Kinetic and Computational Studies of CO Oxidation and PROX on Cu/CeO2 Nanospheres

As supported CuO is well-known for low temperature activity, CuO/CeO2 nanosphere catalysts were synthesized and tested for CO oxidation and preferential oxidation of CO (PROX) in excess H2. For the first reaction, ignition was observed at 95 °C, whereas selective PROX occurred in a temperature window from 50 to 100 °C. The catalytic performance was independent of the initial oxidation state of the catalyst (CuO vs. Cu0), suggesting that the same active phase is formed under reaction conditions. Density functional modeling was applied to elucidate the intermediate steps of CO oxidation, as well as those of the comparably less feasible H2 transformation. In the simulations, various Cu and vacancy sites were probed as reactive centers enabling specific pathways. Supplementary Information The online version contains supplementary material available at 10.1007/s11244-023-01848-x.


Introduction
Hydrogen gas that is produced via steam-reforming of hydrocarbons or alcohols and subsequent water-gas-shift (WGS) still contains high levels of CO.For pure H 2 streams, e.g., required for fuel cells, the CO must be removed, e.g., by preferential oxidation of carbon monoxide, CO-PROX [1][2][3].PROX means that the oxidation of CO (CO + 0.5 O 2 → CO 2 ) is preferred over H 2 oxidation (H 2 + 0.5 O 2 → H 2 O), avoiding the consumption of valuable hydrogen.As mentioned, this process is crucial for using the resulting H 2 stream in polymer electrolyte membrane fuel cells (PEMFCs), because CO poisons the Pt electrodes, and significantly decreases the efficiency [3][4][5][6][7].Accordingly, the CO concentration must be reduced to 10-50 ppm before feeding the H 2 stream to the cells [8][9][10].A variety of heterogeneous catalysts has shown promising performance in CO-PROX, for example, noble metals (Pd, Pt, or Rh) supported on alumina [3,11,12].Nevertheless, the high-cost of precious metals inspired the search for low-cost alternatives.
Wang et al. prepared a catalyst of CuO particles on hollow CeO 2 nanospheres, based on a metal-organic framework (MOF) precursor and using a template-free microwave method.In CO-PROX, 100% CO conversion was reached at about 80 °C [30].In another work, using a hard template method [31] CuO nanoparticles were loaded on the outer surface of hollow CeO 2 nanospheres in a layer-by-layer deposition technique.As reported in literature, the CuO surface plays a vital role as an active site for CO and H 2 oxidation while, after the formation of CO 2 and H 2 O, CeO 2 provides oxygen to fill the vacancies [24,[32][33][34][35].The oxygen transfer potential of the CeO 2 support is also promising for other oxides [36,37].
In the current work, CuO/CeO 2 nanosphere catalysts were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and CO/H 2temperature programmed reduction (TPR), before being examined for CO-PROX in an atmospheric flow reactor.Combining experiment with density functional theory (DFT) modeling of surface structures and adsorbed CO and H 2 on various metal and oxide sites enabled us to identify the active sites and the origin of high selectivity of CuO/CeO 2 .

Catalyst Synthesis and Characterization
The synthesis of CuO/CeO 2 nanosphere catalysts was described in detail in Ref [31].It is based on the hard template method, with CuO nanoparticles loaded on the outer surface of hollow CeO 2 nanospheres via layer-by-layer deposition (Fig. 1a).Details of the preparation (including a final calcination at 500 °C) are presented in the Supporting Information.CO, H 2 , O 2 , and He were obtained from Messer Group GmbH, (purity 4.7 for CO and 5.0 for the others).

XRD
X-ray diffraction (XRD) patterns of oxidized and reduced catalysts were collected on a Philips XPERT-PRO diffractometer using Cu K-α radiation (1.5406 Å; 45 kV; 40 mA) operating in Bragg-Brentano reflection geometry with 2θ scanning from 20-90° (step size of 0.02°).Phase analyses and Rietveld refinements were performed with the High-Score Plus software (JCPDS data base) [38].

TEM
To study the size, morphology, and distribution of CuO/ Cu 0 particles on the ceria support, powder samples were drop-casted on carbon-coated gold grids and examined by transmission electron microscopy (TEM) in a TECNAI F20 microscope operated at 200 kV.Before TEM, the CuO/CeO 2 catalyst was oxidized at 400 °C with 20% oxygen in He.

