Silver-modified cyanamide-modified carbon nitride photocatalyst, preparation method and application thereof

Abstract

The application discloses a silver-modified cyanamide-modified carbon nitride photocatalyst and a preparation method and application thereof, and aims at solving the problems of the existing carbon nitride-based photocatalyst, such as serious photogenerated carrier recombination, insufficient surface active sites, weak NO and O2 adsorption and activation capacity, low NO removal efficiency and insufficient catalytic stability. The catalyst comprises a cyanamide and sulfur co-modified carbon nitride carrier and silver species fixed on the surface of the carrier, and the silver species is anchored on the surface of the carrier through N / S double coordination structures composed of Ag-N bonds and Ag-S-C bonds. The cyanamide and sulfur coordination sites are introduced by loading silver species first and then heat treatment with potassium thiocyanate, so that the interface charge migration and active oxygen species generation are promoted, and the photocatalytic purification efficiency, selectivity and cyclic stability of low-concentration NO are improved.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic air pollutant purification technology, specifically to a silver-modified cyanamide-based carbon nitride photocatalyst, its preparation method, and its application. Background Technology

[0002] Nitrogen oxides are a significant component of air pollution, with nitric oxide (NO) being a typical low-concentration gaseous pollutant and a major precursor to nitrogen dioxide, ozone, nitrate aerosols, and photochemical smog. NO emissions can occur from industrial combustion, vehicle exhaust, indoor gas combustion, and poorly ventilated environments. Given NO's low concentration, high diffusivity, and relatively high reactivity in the air, developing technologies capable of efficiently purifying low-concentration NO at ambient temperature and pressure is crucial for improving air quality and controlling nitrogen oxide pollution.

[0003] Currently, NO and NOx purification technologies mainly include selective catalytic reduction (SCR), adsorption, thermocatalytic oxidation, plasma treatment, and photocatalytic oxidation. Among these, SCR typically requires high reaction temperatures and relies on reducing agents and complex reaction apparatus; adsorption methods suffer from limited adsorption capacity, difficulties in adsorbent regeneration, and secondary treatment issues; thermocatalysis and plasma technologies have relatively high energy consumption, and their economic viability and applicability are limited in low-concentration pollutant treatment scenarios. In contrast, photocatalysis technology can utilize light energy to drive NO oxidation under mild conditions, offering advantages such as mild reaction conditions, low energy consumption, and applicability to the purification of low-concentration gaseous pollutants, thus attracting widespread attention.

[0004] Among existing photocatalytic NO removal materials, semiconductor photocatalysts such as titanium dioxide, bismuth oxide-based materials, tungstates, vanadates, and graphitic carbon nitride have been used for NO oxidation and removal. However, traditional titanium dioxide mainly responds to ultraviolet light, with low utilization of visible light; some metal oxide or halide oxide materials have issues related to cost, stability, or secondary environmental risks. Graphitic carbon nitride, with its abundant raw materials, stable structure, visible light response, and environmentally friendly constituent elements, is a promising non-metallic photocatalytic material. However, ordinary carbon nitride still suffers from limited specific surface area, severe recombination of photogenerated electrons and holes, insufficient surface reaction sites, and weak adsorption and activation capabilities for small molecules such as NO and O2, resulting in its photocatalytic NO removal efficiency and reaction selectivity still failing to meet practical application requirements. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a silver-modified cyanamide-based carbon nitride photocatalyst, its preparation method, and its application. By first loading silver species and then subjecting them to potassium thiocyanate heat treatment, cyanamide groups and sulfur coordination sites are introduced onto the surface of carbon nitride. This allows the silver species to form an N / S dual-coordination anchoring structure through Ag-N and Ag-SC bonds, thereby regulating the electronic structure and surface reaction sites of the catalyst, promoting interfacial charge migration and the generation of reactive oxygen species. This improves the photocatalytic purification efficiency, reaction selectivity, and cycle stability of low-concentration NO, thus solving the problems of limited specific surface area, severe recombination of photogenerated electrons and holes, insufficient surface reaction sites, and weak adsorption and activation capacity for small molecules such as NO and O2 when using carbon nitride as a photocatalytic material in existing technologies.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a silver-modified cyanamide-based carbon nitride photocatalyst, the catalyst comprising a cyanamide-based and sulfur-modified carbon nitride support and silver species immobilized on the surface of the support, the silver species being immobilized on the C3N4 surface through an N / S dual coordination structure.

[0007] Preferably, the silver species simultaneously forms Ag-N bonds with N atoms and Ag-SC bonds with S atoms; more preferably, the silver species are in the form of silver sub-nano clusters.

[0008] Secondly, the present invention provides a method for preparing the above-mentioned silver-modified cyanamide-based modified carbon nitride photocatalyst, the specific steps of which are as follows:

[0009] Step 1: Add silver nitrate to carbon nitride suspension and mix at 80℃~100℃ for 7h~9h. After drying, the product Ag-CN is obtained; wherein the mass ratio of silver nitrate to carbon nitride is (1~3):100.

[0010] Step 2: After thoroughly mixing the Ag-CN obtained in Step 1 and potassium thiocyanate, heat it at 350℃~450℃ for 0.5h~1.5h in an inert atmosphere, then heat it at 450℃~600℃ for 20min~40min. After cooling, wash and dry to obtain the catalyst Ag / NCN-CN; wherein the mass ratio of Ag-CN to potassium thiocyanate is 1:(1~2).

