Near-Barrierless CO Oxidation Using Phosphotungstic Acid-Supported Single-Atom Catalysts.

Efficient CO oxidation at ambient or low temperatures is essential for environmental purification and selective CO oxidation in H2, yet achieving this remains a challenge with current methodologies. In this research, we extensively evaluated the catalytic performance of phosphotungstic acid (PTA)-supported 11 M1/PTA single-atom catalysts (SACs) using density functional theory calculations across both gas phase and 12 common solvents. The Rh1/PTA, Pd1/PTA, and Pt1/PTA systems exhibit moderate CO adsorption energies, facilitating the feasibility of oxygen vacancy formation. Remarkably, the Pd1/PTA and Pt1/PTA catalysts exhibited negligible energy barriers and demonstrated exceptionally high catalytic rates, with values reaching up to (1 × 1010)11, markedly exceeding the threshold for room temperature reactions, set at 6.55 × 108. This phenomenon is attributed to a transition from the high-energy barrier processes of oxygen dissociation in O2 and N-O bond dissociation in N2O to the more efficient dissociation of H2O2. Orbital analysis and charge variations at metal sites throughout the reaction process provide deeper insights into the role of the three metal catalytic sites in CO activation. Our findings not only reveal key aspects of SACs in facilitating CO oxidation at low temperatures but also provide valuable insights for future catalytic reaction mechanism studies and environmental applications.

Reduction of N2O by CO via Mars-van Krevelen [corrected] Mechanism over Phosphotungstic Acid Supported Single-Atom Catalysts: A Density Functional Theory Study.

In general, reduction of N2O by CO is first performed by N2O decomposition over a catalyst surface to release N2 and form an active oxygen species, and subsequently CO is oxidized by the active oxygen species to produce CO2. However, the strong adsorption behavior of CO on the catalyst surface usually inhibits adsorption and decomposition of N2O, which leads to a low activity or poisoning of catalysts. In the present paper, a Mars-van Krevelen (MvK) [correction] mechanism has been probed based on a series of phosphotungstic acid (PTA) supported single-atom catalysts (SACs), M1/PTA (M = Fe, Co, Mn, Rh, Ru, Ir, Os, Pt, and Pd). Although the calculated adsorption energy of CO is exceedingly higher than N2O for our studied systems, the adsorbed CO could react with the surface oxygen atom of the PTA support through the MvK mechanism to form an oxygen vacancy on the PTA surface. N2O acts as an oxygen donor to replenish the PTA support and release N2 in the whole reaction process. This proposed reaction mechanism avoids competitive adsorption and poisoning of the catalyst caused by CO. The calculated adsorption energy, oxygen vacancy formation energy, and the free energy profiles show that the catalytic activity of Pd1/PTA, Rh1/PTA, and Pt1/PTA SACs is quite high, especially for Pt1/PTA and Pd1/PTA systems. Meanwhile, molecular geometry and electronic structure analysis along the favorable reaction pathway indicates that the metal single atom not only plays the role of adsorbing CO and activating surface atoms of the PTA support but also works as an electron transfer media in the whole reaction process. We expect that the present calculated results could provide some clues for the search for appropriate catalyst for reduction of N2O to N2 by CO at low temperature.

Jahn-Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations.

Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)-dinitrogen complexes [PW11O39M(N2)] n- (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N2 molecule, N-N bond length, N-N stretching frequency, and the NBO charge on the coordinated N2 moiety indicate that MoII-, TcII-, WII-, ReII-, and OsII-POM complexes are significant for binding and activation of the inert N2 molecule. The degree of the N2 activation can be classified into the "moderately activated" category according to Tuczek's sense [ J. Comput. Chem. 2006 , 27 , 1278 ]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N2 molecule in these POM-dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn-Teller distorted effects lead to an effective orbital mixture between σ2s* orbital of N2 and d z2 orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N2 into NH3 has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors.

Reduction of N2O by H2 Catalyzed by Keggin-Type Phosphotungstic Acid Supported Single-Atom Catalysts: An Insight from Density Functional Theory Calculations.

In the present paper, the mechanisms of N2O reduction by H2 were systemically examined over various polyoxometalate-supported single-atom catalysts (SACs) M1/PTA (M = Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; PTA = [PW12O40]3-) by means of density functional theory calculations. Among these M1/PTA SACs, Os1/PTA SAC possesses high activity for N2O reduction by H2 with a relatively low rate-determining barrier. The favorable catalytic pathway involves the first and second N2O decomposition over the Os1/PTA SAC and hydrogenation of the key species after the second N2O decomposition. Molecular geometry and electronic structure analyses along the favorable reaction pathway indicate that a strong charge-transfer cooperative effect of metal and support effectively improves the catalytic activity of Os1/PTA SAC. The isolated Os atom not only plays the role of adsorption and activation of the N2O molecule but also works as an electron transfer medium in the whole reaction process. Meanwhile, the PTA support with very high redox stability has also been proven to be capable of transporting the electron to promote the whole reaction. We expect that our computation results can provide ideas for designing new SACs for N2O reduction by using H2 selective catalytic reduction technology.

Calculations of NO reduction with CO over a Cu1/PMA single-atom catalyst: a study of surface oxygen species, active sites, and the reaction mechanism.