CO-and H 2 −TPR
CO-and H 2 -temperature programmed reduction (TPR) techniques were applied to characterize the reducibility of the catalysts at atmospheric pressure.Approximately 20 mg catalyst was loaded in a continuous-flow fixed-bed quartz reactor between two quartz wool plugs.Before each TPR run, contaminants were removed by pretreatment with 20% O 2 in He (50 mL min −1 ) at 400 °C for 30 min (heating rate of 10 °C min −1 ).Samples were then cooled down to 30 °C in a flow of the same composition and purged with He for 30 min at the same temperature.The pretreated samples were then exposed at room temperature to either a mixture of 5 vol% CO in He (CO-TPR) or 5 vol% H 2 in He (H 2 -TPR) at a flow rate of 50 mL min −1 , before being subsequently heated to 400 °C (heating rate of 10 °C min −1 ).CO or H 2 consumption and CO 2 or H 2 O evolution, respectively, were analyzed by an online quadrupole mass spectrometer (QMS, Prisma Plus QMG220, Pfeiffer Vacuum) equipped with a secondary electron multiplier (SEM) detector.

Flow Reactor Studies (CO Oxidation and PROX)
Catalytic reactions of CO oxidation and preferential CO oxidation (PROX) were carried out in the same reactor as for TPR.Concentrations of reactants and products in the outlet stream were monitored by the MS and additionally by gas chromatography (GC, Agilent 6890) using a HP-PLOT Q column, a flame-ionization detector (FID) with a methanizer and a thermal conductivity detector (TCD).
Before CO oxidation and PROX experiments, each fresh catalyst (20 mg) was pretreated as described in the following.In all cases, the heating rate was 10 °C min −1 and the pretreatment time was 30 min each before cool-down to room temperature.For oxidation 20 vol% O 2 in He was used, for reduction 5 vol% H 2 in He, with a total flow rate of 50 mL min −1 .For both a heating rate of 2 °C min −1 was applied.
Temperature-dependent CO oxidation was performed for three differently pretreated catalysts: (i) oxidation at 300 °C, (ii) oxidation at 300 °C followed by reduction at 300 °C, and (iii) oxidation at 500 °C followed by reduction at 500 °C.The feed composition was 5 vol% CO and 10 vol% O 2 in He (heating rate of 2 °C min −1 ).Additionally, to mimic PROX without H 2 , 1 vol% CO and 1 vol% O 2 in He was supplied for a sample oxidized at 400 °C.
For PROX, two different pretreatments were applied: (i) oxidation at 400 °C, (ii) reduction at 300 °C after pretreatment).The PROX reaction was then performed in 1 vol% CO, 1 vol% O 2 , and 50 vol% H 2 in He.
The conversion of CO ( X CO ) and O 2 ( X O 2 ), and the CO 2 selectivity ( S CO 2 ) were calculated from the GC peak areas of CO and O 2 in the inlet and outlet of the reactor using the following formulae: To determine the activation energy ( E a ) of PROX via the Arrhenius equation, the CO conversion (in the range below 30%) was measured for the oxidized and reduced catalyst at 75, 80, 85, and 90 °C.
Furthermore, we measured the reaction orders via varying the concentration of one reactant (at 70 °C and a total flow of 50 mL min −1 ).Accordingly, to determine the CO order, the CO concentration was varied between 0.5 and 2 vol%, while the O 2 and H 2 concentrations were kept constant at 1 vol% and 50 vol% (yielding about 8-9% CO conversion).To determine the O 2 order, the O 2 concentration was varied between 0.5 and 2 vol%, while the CO and H 2 concentrations were kept at 1 vol% and 50 vol%, respectively.

