CO2 Hydrogenation on Cu/Al2O3: Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst

Selective hydrogenation of CO₂ into methanol is a key sustainable technology, where Cu/Al₂O₃ prepared by surface organometallic chemistry displays high activity towards CO₂ hydrogenation compared to Cu/SiO₂, yielding CH₃OH, dimethyl ether (DME), and CO. CH₃OH formation rate increases due to the metal–oxide interface and involves formate intermediates according to advanced spectroscopy and DFT calculations. Al₂O₃ promotes the subsequent conversion of CH₃OH to DME, showing bifunctional catalysis, but also increases the rate of CO formation. The latter takes place 1) directly by activation of CO₂ at the metal–oxide interface, and 2) indirectly by the conversion of formate surface species and CH₃OH to methyl formate, which is further decomposed into CH₃OH and CO. This study shows how Al₂O₃, a Lewis acidic and non‐reducible support, can promote CO₂ hydrogenation by enabling multiple competitive reaction pathways on the oxide and metal–oxide interface.     

Methanol Synthesis by co2 Hydrogenation over Titanium Modified γ -al2o3 Supported Copper Catalysts

A titanium-modified . -Al₂ O₃ supported copper catalyst has been prepared and used for methanol synthesis by CO₂ hydrogenation. The addition of Ti to the CuO/. -Al₂ O₃ catalyst enhances the activity in CO₂ hydrogenation greatly and makes the copper in the catalyst exist in much smaller crystallites. The effect of contact time on the relative selectivity ( 6 CH₃OH /SCO ) and selectivity of methanol was also investigated. The results indicated that methanol was formed directly from the hydrogenation of CO₂ .     

Photo-Induced Switching of CO2 Hydrogenation Pathway towards CH3OH Production over Pt@UiO-66-NH2(Co)

It is highly desired to achieve controllable product selectivity in CO₂ hydrogenation. Herein, we report light-induced switching of reaction pathways of CO₂ hydrogenation towards CH₃OH production over actomically dispersed Co decorated Pt@UiO-66-NH₂. CO, being the main product in the reverse water gas shift (RWGS) pathway under thermocatalysis condition, is switched to CH₃OH via the formate pathway with the assistance of light irradiation. Impressively, the space-time yield of CH₃OH in photo-assisted thermocatalysis (1916.3 μmol g_cat⁻¹ h⁻¹) is about 7.8 times higher than that without light at 240 °C and 1.5 MPa. Mechanism investigation indicates that upon light irradiation, excited UiO-66-NH₂ can transfer electrons to Pt nanoparticles and Co sites, which can efficiently catalyze the critical elementary steps (i.e., CO₂-to-*HCOO conversion), thus suppressing the RWGS pathway to achieve a high CH₃OH selectivity.     

利用第一性原理研究氧化铟在甲醇水蒸气重整反应中的催化作用

基于第一性原理方法,证明了甲醛在In₂O₃(110)表面可以选择性地转化为CO₂. 水分解得到的OH物种有利于甲醛脱氢得到CHO,后者不易直接脱氢,其H原子被周围的OH捕获生成CO和H₂O. 最后,相比较其从表面直接脱附,CO更容易获得一个晶格氧生成CO₂. 计算结果表明,在没有PdIn合金参与催化的甲醇水蒸气重整反应过程中,In₂O₃确实扮演着非常重要的角色,进而从理论上证实了甲醇在氧化铟表面选择性生成CO₂的实验结果.     

Electronic Interactions on Platinum/(Metal-Oxide)-Based Photocatalysts Boost Selective Photoreduction of CO2 to CH4

By supporting platinum (Pt) and cadmium sulfide (CdS) nanoparticles on indium oxide (In₂O₃), we fabricated a CdS/Pt/In₂O₃ photocatalyst. Selective photoreduction of carbon dioxide (CO₂) to methane (CH₄) was achieved on CdS/Pt/In₂O₃ with electronic Pt−In₂O₃ interactions, with CH₄ selectivity reaching to 100 %, which is higher than that on CdS/Pt/In₂O₃ without electronic Pt−In₂O₃ interactions (71.7 %). Moreover, the enhancement effect of electronic Pt-(metal-oxide) interactions on selective photoreduction of CO₂ to CH₄ also occurs by using other common metal oxides, such as photocatalyst supports, including titanium oxide, gallium oxide, zinc oxide, and tungsten oxide. The electronic Pt-(metal-oxide) interactions separate photogenerated electron-hole pairs and convert CO₂ into CO₂^δ−, which can be easily hydrogenated into CH₄ via a CO₂^δ−→HCOO*→HCO*→CH*→CH₄ path, thus boosting selective photoreduction of CO₂ to CH₄. This offers a new way to achieve selective photoreduction of CO₂.     