Density functional theory (DFT) calculations have been employed to probe the reaction mechanism of NO reduction with CO over a Cu1/PMA (PMA is the phosphomolybdate, Cs3PMo12O40) single-atom catalyst (SAC). Several important aspects of the catalytic system were addressed, including the generation of oxygen vacancies (Ov), formation of N2O2 intermediates, scission of the N-O bond of N2O2 intermediates to form N2O or N2, and decomposition of N2O to form N2. Unlike most previous theoretical studies, which tend to explore the reaction mechanism of polyoxometalate (POM) systems based on the isolated anionic unit, here, we build a model of the catalytic system with neutral species by introduction of counter cations to model the solid structure of the Cu1/PMA SAC. The major findings of our present study are: (1) CO adsorption on Cu sites leads to the formation of cationic Cu carbonyl species; (2) the Oc atom at the surface of the PMA support can easily react with the adsorbed CO to generate a Cu-Ov pair; (3) the Cu-Ov pair embedded on PMA is found to be the active site, not only for the formation of N2O2* by the reaction of two NO molecules via an Eley-Rideal pathway but also for the decomposition of N2O to form N2; (4) the adsorption of a NO molecule on the Cu-Ov pair with a bridging model results in charge transfer from the Cu atom to the π* antibonding orbital of the NO molecule; (5) IR spectroscopy of the key intermediates has been identified based on our DFT calculations; and (6) the Cu atom serves as an electron acceptor in Ov formation steps and an electron donor in N2O2 decomposition steps, and thus represents an electron reservoir. These results suggest that the POM-supported SAC with the cheaper Cu element is an efficient catalyst for the reaction between CO and NO.

[The analysis of prognostic factors in patients with thoracic ossification of the ligamentum flavum and thoracic ossification of posterior longitudinal ligament by posterior decompression].

OBJECTIVE: To investigate the prognostic factors for patients with thoracic ossification of the ligamentum flavum (OLF) and thoracic ossification of posterior longitudinal ligament (OPLL). METHODS: Clinical information of 83 patients suffering from thoracic OLF and OPLL was reviewed retrospectively from January 2006 to June 2010. The related factors such as gender, age, preoperative and postoperative Japanese Orthopedic Association (JOA) score, pathological segment, type of thoracic OPLL, degree of thoracic kyphosis, anteroposterior diameter of OPLL, range of circumferential decompression, cerebrospinal fluid leakage or not and dysfunction or not and carotid lumbar disorders or not were analyzed by Chi-square and Logistic regression. RESULTS: All cases were classified into desirable group (58 cases) and undesirable group (25 cases) based on the postoperative JOA score improvement rate. Comparison of physical characteristics between two groups of age, preoperative JOA and the course of the disease had not statistically significant (P > 0.05). Two groups in pathological segment of thoracic OPLL (χ(2) = 6.290, P = 0.043), the ossification type of OPLL (χ(2) = 5.361, P = 0.021) and dysfunction or not in preoperative (χ(2) = 27.711, P = 0.000) had significant difference. Logistic regression analysis showed that the upper thoracic segments (P = 0.044), beak type ossification (P = 0.023) and with dysfunction in preoperative (P = 0.009) were risk factors. There were 24 patients (28.9%) with cerebrospinal fluid leakage, 3 patients with early postoperative deep infection and neurological deterioration of 2 cases in postoperative. CONCLUSIONS: Patients with ossification on the upper section of thoracic have a better prognosis, but the beaked localized longitudinal ligament ossification in patients and associated with preoperative dysfunction show a poor prognosis, combined jumping segmental ossification and cervical or lumbar severe disorders are the influencing factor for poor prognosis.

CO oxidation over the polyoxometalate-supported single-atom catalysts M1/POM (Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; POM = [PW12O40]3-): a computational study on the activation of surface oxygen species.

The discrete anionic structure of polyoxometalates (POMs) at the interface is more like a separate small "island", which effectively prevents the diffusion of single atoms and prohibits the agglomeration and generation of metal particles; thus, POMs can enhance the sintering-resistant behavior and increase metal loading on the surface of single-atom catalysts (SACs). To explore the catalytic performance of POM-supported SACs for CO oxidation, we employed density functional theory (DFT) calculations to gain an understanding of some important aspects, including the CO adsorption, the formation of oxygen vacancies, and the activity of the surface oxygen species, of the catalytic system. Compared to previous theoretical studies, in which the catalytic behavior of POMs has been investigated based on the anionic unit with the highest negative charge, herein, we have constructed a model of the POM-supported SACs, which are neutral species. Our DFT calculations indicated that in the series of the SACs studied herein, (1) upon anchoring of a single metal atom on the POM surface, four key surface oxygen atoms were lifted from the POM surface to form a new interface, and thus, the surface oxygen species were activated; (2) CO adsorbed more strongly on the Ir, Os, Rh, Pt, and Ru sites than on the Fe, Mn, and Co sites; (3) it was easy to form an oxygen vacancy on the POM surface in the case of the Pt system when compared with the other systems; (4) the difference in the surface oxygen species for CO oxidation was remarkable, and the Oc atom at the catalyst interface had higher reactivity for CO oxidation as compared to the Ob atom in the Pt system studied herein; and (5) the single Pt atom served as an electron reservoir in the CO oxidation along the reaction pathway.