[0011] Preferably, the carbon nitride is prepared by the following steps:

[0012] Urea was heated to 500℃~600℃ at a heating rate of 4℃ / min~6℃ / min and held at that temperature for 3h~5h. After cooling to room temperature, the product was collected and ground into powder to obtain the carbon nitride.

[0013] Preferably, in step 1, the silver nitrate content in the carbon nitride suspension is 1wt% to 3wt% by mass percentage.

[0014] Preferably, in step 2, the temperature is increased to 350℃~450℃ at a heating rate of 5℃ / min~10℃ / min; and increased to 450℃~600℃ at a heating rate of 5℃ / min~10℃ / min.

[0015] Thirdly, the present invention provides an application of a silver-modified cyanamide-based modified carbon nitride photocatalyst, wherein the catalyst described above or the catalyst prepared by the above method is used to remove NO from the atmosphere under visible light irradiation.

[0016] Preferably, the removal rate of NO with a concentration ≥200 ppb is stably maintained above 60% under visible light irradiation, and the concentration of byproduct NO2 during the reaction is below 40 ppb.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The catalyst of the present invention has an N / S dual coordination structure composed of Ag-N and Ag-SC bonds, which enables silver species to be highly dispersed in the form of sub-nano clusters on the surface of carbon nitride, while forming a strong electronic coupling interface; electrons are transferred from the nitrogen and sulfur coordination sites in the carbon nitride support to the silver species, so that the silver is in an electron-rich state, which is conducive to capturing photogenerated electrons and activating adsorbed oxygen.

[0019] 2. The preparation method of this invention adopts a process sequence of first loading silver species and then performing potassium thiocyanate heat treatment. This allows the cyanamide species and sulfur atoms generated by the high-temperature decomposition of potassium thiocyanate to preferentially coordinate with the loaded silver species. At the same time, the heat treatment process achieves densification of the interlayer stacking of carbon nitride and improves crystallinity. This avoids the problem that metal atoms have difficulty entering the pre-constructed coordination sites and easily form large-sized nanoparticles in the conventional process of first modifying the support and then loading the metal. The preparation method of this invention is simple, easy to operate and controllable, suitable for industrial-scale production, and has good application prospects.

[0020] 3. The catalyst described in this invention exhibits a synergistic effect of strong interfacial dual coordination in its structure and optimized loading sequence in its preparation method, resulting in simultaneous improvements in light absorption range, photogenerated carrier separation efficiency, and active species generation capacity. This promotes the continuous oxidation of NO to nitrates, reduces the release of NO2 intermediates, and inhibits the aggregation or loss of silver species through strong anchoring, thereby enhancing the stability and reusability of the catalyst during NO photocatalytic purification. It is suitable for the photocatalytic purification of low-concentration NO in the air. Attached Figure Description

[0021] Figure 1In the image: a represents the XRD patterns of CN, Ag-CN, NCN-CN, and Ag / NCN-CN; b represents the corresponding FTIR spectra.

[0022] Figure 2 In the middle: ad are the SEM images of CN, Ag-CN, NCN-CN and Ag / NCN-CN respectively; e is the HRTEM image of Ag / NCN-CN (Ag clusters are marked with red circles); f is the elemental EDS mapping of Ag / NCN-CN.

[0023] Figure 3 In the middle: a) High-resolution C 1s XPS spectra of CN, NCN-CN, and Ag / NCN-CN; b) High-resolution N 1s XPS spectra of CN, NCN-CN, and Ag / NCN-CN; c) Comparison of high-resolution N 1s XPS spectra of CN and Ag-CN.

[0024] Figure 4 In the middle: a is the high-resolution Ag 3d XPS spectrum of Ag-CN and Ag / NCN-CN; b is the high-resolution S 2p XPS spectrum of NCN-CN and Ag / NCN-CN.

[0025] Figure 5 In the image: a) UV-vis DRS spectra of CN, Ag-CN, NCN-CN, and Ag / NCN-CN; b) XPS valence band spectrum; c) corresponding indirect bandgap Tauc plot; d) schematic diagram of band structure calculated based on valence band spectrum and bandgap; e) steady-state PL spectrum; f) time-resolved PL decay curve.

[0026] Figure 6 In the middle: a represents the DMPO content of the sample under visible light irradiation. - •O2 - ESR spectrum of the adduct; b is the DMPO of the sample under visible light irradiation. - ESR spectrum of OH adduct.

[0027] Figure 7 In the middle: a) NO removal efficiency of CN, Ag-CN, NCN-CN, and Ag / NCN-CN under visible light; b) NO removal efficiency of Ag / NCN-CN at different inlet NO concentrations; c) Photocatalytic NO oxidation cycle stability test of Ag / NCN-CN.

[0028] Figure 8 In the middle: a is the in-situ ATR-FTIR spectrum of NO and O2 of Ag / NCN-CN under visible light irradiation; b is the in-situ ATR-FTIR spectrum of NO adsorption photocatalytic oxidation of Ag / NCN-CN under dark conditions. Detailed Implementation

[0029] The present invention will be clearly and completely described in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the present invention are within the scope of protection of the present invention.

[0030] Unless otherwise specified in the specific context, the numerical ranges listed herein include upper and lower limits, as well as all integers and fractions within those ranges, and are not limited to the specific values ​​listed when the range is defined. While only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail. The terms “comprising,” “including,” “having,” “containing,” etc., as used herein are open-ended, meaning they include but are not limited to.

[0031] Unless otherwise specified, the experimental methods used in this invention are all conventional methods.

[0032] Unless otherwise specified, all materials and reagents used in this invention can be purchased or synthesized by known methods.

