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Ing panyulung ning teknolohiya ning hydrogen atyu ya keng pusu ning green economy. Bilang precondition para abalu ing hydrogen storage, kailangan la reng aktibung at matatag a catalysts para keng hydrogenation (de)hydrogenation reaction. Angga ngeni, ing lugal ayni metung yang dominadu ning pamangamit kareng mangamal a metal. keni, magpropose kami keng bayung low-cost cobalt-based catalyst (Co-SAs/NPs@NC) nung nukarin ing matas a distributed a single-metal sites synergistically makayabe la kareng fine nanoparticles para akwa ing efficient a formic acid dehydrogenation. Gamit ing pekamasanting a materyales ning atomically dispersed CoN2C2 units ampo encapsulated nanoparticles a 7-8 nm a sukad, gamit ing propylene carbonate bilang solvent, metung a masanting a gas production a 1403.8 ml g-1 h-1 ing mekwa, at alang pangawala kaybat ning 5 cycles. aktibidad, na 15 besis mas masanting kesa keng commercial Pd/C. Ing in situ assay experiments papakit na, kumpara kareng makasuglung a single metal atom ampo nanoparticle catalysts, ing Co-SAs/NPs@NC papaslag ne ing adsorption ampo ing pamag-activate ning key monodentate intermediate HCOO*, inya sa’t susulung ne ing tutuking CH bond cleavage. Deng theoretical a kalkulasyun papakit da na ing pamisanmetung da reng cobalt nanoparticles susulung ne ing pamagbayu ning d-band center ning metung a Co atom papunta king metung a aktibung lugal, inya sa’t mas masanting ing pamisanmetung ning carbonyl O ning HCOO* intermediate ampo ing Co center, inya sa’t bababa ing energy barrier.
Ing hydrogen metung yang importanting energy carrier para keng kasalukuyan a global energy transition ampo malyari yang maging susi para keng carbon neutrality1. Uling kareng kayang pisikal a katangian antimo ing pangasilab at mababang densidad, ligtas at masanting a pamitipun at pamagdala king hydrogen ila ring susi king pamag-realize king ekonomiya ning hydrogen2,3,4. Deng liquid organic hydrogen carriers (LOHCs), na magtipun ampo magpalwal hydrogen kapamilatan da reng chemical reactions, mipanukala la bilang solusyun. Nung ikumpara la keng molecular hydrogen, deng substansya (methanol, toluene, dibenzyltoluene, atbp.) madali la at manyaman lang gamitan5,6,7. Kareng miyayaliwang tradisyunal a LOHC, ing formic acid (FA) atin yang mababang toxicity (LD50: 1.8 g/kg) ampo H2 capacity a 53 g/L o 4.4 wt%. Ing FA yamu ing LOHC na malyaring mag-store ampo mag-release hydrogen keng mild a kundisyun keng presensya da reng suitable catalysts, inya e na kailangan ing maragul a external energy inputs1,8,9. Ing tutu na niti, dakal lang noble metal catalysts ing megawa para keng dehydrogenation ning formic acid, alimbawa, deng palladium-based catalysts 50-200 besis lang mas aktibo kesa kareng mura nang metal catalysts10,11,12. Makanyan man, nung isipan me ing presyu da reng aktibung metal, alimbawa, ing palladium maigit yang 1000 a besis mas mamasa.
Cobalt, Ing pamanintun kareng matas a aktibu ampo matatag a heterogeneous base metal catalysts patuloy yang makaakit interes kareng dakal a talapanaliksik keng academia ampo industriya13,14,15.
Agyang deng mura a catalysts a makabasi keng Mo ampo Co, ampo reng nanocatalysts a gawa keng noble/base metal alloys,14,16 megawa la para keng FA dehydrogenation, ing karelang gradual deactivation keng reaction e maliaring iwasan pauli ning pamag-okupa kareng aktibung lugal da reng metal, CO2, ampo H2O kareng proton. o formate anions (HCOO-), FA contamination, particle aggregation ampo posibling CO poisoning17,18. Ikami ampo reng aliwa bayu-bayu pa mu pepakit mi na deng single-atom catalysts (SACs) a maki matas a dispersed CoIINx sites bilang aktibung lugal papasanting da ing reactivity ampo ing acid resistance ning formic acid dehydrogenation kumpara kareng nanoparticles17,19,20,21,22,23,24. Kareng Co-NC a materyales a reni, deng N atom magsilbi lang manimunang lugal para isulong ing FA deprotonation kabang papaslag ing katatagan ning istruktura kapamilatan ning pamisanmetung king sentrung Co atom, kabang deng Co atom magsilbi lang H adsorption sites at isulong ing CH22 scission, 25,26. Ing makalungkut, ing aktibidad ampo ing katatagan da reng catalysts a reni marayu la pa kareng modernung homogeneous ampo heterogeneous noble metal catalysts (Fig. 1) 13 .
Ing sobrang enerhiya ibat kareng renewable a pikuanan antimo ing solar o angin malyari yang gawan kapamilatan ning electrolysis ning danum. Ing hydrogen a megawa malyari yang i-store gamit ing LOHC, metung a likidu na ing kayang hydrogenation ampo dehydrogenation malyari yang ibalik. King dehydrogenation step, ing bukud mung produktu ya pin ing hydrogen, at ing carrier liquid mibalik ya king kayang sadyang kabilian at mi-hydrogenate yang pasibayu. Ing hydrogen malyari yang gamitan kareng gasolinahan, baterya, gusali pang-industriya, ampo aliwa pa.
