Operando Investigation of Ag‐Decorated Cu2O Nanocube Catalysts with Enhanced CO2 Electroreduction toward Liquid Products

Abstract Direct conversion of carbon dioxide into multicarbon liquid fuels by the CO2 electrochemical reduction reaction (CO2RR) can contribute to the decarbonization of the global economy. Here, well‐defined Cu2O nanocubes (NCs, 35 nm) uniformly covered with Ag nanoparticles (5 nm) were synthesized. When compared to bare Cu2O NCs, the catalyst with 5 at % Ag on Cu2O NCs displayed a two‐fold increase in the Faradaic efficiency for C2+ liquid products (30 % at −1.0 VRHE), including ethanol, 1‐propanol, and acetaldehyde, while formate and hydrogen were suppressed. Operando X‐ray absorption spectroscopy revealed the partial reduction of Cu2O during CO2RR, accompanied by a reaction‐driven redispersion of Ag on the CuOx NCs. Data from operando surface‐enhanced Raman spectroscopy further uncovered significant variations in the CO binding to Cu, which were assigned to Ag−Cu sites formed during CO2RR that appear crucial for the C−C coupling and the enhanced yield of liquid products.


Introduction
In the quest for developing asustainable energy economy, the electrochemical reduction of carbon dioxide (CO 2 RR) into value-added chemicals and fuels offers the potential to close the anthropogenic carbon cyclea nd store renewable (wind, solar,hydro) energy into chemical bonds. [1] It has been therefore of particular interest to develop efficient and selective electrocatalysts,w hich reduce the reaction overpotential and steer the reaction toward hydrocarbons and multicarbon oxygenates (C 2+ ). Thes elective generation of liquid products such as ethanol, 1-propanol, and acetaldehyde is highly desirable due to their high energy densities and advantages for storage/transport as compared to gaseous products. [2] Av ariety of metal electrodes can be used to catalyze CO 2 RR as demonstrated in the pioneering work by Hori. [3] While some metals primarily reduce CO 2 to CO (Ag, Au,Zn) or formate (Sn, In, Bi), copper is the only metal yielding products such as methane,e thylene,a nd ethanol in considerable amounts. [4] However,t he selective conversion to C 2+ products in the form of liquids (alcohols and carbonyls) still requires high overpotentials,s uffers from low current densities that can be achieved, and the generation of parasitic hydrogen through the competing hydrogen evolution reaction (HER). Va rious strategies have been developed to enhance the performance of Cu-based catalysts,i ncluding nanostructuring Cu (control of exposed facets,d efects and lowcoordinated sites), [5] engineering the Cu-electrolyte interface (change of local pH), [6] and adjusting the Cu oxidation state (compositional change). [7] Fore xample,C u 2 On anocubes (NCs) with well-ordered (100) facets have been shown to lead to an increase in the selectivity toward ethylene,w hile suppressing methane production. [5e,8] Ap romising way to further improve the catalysts performance and selectivity is the introduction of asecondary metal. [9] Recent studies of Cu-Ag bimetallic systems showed enhanced selectivity for C 2+ products.I np articular,aphaseblended Ag-Cu 2 Oc atalyst had at hree times higher Faradaic efficiency (FE) toward ethanol than Ag-free Cu 2 O, but suffered from low activity (j j ethanol j < 1mA). [10] Additionally, Ag-covered Cu 2 Onanowires prepared via agalvanic replacement reaction enabled a1 .4 times higher current density toward ethylene production as compared to pure Cu 2 O nanowires. [11] Furthermore,C uAg surface alloys have been found to be more selective for the formation of multi-carbon products than pure copper. [12] Thef acilitated yield of C 2+ products in the bimetallic system is usually linked to the suppression of HER due to the enhanced coverage of *CO adsorbates as compared to *H, [12b] and the diffusion of CO from Ag sites to Cu sites that enables C À Cc oupling (CO spillover). [10,11] Note here that as hort diffusion path of