant antibodies are quantifiable. An interesting finding is the persistence of clinical improvement for periods of 1 year or more after B-lymphocyte return, which supports the concept that stochastic generation of rare pathogenic B-lymphocyte subsets may be a rate-limiting step in pathogenesis.

scholarly journals Biochemical characterization of a trypanosome enzyme with glutathione-dependent peroxidase activity

2000 ◽  
Vol 352 (3) ◽  
pp. 755-761 ◽  
Author(s):  
Shane R. WILKINSON ◽  
David J. MEYER ◽  
John M. KELLY
Keyword(s):  
Linoleic Acid ◽  
Peroxidase Activity ◽  
Biochemical Characterization ◽  
Linoleic Acid Hydroperoxide ◽  
Butyl Hydroperoxide ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Wide Range ◽  
Acid Hydroperoxide ◽  
Rate Limiting

In most eukaryotes, glutathione-dependent peroxidases play a key role in the metabolism of peroxides. Numerous studies have reported that trypanosomatids lack this activity. Here we show that this is not the case, at least for the American trypanosome Trypanosoma cruzi. We have isolated a single-copy gene from T. cruzi with the potential to encode an 18kDa enzyme, the sequence of which has highest similarity with glutathione peroxidases from plants. A recombinant form of the protein was purified following expression in Escherichia coli. The enzyme was shown to have peroxidase activity in the presence of glutathione/glutathione reductase but not in the presence of trypanothione/trypanothione reductase. It could metabolize a wide range of hydroperoxides (linoleic acid hydroperoxide and phosphatidylcholine hydroperoxide> cumene hydroperoxide>t-butyl hydroperoxide), but no activity towards hydrogen peroxide was detected. Enzyme activity could be saturated by glutathione when both fatty acid and short-chain organic hydroperoxides were used as substrate. For linoleic acid hydroperoxide, the rate-limiting step of this reaction is the reduction of the peroxidase by glutathione. With lower-affinity substrates such as t-butyl hydroperoxide, the rate-limiting step is the reduction of the oxidant. The data presented here identify a new arm of the T. cruzi oxidative defence system.

Kinetics of Base-Catalysed Hydrolysis and Cyclisation of Substituted Acetamide and Benzamide O-(Phenoxycarbonyl)oximes

1999 ◽  
Vol 64 (10) ◽  
pp. 1641-1653 ◽  
Author(s):  
Jaromír Mindl ◽  
Jaromír Kaválek ◽  
Helena Straková ◽  
Vojeslav Štěrba
Keyword(s):  
Reaction Mechanism ◽  
Ph Dependence ◽  
Bond Formation ◽  
Reaction Step ◽  
Rate Limiting Step ◽  
Limiting Step ◽  
Substituted Benzamide ◽  
Ph Range ◽  
Rate Limiting ◽  
Kinetics Of

The reaction kinetics of acetamide O-(4-nitrophenoxycarbonyl)oxime have been studied in aqueous buffers at pH 2-11. At pH > 9, the pH dependence of kobs is linear with slope 1, the cyclisation to 3-methyl-1,2,4-oxadiazol-5(4H)-one and 4-nitrophenol being the only reaction. At pH < 7.5, the only reaction is the hydrolysis giving 4-nitrophenol and acetamidoxime. The dependence of kobs on pH has been used to determine the rate equation and to propose the reaction mechanism. The cyclisation kinetics of substituted benzamide O-(phenoxycarbonyl)oximes have been studied in the pH range from 9.25 to 11. The reaction mechanism has been proposed based on the ρ constants found. In the first reaction step, the proton is split off from the NH2 group; the subsequent, rate-limiting step involves simultaneous N-C bond formation and C-O bond splitting.

Ornithine Decarboxylase Activity in Cultured Endothelial Cells Stimulated by Serum, Thrombin and Serotonin

1978 ◽  
Vol 39 (02) ◽  
pp. 496-503 ◽  
Author(s):  
P A D’Amore ◽  
H B Hechtman ◽  
D Shepro
Keyword(s):  
Endothelial Cells ◽  
Ornithine Decarboxylase ◽  
Decarboxylase Activity ◽  
Aortic Endothelial Cells ◽  
Ornithine Decarboxylase Activity ◽  
Rate Limiting Step ◽  
Bovine Aortic Endothelial Cells ◽  
Limiting Step ◽  
Rate Limiting ◽  
Serum Serotonin

SummaryOrnithine decarboxylase (ODC) activity, the rate-limiting step in the synthesis of polyamines, can be demonstrated in cultured, bovine, aortic endothelial cells (EC). Serum, serotonin and thrombin produce a rise in ODC activity. The serotonin-induced ODC activity is significantly blocked by imipramine (10-5 M) or Lilly 11 0140 (10-6M). Preincubation of EC with these blockers together almost completely depresses the 5-HT-stimulated ODC activity. These observations suggest a manner by which platelets may maintain EC structural and metabolic soundness.

Dynamics of hepatic and peripheral insulin effects suggest common rate-limiting step in vivo

Diabetes ◽  
1993 ◽  
Vol 42 (2) ◽  
pp. 296-306 ◽  
Author(s):  
D. C. Bradley ◽  
R. A. Poulin ◽  
R. N. Bergman
Keyword(s):  
Rate Limiting Step ◽  
Limiting Step ◽  
Rate Limiting
Sign in / Sign up

Export Citation Format

Share Document