Bayu-bayu pa mu, mibalita na ing intrinsic activity da reng specific SACs malyari yang miparangalan keng presensya da reng miyayaliwang metal atoms o karagdagang metal sites a bibie da reng nanoparticles (NPs) o nanoclusters (NCs)27,28. Iti magbukas yang posibilidad para keng karagdagang adsorption ampo activation ning substrate, ampo para keng modulation ning geometry ampo electronic structure da reng monatomic sites. Uli na niti, ing substrate adsorption/activation malyari yang ma-optimize, a mamye mas masanting a pangkabilugan a catalytic efficiency29,30. Iti mamye yang ideya kekatamu para gawan ing ustung catalytic a materyales a maki hybrid active sites. Agyang deng me-improve a SACs pepakit da ing maragul a potensyal keng dakal a catalytic applications, ing karelang papil keng hydrogen storage, keng kekaming beluan, e malino. keng bage ayni, magreport kami keng metung a versatile ampo robust a diskarti para keng synthesis da reng cobalt-based hybrid catalysts (Co-SAs/NPs@NCs) a bibilugan da reng defined nanoparticles ampo individual metal centers. Ing optimized Co-SAs/NPs@NC magpakit yang masanting a formic acid dehydrogenation performance, na mas masanting kesa kareng non-noble nanostructured catalysts (antimo ing CoNx, single cobalt atoms, cobalt@NC ampo γ-Mo2N) ampo pati na noble metal catalysts. Ing in-situ characterization ampo ing DFT a kalkulasyun da reng aktibung catalysts papakit da na deng indibidwal a metal sites magsilbi lang aktibung lugal, ampo deng nanoparticles ning kasalukuyan a imbensyun papaslag de ing d-band center da reng Co atoms, susulung de ing adsorption ampo ing pamag-activate ning HCOO*, inya sa’t bababa ing energy barrier ning reaksyun. .
Deng zeolite imidazolate frameworks (ZIFs) ila reng well-defined a atlung dimensional a precursors a mamye catalysts para kareng nitrogen-doped carbonaceous materials (metal-NC catalysts) para suportan la reng miyayaliwang uri da reng metal37,38. Uli na niti, ing Co(NO3)2 ampo ing Zn(NO3)2 misasanmetung la keng 2-methylimidazole keng methanol para gawang metal complexes keng solusyun. Kaybat ning centrifugation ampo pamag-dry, ing CoZn-ZIF me-pyrolyze ya keng miyayaliwang temperatura (750-950 °C) keng atmosphere a 6% H2 ampo 94% Ar. Antimo ing akakit tamu keng larawan king lalam, deng resultang materyales atin lang miyayaliwang katangian ning aktibung lugal at mipalagyuan lang Co-SAs/NPs@NC-950, Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750 (Figure 2a). ) . Deng partikular a experimental a pamagmasid kareng mapilan a susi keng prosesu ning synthesis makadetalye la keng Figures 1 ampo 2. C1-C3. Ing variable temperature powder X-ray diffraction (VTXRD) megawa ya para abalu ing pamagbayu ning catalyst. Kaybat ning temperatura ning pyrolysis miras yang 650 °C, ing XRD pattern magbayu yang maragul pauli ning pangabagsak ning ordered crystal structure ning ZIF (Fig. S4) 39 . Kabang daragul ya ing temperatura, adwa lang malapad a taluktuk deng lulual kareng XRD patterns ning Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750 keng 20–30° ampo 40–50°, a magrepresenta keng taluktuk ning amorphous carbon (Fig. C5). 40. Dapat papansinan na atlu lang peak deng ikit da keng 44.2°, 51.5° ampo 75.8°, a kayabe keng metallic cobalt (JCPDS #15-0806), ampo 26.2°, a kayabe keng graphic carbon (JCPDS # 41-1487). Ing X-ray spectrum ning Co-SAs/NPs@NC-950 papakit na ing presensya ning graphite-like encapsulated cobalt nanoparticles keng catalyst41,42,43,44. Ing Raman spectrum papakit na ing Co-SAs/NPs@NC-950 mas masikan ya ampo mas makitid a D ampo G peaks kesa kareng aliwang sample, a magpakit mas matas a degree ning graphitization ( Figure S6 ). Dagdag pa, ing Co-SAs/NPs@NC-950 magpakit yang mas matas a Brunner-Emmett-Taylor (BET) surface area ampo pore volume (1261 m2 g-1 ampo 0.37 cm3 g-1) kesa kareng aliwang sample ampo keraklan kareng ZIFs NC derivatives la. materyales (Figure S7 ampo Table S1). Ing atomic absorption spectroscopy (AAS) papakit na ing cobalt content ning Co-SAs/NPs@NC-950, Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@ 2.69 wt%, 2.74 % wt. at 2.73% wt. NC-750 respectively (Table S2). Ing Zn content ning Co-SAs/NPs@NC-950, Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750 bagya-bagya yang daragul, a sangkan ning daragul a pamagbawas ampo pamag-volatilize da reng Zn units. Ing pamagtas ning temperatura ning pyrolysis (Zn, boiling point = 907 °C) 45.46. Ing elemental analysis (EA) pepakit na ing porsyentu ning N bababa ya keng daragul a temperatura ning pyrolysis, at ing matas a O content malyari yang uli ning adsorption ning molecular O2 ibat keng exposure keng angin. (Table S3). keng metung a cobalt content, deng nanoparticles ampo reng isolated coatoms miyabe la, a magresulta keng maragul a pamagtas ning catalyst activity, antimo ing misabi lalam.
Schematic diagram ning synthesis ning Co-SA/NPs@NC-T, nung nukarin ing T ya ing temperatura ning pyrolysis (°C). b TEM image. c Larawan ning Co-SAs/NPs@NC-950 AC-HAADF-STEM. Deng indibidwal a Co atom makamarka la kareng maputing gulis. d EDS Template Co-SA/NPs@NC-950.
Ing transmission electron microscopy (TEM) pepakit na ing presensya da reng miyayaliwang cobalt nanoparticles (NPs) a maki average a sukad a 7.5 ± 1.7 nm mu king Co-SA/NPs@NC-950 ( Figures 2 b and S8). Deting nanoparticles makabalut la keng graphite-like carbon a maka dope keng nitrogen. Ing lattice fringe spacing a 0.361 ampo 0.201 nm makayagpang ya keng graphic carbon (002) ampo metallic Co (111) particles, agpang kareti. Dagdag pa, ing high-angle aberration-corrected annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) pepakit na ing Co NPs keng Co-SAs/NPs@NC-950 makapadurut la keng dakal a atomic cobalt (Fig. 2c). Makanyan man, deng atomically dispersed cobalt atoms mu ing ikit da keng suporta da reng aliwang adwang sample (Fig. S9). Energy dispersive spectroscopy (EDS) HAADF-STEM image papakit na ing pare-pareung pamikalat ning C, N, Co ampo segregated Co NPs keng Co-SAs/NPs@NC-950 (Fig. 2d). Deng eganaganang resulta papakit da na deng atomically dispersed Co centers ampo reng nanoparticles a maka-encapsulate keng N-doped graphite-like carbon matagumpe lang makadikit kareng NC substrates keng Co-SAs/NPs@NC-950, kabang makabukud lamu reng metal center.