CO and therefore ah omogeneous distribution of Cu and Ag at the surface of the catalyst are essential for an effective CO spillover. [10, 12b] Nonetheless,although the spatial arrangement of Cu and Ag in these studies seems to play akey role for the different CO 2 RR selectivity trends obtained, the key parameters for achieving an optimal synergy in Cu-Ag bimetallic systems are still unknown. In particular,o pen questions still remain on the composition and structure of the most active and C 2+ -selective systems under operando CO 2 RR conditions,including the stability of Cu 2 O, which might be modified by introducing Ag. [13] Herein, we prepared well-defined Cu 2 ONCs (35 nm) uniformly covered with Ag nanoparticles (NPs,5nm) by afacile wet-chemical ligand-free synthesis.Employing ex situ, in situ and operando characterization techniques,w eg ained insight into the morphology,chemical state,composition, and adsorbates of the Cu-Ag catalyst under CO 2 RR conditions. In particular,w ed iscuss reaction-induced Ag redispersion, Cu-Ag surface alloy formation, the influence of Ag on the reduction of Cu 2 O, the adsorption of CO on Cu and Ag,and the effect of the former parameters in the CO 2 RR activity and selectivity.  Table S1) reveal an average Cu/O at %r atio of 66:34, which corresponds to the composition of Cu 2 O. In the following,t he Cu 2 ONCs decorated with 3a nd 5at% of Ag will be denoted 3-Ag/ Cu 2 Oa nd 5-Ag/Cu 2 O. TheA gNPs with ad iameter of 4.6 AE 1.1 nm (3-Ag/Cu 2 O, Figure 1e,S2) and of 6.0 AE 2.1 nm (5-Ag/ Cu 2 O, Figure 1i,S3) appear to be uniformly distributed on the surface of the Cu 2 ONCs.T he STEM-EDX maps of the asprepared 3-and 5-Ag/Cu 2 Oc atalysts (Figure 1f,j) indicate ac lear phase separation between the Ag NPs and the Cu 2 ONCs.

Results and Discussion
After 2h of CO 2 RR at À1.0 V RHE in 0.1 m KHCO 3 ,t he cubic morphology appeared less pronounced and hollow structures formed in all cases (Figure 1c,g,k). Simultaneously, the edge length of the NCs decreased in average by 3nm (Table S2) and the size distribution broadened, as reported in the literature for pure Cu 2 ONCs after CO 2 RR. [8] STEM-EDX maps after reaction (Figure 1d,h,l) reveal that Cu 2 Oi s partially reduced to metallic Cu (Table S3) and that the clear phase separation between Ag and Cu is lost. Instead, small Ag clusters are dispersed on the Cu surface.I na ddition, some Ag-rich areas with sizes of 6.3 AE 1.6 nm (3-Ag/Cu 2 O) and 9.5 AE 2.5 nm (5-Ag/Cu 2 O) were also found (Table S2).
X-ray diffraction (XRD) was applied to confirm the phase purity of the catalysts and to track the evolution of the crystal structure after CO 2 RR. Figure S4 shows the XRD pattern of the as-prepared Cu 2 ONCs and Ag/Cu 2 Owith the main Cu 2 O reflections assigned to (111) at 36.48 8 and (200) at 42.38 8.T he presence of metallic Ag can be seen by the Ag(111) at 38.28 8 and Ag(200) at 44.58 8 for the Ag/Cu 2 O. Thel ow intensity of the fcc Ag reflections can be attributed to the low Ag loading and XRD peak broadening due to small particle sizes. Figure 1. STEM-HAADF images with corresponding EDX maps of Cu 2 ONCs, 3-Ag/Cu 2 O, and 5-Ag/Cu 2 Oi nthe upper,m iddle, and lower panels, respectively.As-prepareds amples are shown on the left (a,e,i)w ith EDX maps in (b,f,j) and samples after 2hof CO 2 RR at À1.0 V RHE on the right (c,g,k) with EDX maps in (d,h,l). The scale bars correspond to 50 nm. Table 1shows the as-prepared composition and coherence length derived from Rietveld refinement of the XRD patterns.T he coherence length of Cu 2 Oa grees well with the size distribution obtained by STEM analysis,a lthough it is slightly smaller than the mean NC edge length. Thea tomic fractions of metallic Ag in the as-prepared Ag/Cu 2 Oa gree well with the Cu/Ag composition obtained by ICP-MS (Table S5). This confirms that the majority of the added Ag from the AgNO 3 solution was incorporated in the Ag NPs, and that the initial ratios of the metal salts utilized were maintained.