In-Situ-Formed Potassium-Modified Nickel-Zinc Carbide Boosts Production of Higher Alcohols beyond CH4 in CO2 Hydrogenation

Ni-based catalysts have been widely studied in the hydrogenation of CO₂ to CH₄, but selective and efficient synthesis of higher alcohols (C₂₊OH) from CO₂ hydrogenation over Ni-based catalyst is still challenging due to successive hydrogenation of C1 intermediates leading to methanation. Herein, we report an unprecedented synthesis of C₂₊OH from CO₂ hydrogenation over K-modified Ni−Zn bimetal catalyst with promising activity and selectivity. Systematic experiments (including XRD, in situ spectroscopic characterization) and computational studies reveal the in situ generation of an active K-modified Ni−Zn carbide (K-Ni₃Zn₁C_0.7) by carburization of Zn-incorporated Ni⁰, which can significantly enhance CO₂ adsorption and the surface coverage of alkyl intermediates, and boost the C−C coupling to C₂₊OH rather than conventional CH₄. This work opens a new catalytic avenue toward CO₂ hydrogenation to C₂₊OH, and also provides an insightful example for the rational design of selective and efficient Ni-based catalysts for CO₂ hydrogenation to multiple carbon products.     

Room‐Temperature Activation of H2 by a Surface Frustrated Lewis Pair

Surface frustrated Lewis pairs (SFLPs) have been implicated in the gas‐phase heterogeneous (photo)catalytic hydrogenation of CO₂ to CO and CH₃OH by In₂O_3?x(OH)_y. A key step in the reaction pathway is envisioned to be the heterolysis of H₂ on a proximal Lewis acid–Lewis base pair, the SFLP, the chemistry of which is described as In???In‐OH + H₂ → In‐OH₂⁺???In‐H^?. The product of the heterolysis, thought to be a protonated hydroxide Lewis base In‐OH₂⁺ and a hydride coordinated Lewis acid In‐H^?, can react with CO₂ to form either CO or CH₃OH. While the experimental and theoretical evidence is compelling for heterolysis of H₂ on the SFLP, all conclusions derive from indirect proof, and direct observation remains lacking. Unexpectedly, we have discovered rhombohedral In₂O_3?x(OH)_y can enable dissociation of H₂ at room temperature, which allows its direct observation by several analytical techniques. The collected analytical results lean towards the heterolysis rather than the homolysis reaction pathway.     

A First-Principles Study of CO2 Hydrogenation on a Niobium-Terminated NbC (111) Surface

As promising materials for the reduction of greenhouse gases, transition-metal carbides, which are highly active in the hydrogenation of CO₂, are mainly considered. In this regard, the reaction mechanism of CO₂ hydrogenation to useful products on the Nb-terminated NbC (111) surface is investigated by applying density functional theory calculations. The computational results display that the formation of CH₄, CH₃OH, and CO are more favored than other compounds, where CH₄ is the dominant product. In addition, the findings from reaction energies reveal that the preferred mechanism for CO₂ hydrogenation is thorough HCOOH^*, where the largest exothermic reaction energy releases during the HCOOH^* dissociation reaction (2.004 eV). The preferred mechanism of CO₂ hydrogenation towards CH₄ production is CO₂^*→t,c-COOH^*→HCOOH^*→HCO^*→CH₂O^*→CH₂OH^*→CH₂^*→CH₃^*→CH₄^*, where CO₂^*→t,c-COOH^*→HCOOH^*→HCO^*→CH₂O^*→CH₂OH^*→CH₃OH^* and CO₂^*→t,c-COOH^*→CO^* are also found as the favored mechanisms for CH₃OH and CO productions thermodynamically, respectively. During the mentioned mechanisms, the hydrogenation of CH₂O^* to CH₂OH^* has the largest endothermic reaction energy of 1.344 eV.     