Ing valence state ampo ing chemical composition da reng materyales a mekwa me-aral la kapamilatan ning X-ray photoelectron spectroscopy (XPS). Ing XPS spectra da reng atlung catalysts pepakit na ing presensya da reng elementu Co, N, C ampo O, pero ing Zn atyu yamu keng Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750 (Fig. 2). ). C10). kabang daragul ing temperatura ning pyrolysis, ing kabilugan a nitrogen content bababa ya uling deng nitrogen species maging lang e matatag at mabubuluk la keng NH3 ampo NOx gases keng mas matas a temperatura (Table S4) 47 . Kaya, ing kabilugan a carbon content bagya-bagya yang meragdagan manibat Co-SAs/NPs@NC-750 papunta Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-950 (Figures S11 ampo S12). Ing sample a me pyrolyzed keng mas matas a temperatura atin yang mas mababa a proporsiun da reng nitrogen atoms, na mangabaldugan na ing dagul da reng NC carriers keng Co-SAs/NPs@NC-950 dapat mas ditak ya kesa kareng aliwang sample. Iti magdalang mas masikan a sintering da reng cobalt particles. Ing O 1s spectrum papakit yang adwang peak C=O (531.6 eV) ampo C–O (533.5 eV), agpang (Figure S13) 48 . Antimo ing akakit tamu keng Figure 2a, ing N 1s spectrum malyari yang ma-resolba kareng apat a peak ning pyridine nitrogen N (398.4 eV), pyrrole N (401.1 eV), graphite N (402.3 eV) ampo Co-N (399.2 eV). Deng co-N bonds atyu la kareng atlung sample, a magpakit na deng mapilan a N atoms makacoordinate la kareng monometallic sites, pero deng katangian da miyayaliwa la49. Ing pamangamit king mas matas a temperatura ning pyrolysis malyari yang mabawas ing laman da reng Co-N species manibat 43.7% king Co-SA/NPs@NC-750 angga 27.0% king Co-SAs/NPs@NC-850 ampo Co 17.6%@ NC-950. keng -CA/NPs, na maki kaugnayan keng pamagtas ning C content (Fig. 3a), a magpakit na ing karelang Co-N coordination number malyari yang magbayu at partially mipalitan ya kareng C50 atoms. Ing Zn 2p spectrum papakit na ing elementung ayni atyu ya keng pormang Zn2+. (Figure S14) 51. Ing spectrum ning Co 2p papakit yang adwang prominenting peaks keng 780.8 ampo 796.1 eV, na makayagpang keng Co 2p3/2 ampo Co 2p1/2, agpang (Figure 3b). Nung ikumpara la kareng Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750, ing Co-N peak keng Co-SAs/NPs@NC-950 mipalitan ya keng positibong panig, a magpakit na ing metung a Co atom keng babo -SAs/NPs@NC-950 atin yang mas matas a degree ning electron depletion, a magresulta keng mas matas a oxidation state. Dapat papansinan na ing Co-SAs/NPs@NC-950 mu ing pepakit a mahina a peak ning zero-valent cobalt (Co0) king 778.5 eV, a patune king panga-atyu da reng nanoparticles a bunga ning pamisanmetung ning cobalt SA king matas a temperatura.
a N 1s ampo b Co 2p spectra ning Co-SA/NPs@NC-T. c XANES ampo d FT-EXAFS spectra ning Co-K-edge ning Co-SAs/NPs@NC-950, Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750. e WT-EXAFS contour plots ning Co-SAs/NPs@NC-950, Co-SAs/NPs@NC-850, ampo Co-SAs/NPs@NC-750. f FT-EXAFS fitting curve para keng Co-SA/NPs@NC-950.
Ing time-locked X-ray absorption spectroscopy (XAS) ginamit ya para asuri ing electronic structure ampo ing coordination environment da reng Co species keng mesadiang sample. Cobalt valence states keng Co-SAs/NPs@NC-950, Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750 Edge structure a mipalabas kapamilatan ning normalized near-field X-ray absorption ning Co-K edge ( XANES) spectrum. Antimo ing akakit tamu keng Figure 3c, ing absorption malapit keng gilid da reng atlung sample atyu ya pilatan da reng Co ampo CoO foils, a papakit na ing valence state da reng Co species manibat 0 anggang +253. Dagdag pa, ing pamagbayu papunta keng mas mababang enerhiya meyakit ya ibat Co-SAs/NPs@NC-950 papunta Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750, a magpakit na ing Co-SAs/NPs@NC-750 atin yang mas mababang oxidation state. baliktad ing pamituki-tuki. Agpang keng linear combination fitting results, ing Co valence state ning Co-SAs/NPs@NC-950 metung yang +0.642, na mas mababa kesa keng Co valence state ning Co-SAs/NPs@NC-850 (+1.376). Co-SA/NP @NC-750 (+1.402). Deng resulta a reni papakit da na ing average a oxidation state da reng cobalt particles keng Co-SAs/NPs@NC-950 makabaldugan yang mabawas, na makayagpang kareng resulta ning XRD ampo HADF-STEM ampo malyari yang ipaliwanag keng pamisanmetung da reng cobalt nanoparticles ampo ing metung a cobalt. . Co atoms 41. Fourier transform X-ray absorption fine structure (FT-EXAFS) spectrum ning Co K-edge papakit na ing main peak keng 1.32 Å kayabe ya keng Co-N/Co-C shell, kabang ing scattering path ning metallic Co -Co atyu ya keng 2.18 mu keng Co-SAs Ås mayayakit keng NC/NCs (Fig.95 3d). Dagdag pa, ing wavelet transform (WT) contour plot papakit ne ing maximum a intensity keng 6.7 Å-1 a makayagpang keng Co-N/Co-C, kabang ing Co-SAs/NPs@NC-950 mu ing papakit na ing maximum a intensity a makayagpang keng 8.8. Ing metung pang intensity maximum atyu ya keng Å−1 keng Co–Co bond (Fig. 3e). Dagdag pa, ing EXAFS analysis a gewa ning lessor pepakit na na keng pyrolysis temperatures a 750, 850 ampo 950 °C, ing Co-N coordination numbers 3.8, 3.2 ampo 2.3, at ing Co-C coordination numbers 0. 0.9 ampo 1.8 (Fig. 3. Sf, Table S15 and S15). Mas partikular, ing bayung resulta malyari yang iugne keng presensya da reng atomically dispersed CoN2C2 units ampo nanoparticles keng Co-SAs/NPs@NC-950. Ing pamiyaliwa, keng Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750, CoN3C ampo CoN4 units mu ing atyu. Malino na nung daragul ya ing temperatura ning pyrolysis, deng N atoms keng CoN4 unit bagya-bagya lang mipalitan kareng C atoms, ampo ing cobalt CA aggregates para gawang nanoparticles.
