Chemical looping of metal nitride catalysts: low-pressure ammonia synthesis for energy storage

Design principles for reducible metal nitride catalysts are developed and demonstrated for ambient-pressure solar-driven N₂ reduction into NH₃.

The activity of many heterogeneous catalysts is limited by strong correlations between activation energies and adsorption energies of reaction intermediates. Although the reaction is thermodynamically favourable at ambient temperature and pressure, the catalytic synthesis of ammonia (NH₃), a fertilizer and chemical fuel, from N₂ and H₂ requires some of the most extreme conditions of the chemical industry. We demonstrate how ammonia can be produced at ambient pressure from air, water, and concentrated sunlight as renewable source of process heat via nitrogen reduction with a looped metal nitride, followed by separate hydrogenation of the lattice nitrogen into ammonia. Separating ammonia synthesis into two reaction steps introduces an additional degree of freedom when designing catalysts with desirable activation and adsorption energies. We discuss the hydrogenation of alkali and alkaline earth metal nitrides and the reduction of transition metal nitrides to outline a promoting role of lattice hydrogen in ammonia evolution. This is rationalized via electronic structure calculations with the activity of nitrogen vacancies controlling the redox-intercalation of hydrogen and the formation and hydrogenation of adsorbed nitrogen species. The predicted trends are confirmed experimentally with evolution of 56.3, 80.7, and 128 μmol NH₃ per mol metal per min at 1 bar and above 550 °C via reduction of Mn₆N_2.58 to Mn₄N and hydrogenation of Ca₃N₂ and Sr₂N to Ca₂NH and SrH₂, respectively.

Hybrid SPECT-CT and PET-CT imaging of differentiated thyroid carcinoma

Hybrid imaging modalities such as radioiodine single photon emission CT with integrated CT ((131)I SPECT-CT) and 2-(fluorine-18)-fluoro-2-deoxy-D-glucose positron emission tomography with integrated CT (FDG PET-CT) allow the rapid and efficient fusion of functional and anatomic images, and provide diagnostic information that may influence management decisions in patients with differentiated thyroid carcinoma (DTC). Diagnostic localisation and therapy of these tumours are dependent upon their capacity to concentrate radioiodine ((131)I) via uptake through the sodium-iodide symporter and retention within the tumour. The prognosis for most patients with DTC is favourable, although controversy exists regarding the role of post-operative (131)I therapy in patients at low-risk for disease. Accurate identification of functional thyroid tissue (benign or malignant) using diagnostic (131)I planar scintigraphy complemented by SPECT-CT imaging enables the completion of post-operative staging and patient risk stratification prior to (131)I therapy administration. In patients with non-iodine-avid tumours (negative (131)I scan but elevated thyroglobulin indicative of persistent or recurrent disease), FDG PET-CT is used to identify tumours with enhanced glucose metabolism and to localise the source of thyroglobulin production. The CT component of this hybrid technology provides anatomic localisation of activity and allows CT-based attenuation correction of PET images. Images from 15 patients illustrate the applications of (131)I SPECT-CT and FDG PET-CT.

Euglycemic clamp study in clozapine-induced diabetic ketoacidosis

OBJECTIVE

To describe the fifth case of clozapine-induced diabetic ketoacidosis (DKA) with complete resolution of abnormal glucose metabolism after discontinuation of clozapine as assessed by oral glucose tolerance testing (OGTT) and the first to be serially studied with markers of pancreatic autoimmunity; to demonstrate insulin resistance using the euglycemic clamp study and reduced pancreatic insulin reserve using intravenous glucose tolerance testing (IVGTT) in clozapine-induced diabetes mellitus and DKA, when the OGTT was normal; and to systematically review the previously described cases of clozapine-induced diabetes mellitus and DKA.

CASE SUMMARY

A 33-year-old white man without past or family history of diabetes mellitus presented with DKA after eight months of clozapine therapy (50 mg twice daily). After treatment of DKA and discontinuation of clozapine, glucose tolerance and concurrent serum insulin concentrations reverted to normal as measured by two OGTT performed 60 and 320 days after resolution of DKA.

DISCUSSION

Antiislet-cell antibodies, antiglutamic acid decarboxylase antibodies, and human insulin antibody were negative on two separate occasions. Euglycemic clamp study demonstrated insulin resistance manifested by a glucose disposal rate of approximately 55% of mean normal values. IVGTT demonstrated a low rate of glucose disappearance (KG = 0.95) and diminished first-phase insulin response when OGTT was normal, indicating impairment in insulin sensitivity and reduction in beta cell function 323 days after discontinuance of clozapine. This adverse reaction is considered probable according to the Naranjo probability scale.

CONCLUSIONS

The occurrence of cases of DKA and new or worsening diabetes mellitus in patients using clozapine suggests a causal relationship. We hypothesize that the mechanism by which clozapine may produce glucose intolerance may require a preexisting latent defect in insulin secretion and insulin action. With the administration of clozapine, some of these patients may develop worsening insulin resistance and may fail to mount an appropriate compensatory beta cell insulin secretion for the degree of insulin resistance. As a consequence, hyperglycemia develops and its persistence results in glucose toxicity, further suppressing beta cell insulin secretion. Such combined defects in insulin secretion and sensitivity are known to be synergistic, leading to the development of abnormal glucose tolerance, which can be clinically manifested as a spectrum ranging from impaired glucose tolerance through severe hyperglycemia to DKA. Patients being started on clozapine should be carefully followed for the development or worsening of diabetes mellitus, regardless of the dose of the drug.