Theoretical insight into methanol steam reforming on indium oxide with different coordination environments

Indium oxide(In_2O_3) has demonstrated to be an effective non-noble metal catalyst for methanol steam reforming reaction(MSR).However, the reaction mechanism of MSR and crucial structure-activity relations determining the catalytic performance of In_2O_3 are still not fully understood yet. Using density functional theory(DFT) calculation, we systematically investigate the MSR process over a high-index In_2O_3(211) and a favoured catalytic cycle of MSR is determined. The results show that In_2O_3(211) possesses excellent dehydrogenation and oxidizing ability, on which CH_3 OH can readily adsorb on the In4 c site and be easily activated by the reactive lattice oxygens, resulting in a total oxidation into CO_2 rather than CO, while the H_2 formation through surface H–H coupling limits the overall MSR activity because of the strong H adsorption on the two-coordinated lattice O(O_(2c)). Our analyses show that the relatively inert three-coordinated lattice O(O_(3c)) could play an important catalytic role. To uncover the influence of the local coordination of surface In atoms and lattice O on the catalytic activity, we evaluate the activity trend of several types of In_2O_3 surfaces including(211),(111), and(100) by examining the rate-limiting, which reveals the following activity order:(211)(111)(100). These findings provide an in-depth understanding on the MSR reaction mechanism over In_2O_3 catalysts and some basic structure-activity relations at the atomic scale, could facilitate the rational design of In_2O_3-based catalysts for MSR by controlling the local coordination environment of surface active sites.     

Photofragmentation of alpha-oxocarboxylic acids in aqueous solution. An EPR study. Formation of semidione radicals by decarboxylative substitution of alpha-oxocarboxylic acids by acyl radicals--II. Oxaloacetic acid (oxosuccinic acid).

Abstract— –On in situ photolysis (Λ= 250–400 nm) of aqueous oxaloacetic acid solutions, between pH 5 and 10, the radicals ^-O₂CCH₂C(O^->=C(O⁺)CH₂CO₂^- and ^-O₂CCH₂C(OH)CO₂ are identified. With acetone present, CH₂CO₂, CH₃C(OH)CO^-₂, CH₃C(O^-)=C(O)CH₂CO₂ and -O₂CCHCOCO₂ are also observed. CO₂ and CO are identified as reaction products. The experimental results are explained in terms of α-cleavage of the electronically excited keto-isomer dianion of oxaloacetic acid to yield O₂CCH₂CO and CO₂^-. ^-O₂CCH₂CO adds to the keto-isomer of oxaloacetic acid and to pyruvic acid, which is formed from oxaloacetic acid by thermal decarboxylation, to yield ^-O₂CCH₂C(O^-)= C(O^-)CH₂C0^- and CH₃C(O^-)=C(O)CH₂CO^-₂, respectively, via a decarboxylase substitution reaction. CH₂CO₂ is derived from ^-O₂CCH₂CO by decarbonylation. CO₂^- is scavenged by oxaloacetic acid and pyruvic acid to yield O₂CCH₂C(OH)CO₂ and CH₃C(OH)CO₂^-, respectively.     

Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation

Methanol synthesis by CO₂ hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In₂O₃ supported on ZrO₂ under industrially relevant conditions. This strongly contrasts to the benchmark Cu‐ZnO‐Al₂O₃ catalyst, which is unselective and experiences rapid deactivation. In‐depth characterization of the In₂O₃‐based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co‐feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.     

Selective Electrochemical Reduction of Carbon Dioxide to Ethanol on a Boron‐ and Nitrogen‐Co‐doped Nanodiamond

Electrochemical reduction of CO₂ to ethanol, a clean and renewable liquid fuel with high heating value, is an attractive strategy for global warming mitigation and resource utilization. However, converting CO₂ to ethanol remains great challenge due to the low activity, poor product selectivity and stability of electrocatalysts. Here, the B‐ and N‐co‐doped nanodiamond (BND) was reported as an efficient and stable electrode for selective reduction of CO₂ to ethanol. Good ethanol selectivity was achieved on the BND with high Faradaic efficiency of 93.2 % (−1.0 V vs. RHE), which overcame the limitation of low selectivity for multicarbon or high heating value fuels. Its superior performance was mainly originated from the synergistic effect of B and N co‐doping, high N content and overpotential for hydrogen evolution. The possible pathway for CO₂ reduction revealed by DFT computation was CO₂→*COOH→*CO→*COCO→*COCH₂OH→*CH₂OCH₂OH→CH₃CH₂OH.     