Deng minunang me-aral a kundisyun ning reaksyun ginamit la para abalu ing epektu da reng kundisyun ning pamagsadya kareng katangian da reng miyayaliwang materyales (Fig. S16)17,49. Antimo ing akakit tamu keng Figure 4 a, ing aktibidad da reng Co-SAs/NPs@NC-950 mas matas ya kesa kareng Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750. Ing makabaldugan, deng atlung mesadiang Co samples pepakit lang mas matas a performance kumpara kareng standard commercial precious metal catalysts (Pd/C ampo Pt/C). Dagdag pa, ing Zn-ZIF-8 ampo Zn-NC samples e la aktibo keng formic acid dehydrogenation, a magpakit na deng Zn particles ali la aktibo a lugal, pero ing karelang epektu keng aktibidad e ya maragul. Dagdag pa, ing aktibidad da reng Co-SAs/NPs@NC-850 ampo Co-SAs/NPs@NC-750 dinalan lang secondary pyrolysis keng 950°C kilub ning 1 oras, pero mas mababa ya kesa keng Co-SAs/NPs@NC-750 . @NC-950 (Fig. S17). Ing structural characterization da reng materyales a reni pepakit na ing presensya da reng Co nanoparticles kareng re-pyrolyzed a sample, pero ing mababang specific surface area ampo ing alang graphite-like carbon meging sangkan ning mas mababang aktibidad kumpara kareng Co-SAs/NPs@NC-950 (Figure S18–S20). Ing aktibidad da reng sample a miyayaliwang dagul ning Co precursor mekumpara ya, ing pekamatas a aktibidad pepakit ya keng 3.5 mol a karagdagan (Table S6 ampo Figure S21). Malino na ing pamaglalang da reng miyayaliwang sentru ning metal makaimpluwensya ya keng hydrogen content keng pyrolysis atmosphere ampo ing pyrolysis time. Uli na niti, deng aliwang Co-SAs/NPs@NC-950 a materyales me evaluate la para keng formic acid dehydrogenation activity. Deng eganaganang materyales pepakit lang katamtaman anggang masanting a performance; makanyan man, ala metung man karela ing mas masanting kesa keng Co-SAs/NPs@NC-950 (Figures S22 ampo S23). Ing structural characterization ning materyal pepakit na na nung daragul ya ing pyrolysis time, ing laman ning monoatomic Co-N positions bagya-bagya yang bababa pauli ning pamisanmetung da reng metal atoms kareng nanoparticles, a magpaliwanag king pamiyaliwa ning aktibidad da reng sample a maki pyrolysis time a 100-2000. aliwa. 0.5 h, 1 h, ampo 2 h (Figures S24-S28 ampo Table S7).
Graph ning gas volume kumpara keng oras a mekwa keng dehydrogenation da reng fuel assemblies gamit deng miyayaliwang catalysts. Reaction conditions: FA (10 mmol, 377 μl), catalyst (30 mg), PC (6 ml), Tback: 110 °C, Tactical: 98 °C, 4 parts b Co-SAs/NPs@NC-950 ( 30 mg), miyayaliwang solvents. c Pamagkumpara kareng gas evolution rates da reng heterogeneous catalysts kareng organic solvents keng 85-110 °C. d Co-SA/NPs@NC-950 recycling experiment. Reaction conditions: FA (10 mmol, 377 μl), Co-SAs/NPs@NC-950 (30 mg), solvent (6 ml), Tback: 110 °C, Tactical: 98 °C, balang reaction cycle tatagal yang metung oras. Deng error bars mangabaldugan la kareng standard deviations a mekwa ibat kareng atlung aktibung test.
Keraklan, ing efficiency da reng FA dehydrogenation catalysts makadependi ya keng kundisyun ning reaksyun, lalu na ing solvent a ginamit8,49. Nung gamitan me ing danum bilang solvent, ing Co-SAs/NPs@NC-950 pepakit na ing pekamatas a initial reaction rate, pero ing deactivation milyari ya, mekad uli da reng protons o H2O18 a sasakupan da reng aktibung lugal. Ing pamagsuri keng catalyst kareng organic solvents antimo ing 1,4-dioxane (DXA), n-butyl acetate (BAC), toluene (PhMe), triglyme ampo cyclohexanone (CYC) ala lang pepakit a pamagbayu, ampo keng propylene carbonate (PC) ) (Fig. 4b ampo Table S8). makanyan mu naman, deng additives antimo ing triethylamine (NEt3) o sodium formate (HCCONa) ala lang aliwang positibong epektu keng catalyst performance (Figure S29). keng lalam ning masanting a kondisyun ning reaksyun, ing gas yield miras yang 1403.8 mL g−1 h−1 (Fig. S30), na mas matas kesa kareng sablang minunang mibalita a Co catalysts (kayabe na ing SAC17, 23, 24). kareng miyayaliwang eksperimentu, e la kayabe deng reaksyun keng danum ampo kareng formate additives, ing selectivity anggang 99.96% para keng dehydrogenation ampo dehydration mekwa ya (Table S9). Ing mekwa nang activation energy 88.4 kJ/mol ya, na makayagpang ya keng activation energy da reng noble metal catalysts (Figure S31 ampo Table S10).