Thes tructural evolution of the 5-Ag/Cu 2 Oc atalyst was investigated before and after 2h of CO 2 RR using ex situ grazing incidence (GI) XRD ( Figure 2, Table 1) and shows the reduction of Cu 2 Ot om etallic Cu as prominently seen in the Cu(111) reflection at 43.28 8.T he background arises from the carbon paper support ( Figure S5). Rietveld refinement of 5-Ag/Cu 2 Oa fter CO 2 RR suggests am ixture of Cu 2 Oa nd metallic Cu and as ignificantly increased Ag fraction, while ICP-MS did not show changes in the catalyst composition (Table S6). Thus,w ea re missing ac onsiderable fraction of Cu/CuO x after reaction according to XRD.T his suggests that Cu might be present in non-crystalline domains,r esulting in an increased Ag/Cu ratio.Additionally,t here is as light contraction of the Ag lattice (4.092 AE 0.003 to 4.088 AE 0.003 ), which could be explained by the incorporation of Cu into the Ag lattice (Table S4). Notably,t he coherence length of Cu 2 Od ecreased strongly from 23.1 nm (as-prepared) to 6.8 nm (after CO 2 RR), reaching as imilar value to the coherence length of the metallic Cu phase (9 nm). We conclude that the hollow structures after CO 2 RR ( Figure 1) might consist of am ixture of Cu 2 Oa nd Cu crystallites as seen in XRD.T he coherence length of the metallic Ag phase is on average slightly decreased after CO 2 RR, which agrees with the partial re-dispersion of Ag on the Cu surface and the Ag-rich domains likely being formed from multiple Ag NPs.
Quasi-in situ X-ray photoelectron spectroscopy (XPS) measurements were performed to gain deeper insight into the surface composition and chemical state of the Ag NPdecorated and pure Cu 2 ONCs before and after 2h of CO 2 RR. Figure 3p resents the Ag 3d and Cu Auger LMM spectra, while Figure S6 shows the Cu 2p spectra. TheA g3d core level regions of 3-and 5-Ag/Cu 2 O ( Figure 3a,b) reveal that Ag is in the metallic state before and after CO 2 RR, which is also consistent with the XRD results.T he Ag/Cu surface composition ratios were determined by integrating the peak areas of Ag 3d 5/2 and Cu 2p 3/2 .A se xpected for Ag NPs decorating the surface of Cu 2 ONCs,t he Ag/Cu ratio extracted from the more surface-sensitive XPS technique was higher than the bulk composition obtained from the XRD and ICP-MS analysis.A fter CO 2 RR, the Ag/Cu XPS ratios further increased from 4:96 to 7:93 at %i n3 -Ag/Cu 2 Oa nd from 9:91 to 11:89 in 5-Ag/Cu 2 O. Thus,w ec onclude that amore homogeneous distribution of Ag (redispersion) on the Cu 2 Os urface takes place during CO 2 RR. Furthermore,w e can exclude significant preferential dissolution of Cu during CO 2 RR due to the constant bulk Ag/Cu ratio before and after CO 2 RR as demonstrated by ICP-MS (Table S6). Theh igher Ag signal observed after CO 2 RR agrees with the formation of smaller Ag NPs and clusters and an enhanced Ag dispersion, as also revealed by the STEM and XRD data.
Additionally,d econvolution of the Cu LMM spectra (Figure 3c,d, Table S7) was carried out to distinguish Cu 0 , Cu I ,a nd Cu II near-surface species.I nt he as-prepared state, the samples consisted mainly of Cu 2 Ow ith ac ontribution of CuO.A fter 2h of CO 2 RR, the near-surface regions of the Cu 2 ONCs and Ag/Cu 2 Os amples were fully reduced to metallic Cu within the error margin. In contrast to prior studies,o ur samples were not exposed to air after CO 2 RR, since our electrochemical cell is directly connected to the XPS chamber.Thus,even though no potential is applied during the XPS measurement, re-oxidation in air can be excluded.