[0033] In the quantitative experiments of this invention, each experiment was repeated three times, and the average value of the results was taken.

[0034] I. A silver-modified cyanamide-based carbon nitride photocatalyst

[0035] To address the technical challenges of existing carbon nitride-based photocatalysts, such as the lack of strong coordination anchoring sites leading to easy migration and aggregation of active metal components, weak interfacial electronic coupling, and the inability of a single modification strategy to simultaneously achieve catalytic activity, product selectivity, and cycle stability, this invention proposes a process route involving first loading silver species followed by co-modification with cyanamide groups and sulfur. This constructs a dual-coordination environment containing both nitrogen and sulfur atoms on the carbon nitride surface. The nitrogen coordination sites provided by the cyanamide groups and the sulfur coordination sites introduced by doping jointly anchor the silver atoms, thereby forming a stable interfacial bonding structure between silver and the carbon nitride support. Based on this, the catalyst prepared in this invention consists of Ag clusters fixed on the C3N4 surface through an N / S dual-coordination structure. Specifically, Ag atoms simultaneously form Ag-N bonds with N atoms and Ag-SC bonds with S atoms.

[0036] The catalyst described in this invention has achieved many unexpected technical effects in practical applications: When applied to the treatment of low-concentration nitrogen oxide waste gas, under visible light irradiation, its removal efficiency for NO waste gas with a concentration of not less than 200 ppb can be consistently maintained above 60%, and its activity did not show significant decay during long-term continuous flow reaction tests, indicating that the active sites have extremely high structural stability; more importantly, in the process of deep oxidation of NO, the catalyst exhibits unique reaction pathway guidance, and the NO2 concentration detected in the reaction tail gas is always controlled below the safe threshold of 40 ppb. This means that the catalyst can preferentially convert nitric oxide into nitrates and enrich them on the surface, rather than causing the escape of highly toxic intermediate nitrogen dioxide due to over-oxidation or poor selectivity, as is the case with traditional catalysts. Thus, while achieving highly efficient purification, the risk of secondary pollution is completely eliminated.

[0037] II. A method for preparing silver-modified cyanamide-based carbon nitride photocatalysts

[0038] Step 1: Add silver nitrate to carbon nitride suspension and mix at 80℃~100℃ for 7h~9h. After drying, the product Ag-CN is obtained; wherein the mass ratio of silver nitrate to carbon nitride is (1~3):100.

[0039] Step 2: After thoroughly mixing the Ag-CN obtained in Step 1 and potassium thiocyanate, heat it at 350℃~450℃ for 0.5h~1.5h in an inert atmosphere, then heat it at 450℃~600℃ for 20min~40min. After cooling, wash and dry to obtain the catalyst Ag / NCN-CN; wherein the mass ratio of Ag-CN to potassium thiocyanate is 1:(1~2).

[0040] In some embodiments of the present invention, the carbon nitride is prepared by the following steps:

[0041] Urea was heated to 500℃~600℃ at a heating rate of 4℃ / min~6℃ / min and held at that temperature for 3h~5h. After cooling to room temperature, the product was collected and ground into powder to obtain the carbon nitride.

[0042] In some embodiments of the present invention, in step 1, the content of silver nitrate in the carbon nitride suspension is 1wt% to 3wt% by mass percentage.

[0043] In some embodiments of the present invention, in step 1, the mass ratio of silver nitrate to carbon nitride is (1~3):100. This ratio ensures that silver species are highly dispersed as sub-nano clusters, rather than forming large nanoparticles. When the amount of silver nitrate is too low, the silver species coverage is insufficient, resulting in a low density of sub-nano clusters in the final catalyst, a reduction in photogenerated electron capture sites, and limited catalytic activity. When the amount of silver nitrate is too high, the silver atom concentration is too high, exceeding the anchoring ability of cyanamide groups and sulfur sites, leading to the formation of large silver particles or sintering during heat treatment, reducing atom utilization efficiency.

[0044] In some embodiments of the present invention, in step 1, mixing is performed at 80°C to 100°C for 7 to 9 hours. Temperature control during this time is beneficial for preparing the precursor, the purpose of which is to allow Ag to... + Ions are fully adsorbed on the surface of carbon nitride while avoiding damage to the carbon nitride framework. Too low a temperature will cause Ag... + Insufficient ion diffusion kinetics lead to uneven adsorption and inconsistent precursor distribution, affecting the consistency of interfacial reactions during subsequent heat treatment. Excessively high temperatures cause dehydration or desorption of oxygen / nitrogen functional groups (such as -OH, -NH2) on the carbon nitride surface, reducing initial adsorption sites for silver species and weakening subsequent binding strength with cyanamide groups. Conversely, excessively short treatment times prevent the adsorption process from reaching equilibrium, leaving residual Ag in the solution. + Drying may cause silver salts to accumulate locally on the carrier surface, resulting in uneven, large particles after heat treatment; excessively long treatment times may cause some Ag to be affected. + Excessive reduction or ripening can lead to the formation of unstable silver clusters, increasing the difficulty of subsequent dispersion.