Dagdag pa, ikumpara mi la reng aliwang heterogeneous catalysts para keng formic acid dehydrogenation keng parehung kundisyun (Fig. 4c, tables S11 ampo S12). Antimo ing akakit tamu keng Figure 3c, ing gas production rate ning Co-SAs/NPs@NC-950 lalagpus ya keng pekakilalang heterogeneous base metal catalysts ampo 15 ampo 15 a besis mas matas kesa keng commercial 5% Pd/C ampo 5% Pd/C, respectively. minsan. % Pt/C catalyst.
Ing metung a importanting bage keng nanu mang praktikal a aplikasyun da reng (de)hydrogenation catalysts yapin ing karelang katatagan. Uli na niti, metung a serye da reng pamag-recycle a eksperimentu gamit ing Co-SAs/NPs@NC-950 ing megawa. Antimo ing akakit tamu keng Figure 4 d, ing mumunang aktibidad ampo ing selectivity ning materyal menatili yang e mibayu kilub ning limang sunod-sunod a pamagdalagan (lawen ya ing Table S13). Deng makabang panaun a pamagsuri megawa la at ing pamaglalang gas meragdagan yang linear kilub ning 72 oras (Figure S32). Ing cobalt content ning ginamit a Co-SA/NPs@NC-950 2.5 wt% ya, na malapit ya keng bayung catalyst, a magpakit na alang malino leaching ning cobalt (Table S14). Ala yang malino pamagbayu kule o pamisanmetung da reng metal particles bayu at kaybat ning reaksyun ( Figure S33 ). Ing AC-HAADF-STEM ampo ing EDS da reng materyales a ginamit kareng makabang panaun a eksperimentu pepakit da ing pamag-retain ampo ing pare-pareung pamikalat da reng atomic dispersion sites ampo alang makabaldugan a pamagbayu keng istruktura (Figures S34 ampo S35). Deng katangian a peak ning Co0 ampo Co-N atyu la pa keng XPS, patunayan na ing pamisanmetung da reng Co NPs ampo reng indibidwal a metal sites, a magpatutu mu naman keng katatagan ning Co-SAs/NPs@NC-950 catalyst (Figure S36).
Para abalu la reng peka aktibung lugal a maki pakibatan keng formic acid dehydrogenation, mepili lang materyales a ating metung mung metal center (CoN2C2) o Co NP a misadia base kareng minunang pamagaral17. Ing order ning formic acid dehydrogenation activity a mayayakit keng pareung kundisyun yapin ing Co-SAs/NPs@NC-950 > Co SA > Co NP (Table S15), a magpakit na mas active la reng atomically dispersed CoN2C2 sites kesa kareng NPs. Ing reaction kinetics papakit na ing hydrogen evolution tutuki ya keng first-order reaction kinetics, pero deng slope da reng mapilan a curves kareng miyayaliwang cobalt contents ali la parehu, papakit na ing kinetics e ya mu makadependi keng formic acid, nune pati na keng active site (Fig. 2). C37). Deng karagdagang pamagaral king kinetic pepakit da na, uling alang cobalt metal peaks keng X-ray diffraction analysis, ing kinetic order ning reaksyun keng cobalt content meyakit yang 1.02 keng mas mababang antas (kulang keng 2.5%), a magpakit keng halus pare-pareung pamikalat da reng monoatomic cobalt centers. mismu. aktibung lugal (figs. S38 ampo S39). Potang ing laman da reng Co particles miras yang 2.7%, bigla yang daragul ing r, a magpakit na deng nanoparticles makipag-ugnayan lang masalese kareng indibidwal a atom para mika mas matas a aktibidad. Kabang daragul ya ing laman da reng Co particles, ing curve maging yang nonlinear, a maki kaugnayan king pamagtas ning bilang da reng nanoparticles ampo ing pamagbaba da reng monatomic positions. Kaya, ing mas masanting a LC dehydrogenation performance ning Co-SA/NPs@NC-950 manibatan ya keng cooperative behavior da reng indibidwal a metal sites ampo nanoparticles.
Metung a malalam a pamagaral ing megawa gamit ing in situ diffuse reflectance Fourier transform (in situ DRIFT) para akilala la reng reaction intermediates keng prosesu. Kaybat dang pepainit deng sample kareng miyayaliwang temperatura ning reaksyun kaybat dang dinan formic acid, adwa lang set da reng frequency ing ikit (Fig. 5a). Atlung katangian a peak ning HCOOH* ing 1089, 1217 ampo 1790 cm-1, na mipapag-ugne keng out-of-plane CH π (CH) stretching vibration, CO ν (CO) stretching vibration ampo C=O ν (C=O) stretching vibration, 54, 55 . Ing metung pang set da reng peaks keng 1363 ampo 1592 cm-1 makayagpang ya keng symmetric OCO vibration νs(OCO) ampo ing asymmetric OCO stretching vibration νas(OCO)33.56 HCOO*, respectively. Kabang susulung ya ing reaksyun, ing relatibung peak ning HCOOH* ampo HCOO* bagya bagya yang mangaina. Keraklan, ing pamaglaso ning formic acid atin yang atlung lakbang: (I) pamag-adsorb ning formic acid kareng aktibung lugal, (II) pamaglako ning H kapamilatan ning formate o carboxylate pathway, ampo (III) pamisanmetung da reng adwang me-adsorb a H para mika hydrogen. HCOO* ampo COOH* ila reng susi a pilatan keng pamangilala kareng formate o carboxylate pathways, respectively57. keng kekaming catalytic system, ing characteristic HCOO* peak mu ing linwal, a magpakit na ing decomposition ning formic acid malilyari yamu keng formic acid pathway58. Deng parehung observasyun megawa la keng mas mababang temperatura a 78 °C ampo 88 °C (fig. S40).
In situ DRIFT spectra ning HCOOH dehydrogenation keng Co-SAs/NPs@NC-950 ampo b Co SAs. Ing alamat papakit na ing on-site reaction times. c Dynamics ning gas volume a mekwa gamit ing miyayaliwang isotope labeling reagents. d Kinetic isotope effect data.