Theproduction of ethanol at À1.0 V RHE increases significantly with the amount of Ag. Cu 2 ONCs,3-, and 5-Ag/Cu 2 O reach 10 %, 13 %, and 17 %F Ef or ethanol at À1.0 V RHE , respectively (Figures 4a,S 7a). Thus,F E ethanol increased 1.5 times by the addition of 5at% of Ag to Cu 2 ONCs. Additionally,t he production of 1-propanol is doubled in the Ag/Cu 2 Os amples with FE of 5-6 %a tÀ0.9 V RHE compared to 3% for the Cu 2 ONCs (Figure 4b). This increase in alcohol yield has been previously linked to CO spillover from Ag to Cu, since the weaker binding between Ag and the *CO intermediate is considered to facilitate CO production as compared to the moderate binding energy between Cu and the *CO intermediate leading to the formation of hydrocarbons. [4,10] Also in our case,t he Ag/Cu 2 Os amples show higher FEs for CO at low overpotentials,with almost 40 %FE at À0.7 V RHE in comparison to 20 %F Ef or the pure Cu 2 ONCs ( Figure S8b). At higher overpotentials,t he FE CO decreases for all catalysts,a nd starting from À1.0 V RHE the selectivity for CO of the Ag/Cu 2 Oiseven similar to that of the bare Cu 2 ONCs (FE CO = 3-5 %). We note that 3-Ag/Cu 2 Os hows ab etter ability to transform CO 2 into C 2+ products at lower overpotentials than 5-Ag/Cu 2 O, which is also related to al ower FE of CO starting from À0.83 V RHE ( Figure S8b). These differences might originate from the different sizes of the Ag NPs in the two samples as extracted by STEM analysis (Table S2), which can affect the electrocatalytic reduction of CO 2 to CO as well as the CO spillover to the Ag/Cu interface.
Furthermore,t he parasitic HER ( Figure S8a) and the production of formate (the only C 1 liquid, Figure S8d) were (slightly) suppressed. Thel atter has also been recently observed for AgCu foam catalysts as compared to pure Cu foams. [14] In contrast, the formation of acetaldehyde (Figure S9a) and propionaldehyde ( Figure S9b) is significantly increased for 5-Ag/Cu 2 O, which results in an enhancement of the carbonyl C 2+ products by afactor of three (Figure 4f,S7f). Overall, introducing Ag increases the FE of the desired C 2+ liquid products (Figure 4e,S 7e) for 5-Ag/Cu 2 Ob y1 5% in comparison to Cu 2 ONCs at À1.0 V RHE .This is assigned to the enhanced production of ethanol, 1-propanol, allyl alcohol, and carbonyl C 2+ products (Figure 4a,b,f,S 9c). We additionally performed long-term CO 2 RR measurements for 12 ha t À1.0 V RHE to track the stability of the catalytic performance ( Figure S12) and found ag ood stability for all catalysts.F or the Ag/Cu 2 Os amples,t he amount of C 2+ liquid products remains stable over time with adecrease of the aldehydes and increase of 1-propanol as well as acetate for the 5-Ag/Cu 2 O catalyst.
In order to further examine the Cu-Ag interaction and its role in the CO 2 RR selectivity,w ea lso investigated the catalytic performance of the pure Cu 2 ONCs deposited on ap olished Ag foil (Cu 2 O/Ag). In this configuration (Figure S13), the production of C 2+ liquids is also enhanced on the Cu 2 O/Ag sample at À1.0 V RHE ,i ndicating again the importance of the CO generated at the Ag sites for the further hydrogenation of the CÀCproducts generated on the Cu sites. However,t he Cu 2 O/Ag sample shows ad rastic increase of HER as compared to the 5-Ag/Cu 2 O, namely from 20 to 40 % FE, and asuppression of the ethylene FE from 30 %to20%. This might be correlated to the decrease of the total area of the Ag/Cu interface,w hich decreases the atomic interaction and synergy between Ag and Cu. Consequently,t he welldistributed Ag NPs,w hich redispersed on the surface of our Cu 2 ONCs,a ppear to play as ignificant role in the observed synergistic effect and the C À Ccoupling mechanism.
Further insight into the Ag-Cu interaction can be extracted from operando X-ray absorption spectroscopy (XAS) data. Figure 5d epicts the normalized Cu K-edge and Ag K-edge X-ray absorption near edge structure (XANES) spectra of the Ag NP-decorated and pure Cu 2 ONCs in their as-prepared state (Figure 5a,c) and during CO 2 RR in as teady-state after 5h of the reaction (Figure 5b,d). The position of the absorption edge in the Cu K-edge XANES spectra of the as-prepared Cu 2 ONCs compared to the reference spectra shows that the NCs mainly exhibit aC u I oxidation state with the characteristic pre-edge feature at 8981 eV.L inear combination analysis (LCA) of Cu K-edge XANES data revealed the presence of Cu I and Cu II species ( Figure S14), as already seen in the more surface-sensitive XPS analysis.T he decoration with the Ag NPs does not change the Cu XANES spectra (Figure 5c). Ag K-edge XANES spectra, in turn, confirm the metallic state of Ag with all XANES features resembling those of the Ag foil. These findings are consistent with the STEM, XRD,and XPS results,e mphasizing the lack of significant interaction between the Cu 2 Oand Ag species in the as-prepared state.