Computational Methods
Density functional theory calculations were carried out in spin-polarized fashion using the Vienna Ab initio Simulation Package (VASP) [39] utilizing the projector augmentedwave method (PAW) [40,41].The generalized gradient approximation (GGA) was used in the parameterization according to Perdew, Burke, and Ernzerhof (PBE) [42].The plane wave cutoff energy was set to 450 eV for all surface calculations.The following electron configurations were calculated explicitly, Cu(3d 10 4s 1 ), O(2s 2 2p 4 ), C(2s 2 2p 2 ), and H(1s 1 ).The strong correlations of d states were treated by a Hubbard model using the Dudarev formalism (DFT + U) [43].The electronic self-consistent loop was considered converged when the energy changes became smaller than 1 × 10 −8 eV.Atomic positions were optimized until the Hellmann-Feynman forces acting on each atom dropped below 0.02 eV/Å.According to the role of CeO 2 described in the Introduction, our DFT study focused on the CuO surface solely.
For these calculations, the most stable CuO(111) surface was chosen, as it has the lowest surface energy (see Tables S1, S2 in the Supplementary Information (SI)).The CuO(111) surface was cut from the calculated CuO unit cell, using 8 × 8 × 8 k-points and a cut-off of 600 eV.In this study we used a 2 × 1 periodic surface supercell p(2 × 1)-CuO(111) with a slab thickness of 5 layers and a vacuum gap of 15 Å keeping both "bottom" layers fixed at the bulk distance.A 2 × 3 × 1 Γ-centered k-point grid was sampled for surface calculations of CuO(111).van-der Waals interactions were included using the DFT-D3 method [44].
We chose an effective U value ( U eff ) of 7 eV as it tested quite reliable to describe the lattice constant and electronic properties, e.g., magnetic moment, derived from experimental work, see Fig. S1 and Table S4 of the SI.
The oxygen vacancy formation energy, E vac , on a surface slab, was computed according to where E defect∕surf is the total energy of a surface slab with an oxygen vacancy, V O , and E O 2 is the total energy of isolated triplet O 2 in the gas phase.The adsorption energy, E(X) ads , of an adsorbed molecule X (i.e., CO or H 2 ) was calculated in the following way where E X∕surf is the total energy of the molecule adsorbed on the surface, E surf is the total energy of the bare surface, and E X is the total energy of an isolated molecule in the gas phase.A negative E ads indicates a binding interaction.The desorption energy of molecule X, i.e., CO 2 and H 2 O, E des (X) is the negative value of E ads (X) .A charge analysis according to the procedure suggested by Bader [45] was carried out to evaluate the change in charge Δq as described in the SI.All simulation files used in this study can be found on the ioChem-BD platform [46].

Catalyst Characterization
Figure 1a illustrates the synthesis of the CuO/CeO 2 nanosphere catalysts.Figure 1b-d shows TEM images, displaying CeO 2 hollow spheres covered outside by CuO nanoparticles (partly decorated by CeO 2 ).The average size/diameter of the CuO/CeO 2 nanospheres was about 180 nm.The maps of energy dispersive X-ray analysis (EDX) (Fig. 1e and Table S1) display that cerium, copper, and oxygen are very homogeneously distributed over the hollow spheres and indicate a loading of ~ 10.5 wt% Cu or ~ 13 wt% CuO.
To determine the catalyst reducibility, CO-and H 2 -TPR were performed (Fig. 2a, b).For CO-TPR, a two-step reduction (Cu 2+ → Cu + → Cu 0 ) was indicated by peaks at 180 and 215 °C.In contrast, for H 2 -TPR, rather a one-step reduction (Cu 2+ → Cu 0 ) was suggested by a dominant peak at 205 °C.Overall, this behavior is quite similar to that of Co 3 O 4 reported before [27].As discussed below, these temperatures (representative of bulk reduction) are much higher than the typical reaction temperatures of CO oxidation and PROX.Nevertheless, surface reduction may still occur at lower temperatures.Figure 2c shows XRD patterns of the pre-oxidized (20 vol% O 2 in He, at 400 °C) CuO/CeO 2 catalyst, as well as of Cu 0 /CeO 2 , obtained by the reduction at 300 °C in 5 vol% H 2 (i.e., much higher than the 205 °C in Fig. 2b).The crystallite size was calculated using the Rietveld refinement and was 6 nm for CuO (PDF no; 00-006-2679, monoclinic) and 17 nm for Cu 0 (PDF no; 04-009-2090, cubic).The diffractogram confirms a complete reduction of CuO at 300 °C, while CeO 2 (PDF no; 04-003-1755, cubic) remained unaffected in both oxidized and reduced samples.The nanosphere shell is composed of CeO 2 crystals of about 10 nm in size (9.6 and 10 nm for oxidized and reduced, respectively).