Deng parehung in situ DRIFT experiments megawa la kareng makaugne a materyales Co NP ampo Co SA para abalu ing synergistic effect keng Co-SA/NPs@NC-950 ( Figures 5 b ampo S41). Parehu lang magpakit trend deng adwang materyales, pero deng characteristic peaks ning HCOOH* ampo HCOO* medyu mibayu la, papakit na ing pamaglub da reng Co NPs magbayu ya keng electronic structure ning monoatomic center. Ing metung a katangian a νas(OCO) peak mayayakit ya keng Co-SAs/NPs@NC-950 ampo Co SA pero ali keng Co NPs, a magpakit pa na ing intermediate a mebuu keng pamagdagdag ning formic acid yapin ing monodentate formic acid a perpendicular keng plane salt surface. at me-adsorb ya keng SA bilang aktibung lugal 59 . Dapat papansinan na ing maragul a pamagtas da reng vibrations da reng characteristic peaks π(CH) ampo ν(C = O) meyakit, na meging sangkan ning distortion ning HCOOH* ampo meging sangkan ning reaksyun. Uli na niti, ing katangian a peak ning HCOOH* ampo HCOO* keng Co-SAs/NPs@NC halus mewala ya kaybat ning 2 min a reaksyun, na mas mabilis kesa keng monometallic (6 min) ampo nanoparticle-based catalysts (12 min). . Deng sablang resulta a reni papatutuan da na ing nanoparticle doping papaslag ne ing adsorption ampo ing pamag-activate da reng intermediates, inya pin mabilis ing reaksyun a mipanukala babo.
Para mas a-analisa ing dalan ning reaksyun at abalu ing rate determining step (RDS), ing KIE effect megawa ya keng presensya ning Co-SAs/NPs@NC-950. keni, miyayaliwang formic acid isotopes antimo ing HCOOH, HCOOD, DCOOH ampo DCOOD ing gagamitan da para keng KIE a pamagaral. Antimo ing akakit tamu keng Figure 5c, ing dehydrogenation rate bababa ya keng makatuking order: HCOOH > HCOOD > DCOOH > DCOOD. Dagdag pa, ing mekwa nang ulaga ning KHCOOH/KHCOOD, KHCOOH/KDCOOH, KHCOOD/KDCOOD ampo KDCOOH/KDCOOD 1.14, 1.71, 2.16 ampo 1.44, agpang (Fig. 5d). Kaya, ing CH bond cleavage keng HCOO* magpakit yang kH/kD values ​​>1.5, a magpakit keng maragul a kinetic effect60,61, at balamu magrepresenta ya keng RDS ning HCOOH dehydrogenation keng Co-SAs/NPs@NC-950.
Dagdag pa, ing DFT a kalkulasyun megawa ya para aintindian ing epektu da reng doped nanoparticles keng intrinsic activity ning Co-SA. Ing Co-SAs/NPs@NC ampo Co-SA models megawa la base kareng experimentung pepakit ampo kareng minunang obra (Figs. 6a ampo S42)52,62. Kaybat ning geometric optimization, deng malating Co6 nanoparticles (CoN2C2) a kayabe da reng monoatomic units mekilala la, at ing Co-C ampo Co-N bond lengths keng Co-SA/NPs@NC meyakit lang 1.87 Å ampo 1.90 Å, agpang kareti. , na makayagpang kareng resulta ning XAFS. Ing mekwa nang partial density of states (PDOS) papakit na ing metung a Co metal atom ampo ing nanoparticle composite (Co-SAs/NPs@NC) magpakit lang mas matas a hybridization malapit keng Fermi level kumpara keng CoN2C2, a magresulta keng HCOOH. Ing decomposed electron transfer mas efficient ya ( Figures 6b and S43). Deng katumbas a d-band centers ning Co-SAs/NPs@NC ampo Co-SA mekwa la -0.67 eV ampo -0.80 eV, agpang kareti, nung nukarin ing pamagtas ning Co-SAs/NPs@NC 0.13 eV ya, na meging sangkan nung bakit kaybat ning pamaglub ning NP, ing pamag-adsorb kareng HCOO* particles ning electronic a istruktura ning Co-2Cs milyari. Ing pamiyaliwa ning charge density papakit na ing maragul a electron cloud makapadurut keng CoN2C2 block ampo ing nanoparticle, a magpakit keng masikan a pamiugnayan da pauli ning electron exchange. Kayabe ning Bader charge analysis, ikit da na ing atomically dispersed Co mewala yang 1.064e keng Co-SA/NPs@NC ampo 0.796e keng Co SA ( Figure S44 ). Deng resulta a reni papakit da na ing pamisanmetung da reng nanoparticles magdalang electron depletion kareng Co sites, a magresulta keng pamagtas ning Co valence, na makayagpang kareng resulta ning XPS (Fig. 6c). Ing Co-O interaction characteristics ning HCOO adsorption keng Co-SAs/NPs@NC ampo Co SA me-analisa ya kapamilatan ning pamagkwenta keng crystalline orbital Hamiltonian group (COHP)63. Antimo ing akakit tamu keng Figure 6 d, deng negatibo ampo positibong ulaga ning -COHP makayagpang la keng antibonding state ampo binding state, agpang kareti. Ing sikanan ning Co-O a me-adsorb keng HCOO (Co-carbonyl O HCOO*) me-assess ya kapamilatan ning pamisanmetung da reng -COHP values, na 3.51 ampo 3.38 para kareng Co-SAs/NPs@NC ampo Co-SA, agpang kareti. Ing HCOOH adsorption pepakit na ing parehung resulta: ing pamagtas ning integral value ning -COHP kaybat ning nanoparticle doping pepakit na ing pamagtas ning Co-O bonding, inya sa’t pepakit na ing pamag-activate ning HCOO ampo HCOOH ( Figure S45 ).
Lattice structure ning Co-SA/NPs@NC-950. b PDOS Co-SA/NP@NC-950 ampo Co SA. c 3D isosurface ning pamiyaliwa da reng charge densities ning HCOOH adsorption keng Co-SA/NPs@NC-950 ampo Co-SA. (d) pCOHP ning Co-O bonds a me-adsorb keng HCOO keng Co-SA/NPs@NC-950 (kayli) ampo Co-SA (wanan). e Reaction pathway ning HCOOH dehydrogenation keng Co-SA/NPs@NC-950 ampo Co-SA.