Ther eduction of the Cu 2 ONCs was investigated during CO 2 RR at À1.0 V RHE for 5h( Figure S14). In the final state, the Cu K-edge XANES spectrum resembles that of the metallic Cu foil reference (Figure 5b). We tracked the evolution of the Cu 0 /Cu I /Cu II ratios by LCA ( Figure S14b,c) and our analysis revealed that asignificant fraction of Cu I was preserved under CO 2 RR reaction conditions with 15-25 %of Cu I present in all samples.F igure 5d shows the Ag K-edge XANES spectra under CO 2 RR conditions at À1.0 V RHE after reaching stationary conditions.Agr emained metallic,b ut an attenuation of the post-edge oscillations is visible during CO 2 RR, explainable by adecrease in the size of the Ag NPs due to their redispersion on the Cu 2 ONCs urface. [15] To achieve ad eeper understanding of the local atomic structure,F ourier transformed extended X-ray absorption fine structure (FT-EXAFS) spectra of the Ag NP-decorated and pure Cu 2 ONCs are shown in Figure 6w ith the corresponding Fourier-filtered EXAFS spectra in Figure S15. The peaks at 1.5 and 2.8 (phase uncorrected) in Cu K-edge FT-EXAFS and at 2.8 in Ag K-edge FT-EXAFS for the as-  Under CO 2 RR conditions at À1.0 V RHE ,as trong peak at 2.2 (phase-uncorrected) appeared in the Cu K-edge FT-EXAFS,w hich can be attributed to the CuÀCu contribution in metallic Cu. TheA g ÀMp eak position at 2.8 in Ag Kedge FT-EXAFS did not change significantly,a nd no additional peaks appeared. Nonetheless,t he comparison of the FT-EXAFS before and during CO 2 RR shows ad ecrease in the Ag À Mp eak intensities.M oreover,t he intensity of both, the CuÀCu and AgÀMpeaks decreases with an increase in Ag loading ( Figure 6). These differences suggest structural changes in the Ag and Cu atomic environment under CO 2 RR conditions,w hich we further investigated using quantitative fitting of the EXAFS spectra to obtain the structural parameters presented in Tables S10-S11. Forthe as-prepared state,w eo btained aC u À Oc oordination number (N CuÀO )o f circa 2a nd aC u ÀOd istance (R CuÀO )o fc irca 1.87 ,w hich agree with the values of these parameters in bulk Cu 2 O. Furthermore,i nt he as-prepared samples,t he N AgÀAg de-creased from 12 in the Ag foil over 11.4 AE 0.4 in 3-Ag/ Cu 2 Ot o1 0.5 AE 0.3 in 5-Ag/ Cu 2 O, indicating an enhanced disorder within the Ag NPs.H owever,t he R AgÀ Ag distances in all samples are comparable to that of the Ag foil, 2.833 AE 0.003 , which means that there is no significant lattice contraction due to alloying in the as-prepared state.
Under stationary CO 2 RR conditions,t he Cu À Cu coordination number of approx. 9i sl ower than that of the Cu foil reference (12). We also observe CuÀOb onds with N CuÀO % 0.3. TheC u ÀCu distance (R CuÀCu )d uring CO 2 RR, 2.524 AE 0.003 ,i s comparable to that in the bulk Cu foil reference (2.527 AE 0.002 ). TheC u 0 / Cu I ratio under CO 2 RR, as obtained from the EXAFS analysis,a grees well with the XANES data (Table S9), showing the partial reduction of the Cu 2 Ot o metallic Cu under CO 2 RR conditions.O ur primary finding from the Cu K-edge is the increasing disorder of the CuÀCu bonds with higher Ag loading and thus reveals the decoration of the Cu 2 ONCs with Ag NPs during CO 2 RR, but one cannot exclude that this could also be aresult of incomplete CuO x reduction.