CO Oxidation
Figure 3a, b displays results of CO oxidation (1 vol % CO and 1 vol % O 2 ) on nanosphere CuO/CeO 2 , analyzed simultaneously by MS and GC.An ignition behavior [47,48] was observed at 95 °C, leading to more than 90% CO conversion.Under similar feed conditions, a maximum CO conversion was observed at ~ 150 °C for Co 3 O 4 [27].For a conventional CuO/CeO 2 morphology, a temperature of > 100 °C was needed for 90% CO conversion [49,50].For pure CeO 2 , CO oxidation sets in above 325 °C [51,52].Interestingly, for the nanosphere catalysts, the reaction onset temperature was independent of the catalyst pretreatment, i.e., the preoxidized and reduced (both at 300 and 500 °C) catalysts behaved identically (Fig. 3c).It seems that the same reactive phase is obtained under reaction conditions.The similar activity despite the ~ 2.5 times lower dispersion of Cu 0 /CeO 2 may point to interface sites as active centers, as their number is less affected by particle size.

PROX
Analogous experiments were carried out for the PROX reaction.Figure 4a, b compares MS results for CuO/CeO 2 and Cu 0 /CeO 2 , with the reaction properties being basically identical: up to 100 °C, the catalyst shows 100% CO 2 selectivity.Once more, the same reactive state seems to manifest under reactive conditions.GC analysis for CuO/CeO 2 is shown in Fig. 4c, d, confirming the MS results (GC is less sensitive to small amounts of water than MS, though).Caputo et al. reported 100% CO 2 selectivity below 144 °C on 4 wt% CuO/CeO 2 catalyst (both oxidized and reduced) for preferential CO oxidation [53].Above 125 °C, significant water formation sets in, leading to a drop in selectivity, although methane formation did not occur.Above 175 °C, the CO conversion X CO and CO 2 selectivity S CO 2 further decreased, likely due to the onset of a reverse Water-Gas shift reaction (CO 2 + H 2 ↔ CO + H 2 O), for which the Cu-based catalyst supported on CeO 2 is known to be activated at temperatures around 200-300 °C [54,55].
It is noteworthy that pure CeO 2 shows PROX performance at high temperature with CO oxidation starting at around 270 °C (Fig. S2).This confirms that CuO on CeO 2 strongly decreases the temperature of CO oxidation, enabling PROX performance at low temperature for this type of catalyst.
The Arrhenius plots in Fig. 5a, b display the activation energies ( E a ) for CuO/CeO 2 (60.2 ± 1.7 kJ mol −1 ) and Cu 0 / CeO 2 (58.1 ± 2.1 kJ mol −1 ); note that the fitted values for both catalysts exhibit only a small difference (2.1 kJ mol −1 ) which is within the measurement accuracy.This once more demonstrates that the initial state of the catalyst after pretreatment is not decisive for PROX.The E a values agree with those reported in the literature for selective CO oxidation over copper-ceria catalysts ( E a range of 55-57.2kJ mol −1 ) [56,57].The E a of CO oxidation for copper-ceria catalysts is ca.20 kJ mol −1 lower than that of pure CuO catalysts, confirming that ceria aids in catalytic CO oxidation [58].
The active state of the current CuO/CeO 2 nanosphere catalysts has not yet been determined by in situ/operando studies [62,63].Nevertheless, sites at the Cu-O-Ce interface were often invoked previously to rationalize the high activity and selectivity [21][22][23][24].In order to differentiate the contributions of Cu 2+ and Cu + on the CuO nanoparticles for low-temperature oxidation, we will take up an alternative approach herein.We employ computational modeling to rationalize the experimental results, with a focus solely on the active surface of CuO(111) [24].

Computational Modeling
In order to obtain atomistic insights into the PROX reaction and the CO 2 selectivity, density functional theory (DFT + U) calculations were carried out to study the formation of an oxygen vacancy, its re-oxidation by O 2 , adsorption of CO and H 2 and reaction at various sites on a p(2 × 1)-CuO(111) model surface.Finally, PROX reaction pathways were evaluated, with a focus on the CO 2 selectivity over CuO at low temperature.