Para mas aintindian ing superior dehydrogenation performance ning Co-SA/NPs@NC, ing reaction path ampo ing enerhiya mitatag ya. Partikular, ing FA dehydrogenation atin yang limang lakbang, kayabe na ing pamagbayu ning HCOOH papunta HCOOH*, HCOOH* papunta HCOO* + H*, HCOO* + H* papunta 2H* + CO2*, 2H* + CO2* papunta 2H* + CO2, ampo 2H* keng H2 (Fig. 6e). Ing adsorption energy da reng formic acid molecules keng catalyst surface kapamilatan ning carboxylic oxygen mas mababa ya kesa keng hydroxyl oxygen (Figures S46 ampo S47). Kaybat, pauli ning mas mababang enerhiya, ing adsorbate mas buri na ing dumalan ya keng OH bond cleavage para maging HCOO* kesa keng CH bond cleavage para maging COOH*. At ing HCOO* gagamit yang monodentate adsorption, na magpromote keng pamamutut da reng bonds ampo ing pamaglalang ning CO2 ampo H2. Deting resulta makayagpang la keng presensya ning νas(OCO) peak keng in situ DRIFT, a magpakit pa na ing FA degradation malilyari ya kapamilatan ning formate pathway keng kekaming pamagaral. Importanti ing papansinan na agpang keng KIE measurements, ing CH dissociation atin yang mas matas a reaction energy barrier kesa kareng aliwang reaction steps at magrepresenta yang RDS. Ing energy barrier ning optimal Co-SAs/NPs@NC catalyst system 0.86 eV mas mababa ya kesa keng Co-SA (1.2 eV), na makabaldugan a magpasanting keng kabilugan a dehydrogenation efficiency. Ing presensya da reng nanoparticles magregulate ya keng electronic structure da reng atomically dispersed coactive sites, na mas magpasanting keng adsorption ampo activation da reng intermediates, inya sa’t babawasan ne ing reaction barrier ampo papalwal ne ing hydrogen production.
keng pamamutut, pepakit mi keng mumunang besis na ing catalytic performance da reng hydrogen production catalysts malyari yang maging mas masanting kapamilatan ning pamangamit kareng materyales a ating matas a monometallic centers ampo mangalating nanoparticles. Ing konseptung ini me validate ya kapamilatan ning synthesis da reng cobalt-based single-atom metal catalysts a mibayu kareng nanoparticles (Co-SAs/NPs@NC), ampo reng aliwang materyales a maki single-metal centers (CoN2C2) o Co NPs. Deng eganaganang materyales mesadya la kapamilatan ning simpleng one-step pyrolysis a paralan. Ing pamagsuri keng istruktura papakit na ing pekamasanting a catalyst (Co-SAs/NPs@NC-950) atin yang atomically dispersed CoN2C2 units ampo mangalating nanoparticles (7-8 nm) a maka dope keng nitrogen ampo graphite-like carbon. Atin yang masanting a gas productivity anggang 1403.8 ml g-1 h-1 (H2:CO2 = 1.01:1), H2 ampo CO selectivity a 99.96% ampo malyari yang manatili ing constant activity kilub ning mapilan a aldo. Ing aktibidad na niting catalyst lalagpusan ne ing aktibidad da reng mapilan a Co SA ampo Pd/C catalysts king 4 ampo 15 a besis, agpang kareti. Ing in situ DRIFT experiments papakit na kumpara keng Co-SA, ing Co-SAs/NPs@NC-950 magpakit yang mas masikan a monodentate adsorption ning HCOO*, na importanti para keng formate pathway, ampo deng dopant nanoparticles malyari lang magpromote HCOO* activation ampo C–H acceleration. Ing bond cleavage mekilala ya bilang RDS. Deng theoretical a kalkulasyun papakit da na ing Co NP doping daragdagan ne ing d-band center da reng metung a Co atoms king 0.13 eV kapamilatan ning pamiugnayan, papaslag ne ing adsorption ning HCOOH* ampo HCOO* intermediates, inya sa’t babawasan ne ing reaction barrier manibat 1.20 eV para king Co SA angga 0 .86 eV. Iya ing maki pakibatan keng masanting a pamagperform.
Mas malawak, ing pamanyaliksik ayni mamye yang ideya para keng pamagdisenyu kareng bayung single-atom metal catalysts ampo susulung ing pamanintindi nung makananu yang asundu ing catalytic performance kapamilatan ning synergistic effect da reng metal centers a miyayaliwang sukad. Maniwala kami na ining paralan ayni malyari yang ipalapad kareng aliwang catalytic systems.
Co(NO3)2 6H2O (AR, 99%), Zn(NO3)2 6H2O (AR, 99%), 2-methylimidazole (98%), methanol (99.5%), propylene carbonate (PC, 99%) ethanol (AR, 99.7%) mesali la keng McLean, China. Ing formic acid (HCOOH, 98%) mesali ya keng Rhawn, China. Deng eganaganang reagents ginamit lang diretsu a alang karagdagang pamaglinis, at ing ultrapure a danum mesadya ya gamit ing ultrapure a sistema ning pamaglinis. Pt/C (5% mass loading) ampo Pd/C (5% mass loading) mesali la keng Sigma-Aldrich.
Ing synthesis da reng CoZn-ZIF nanocrystals megawa ya base kareng minunang paralan a maki mapilan a pamagbayu23,64. Mumuna, 30 mmol Zn(NO3)2·6H2O (8.925 g) ampo 3.5 mmol Co(NO3)2·6H2O (1.014 g) ing mipagkalub at milaso keng 300 ml ning methanol. Kaybat, 120 mmol ning 2-methylimidazole (9.853 g) ing metunaw keng 100 ml ning methanol at miyabe ya keng solusyun babo. Ing timpla me-stir ya keng room temperature kilub ning 24 oras. Katataulian, ing produktu mekawani ya kapamilatan ning centrifugation king 6429 g kilub ning 10 min at mehugas yang masalese king methanol atlung besis. Ing resulta ning pulbura mepatuyu ya keng vacuum keng 60°C magdamag bayu ya gamitan.