[0045] In some embodiments of the present invention, in step 2, the temperature is maintained at 350℃~450℃ for 0.5h~1.5h in an inert atmosphere. This stage corresponds to the decomposition of KSCN and the grafting of cyanamide groups, causing KSCN to decompose and generate cyanamide species (-NC≡N) and active sulfur species, thus constructing an N / S co-modified support environment. If the temperature is too low, the KSCN decomposition will be incomplete, and the introduction of cyanamide groups will be insufficient, resulting in a weak anchoring ability of the support for silver and an inability to effectively induce electron transfer from N / S atoms to Ag. If the temperature is too high, the carbon nitride framework will be over-denitrified, destroying the conjugated structure of the support and potentially causing early migration of silver species. If the time is too short, the amount of cyanamide group grafting will be insufficient, resulting in uneven surface modification. If the time is too long, it may lead to over-etching, increasing the specific surface area but decreasing the crystallinity, and the sulfur species may excessively cover the surface active sites. Then, the temperature is maintained at 450℃~600℃ for 20min~40min. This stage is crucial for the formation of Ag-NC / Ag-SC interfacial bonds. This temperature promotes strong electron interactions between the silver species and the support, forming unique electron-rich silver clusters (Ag). δ-Instead of traditional Ag-N or Ag-S chemical bonds, silver exhibits a unique characteristic: insufficient interfacial activation energy prevents strong electronic coupling between silver species and the support, resulting in no significant negative shift in the Ag 3d spectrum and predominantly physical adsorption, leading to poor stability. Excessively high temperatures cause the breakage of carbon nitride tris-triazine units and excessive kinetic energy of silver atoms, resulting in severe aggregation and disrupting the sub-nanometer dispersion. Insufficient holding time leads to inadequate interfacial electronic coupling, incomplete establishment of the electron transfer chain (N 1s positive shift, Ag 3d negative shift), and low photogenerated carrier separation efficiency. Excessively long holding time results in thermodynamically driven migration of silver atoms, causing sub-nanometer clusters to grow and reducing specific surface area.

[0046] In some embodiments of the present invention, in step 2, the mass ratio of Ag-CN to potassium thiocyanate is 1:(1~2), ensuring that KSCN can provide sufficient cyanamide and sulfur sources to modify the support while avoiding excessive residue. A ratio that is too low (<1:1) results in insufficient modification, failing to build a sufficient N / S coordination environment around the silver species, leading to weak silver anchoring; however, a ratio that is too high (>1:2) may cause excessive KSCN decomposition to produce a carbon-sulfur impurity layer covering the carbon nitride surface, hindering the transfer of photogenerated carriers to the silver clusters, and simultaneously blocking the pores and affecting NO adsorption.

[0047] In some embodiments of the present invention, in step 2, the temperature is increased to 350°C to 450°C at a heating rate of 5°C / min to 10°C / min; and further increased to 450°C to 600°C at a heating rate of 5°C / min to 10°C / min. By controlling the heating rate, the synchronicity of KSCN decomposition and silver cluster formation is controlled. If the heating rate is too slow, the residence time in the low-temperature section is prolonged, which may lead to the volatilization and loss of KSCN decomposition products, and silver species are easily oxidized or lost at low temperatures. If the heating rate is too fast, it causes violent decomposition of KSCN, generating a large number of bubbles or localized overheating, destroying the layered structure of the carrier, and resulting in uneven silver distribution.

[0048] III. Application of a Silver-Modified Cyanamide-Based Carbon Nitride Photocatalyst

[0049] The catalyst described in this invention or the catalyst prepared by the method described herein is used to remove NO from the atmosphere when irradiated with visible light.

[0050] In some embodiments of the present invention, the removal rate of NO with a concentration ≥200 ppb under visible light irradiation is stably maintained at over 60%, and the concentration of the byproduct NO2 during the reaction is less than 40 ppb.

[0051] IV. Examples and Comparative Examples

[0052] Example 1

[0053] Step 1: Preparation of CN: Carbon nitride was synthesized by urea thermal polymerization. 10 g of urea was weighed and placed in an alumina crucible, covered, and placed in a muffle furnace. The crucible was heated to 500 °C at a heating rate of 5 °C / min and held at this temperature for 4 h. After natural cooling to room temperature, the resulting light yellow product was ground into a fine powder to obtain carbon nitride, denoted as CN.

[0054] Step 2: Preparation of Ag-CN: Take 2 g of CN and add it to distilled water to form a suspension. Add AgNO3 to the suspension to make the mass fraction of AgNO3 in the suspension 2%. Stir the mixture at 90℃ for 8 h. Take it out and dry it in an oven to obtain the product Ag-CN.

[0055] Step 3: Preparation of Ag / NCN-CN: Using the Ag-CN prepared in Step 2 as raw material, take 1g of Ag-CN and 1.5g of KSCN and grind them thoroughly in a mortar. Place the ground sample in a tube furnace and heat it to 400℃ at a rate of 6℃ / min in a nitrogen atmosphere and maintain it for 1h. Then heat it to 500℃ at a rate of 6℃ / min and maintain it for 0.5h. After natural cooling, a green product is obtained. Wash and dry it and name it Ag / NCN-CN.

[0056] Example 2

[0057] The process is basically the same as in Example 1, except that: in step 2, 1 g of CN is added to distilled water to form a suspension, and 0.01 g of AgNO3 is added to the suspension so that the mass ratio of AgNO3 to CN is 1:100; in step 3, 1 g of Ag-CN and 1 g of KSCN are thoroughly ground, heated to 350°C at 5°C / min under a nitrogen atmosphere and maintained for 0.5 h, then heated to 450°C at 5°C / min and maintained for 20 min, cooled, washed and dried to obtain the silver-modified cyanamide-based modified carbon nitride photocatalyst.