One of the main goals of this study is to explore the interaction between Cu and Ag at the atomic scale under CO 2 RR conditions.D ue to the low concentration of Ag as compared to Cu, information on the interplay between Ag and Cu atoms can be best extracted from the analysis of Ag EXAFS data (Figure 6c,d, S16). During CO 2 RR, we identified an additional contribution of Ag À Cu bonds with R AgÀCu of 2.623 AE 0.005 (3-Ag/Cu 2 O) and 2.596 AE 0.008 (5-Ag/ Cu 2 O), while the bond lengths in the more prominent AgÀ Ag component did not contract significantly for 3-Ag/Cu 2 O (2.840 AE 0.005 ), but did so for 5-Ag/Cu 2 O( 2.787 AE 0.007 ). TheA g ÀAg coordination number decreased from 12 (bulk) to 9, and an Ag À Cu coordination number (N AgÀCu ) of up to 1was obtained under CO 2 RR conditions,being larger for the 3-Ag/Cu 2 Osample than for 5-Ag/Cu 2 O. Theobtained AgÀCu bond length is in between the values for the CuÀCu distance in bulk Cu (2.527 AE 0.002 )and the AgÀAg distance in bulk Ag (2.833 AE 0.003 ). TheA g ÀCu bond lengths as well as the coordination numbers suggest the partial incorporation of Ag into Cu-rich domains either as an AgCu phase or as dispersed particles or clusters on the Cu surface under CO 2 RR conditions.F urthermore,t he reduced total Ag À M coordination numbers with respect to those in the as-prepared samples suggest an increase in the disorder in the AgÀAg component during CO 2 RR, which agrees well with as maller particle size and/or Ag redispersion during CO 2 RR, as shown by the STEM results.T hus,weobserve the partial miscibility of Cu and Ag under CO 2 RR conditions by using operando Ag K-edge XAS,w hich was not provided in comparable studies so far.
We furthermore followed the reduction of the Cu 2 O species and the CO or CO-like intermediates chemisorbed on Cu and Ag at different potentials and during CO 2 RR via operando surface-enhanced Raman spectroscopy (SERS). Figure 7p resents potential-dependent SERS spectra of the Ag NP-decorated and pure Cu 2 ONCs acquired at open circuit potential (ocp) and between À0.4 and À1.1 V RHE from 150-700 cm À1 and 1850-2350 cm À1 (see also Figure S17).
In the as-prepared state, the characteristic peaks of Cu 2 Oa re observed at 415 cm À1 ,5 27 cm À1 ,a nd 623 cm À1 . [16] TheC u 2 O peaks vanished for all samples after applying ar eductive potential of À0.4 V RHE , which indicates ap rompt reduction of the surface Cu 2 Ospecies independently from the Ag loading.T he latter is in agreement with the quasi-in situ XPS analysis.
At more negative potentials,t he operando SERS data of the Ag NPdecorated Cu 2 ONCs exhibit significant differences compared to the bare Cu 2 ONCs during CO 2 RR: 1) At higher Raman shifts, ab road band of the C À O vibrations appeared at 2088 cm À1 for Cu 2 ONCs during CO 2 RR, while for Ag/Cu 2 Oashoulder at 2031 cm À1 is more pronounced. This might be assigned to the multisite binding mechanism on the Ag-Cu surface,w here each binding configuration has ad ifferent electron backdonating ability. [17] 2) For the Ag NP-decorated Cu 2 ONCs amples,t wo additional peaks at 490 and 530 cm À1 appeared, which might be assigned to AgÀCO vibrations. [11,18] 3) Tw op eaks evolve differently at 280 cm À1 and 366 cm À1 ,c aused by the Cu À CO frustrated rotation and stretching vibration, respectively. [16b,18] Thus,wecan track the CO adsorbed on Cu and Ag separately by using operando SERS and identified significant differences in the CO adsorption characteristics on Cu in the presence of dispersed Ag atoms during CO 2 RR as,i nterestingly,t he peak intensity ratio of the two Cu À CO Raman peaks shifts towards the Cu À CO stretching vibration with decreasing electrode potential. Asimilar trend can also be seen with increasing Ag content at À1.0 V RHE .T hese drastic changes might originate from the way that CO predominantly binds to Cu. While on pure Cu 2 ONCs the CO binding configuration is similarly prone to Cu À CO rotation and stretching,the presence of Ag sites gives rise to aCObinding configuration that facilitates the Cu À CO stretching with thus stronger lateral confinement. We anticipate that the latter plays acritical catalytic role in enhancing the CÀCc oupling of neighboring CO adsorbates and thus increases the C 2+ liquid product formation. From the ex situ and operando studies performed under CO 2 RR we gained ad etailed insight into the structural evolution of the Ag NP-decorated Cu 2 ONCcatalysts.Intheir as-prepared state,t he Cu 2 ONCs as well as the Ag NPdecorated samples mainly consist of Cu 2 O, with some contribution from CuO.D uring CO 2 RR, Cu 2 Oi sp artially reduced to metallic Cu, while the contribution of CuO vanishes.I nf act, the operando XANES data evidenced the incomplete reduction of Cu 2 Oi na ll samples after five hours of CO 2 RR (up to 30 %o fC u 2 Or emained in the 3-Ag/Cu 2 O sample). However,wedid not find acorrelation between the content of Cu 2 O( in the bulk, XAS data) and the increased selectivity for C 2+ liquid products,which might be due to the complete reduction (XPS,SERS) of the surface Cu 2 Ospecies to metallic Cu during CO 2 RR.