Gas-sensitivity properties of nanoscale Au-In<Subscript>2</Subscript>O<Subscript>3</Subscript> materials

A possibility was demonstrated of producing the chemical sensors based on Au-In₂O₃ obtained using a sol-gel technology. The sensors exhibit high sensitivity and selectivity toward CO. The differences in gas-sensitivity properties of In₂O₃ sensor with respect to CO and CH₄ at different ways of doping with Au(III) was examined. The effect of the gold nanoparticles size and the state of the indium oxide surface on the characteristics of Au-In₂O₃ and Au/In₂O₃ sensors at the detection of CO and CH₄ was examined.     

A Doping Technique that Suppresses Undesirable H2 Evolution Derived from Overall Water Splitting in the Highly Selective Photocatalytic Conversion of CO2 in and by Water

Photocatalytic conversion of CO₂ to reduction products, such as CO, HCOOH, HCHO, CH₃OH, and CH₄, is one of the most attractive propositions for producing green energy by artificial photosynthesis. Herein, we found that Ga₂O₃ photocatalysts exhibit high conversion of CO₂. Doping of Zn species into Ga₂O₃ suppresses the H₂ evolution derived from overall water splitting and, consequently, Zn‐doped, Ag‐modified Ga₂O₃ exhibits higher selectivity toward CO evolution than bare, Ag‐modified Ga₂O₃. We observed stoichiometric amounts of evolved O₂ together with CO. Mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO₂ introduced in the gas phase. Doping of the photocatalyst with Zn is expected to ease the adsorption of CO₂ on the catalyst surface.     

PHOTOFRAGMENTATION OF HYDROXYACETONE, 1.3-DIHYDROXYACETONE, AND 1.3-DICARBOXYACETONE IN AQUEOUS SOLUTION. AN EPR STUDY

Abstract —On photoexcitation, hydroxyacetone undergoes a Norrish-type-1 fragmentation to yield CH₃CO and CH₂OH. CH₂OH is identified by its EPR spectrum. The existence of CH₃CO is inferred from the presence of diacetyl and acetaldehyde in irradiated solutions. Above pH 5, in addition to CH₂OH, the cis and trans forms of the hydroxyacetone enediol radical anion, CH₃C(O^-)=C(O***)H, are detected. 1.3-Dihydroxyacetone is photodecomposed to HOCH₂C?O and C?H₂OH. The former radical decarbonylates to yield CH₂OH and CO. At 254 nm the overall quantum yield of CO production is 0.75. Above pH 5, in addition to CH₂OH, the cis and trans forms of the 1.3-dihydroxyacetone enediol radical anion, HOCH₂C(O^-)C(O***)H, are observed. Electronically excited hydroxyacetone and 1.3-dihydroxyacetone react exclusively by C-C fragmentation, and no H-abstraction from H-donors is observed. In contrast, electronically excited 1.3-dicarboxyacetone shows H-abstraction from H-donors in competition with C-C fragmentation. In the absence of H-donors, fragmentation resulting in CH₂CO₂^- and ^-O₂CCH₂C?O occurs followed by decarbonylation of ^-O₂H₂C?O. At 254 nm the quantum yield of CO production is 0.02. In the presence of H-donors, H-abstraction, yielding HO₂CCH₂C(OH)CH₂CO₂, predominates.     

CO hydrogenation on stepped Cu and CuZn alloy surfaces: Competition between methanol synthesis and methanation pathways

Comprehensive fundamental understanding of CO hydrogenation reactions over Cu and ZnCu alloy surfaces is of great importance. Herein, we report a comparative DFT calculation study of elementary surface reaction network of CO hydrogenation reactions on stepped Cu(211), Cu(611), ZnCu(211) and ZnCu(611) surfaces. On ZnCu(211) and ZnCu(611) surfaces, the energetic favorable reaction path of CO hydrogenation reaction follows CO* → HCO* → H₂CO* → H₃CO* → CH₃OH* → CH₃OH with H₃CO* hydrogenation as the rate-limiting step and proceeds more facilely on ZnCu(611) surface than on ZnCu(211) surface. On Cu(211) and Cu(611) surfaces, the energetic favorable reaction path of CO hydrogenation reaction follows CO* → HCO* → HCOH* → H₂COH* → H₃COH* → CH₃* → CH₄* → CH₄ with H₂COH* hydrogenation as the rate-limiting step and proceeds more facilely on Cu(611) than on Cu(211). The key difference of CO hydrogenation reaction on ZnCu alloy surface and Cu is that the resulting CH₃OH* species desorbs to produce CH₃OH on ZnCu alloy but undergoes H*-assisted decomposition to CH₃* and eventually to CH₄ on Cu surface. These results successfully unveil elementary surface reaction networks and structure sensitivity of Cu and ZnCu alloy-catalyzed CO hydrogenation reactions.     