[0058] Example 3

[0059] The process is basically the same as in Example 1, except that: in step 2, 1 g of CN is added to distilled water to form a suspension, and 0.03 g of AgNO3 is added to the suspension so that the mass ratio of AgNO3 to CN is 3:100; in step 3, 1 g of Ag-CN and 2 g of KSCN are thoroughly ground, heated to 450°C at 10°C / min under a nitrogen atmosphere and maintained for 1.5 h, then heated to 600°C at 10°C / min and maintained for 40 min, cooled, washed and dried to obtain the silver-modified cyanamide-based modified carbon nitride photocatalyst.

[0060] Comparative Example 1

[0061] The CN prepared in Example 1 was used as a raw material, and cyanamide groups were introduced through KSCN. The specific method was as follows: 1g of CN and 1.5g of KSCN were added to a mortar and ground thoroughly. The ground sample was placed in a tube furnace and heated to 400°C in a nitrogen atmosphere and maintained for 1 hour, then heated to 500°C and maintained for 0.5 hours. After natural cooling, a green product was obtained, which was washed, dried and named NCN-CN.

[0062] Comparative Example 2

[0063] The Ag-CN prepared in step 2 of Example 1 was used as Comparative Example 2. Specifically, 1 g of CN was added to distilled water to form a suspension, and 0.02 g of AgNO3 was added to the suspension to make the mass ratio of AgNO3 to CN 2:100. The mixture was stirred at 90°C for 8 hours, and then dried in an oven to obtain Ag-CN. This comparative example only loaded silver species onto the CN surface by impregnation, without KSCN heat treatment to introduce cyanamide groups and sulfur coordination sites, to illustrate the role of the N / S dual coordination structure in anchoring silver species and interfacial electronic coupling.

[0064] V. Performance Analysis

[0065] The catalysts prepared in the examples can all be used to remove NO from the atmosphere under visible light irradiation. Specifically, they can all achieve a stable removal rate of over 60% for NO concentrations ≥200 ppb under visible light irradiation, and the concentration of the byproduct NO2 during the reaction is below 40 ppb. Therefore, in order to explore the catalytic performance of the catalysts, only Example 1 and the comparative example are used as controls for explanation here.

[0066] 1. Morphological and structural characteristics

[0067] The crystal structure of each sample was analyzed by X-ray diffraction (XRD). Figure 1 As shown in Figure a, the original CN exhibits characteristic diffraction peaks at 13.2° and 27.5°. After KSCN heat treatment, the (100) diffraction peak disappeared in the XRD patterns of NCN-CN and Ag / NCN-CN, while the (002) diffraction peak shifted to higher angles and became sharper. Most importantly, no diffraction peaks belonging to Ag or Ag₂S were observed in any of the samples. This demonstrates that the preparation method described in this invention successfully optimized the crystallinity of carbon nitride, and that the silver species in the catalyst prepared by this invention do not exist in the form of large-sized crystal particles (such as metallic silver or silver sulfide), but are anchored on the support in a highly dispersed state, which is beneficial for exposing more active sites.

[0068] Fourier transform infrared spectroscopy (FTIR) was used to analyze the surface functional groups and chemical bonding of the samples. Figure 1As shown in b, all samples retained the characteristic skeletal vibrational peaks of g-C3N4. Compared to CN and Ag-CN, NCN-CN and Ag / NCN-CN showed peaks at 2178 cm⁻¹. -1 A new absorption peak appeared, and the peak intensity was stronger in Ag / NCN-CN. This directly proves that the preparation method described in this invention successfully introduced the cyanamide group (-C≡N) onto the carbon nitride surface. The enhanced peak intensity in Ag / NCN-CN proves that the introduction of silver species further enhanced the characteristics of the cyanamide group, indicating a strong interaction between silver and the cyanamide group, forming a stable coordination environment.

[0069] The microstructure of four samples was observed using scanning electron microscopy (SEM). Figure 2 As shown in a-2d, after KSCN heat treatment, the material changes from a bulk structure to loose, fine particles. Figure 2 The image shows that a large number of high-contrast nanoparticles are uniformly distributed on the Ag / NCN-CN surface. This indicates that the present invention effectively disrupts the layered structure of carbon nitride through the method described, forming a particulate morphology that is more conducive to reactant diffusion. More importantly, silver species are highly dispersed on the support surface in ultra-small sizes (sub-nanometer clusters), and this morphological feature greatly improves the utilization rate of silver atoms and catalytic activity.

[0070] The elemental distribution of Ag / NCN-CN was analyzed using energy-dispersive X-ray spectroscopy (EDS) surface scanning. Figure 2 As shown in f, C, N, S, and Ag elements are uniformly distributed in Ag / NCN-CN, with the distribution positions of Ag (blue) and S (pink) elements highly overlapping. This indicates a close spatial correlation between sulfur and silver species, visually confirming the strong interfacial bond between silver and sulfur sites. This structure helps stabilize silver clusters and prevents them from being lost or agglomerated during the reaction.

[0071] 2. Surface chemical state analysis

[0072] The chemical states of elements on the sample surface were precisely analyzed using X-ray photoelectron spectroscopy. Figure 3 High-resolution C 1s and N 1s spectra of CN, NCN-CN, and Ag / NCN-CN are presented, with the N 1s spectrum of Ag-CN included as a control to clarify the respective contributions of Ag loading and cyanamide functionalization. Figure 3 As shown in figure a, both NCN-CN and Ag / NCN-CN exhibit a new shoulder peak at 286.5 eV. Figure 3As shown in b, a new peak (399.0 eV) belonging to the cyanamide group (-NC≡N) appeared in the N 1s spectrum of Ag / NCN-CN, and compared with Ag-CN, the N 1s peak position of Ag / NCN-CN underwent a systematic binding energy shift. This proves that the preparation method described in this invention successfully introduced the cyanamide group. The shift of the N 1s peak in Ag / NCN-CN proves that electrons have been transferred from the carbon nitride framework to the silver species, forming Ag-N coordination bonds. This strong interaction is the key to stabilizing silver sub-nano clusters and is also an essential characteristic that distinguishes it from simple physical adsorption (such as Ag-CN).