Surface Structure and Oxygen Vacancy Formation
The CuO(111) surface model has antiferromagnetic coupled spins in which the magnetic moment per Cu remained at 0.658 B , alike the bulk structure.This is consistent with previous theoretical and experimental reports of 0.63 and 0.68 B , respectively [64,65].Four distinct surface sites of Cu and O atoms exist at the CuO (111) surface, as indicated in Fig. 6a, b.A three-fold coordinated O atom, O 3c , represents the outermost surface atom while the four-fold coordinated O atom, O 4c , represents the innermost surface atom.
CO oxidation and PROX on reducible oxides are wellknown to proceed via a Mars-van-Krevelen mechanism, which involves the creation of oxygen vacancies [3, 24,  6c, d.The oxygen vacancy formation energies E vac were calculated using Eq. ( 1) and are listed in Table 1.Forming an oxygen vacancy at O 3c site (V O @O 3c ) is by 0.47 eV energetically more favorable than at the site O 4c (V O @O 4c ), most likely because the latter is coordinatively more saturated by four nearest neighbors (4NN) [70,71].
When one V O is created ( =1/8), two excess electrons localize at and reduce specific Cu atoms (Cu 2+ + e − -> Cu + ), as drawn in ocean green spheres in Fig. 6c, d.In the calculations, these reduced species are indicated by a change in the local magnetic moment and their modified atomic charge Δq , gaining 0.41 e − , Table S6.To create metallic Cu (Cu 0 ) on the CuO (111) surface, a V O coverage of ≥ 5/8 is needed, resulting in a change in charge Δq of 0.94 e − [64].Furthermore, we modeled the re-oxidation of the energetically favorable vacancy site (V O @O 3c ) by an O 2 molecule, in addition creating the new site O ad at the defective surface (Fig. 6e).Adsorption of O 2 is strongly favorable with an energy of − 1.99 eV compared to the adsorbed CO and H 2 species on bare surface (Sect.3.4.2).This is in line with the calculated V O formation energy.Notably, the adsorbed O 2 molecule shows an O-O bond length of 1.50 Å, which is elongated as compared to gas phase O 2 , 1.24 Å, suggesting that adsorbed O 2 is activated and nearly dissociated.In more detail, upon O 2 adsorption, electrons from reduced Cu (Cu + ) sites transfer to the oxygen moiety and occupy 2π * orbitals, leading to an elongation of the O-O bond, so that O 2 becomes a peroxo species, O 2 2− (see also the SI) [72][73][74].Such peroxo species have also been discussed in the context of tuning the surface reactivity [75].As reported in the literature, V O thus plays a vital role in the adsorption of the O 2 molecule on metal oxide surfaces [73,76,77].In view of the energetically demanding V O formation energies, promotion by interaction with CO or H 2 is required.Thus, molecular adsorption of CO and H 2 , as well as surface reactions, are discussed in the next section.