Since the fraction of Ag in the near-surface is low (< 11 at %), intermixing is plausible although aprecise determination of the Ag/Cu ratio in the Ag x Cu 1Àx regions is not possible.T herefore,o ur system consists of three classes of potentially active sites during CO 2 RR:Ag/AgCu/Cu.
To further understand how the structural/chemical rearrangements influence the reaction mechanism, it is useful to correlate such changes with the formation and stability of different intermediates during CO 2 RR. Thep roduction of CO increased at lower overpotentials with increasing Ag content in the samples. [19] It has been suggested that *CO, which is ak ey intermediate for the C À Cc oupling mechanism, [19] might couple with *CH x [20] to form the *CH 2 CHO intermediate,w hich plays ac entral role in the formation of C 2+ liquid products. [ If *CH 3 CHO couples with another *CO,propionaldehyde [Eq. (3)] and 1-propanol [Eq. (4)] can be formed.
On Cu surfaces,a cetaldehyde and propionaldehyde are usually hydrogenated to ethanol, and 1-propanol is only detected with FE % 1%,asseen for the Cu 2 ONCs.Therefore, we can relate the enhancement of the two aldehyde selectivities to the dispersion of the Ag atoms/small clusters on the Cu surface during CO 2 RR, which induces locally strained Cu sites with expanded CuÀAg distances compared to CuÀCu and, most importantly,d ifferences in the predominant CO binding motifs.D FT calculations of our previous study showed that an expanded Cu lattice increases the binding energies for the intermediates of CO 2 RR (e.g., *CO on Cu). [9c] Additionally,A gi ncorporation in Cu weakens the binding energies of the reduced aldehyde intermediates and inhibits their further reduction to ethanol and 1-propanol as demonstrated in ar ecent DFT study. [22] However, the structural analysis of the catalysts showed that the surface partially consists of Cu/Ag areas,w hich also leads to the formation of ethanol and 1-propanol at the Cu sites,since they are expected to have higher binding energies for the oxygenated intermediates as compared to the Ag-Cu mixed regions.T herefore,h aving Ag/AgCu/Cu interfaces as active surface sites appears to enhance the yield of C 2+ liquid products.

Conclusion
In summary,AgNP-decorated Cu 2 Onanocubes displayed enhanced selectivity toward C 2+ liquid products,w hile the production of formate and hydrogen was suppressed. By means of ex situ, quasi-in situ, and operando spectroscopy studies under CO 2 RR conditions,w ec ould gain insight into the structural and chemical transformations of these catalysts that were shown to influence the selectivity trends.I n particular,w ef ound the redispersion of the Ag NPs on Cu and acertain Cu-Ag miscibility,leading to expanded Cu-Ag distances compared to metallic Cu-Cu distances.S uch structural rearrangements were found to result in an enhanced formation of alcohols and aldehydes.
By comparing the selectivity of pure Cu 2 Oa nd Ag NPdecorated Cu 2 ONCs we concluded that even though Cu 2 O species were partially preserved under reaction conditions, they are not the sole species responsible for the enhancement in the C 2+ liquid products,which is favored when large Ag/Cu interfaces are formed. Importantly,wecorrelate the enhancement to variations in the predominant CO adsorption motif on Cu in the presence of dispersed Ag atoms.
Our work contributes to the fundamental understanding of the CO 2 RR by highlighting the intricate interplay of different parameters affecting the selectivity.T hese include the content of residual Cu 2 Os pecies,t he presence of as econdary metal near Cu where efficient CO spillover can take place,a nd the alloying of Cu with another metal that is able to locally increase the interatomic distance,l eading to ac hange in the binding energies of adsorbates and intermediates,t hus favoring the formation of C 2+ liquid products.