[0073] The chemical states of silver species in the sample were further analyzed using Ag 3d high-resolution XPS spectroscopy. Figure 4 As shown in figure a, the Ag 3d peak of Ag / NCN-CN is negatively shifted by 0.7 eV compared to Ag-CN, and the peak shape becomes sharper. Figure 4 As shown in b, the S 2p peak of Ag / NCN-CN is located at 163.8 eV, rather than the typical binding energy of Ag2S (~161.5 eV). This indicates that the negative shift of Ag 3d proves that silver is in an electron-rich state (Ag2S). δ- This is beneficial for activating oxygen molecules. The binding energy position of S 2p clearly excludes the formation of Ag2S, confirming that silver and sulfur form an Ag-SC interfacial bond structure, further demonstrating that the present invention constructs a unique N / S dual coordination interface, enabling silver to exist stably in the form of sub-nanometer clusters.

[0074] 3. Optical properties and carrier dynamics analysis

[0075] The light absorption characteristics of the sample were investigated using ultraviolet-visible diffuse reflectance spectroscopy. Figure 5 a) The absorption edge of Ag-CN is basically the same as that of CN, indicating that pure Ag... + Impregnation has a limited effect on the intrinsic light absorption of carbon nitride. After KSCN treatment, the absorption edge of NCN-CN shows a significant red shift, and the absorption intensity is significantly improved throughout the visible light region (450-700 nm). This is attributed to the introduction of defect energy levels in the band gap by the cyanamide groups, which expands the photoresponse range. Ag / NCN-CN further exhibits the strongest visible light trapping ability, with its absorption intensity in the visible light region surpassing that of NCN-CN. This enhancement can be attributed to the localized surface plasmon resonance effect of Ag sub-nano clusters and the charge transfer transition induced by Ag-N coordination. The valence band top position of each sample was determined by XPS valence band spectroscopy. Figure 5b). Relative to the standard hydrogen electrode, the valence band peak of CN is located at 1.92 eV. The valence band peak of Ag-CN shifts positively to 2.11 eV, while that of NCN-CN shifts significantly negatively to 1.46 eV, reflecting the band-gap modulation effect of the cyanamide group as a strong electron-withdrawing group. The valence band peak of Ag / NCN-CN shifts further positively to 2.32 eV, 0.86 eV higher than that of NCN-CN. Indirect band gap diagram calculated based on the Kubelka-Munk function ( Figure 5 c) shows that the optical band gaps of CN, Ag-CN, NCN-CN, and Ag / NCN-CN are 2.59 eV, 2.54 eV, 2.48 eV, and 2.41 eV, respectively. The gradual narrowing of the band gap perfectly matches the redshift trend of the absorption edge, confirming the synergistic effect of the introduction of the cyanamide group and Ag loading on the band structure, which is beneficial to exciting more photogenerated carriers. The conduction band bottom position is calculated based on the optical band gap, resulting in a schematic diagram of the band structure (…). Figure 5 d). The CN conduction band bottom is located at -0.67 eV, Ag-CN shifts down to -0.43 eV, NCN-CN shifts up to -1.02 eV, and Ag / NCN-CN shifts down to -0.09 eV. This unique band arrangement gives Ag / NCN-CN both strong oxidizing power (correcting the valence band) and moderate reducing power.

[0076] Steady-state photoluminescence spectroscopy was used to study carrier recombination behavior. Figure 5 e). CN exhibits the strongest emission peak at 449 nm, originating from radiative recombination caused by high-density structural defects. The Ag-CN emission peak blue-shifts to 448 nm, with a decrease in intensity, indicating that Ag... + It has some inhibitory effect on recombination, but the effect is limited. The emission peak of NCN-CN redshifts to 466 nm and the intensity decreases significantly. The redshift is consistent with the narrowing of the band gap, and the decrease in intensity indicates that the cyanamide group, as an electron acceptor, provides an additional transfer channel for photogenerated electrons. The emission peak of Ag / NCN-CN redshifts to 458 nm, but its PL intensity is slightly higher than that of NCN-CN but much lower than that of CN. This phenomenon needs to be discussed in detail in conjunction with time-resolved results. Time-resolved photoluminescence spectral fitting analysis ( Figure 5f) shows that the average fluorescence lifetimes of CN, Ag-CN, NCN-CN, and Ag / NCN-CN are 8.94 ns, 7.91 ns, 0.83 ns, and 0.92 ns, respectively. The lifetime of NCN-CN is nearly an order of magnitude shorter than that of CN, which is attributed to the large number of structural defects introduced by the cyanamide group, which act as non-radiative recombination centers to accelerate carrier quenching. This is completely consistent with the appearance of the new peak at 399.0 eV and the decrease in PL intensity in XPS. The lifetime of Ag / NCN-CN is slightly higher than that of NCN-CN (+0.09 ns), yet it exhibits the best photocatalytic activity. This seemingly contradictory phenomenon contains a profound mechanism: atomically dispersed Ag sub-nano clusters preferentially coordinate with NCN-CN surface defect sites to form Ag-N and Ag-SC bonds, partially passivating deep-level defects and reducing non-radiative recombination centers, thus slightly extending the photocatalytic lifetime. However, Ag also acts as a highly efficient electron trap, rapidly capturing photogenerated electrons and transferring them to the surface to participate in the reaction. This charge separation effect far outweighs the small contribution of defect passivation, thus macroscopically exhibiting the highest activity. The lifetime difference revealed by TRPL, combined with the electron transfer confirmed by XPS, jointly outlines the unique carrier dynamics of Ag / NCN-CN: photogenerated electrons are rapidly captured and transferred to Ag clusters, and efficiently injected into surface reaction sites through the Ag-N / Ag-S interface. Therefore, it can be concluded that the introduction of cyanamide groups narrows the band gap, expands the light absorption range, and introduces a non-radiative recombination channel; the loading of Ag sub-nano clusters further optimizes the band structure and achieves efficient charge separation and transfer through the Ag-N / Ag-S interface. These synergistic effects lay a solid physical foundation for the significant improvement in the photocatalytic NO oxidation performance.