Water‐Enhanced Synthesis of Higher Alcohols from CO2 Hydrogenation over a Pt/Co3O4 Catalyst under Milder Conditions

The effect of water on CO₂ hydrogenation to produce higher alcohols (C₂–C₄) was studied. Pt/Co₃O₄, which had not been used previously for this reaction, was applied as the heterogeneous catalyst. It was found that water and the catalyst had an excellent synergistic effect for promoting the reaction. High selectivity of C₂–C₄ alcohols could be achieved at 140 °C (especially with DMI (1,3‐dimethyl‐2‐imidazolidinone) as co‐solvent), which is a much lower temperature than reported previously. The catalyst could be reused at least five times without reducing the activity and selectivity. D₂O and ¹³CH₃OH labeling experiments indicated that water involved in the reaction and promoted the reaction kinetically, and ethanol was formed via CH₃OH as an intermediate.     

Some transesterification reactions of [Ir(CO2Me)(CO)2(PPh3)2]

The iridium(I) complex [Ir(CO₂Me)(CO)₂(PPh₃)₂] undergoes a transesterification reaction with the alcohols CH₂C(R)CH₂OH (R = H, Me), MeCCCH₂CH₂OH, and HOCH₂CH₂OH to afford the complexes

[Ir(CO₂CH₂CH₂CMe)(CO)₂(PPh₃)₂] and [Ir(CO₂CH₂CH₂OH)(CO)₂(PPh₃)₂], respectively. In contrast the acetylenic alcohol HCCCH₂CH₂OH gives [Ir(CCCH₂CH₂OH)(CO)PPh₃)₂]. Some reactions of the new complexes are described.     

Experimental Evidence of CO2 Photoreduction Activity of SnO2 Nanoparticles

Tin dioxide (SnO₂) has intrinsic characteristics that do not favor its photocatalytic activity. However, we evidenced that surface modification can positively influence its performance for CO₂ photoreduction in the gas phase. The hydroxylation of the SnO₂ surface played a role in the CO₂ affinity decreasing its reduction potential. The results showed that a certain selectivity for methane (CH₄), carbon monoxide (CO), and ethylene (C₂H₄) is related to different SnO₂ hydrothermal annealing. The best performance was seen for SnO₂ annealed at 150 °C, with a production of 20.4 μmol g⁻¹ for CH₄ and 16.45 μmol g⁻¹ for CO, while for SnO₂ at 200 °C the system produced more C₂H₄, probably due to a decrease of surface −OH groups.     

Fischer–Tropsch Synthesis by Carbon Dioxide Hydrogenation on Fe-Based Catalysts

Impregnated and co-precipitated, promoted and unpromoted, bulk and supported iron catalysts were prepared, characterized, and subjected to hydrogenation of CO₂ at various pressures (1–2 MPa) and temperatures (573–673 K). Potassium, as an important promoter, enhanced the CO₂ uptake and selectivity towards olefins and long-chain hydrocarbons. Al₂O₃, when added as a structural promoter during co-precipitation, increased CO₂ conversion as well as selectivity to C₂₊ hydrocarbons. Among V, Cr, Mn and Zn promoters, Zn offered the highest selectivity to C₂–C₄ alkenes. The different episodes involved in the transformation of the catalyst before it reached steady-state were identified, on the co-precipitated catalyst. Using a biomass derived syngas (CO/CO₂/H₂), CO alone took part in hydrogenation. When enriched with H₂, CO₂ was also converted to hydrocarbons. The deactivation of impregnated Fe–K/Al₂O₃ catalyst was found to be due to carbon deposition, whereas that for the precipitated catalyst was due to increase in crystallinity of iron species. The suitability of SiO₂, TiO₂, Al₂O₃, HY and ion exchanged NaY as supports was examined for obtaining high activity and selectivity towards light olefins and C₂₊ hydrocarbons and found Al₂O₃ to be the best support. A comparative study with Co catalysts revealed the advantages of Fe catalysts for hydrocarbon production by F–T synthesis.