CO and H 2 Adsorption on the CuO(111) Surface
The stability of surface adsorption sites was probed in the presence and absence of oxygen vacancies.First, we studied the interaction of CO and H 2 with surface oxygen species, O 3c , and O 4c .CO adsorbs exothermically on the activated O 3c site, Fig. 7a, whereas at the O 4c site, CO moves away from the surface.The adsorption of CO on activated O 3c is quite exothermic E ads (CO) = − 1.13 eV.The activated lat- tice O 3c atom is pulled out of the surface, moving by 2.43 Å and reacting with CO, forming an adsorbed CO 2 molecule.Consequently, CO 2 formation produces an oxygen vacancy V O (V O @O 3c ), yielding two excess electrons which localize at Cu 2+ to form Cu + species.The adsorbed CO 2 molecule is linear with an OCO angle of 179.2° and an average C-O distance of 1.18 Å close to the typical CO 2 distance of 1.16 Å in the gas phase.The calculated energy E des for subsequent desorption of such a CO 2 molecule is 0.28 eV.Keeping in mind that entropy effects are not included in the presented energetics, we can safely assume that the desorption of the CO 2 molecule would be favorable in free energy.The adsorption of CO at Cu 2 O(100) surfaces has also been modeled, with a CO 2 formation energy of ~ 1.7 eV [78,79].Alike molecular CO adsorption, the dissociative adsorption of H 2 at O 3c forms a H 2 O molecule, as shown in Fig. 7e.The adsorption energy is − 0.87 eV, i.e., 0.26 eV less favorable than for CO.Furthermore, the desorption energy of the formed water molecule, E des H 2 O = 0.77 eV, is substan- tially larger than that of CO 2 .This rationalizes the more facile conversion of CO to CO 2 as the formed water would remain on the surface, blocking further H 2 activation.The calculated binding and desorption energies agree with previously published values of − 0.83 and 0.78 eV, respectively [64].The formed water molecule binds to a surface Cu atom at a distance of 2.05 Å and the measured bond angle and average H-O distance are 108.16°and 0.99 Å, respectively.These values are close to reported theoretical work with a bond angle and H-O distance of 107.5° and 1.0 Å, respectively [64].
Adsorption of CO and H 2 on Cu sites of the CuO(111) surface were studied next, locating two possible sites: Cu 3c (Cu 2+ ) and reduced Cu 3c (Cu + ) as listed in Table 1.The calculated adsorption energies support a spontaneous adsorption process for both molecules.CO likely adsorbs at the Cu 3c (Cu 2+ ) site with an adsorption energy of − 0.62 eV, Fig. 7b.In contrast, CO does not adsorb at the Cu 4c (Cu 2+ ) site and moves away, likely due to bond competition.As described above, when a vacancy is introduced (e.g., by a previous reaction with CO), two Cu 3c (Cu 2+ ) atoms are reduced and transformed to Cu + .In contrast to Cu 2+ , these Cu + ions adsorb CO twice as strong with E ads (CO) = − 1.29 eV, forming a CO-Cu + carbonyl species, Fig. 7c.This is in line with experimental reports of Cu + suggested as active centers for CO oxidation [10,[80][81][82][83][84] while Cu 2+ was also reported [53].The CO molecule binds to metallic Cu (Cu 0 ) on the surface exothermically with an adsorption energy of − 0.92 eV (Fig. S4d) that is in between the other two oxidation states.However, in oxygen-containing feed (high Δ O ), Cu 0 is hardly stable as the formation of CuO x species occurs [85].
The interaction of H 2 with Cu sites on CuO(111) was explored as well, (Fig. 7f, g and Fig. S4i of the SI), with H 2 placed on Cu 3c (Cu 2+ ), Cu + , and Cu 0 atoms.The H 2 molecule binds exothermically to the Cu + site with an adsorption energy of − 0.46 eV, 0.30 eV stronger than the Cu 2+ site, and agrees with a previously reported value of E ads = − 0.47 eV [86].In contrast, it shows very weak adsorption at Cu 0 , E ads H 2 = − 0.07 eV, which is in good agree- ment with a previous study of H 2 adsorption on metallic Cu(111), E ads H 2 = − 0.07 eV [87].Overall Cu + adsorbs both H 2 and CO better than the other oxidation states of Cu.
During the reaction the formed vacancies on the surface will be spontaneously replenished by gaseous O 2 forming an active oxygen species at the surface, O ad .Thus, the adsorption of CO and H 2 at O ad was studied accordingly.O ad reacts with CO and spontaneously forms CO 2 with a strongly exothermic reaction energy of − 4.02 eV (Fig. 7d).In the same way, the adsorption of H 2 also forms H 2 O with an adsorption energy of − 3.85 eV (Fig. 7h).As expected, the O ad species at the V O -CuO (111) surface is quite reactive in the oxidation process.A study of oxygen vacancy rich La 0.8 Sr 0.2 CoO 3 also suggested that the presence of nearby V O enhances the CO oxidation/interaction with adsorbed O ad species [73].Note that the possibility of hydroxyl formation [88] from H 2 adsorption at O 3c was also observed at a V O -CuO(111) surface (see Fig. S5).To complete the catalytic cycle, the formed species CO 2 and H 2 O desorb from the surface with energies of 0.30 and 0.88 eV.This again indicates a strongly favored CO 2 desorption, which rationalizes the experimentally observed high CO 2 selectivity.A possible reaction mechanism is described in the following section.