[0077] Using electron spin resonance technology and DMPO as a spin trapping agent, the ability of various samples to generate reactive oxygen species under visible light excitation was detected. Figure 6 As shown, no signal was detected in the dark, but all samples produced characteristic ESR responses after illumination. For DMPO-•O2 - adducts ( Figure 6 a) CN only exhibits a weak sextet characteristic peak, reflecting its O2 - The generation capacity is limited. The signal intensity of NCN-CN is significantly enhanced, thanks to the optimized band structure and enhanced charge separation following the introduction of the cyanamide group (see TRPL results). Ag / NCN-CN exhibits the strongest DMPO-•O2. - The signal strength is approximately 5 times that of CN and 2 times that of NCN-CN. The intensity trend of the DMPO-•OH signal is similar to that of •O2. - exactly the same ( Figure 6 b), that is, Ag / NCN-CN > NCN-CN > CN, but the •O2 of all samples - The signals are all stronger than •OH, confirming the presence of •O2.- It is the dominant active species in the photocatalytic process. The enhanced ability of active species to generate is inseparable from the efficient charge separation and transfer of Ag / NCN-CN: the electron transfer channel at the Ag-N / Ag-S interface, confirmed by XPS, enables photogenerated electrons to be rapidly enriched in Ag clusters, and then captured by adsorbed O2 to generate •O2. - Simultaneously, the band structure regulated by the cyanamide group (corrected valence band) also moderately promoted the generation of •OH. This result provides direct evidence at the level of active species for subsequent optimization of the photocatalytic NO oxidation performance.

[0078] 4. Photocatalytic NO oxidation performance and mechanism

[0079] The NO oxidation performance of each sample under visible light-driven conditions (600 ppb, 50% relative humidity) was evaluated in a continuous flow reactor, and the results are as follows: Figure 7 As shown in figure a, the original CN removed only 15.6% of NO, due to its rapid carrier recombination (TRPL lifetime 8.94 ns). The removal rate of Ag-CN was improved to 37.5%, attributed to Ag. + While it promotes charge separation to some extent (TRPL 7.91 ns), weak interfacial interactions limit the effect. NCN-CN further increased to 43.6%, thanks to the comprehensive optimization of band structure, charge separation, and adsorption capacity brought about by the introduction of cyanamide groups. Ag / NCN-CN achieved the highest removal rate of 68.1%, which is 4.3 times higher than CN, due to the synergistic effect of Ag sub-nano clusters and cyanamide-functionalized carrier: the strong coupling interface efficiently captures photogenerated electrons, the LSPR effect enhances light absorption, and the enriched electrons activate O2 to generate a large amount of •O2. - (Strongest ESR), while partially passivating deep-level defects to optimize carrier dynamics. Ag / NCN-CN adapts well to concentration fluctuations. Figure 7 (b) Within the range of 200-600 ppb, the removal rate remained stable at over 60%. After six cycles of testing, the removal rate still remained at 62.5%, decreasing by only about 5.6 percentage points. Figure 7 c) The strong anchoring of Ag clusters effectively inhibited the loss of active components. The concentration of byproduct NO2 was below 40 ppb throughout the test, far exceeding the safety threshold (53 ppb).

[0080] The dynamic process of NO oxidation on the Ag / NCN-CN surface was tracked in real time using in-situ attenuated total reflectance Fourier transform infrared spectroscopy. Under a continuously purged atmosphere of mixed NO and O2, spectra were collected for the dark adsorption and visible light irradiation stages to reveal the evolution of reaction intermediates. (Under dark conditions...) Figure 8 a) Multiple infrared absorption peaks gradually appear on the Ag / NCN-CN surface, indicating its strong adsorption capacity for NO and related species. Located at 1556 cm⁻¹-1 The peak is attributed to adsorbed NO molecules; 1401 cm⁻¹ -1 The peaks at 1190, 1150, and 997 cm⁻¹ correspond to NO₂ species; -1 One set of peaks is attributed to nitrite (NO2). - ); 1263cm -1 The peak at that point is attributed to nitrate (NO3). - NO, NO2, and NO2 were detected simultaneously in the dark. - The characteristic peaks of NO⁻ and NO₃⁻ indicate a complex adsorption and spontaneous oxidation process on the Ag / NCN-CN surface. This phenomenon stems from the abundant surface sites provided by the cyanamide groups and Ag clusters (XPS confirmed Ag-N / Ag-S coordination), which enhance the co-adsorption of NO and O₂. Furthermore, the Ag clusters may weakly activate O₂ even under light-free conditions, initiating partial NO oxidation. - and NO3 - The presence of these substances indicates that preliminary oxidation has occurred during the adsorption stage, which is beneficial for the rapid initiation of subsequent photocatalytic reactions.