Para maka-synthesize yang Co-SAs/NPs@NC-950, ing dry CoZn-ZIF powder me-pyrolyze ya keng 950 °C kilub ning 1 h keng gas flow a 6% H2 + 94% Ar, a maki heating rate a 5 °C/min. Ing sample pepalamig de keng room temperature para mika Co-SA/NPs@NC-950. Para kareng Co-SAs/NPs@NC-850 o Co-SAs/NPs@NC-750, ing temperatura ning pyrolysis mibayu ya keng 850 ampo 750 °C, agpang kareti. Deng sample a makasadya malyari lang gamitan a alang karagdagang pamagprosesu, antimo ing acid etching.
Ing TEM (transmission electron microscopy) a sukad megawa ya keng Thermo Fisher Titan Themis 60-300 “cube” microscope a maki aberration corrector para keng imaging ampo 300 kV probe shaping lens. Deng HAADF-STEM experiments megawa la gamit ing FEI Titan G2 ampo FEI Titan Themis Z microscopes a ating probes ampo image correctors, ampo DF4 four-segment detectors. Deng EDS elemental mapping images mekwa la naman keng FEI Titan Themis Z microscope. Ing XPS analysis megawa ya keng X-ray photoelectron spectrometer (Thermo Fisher model ESCALAB 250Xi). XANES ampo EXAFS spectra ning Co K-edge mekolekta la gamit ing XAFS-500 table (China Spectral Instruments Co., Ltd.). Ing laman ning co meyakit ya kapamilatan ning atomic absorption spectroscopy (AAS) (PinAAcle900T). Ing X-ray diffraction (XRD) spectra mitala ya keng X-ray diffractometer (Bruker, Bruker D8 Advance, Germany). Deng nitrogen adsorption isotherms mekwa la gamit ing physical adsorption apparatus (Micromeritics, ASAP2020, USA).
Ing dehydrogenation reaction megawa ya keng argon atmosphere ampo ing angin mewala ya agpang keng standard Schlenk method. Ing reaction vessel me-evacuate ya at me-refill ya keng argon 6 a besis. I-on me ing condenser water supply at dagdagan me ing catalyst (30 mg) ampo ing solvent (6 ml). Painitan me ing lulanan keng buri mung temperatura gamit ing thermostat at paburen meng mag-equilibrate kilub ning 30 minutus. Ing formic acid (10 mmol, 377 μL) miyabe ya keng reaction vessel lalam ning argon. Ibalik me ing three-way burette valve para i-depressurize ing reactor, isara me pasibayu, at magumpisa kang sukatan ing volume ning gas a megawa gamit ing manual burette (Figure S16). Kaybat ning oras a kailangan para mayari ing reaksyun, metung a sample ning gas ing mekolekta para keng GC analysis gamit ing gas-tight syringe a me-purge keng argon.
Ing in situ drift experiments megawa ya keng Fourier transform infrared (FTIR) spectrometer (Thermo Fisher Scientific, Nicolet iS50) a maki mercury cadmium telluride (MCT) detector. Ing catalyst powder mibili ya keng reaction cell (Harrick Scientific Products, Praying Mantis). Kaybat nang me-treat ing catalyst keng stream ning Ar (50 ml/min) keng room temperature, ing sample me-init ya keng metung a temperatura, kaybat me-bubble ya keng Ar (50 ml/min) keng HCOOH solution at mibubu ya keng in-situ reaction cell. para keng reaksyun. Model heterogeneous catalytic processes. Deng infrared spectra merecord la keng intervals manibat 3.0 seconds anggang 1 oras.
HCOOH, DCOOH, HCOOD ampo DCOOD ila reng gagamitan bilang substrate keng propylene carbonate. Deng mitagan a kundisyun makayagpang la keng HCOOH dehydrogenation procedure.
Ing mumunang prinsipyung kalkulasyun megawa ya gamit ing density functional theory framework kilub ning Vienna Ab initio modeling package (VASP 5.4.4) 65,66. Ing superunit cell a maki graphene surface (5 × 5) a maki transverse dimension a manga 12.5 Å ing ginamit bilang substrate para king CoN2C2 ampo CoN2C2-Co6. Ing vacuum distance a maigit 15 Å miragdagan ya para aiwasan ing pamiugnayan da reng makalapit a substrate layers. Ing pamiugnayan da reng ions ampo electrons makalarawan ya keng projected amplified wave (PAW) method65,67. Ing Perdue-Burke-Ernzerhoff (PBE) generalized gradient approximation (GGA) function a mipanukala nang Grimm a maki van der Waals correction68,69 ing ginamit. Ing convergence criteria para keng kabilugan a enerhiya ampo sikanan 10−6 eV/atom ampo 0.01 eV/Å. Ing energy cutoff mitakda ya keng 600 eV gamit ing Monkhorst-Pack 2 × 2 × 1 K-point grid. Ing pseudopotential a ginamit kening model ayni megawa ya ibat keng electronic configuration papunta keng C 2s22p2 state, N 2s22p3 state, Co 3d74s2 state, H 1 s1 state, ampo O 2s22p4 state. Ing adsorption energy ampo ing electron density difference makalkula la kapamilatan ning pamagbawas keng energy ning gas phase ampo reng surface species ibat keng energy ning adsorbed system agpang keng adsorption o interface models70,71,72,73,74. Ing Gibbs free energy correction magagamit ya para i-convert ing DFT energy keng Gibbs free energy at kukunan ne ing vibrational contributions keng entropy ampo zero point energy75. Ing ascending image-nudging elastic band (CI-NEB) a paralan ginamit ya para panintunan ing transition state ning reaksyun76.
Deng eganaganang data a mekwa ampo me-analisa keng pamagaral ayni kayabe la keng artikulu ampo kareng supplementary materials o malyari la keng corresponding author nung makatuliran ing anyaran. Ing penibatan a data makabili ya para keng artikulung ayni.
Deng eganaganang code a ginamit kareng simulation a kayabe na niting artikulu malyari lang akwa kareng corresponding authors nung aduan.
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