Suggested Reaction Pathways on the CuO(111) Surface
Following the results on the interaction of O 2 , CO, and H 2 at CuO(111) we suggest two reaction pathways (Fig. 8) based on a Mars-van-Krevelen (MvK) mechanism [24,89,90].Adsorbing CO and H 2 molecules from the gas phase on Cu 2+ (a1, b1) is slightly exothermic, but to form CO 2 / H 2 O and continue with the catalytic cycle the adsorption at an oxygen site is needed.Adsorption at O 3c forms CO 2 (a2)/H 2 O (b2) and one V O , with the CO 2 formation being slightly preferred.The formed CO 2 easily desorbs into the gas phase, while H 2 O desorption is substantially more challenging.These energies are compatible with the experimental results where CO 2 is preferentially produced up to ~ 100 °C, whereas above H 2 O formation sets in and dominates > 175 °C (Fig. 4).Note that the transformation of CO to CO 2 and H 2 to H 2 O on activated O 3c occurs without barrier as a downhill process except for the very first activation.In line with previous studies [91,92], our nudged elastic band calculations [93,94] showed these barriers to be smaller than ~ 0.5 eV for CO and smaller than ~ 0.7 eV for H 2 , around/below the "barrier" for the desorption process.
The reaction rate will thus be controlled by the desorption process, for which we have determined a significant difference between H 2 O and CO 2 .
The formed vacancy V O has two Cu + neighbors allowing further adsorption of CO (a4)/H 2 (b4).Alternatively, O 2 may repopulate the vacancy site (a5-b5), resulting in an added oxygen species, O ad .At this special oxygen another CO or H 2 may adsorb and form CO 2 (a6) or H 2 O (b6).Yet again, CO 2 (a7) desorbs more favorably by 0.6 eV into the gas phase than H 2 O (b7), completing the reaction cycle.

Summary
CuO/CeO 2 nanosphere catalyst particles are active in CO oxidation and CO-PROX and were tested in the present work via experimental and computational approaches.CO oxidation ignited at 95 °C, while under PROX conditions, 100% selectivity to CO 2 was maintained up to 100 °C, overall showing that the nanosphere catalyst works fairly well.Oxidative or reductive pretreatments, initially forming Cu 2+ or Cu 0 moieties, respectively, yielded no discernable difference in catalytic steady-state performance and activation energies.Hence, the same active catalyst phase is formed under reaction conditions.
Using DFT modeling, the energetics of likely reaction pathways on Cu oxide surfaces was further studied, indicating a conventional Mars-van-Krevelen (MvK) type mechanism.Upon CO adsorption and conversion to CO 2 , oxygen vacancies on Cu oxide surfaces were easily formed, rather independent of the oxidation state of neighboring Cu centers.For further adsorption, we determined that Cu + is the preferred binding site, with CO adsorption being much stronger than H 2 adsorption.Molecular O 2 adsorption and subsequent activation proceeds at a vacancy, creating highly active O species.Upon interaction with CO the formed CO 2 binds only weakly at the surface allowing for rapid desorption and freeing sites for further turnover, promoting CO 2 selectivity as observed experimentally.The adsorption/activation of H 2 is less feasible and the resulting water binds much stronger to the surface.This explains the undesired water formation only at high CO conversions and above 100 °C.Ceria is certainly beneficial due to its oxygen vacancies, but the support was not considered herein.Operando studies are planned for the future to elucidate active phases under reaction conditions.

Fig. 1
Fig. 1 Synthesis and electron microscopy of CuO/CeO 2 : a schematic illustration of the synthesis, b-d TEM images, and e EDX elemental mapping of the CuO/CeO 2 catalyst (oxidized at 300 °C)

Fig. 2
Fig. 2 Temperature-programmed reduction and X-ray diffraction of catalysts: Mass spectrometry profiles obtained during a CO-TPR of CuO/ CeO 2 , b H 2 -TPR of CuO/CeO 2 , and c XRD patterns of oxidized CuO/CeO 2 and reduced Cu 0 /CeO 2

Fig. 3
Fig. 3 Temperature programmed CO oxidation on CuO/CeO 2 (oxidized at 300 °C, 1 vol% CO and 1 vol% O 2 in He) a MS and b GC of CuO/CeO 2 .c The same for different pretreatments: oxidized at

Fig. 7
Fig. 7 Adsorption complexes at bare surfaces and those with oxygen vacancies: Top views of CO and H 2 adsorption on CuO 3c sites a, e, while b, c and f, g show adsorption on Cu sites.d and h represent CO 2 and H 2 O formed on the extra oxygen species, O ad , generated

Fig. 8
Fig. 8 Reaction pathways of a CO and b H 2 oxidation.Energy baseline of CO 2 and H 2 O products are marked in red and blue.Energies are reported relative to the bare surface and CO/H 2 in the gas phase.

Table 1
Oxygen vacancy formation energies E vac , of three-fold O 3c (V O @O 3c ) and four-fold O 4c (V O @O 4c ) coordinated O atoms, and adsorption energies E ads (X) , of CO, H 2 , and O 2 .The desorption energies E des (X) of CO 2 and H 2 O are included.Cu species refer to Cu 3c .Energies are given in eV