[0081] After turning on visible light irradiation ( Figure 8 (b) The infrared spectrum underwent a significant evolution. The characteristic peaks of nitrate were markedly enhanced, and several split peaks appeared: 1321, 1307, 1231, and 1272 cm⁻¹. -1 NO3 belonging to different coordination configurations - Species; 1462 cm -1 The corresponding NO2 species; 1556 cm -1 The intensity of the NO adsorption peak at that location did not change significantly. The key observation points are: the nitrate peaks after illumination (especially at 1272, 1307, and 1321 cm⁻¹). -1 The intensity increased rapidly, while the intermediate NO2 (1462 cm) -1 The signal intensity of the Ag / NCN-CN sample is relatively weak and its growth is gradual. This indicates that the intermediate generated on the Ag / NCN-CN surface can be rapidly converted into the final product nitrate, and the reaction pathway is continuous and complete. Combining the infrared spectral evolution of dark-state adsorption and illumination processes, the NO oxidation pathway on the Ag / NCN-CN surface can be deduced: NO molecules first adsorb onto surface active sites (Ag clusters, cyanamide groups), and are partially weakly oxidized to NO2. - With NO2; under illumination, photogenerated electrons rapidly transfer to Ag clusters through the Ag–N / Ag–S interface, activating adsorbed O2 and generating a large amount of •O2. - •O2 - The adsorbed NO and its intermediates are rapidly oxidized to the final product NO3. - This path can be summarized as NO → NO2. - / NO2→ NO3 -.

[0082] In-situ infrared results and photocatalytic activity (68.1% removal rate), ESR (•O2) - The results of the study (dominant) and XPS (Ag–N / Ag–S interface electron transfer) are highly consistent, revealing the microscopic mechanism of efficient NO oxidation in Ag / NCN-CN: the coordination environment constructed by cyanamide groups stabilizes Ag sub-nano clusters, forming a strong electronic coupling interface; photogenerated electrons are rapidly captured by Ag clusters and transferred to the surface, activating O2 to generate •O2. - ; Adequate •O2 - Drives the continuous oxidation of NO to NO3 - It results in less intermediate accumulation and lower release of NO2 byproduct.

[0083] This invention successfully constructed a cyanamide-functionalized carbon nitride (Ag / NCN-CN) photocatalyst anchored in Ag sub-nano clusters. The introduction of cyanamide groups not only provides an N / S coordination environment but also narrows the band gap and enhances visible light absorption. Ag, in the form of sub-nano clusters with an average size of 0.943 nm, is highly dispersed on the support surface through Ag-N and Ag-SC bonds, forming a strong electronic coupling interface. The negative shift of Ag 3d, positive shift of N 1s, and blue shift of S 2p in XPS constitute a complete chain of evidence for electron transfer, confirming electron enrichment from the support to Ag. This unique interface structure allows photogenerated electrons to be rapidly captured and transferred to the Ag clusters, efficiently activating O2 to generate •O2⁻ and driving the continuous oxidation of NO to NO3⁻. Ag / NCN-CN achieved a NO removal rate of 68.1% under visible light, and its activity remained above 62% after six cycles, exhibiting excellent activity, stability, and selectivity. This invention provides a new approach for the design of carbon nitride cluster catalysts.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A silver-modified cyanamide-based carbon nitride photocatalyst, characterized in that, The catalyst comprises a cyanamide- and sulfur-modified carbon nitride support and silver species immobilized on the surface of the support, wherein the silver species are immobilized on the C3N4 surface through an N / S dual coordination structure.

2. The catalyst according to claim 1, characterized in that, The silver species simultaneously forms Ag-N bonds with N atoms and Ag-SC bonds with S atoms.

3. A method for preparing the silver-modified cyanamide-based modified carbon nitride photocatalyst according to claim 1 or 2, characterized in that, The specific steps are as follows: Step 1: Add silver nitrate to carbon nitride suspension and mix at 80℃~100℃ for 7h~9h. After drying, the product Ag-CN is obtained; wherein the mass ratio of silver nitrate to carbon nitride is (1~3):

100. Step 2: After thoroughly mixing the Ag-CN obtained in Step 1 and potassium thiocyanate, heat it at 350℃~450℃ for 0.5h~1.5h in an inert atmosphere, then heat it at 450℃~600℃ for 20min~40min. After cooling, wash and dry to obtain the catalyst Ag / NCN-CN; wherein the mass ratio of Ag-CN to potassium thiocyanate is 1:(1~2).

4. The method according to claim 3, characterized in that, In step 1, the silver nitrate content in the carbon nitride suspension is calculated to be 1 wt% to 3 wt% by mass percentage.

5. The method according to claim 3, characterized in that, In step 2, the temperature is increased to 350℃~450℃ at a heating rate of 5℃ / min~10℃ / min; and increased to 450℃~600℃ at a heating rate of 5℃ / min~10℃ / min.

6. The application of a silver-modified cyanamide-based carbon nitride photocatalyst, characterized in that, The catalyst according to claim 1 or 2, or the catalyst prepared by the method according to any one of claims 3 to 5, is used to remove NO from the atmosphere under visible light irradiation.

7. The application according to claim 6, characterized in that, Under visible light irradiation, the removal rate of NO with a concentration ≥200 ppb remained stable at over 60%, and the concentration of the byproduct NO2 during the reaction was below 40 ppb.

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